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• Nano-modified natural ester- paper cellulose interface can adsorb more H 3 O+. • Acids in paper cellulose is significantly reduced after nano-modification. • Nano-modification enhanced the ageing resistance of natural ester- paper cellulose system. The oil-paper insulation system will be affected by the synergistic effects of moisture and acids during long-term stable operation of the transformer. The moisture in the oil can be combined with the H+ionized from the acid molecules in the form of H 3 O+ by coordination, thus changing the proportion of acid distribution in the insulating oil and paper cellulose. Natural ester insulating oil, as a new development trend to replace mineral oil, can be significantly improved by adding nanoparticles to them. The diffusion behavior of H 3 O+ in paper cellulose and nano-SiO 2 -modified soybean oil-based natural ester insulating oil insulating system has been investigated using molecular dynamics in this article. The results indicate that the incorporation with nanoparticles is effective in mitigating the impact of temperature on the diffusion behavior of H 3 O+ in oil-paper insulation. Compared with the unmodified model, the nano-modified model results in a significant reduction in the concentration distribution of H 3 O+ in paper cellulose. The hydroxyl and ether bonds in paper cellulose will interact with H 3 O+ to generate hydrogen bonds and destroy its original hydrogen bonding network, while nanoparticles can inhibit diffusion by adsorbing H 3 O+ through hydroxyl oxygen atoms. [ABSTRACT FROM AUTHOR]

The transport properties of gas diffusion layer (GDL) in proton exchange membrane fuel cell are important parameters in fuel cell modelling, and one method to measure them is to simulate the transport of each species at spatial resolutions of a few microns in a microstructure of the GDL. One issue in this method is the size of the microstructure as using an unnecessarily big sample could substantially increase the computational cost. Both carbon-paper and carbon-cloth GDLs are investigated and their microstructures are obtained using numerical generation and X-ray micro-tomography respectively at a resolution of 1.733 µm. For each reconstructed GDL, we use the orthorhombic lattice Boltzmann model to simulate fluid flow through some subsamples taken from it in both the in-plane and the through-plane directions; the results show that the permeability of all subsamples is anisotropic in that their permeability in the in-plane direction is higher than in the through-plane direction. For each GDL, we compare its permeability calculated using samples of different sizes and find its representative elementary volume (REV) – a volume above which the calculated permeability represents the average ability of the GDL to conduct fluids. [ABSTRACT FROM AUTHOR]

Highlights • The applicability of the continuum hypothesis to transport in carbon-fiber papers is analyzed. • The LBM is combined with X-ray tomograms to simulate diffusion, convection and conduction. • A representative elementary volume cannot be defined due to the thin nature of these materials. • The anisotropic microstructure strongly reduces through-plane compared to in-plane transport. • The reduction is critical for solid-phase conductivity due to the bottleneck effect of the surface region. Abstract Understanding the transport processes that occur in carbon-fiber papers (CFPs) used in fuel cells, electrolyzers, and metal-air/redox flow batteries is necessary to help predict cell performance and durability, optimize materials and diagnose problems. The most common technique used to model these thin, heterogeneous, anisotropic porous media is the volume-averaged approximation based on the existence of a representative elementary volume (REV). However, the applicability of the continuum hypothesis to these materials has been questioned many times, and the error incurred in the predictions is yet to be quantified. In this work, the existence of a REV in CFPs is assessed in terms of dry effective transport properties: mass diffusivity, permeability and electrical/thermal conductivity. Multiple sub-samples with different widths and thicknesses are examined by combining the lattice Boltzmann method with X-ray tomography images of four uncompressed CFPs. The results show that a meaningful length scale can be defined in the material plane in the order of 1–2 mm, which is comparable to the rib/channel width used in the aforementioned devices. As for the through-plane direction, no distinctive length scale smaller than the thickness can be identified due to the lack of a well-defined separation between pore and volume-averaged scales in these inherently thin heterogeneous materials. The results also show that the highly porous surface region (amounting up to 20% of the thickness) significantly reduces the through-plane electrical/thermal conductivity. Overall, good agreement is found with previous experimental data of virtually uncompressed CFPs when approximately the full thickness is considered. [ABSTRACT FROM AUTHOR]

A paper-based channel is a useful platform for the facile development of analytical devices implementing various chemical or biological reactions. To improve the analytical performance for various applications, it was necessary to analyze the detailed mixing characteristics within a paper-based channel. In this paper, we proposed a mathematical model to predict a concentration field created as a result of the imbibition of multiple fluids within a porous material. Interestingly, we found that the model exhibited a constant interdiffusion width within a paper-based channel even though the flow front velocity decreased over time. We were able to verify that our model accurately predicted the concentration field by comparing the experimental and numerical results for mixing in a 2 inlet-channel. Finally, we designed and fabricated paper-based channels to generate two (linear and non-linear) concentration gradients based on predictions made by the model. Both the experimental and numerical results were in good agreement, demonstrating that our model was accurate and useful for developing a paper-based analytical device utilizing the mixing characteristics of a sample and reagent flow system. [ABSTRACT FROM AUTHOR]

• Innovative concept for drying of a moist porous medium, namely paper, with a PCM is proposed. • The presence of PCM is studied numerically in two different configurations: a layer of PC embedded in the middle of a paper sheet, and uniformly dispersed PCM particles within the paper sheet. • Presence of an infrared emitter within the drying process is studied where the corresponding absorption coefficient of PCM is measured experimentally. • The numerical results indicate that the PCM acts as a heat source and a heat sink throughout the drying process, enhancing the drying rate. This work numerically investigates the transport characteristics associated with the drying process of paper in the presence of a phase change material (PCM). The presence of PCM is studied in two different configurations: a layer of PCM embedded in the middle of paper and the PCM particles dispersed uniformly within the paper. This unique concept offers a new approach to paper drying by utilizing the latent-heat of fusion associated with the PCM, favorably impacting the energy efficiency of the drying process. The results indicate that the PCM acts as a heat source and a heat sink throughout the drying process, enhancing the drying rate. This study also includes the presence of an infrared emitter within the drying process. The corresponding absorption coefficient of PCM is measured experimentally. This paper is not concerned with the impact of presence of PCM on paper sheet properties.The proposed operating conditions and configuration presented here are solely for the purpose of illustration of the concept of drying in the presence of PCM. [ABSTRACT FROM AUTHOR]

• A comprehensive review on numerical simulations of boiling heat transfer from atomistic to mesoscopic and macroscopic scales is presented. • Critical issues related to nanoscale bubble nucleation mechanisms, pool boiling, and flow boiling are highlighted. • Potential solutions and future research in the field of boiling heat transfer using the MDS, LB and CFD methods are proposed. Boiling is an efficient heat transfer mode with significant potential for thermal management in high-power electronic equipment. However, a comprehensive understanding of the boiling process, which encompasses bubble nucleation, growth, coalescence, slipping, and detachment across various scales, remains challenging. Molecular dynamics simulation, lattice Boltzmann, and computational fluid dynamics methods are popular and powerful tools for investigating boiling heat transfer phenomena at microscopic, mesoscopic, and macroscopic scales. These methods enable researchers to uncover the underlying boiling mechanisms and propose heat transfer enhancement techniques. Therefore, this paper provides a comprehensive review of boiling heat transfer, spanning from atomistic to mesoscopic and macroscopic scales, utilizing these three numerical methods. It addresses critical issues related to nanoscale bubble nucleation mechanisms, pool boiling, and flow boiling, and proposes potential solutions and future researches, supplementing our previous review [Some advances in numerical simulations of multiscale heat transfer problems and particularly for boiling heat transfer, Annu. Rev. Heat Transf., 6 (2022) 217–269]. Besides, by shedding light on the characteristics of these numerical methods in studying boiling heat transfer, this paper aims to foster their development and advance enhanced heat transfer technologies. [ABSTRACT FROM AUTHOR]

• A new radiometric temperature measurement platform under particulate interference was established. • Effects of water spray, smoke, soot, and flame on single-color and two-color radiation thermometry were studied qualitatively. • Effects of water flow rate, gas pressure, water spray layer's thickness, and target temperature on radiometric temperature measurement errors were investigated quantitatively. This paper established a radiation temperature measurement system with its optical path interfered with particulates and investigated the influence of particulate interference on single-color and two-color radiation thermometry qualitatively and quantitatively. The investigation revealed some experimental phenomena never reported before, and theoretical analysis was used to explain these observations. When path between pyrometer and target contained particulates, both single-color and two-color temperature measurements can either be lower or higher than true target temperature, and two-color temperature measurement errors may be greater or smaller than single-color temperature measurement errors, depending on the thermal properties, geometric properties, and radiation characteristics of the particulates. When the directions of temperature fluctuations caused by the particulates in single-color and two-color measurements were opposite, the true temperature value unaffected by the particulate lay between the two temperature values. When the fluctuation directions were identical, the temperature value with smoother fluctuation was closer to the true temperature value. The experimental conclusions presented in this paper contributed to a better understanding of the impact of particulates on radiation temperature measurement and can be used to further guide the correction of radiation temperature measurement errors caused by particulate interference. [ABSTRACT FROM AUTHOR]

