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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]

Abstract: In this paper, we present an experimental study on measurement of the thermal conductivity and heat pipe effect in both hydrophilic and hydrophobic (Toray TGP-H60) carbon papers (around 200μm thickness) with/out liquid water. An experimental setup is developed for measuring thermal conductance at different liquid water contents and temperatures without dissembling the testing device for water addition. Theoretical analysis is also performed to evaluate the apparent conductance of heat pipe effect. We found that liquid water presence inside these materials increases the overall thermal conductivity. At high temperature around 80°C, the heat pipe effect is evident for the hydrophilic paper; while for the hydrophobic one, the heat pipe effect is found to be smaller. The distinction is likely due to the different patterns of the capillary liquid flow in the two media. For the hydrophobic paper, liquid water flows back to the evaporation side when the breakthrough pressure is reached and flow is through preferred routes of small flow resistance. As a result, heat pipe effect is active only in part of the medium, therefore smaller than that in the hydrophilic one. The results are important for understanding the heat transfer phenomena occurring in porous media and effects of material surface property. [Copyright &y& Elsevier]

Abstract: This paper presents the methodology used and the results obtained in the determination of moisture diffusion coefficient of non-impregnated transformer insulating paper. In order to establish the diffusion coefficient, drying curves of paper samples were obtained by means of thermogravimetric experiments. The diffusion coefficient parameters were found by applying an optimization process based on genetic algorithms. The error function between measured and simulated curves was determined, and the parameters achieving the best correspondence between measured and estimated values were obtained. As a result, a new equation for the diffusion coefficient of non-impregnated insulating paper is proposed, depending on average moisture concentration, temperature and insulation thickness. The proposed coefficient was validated through experimental cases finding a good agreement between the experimental drying curves and those obtained by simulation using the diffusion coefficient. The proposed diffusion coefficient can be used for the determination of the time required to dry power transformers in factory. [Copyright &y& Elsevier]

Abstract: Capillary pressures in a carbon paper gas diffusion layer (GDL) having hydrophilic and hydrophobic pores of a polymer electrolyte membrane fuel cell (PEMFC) are investigated by both lattice Boltzmann simulations and experimental measurements. The simulated and measured capillary pressures as a function of water saturation for water drainage and imbibition processes in the GDL are presented and compared. It is shown that the pore-scale simulated drainage and imbibition capillary pressure curves are in good agreement with that obtained by experiment, both indicating the coexistence of hydrophilic and hydrophobic properties in the polytetrafluoroethylene (PTFE) treated carbon paper GDLs. The fitted capillary pressure curves, obtained from this paper, can provide more accurate predictions of the capillary pressure in carbon paper GDLs with non-uniform porosity and wettability than the standard Leverett–Udell relationship which was obtained for soil with more uniform porosity and wettability. [Copyright &y& Elsevier]

Abstract: In a previous work, a moisture diffusion coefficient for non-impregnated transformer insulation paper was proposed, which is useful for analysing the moisture dynamics in electrical power transformer insulation during its construction stage. This diffusion coefficient was determined using a set of experimental drying curves obtained in a thermogravimetric analyzer and an optimization process that employs genetic algorithms. The expression for the moisture diffusion coefficient proposed in that previous work is dependent on the temperature, insulation thickness and average moisture concentration. The use of average moisture concentration is not common in diffusion models. In this paper, a new expression for the moisture diffusion coefficient is proposed in which the dependence on average moisture has been substituted by a dependence on the local concentration. This new expression was obtained that is more suitable for diffusion models. To obtain that expression, the same experimental data used in the previous study have been used, but the genetic algorithm’s optimization process was modified. In this paper, a description of the applied procedure is detailed, and the new proposed expression is presented. Finally, a comparison between both equations, the one dependent on average moisture and the one dependent on the local moisture, is performed using a drying diffusion model implemented in a finite element commercial software package. [Copyright &y& Elsevier]

Abstract: Effects of suction pressure and permeability on the steam heating characteristics of the wet paper are studied. Experimental results show that suction pressure enhances the energy absorption in the wet paper and effects of suction pressure strongly appear in the high-permeability paper, and also absorbed energy rate is decreased with increasing in heating time. From the numerical simulation results it is found that increase in moisture content and decrease of pressure gradient reduce the absorbed energy rate. Dimensionless numbers are derived from the basic equations to summarize the experimental and numerical simulation results. [Copyright &y& Elsevier]

