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The fabrication of most current thermal insulation materials from polymeric foams has created an unfavorable impact on the environment. With the aim of reducing the use of foam plastic, this paper focuses on honeycomb paperboard, an alternative thermal insulation material which is recyclable and cost-effective, and evaluates its heat transfer performance under different conditions. The experimental results show the effective thermal conductivity of honeycomb paperboard increases as the honeycomb height or the side length increases. This paper also establishes a novel theoretical model based on a newly developed convective heat transfer correlation to predict the effective thermal conductivity under different conditions. The model's results conform well to the experimental results, and the effect of convective heat transfer is investigated quantitatively using this model. As the vertical adiabatic boundary surfaces are replaced by honeycomb walls, the convection heat transfer strictly depends on the thermal boundary condition of the honeycomb walls and highly depends on honeycomb height and side length. • The novel theoretical models involving convective heat transfer are established under different conditions. • The different heat transfer paths are quantitatively investigated based on the proposed models. • Under the condition of hot-above and cold-below, convective heat transfer in honeycomb paperboard increases as the honeycomb height increases, but decreases as the side lengths increases. • Under the condition of cold-above and hot-below, convection heat transfer in honeycomb paperboard increases as the honeycomb height or the side length increases. [ABSTRACT FROM AUTHOR]

This paper describes an accurate model of the Electrical Power Subsystem (EPS) of the Thermal Analysis Support and Environment Characterization Laboratory (TASEC-Lab), a university experiment developed for using in high-altitude balloon missions to measure and analyze the convection heat transfer. A Li-ion battery and two high efficient DC–DC converters have been characterized through laboratory tests and fitting to the experimental results. The developed mathematical models described in this paper are used to carry out accurate EPS simulations and are validated by comparison with the experimental data. • Design and modeling of the electrical power subsystem of a stratospheric balloon experiment. • New Li-lion battery discharging model that describes the three main parts: initial decay, linear, and exponential decay. • Formulation of a new DC/DC converter efficiency model. • Simulation and model validation of the electrical power subsystem. [ABSTRACT FROM AUTHOR]

Nanochannels consisting of hot and cold walls with periodic rectangular and triangular nanostructures were simulated by molecular dynamics in this paper, the periodic size dimensionless parameters φ was defined to describe the dimension of nanostructures on the wall. The results show that the addition of nanostructures leads to the variation of mass density distribution of fluid near the wall. A hot wall with nanostructures can promote the appearance of a fluid high-potential area near it. The discrepant attraction for fluid from the wall caused by the different wall conditions leads to different flow characteristics in nanochannels. Fluid velocity growth rate near the hot wall is 10.5% higher than that near the cold wall. Average velocity of the fluid near the hot wall is 5.1% higher than that of fluid near the cold wall. A wall with rectangular nanostructures at φ = 0.28 shows minimum flow resistance among rough hot walls, while a wall with triangular nanostructures at φ = 0.14 has minimum flow resistance among rough cold walls. Heat is transferred from the hot wall to the cold wall through fluid flow. Wall conditions impact the heat transfer situation of fluid in flow boundary by changing the fluid mass density contribution. The addition of nanostructures and decrease of nanostructures size is beneficial for heat transfer while unfavorable for fluid flow, convective heat transfer was influenced by their combination. For a wall with 300 K, the addition of rectangular nanostructures at φ = 0.07 has the best convective heat transfer performance. For a wall with 200 K temperature, increasing triangular nanostructures at φ = 0.14 on the wall can reach the best convective heat transfer capacity. This paper can provide a theoretical foundation for the optimization of nanochannels to achieve best convective heat transfer ability. [ABSTRACT FROM AUTHOR]

The presence of magnetic field results in a reduction in natural convective heat transfer and entropy generation in cavities. The use of nanofluids improves the performance of thermal systems. The present paper discusses the impact of uniform and non-uniform magnetic fields on the second-law performance of nanofluid-filled and porous cavities, introduces artificial intelligence (AI) approaches, and examines the applications of AI in optimizing heat transfer and entropy generated in these systems. The mathematical formulation is presented in terms of the entropy generation for natural convection in cavities. This paper demonstrates the influences of various important non-dimensional parameters on entropy generation in cavities. Besides, advances in the second-law performance of cavities and the role of AI techniques in the field are discussed and future directions are provided. [ABSTRACT FROM AUTHOR]

Unsteady oblique stagnation-point flow and heat transfer of fractional Maxwell fluid with convective derivative towards an oscillating tensile plate are discussed in this paper. The fractional operator is introduced to material derivative for the first time to obtain a newly defined constitutive equation of Maxwell fluid. The Fourier's law is modified accordingly. The pressure gradient is innovatively obtained by solving the differential equation, which bases on the momentum equation away from the plate. Furthermore, the numerical solutions are acquired by virtue of the finite difference method combined with L1-algorithm. It turns out to be convergent through constructing numerical example. The influence of related parameters on the velocity and temperature are performed graphically in detail. Results show that the fluid velocity and temperature distributions tend to periodic oscillation near the plate. Both the velocity and the temperature reduce with the augment of fractional derivative parameters, which indicates the velocity and the temperature boundary layer become thinner. The smaller velocity relaxation time parameter leads to enhanced velocity, meaning that the pressure promotes the fluid velocity. It is evident that all the temperature curves have a tendency to increase first and then decrease with the diverse parameters due to the thermal relaxation characteristic. [ABSTRACT FROM AUTHOR]

Combustion of iron powder is a promising option for clean and sustainable generation of heat and power. However, a fundamental understanding of the flame propagation mode is missing, especially under strong convection effects. In part I of this work, we analyzed the modes of flame propagation under weak convection effects and estimated the discreteness parameter as a ratio of the particle burn time to the conduction timescale. We found that both, the selection of temperature at which thermal properties are evaluated, and the selected length scale, are critical for a reliable prediction of the transitional particle spacing indicating the transition from the continuous to the discrete mode. In this paper, we extend our previous work by studying the effect of convection on inter-particle heat and mass transfer using boundary layer resolved numerical simulations. It is shown that for increasing convection, iron flames transition from the continuous to the discrete mode at higher particle distances. Taking into account both diffusion and convection-dominated heat transfer, an extension for the scaling of transitional particle spacing of part I is proposed. Furthermore, since iron flames could experience varying ranges of particle distances at different velocities in a practical flame, a regime diagram is constructed which is based on the Péclet number and the particle spacing with a boundary separating the two modes of flame propagation. The regime diagram offers orientation for simulations of large-scale flames using Euler–Lagrange methods where boundary layers around the particles are not resolved. [ABSTRACT FROM AUTHOR]

To deal with inverse nonlinear heat transfer problems in a two-dimensional heat conduction problem, this paper presents a hybrid approach combining fuzzy logic and the extended Kalman filter (EKF). The proposed algorithm has been applied to the real-time reconstruction of the temperature field, time-varying convective heat transfer coefficient, and time-varying boundary heat flux using temperature data from a set of sensors. The effects of the covariance of the initial estimation error, the time step of data acquisition by the sensors, and the number of installed sensors on the precision and stability of the estimation results has been investigated by numerical experiments. To compare and validate the results, all these parameters were investigated using the EKF method. The results show that the adaptive fuzzy extended Kalman filter (FEKF) method for estimating temperature, convection coefficient, and heat flux at the boundaries is precise and stable. The comparison shows that the performance of the FEKF method in parameter estimation is better than the EKF method, and the highest increase in precision and stability of the results is related to the convective heat transfer coefficient. [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]