• Stabilization of boundary conditions obtained from the solution of the inverse problem. • Cooling inside a furnace intended for thermo-chemical treatment. • The impact of the regularizing term on the solution of the inverse problem. Heating and cooling inside a furnace intended for thermochemical treatment is the basis for the execution of processes characterized by lower energy consumption, and, thus, lower costs. Precise control of the thermochemical treatment requires knowledge of the temperature, heat flux, and the heat transfer coefficient (HTC) on the boundaries of the treated elements. The mentioned boundary conditions have been established in this paper for the cooling stage inside the furnace for thermochemical treatment by solving the inverse heat conduction problem. The paper analyzes the impact of regularization of the inverse problem on the stability of the calculated boundary conditions during cooling. Numerical tests were performed for the Tikhonov and Tikhonov-Phillips regularization as well as their modification. The computational method presented in the paper was developed for online control of thermochemical treatment processes. Online control necessitates minimizing computation time. The paper investigated the impact of the regularization term on the solution of the inverse problem with a constant value of the regularization parameter. The choice of regularization method was based on a norm that measures the oscillation of the solution of the inverse problem. The approach presented in the paper allowed for a reduction in computation time and the obtaining of a stable solution to the inverse heat conduction problem for both numerical tests and experimental data. [ABSTRACT FROM AUTHOR]

• Granular PCMs are proposed to be used in respirators for breathing air cooling. • Commercially available PCMs can be used to cool the inhaled air in indoor fires. • An increase in the melting temperature worsens the cooling efficiency of the respirator. • Time variations of the outlet air flow rate can considerably affect the device characteristics. • Partial closure of the respirator outlet can decrease the operating time of the device. In this paper, we propose to use granular phase change material (PCM) as a heat accumulating material in a thermal protective respirator also called a heat exchanger to cool the breathing air. Such device is needed to protect human respiratory system from thermal injuries at indoor fires. Using the original solver for OpenFOAM software developed and validated previously, the ability of the commercially available granular PCM in the respirator to cool hot air for breathing during the time required for the human evacuation has been shown numerically: mass of 130 g is enough to cool the air from 177 °C to a temperature of less than 50 °C within 10 min. This result gives hope that such cooling element can be combined with "conventional" gas-and-smoke protective element, resulting in the creation of respirators for the protection of a person from both hot air and toxic combustion products. A parametric study of the performance of the thermal protective respirator is carried out in the paper: effect of PCM melting temperature, time variation of the outlet air flow rate and partial closure of the outlet from the cooling element have been analyzed. It has been shown that an increase in the PCM melting temperature while keeping the other parameters unchanged worsens the thermal performance of the breathing air cooler. The cooling efficiency is also worsened when the cooling element is narrowed or its outlet is partially closed, as this leads to an increase in the maximum temperature of the inhaled air. It has been shown that the time variation of the inhaled air flow rate, which is used in the calculations of the thermal protective respirator, must be close to the parameters of real human breathing, and the use of the assumption of constant air flow rate is unacceptable, since this can significantly affect the output characteristics of the respirator. [ABSTRACT FROM AUTHOR]

• The diffusion and mixing characteristics of liquid jet with phase-transition effected by shock waves were analyzed. • The influence law of shock waves on the kerosene diffusion and mixing process effected by evaporation was obtained. • The mechanism that shock waves enhance the diffusion and mixing capability of liquid kerosene was revealed. The diffusion characteristics of liquid jets with phase transition under the action of shock waves in supersonic high-enthalpy crossflow were discussed in this paper. Numerous numerical simulations were carried out in high enthalpy inflow (T t = 1680 K) under different shock wave intensity and injection parameter conditions. The Euler-Lagrangian method was employed to solve the gas-liquid two-phase flow coupling, and the evaporation, diffusion and mixing processes of kerosene were obtained. By analyzing simulation results, the influence and the mechanism of shock waves on enhanced kerosene mixing were revealed. The results show that increasing the shock wave intensity effectively improves the penetration and mixing of kerosene in general by changing the flow state and evaporation process. The penetration depth can be increased by up to 63.54 % in this paper. To balance the contradiction between total pressure loss and mixing efficiency, it is necessary to select an appropriate shock wave intensity. The enhanced diffusion and mixing process of kerosene by shock waves are related to the injection momentum ratio, evaporation momentum ratio and streamwise vortex intensity. Weakening mainstream momentum while boosting evaporation and vortex intensity effectively strengthens kerosene diffusion and mixing. This research significantly contributes to enhancing the diffusion and mixing of liquid kerosene and improving combustor performance. [ABSTRACT FROM AUTHOR]

This paper presents new experimental data on pressure drop, heat transfer coefficient, and critical heat flux during flow boiling of low-pressure fluid R123 in microchannels. High mass velocities were used, above the values found in the literature in tests with this fluid. Experiments were conducted in a single smooth and horizontal microchannel, with an internal diameter of 1.1 mm and heating sections of 46 and 100 mm. The mass velocities used were between 1500 and 7500 kg/m2s. Saturation temperature ranged from 40.1 to 85.1 o C. The outlet vapor quality varied between -0.54 and 0.53 for the CHF results, between 0 and 0.5 for the pressure drop results, and between 0 and 0.35 for the HTC results, respectively. Record critical heat flux values for flow boiling of R123 in smooth microchannels were achieved, reaching 1.44 MW/m2. An analysis was made of the influence of an upstream pressure drop in avoiding instabilities during critical heat flux experiments. The highest heat transfer coefficient values during convective boiling of R123 in smooth microchannels were also obtained, with values close to 43.3 kW/m2K. The saturation temperatures used were among the highest for flow boiling of R123 in smooth microchannels found in the literature. Correlations for frictional pressure drop, heat transfer coefficient, and critical heat flux were evaluated in predicting the experimental data. An analysis was made on the influence of the void fraction correlation when estimating both the accelerational and frictional pressure drop. • Flow boiling tests at high mass fluxes for R123 in microchannel are presented. • New record Critical Heat Flux for R123 in smooth microchannels was obtained. • The highest value of heat transfer coefficient for R123 was measured. • This paper presents the first available diabatic pressure drop data for R123 in microchannels. • Experimental data are compared against predictive methods. [ABSTRACT FROM AUTHOR]

This paper offers a comprehensive exploration of thermal analysis for space systems operating in low Rayleigh atmospheres, with a primary focus on enhancing the accuracy and reliability of convective heat transfer modeling. The study centers around an analysis of a vertical heated plate experiment, where a lumped parameter model is employed to encompass conductive, radiative, and convective heat transfer mechanisms. To advance the accuracy of convective heat transfer coefficient calculations, this paper introduces a novel approach involving the utilization of Nusselt number correlations. These correlations are developed through an analytical solution derived from the Navier-Stokes equations, tailored specifically for the internal natural convection problem. This approach treats convection as a heat exchange process between the heated plate and the surrounding air, thereby improving the precision of modeling internal convection within a lumped parameter framework. A specific correlation (González-Bárcena et al.) is obtained for the proposed problem, derived from experimental tests conducted in a thermal vacuum chamber, under varying pressure conditions to control the Rayleigh number. Additionally, the findings are validated using Computational Fluid Dynamics (CFD) tools to calculate the velocity and temperature fields within the cavity. The González-Bárcena et al. correlation significantly enhances temperature predictions, demonstrating an improvement of up to 10 ∘C when compared to conventional literature correlations. The methodology presented in this paper holds promise for extension to more complex experiments in low Rayleigh atmospheres, including those conducted in the Earth's stratosphere or on the Martian surface, thereby contributing to the advancement of thermal analysis in challenging space environments. • CFD and lumped parameter models have been used for thermal analyses. • Atmospheric tests have been conducted to characterize convection. • A new correlation for internal convection has been experimentally validated. • Different author correlations for vertical plates have been compared. [ABSTRACT FROM AUTHOR]

· A new numerical model for frosting is developed. · Phase change model based on nucleation theory is modified. · Packing limit value of frost is corrected. · A new method for calculating two-phase average temperature of frost is proposed. · Multiple parameter verification shows that the error of the new model is within 15 %. Cold surface frosting is a complex process involving multi-component phase change heat and mass transfer and variable void structure. It is very challenging to deeply understand the frosting law and mechanism and accurately simulate the frosting process. Existing numerical models of frost growth have many shortcomings, such as a narrow application range, few verification parameters and low reliability. For these reasons, this paper developed a numerical model of frost layer growth based on nucleation theory, modified the phase transition model, and proposed a formula for constraining the picking limit of the frost layer. Additionally, it introduced a formula for calculating the average temperature of the two phases in the frost layer. The simulation results for frost layer thickness, density, temperature, and distribution were extensively verified using experimental data obtained by different scholars under various conditions and compared with simulation results in related literatures. The results show that the simulation results of all parameters under different conditions are in good agreement with the experimental data, with a maximum deviation of less than 15 %. Notably, the simulation results for frost thickness and distribution are more accurate than those found in existing literatures. The model developed in this paper offers a higher number of validation parameters, a wider validation range, and greater accuracy and reliability. This model can provide a solid theoretical foundation for studying frost characteristics under complex conditions and for the optimal design of frost-prone heat exchangers. [ABSTRACT FROM AUTHOR]

• A superimposed model applicable for predicting the growth rates of both stationary and sliding bubbles is proposed. • The force balance model for predicting bubble departure diameter and lift-off diameter is modified. • Present modified model predicts both departure and lift-off diameters with mean absolute percentage errors of 25.0 % and 24.0 %, respectively. The calculation of heat flux for each component in the wall heat flux distribution model involves a crucial parameter, namely the characteristic bubble diameter. The bubble departure diameter and lift-off diameter are two of the most significant characteristic bubble diameters. In this paper, the bubble growth rate and bubble force analysis in subcooled flow boiling under various inclination angles of the heating surface are investigated, and the bubble departure and lift-off diameters are calculated based on force balance. According to the energy balance method, a superimposed bubble growth model is proposed that incorporates the effects of superheated layer evaporation around the bubble, microlayer evaporation, and condensation at the top of the bubble on the bubble growth rate. This paper proposes a force balance model for heating surfaces with varying inclination angles, based on accurate calculations of wall superheat. The model has been validated by a large amount of experimental data and exhibits superior accuracy and stability compared to existing models. [ABSTRACT FROM AUTHOR]