A model of simultaneous heat and mass transfer through a porous material is presented to explain transient moisture sorption by paper sheets from humid air. There are three primary resistances to moisture transport: (1) diffusion through an external boundary layer; (2) diffusion through the pore system and, (3) diffusion from the pore system into the fibers. It is found that diffusion through the fiber phase perpendicular to the plane of the sheet is not significant for the moisture content range considered here. The mass transport model is able to predict the results of transverse moisture gradient experiments which show that moisture content gradients in paper are not as large as previously thought during transient periods. The model shows that the sigmoidal temperature response of paper to a linear change of relative humidity is due to non-linearities of the moisture content isotherm and heat of sorption. [Copyright &y& Elsevier]

Scaling in conventional black liquor evaporators has presented problems for decades, impeding the improvement of productivity in paper mills. Recent investigations suggest that falling film technology may effectively minimize black liquor fouling and improve productivity in a paper mill. This finding motivates the current work to analyze the transport phenomenon, enrichment and scale fouling of black liquor in a falling film evaporator. In the paper, a mathematical model based on a turbulent two-phase flow with multiple components is presented to investigate the transport processes of black liquor in a falling film evaporator. A phenomenological model of crystallization fouling is used to predict the fouling process. The results show the relationship between heat and mass transfer occurring within a very thin viscous sublayer close to the heat transfer surface, and the influence of soluble solids concentration and thermal boundary condition on the enrichment and scale fouling of black liquor. [Copyright &y& Elsevier]

• 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]

• Mixed convection for a viscoelastic fluid within a square enclosure containing a rotating cylinder was numerically simulated. • The Phan-Thien-Tanner (PTT) model was used to describe the viscoelastic fluid behavior via the OpenFOAM software. • The numerical divergence at high Weissenberg numbers was addressed using a log-conformation approach. • Elevating the Weissenberg number reduced the frictional torque by 54 % and raised the Nusselt number from −1.33 to 3.32. • As the Weissenberg number increased from 1 to 100, there was a significant decline in the tangential normal stress. This paper for the first time conducted numerical simulations for the mixed convective flow and heat transfer with a viscoelastic fluid within a square enclosure containing a rotating cylinder. The flow regime was considered laminar, two-dimensional, and transient, and the Phan-Thien-Tanner (PTT) model, recognized as one of the most accurate nonlinear models for viscoelastic fluids, was used to describe the viscoelastic fluid behavior, mathematically. Solving nonlinear fluid equations, predicting the complex properties of viscoelastic fluids using the constitutive equation of the PTT model, and considering viscous dissipation were the challenges of this research. Additionally, the paper addressed the numerical divergence at high Weissenberg numbers using a log-conformation approach based on a logarithmic variable change in the polymer stress equation. The effects of the Richardson, Weissenberg, and Brinkman numbers and the ratio of polymeric viscosity to total viscosity on fluid flow and heat transfer were investigated by solving the nonlinear equations via the finite volume method. This was accomplished using the rheoHeatFoam solver within the OpenFOAM software. Local and average heat transfer characteristics around the rotating cylinder were fully studied. The results showed that elevating the Weissenberg number enhances the shear-thinning effect and diminishes the effective viscosity of the fluid by 63 %, resulting in a 54 % reduction in both the shear stress around the cylinder and the frictional torque applied to it, as well as a decrease in the viscous dissipation. Furthermore, increasing the Weissenberg number raised the Nusselt number from -1.33 to 3.32, signifying a complete reversal in the direction of heat transfer. However, the shear-thinning effect became negligible when the Weissenberg number exceeded 100. Moreover, the tangential normal stress significantly increased with an increase in the Weissenberg number up to one. However, as the Weissenberg number increased from 1 to 100, there was a significant decline in the tangential normal stress. Consequently, at very high Weissenberg numbers, the tangential normal stress approached zero, resembling a Newtonian fluid. [ABSTRACT FROM AUTHOR]