• Presents an analytical solution for thermal spreading in a cylindrical body with spatially varying side wall cooling. • A series solution is derived, with coefficients shown to be the solution of a set of linear algebraic equations. • Impact of key non-dimensional parameters on temperature field and thermal resistance is investigated. • Results are shown to be in excellent agreement with past work for special cases. • Results are of relevance for edge cooling of microprocessor chips as well as batteries. Thermal spreading and constriction resistance play an important role in thermal management of a variety of engineering systems, including semiconductor devices. Moreover, thermal contact resistance at interfaces between materials is also governed by spreading and constriction processes. Most of the past literature on theoretical modeling of thermal spreading from a source into a larger body assumes an adiabatic side wall, whereas several recent thermal management problems involve active cooling at the side wall. This work addresses such scenarios by solving the thermal spreading problem in a finite thickness cylinder with spatially varying convective heat transfer on the side wall. An eigenfunction-based series solution for the temperature field is written, the coefficients of which are shown to be governed by a set of coupled algebraic equations. Results are shown to agree well with past work for special cases of the general analysis presented here. Thermal spreading resistance is computed for a number of practical scenarios, including step function, periodic and sinusoidal convective cooling on the side wall. In each case, it is shown that the temperature distribution is consistent with the nature of the spatially varying boundary condition. A key theoretical insight from this work is that under certain conditions, the classical definition of thermal spreading resistance may become negative due to the presence of two heat sinks in the problem. Therefore, analysis based on total resistance may be more appropriate. This work offers a significant generalization of past papers, making it possible to analyze realistic thermal management problems, in which, side wall cooling plays an important role. [ABSTRACT FROM AUTHOR]

This review paper presents an overview of transpired solar collectors (TSCs) - a clean energy technology that has gained considerable attention due to its potential to minimise building energy demand and contribute to achieving net zero emissions. The review is mainly focused on scrutinising the disparities in heat and mass transfer mechanisms, specifically the impact of homogeneous and inhomogeneous suction flow distributions on the TSC, encompassing performance improvement methods, performance assessment, and optimisation strategies for TSCs. The inconsistency between the theoretical and real-world performance of TSCs was observed, attributed to the high turbulence wind with non-uniform speed distribution and suction flow interactions with the main flow in the plenum. Corrugations on the absorber improved thermal performance but increased wind heat loss on the exterior surface, which could be mitigated by adding glazing to corrugated TSCs. The review highlighted the flexibility of integrating TSCs with other technologies, including coupling with separate phase change material (PCM) thermal energy storage units, which outperformed those placing the PCM plate into the plenum. Moreover, integrating TSCs with desiccant wheel cooling systems showed some interesting results in terms of cost and payback period, while integration with heat pump systems could reduce electricity consumption, but it was not recommended in cold climates. Exergetic analysis has been frequently used to determine thermodynamic losses of TSCs and it could provide a more accurate performance assessment when integrating TSCs with power generation systems. The review identified several research gaps in related study fields and provided recommendations, encompassing dimensionless analysis of the relationships between wind effect and heat transfer of TSC absorbers, the development of convective heat transfer correlations for TSC's absorber, and the exploration of cost-effective design and optimisation strategies for hybrid TSCs. • Heat and mass transfer mechanisms of transpired solar collectors (TSCs) were analysed. • TSCs can be integrated with different energy technologies to improve flexibility. • Various designs of TSC systems and their main design parameters were discussed. • It provides some insights into developing convective heat transfer correlations. [ABSTRACT FROM AUTHOR]

• Performance evaluation of three different cooling methods for data centers is conducted. • Single-phase immersion cooling has advantages in heat transfer and energy consumption. • Correlation between oil properties and immersion cooling performance is established. • Dynamic viscosity has the greatest impact on the flow and thermal performance of oil coolants. This paper conducts a comprehensive performance evaluation and optimization of single-phase immersion cooling (SPIC) systems to address challenges in data center server cooling. A three-dimensional model of blade servers with different cooling methods is constructed and numerically solved to compare thermal transport performance, flow characteristics, and energy consumption performance. Furthermore, the influence mechanisms of different oil coolants and operating conditions on SPIC system performance are studied. The results indicate that the SPIC scheme exhibits significant advantages in terms of heat transfer capability, heat transfer uniformity, and flow resistance loss. Moreover, the power usage effectiveness (PUE) of SPIC systems decreases by 20.8 % and 17.6 % compared to forced air cooling and water cooling plate solutions, respectively. The dynamic viscosity of oil coolants has the greatest impact on the SPIC performance. Moreover, high-viscosity silicone oil increases the flow resistance of SPIC systems and weakens the convective heat transfer capability between the coolant and components. By ensuring safe operation, increasing the inlet temperature of oil coolants reduces flow resistance losses and enhances heat transfer capacity. Additionally, initially increasing the mass flow rate improves convective heat transfer capacity. However, the heat transfer improvement gradually diminished beyond a certain threshold while flow resistance losses increased significantly. [ABSTRACT FROM AUTHOR]

The electronics in logging tools are susceptible to thermal failure due to the extreme thermal environment. Passive thermal management system (PTMS) has been proven to protect the electronics for several hours. In this paper, a numerical model based on the equivalent thermal conductivity method was developed to determine the transient temperature of downhole electronics. Compared with the traditional analytical model or numerical model considering only one single heat transfer process, the complex heat transfer in the logging tool were comprehensively considered through the proposed model. Briefly, the heat transfer processes in the vacuum bottle were theoretically calculated rather than empirically acquired, and the convective and radiative heat transfer inside the vacuum bottle was equated to the temperature-dependent thermal conductivity of air. In addition, the proposed numerical model was compared with experiments and previous numerical models to verify the accuracy of the model, and the temperature deviation of the electronics was within 5 °C. Moreover, the computational time was greatly reduced compared with the traditional three-dimensional numerical simulation process while ensuring the accuracy of calculation. Therefore, the proposed numerical model achieved both computational accuracy and efficiency, which was expected to be widely used in the development of PTMS for logging tools. • A numerical model based on equivalent thermal conductivity method was proposed. • The complex heat transfer processes in PTMS were comprehensively considered. • The proposed model possessed high computational accuracy and efficiency. [ABSTRACT FROM AUTHOR]

This paper presents a model of heat and moisture transfer through multilayer firefighter protective clothing, which for the first time has taken into account all the major heat and moisture transport mechanisms including conductive and convective heat transfer, two-flux radiative heat transfer, moisture bulk flow, moisture absorption/desorption by fibers, moisture diffusion and phase change. The model was solved by Crank–Nicolson scheme of the finite difference method. Numerical analysis showed that our model can accurately predict the second-degree burn time with an error ranging from 0.37% to 3.55% for different types of clothing assemblies. With this model, the distributions of heat and moisture within the protective clothing system are elucidated and the relative importance of key clothing parameters on thermal protection better understood. It revealed that the thickness of the moisture barrier layer has the greatest influence on the skin burn time. This study is significant in providing a theoretical framework for optimizing the design of thermal protective clothing. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive investigation of heat transfer enhancement and pressure drop in a Coiled Flow Reverser (CFR) with a twisted tape insert. Both laminar (10 < Re < 2000) and turbulent (12,000 < Re < 18,000) regimes have been studied using both experimental and numerical methods. The effects of twist ratio of twisted tape, inlet Reynolds number, and perforating the twisted tapes have been investigated. The study found that the best performance in the laminar regime is achieved with a twisted tape with a twist ratio of 15.73 and no holes on it at a Reynolds number of 2000. In the turbulent regime, the same twisted tape performs the best at a Reynolds number of 17,000. The highest Thermal Performance Ratio (TPR) in laminar regime is 1.55 and in turbulent regime is 0.99. In comparison to the straight pipe, all types of CFRs show better performance in heat transfer enhancement. Also, as the Reynolds number increases, the friction coefficient decreases in all configurations. The study reveals that the twisted tape insert enhances heat transfer performance while increasing pressure drop. This study provides valuable insights into the design and optimization of heat exchangers with CFR. The findings can be used to improve the efficiency and performance of various industrial processes, such as refrigeration systems, air conditioning, and power plants. • Fluid flow and heat transfer in a coiled flow reverser (CFR) were studied. • Effects of tube insert on the heat transfer enhancement of CFR were investigated. • Effects of perforation of tube insert on heat transfer enhancement of CFR were examined. • The effects of tube insert at laminar flow are more than turbulent flow. • Perforation has no significant effects on the thermal performance ratio. [ABSTRACT FROM AUTHOR]

