As an important and effective indicator of contact heat transfer, thermal contact resistance is a widespread phenomenon in engineering. It can directly affect product reliability, full-load performance, power consumption and even life cycle in energy, aerospace, electronic packaging, cryogenic refrigeration, etc. Therefore, enhancing the interface heat transfer and suppressing thermal contact resistance have become increasingly important. Against this background, this paper seeks to elaborate on conceptions of thermal contact resistance and the ways to reduce it. After reviewing the existing methods of measuring thermal contact resistance and characterizing the interface morphology, we highlight the theoretical underpinnings of thermal contact resistance, including the two-dimensional mathematic characteristics of the contact interface and the theoretical and empirical models for quantifying it. Three categories of influencing factors, i.e., thermal, geometrical and mechanical states, are then presented. Based on the macroscopic formation mechanism, the paper summarizes the existing methods for suppressing thermal contact resistance, with close attention paid to polymer composite thermal interfacial materials and metal interfacial materials filled with high thermal conductivity filler. In light of the findings, this review provides five promising directions for future research on thermal contact resistance. It suggests that the failure modes and service life of interface materials are essential to apply such technologies to suppress thermal contact resistance in practice. This review will be a guide for future research in thermal contact resistance and for the widespread use of composite interface materials.

In the field of nano energy, investigating the specific heat capacity and coordination number of nano-confined water is highly significant for gaining a better understanding of the energy and microstructure of confined water. In this work, we employed the method of molecular dynamics (MD) simulation to calculate the specific heat capacity at constant volume and coordination number of water molecules confined in carbon nanotubes (CNTs) under different conditions (T=600–700 K, P=21.776 and 25 MPa, CNT diameter=0.949–5.017 nm). The results showed that near the critical point, the specific heat capacity at constant volume of confined water was lower than that of bulk water, and the energy fluctuation showed a trend of first increasing and then remaining unchanged with the increase of temperature and CNT diameter. Among them, the saturation point of temperature is 650 K (reduced pressure P_r=1) and 660 K (P_r=1.15), and the saturation point of CNT diameter is 2.034 nm. Additionally, the pseudo-critical temperature of confined water was the same as bulk water, and it increased with the increase of critical pressure. Moreover, with the increase of CNT diameter, the coordination number of confined water increased rapidly, and reaches the saturation state when the CNT diameter is 2.034 nm. This investigation revealed the mass and energy characteristics of nano-confined water near the critical point, which could provide guidance for the critical phase transition of nano-confined water.

Backward Monte Carlo method of the complicated and exact three-dimensional turbine with the spectral emission and reflection characteristics of the turbine blades materials and the spectral absorption and emission characteristics of combustion gas is established. The factors affecting the accuracy of the radiation temperature measurement are analyzed. The results show that reducing the distance from the probe to the target surface can reduce the effect of the environment on the measurement accuracy. Increasing the temperature and emissivity of the target surface can improve the measurement accuracy. The reflection characteristics of the surfaces have little influence on the radiation temperature measurement, so the blades can be considered as diffuse reflectors in order to improve the calculation efficiency. The temperature measurement accuracy decreases rapidly as the temperature of the combustion gas increases. The temperature measurement accuracy decreases with the increase of total gas pressure and H₂O concentration. When measuring the temperature of rotating blades, the apparent emissivity of the target surface is inversely proportional to the measurement accuracy.

SCRamjet is exposed to severe thermal environments during hypersonic flights, which poses a serious challenge to the engine cooling technology. Regenerative cooling with hydrocarbon fuel is considered promising, in which the hydrocarbon fuel flows through micro channels (200 μm^–3 mm) to absorb the combustion heat release. With strictly limited hydrocarbon fuel onboard, heat transfer deterioration and over-temperature are highly possible. In this paper, micro ribs with staggered side gaps are introduced and numerically studied to enhance the heat transfer. Compared with the straight channel and channel with straight micro ribs, the staggered side gaps alleviate the local low velocity zone and intensify the longitudinal and transverse vortexes. The heat transfer is obviously enhanced. Larger rib height enhances the heat transfer by stronger side gap effects at the cost of larger pressure loss. The best overall heat transfer factor η is achieved in the case of h_rib/H=0.1, which increases by 204.5% comparing to the straight channel. When the rib interval is too small or too large, it approaches to the straight channel. The best overall heat transfer factor η is achieved in the case of L/p_rib=100, which increases by 212.9% comparing to the straight channel. It is known the improvement in the geometry of the ribs, i.e., the staggered-side-gap micro ribs, induces extra transverse vortex and improves the heat transfer performance more effectively. The research of this paper provides support for the cooling design of the SCRamjet.