• Local condensation heat transfer in smooth channels and pin fin array is presented. • The effect of operating parameters on the heat transfer characteristics was analyzed. • Sensitivity analysis is performed of heat transfer to identify the critical parameters. • Condensation heat transfer increases with heat flux in the range of vapor quality above 0.4. In this paper, an experimental study of flow condensation was carried out of R134a in a multiport mini channel and a mini channel with pin fin array. The former comprised 20 parallel rectangular channels with an equivalent diameter of 0.64 mm. The latter is a narrow rectangular mini channel containing 10 rows of diamond pin fins with a staggered configuration, and height and longitudinal/transverse pitch of 0.5 mm and 2 mm respectively. To acquire the local value of heat flux and heat transfer coefficient, air jet impingement cooling devices were adopted to condense the vapor of refrigerants. The effects of vapor quality, mass flux, heat flux, and saturation pressure on flow condensation heat transfer coefficient were investigated with the operating conditions: vapor quality from 1 to 0, mass flux from 160 to 450 kg/(m2s), heat flux from 10.0 to 39.8 kW/m2, and saturation pressure from 600 to 1500 kPa. The experimental results indicated that the pin fin array significantly improves the condensation heat transfer coefficient up to nearly 186 % compared to the multiport mini channel in the high vapor quality region. For both channels, the local heat transfer coefficient increases with an increase in vapor quality, mass flux, and heat flux whereas decreases with increase in saturation pressure. The influence of heat flux and mass flux on heat transfer coefficient was more pronounced in the high vapor quality region than in the low vapor quality region. A sensitivity analysis was performed at different vapor quality levels. The most influential parameters in the smooth channel and the pin fin array are saturation pressure and mass flux, respectively. The relative contribution of heat flux is more obvious in the high vapor quality region. Some of the existing correlations have good prediction performance for the smooth channel experimental data in this paper, and the corresponding MAD is within 20 %. However, their deviations are much greater for the applications of pin fin array. [ABSTRACT FROM AUTHOR]

• Applied the finite data analysis method to optimize the diffusion layer structure. • Obtained and provided the optimal tapered diffusion layer structure. • Improved the overall performance of the PEMFC about 3.5 %. • Analyzed the synergy performance of the new diffusion layer structure by the multi-physical field synergistic. Optimized design of the full structure is a key solution to improve the overall performance of proton exchange membrane fuel cells and advance their commercialization. This paper established a three-dimensional mathematical model of PEMFC, and obtained a series of simulation data collection under different structural parameters. On this basis, we derived the mapping relationship between the thickness of the diffusion layer and the output power using response surface analysis, and then obtained the optimal structural parameters by the BP neural network. The optimization provided a new type of gradient diffusion layer used in the PEMFC, i.e., a tapered diffusion layer. Compared with the fuel cell using a conventional diffusion layer under the same flow channel structure and operating conditions, the results show that the average power of the fuel cell using a gradient diffusion layer has been increased by 3.5 %. The overall heat transfer capacity increased by 10.3 %, and the oxygen utilization increased by 8.7 %. Finally, this paper analyzed the multi-physics field synergistic performance of PEMFCs with different diffusion layer structures, and found that the fuel cell with tapered diffusion layer has better synergistic performance in terms of temperature and concentration fields, and temperature and velocity fields. [ABSTRACT FROM AUTHOR]

• The liquid metal cross flow tube bundle under pulsation condition is simulated. • The effect of different pulsation frequency and amplitude are analyzed. • Circumferential flow field and temperature field of the single tube are analyzed. • The vortex characteristics and turbulent structure are analyzed with PSD and CWT. Liquid metal has a broad application prospect in tube bundle heat exchangers because of its excellent thermal conductivity. However, conventional in-line tube bundle arrangements often exhibit limited secondary flow, so new methods are urgently needed to improve heat transfer. The topic of this paper is to combine two methods, liquid metal cross flow tube bundle and pulsating flow, to improve the heat transfer performance in-line tube bundles. Firstly, the applicability of the model was validated through experimental data from existing literature, encompassing overall flow dynamics, heat transfer performance, and circumferential pressure distribution around the tubes. Subsequently, numerical simulations are conducted involving liquid metal cross flow in-line tube bundles at varying pulsating frequencies, amplitudes, and Reynolds numbers. Circumferential pressure distributions and temperature profiles for individual tubes were analyzed and compared. The amplitude of reduction in thermal resistance compared to the no-oscillation condition increases gradually from a minimum of 8 % in the first row to a maximum of 40 % in the third row, while the reduction in the rear row of tubes is similar to that of the third row. Remarkably, a reversal in the trend of thermal resistance for the first three rows of tubes under pulsating flow were observed, contrasting conventional flow except for the case with an amplitude of 0.1. By analyzing the local transient flow field distribution near single tube and the corresponding circumferential heat transfer performance, the influence of the vortex evolution characteristics on the heat transfer performance at different phases of the pulsating velocity is revealed, which transient Nu is up to three times that of flow with no pulsating. Finally, a series of coherence analyses reveal that as frequency rises, turbulence exhibits a richer spectrum of small-scale vortex structures, intricately linked to enhanced energy cascade efficiency. Amplitude augmentation intensifies velocity fluctuations, particularly at high amplitudes, resulting in substantial energy peak amplification. The integrated heat transfer performance PEC factor varies in the range of 1.25 to 1.51 for all the conditions simulated in this paper, which indicates that the liquid metal coupled pulsating flow approach can significantly increase the comprehensive performance of the tube bundle heat exchangers. These findings hold significant promise for advancing heat exchanger design and enhancing thermal efficiency, with implications for various engineering applications. [ABSTRACT FROM AUTHOR]

• The jet interaction between hot and cold fluids is accompanied by strong heat exchange and phase change processes, and such dramatic variations lead to the eventual dispersion of numerous fragmented bubbles in the high-temperature liquid pool. • The test result shown that local vapor explosion is the primary mechanism leading to bubble fragmentation. • The fragmented bubble size distribution was modeled, which is promising to provide guidance for the bubble behavior characteristics after steam generator tube rupture accidents in lead-cooled fast reactors. Bubble fragmentation and size distribution in hot and cold fluid jet interaction are fundamental for industries such as energy and metallurgy, especially for safety analysis after steam generator tube rupture (SGTR) accidents in lead-cooled fast reactors. The opaqueness to light of the molten lead pool makes it difficult to capture the gas-liquid interface behavior and thus lacks relevant data. In this paper, silicone oil was used instead of lead to simulate the injection process of the SGTR accident. Then the jet visualization experiments were carried out to investigate the bubble fragmentation characteristics. The visualization results show that the jet between the hot and cold fluids process is accompanied by strong thermal interactions, eventually leading to many bubbles dispersed in the high-temperature liquid pool. Secondly, four mechanisms of action affecting bubble size were elaborated in this paper, among which local vapor explosion is the primary mechanism of bubble fragmentation. Finally, the data revealed that bubble size presented a lognormal distribution, and the fragmented bubble size distribution was modeled. This study is promising to provide guidance for the bubble behavior characteristics after SGTR accidents in lead-cooled fast reactors. [ABSTRACT FROM AUTHOR]

In this paper the aerothermal heating of a reusable launch vehicle is reconstructed on the basis of temperature measurements taken in the thermal protection system of this vehicle. The discussed integrated thermal protection system is composed of three layers. Mathematical model, describing the problem, takes into account the dependence on temperature of the material parameters as well as the thermal resistances occurring in the contact zones of the layers, which is a novelty in the proposed approach. For solving the direct problem, the implicit scheme of the finite difference method is applied. Next, by using the solution of the direct problem, the Tikhonov functional is created, which describes the error of the current approximate solution. Whereas for determining the solution of the inverse problem the Levenberg-Marquardt method, modified and adapted to the Tikhonov functional, is used. The paper presents the mathematical model of the problem and the method of solution together with the selected examples illustrating its exactness and stability. In order to better examination of the solution method some various values of parameters are taken in the demonstrated examples. • Reconstructing an aerothermal heating in space vehicle protection system with temperature dependent material properties. • Reconstructing an aerothermal heating taking into account the thermal resistance between layers. • Solving the problem for three layers model of the thermal protection system. • Inverse problem solved for input data taken from NASA measurements. • Heat flux measured by NASA reconstructed with good precision and stability. [ABSTRACT FROM AUTHOR]

• Optimize the thermosyphon loop thermal performance by controlling loop wettability. • Long-term effective hydrophilic and hydrophobic modification method was determined. • The optimum wall wettability under different filling ratios was clarified. • The optimized R loop is reduced by 6.9% to 53.5% under different filling ratios. The behavior of vapor-liquid two-phase transport within the riser of the two-phase thermosyphon loop significantly impacts the heat transfer process from the evaporation to the condensation, as well as the circulation velocity, thus playing a crucial role in the heat transfer performance. However, existing research has predominantly focused on optimizing the performance of the evaporator, overlooking the optimization of vapor-liquid two-phase flow patterns and transport characteristics within the loop. This paper aims to optimize the heat transfer performance of the two-phase thermosyphon loop by controlling the wall wettability of the loop. The strategy employed involves modifying the wettability of the riser to regulate the two-phase flow within the loop, while visualization techniques are utilized to investigate the influence of riser wettability on the vapor-liquid two-phase distribution and heat transfer performance of the two-phase thermosyphon loop. The findings reveal that the impact of wall wettability on flow behavior and heat transfer performance within the loop varies significantly with changes in the filling ratio. This paper sheds some light on the riser wettability of two-phase thermosyphon loops to optimize the thermal performance. [ABSTRACT FROM AUTHOR]