As the core component of ejector refrigeration systems, the research of ejector mechanisms has always been a hot topic. Based on the compound-choking theory, this paper studies the relationship between the entrainment ratio and critical back pressure and designs a neural network-based online error compensation mechanism for supersonic ejectors. Firstly, a thermodynamic model of the ejector is developed by combining the ideal gas model, the compound-choking theory, and the constant-pressure mixing theory. Then, the thermodynamic relationship between the entrainment ratio and critical back pressure is derived by introducing the idea of a hypothetical mixing cross section. Compared with the existing results, our method is able to improve critical back pressure prediction accuracy by virtue of the actual entrainment ratio. Therefore, a new adaptive error compensation algorithm is proposed for supersonic ejectors based on neural networks. Thus, the proposed scheme can be regarded as a hybrid modeling method for supersonic ejectors, which combines thermodynamic laws and data-driven artificial intelligence algorithms. Experimental data from ejector refrigeration systems with R141b, R245fa, water vapor and R134a are used in this paper to verify the validity of the model. The simulation results show that our method can effectively predict the critical back pressure, and all the prediction errors are less than 10%. • A novel hybrid modeling method is proposed for supersonic ejectors based on the compound-choking theory. • The idea of hypothetical mixing cross section is introduced to develop the ejector model. • The predictive error of the thermodynamic model is compensated by using adaptive neural scheme. • The simulation results show that all prediction errors are less than 10%. [ABSTRACT FROM AUTHOR]

• The outdoor air relative humidity also has an impact on the threshold temperature of frost accumulation. • Under certain conditions, higher efficiency of heat recovery may be safer in terms of frost risk. • The largest ' frost ' accumulation area is formed for the middle values of the return air relative humidity (in analyzed case RH 2 i =38.5 %). In this paper, a process of frost accumulation inside the non-hygroscopic rotary heat exchanger depending on operating conditions is investigated. The study was based on the results of the numerical simulations conducted in the original computer program. The analysis considers both the threshold conditions of frost accumulation and the conditions of frost layer growth. For this purpose, three indicators related to frost accumulation were distinguished: a frost accumulation limit temperature, a surface of the matrix covered with the accumulated frost layer, a maximum thickness of the accumulated frost layer. The analyzes assessed the influence of operating parameters on these indicators, focusing on the return air relative humidity. It was revealed that the highest values of this parameter cause the greater thickness of the frost layer. However, the largest ' frost ' accumulation area was created in the range of average return air relative humidity values – in the analyzed cases it was RH 2 i =38.5 %. The paper also includes a detailed analysis of the threshold temperature of frost accumulation. These limits of outdoor air temperature were determined for a wide range of operating parameters' values: both airstream relative humidity and nominal temperature effectiveness. Additionally, eight probable configurations of active heat and mass transfer areas within rotary heat exchanger channels were revealed, which are related to the initiation of frost accumulation. Consequently, the locations of the initial ' frost ' accumulation area were predicted. It was also found that the greatest impact on the frost accumulation threshold temperature has the return air relative humidity RH 2 i , although this influence is not unequivocal. The most unfavorable conditions regarding nominal temperature effectiveness in terms of frost accumulation are also unclear. In some cases, it was even demonstrated that an increase in the effectiveness can eliminate the ' frost ' accumulation area. [ABSTRACT FROM AUTHOR]

• LaplacianFoam in OpenFOAM® solves the scalar diffusion equation. • Thermal diffusivity may be often incorrectly used in heat conduction problems. • It is tempting to make just a little change from a constant to a scalar field. • To correctly model heat conduction in heterogeneous media, ρ , c p and λ is needed. In the paper "Numerical study with OpenFOAM on heat conduction problems in heterogeneous media" by K. Zhang, C.-A. Wang, J.-Y. Tan, International Journal of Heat and Mass Transfer, Vol. 124 (2018) 1156–1162 the authors modify laplacianFoam from open-source CFD package OpenFOAM® to solve heat conduction problems in heterogeneous media by changing the diffusion coefficient from a constant value to a scalar field. Their paper clearly demonstrates a fundamental, overlooked, and common mistake in rearranging the heat conduction equation, which cannot be replaced with the temperature diffusion equation. In this comment, we provide clear evidence through mathematical derivations and numerical examples calculated in OpenFOAM®. [ABSTRACT FROM AUTHOR]