Tubular moving bed heat exchangers (MBHEs) present inherent advantages for efficiently and stably recovering sensible heat from high-temperature granular bulk. In this study, we introduce a viable and practical approach based on the combined approach of Computational Fluid Dynamics with Discrete Element Method (CFD-DEM) and employ it to conduct a comprehensive investigation into the effects of operation parameters on tubular MBHEs. These parameters include inlet particle temperature (ranging from 500°C to 700°C), tube wall temperature (ranging from 50°C to 250°C), and particle descent velocity (ranging from 0.5 mm/s to 12 mm/s). Our analysis reveals that the heat radiation and gas film heat conduction predominantly govern the heat transfer process in the particle-fluid-wall system, collectively contributing to approximately 90% of the total heat flux of tube wall (Q_w^simu). The results indicate that increasing the inlet particle temperature and reducing the tube wall temperature intensify heat transfer by enlarging the temperature difference. More interestingly,   Q_w^simu exhibits three distinct stages as particle descent velocity increases, including an ascent stage, a descent stage, and a stable stage. Furthermore, the simulation attempts suggest that the optimal descent velocity for maximizing Q_w^simu  falls within the range of 1.3–2.0 mm/s. These findings not only uncover the precise influence mechanisms of operation parameters on heat transfer outcomes but also offer valuable insights for heat transfer enhancement efficiency in MBHE system.

In this work, the interactions between the environmentally friendly refrigerant propane (R290) and Polyol Ester (POE) including solubility parameters, diffusion coefficients, binding energies, and radial distribution functions were investigated using molecular dynamics (MD). Specifically, the effect of chain length of Pentaerythritol esters (PEC) as the representative component of POE on the interaction of PEC/R290 was discussed. The solubility parameters difference exhibits the PEC and R290 are more easily miscible as increasing chain length of PEC, and there is plateau as the chain lengths is above 8 units. In addition, it was also found that solubility parameters are various for the isomers due to the different spatial structure. Moreover, the presence of PEC would reduce the diffusion coefficient of R290 in the mixed system of R290/lubricant with the reduction of 20% on average. It is also found that van der Waals forces are dominant in the R290/PEC system. The PEC molecules start to be bound to the H atoms of R290 at the first neighbor shell layer with a radius of 0.219 nm. Finally, the molecular simulation model of POE22 considering various actual components was innovatively developed. The results showed that the solubility of R290 with typical POE lubricant is affected by the composition and proportions of based oil and additives.

Understanding the residence time distribution (RTD) of a continuous hydrothermal reactor is of great significance to improve product quality and reaction efficiency. In this work, an on-line measurement system is attached to a continuous reactor to investigate the characteristics of RTD. An approach that can accurately fit and describe the experimental measured RTD curve by finding characteristic values is proposed for analysis and comparison. The RTD curves of three experiment groups are measured and the characteristic values are calculated. Results show that increasing total flow rate and extending effective reactor length have inverse effect on average residence time, but they both cause the reactor to approach a plug flow reactor and improve the materials leading. The branch flow rate fraction has no significant effect on RTD characteristics in the scope of the present work except the weak negative correlation with the average residence time. Besides, the natural convection stirring effect can also increase the average residence time, especially when the forced flow is weak. The analysis reveals that it is necessary to consider the matching of natural convection, forced flow and reactor size to control RTD when designing the hydrothermal reactor and working conditions.