Highlights • A paper-based TIM is fabricated using graphene and p-polymer. • The thermal conductivity is measured using laser flash method. • The fabricated graphene paper has significantly enhanced in-plane thermal conductivity. • The fabricated graphene paper has good mechanical, thermal and electrical characteristics. Abstract In this study, an innovative thermal interface material (TIM) paper based on a composite of cellulose and graphene is investigated experimentally. Six types of commercially-available papers: a wool paper; an aqua satin; a merit paper; a new craft board; and two oriental traditional papers (Bulgyeong and Daerye) are used to fabricate the paper-graphene composites via bar coating and a slot die coating. The fabricated TIM papers are lightweight, flexible and robust against tensile strength. The in-plane and through-plane thermal conductivities of the TIM papers are measured using a laser-flash-method (LFM). The measured in-plane thermal conductivities are of the order of 5 W/m-K, whereas the through-plane thermal conductivities are of the order of 0.1 W/m-K. These results suggest that the addition of graphene significantly enhance the in-plane thermal conductivity of papers, while the through-plane thermal conductivities are not significantly improved. The mechanical properties of the TIM papers are also tested. This work provides a new possibility for development of next-generation thermal interface materials with good thermal and mechanical properties. [ABSTRACT FROM AUTHOR]

Highlights • The rough and hydrophobic coating surface retarded the frost growth. • The optimum condition between the frost layer thickness and the sandpaper grit size was obtained. • The effect of hydrophobic surfaces was observed with enhanced roughness. Abstract Frost retardant of glass fiber (GF)/unsaturated polyester (UP) composites was evaluated with surface roughness. Glass fiber reinforced composites (GFRCs) were manufactured, using 8 ply fabric type GF mats and UP at 50 vol% volume fraction, using a vacuum assisted resin transfer molding (VARTM). Surfaces of the GFRCs were sanded using different grit sand papers. Static and dynamic contact angle (CA) measurements were performed to determine the hydrophobicity of specimens with different surface roughness. Observation of frost thickness versus time was performed to evaluate frost retardant as a function of surface roughness. An atomic force microscope (AFM) was used to quantify the surface roughness produced by sanding with different grit sand paper. It was found that surface roughness, CA and frost formation thickness were all-dependent on the grit of sand paper used in the sanding process. These behaviors appeared to be interrelated with an optimal sanding grit size of 320 CC. [ABSTRACT FROM AUTHOR]

• Algorithm for PLIC-based interfacial area concentration on 3D non-orthogonal grids. • Thermal phase change model with geometrical basis and free of empirical input. • Oscillating behavior of GOX jet plume in LOX crossflow is reproduced in simulation. • Imbalance between the mass transfer rate and the supply causes the oscillation. • Methodology developed is verified by the Stefan problem and the bubble experiment. In order to enhance the physical basis of the mass transfer model in numerical simulation of gas-liquid direct contact condensation and improve the fidelity, a set of methods and models are developed. Based on the concept of the piecewise-linear interface calculation (PLIC), an interface reconstruction algorithm is proposed to calculate the interfacial area concentration in three-dimensional non-orthogonal structured grids. The actual interface in the cell is replaced by a plane, whose position is determined iteratively to satisfy the local volume fraction and orientation of the actual interface. The area of the polygonal cut by the plane is taken as the interfacial area in the cell. The proposed algorithm is validated by calculating the surface area of a series of basic geometries. Obvious superiority of the proposed algorithm over the traditional models (gradient model and algebraic model) is seen. Then, the reconstructed area is used to determine the interfacial area concentration in the thermal phase change model. The process is implemented by ANSYS Fluent user-defined functions. The Stefan problem and steam bubble condensation experiment are used to test the performance of the developed methods and models. The simulation results agree well with the theoretical and experimental results with the quantitative relative errors no more than 4.3 % and 15 %, respectively. The oxygen jets condensation experiments are also used as test cases. The primary dominant frequencies predicted by the simulations are 163.01 Hz and 131.65 Hz, which agree well with the 139.72 Hz and 125.75 Hz in the experiments. The relative errors are only 17 % and 4.7 %. The unstable flow pattern of oxygen jet in liquid oxygen crossflow is explained based on the dynamic characteristics of turbulence, heat and mass transfer. The periodic variation of mass transfer rate is considered as the cause of condensation oscillation. The method for calculating the interfacial area proposed in this paper can serve as an optional and practical improvement to the numerical simulation of direct contact condensation. [ABSTRACT FROM AUTHOR]

Proton Exchange Membrane Fuel Cell (PEMFC) is widely recognized for its cleanliness and high efficiency, but is still facing challenges in cold environments. At low temperatures, the formation of ice and repeated freezing/thawing cycles may cause cell performance reduction and irreversible degradation. The cathode flow field of PEMFCs has a significant effect on the performance. In contrast to the conventional "channel-ridge" flow field, the metal foam has the advantages of excellent pre-distribution of gases and water drainage, which make it a promising candidate for the cold start. This paper examines the cold start of PEMFCs with metal foam flow field (MFFF) and serpentine flow field (SFF), and the influence of constant current mode, constant voltage mode, and ramping current mode is investigated experimentally through performance test and electrochemical characterization. The results show that lowering the voltage and increasing the current can enhance the cold-start performance of fuel cells. The MFFF fuel cell has superior cold start performance compared to the SFF fuel cell under the constant voltage mode of 0.3 V. Furthermore, the variable current mode is developed by considering the distinct properties of heat and water production during various phases, and the results indicate that increasing the current density at the unsaturated stage leads to an elevated rate of heat production and a reduced rate of water production, which can improve the cold start of PEMFCs. • The cold start of PEM fuel cells with metal foam flow fields is studied. • The influence of startup modes is investigated through experimental testing. • Increasing current and decreasing voltage result in higher rate of temperature rise. • Variable current mode is designed to balance water and heat generation in cold start. • Increasing current density in the unsaturated stage improves cold start performance. [ABSTRACT FROM AUTHOR]

• A defect model modification method with surface heat flux identification is proposed for thermal protection system. • Levenberg–Marquardt method is adopted to identify surface heat flux, and whether the defect has occurred is determined. • A defect model modification method is proposed from the knowledge of the identified heat flux. • An experiment is designed to prove good accuracy of the proposed method. Thermal protection systems (TPS) have become a key issue in ensuring the reliability of hypersonic flights. Aimed at monitoring the thermal loads and possible defects for TPS, a defect model modification method with surface heat flux identification is proposed in this paper. Firstly, by the measured temperature from a distributed sensor network and the initial TPS model, the surface heat flux was identified with the Levenberg–Marquardt Method (LMM). If there existed outliers in the identified heat flux distribution, it can be visually indicated that the defect occurred. Then, defects were represented by shape parameters and can be determined by the model modification from the knowledge of the identified heat flux and the measured temperature. The effectiveness of the proposed method was verified numerically using TPS containing a surface defect. Meanwhile, the effects of various factors on the identified results were investigated in detail, such as defect sizes, defect shapes, sensor locations, and real aerodynamic heating. Finally, the proposed method was successfully applied to an experiment, and results demonstrated that the proposed method can achieve good identification results by experimentally measured temperature. [ABSTRACT FROM AUTHOR]

• The transverse thermal resistance of chip was 1.25 times of the longitudinal thermal resistance. • The max temperature is 72.45 ℃, 43.15 ℃, 37.02 ℃ and 36.58 ℃ while the defect is missing bump and pad, missing bump, normal bump and bridge bump, respectively. • The normal bump and bridge bump are inspected by the max amplitude which the normal is 22.5 and the bridge is 26.5. • The state changed from solid to mushy and mushy to liquid when the time was 3.5 s and 8.45 s, respectively. Flip chip technology has been used extensively in microelectronic packaging due to the high density, fine spacing, smaller size. However, the size and spacing of the solder bumps are decreasing gradually, defect detection is getting more and more difficult. Thus, the growing demand for high reliability has generated considerable attention on the importance of defect inspection. This paper proposes an in-situ infrared thermography monitoring method with in situ monitoring system bases on modified thermal resistance network model that added the phase transition for the non-destructive analysis of packaging process. The different defects, such as missing bump and pad, missing bump, and bridge are inspected with an accuracy of up to 90 % based on in situ monitoring system. The maximum error between the model and the experiment is 27.6 %, while the minimum error is 13.6 %. The experiment results show that the max temperature of missing bump and pad, missing bump is 35.43 °C and 6.13 °C higher than normal, and the bridge is 0.44 °C lower than normal, respectively. The maximum amplitude by the Fourier transform is used to inspect due to the max temperature of normal and bridge are indiscernible, the results show that the bridge is 4 lower than normal. The model and experimental results show that the in-situ infrared thermography monitoring method is effective for detecting defects in high density electronic devices. The proposed method of in situ monitoring is expected to provide a new strategy for next generation of three-dimensional heterogeneous integrated chips defect inspection. [ABSTRACT FROM AUTHOR]