This paper presents pioneer experimental data on critical heat flux (CHF) during flow boiling of low-pressure fluid R514a in a microchannel. The authors conducted experiments in a single smooth and horizontal microchannel, with an internal diameter of 1.1 mm and heating sections of 46 and 100 mm, a saturation temperature range of 56 to 122.7 °C, and a mass velocity range of 1500 to 7500 kg/m2s. This research achieved high critical heat flux values, up to 1.85 MW/m2. The results were compared with the CHF of R123 under similar conditions and a higher CHF for R514a was observed. The performance of CHF predictive methods against the experimental data was performed. Since predictive methods for the CHF in microchannels need surface tension data, but there is a lack of experimental information about this property for R514a in the literature, this paper also presents experimental results for the surface tension of R514a and an expression for calculation of this property as a function of the temperature. • Pioneer critical heat flux experimental results for R514a are presented. • High values of mass velocities and saturation temperatures were tested. • Critical heat flux up to 1.85 MW/m2 were obtained. • Pioneer surface tension experimental data for R514a were measured. • Results are compared against predictive methods. [ABSTRACT FROM AUTHOR]

This paper presents a proposition of a dual grid (DG)-based coupled deep reinforcement learning (DRL) and computational fluid dynamics (CFD) method for active flow control. The DG-DRL-CFD method uses a dual-resolution grid, with a coarser grid for the training phase and a finer grid for the testing phase of the DRL-CFD method. Further, after a validation study for our DRL-CFD-based parallelized in-house solvers, this paper presents a performance study for a periodic suction/ejection-based drag reduction for a cylinder in a channel-confined flow. Using an immersed boundary method for CFD on a Cartesian grid, this novel DG-DRL-CFD method is shown to result in almost same accuracy (within 1%) in a substantially reduced computational time as compared to the traditional DRL-CFD method (on the finer uniform grid). Finally, using a sharp interface level set method for an axisymmetric two-phase flow, this paper presents an application of the DR-DRL-CFD method for an oscillating base plate-based enhancement of heat transfer during nucleate pool boiling. As compared to our recent CFD study-based parametric study for the nucleate pool boiling problem, the present DG-DRL-CFD method leads to a profile and frequency of plate-oscillation that results in a larger value of average Nusselt number Nu avg for the plate. The coupled DRL-CFD method leads to plate oscillation profile giving higher enhancement in Nu avg , compared to an open-loop control strategy that involves a parametric sweep involving multiple simulations. • A dual grid (DG) based novel method is proposed which couples deep reinforcement learning and CFD for active flow control. • The method is validated on the problem of flow past cylinder with suction/ejection based drag reduction. • The method is applied to nucleate boiling with oscillating base plate where plate position is controlled using DRL. • The enhancement of Nusselt number is found to be higher than open-loop control strategy that involves a parametric sweep. [ABSTRACT FROM AUTHOR]

• Empirically estimated SMD and measured values by PDA were compared and evaluated. • Droplet size distribution, velocity, and their correlations are presented. • The Gamma PDF provided the best droplet size fit with less than 10 % SMD deviation. • Velocity fluctuations were similar, while its decay depended on the fuel type. • Overshooting dominated the majority of the spray in all cases. Understanding spray evolution in a reacting environment is critical to designing advanced, clean combustion systems. The processes in the upstream region determine flame shape, stability, ignition characteristics, pollutant emission, and combustion efficiency. The developed spray is never achieved in combustion since the early regions feature primary and secondary atomization, while droplets evaporate as they approach the flame. Consequently, there is no thermodynamic equilibrium before the flame front. The principal goal of this paper is to provide detailed information to model developers on various sprays measured by a Phase Doppler Anemometer; the processed measurement data is available as supplementary material, while the raw data will be provided upon request. Four different fuels were tested: diesel fuel, aviation kerosene type JP-8, biodiesel, and a 50 % biodiesel-diesel blend by volume. The plain-jet airblast atomizer was tested at four atomization gauge pressures (0.3, 0.45, 0.6, 0.75 bar g). Therefore, sixteen different sprays were measured along one spray diameter at each of four downstream distances of 15, 25, 35, and 45 mm, measured from the nozzle tip. The paper details the droplet size distribution, droplet axial velocity, fluctuations, and correlation between size and velocity to facilitate a comprehensive understanding of liquid fuel sprays. This latter measure helps identify the overshooting phenomenon, i.e., localizing the regions where the large droplets move faster than the gas phase. [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]