The loop heat pipe (LHP) is an advanced, efficient two-phase heat transfer unit, whose operational performance may be affected by microgravity conditions in contrast to ground-based applications. The performance of on-orbit temperature data and ground test of a copper-propylene LHP with a condenser temperature range of 243.15 K to 303.15 K were employed to compared and analyzed. The LHP has successfully started up for more than 193 times with a good heat transfer performance and a stable start-up stabilization on-orbit under a complex orbital heating environment for more than eight months. With a small heat load (10.0 W), the average start-up time is 110.0 s while the start-up temperature ranges from 5.71 K–12.78 K. The start-up time at large temperature differences in the high temperature zone will be higher than the time required for start-up at smaller temperature differences in the low one. When the condenser temperature is 250.0 K, the stable temperature difference on orbit is 3.83 K, which is generally consistent in heat transfer compared to 2.20 K in the ground test. In this paper, we can conclude that the on-orbit flight data up to now can provide a reference to the design of subsequent LHP space applications.

Near-field thermophotovoltaic (NTPV) devices comprising a SiC-hBN-graphene emitter and a graphene-InSb cell with gratings are designed to enhance the performance of the NTPV systems. Fluctuational electrodynamics and rigorous coupled-wave analysis are employed to calculate radiative heat transfer fluxes. It is found that the NTPV systems with two graphene ribbons perform better due to the graphene strong coupling effects. The effects of graphene chemical potential are discussed. It is demonstrated that near-field radiative heat transfer of thermophotovoltaic devices is enhanced by the coupling of surface plasmon polaritons, surface phonon polaritons, hyperbolic phonon polaritons, and magnetic polaritons caused by the graphene strong coupling effects. Rabi splitting frequency of different polaritons is calculated to quantify the mutual interaction of graphene strong coupling effects. Finally, the effects of cell grating filling ratio are investigated. The excitation of magnetic polaritons is affected by the graphene ribbon and the cell filling ratio. This investigation provides a new explanation of the enhancement mechanism of graphene-assisted thermophotovoltaic systems and a novel approach for improving the output power of the near-field thermophotovoltaic system.

A numerical study is conducted to elucidate the impact of hole shapes and additional flow angles on the flow structure of the coolant and temperature field in the leading edge area of the gas turbine rotor. Four typical hole shapes are considered for the GE-E3 blade. The impact of the additional flow angle (E) within each hole shape on the temperature field is investigated. The results indicate that for the leading edge area and suction surface, the fan-shaped hole case performs best in decreasing temperatures, with a decrease of about 43 K. This is mainly due to the fact that the fan-shaped hole has the maximum expansion in hole spanwise direction. For the pressure surface, the console hole case performs best in decreasing temperatures, with a maximum reduction of about 47.2 K. The influence of E on the surface temperature at leading edge area varied between the different hole shapes. For the cylinder hole and console hole, the E=–20° case has the lowest area-averaged temperature. Because both the fan-shaped hole and the 7-7-7 shaped hole are expansion holes, the pattern of variation of the leading edge area temperature with increasing E is similar for the fan-shaped hole case and 7-7-7 shaped hole case. The E=20° case shows the lowest spanwise-averaged temperature near the hole outlet, and the E=–20° case shows the lowest spanwise-averaged temperature further downstream.

Molten salt and supercritical carbon dioxide (sCO₂) are considered to be one of the most promising combined heat transfer refrigerants for third-generation solar thermal power generation. To evaluate the potential of chloride salts and carbonates in third-generation solar thermal power generation, this paper uses molten salts and sCO₂ as the working media of printed circuit board heat exchangers (PCHE), and uses numerical simulation to study the heat transfer and friction of PCHE channels with different molten salts and sCO₂, and establishes predictive correlations respectively. A local heat transfer and friction study was conducted on the sCO₂ side of the airfoil channel, and it was found that the inlet mass flow rate has a significant impact on it, while the inlet temperature has a relatively small impact. A comprehensive comparison was made between the heat transfer and friction of two molten salts, and the comprehensive performance of chloride salts was 70%–80% higher than that of carbonates. The results indicate that the potential of chloride salts in third-generation solar thermal power generation is much greater than that of carbonates.

Supercritical carbon dioxide printed circuit board heat exchangers are expected to be applied in third-generation solar thermal power generation. However, the uniformity of supercritical carbon dioxide entering the heat exchanger has a significant impact on the overall performance of the heat exchanger. In order to improve the uniformity of flow distribution in the inlet header, this article studies and optimizes the inlet header of a printed circuit board heat exchanger through numerical simulation. The results indicate that when supercritical carbon dioxide flows through the header cavity, eddy currents will be generated, which will increase the uneven distribution of flow rate, while reducing the generation of eddy currents will improve the uniform distribution of flow rate. When the dimensionless factor of the inlet header is 6, the hyperbolic configuration is the optimal structure. We also reduced the eddy current region by adding transition segments, and the results showed that the structure was the best when the dilation angle was 10°, which reduced the non-uniformity by 21% compared to the hyperbolic configuration, providing guidance for engineering practice.