Integrating nanotechnology into phase change materials (PCMs) has emerged as a novel approach to improving PCM thermal properties and performance in various thermal energy storage applications. Nanofluids, nanoparticle suspensions in a base fluid, have been identified as a promising method of increasing the thermal conductivity of PCMs and thus reducing thermal energy charging and discharging duration. This review paper investigates recent advances in incorporating nanotechnology into PCMs, focusing on their applications in the solar energy sector. Thermal conductivity, specific heat, viscosity, density, and convective heat transfer coefficient of nanofluids are discussed, as are the thermal properties of nano-enhanced PCMs (NEPCMs), including thermal conductivity, latent heat, specific heat, viscosity, supercooling, and phase-change temperature. The paper also gives an in-depth look at the uses of nanofluids and NEPCMs in solar collectors, photovoltaic/thermal (PV/T) systems, coolants, and desalination, emphasizing their potential to improve system efficiency. The review also considers the potential environmental and human health concerns connected with using nanofluids and NEPCMs, emphasizing the importance of rigorous evaluation and risk assessment in developing and applying these materials. Overall, the review provides valuable insights into the potential benefits and challenges of incorporating nanotechnology into PCMs and emphasizes the importance of ongoing research and development in this field to further advance the use of thermal energy storage technologies in various applications, including solar energy systems. [Display omitted] • Nanofluids and nanomaterials are considered as innovative heat transfer medium. • Phase change materials (PCMs) for harvesting thermal energy are scrutinized. • Recent developments and properties of nanofluids and nano-enhanced PCM. • Potential applications of nanofluids and nano-enhanced PCM are fully presented. • Environmental impacts and human health risks are comprehensively discussed. [ABSTRACT FROM AUTHOR]

Abstract This paper presents analyses of complex flows and heat transfer induced by twin oblique turbulent slot-jets of different directions (divergent, convergent or parallel) impinging a heated wall. A comparison of the heat transfer characteristics between perpendicular and three cases of twin oblique jets (parallel, convergent and divergent). The twin slot jets are located on a confining adiabatic wall at a distance of 8 slot jet width. Convective heat is investigated numerically examining the effect of Reynolds number (Re) and jet inclination angle (α). This problem is relevant to a wide range of practical applications including nuclear engineering devices, manufacturing, material processing, electronic cooling, drying paper or textile, tempering of glass, etc. All computations are performed using two dimensional large eddy simulations (LES) approach with Smagorinsky sub-grid scale (SGS) models. For all directions and inclinations of the jets, the location of the stagnation points is changed and hence, the location and magnitude of the maximum Nusselt number on the heated wall vary. When Reynolds number increases, Nusselt number is improved for all types of inclination. The averaged Nusselt number shows that the perpendicular impingement gives better heat transfer than that of the oblique jets. The poor heat transfer is obtained for the parallel oblique jets. For the same angle, divergent jets give smallest heat transfer than the convergent jets. Graphical abstract Twin-jets flow configuration. (a) Perpendicular (b) Divergent (c) Parallel (d) Convergent. Image 1 Highlights • Four types of flow patterns of impinging twin jet are studied by Large Dissipation Simulation. • Nusselt number is enhanced for all types of inclination for increasing Reynolds number. • Averaged Nusselt number of the perpendicular impingement gives better heat transfer than that of the oblique jets. • The poor heat transfer is obtained for the parallel oblique jets. • For the same angle, divergent jets give smallest heat transfer than the convergent jets. [ABSTRACT FROM AUTHOR]

Highlights • Transport characteristics in porous media are reviewed, including flow and heat transfer. • Transport phenomena of nanofluids are comprehensively summarized, involving heat conduction, convection and radiation. • Transport literatures for the combination of nanofluid and porous medium are classified and concerned. • Phase change heat transfer of nanofluids and porous media are summarized for liquid-gas type and solid-liquid type. • This review involves conduction, convection, radiation and phase change for specified media. Abstract Increasing the heat transfer rate of heat transfer equipment is an ever-lasting topic in thermal engineering. Due to the advantages of light weight, high specific surface, high thermal conductivity, metal foam is a good extending surface for heat transfer enhancement. Nanofluid has a higher thermal conductivity than the traditional base fluid, so it can be used as an efficient heat transfer medium. This paper focuses on various flow and heat transfer modes of nanofluid, metal foam and the combination of the two, with the physical properties of nanofluid and metal foam summarized. The characteristics of flow and heat transfer are introduced. The motivation of this review paper is to arouse the researchers to pay attention to the basic transport understandings for the heat transfer enhancement of nanofluids in porous media. The knowledge reviewed in this paper is useful for improving the performance of compact heat exchangers, and heat sinks for cooling electronics with porous media and nanofluids. [ABSTRACT FROM AUTHOR]

Highlights • Vertical fins contribute to melting by enhancing natural convection mechanism. • Extending fin length is more beneficial than increasing number of fins. • The cases with longer fins could have a higher overall heat transfer coefficient. • The compromise in energy density could be minor due to sensible energy gain. Abstract This paper investigates the effect of vertical fins, as an enhancement technique, on the heat transfer rate and energy density of a latent heat thermal energy storage system. This contributes with knowledge on the interaction of heat transfer surface with the storage material for optimizing storage capacity (energy) and power (heat transfer rate). For the assessment, numerical modeling is employed to study the melting process in a two-dimensional rectangular cavity. The cavity is considered heated isothermally from the bottom with surface temperatures of 55 ˚C, 60 ˚C or 70 ˚C, while the other surfaces are insulated from the surrounding. Aluminum and lauric acid are considered as fin/enclosure material and phase change material, respectively. Vertical fins attached to the bottom surface are employed to enhance the charging rate, and a parametric study is carried out by varying the fin length and number of fins. Thus, a broad range of data is provided to analyze the influence of fin configurations on contributing natural convection patterns, as well as the effects on melting time, enhanced heat transfer rate and accumulated energy. The results show that in addition to increasing the heat transfer surface area, the installation of vertically oriented fins does not suppress the natural convection mechanism. This is as opposed to horizontal fins which in previous studies have shown tendencies to reduce the impact of natural convection. This paper also highlights how using longer fins offers a higher rate of heat transfer and a better overall heat transfer coefficient rather than increasing the number of fins. Also, fins do not only enhance the heat transfer performance in the corresponding melting time, but also maintain similar total amount of stored energy as compared to the no-fin case. This paper discusses how this is the result of the enhanced heat transfer allowing a larger portion of sensible heat to be recovered. For example, in the case with long fins, the relative mean power enhancement is about 200% with merely 6% capacity reduction, even though the amount of PCM in the cavity has been reduced by 12% as compared to the no-fin case. Although the basis for these results stems from the principles of thermodynamics, this paper is bringing it forward with design consideration. This is because despite its importance for making appropriate comparisons among heat transfer enhancement techniques in latent heat thermal energy storage, it has not been previously discussed in the literature. In the end, the aim is to accomplish robust storage systems in terms of power and energy density. [ABSTRACT FROM AUTHOR]

Abstract In this paper, a simple one-dimensional heat transfer model for the nanofluid-based direct absorption parabolic trough collector (NDAPTC) was developed, which was quick to solve, easy to understand, and unnecessary to solve the complicated differential equations. In an actual system, the vacuum condition between the inner tube and glass cover may be changed gradually over time. Therefore, a continuous function for calculating the convective heat transfer in the annulus from near vacuum to atmospheric pressure was established. Combined with it, the derived heat transfer model was suited to evaluating the long-running thermal performance of a NDAPTC quickly. The heat transfer model for a conventional indirect absorption parabolic trough collector (CIAPTC) was also constructed, aiming at comparing the heat transfer processes of this two kinds of collectors. Both the heat transfer models were validated by the test data from the reference. Additionally, the effects of physical parameters, operational and weather conditions on the collector thermal performance were analyzed. It was shown that improvement of the glass transmittance and emittance was crucial to raise the collector efficiency, especially at a higher inlet temperature. The pressure change in the annulus affected the collector thermal performance greatly, thus it tried to avoid the vacuum damage. The collector efficiency was raised with the solar irradiance and ambient temperature increasing and wind velocity reducing, which was more sensible to these weather conditions at a higher inlet temperature and pressure in the annulus. The results in this paper were useful for guiding the collector improvements and engineering designs theoretically, and were beneficial for cleaner production in civil and industrial heat utilization further. Highlights • A one-dimensional heat transfer model for the novel NDAPTC was developed. • A continuous convective heat transfer function in the annulus was derived. • The model was simple and feasible compared to the results from other research. • Effects of glass properties, operation and weather conditions on η c were studied. • The results were useful for evaluating collector long-running thermal performance. [ABSTRACT FROM AUTHOR]