• Internal structure of the composite material is adjusted by AC electric field. • The high in-plane thermal conductivity of h-BN is fully exploited. • New thermal conductivity model on h-BN orientation was established. The excellent electrical insulation properties and mechanical properties of epoxy resin (EP) have led to its rapid development in the field of microelectronic packaging. However, the poor thermal conductivity of epoxy resins (thermal conductivity of only 0.14 W/(m·K)) limits the development of components for high power. Adding h-BN to epoxy resin to prepare composites can effectively improve the thermal conductivity of epoxy resin. In this paper, h-BN/EP high thermal conductivity composites were prepared by means of AC electric field induction with the goal of making the composites low loading and high thermal conductivity (loading of 10 wt%). It was shown that the h-BN/EP composite had the highest thermal conductivity of 0.310 W/(m·K) when applied to an AC electric field of 120 V/mm and 50 Hz for 1 h. It was 2.214 times higher than that of EP and 1.325 times higher than that of the same-loaded h-BN/EP composite (0.234 W/(m·K)). A thermal conductivity model that can predict the thermal conductivity of composites prepared using the same type of method is also developed, which extends the applicability of the thermal conductivity model. The prepared composites have good thermal management capability. The method is easy to be engineered and has great potential for use in electronic packaging and even other high thermal conductivity applications. [ABSTRACT FROM AUTHOR]

• The dynamics of the thermal runaway front in large-scale Li-ion batteries are deduced theoretically. • The velocity of the front yields a 1/2 power relationship between the thermal conductivity and reaction rate. • Freezing thermal runaway front is possible in theory and validated by experiments. • Heat dissipation enlarged by heat transfer time delay is critical for the freeze. Large-scale lithium-ion batteries are favored in electric vehicles and energy storage stations; for instance, BYD blade batteries and CATL Kirin batteries are popular. A tiny defect will trigger thermal runaway from a local point towards the other parts of the battery, and the boundary between the failure region and the intact region is called the "thermal runaway front," of which the dynamics are still unclear. Herein, this paper deduces two formulas for the moving speed (v TR) of the thermal runaway front by applying the principles of energy conservation (Formula I) and flame front (Formula II); formulas for the thermal runaway front velocity (v TR) are derived. The velocity v TR conforms to the 1/2 power of the heat conductivity and reaction rate in both formulas. Data regression for the experimental data demonstrated that the overall estimation error was 10 % using Formula I and 25 % using Formula II. As the zero velocity value indicates the termination of thermal runaway propagation, further derivations reveal that forced cooling can suppress thermal runaway as long as the heat dissipation power is higher than the heat generation power. Sufficient heat dissipation time is achieved by the time delay of heat transfer in the long run of thermal runaway propagation. The formula for the dynamics of thermal runaway propagation universally applies to various large-scale lithium-ion batteries, assisting the safe design of high-energy batteries and providing new insights into suppressing battery thermal runaway by the heat transfer delay effect. [ABSTRACT FROM AUTHOR]

• Pulsating heat pipes with good thermal performance were designed for battery module heating management at low–temperatures. • The low–temperatures thermal management system for the battery module with pulsating heat pipes was proposed. • The thermal and electrical performance of the battery module with low–temperatures thermal management system was investigated. To effectively solve the problem that the capacity of lithium power batteries decreases significantly and is prone to irreversible damage under low–temperature charge and discharge scenarios, which seriously affects the driving range, cycle life and service safety of electric vehicles (EVs). In this paper, a low–temperatures thermal management system of pulsating heat pipes with ethanol as the base fluids and TiO 2 nanofluids as the working fluids (TiO 2 –CLPHP TMS) is constructed. On this basis, the thermal performance analysis of TiO 2 nanofluids pulsating heat pipes (TiO 2 –CLPHP) is carried out under different volume concentrations (0 %, 0.5 %, 1.0 %, 2.0 %, 4.0 %, and 8.0 %) and different fluid filling rates (30 %, 50 %, 70 %, and 90 %) in a low–temperature environment. Aiming at the low–temperatures charging scenario of the battery module, the performance test of the TiO 2 –CLPHP TMS for the battery module heating management is carried out under different low–temperature environments (−5 °C, −15 °C, and −25 °C) and different charging rates (0.3C, 0.5C, 0.7C, and 1.0C). The experimental results show that TiO 2 –CLPHP with fluids filling rate of 50.00 % and TiO 2 nanofluids concentration of 2.00 % has good performance in start–up and heat transfer, and can optimize the thermal performance of TiO 2 –CLPHP. Meanwhile, TiO 2 –CLPHP TMS can ensure that the battery module has superior thermal performance. the battery module can be charged within the allowed temperature range (greater than 0 °C). under different low temperature environments (−5 °C, −15 °C, and −25 °C), the temperature of the battery module is basically maintained at 3.30 °C, 8.00 °C and 16.00 °C. The temperature difference between different areas of the battery module is within the allowable temperature difference range of 5 °C, which reduces the adverse effects of the battery aging caused by low–temperature charging and uneven temperature. Moreover, the battery module also has good electrical performance, the battery module voltage is in a low–level range in the early and middle stages of charging. During the complete charging process, the low–temperature charging capacity of the battery module has almost no attenuation, and the charging capacity ratio is as high as 95.92 %. Compared with the pure battery module, the charging capacity and charging efficiency of the battery module with the TiO 2 -CLPHP TMS has been improved to different degrees. Especially at −25 °C, the maximum charging capacity and charging efficiency can be increased to 24.51 %, and the minimum can be increased to 13.17 %. which ensures the efficient charging of the battery module. This proves that the designed TiO 2 –CLPHP TMS is feasible and effective for battery module heating management. [ABSTRACT FROM AUTHOR]

Since the landmark paper by Hicks and Dresselhaus [Phys. Rev. B 47 , 16631(R) (1993)], there has been a general consensus that one-dimensional nanoscale conductors, i.e. nanowires, provide the long sought paradigm to implement the so-called phonon-glass electron-crystal material, which results in large improvements in the thermoelectric figure of merit ZT. Despite some encouraging—though isolated—experimental results, this idea has never been subjected to a rigorous scrutiny and the effect of the coupled dynamics of electrons and phonons has usually been oversimplified. To bypass these limitations, we have calculated the effective thermoelectric parameters for silicon nanowires (SiNWs) by iteratively solving the coupled electron-phonon Boltzmann transport equation (EPBTE) supplied with first-principles data. This allows for an unprecedented precision in determining the correct dependence of the thermoelectric parameters with system size; including, but not limited to, the figure of merit and its enhancement or degradation due to nanostructuring. Indeed, we demonstrate that the commonly used relaxation time approximation (RTA), or the uncoupled beyond the RTA (iterative) solution fail to describe the correct effect of nanostructuring on the thermoelectric properties and efficiency in SiNWs due to the strong contribution of phonon drag to the Seebeck coefficient, so that the use of fully coupled solution of the EPBTE is essential to obtain the correct effect of nanostructuring. Most importantly, we show that, contrarily to what commonly argued, resorting to NWs is not necessarily beneficial for ZT. Indeed, in a wide range of diameters nanostructuring diminishes the Seebeck coefficient faster than the decrease in thermal conductivity, due to the suppression of very long wavelength phonons responsible for the largest contribution to the phonon drag component of the Seebeck coefficient. This penalty to ZT can be mitigated if the NWs have a very rough surface, providing additional reduction to the thermal conductivity. Additionally, we demonstrate that our methodology provides improved data sets for an accurate determination of doping concentration in NWs through electrical-based inference and excellent agreement with the available experimental data. • Proper inclusion of electron-phonon coupling is required for accurate computational prediction of thermoelectric properties. • Large- and medium-thickness (R ≳ 30 nm) smooth Si nanowires may have a ZT lower than bulk. • Above is due to the suppression of the phonon drag contribution to S , illustrating that nanostructuring not always benefits ZT. • Large surface roughness diminishes the reduction of ZT when going to the nanowire form. • Expressions are provided to extract doping from the measurement of the radius and Seebeck coefficient of a Si nanowire. [ABSTRACT FROM AUTHOR]

The paper is devoted to study the frequency shift in the solution of the generalized telegraph equation with a moving point-wise harmonic source. This equation contains the nonlocality in time derivatives which is expressed by the memory functions η (t) and γ (t) , where η (t) smears the second time-derivative and γ (t) the first one. Moreover, in the Laplace domain we have η ˆ (s) = γ ˆ 2 (s). The generalized telegraph equation with an external source is solved by using the integral decomposition which allows us to write this solution as a product of the solution of the telegrapher equation with harmonic source and f γ ˆ (ξ , t) which is a function of the Laplace transform of memory function γ. Such obtained solution manifests the frequency shift which is illustrated in three examples of the memory functions γ (t) : the localized case, its mixture with power-law, and the power-law case only. We show that only the first two cases have the wave front and the Doppler-like shift. The third example, despite the lack of wave fronts, also manifests the frequency shift. Thus it turns out that the frequency shift occurs regardless of the existence of a wave front, but it is more visible when such a front exists. • The method of integral decomposition is applied to get solutions of the memory dependent evolution equations. • Jugging existence/non-existence of the Doppler shift is separated from the existence/non-existence the wave fronts. • It is shown that the Cattaneo-Vernotte equation may be used to describe the wave properties of the heat transport. [ABSTRACT FROM AUTHOR]