By investigating heat transfer and flow structures of dimples, orthogonal ribs, and V-shaped ribs in the impingement/effusion cooling, the article is dedicated to selecting a best-performing internal cooling structure for a turbine vane. The overall cooling effectiveness and coolant consumption are adopted to evaluate the cooling performance. To analyze the influence of structural modification, the flow field is investigated on chordwise/spanwise sections and the target surface. The blockage effect on crossflow can protect jet flow, resulting in higher heat transfer performance of the target surface. Ribs own a stronger blockage effect than dimples. Compared with the blockage effect, the influence of the rib shape is negligible. By installing dimples between ribs, heat transfer is augmented further. The introduction of ribs/dimples leads to higher discharge coefficients of jet nozzles but lower discharge coefficients of film holes. Thus, the film cooling deteriorates. Meanwhile, the installation of the ribs and dimples decreases total coolant consumption. The effect of ribs/dimples on heat transfer and effusion condition of internal and external cooling is analyzed. The best-performing cooling structure is the target surface with dimples and orthogonal ribs, which decreases the wall temperature and coolant consumption by 14.57–28.03 K and 1.19%–1.81% respectively. This article concludes the flow mechanism for dimples and influence factors on the cooling performance, which may serve as guidance for the turbine vane design.

The turbine blades of aircrafts must be properly cooled to prevent engine failure. Thus, to investigate the influence of the tip structure on the film cooling effect, pressure-sensitive paint test technology was used to determine the adiabatic film cooling effectiveness in this study. The experiment was completed in a cascade comprising three straight blades. The effects of the blowing ratio, density ratio, tip clearance, and tip structure on film cooling efficiency were analyzed. The experimental results demonstrated that, as the blowing ratio increased, the film coverage area and film cooling efficiency increased under most experimental conditions. However, the film cooling efficiency was found to initially increase, and subsequently decrease, as the blowing ratio increased. The respective influences of the density ratio and tip clearance on the film cooling efficiency were found to be significant. The density ratio experiments revealed that a high-density ratio can result in better film coverage than the low-density-ratio air. The tip clearance experimental results indicated that a small tip clearance promotes an increase in film cooling efficiency; this is because the small tip clearance negatively affects the main stream leakage flow, which can reduce the film coverage area. Under the conditions of the Base case 2 configuration, a blowing ratio of 2.1, and a tip clearance of 0.6%h, the average film cooling efficiency of the blade tip was 0.22. Among the three blade tip structures applied in this study, Base case 2 demonstrated higher film cooling efficiency than the other two blade tip structures under the conditions of the same blowing ratio, tip clearance, and density ratio.

The air precooler of SABRE engine (Synergetic Air-Breathing Rocket Engine) is a tube bundle type of heat exchanger consisting of thousands of small tubes. The cold helium gas flows in the small tube bundle, and the high-speed hot air circulates outside the tubes and exchanges heat with the helium in the tubes. In this study, the cross-arrangement tube bundle consisting of small tubes with outer diameter of 1 mm was taken as the research object. The system simulation calculation and data sorting were carried out. It is found that the empirical correlation of the conventional tube bundle is no longer applicable to the small tube and the reasons for inapplicability are explained by analysis of flow field. In addition, the results show that the average heat transfer coefficient of cross small/micro tube bundle isn’t affected by heat flux, but increases with the increase of Reynolds number, increases with the increase of transverse spacing, and decreases with the increase of longitudinal spacing. The flow resistance coefficient isn’t affected by heat flux and longitudinal spacing, but decreases with the increase of Reynolds number, and decreases with the increase of transverse spacing.