The combination of the high conductivity of nanofluids with the huge accessible surface area of open-cell metal foam would indeed be a superb heat exchange technology. While individual ingredients of this technology have been separately investigated, the combination of the two has received only scant attention. Actually, there are only few studies that are focused on nanofluids flow and heat transfer in metal foam. This paper is intended to serve as an orientation for researchers who are interested in both nanofluids heat transfer and metal foam. The paper thus includes discussion of various aspects of nanofluids and their heat-transfer applications, as well as fundamentals of flow and heat transfer of ordinary fluids in metal foam. A section on recent heat transfer studies in metal foam is included, and another section on pore-scale modeling of transport in metal foam is provided. Nanofluids are presented in terms of their definition, brief history, preparation methods, theoretical models, properties, and engineering applications related to heat transfer. The paper then moves to describe metal foam and its morphology, as well as current understanding and theoretical treatment of flow and heat transfer in this class of highly-porous media. Key heat transfer and flow properties of the foam are outlined. The paper also presents the case for nanofluids forced convection heat transfer in open channels to serve as a background for the more complex case that involves heat transfer in porous media in general. The case of forced convection of nanofluids in metal foam in particular is presented next. Studies including complex effects, e.g. magnetism, are briefly described. Based on the above information, some key findings, conclusions and recommendations are made for interested researches. [ABSTRACT FROM AUTHOR]

In this paper Double Multi Relaxation Time Lattice Boltzmann method (DMRT-LBM) is utilized to investigate the particle motion in a modeled room for the first time. The effect of indoor heating system location has been considered on both particle deposition and convective heat transfer so six different positions have been selected for the heating system. Results are shown for two Rayleigh numbers (104 and 105) and different particle sizes (1 μ m and 10 μ m). To study the particle motion, after the flow reaches steady state, 50 000 particles are randomly injected to the room and the number of deposited particles on the walls has been reported for 100 s. Considering the present results, when the heating system is in the middle of the wall, the maximum heat transfer rate is seen for both Rayleigh numbers. Also, the number of deposited 1 μ m particles on the walls for all locations of the heating system is zero for R a = 104, so the number of suspended particles is the maximum. For both Rayleigh numbers and 10 μ m particles, the number of deposited particles is maximum when the heating system is fixed on the top of the wall. It is also concluded that when the heating system is located on the top of the wall, the number of suspended 1 μ m particles for R a = 105 reaches its maximum value. [ABSTRACT FROM AUTHOR]

Deep reinforcement learning (DRL) has gradually emerged as a novel and effective method for intelligent control of conjugate heat transfer. Through proper training, DRL agent usually can find a better control strategy than the one optimized manually. For numerous numerical simulation-based investigations, the promising results have only been obtained on 2-dimensional models due to the heavy burden of data acquisition. This paper proposes a novel strategy of transferring the control policy learned in 2-dimensional environment into a new 3-dimensional environment. PDD-DQN (Prioritized Dueling Double Deep Q-Networks) algorithm is used to recognize the underlying relationship between the forced fluid flow and heat transfer performance to produce a control strategy. The DRL controller is first trained on a simple 2-dimensional cavity with one heat source, and the DRL controller is able to reduce the maximum temperature to 315 K which is 2 K lower than the manually optimized control strategy; then the control strategy is transferred to 3-dimensional models where the maximum temperature is further reduced to 308 K; and compared to training the DRL agent directly on 3-dimensional model, the policy-transfer strategy requires only 10% computational expenditure. The proposed strategy is then applied to a more complex testbed with multiple heat sources for further investigating its ability; in both 2D and 3D environment, the DRL controller can cool the maximum temperature to be 307 K which is 8 K lower than the manually optimized control policy; and the computational cost drop to be 3%. The generalization ability of the trained DRL agent in inter-dimensional geometries is confirmed. For the cases with huge grid size, the policy-transfer strategy can economize exponential computational cost, which is of great significance for applying DRL methods to practical thermal control problems. [ABSTRACT FROM AUTHOR]

• Optimizing Permanent Magnets for Ferrofluid Cooling Breakthroughs. • Ferrofluid Cooling with Permanent Magnet Configurations for Efficient Thermal Management. • Unravelling the Impact of Permanent Magnet Configurations on Ferrofluid Cooling Efficiency. • Optimizing Magnet Configurations for Ultra-Efficient Heat Transfer in Ferrofluids. Many industries, such as microelectronics and power electronics, face challenges in removing large amounts of heat from small areas to prevent overheating. Recent research has shown that microchannel flow of ferrofluids driven by an external magnetic field (MF) has the potential to deliver efficient cooling systems for modern electronics. Despite this potential, robust optimisation of the MF has not been undertaken. This paper addresses this shortcoming by reporting the effect of the MF produced by three permanent magnets in various configurations on the heat transfer of ferrofluids undergoing laminar flow through a microchannel under constant heat flux boundary conditions at Peclét number Pe=73. Observations from numerical simulation show that the rate of convective heat transfer varies strongly with position and orientation of the magnets. We show that for the optimum magnet configuration, the maximum and average Nusselt numbers increase by more than 146% and 51% over the unperturbed flow, some of the highest values reported in the literature. These results show that significantly accelerated heat transfer is possible in magnetically driven ferrofluids, with potential for further enhancement with more rigorous optimisation. [ABSTRACT FROM AUTHOR]

Fluid transport through microvascular networks—a hallmark for homeostasis in living systems—has transcended to engineered materials, primarily made possible because of modern manufacturing advancements. Vascular-enabled multifunctionality, including thermal regulation and self-healing, holds great potential for extending the lifetime of structural materials and expanding the operational envelope. Prior studies on vascular-based active cooling use a "combined" heat transfer coefficient (HTC): a single parameter lumps convection and radiation effects. Although the resulting mathematical models are linear—an attractive feature for computational modeling, the combined coefficient approach may not be accurate or even applicable if the operating temperature is unknown, which is the case with many thermal regulation applications (e.g., space probes). In this paper, we illustrate the remarked limitations of the lumped approach and advocate the need to use a decoupled HTC by splitting convective and radiative heat transfer modes. We show the broad applicability of the proposed method by applying it to three material systems: glass and carbon fiber-reinforced polymer composites and an additive manufactured metal. We show, using numerical simulations, the differences in the predictions from the decoupled approach with that of the combined HTC; these differences are prominent at higher heat fluxes. Also, the decoupling has enabled us to establish a scaling law that allows transferring of solutions fields across material systems, strengthening further the validity and utility of our approach. This work's significance is two-fold. First , the research is fundamental, providing accurate measurement protocols for critical model parameters. Second , this work facilitates the development of mathematical models for vascular-based thermal regulation that are predictive even for hostile environments (which are often difficult to realize in laboratories), such as outer space. • Theoretically consistent heat transfer coefficient (HTC) measurements and analyses. • A heat transfer framework for improved prediction of microvascular active-cooling. • Multi-material experimental validation of a reduced-order model. • Self-similar scaling strategy to transfer solution fields across material systems. [ABSTRACT FROM AUTHOR]

Nowadays, a bio-inspired heat exchanger incorporating a triply periodic minimal surface (TPMS) structure has demonstrated great potential in the fields of new energy research and aerospace, which necessitates achieving a balance between low volume and high heat transfer efficiency while maintaining low pressure drop. In this paper, the adjustable heat efficiency of the heat exchanger with TPMS in gradient thicknesses is specially designed. A steady-state conjugate heat transfer (CHT) model is coupled with computational fluid dynamics (CFD) analysis. In addition, the temperature profile and velocity streamline are also checked to analyze the fluid flow behavior of the radiator. The results show that the convective heat transfer coefficient of the Gyroid with gradient level set values is 26.02–60.10% higher than that of the uniform Gyroid model, and the pressure drop is decreased by 9.66–18.05%. Both high heat transfer efficiency and low pressure drop can be achieved when the thickness is 0.2–0.3 mm and Re is 100–125. The heat exchangers with a TPMS thickness gradient in the ratio of 2:4:6 demonstrate a remarkable enhancement in overall heat transfer efficiency, achieving a 30.22% improvement compared to those with a TPMS thickness gradient in 6:4:2. [ABSTRACT FROM AUTHOR]