In the state of the art of steel production, the temperature evolution of steel strips is typically controlled to regulate the phase contents indirectly and, with this, their material properties. This paper proposes a novel computationally efficient, real-time capable dynamic model that captures both the temperature evolution and the phase transformations in the steel strip. The steel strip is processed in a cooling section after a continuous annealing furnace. The phase transformations cover the austenite decomposition which is mainly controlled by specifically decreasing the temperature during the cooling process. For this, a phenomenological state-space model is derived, which is inspired by the Johnson-Mehl-Avrami-Kolmogorov and the Koistinen-Marburger model. Phase transformations generally change the specific latent heat of the material, which is captured in the proposed distributed-parameter model of the strip temperature by an energy balance. Lumped-parameter models are used for the temperature evolution of the wall, the rolls, and the radiant tubes. Heat transfer due to convection, radiation, and conduction couple the individual thermal submodels. A comparison of simulation results and measurements from both experimental material tests and the real plant operation demonstrate the accuracy and feasibility of the proposed model. The model is computationally inexpensive and serves as a solid basis for advanced real-time control and optimization. • A mathematical model is proposed to describe the evolution of temperature and phases in a steel strip during cooling. • The austenite decomposition is captured by a phenomenological phase transformation model. • The model is computationally efficient and real-time capable. • The accuracy and the suitability of the proposed model is demonstrated by comparing simulation results with measurements. [ABSTRACT FROM AUTHOR]

• A Bayesian optimization is conducted utilizing an automated framework to enhance the trade-off between heat transfer and pressure drop using spline-based two-dimensional pin-fin arrays. • The optimization objectives include minimization of pressure drop, maximization of heat transfer, and maximization of efficiency. • The maximization of efficiency optimization yields designs which balanced aerodynamic characteristics with total surface area, leading to enhanced thermal performance. • Pin-fin arrays using four splines outperform all other geometries due to the symmetric nature of the shape generation. Pin-fin arrays are crucial in the cooling of aerospace components. Specifically, pin-fins find extensive usage in the trailing edge of turbine blades, primarily due to the harsh operating conditions these blades experience. With the development of additive manufacturing, complex internal geometries that were previously too expensive to manufacture are now feasible. This advancement has created a new pin-fin design space to be explored. In this paper, Bayesian optimization (BO) is conducted to find the pin-fin array that optimizes the trade-off between heat transfer and pressure drop in a two-dimensional channel. The complexity of the pin-fin geometry varies, and is controlled by the number of connecting piece-wise cubic splines used to close the shape. Different designs are obtained using 3-, 4-, 5-, and 6-splines in a staggered pin-fin array orientation. The optimization objectives included the minimization of pressure drop, the maximization of heat transfer, and the maximization of efficiency. It is found that the pin-fins which utilize 4-splines outperform all others due to their symmetric characteristics. The performances of the pin-fin array plateaus after increasing the complexity beyond 5-splines. It is concluded that the optimization objective significantly influences the final shape characteristics and behavior. As expected, the pressure drop minimization finds aerodynamic shapes while heat transfer maximization increases the total surface area in the channel. The maximization of efficiency results in a balance of aerodynamic shapes and total surface area. [ABSTRACT FROM AUTHOR]

• Thermal behaviors of vapor chamber are explored by hybrid Lattice Boltzmann method. • Surface wettability has a non-monotonic effect on the thermal performance of VC. • Hydrophilic boiling surface is recommended for higher boiling heat transfer limits. • Hydrophobic boiling surface is recommended for lower heat flux conditions. • Hydrophilic condensation surface is beneficial for faster boiling thermal response. Vapor chamber (VC) has been recognized as an effective passive cooling solution for thermal management. Due to complexity of phase-change processes in VC, boiling and condensation have been the focus of attention. In this paper, a hybrid lattice Boltzmann method was employed to explore the effects of surface wettability on thermal behavior/performance of VC. Taking the bubble/droplet dynamics, boiling heat transfer coefficient (BHTC), critical heat flux (CHF), thermal response and resistance as key factors, selections of proper surface wettability to address heat dissipation requirements were discussed. The results show that the influences of wettability on phase-change processes are not monotonic. For high heat fluxes, compared with neutral surface of 89.0° contact angle (CA), the CHF of the hydrophilic boiling surface (CA=23.9°) is enhanced by 92.57%. For low heat fluxes, the VC with a hydrophobic boiling surface (CA=115.5°) results in the highest BHTC by 260.34% compared with the neutral one. The heat flux should be carefully applied to the hydrophobic surface to prevent burnout. For condensation surface modification, the hydrophilic condensation surface is preferred for the faster thermal response. These findings are expected to provide a promising method for enhancing thermal performance of VC by surface modification. [Display omitted] [ABSTRACT FROM AUTHOR]

The rapid and precise extraction of thermophysical properties remains an enormous challenge in pump-probe thermoreflectance, and quantum algorithms present significant potential for addressing thermal transport problems. This paper introduces an innovative application of the quantum genetic algorithm to the fitting of experimental data from pump-probe thermoreflectance. Initially, a physical model based on the thermal transmission matrix is developed to compute the heat transport of multilayer micro-/nano-structures. The quantum genetic algorithm is then employed for multiparameter optimization in the extraction of thermophysical properties and tested on three samples. Both simulation and experimental results demonstrate that the quantum genetic algorithm assisted pump-probe thermoreflectance, capable of extracting thermophysical properties with high precision in just one minute, is approximately 30 times faster than the conventional method and 10 times faster than the genetic algorithm. Furthermore, measurement sensitivity and algorithmic randomness are analyzed. The reasons behind the phenomenon of equivalent fitting results and the indication of non-convex optimization are also presented. • This study pioneers the application of quantum algorithms in data processing for pump-probe thermoreflectance. • The quantum algorithm-assisted approach surpasses traditional methods by a factor of 30 in terms of fitting efficiency. • The fusion of quantum algorithms opens up new frontiers in tackling complex thermal transport problems. [ABSTRACT FROM AUTHOR]

• Flow boiling of refrigerants in small to micro tubes and channels. • Two-phase pressure was obtained for different refrigerants, geometries and operating conditions. • Past pressure drop correlations did not compare well with current data bank on two-phase pressure drop. • A new correlation for predicting two-phase pressure drop in flow boiling in small to micro passages is proposed. An accurate and acceptable correlation for the prediction of two-phase pressure drop is considered a crucial step in heat exchanger design. Although many existing models and correlations were developed and proposed in the past, their ability to predict within acceptable error bands when applied in general to flow boiling flows is limited, even within their originally recorded ranges. The discrepancies are worse when predicting two-phase pressure drop in small to micro-scale passages. Therefore, the aim of the work described in this paper is to assess the most well-known models and correlations using a large experimental data-bank. The data-bank includes four refrigerants, namely R134a, R245fa, HFE-7100 and HFE-7200, small to micro heat exchangers made of different metals and different channel configurations. In addition, the data cover a large range of operating conditions, which can allow generally applicability of new correlations developed. This range covers channel hydraulic diameter of 0.46−4.26 mm, heated length of 20−500 mm, system pressure of 1 − 14 bar, mass flux of 50−700 kg/m2 s, wall heat flux ranging from 2 to 234 kW/m2 and exit vapour quality up to one. Twenty six existing models and correlations that were developed and proposed for vertical/horizontal flow, single/multi-channels and circular/non-circular channels were evaluated. Moreover, the effect of using different equations for calculating the two-phase mixture viscosity, void fraction, Fanning friction factor and Lockhart–Martinelli parameter was assessed and discussed. The mean absolute error of the existing correlations when compared with our data-bank was more than 30 %. Therefore, a new correlation for calculating the two-phase multiplier, which was strongly dependent on the Boiling number and the Lockhart–Martinelli parameter, was developed and then the frictional component and total two-phase pressure drop relationships were completed. [ABSTRACT FROM AUTHOR]

• The new BTMS has two heat transfer paths, including OHP and liquid cooling plate. • Pulsating flow improves OHP stability through periodic enhanced thermal loads. • Optimal OHP stability lies in 38% or 62% duty cycles, 0.03–0.05 Hz frequencies. • Stably operating OHP enhances the new system's heat transfer performance. After a comprehensive review of oscillating heat pipe (OHP) based on battery thermal management system (BTMS), a novel battery liquid-cooling system with a ⊥-shaped OHP is presented to increase the volume utilization efficiency of battery module and the total amount of heat dissipation. The new system retains the heat transfer path of the OHP while incorporating an additional heat dissipation route where the liquid cooling plate is in direct contact with the cells. In this paper, pulsating flow is used to tackle the problem of the OHP failing to operate stably due to the insufficient heat load achieved in the new system. The parametric effects of pulsating flow frequencies, cooling initiation temperatures, and duty cycles on the operational performance of the OHP and the average surface temperature of the battery module ( T b ) within the new system were investigated. The results indicate that pulsating flow periodically elevates the heat load on the battery surface, enhancing the OHP working properly. The smaller the pulsating flow frequency, the greater the impact on the temperature fluctuations of the new system. The T b is lowest when the pulsating flow frequencies range from 0.03 Hz to 0.05 Hz. Beyond a frequency of 0.06 Hz, the OHP cannot operate stably. Additionally, moderately increasing the cooling initiation temperature is beneficial for enhancing the average temperature oscillation amplitude of the OHP. The cooling initiation temperature between 45 °C and 47 °C results in a better T b . Both pulsating flow duty cycles of 38% and 62% contribute to OHP running effectively. The new system with OHP under optimum pulsating flow condition has the optimal T b , eventually rising by 17.5 °C, which is a 9.8% decrease in temperature rise compared to the system without OHP. And the battery module's surface temperature uniformity has been improved through a reduction of 1.3 °C in the maximum temperature difference ( ΔT max ), amounting to a 17.8% decrease. It has been proved that the improved thermal performance of the new system under pulsating flow is attributed to the stable operation of the OHP. [ABSTRACT FROM AUTHOR]