Cryogenic loop heat pipes are highly efficient heat transfer devices at cryogenic temperature range, which have promising application prospects in satellites, spacecrafts, electronics, and so on. Cooling down process is a most critical process for a CLHP before startup. At present, secondary loop is a major way for a CLHP to fulfil cooling down and most studies are concentrated on heat transfer characteristics during normal operation. However, few investigations have been carried out on the cooling down process. In this paper, the cooling down process of a nitrogen-charged CLHP assisted with a secondary loop was experimentally investigated. A simple qualitative approach to estimate the cooling down time was proposed according to the law of conservation of energy. The two flow paths of the working fluid in the CLHP during the cooling down process were described. Experimental studies on the cooling down process with various secondary heat loads and working fluid inventory were presented in detail. With the increase of secondary heat load, the elapsed time of Stage III decreased significantly due to the larger mass flow rate in Path I. In addition, the effect of the working fluid inventory on the cooling down time was generally small in the range from 2.99 MPa to 3.80 MPa. However, with 2.80 MPa working fluid inventory, it required much longer cooling down time, which was because of the lack of liquid in the CLHP with low working fluid inventory. Moreover, the influence of gravity on the temperature variation of the components during the experiments was analyzed. This work is beneficial for better understanding of the cooling down process and optimizing of CLHPs.

The experimental and theoretical research on the critical heat flux (CHF) in a uniformly heated water-wall tube of the efficient ultra-supercritical circulating fluidized bed (USCFB) boiler has been conducted. In particular, the experimental pressure varies from 18 MPa to 21 MPa, which is from 0.814Pcr–0.95Pcr (Pcr: critical pressure). The mass flux varies from 310 kg·m^–2·s^–1 to 550 kg·m^–2·s^–1. The inlet sub-cooling temperatures vary from 5°C to 10°C. The material of the tube is 12Cr1MoVg. From experimental investigation, the near critical pressure CHF test data of water are obtained. We find that the CHF mainly occurs when the vapor qualities are less than 0.4, and it occurs earlier (at lower vapor quality) when the pressure is closer to 22.115 MPa or the mass flux is smaller. From the experimental data, a correlation function for the CHF is established via regression and machine learning. Correlations established via machine learning greatly improved the regression accuracy. To study the CHF phenomenon mechanically, a theoretical model is established based on the near-surface bubble crowding model describing the DNB-type CHF. In the development of the CHF model, the friction resistance coefficient is determined according to our test results. By comparison with different experimental results, the near-surface bubble crowding model is well suited to describe DNB-type CHF. The calculation results of the model can provide reference for the optimal design of the USCFB boiler.

The evaporating section of the pulsating heat pipe (PHP) is in direct contact with the electronics when it is used for heat dissipation, and thus the evaporating temperature uniformity has an important effect on the safe and reliable operation of electronic equipment. On the basis of these conditions, an experimental study on the evaporating temperature uniformity of the PHP with surfactant solutions at different concentrations was conducted at the heat fluxes of (1911–19 427) W/m². Sodium stearate was utilized for the solute; the surfactant solutions were prepared with the concentrations of 0.001 wt%, 0.002 wt%, and 0.004 wt%, respectively, and the filling ratios of the PHP were 0.31, 0.44 and 0.57, respectively. The experimental results revealed that under all tested working conditions, the highest temperature always appeared in the intermediate zone of the evaporating section. As the heat flux increased, the temperature differences among different zones rose initially and then reduced due to the change of the flow motion and the flow pattern. The evaporating temperature uniformity of the sodium stearate solutions-PHP was better than that of the deionized water-PHP, which suggested that the evaporating temperature uniformity might be improved through decreasing the surface tension. Furthermore, combined with the effect of surface tension and viscosity, for different filling ratios, the required concentration was different when the best evaporating temperature uniformity was achieved. To be specific, when the filling ratio were 0.31 and 0.44, the best evaporating temperature uniformity was achieved at the concentration of 0.004 wt%, while at the filling ratio of 0.57, the best evaporating temperature uniformity was attained at the concentration of 0.002 wt%.