• The two-way coupled transient multiphase flow and heat transfer modeling framework. • Validation of the heat transfer models based on full-scale distributed temperature sensing (DTS) data. • Benchmarking of convective heat transfer correlations for transient multiphase flow against DTS data. • Analysis of coupling methods for the energy conservation equation with the Drift-flux model. Effective management of gas influxes during drilling for either oil-gas or geothermal wells is crucial for ensuring operational safety and minimizing non-productive times, especially for high-pressure, high-temperature (HPHT) situations. In recent years, there has been a growing demand for comprehensive modeling techniques that can accurately predict gas influx behaviors during Managed Pressure Drilling (MPD). However, many existing approaches have neglected the heat transfer processes between wellbore fluids and the surrounding formation during gas influx management, which can significantly impact the estimation accuracy. To address this issue, this paper presents an advanced transient multiphase flow and heat transfer modeling framework that integrates the solution of the energy conservation equation with the multiphase mass and momentum equations using a two-way coupled approach. A set of full-scale experimental data, including fiber-optic-based Distributed Temperature Sensing (DTS) measurements, were used to perform thorough validation and verifications of the developed modeling framework. Multiple existing heat convection correlations were benchmarked against the DTS measurements to determine the best match. The data from multi-depth downhole temperature sensors and Distributed Acoustic Sensing (DAS) were also compared to the results from numerical simulations. The estimation results of the wellbore fluids temperature profiles showed good agreement with the DTS data for both transient single-phase flow cases and gas influx management scenarios. A comparative analysis revealed that the selection of convective heat transfer correlations could significantly affect the simulation accuracy of fluid temperature distributions and gas influx behaviors. The benchmarking and optimized selection of convection correlations help to improve the performance of transient heat-transfer modeling. In addition, the two-way coupled approach shows improved accuracy compared to the decoupled methods, particularly for HPHT situations and more transient processes. [ABSTRACT FROM AUTHOR]

This paper studied the buoyance-aided mixed convective heat transfer of air-cooling flow in a 5 m long and 1.2mX0.3 m cross-section vertical channel. One of the channel walls was uniformly heated and the rest three walls were unheated and insulated. Heat was transferred from the heated wall by convection and radiation, and heat was also transferred from unheated walls to the air flow by convection originated from their net radiation gain from the heated wall. The air flow rate, the inlet and outlet air flow temperatures, and local surface heat fluxes and temperatures on the heated wall were measured. The Reynolds number of air flow was from 2900 to 221,000 and Richardson number was changed from 0.006 to 90.6. The results showed that for walls with larger emissivity the total convection heat transfer was not deteriorated in mixed convection zone. Therefor a modified heat transfer correlation was developed for the aiding mixed convection in the asymmetrically heated channel considering both mixed convection and radiation. It predicted the experiment results with a deviation less than 18%. [ABSTRACT FROM AUTHOR]

The following paper presents a simple method of determining the presence, distribution and values of heat losses from external building walls as thermodynamic partitions using a Thermal Imaging Camera (TIC). According to Fourier's equation, the value of heat loss is proportional to the temperature gradient ∂ t /∂ y | y=0 in air in the y direction perpendicular to the heated surface. Unfortunately, air temperature cannot be measured with a TIC, as gases do not emit thermal radiation. It is therefore suggested that a grid placed vertically in the air and perpendicularly to the heated surface should be used as a detector of temperature field. Warmed by convective air flows, such a grid becomes a source of infrared radiation. The temperature distribution reproduced on the grid constructed from thin strands of a low thermal conductivity material is sufficiently distinct and sharp to be recorded with a TIC. The correctness of the TIC method has been tested in laboratory conditions on the example of a vertical heating plate, as well as in real-life conditions − on a single pane of glass serving as a building partition. When compared to the works of other authors, the result obtained as Nu  = 0.571· Ra 0.25 relation reveals a satisfactory correlation for the heating plate, while estimated temperature distributions and heat fluxes from the room to the surface of thermodynamic partition wall Q loss,conv,in and on the outside of the building Q loss,conv,ext successfully and quite precisely correlate (5–7% for both internal and external sides) with theoretical calculations. Therefore, the TIC method proposed in this paper can significantly extend the range of application of TICs in energy audits, especially for buildings [ABSTRACT FROM AUTHOR]

The aim of the present paper is to establish the detailed numerical results for bioconvection boundary-layer flow of a two-phase dusty nanofluid. The dusty fluid contains gyrotactic microorganisms along an isothermally heated vertical wall. The physical mechanisms responsible for the slip velocity between the dusty fluid and nanoparticles, such as thermophoresis and Brownian motion, are included in this study. The influence of the dusty nanofluid on heat transfer and flow characteristics are investigated in this paper. The governing equations for two-phase model are non-dimensionalized and then solved numerically via two-point finite difference method together with the tri-diagonal solver. Results are presented graphically for wall skin friction coefficient, rate of heat transfer, velocity and temperature profiles and streamlines and isotherms. To ensure the accuracy, the computational results are compared with available data and are found in good agreement. The key observation from the present analysis is that the mass concentration parameter, D ρ , extensively promotes the rate of heat transfer, Q w , whereas, the wall skin friction coefficient, τ w , is reduced by loading the dust parameters in water based dusty nanofluid. [ABSTRACT FROM AUTHOR]

• An adaptive heuristic fluid-solid interface condition is proposed for the first time. This new condition, called "X-R", has been developed to switch automatically between the Dirichlet-Robin (D-R) and Neumann-Robin (N-R) conditions on the basis of the Biot number. • A temporal interpolation of the fluid conditions for dealing with "non-coupling instants" is defined on the basis of a physics-based approach by naturally considering the flux-temperature linearity. This approach replaces the generally adopted time-dependent linearity, which cannot be justified. • The first test case demonstrates that the adaptive interface condition always produces smooth solutions, which is not the case when fixed conditions like D-R or N-R are adopted. • The second test case shows that the physics-base approach based on the flux-temperature linearity is by far the most satisfactory. However, this interpolation method remains valid only if each ramp defined initially remains in a linear domain. • Indeed, a full transient cycle, entirely based on the existence of particular time points can lead to non-linear problems, difficult to deal with. • As a direct consequence of the previous point and shown in the third test case, the initial problem can be not well-posed. In this case, adding a small number of temporal points can keep the coupling algorithm in a quasi-linear domain. This paper is devoted to the study of numerical methods used in the analysis of a thermal transient flight cycle. Two priorities are stressed. The first one concerns a stability issue, namely the fluid-solid interface conditions. The main properties of the Dirichlet-Robin and Neumann-Robin conditions are first recalled, before proposing for the first time an adaptive heuristic condition combining them. The second line of research is dedicated to an accuracy issue, the temporal interpolation of the fluid conditions for dealing with non-coupling instants. We propose here to adopt a physics-based approach by naturally considering the flux-temperature linearity. Then, using a simple numerical test under severe thermal boundary conditions, three test cases are proposed. The first one demonstrates that the adaptive interface condition always produces smooth solutions. The other two cases show that the physical interpolation approach is much more accurate than the one based on temporal linearity, using a costly but accurate reference solution built for the validation. Moreover, important improvements were made by adequately defining the integral contribution of the boundary condition using the Gauss points in the finite element solid code. It was also revealed that a relevant approximation of the heat transfer requires a sufficiently fine solid mesh at the interface. Finally, potential improvements are proposed at the end of this paper and more particularly two other interface conditions. One of them has the remarkable property of not using the convective heat transfer coefficient. [ABSTRACT FROM AUTHOR]