• A Reference-Standard-Volume optimization method is proposed to improve CESPB. • The best aspect ratio of CESPB is 2–4 considering efficiencies and pressure drop. • The volume multiple is given at 1.1 with the highest exergy efficiency of 63.72 %. • The thermal and exergy efficiencies are increased by 2.03 % and 3.65 %, respectively. Cryogenic liquids (e.g., liquid air, liquid hydrogen, liquid carbon dioxide) have gained popularity in electricity storage due to their high energy density, no geographical constraints, and environmental friendliness. The cryogenic energy storage packed bed (CESPB) is widely employed as a cold recovery device to enhance the round-trip efficiency of cryogenic energy storage systems. Nonetheless, the cycle efficiencies of CESPB remain relatively low, with limited research investigating efficient methods for determining the design parameters. To address this, a novel optimization method named as the Reference-Standard-Volume (RSV) approach is introduced to enhance the thermodynamic performance of CESPB. In this paper, the complete optimization process of CESPB is illustrated, presenting the optimization outcomes in the view of heat and mass transfer. The findings reveal that a larger aspect ratio leads to the higher efficiencies but increased pressure drops. An optimal aspect ratio of 3 and a volume multiple of 1.1 is identified, achieving the highest exergy efficiency of 63.72 %. Meanwhile, increasing charging/discharging time enhances heat transfer in CESPB, however, it leads to an increase in external cold dissipation, which gives an optimal charging/discharging time of 3/3 h. By implementing the RSV optimization method, the percentage increase of discharging efficiency, thermal efficiency, and exergy efficiency of CESPB is 2.08 %, 2.03 %, and 3.65 %, respectively. These research outcomes offer valuable reference and guidance for future CESPB design and research. [ABSTRACT FROM AUTHOR]

• Steady-state measurement set-up for TCR under high-temperature vacuum is built. • Influence of liquid metal, thermal grease and graphite film on the TCR is analyzed. • The TCR between copper and graphite was reduced by 91 % by brazing method. • The TCR between palladium and graphite is reduced by 95 % by adding gallium alloy. • The temperature of the target is successfully controlled within 900 ℃. The high thermal contact resistance (TCR) between the target and the heat exchanger in the Isotope Neutron Source Accelerator will cause the high-energy charged particle beam to stay in the target, resulting in a remarkable increase of temperature which affects the production of stable isotopes. At present, the usage of thermal interface materials (TIMs) can fill the gap at the contact surface and thus reduces the TCR. In this paper, aiming at the actual working condition of neutron source, the steady-state TCR measurement experiment is designed under vacuum and high temperature environment. The TCR at the contact surface was reduced after adding graphite films, thermal greases, liquid metals and preloads. The improvement of TCR of the contact surface between palladium (Pd), copper (Cu) and graphite (Gr) were measured respectively. The results show that the TCR between palladium and graphite is reduced by 95 % by adding liquid metal (Gallium alloy). The TCR between copper and graphite was reduced by 91 % by brazing method. Compared with other TIMs, graphite film has the advantages of high thermal conductivity, high temperature resistance and no fluidity. Adding 0.08 mm thick graphite film between Pd/Gr and Cu/Gr contact surface can reduce the TCR by 50 % and 70 %, respectively. The improvement effect between the two contact surfaces was not obvious when 15.9–318.3 kPa preload was applied. [ABSTRACT FROM AUTHOR]

• Analysis of single and double pass solar air heater-phase change material. • Evaluation based of PCM thickness to length ratio. • Improved outlet temperature during non-sunshine hours. • Improvement in useful energy during discharging period. • System's maximum efficiency of 63.34% achieved during PCM discharging. This paper presents a two-dimensional transient model for a solar air heater with phase change material (SAH-PCM), focusing on the thickness-to-length ratio (t/L) of the PCM container. Verified through experiments, the model considers single (SP) and double pass (DP) flow configurations, assessing liquid fraction, dead length, outlet temperature, energy, exergy, and system efficiency during PCM charging and discharging. Increasing the t/L ratio improves peak liquid fraction, reduces dead length, and delays PCM discharge in both configurations, enhancing system energy and exergy efficiencies. In a SP flow system, optimizing t/L at 0.042 yields a peak liquid fraction of 0.928, a 92.28% dead length reduction, and a 2 h and 22 min delay for PCM discharge (base-case t/L ratio: 0.0182), achieving a highest discharging efficiency of 39.1%. For a DP flow system, the optimized t/L ratio of 0.058 results in a peak liquid fraction of 0.976 and a 1 hour and 34 min delay for PCM discharge, attaining the highest discharging efficiency of 63.34%. Moreover, an improvement in the energy payback time (EPBT) is observed for both SP and DP flow as the t/L is increased. For the DP flow, a minimum EPBT of 0.68 years (energy basis) and 6.55 years (exergy basis) is observed at a t/L ratio of 0.058. In terms of energy cost, SP flow reaches a minimum of Rs. 20.824 Rs/kWh at a t/L ratio of 0.042. For DP flow, the minimum energy cost is 13.033 Rs/kWh at a t/L ratio of 0.058, representing a noTable 37.42% improvement compared to SP flow. These findings underscore the critical role of optimizing system parameters, specifically the t/L ratio, for enhanced energy and exergy performance, and economic viability of SAH-PCM. [ABSTRACT FROM AUTHOR]

• A reservoir-assisted loop thermosyphon with a micro-grooved evaporator (MGTPLT) was fabricated. • Micro-grooved structures allowed faster startup and more stable operation of the MGTPLT. • The convection heat transfer ratio was proposed to evaluate the heat transfer enhancement. • The MGTPLT reduced evaporator temperatures and achieved thermal performance enhancement. The two-phase loop thermosyphon (TPLT) offers significant advantages in high-efficiency and long-distance heat transport for various thermal and energy-related applications without external power consumption. In this paper, the startup and heat transfer characteristics of a reservoir-assisted loop thermosyphon with a micro-grooved evaporator (MGTPLT) were experimentally investigated and compared with a loop thermosyphon having a smooth-tube evaporator (STPLT). R245fa was used as a working fluid at volumetric filling ratios of 50–80 %. The results demonstrated that the additional microgrooves allowed a shorter startup time characterized by a lower maximum temperature with respect to the temperature overshoot. Compared to the STPLT, the MGTPLT exhibited lower evaporator temperatures and better temperature uniformities; the micro-grooved structures could alleviate local intermittent dry-out at moderate and high levels of power input for filling ratios of 50 and 60 %. The addition of micro-grooved structures enhanced the heat transfer performance of the TPLT characterized by lower thermal resistances. The convection heat transfer ratio (CHTR) was proposed to evaluate the enhancement of the convection heat transfer within the evaporator region after using microgrooves. The CHTR showed a decreasing tend with increasing the power input. At moderate filling ratios, the value of the CHRT was greater than 1.1, and an approximately 18 % increase in the convection heat transfer was achieved under the combined condition of 60 % filling ratio and 250 W power input. The fundamental mechanism of heat transfer performance enhancement with the aid of micro-grooved structures was analyzed and elucidated. [ABSTRACT FROM AUTHOR]

• Thermal behavior of rectangular fins located within a square cavity were analyzed. • Inverse CFD method and experimental data were adopted for heat transfer prediction. • Suitable turbulence model based on the RMSE of the flow model were selected. • Effect of fin geometry on the flow and heat transfer within a cavity was evaluated. • Effect of PCM heat sink on the flow and heat transfer within a cavity was studied. This study examines the fluid flow and heat transfer characteristics of rectangular fins positioned within a square cavity through the use of the inverse three-dimensional computational fluid dynamics (CFD) method, experimental analysis, and the constant heat transfer rate assumption. This paper proposes a heat sink that uses a phase change material (PCM) made of paraffin within a small cavity. The inverse CFD method, combined with the least squares method, root mean square error, and excessive experimental data, is adopted to predict the unknown heat transfer rate Q and absorption heat Q ab. One of the objectives of this study is to investigate the effect of the PCM heat sink on fluid flow and heat transfer characteristics within a cavity. A sequence of tests on various flow models indicates that employing the RNG k-ε turbulence model with the standard wall function is the most suitable choice for all scenarios in the three-fin model. Conversely, the zero-equation model proved to be a more adequate fit for the PCM heat sink. Another objective of this study is to study the effect of the height of the PCM heat sink on Q and Q ab. The incorporation of a PCM heat sink results in a further improvement in the efficiency of heat dissipation. Part of the heat created is absorbed by the PCM heat sink, which absorbs 22.5 % of the thermal energy. Under the same volume, the PCM heat sink with a lower height (H p = 0.012 m) absorbs 14 % more heat compared to H p = 0.02 m. Thus, a lower-height PCM heat sink has a better heat dissipation effect. [ABSTRACT FROM AUTHOR]

• Verified rivulet behavior during dropwise condensation on vertical surface • Identified effects of surface length and rivulet flow on heat transfer • Related rivulet-covered area and dropwise condensation heat transfer Due to its superior heat transfer performance, dropwise condensation has been studied for industrial applications. This paper experimentally reports the effects of vertical length on dropwise condnsation in the presence of noncondensable gas. The formation of rivulet flows, which reduce heat transfer due to droplet coalescence, was observed. Compared to the upper side, rivulet flows on the lower side of the condensing surface were faster and more frequent. Lower heat flux was also measured on the lower side, indicating a correlation between heat transfer and rivulet flow parameters during dropwise condensation on the vertical surface. These findings represent the first study of the effects of rivulet flows on dropwise condensation heat transfer, and they can be applied to evaluate condensation heat transfer on long vertical surfaces, such as those found in small modular reactor containment systems as well as large containment building with passive containment cooling systems. [ABSTRACT FROM AUTHOR]