Electric double layer capacitors (EDLCs) as promising electrical energy storage devices are faced with thermal management issues, which concern the performance and lifetime of the devices. Heat transfer at the solid-liquid interface has a crucial impact on the thermal management of EDLCs. In this work, the interfacial thermal resistance (Kapitza resistance) of the interface between ionic liquid (IL) and graphite electrode is determined, and the heat transfer resistance in the uncharged/charged system with different temperatures is investigated via molecular dynamics simulations. It is found that Kapitza resistance near the negative-charged interface decreases by 23% compared to that in the uncharged system, while the temperature effect on Kapitza resistance is little in our simulation. The unique ion layer structure of ILs formed at the interface may influence the thermal transport performance. Simulations are performed to investigate the effects of surface charge and working temperature on the heat transfer resistance of interfacial ILs from three aspects: ionic spacing, inter-ion interaction, and heat capacity. With the influence of surface charge, ionic spacing in the electric double layer is found to decrease while the inter-ion interaction and heat capacity increase, leading to the reduction in thermal resistance of interfacial ILs. However, rising temperature has small effects on the three thermal properties, with a slight tendency to increase the thermal resistance of ILs.

Since the resin-based composite materials are of essential importance in many key engineering fields, the manufacture processes are highly worth studying and optimizing for satisfying quality control at the highest possible production rate. In this paper, combined with the impregnation theory, the flow-thermal-mechanical multiphysics coupling model is built to characterize, investigate and optimize the osmotic flow process of hot-melt resin in fiber fabrics with the uniformity and adequacy of resin impregnation as the evaluation criteria. First, the osmotic flow process is characterized by the osmotic flow front of resin, which is tracked by the phase-field method. Then, the influencing factors of roller clearance, temperature and speed are comprehensively investigated. After that, the simulation data of resin impregnation degree are fitted by polynomial curves, with accuracy up to 96.13%, for further investigation of interaction between influencing factors. Finally, based on the above results, the operation parameter combination for impregnation process is optimized with the response surface method and provided as the guidance for practical application.

Investigations on entropy generation and thermal irreversibility analysis are conducted for liquid lead-bismuth eutectic (LBE) in an annular pipe. To find better performance in convective heat transfer, the computational fluid dynamics (CFD) code based on the finite volume method (FVM) is adopted to solve this problem. The elevated temperature LBE flows in the annular pipe, and four types of heat flux, including constant, linear increase and decrease, and parabolic distributions are imposed at the inside wall of the annular pipe. The investigations are conducted for the specific average heat input of 200 kW/m², and the different Peclet number Pe is set from 1200 to 3200. The SST k-ω turbulent model and Cheng-Tak P_rt model are adopted. The mesh independence validation and models verification are also conducted and the maximum Nu error is 5.43% compared with previous experimental correlations. The results from the local and system scales, respectively, including volumetric dimensionless entropy generation, Ns, Be, and E_p, are discussed. The results indicate that the viscous friction and heat transfer caused by entropy generation can be found in the viscous sub-layer and buffer layer respectively. Heat transfer is the primary factor that leads to irreversible losses. Besides, the results show that the best thermodynamic performance occurs under parabolic distributed heat flux in the research scope.

Jet impingement cooling with supercritical pressure carbon dioxide in a multi-layer cold plate during the heat flux of 400 W/cm² is investigated numerically. The generation and distribution of pseudocritical fluid with the high specific heat of supercritical pressure carbon dioxide and the mechanism of the heat transfer enhancement led by the high specific heat are analyzed. For a given nozzle diameter, the effects of the geometric parameters of a multi-layer cold plate such as the relative nozzle-to-plate distance, relative plate thickness, and relative upper fluid thickness on the average heat transfer coefficient are studied. The results show that the target surface is cooled effectively with supercritical pressure carbon dioxide jet impingement cooling. When the radial distance is less than 6 mm, the maximum wall temperature is 368 K, which is 30 K lower than the maximum junction temperature for a silicon-based insulated gate bipolar transistor, a typical electronic power device. There is a pseudocritical fluid layer near the target surface, where specific heat reaches above 34 kJ/(kg·K) locally. The drastic rise of the specific heat leads to obvious heat transfer enhancement. Within a certain range, the local heat transfer coefficient and the specific heat are linearly correlated and Stanton number remains constant over this range. The heat transfer coefficient is at a maximum when the relative nozzle-to-plate distance is 1. As the relative plate thickness increases from 0.5 to 3.5 or the relative upper fluid thickness increases from 0.5 to 2.5, the average heat transfer coefficient decreases monotonically.