Pulsed Surface Arc Discharge (PSAD) actuators can improve the aerodynamic performance of aircraft due to their fast response speed and wide frequency band. Understanding the flow structure induced by arc discharge and its transfer process is the key to the sensible use of plasma excitation for flow control. This paper has investigated the dynamic heat and mass transfer process of charged particle clusters generated by arc discharge theoretically and experimentally. The results show that the clusters are composed of charged particles, and the thermal effect is the macroscopic manifestation of the thermal motion of charged particles. The heat and mass transfer processes of charged particle clusters can be expressed as a function of temperature. During heat and mass transfer, the transfer mode of the plasma cluster composed of charged particles will change. The plasma cluster will change from forced convection in the initial stage to natural convection in the final stage, and the Richardson number is different in different stages. The heat released by the gas discharge and the heat released by the plasma mass transfer will form accumulated heat in the space and change the temperature difference, thereby affecting the process of heat and mass transfer. This paper has constructed a model describing pulsed plasma clusters' dynamic heat and mass transfer. Furthermore, the dynamic mode decomposition results of the theoretical model function show that plasma clusters mainly conduct the mass transfer through low-frequency and heat transfer through high-frequency modes. [ABSTRACT FROM AUTHOR]

Unsteady natural convection in a fluid saturated porous enclosure with a pair of vertical diathermal partition separated at equal distance from centre of enclosure is studied in this paper. The purpose is to curtail the convection heat transfer rate across a differentially heated enclosure. The governing equations are based on Darcy's model and solved using Successive Accelerated Replacement and explicit scheme. Streamlines and isotherms are observed for various values of partition ratio, where partition ratio is the ratio of distance of from enclosure centre to either of the partition to the total length of the enclosure; while Nusselt number is evaluated to manifest the heat transfer rate of each configuration. It is found that, employing partition in the porous enclosure substantially attenuates the value of Nusselt number. It has been also found that lower the value of partition ratio, higher will be the reduction in Nusselt number. [ABSTRACT FROM AUTHOR]

• Study the Buoyancy-Marangoni convection in a porous cubic cavity with complex surface. • The Weierstrass–Mandelbrot (W-M) function is applied to simulate the shape of the complex upper surface. • Find the effects of fractal dimension on fluid flow, heat and mass transfer. • Simulate the 3D convection with the high order accurate compact finite volume method. In this paper, a convection and heat transfer problem of power-law fluid in a three-dimensional porous media with complex surface is studied. The Buoyancy-Marangoni convection for non-Newtonian power-law fluids in porous media is solved using a compact high-order finite volume method. For this single-phase model, the left wall is kept at high temperature and high concentration, the right wall is affected by lower temperature and lower concentration, the upper wall is a complex surface, and the remaining walls are considered to be adiabatic and impermeable. The Weierstrass–Mandelbrot function is used to simulate the shape of the upper surface. The fluid in the porous cavity is a power-law nanofluids containing copper oxide nanoparticles. The solid material of the porous medium is aluminum foam. Numerical simulations can be used to determine the power law exponent, Marangoni number, Rayleigh number, and aspect ratio on the flow, heat transfer, and mass transfer rate. [ABSTRACT FROM AUTHOR]

• A heat transfer model was developed under the non-constant wall temperature condition. • Non-constant wall temperature was built based on field test results. • The developed model prediction accuracy is higher than the existing calculation model. • The developed model was extended for the hydropower tunnel ventilation performance. Underground tunnel ventilating has been used in many hydropower stations to reduce the building energy consumption of the underground space by precooling or preheating air. The tunnel wall temperature has an important influence on the cooling or preheating performance of the deeply buried underground tunnels. To solve the problem that the heat balance equation is difficult to solve due to the wall temperature variation along the tunnel, a simple heat transfer model under the non-constant wall temperature condition was developed. The developed model is validated against field tests which showed good agreement between the model results and test data. Field tests of underground traffic tunnels in 5 hydropower stations show that the maximum and minimum attenuations reach 13.0 °C and 2.2 °C, respectively. The comparison results show that the relative deviation of the prediction value of the developed model is 3.72% lower than that of the existing constant wall temperature model. Furthermore, the developed model is used for performance simulation of tunnel ventilation for real great hydropower stations, and the conclusions show that the maximum deviations between the calculated model and tested values range from 0.34 °C to 2.76 °C, and the maximum relative deviations range from 1.89% to 9.69%. In addition, the average relative deviations range from 1.13% to 2.40%. Data comparison verifies that the developed model established in this paper has accurate prediction performance, which is conducive to the design optimization of air conditioning and utilization of natural thermal for underground hydropower stations. [ABSTRACT FROM AUTHOR]

In order to improve the heat transfer performance of microchannels, considering from balance flow resistance and increasing heat transfer area, a non-closed droplet pin fin array microchannel has been designed. The purpose of this paper is to study the inner influence of non-closed droplet structure on convection heat transfer by means of simulation. Through the simulation tool ANSYS Fluent, the working conditions of heat flux density from 5 W/cm2 to 85 W/cm2 are simulated respectively, with the inlet flow as 1.5 kg/h and the inlet fluid temperature as 40 °C unchanged. Along with the increase of heat flux density (5 W/cm2 ⩽ q ⩽ 85 W/cm2), the pressure drop, the bottom temperature as well as the heat transfer coefficient continue to rise, but the rising rates are various. Such variation could be explained by the different heat exchange processes: single phase heat transfer, bubble nucleation, growth and migration and then dry phenomenon, which can be observed from the vapor phase distribution results. The distribution of vapor phase under multiple heat flux explains the variation of pressure and temperature in different stages. It could be concluded that the non-closed droplet pin fin has a strong fractal effect on the fluid. After multiple shunting and confluence, the flow velocity of the fluid on both sides is significantly greater than that between the fins, whose average velocity is 0.135 m/s faster than that between the pins. Besides, the heat transfer effect of the corresponding walls on both sides is also better than that between the pin fins. Because the pin fin has the characteristics of droplet structure, the boundary layer on both sides of the pin fin will be separated and the heat transfer performance will be enhanced. The temperature of the pin fin head is lower, and the temperature of the pin fin tail is higher, with an average difference as about 9.32 °C. The comparison research between non-closed droplet and traditional droplet pin fin has been conducted, as well. It could be concluded that when 5 W/cm2< q < 60 W/cm2, the heat transfer coefficient of the opening droplet microchannel is higher, with pressure drop lower. The main reason is that the improvement of the non-closed structure reduces the kinetic energy loss caused by the collision, and promotes the emergence and growth of the vaporization core. • With the increase of heat flux, the pressure drop increases slowly, rapidly and slowly. • With the increase of heat flux, the initial point of phase transition gradually moves forward from the rear section. • The non-closed structure has a strong fractal effect on the fluid. • The boundary layer on both sides of pin fin will be damaged and the heat transfer performance will be enhanced. [ABSTRACT FROM AUTHOR]

This paper presents a computational heat transfer model of a rotary kiln used for the production of rutile titanium dioxide by the calcination of paste-like hydrous titanium dioxide. The work details the modelling of several chemical reactions occurring in the solid bed region along with turbulent convection of gas, radiation heat exchange among hot gas, refractory wall and the solid surface, and conduction in the refractory wall. Finite-difference techniques are used and the steady state thermal conditions are assumed. The kiln is divided into axial segments of equal length. The solution is of marching type and proceeds from the solid inlet to the solid outlet. The direction of gas flow is opposite to that of the solids. Mass balance of each species in the solid charge, and mass and energy balances of the solid and gas in an axial segment are used to obtain solids and gas temperatures, and species concentration at the exit of that segment. The kiln length predicted by the present model is 45.75 m as compared to 45 m of an actual kiln reported by Ginsberg and Modigell (2011). The steady-state axial gas and solid temperature profiles have been also satisfactorily validated with the numerical results of the aforementioned paper. The output data consist of refractory wall temperature distribution, the axial solids and gas temperature profiles, axial solids composition profile, the length required for drying of the solid charge and the total kiln length required to achieve 98% conversion of anatase TiO 2 to rutile TiO 2 . A detailed parametric study with respect to the controlling parameters such as percent water content (with respect to dry solids), solids flow rate, gas flow rate, kiln inclination angle and kiln rotational speed lent a good physical insight into the rutile-TiO 2 production process in a rotary kiln. [ABSTRACT FROM AUTHOR]