• Gradient narrowing and gradient droplet flow fields is proposed considering gas-water distribution characteristics. • Performance and gas-water distribution are greatly improved with gradient flow fields. • Effect of contact resistivity on performance varies in ratio of channel to rib area. • Gradient narrowing flow field performs best and its geometric parameters are analyzed. Ensuring sufficient gas supply and high distribution uniformity are crucial issues in the flow field design to improve the performance and durability of proton exchange membrane fuel cell (PEMFC). However, previous studies often ignore the gas-water distribution variations along the flow direction and adopt identical design for the entire flow field, resulting in unsatisfactory performance. To fill in this gap, this paper innovatively proposes two types of gradient flow field (gradient narrowing and gradient droplet) considering the gas-water distribution characteristics. The channel of the gradient narrowing flow field gradually decreases in all directions along the flow direction, and the gradient droplet flow field is composed of staggered droplet blocks, and the distribution density gradually decreases along the flow direction. The Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS) method is used for the comprehensive evaluation of flow fields. The results demonstrate that the gradient flow field can significantly improve the performance and physical field distribution. The gradient narrowing flow field has the best performance, with 11.39 % improvement in peak power density, 77.96 % increment in average oxygen mass fraction, and 43.11 % improvement in oxygen uniformity compared to parallel flow field. Further, the key geometric parameters sensitivity analysis of gradient narrowing flow field is carried out. This study contributes to understanding the underlying mechanism of mass transfer properties in the novel gradient flow field, and exploring its potential for high-performance flow fields design. [ABSTRACT FROM AUTHOR]

• A new wall boiling heat transfer model is developed to offer a fresh approach for CFD simulations within the Eulerian two-fluid framework. • Each parameter within the new model is calculated based on local physical properties and macroscopic parameters at cell level. • The new model reasonably predicts the wall temperature in flow boiling cases. • The sensitivity of the coefficients introduced in the proposed model are analyzed. Accurate prediction of flow boiling heat transfer is prominently dependent on the modeling of wall heat flux partitioning. In this paper, a new wall boiling heat transfer model was developed for three-dimensional Computational Fluid Dynamics (CFD) code to predict the wall heat flux and wall temperature. The proposed model partitioned the wall heat flux into convective heat flux and nucleate boiling heat flux, which were further modified by two correction factors. The key feature is that the new wall boiling heat transfer model was derived from bubble growth mechanism, incorporating reasonable assumptions, and each parameter within the model was calculated based on local physical properties and macroscopic parameters at the cell level. On this basis, the new wall boiling heat transfer model was coupled into ANSYS-Fluent and validated against various public experiments as well as the KIMOF experiments conducted under different conditions. Simulation results indicated that the proposed model could predict reasonable results for wall temperature and cross-section average void fraction. Finally, a comprehensive investigation was carried out to assess the sensitivity of the computational grids and the coefficients introduced in the new model. [ABSTRACT FROM AUTHOR]

· Different change rate of cross-sections along the flow field is investigated. · Microchannel condenser is used in loop heat pipe. · Effects of operating height situation and microchannel structure are studied. · Smooth operation and anti-gravity performance are effectively improved with the help of tapered channel. The microchannel condenser has the advantages of high condensation efficiency and low flow resistance. The vapor can liquefy and return to the evaporator rapidly, and the heat transfer performance and the smooth operation of the loop heat pipe can be improved. Suitable change rate of the cross-section along the microchannel flow field can adjust the pressure drop and flow rate of working fluid, and improve the anti-gravity performance of the loop heat pipe. In this paper, microchannels with different change rates of cross-section along the flow field, including 0.5 mm straight channel, 1 mm straight channel, divergent channel, and tapered channel, are designed and applied as condensers in loop heat pipes. The entrance size of the gradient microchannel is 0.5 mm or 1 mm, which uniformly expands to 1 mm or shrinks to 0.5 mm along the flow field. The vapor and cooling water form a countercurrent direction of motion in the microchannel, and the heat exchange is carried out through the bottom of the channel. The influence of the microchannel condenser structure and operating height situation between the evaporator and condenser on the heat transfer performance of the loop heat pipe are experimentally investigated. The results show that the loop heat pipe achieves an extremely low thermal resistance of 0.004℃/W with gravity assist. Under the gravity-free operation situation, the maximum evaporator temperature of the loop heat pipe with 0.5 mm straight channel and 1000 W heat load is 128.1 °C, and the vapor pressure drop fluctuated is in the range of -1000 ∼ 2500 Pa. Under the anti-gravity operation, the loop heat pipe with the tapered channel has effective transfer performance at 500 W, and the influence factor is only 28.1 %. [ABSTRACT FROM AUTHOR]

• A semi-analytical model of fractured horizontal well with secondary fractures and SRV is presented. • The flow regime of horizontal well with secondary fractures and SRV can be classified into eleven regimes. • A new coupling solving method of fracture and reservoirs model is presented by BEM. • Fracture parameter influence pressure response curve of early and middle regime, while SRV parameter and shape is contrary. Horizontal well volume fracturing plays an important role on enhancing gas recovery. Microseismic map indicates that the secondary fracture has been observed obviously and irregular stimulated reservoir volume (SRV) region created. In order to simulate pressure response of this phenomenon, the aim of this paper presented the semi-analytical model of multi-stage fractured horizontal well with secondary fracture and SRV region. Composite model is used to describe irregular SRV, and high-precision boundary element method and coupling method are employed to solve the semi-analytical mathematical model. Compared with conventional fractured horizontal well model, pressure transient behavior of multi-stage fractured horizontal well with secondary fracture and irregular SRV region has unique flow regime. The supply regime from secondary fracture to hydraulic fracture, second bilinear flow regime. Pressure derivative curve shows obvious 'dip' for supply regime from secondary fracture to hydraulic fracture, and 1/2 slope line for second bilinear flow regime reflecting formation gas flow towards hydraulic fractures and secondary fractures. Fracture parameter (such as conductivity, length and number) have an obvious influence on pressure transient curve of early and middle flow regime, and they can cover up some flow regime of early flow regime. SRV parameter and shape have an obvious influence on pressure transient curve of late and middle flow regime, and they can cover up some flow regime of late and middle flow regime. Model verification indicates that semi-analytical solving method combining with high-precision boundary element method can enhance calculation accuracy of early flow regime. [ABSTRACT FROM AUTHOR]

• A thermodynamics model for CO 2 hydrate heterogeneous nucleation has been developed. • Effects of pressure, temperature, wall characteristics and solution activity are illustrated. • The critical radius and nucleation barrier for CO 2 hydrate are predicted for the first time. • Heterogeneous nucleation mechanisms of CO 2 hydrates on fractal surfaces are presented. The massive emission of greenhouse gas CO 2 has significant effects on global climate and environment, causing worldwide concern. It has been proposed to store CO 2 in clathrate hydrates as a possible strategy for reducing atmospheric CO 2 emission. Nevertheless, the CO 2 hydrate formation rate is slow and its associated heterogeneous nucleation mechanisms are not well understood, which limit commercial applications of this technology. This study developed a thermodynamic model based on the change in availability function approach, considering effects of surface characteristics (including surface topography, roughness, curvature radius, and wettability), and CO 2 dissolution on the solution's activity. The critical nucleation radius and energy barrier of CO 2 hydrate nucleation are determined. Effects of pressure, temperature, wall characteristics and solution activity on the nucleation radius of CO 2 hydrate are discussed. The results of this paper contribute to advancing carbon capture and storage technology based on hydrate methods. [ABSTRACT FROM AUTHOR]

This paper presents a set of high-performing heat sinks that exhibit twice the thermohydraulic performance in terms of conductance compared to conventional rectangular fin heat sinks. The heat sinks presented here are designed through three-dimensional topology optimization (TO), manufactured using additive manufacturing (AM), and their performance is validated through experimental testing. The heat sink design is governed by steady-state Navier-Stokes equations and the energy equation. The objective is to minimize the average temperature of the heat source surface with a constant heat flux. Our design process incorporates two constraints: the pressure drop constraint and the project undercut perimeter (PUP) based overhang angle constraint. The incorporation of the overhang angle constraint ensures that the optimized heat sink design is self-supported and amenable to additive manufacturing without the need for additional support structures. Post-optimization CFD investigations revealed that the optimized heat sink offers improved thermal performance, attributed to 2 kinds of three-dimensional convection effects, thermal boundary layer re-initialization, and efficient mixing. The optimized heat sink designs are manufactured using laser-powder bed fusion process, an additive manufacturing technique, and their superior performance relative to a conventional rectangular heat sink is validated through experimental measurements. The experimental tests are in good agreement with CFD simulations, confirming a remarkable 100% increase in conductance for the TO designs compared to a conventional heat sink. • Geometrically intricate heat sink design: achieved through topology optimization and additive manufacturing. • Improved thermohydraulic performance: twice the heat conductance over rectangular fin design. • Cooling mechanism: optimized designs disrupt boundary layer, enables full-field mixing of cold and hot flow. • Smaller surface area: optimized designs possess smaller surface area than rectangular fins. [ABSTRACT FROM AUTHOR]