In this paper, enhancement for convection heat transfer of turbulent flow in a solar central receiver tube with porous medium and non-uniform circumferential heat flux was numerically investigated. A new method was introduced to build different porous medium configurations in a unified grid system. Four kinds of enhanced receiver tubes (ERTs) with different porous insert configurations were modeled to optimize the performance of ERT. Furthermore, parameters including filling ratio of porous medium, thermal conductivity ratio (thermal conductivity of porous medium versus that of working fluid), porosity and Reynolds number were analyzed. The results showed that ERT partially filled with porous medium has better heat transfer performance than that fully filled with porous medium. The configuration of porous insert for optimal thermal or thermo-hydraulic performance is interactively affected by all the parameters discussed in this paper. The thermal conductivity ratio is the most crucial parameter to the thermal or thermo-hydraulic performance of ERT. The value of thermal conductivity ratio should be greater than 100 to obtain a good thermo-hydraulic performance. The ERTs with horizontal cylindrical segment shaped porous inserts and hollow cylinder shaped porous inserts are proposed because they can obtain optimal thermal or thermo-hydraulic performance. [ABSTRACT FROM AUTHOR]

Vehicle braking systems use sliding friction between discs and pads to convert large amounts of kinetic energy into heat. Thus, sufficient cooling of disc brakes is needed. Convective heat transfer is an efficient way to cool disc brakes and prevent very high temperatures. To enhance convective heat transfer, ventilated disc brakes are commonly used. Therefore, the heat transfer characteristics of the internal passages of ventilated discs are of special interest. This paper describes a new experimental method to study the convective heat transfer characteristics of the interior passages of ventilated disc brakes, in which a wheel rolled a ventilated disc with tapered radial vanes on a circular track to simulate the real working conditions of a ventilated disc brake. The naphthalene sublimation method was used to obtain convective heat transfer characteristics by the heat/mass transfer analogy principle. Different data reduction methods were used to present the experimental results. The correlations among the Nusselt number, Reynolds number and other geometric parameters of the tapered radial vanes are reported in this paper. The results indicate that the heat transfer coefficient has a close relationship with Coriolis acceleration. Thus, this paper reports a correlation between Coriolis acceleration and the convective heat transfer coefficient, which can be used to extend the results obtained by the tested discs to realistic cases. • A new model to study the heat transfer ability of the interior passage of ventilated disc brake is provided. • Correlations of the interior passage of a ventilated disc brake with tapered radial vanes is obtained. • Relationship between Coriolis acceleration and heat transfer capacity is addressed. • Relationship between Coriolis acceleration and heat transfer capacity can extend the experimental results to real case. [ABSTRACT FROM AUTHOR]

• Static mixers for enhancing convective heat transfer were additively manufactured. • Additive manufacturing enabled novel flow geometry, chevron-shaped structures. • Static mixers were monolithically integrated into liquid water channel. • Static mixers enhanced convective heat transfer coefficient about 100%. • A finite volume model suggests a guidance on the static mixer design. This paper investigates heat transfer enhancement by means of additively manufactured static mixers during liquid water cooling of a horizontal, heated flat plate. The static mixers disrupt the thermal boundary layer and induce mixing, resulting in an increased heat transfer rate of about 2X larger than flows without mixers. Simulations of the flows provided insights into the flows near the mixers, and guided selection of specific mixer geometries. The mixers were fabricated directly into the flow channels using additive manufacturing and then assembled onto the heated plate. Two types of mixing structures were analyzed: twisted tape structures that are similar to conventional static mixers; and novel chevron-shaped offset wing structures. Heat transfer performance was measured for liquid water (510 ≤ Re ≤ 1366) cooling the heated section with convective heat flux ranging between 0.1 and 0.8 W/cm2. This work demonstrates the potential of additive manufacturing to enable novel flow geometries that can enhance convection heat transfer whilst minimizing pressure drop penalties and volumes. [ABSTRACT FROM AUTHOR]

Highlights • Thermal convection was suppressed by the dielectrophoretic force. • The electroconvection has a strengthening effect on free convection heat transfer. • Strengthening effect of electroconvection increases with increasing electric field. • The strengthening effect at a low heat flux is better than that at a high heat flux. Abstract In this paper, the electrohydrodynamic (EHD) enhancement of free convection heat transfer is studied experimentally. A smooth copper heating surface is submerged in R113 liquid, the heat fluxes ranging from 4461 W/m2 to 17805 W/m2, and a high voltage ranging from 1 kV to 5 kV are applied on the heating surface. High speed photography is used to record the images of electroconvection. The results show the relationship between the intensity of the electric field, the height of the electroconvection layer and the enhancement of the heat transfer coefficient. Electric field can enhance free convection heat transfer, with the increase of the electric field the fluid disturbance on the heating surface increases obviously, and with the increase of the heat fluxes the enhancement of electric field gradually decreases. [ABSTRACT FROM AUTHOR]

Highlights • CFD study of natural convection around heated inclined plates. • Location of lift-off of plume approximately coincides with minimum heat flux point. • Relative importance of horizontal and vertical convection mechanisms is discussed. • Subtle and complex thermo-fluid-dynamics in near-horizontal configurations is revealed. • Nusselt number passes through a minimum as inclination changes from horizontal to vertical. Abstract The paper invokes the power of computational fluid dynamics (CFD) for accurate determination of the detailed thermo-fluid-dynamics of natural convective flow around heated inclined plates, with an extensive experimental validation of the computed heat transfer results. The study provides fundamental physical insight through a comprehensive understanding of the behaviour of the contours of velocity, temperature and pressure as a function of inclination angle over the entire range from the vertical (γ = 90 °) to the horizontal (γ = 0 °). In particular, the present study documents, for the first time, qualitative and quantitative behaviour of the lift-off point at which the natural convective boundary layer converts into a free plume. Similarly, the details of the spatial evolution of the velocity profile and temperature profile in the plume as a function of the inclination angle of the plate are determined for the first time. It is shown how the relative importance of indirect pressure difference and direct buoyancy, as mechanisms of natural convection, changes as the inclination angle is gradually altered from the horizontal to the vertical. Through accurate computation (and ingenious representation) of the velocity, temperature and pressure fields at small intervals of the inclination angle, the subtle and complex thermo-fluid-dynamics in near-horizontal configurations is revealed. It is shown that the non-dimensional lift-off distance changes from 0.5 (middle of the plate) to 0.9816 (nearly the trailing edge) as the angle of inclination is changed from 0 ° to 15 °. It is established that as the inclination angle is increased gradually from the horizontal position, the value of average Nusselt number Nu ‾ initially decreases slightly, passes through a minimum point and then onward increases continuously up to the vertical position of the plate. [ABSTRACT FROM AUTHOR]

Abstract The transfer of heat by the fins is influenced by the change of the direction of the fins. This paper investigates study the effect of the direction of longitudinal fins on a three-dimensional convection heat transfer in a rectangular channel and also study the effect of the lateral and longitudinal inclination of the rectangular channel. The Grashof range from 5 × 108 to 109, Reynolds from 1000 to 2300 and Prandtl 0.71. The bottom surface of the channel is exposed to constant heat flux, while other walls are isolated. Two cases are investigated. In case one, measurements were conducted for a lateral inclination of the channel, with a range of α = 0°,30°,60°, and 90°. Case two studied the longitudinal inclination of the channel, with the lateral inclination angle fixed at α = 90° and the longitudinal inclination angle Ө = 0°, 30°, 60°, and 70°. The dimensionless of fin height was Hf/H = 0.6, and the fin spacing was S/H = 0.17. The experimental results show that the coefficient of the heat transfer for lateral inclination (α = 0°) was greater than that for a sideways orientation (α = 90°). Additionally, the average coefficient of heat transfer for both the lateral and longitudinal inclination case is increased with the longitudinal inclination angle. The empirical equations are obtained based on the experimental results. These equations correlated the Nusselt number as a dependent variable of the orientations angles, Reynolds number, and the Grashof number, these equations are consistent with experimental results. Graphical abstract Image 1 Highlights • Investigate the effect of inclination and rotation angles of rectangular duct. • Experiments conducted on laminar mixed convection heat transfer from fins array. • Optimized a constant heat flux condition by tuning inclination and rotation angle. • A novel correlations for mixed convection heat transfer for various orientations. • The higher heat transfer coefficient is observed at the highest inclination angle. [ABSTRACT FROM AUTHOR]

Abstract In the process of drilling, the tubing-casing annulus are easily to be eccentric. Heat transport phenomenon occurring in such tubes and annulus can be complex, and this paper investigates forced convection in a vertical eccentric annulus with different radius ratios and eccentricity (normalized by the radius difference). Physical experiments and numerical simulation have been conducted with constant inner tube inlet temperature and flow rate of hot water. Results show that with the increase of radius ratios, the convection heat transport coefficient increases. For a radius ratio of 1.875, the heat transport coefficient increases as the eccentricity increases when the Re > 2000 and it decreases when the Re < 2000. For a radius ratio of 2.25 and Re < 1500, the influence of eccentricity is trivial, but for Re > 1500, the increase of eccentricity strengthens the heat convection. For a radius ratio of 3.06, the heat transport coefficient increases as the eccentricity increases. An empirical correlation for Nusselt number is derived as a function of Reynolds number, Prandtl number and eccentricity. Highlights • For D o / D i = 1.875, the effect of eccentricity is identified into two zones. • For D o / D i = 3.06, the increase of eccentricity enhances the convective heat transport. • The increase of radius ratio strengthens the heat transport of annular channels. • With low eccentricity and low Reynolds number, the increase of radius ratio can hardly improve the heat exchange efficiency. • Three kind of empirical correlations are regressed through parametric analysis. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract The interaction of two drops is studied experimentally: a small droplet falls on a large sessile drop located on a hot wall. The temperature of the falling droplet is 20 °C, and the wall temperature is 80 °C. It is well known that the surfactant suppresses the Marangoni flow (Ma). This paper considers the influence of surfactant in the presence of several key factors – dynamic and thermal ones. When a droplet impacts a liquid layer, the heat and mass transfer is associated with both the dynamic factor (inertial forces and pressure "jumps" inside the droplet) and the thermal factor (an increase in the surface temperature gradient). The novelty of this work is that it for the first time shows that at the moment of extremely short-term interaction of drops, there is 7–8 times increase of velocity inside the drop. In this case, the determining effect on the convection enhancement is associated with thermocapillary convection, and the role of the dynamic factor is insignificant. The influence of graphite particles and surfactant of sodium dodecyl sulphate (SDS) has been investigated. The velocity "jump" resulting from the fall of the water droplet with surfactant is 4 times smaller as compared to the fall of surfactant-free droplet. The instantaneous velocity fields inside the sessile drop have been experimentally studied using Micro Particle Image Velocimetry (Micro PIV). The obtained results are of great importance for the correct modeling of heat and mass transfer and have a wide application for sprays, as well as for the multiphase flows. [ABSTRACT FROM AUTHOR]

Abstract In this paper the relative performance of two different differential stress and flux models for computing natural convection in an enclosure is examined. One model is the low-Reynolds number differential stress and flux model and the other model is a recently developed elliptic blending based differential stress and flux model. The primary emphasis of the study is placed on investigating the accuracy of the both models for the turbulent natural convection in an enclosure. Both models are used to predict turbulent natural convections in a square cavity with hot and cold side walls and conducting top and bottom walls (Ra = 1.58 × 109). The relative performance between both models is examined through comparing their solutions with the experimental data. It is shown that both models predict the vertical mean velocity component and mean temperature well. It is also shown that the low-Reynolds number differential stress and flux model slightly under-predicts the turbulent kinetic energy and vertical turbulent heat flux. It is also noted that the implementation of the elliptic blending second moment model is easier since this model does not contain any wall-related parameters and damping functions. [ABSTRACT FROM AUTHOR]

Abstract This paper deals with the modeling of weakly rarefied and dilute gas flows in micro channels by the continuum approach, valid for Knudsen numbers smaller than about 0.1. It particularly focuses on the modeling of the associated heat transfer. The models proposed in the literature for the forced convection of gas flows in long micro channels between two infinite plates are more specifically discussed. The complete model for such flows is reminded after a compilation and a brief description of their possible applications in industries. The compatibility of the pressure work and viscous dissipation in the energy equation with the power of the viscous stress at the walls is discussed in detail. A dimensional analysis is proposed in the context of long micro channels. Analytical solutions for the velocity and temperature fields and for the Nusselt number are provided in the case of compressible micro-flows in isothermally heated flat plate channels, with pressure work and viscous dissipation included. The choice of an appropriate Nusselt number, including the power of the viscous stress at the wall, is particularly discussed. It is shown analytically and numerically, by solving the complete model for an isothermal wall micro-channel, that the Nusselt number tends to zero when the hydraulic diameter decreases, that is when the Reynolds number decreases and the Knudsen number increases. This could theoretically explain the very small values of the Nusselt number obtained in the experiments by Demsis et al. (2009, 2010). Highlights • Isothermally heated gas slipping flows in long plane micro-channels are considered. • Analytical solution of flow and heat transfer established from an asymptotic analysis. • Viscous stress power at the wall must be included in the total wall heat flux. • Wall Nusselt number tends to zero or to very small values for this flow type. • The very small values of the Nusselt number obtained in experiments are explained. [ABSTRACT FROM AUTHOR]

Highlights • Constructal design of the first to fifth order line-to-line vascular channels. • Application of entropy generation minimization principle. • Angles of line-to-line vascular channels are optimization variables. • Fixed vascular channel areas and total volumes constraints. • Optimal constructs and influences of the mass flow rate are obtained. Abstract Constructal optimizations of the first to fifth order line-to-line vascular channels (LVCs) with convective heat transfer are carried out in this paper by taking minimum entropy generation rate (EGR) and minimum entropy generation number (EGN) as the optimization objectives, respectively. The angles of the LVCs are taken as the optimization variables, and the fixed vascular channel areas and total volumes of LVCs are taken as the constraints. The optimal constructs corresponding to minimum EGR and minimum EGN are obtained, respectively. The influences of the mass flow rate (MFR) on the optimal results are analyzed. The results show that the EGR of the LVCs decreases when the angle freedom increases with the constant dimensionless MFR, which is more obvious when the order increases. When the order increases, the EGR increases but the EGN decreases. Both the EGR and the EGN increase with the increase of the dimensionless MFR. The results obtained herein can provide some thermal design and management guidelines for the applications of LVCs. [ABSTRACT FROM AUTHOR]

This paper presents a direct numerical simulation (DNS) technique, the Finite Element Fictitious Boundary Method (FEM-FBM) for the simulation of fluid-solid two-phase flows with heat transfer. The heat transfer equation is introduced to study thermal convection in fluid-solid two-phase flows. The Boussinesq approximation is considered for the coupling of momentum and temperature flow fields. Multi-grid finite element solver is used to compute flow equations of mass, momentum, and energy on a fixed Eulerian mesh which is independent of time and the solid particles are allowed to move freely in the whole computational domain. Fictitious boundary method (FBM) is used to treat the particles inside the fluid, FBM takes account of the thermal and momentum interaction between the fluid and the particles. The accuracy and stability of presented method are validated by comparing our test cases results with the results reported in the available literature. Numerical tests are performed to show that this method is potentially powerful and provides an efficient approach to simulate complex thermal convective particulate flows with a large number of particles. [ABSTRACT FROM AUTHOR]

Helical tube has been widely used in liquefied natural gas (LNG) liquefying applications. However, studies on the heat transfer performance of hydrocarbon mixture refrigerant in a helical tube have rarely been conducted. In this paper, condensation heat transfer characteristics of hydrocarbon mixture refrigerant in a helical tube were investigated numerically. The tests were conducted at saturation pressure of 2–4 MPa, mass flux of 100–400 kg/(m 2 ·s) and vapor quality 0–1. The numerical model was well verified by experimental data. And the effects of different structural parameters (tube diameter, helix angle and curvature diameter) and operating parameters (mass flux, heat flux and saturation pressure) on the heat transfer of hydrocarbons refrigeration were considered. The results showed that the tube diameter and mass flux had significant effect on condensation heat transfer coefficient. Meanwhile, stratified flow, half-annular flow and annular flow were observed during the condensation of methane/propane mixtures refrigerant by simulations, and a new flow pattern transition line for hydrocarbon refrigerants condensation process was proposed. Finally, the simulation results were compared with several well-known condensation heat transfer correlations. The research results will provide some constructive instructions for more effective design of LNG heat exchanger. [ABSTRACT FROM AUTHOR]