Printed Circuit Heat Exchanger (PCHE) with high-efficiency and compact structure has great application prospect in the supercritical carbon dioxide (S-CO₂) power systems for the next generation of high-temperature concentrated solar and advanced nuclear energy. However, the high operating temperature and pressure require PCHE to maintain good heat transfer performance, as well as reliable mechanical performance at the same time. It is necessary to carry out the fluid-thermal-mechanical coupled analysis of PCHE for the safe and efficient operation of the S-CO₂ cycle. In this paper, a three-dimensional fluid-structure coupled numerical model was established to study the fluid-thermal-mechanical coupled characteristics of PCHE under different airfoil fin arrangements. The stress distribution of the single airfoil fin was studied, and a better airfoil arrangement that comprehensively considers heat transfer characteristics and stress distribution was obtained. Aiming at the high stress caused by the stress concentration at both ends of the airfoil fin, an optimized configuration combining straight channel and airfoil channel was proposed. The results show that the difference between the flow and heat transfer performance of the two optimized structures and the reference structure is only within 1.5%, but the maximum stresses of the two optimized structures are respectively reduced by 69.4% and 70.0% compared with that of the reference structure, which significantly reduces the stress intensity of PCHE. The result provides a new method to develop the airfoil PCHE with uniform stress distribution and good thermo-hydraulic performance.

Printed circuit heat exchangers (PCHEs) have great potential to be employed in the advanced nuclear reactor systems. In this work, the equivalent thermal conduction resistance of PCHE is studied. The influences of thermal convection resistance are analyzed. The results indicate that the equivalent thermal conduction resistance of PCHEs with unequal numbers of hot plates and cold plates are sensitive to the thermal convection resistance of hot side and cold side. Specifically, for case C which has unequal number of hot and cold channels, the maximum value of equivalent thermal conduction resistance can be 1.7–2.4 times the minimum value. The equivalent thermal conduction resistance is underestimated under the isothermal boundary. In addition, the non-uniformity of the lengths of all the heat flux lines determines the influence degree of thermal convection resistance on the equivalent thermal conduction resistance. For further investigation, Latin hypercube sampling method is adopted to generate a large number of design points for each PCHE configuration. Based on the sample data, mathematical correlations and artificial neural network (ANN) for prediction of equivalent thermal conduction resistance for each case are developed. The proposed correlations of equivalent thermal conduction resistance for each case have acceptable accuracy of prediction with a wide range covering general engineering applications. The ANN model can achieve much better prediction accuracy than the proposed correlations thus it is recommended in the cases that the prediction accuracy is considered as the priority need.

In the present study, pool boiling heat transfer performance and bubble behaviors of hybrid structures with metal foam and square column are investigated by lattice Boltzmann method. By using the vapor-liquid phase change model of Gong-Cheng and Peng-Robinson equation of state, the effects of structural parameters, including metal foam thickness, porosity, column height and ratio of column width (W) to gap spacing (D) are investigated in details. The results show that hybrid structure performs better than pure columnar structure in pool boiling heat transfer. The hybrid structure accelerates bubble growth by fluid disturbance while metal skeletons prevent the bubble escaping. The optimum ratio of column width to gap spacing decreases with the increase of heat flux and HTC (heat transfer coefficient) can achieve an increase up to 25% when W/D change from 5/3 to 1/3. The increase of column height enhances heat transfer by expanding surface area and providing space for bubble motion. The metal foam thickness and porosity have a little influence on pool boiling heat transfer performance, but they have an important effect on bubble motion in the regime.

The Finite-Element Method (FEM) is mainly used to design Compact Heat Exchanger structures. However, the narrow channel structures require fine mesh to accurately compute stress and strain. Combined with the dimensions of the overall component, this leads to an excessively large numerical model and therefore long computing time. This is especially true for transient thermal analyses, which require taking into account the full geometry. It is therefore interesting to reduce the size of the mesh. Periodic homogenization is an efficient method for achieving this goal. It can be applied to the core of the structure when periodic patterns (identified on basis of the arrangement of the channels) exist. A method based on this technique, with some improvements, is proposed in this paper for thermal loading without internal pressure. In addition to the Equivalent Homogenous Medium (EHM) replacing the periodic patterns, some explicit channels are interposed between the EHM and the cover plates. This has two advantages. The first is to smooth the transition of stiffness between the cover plates and the homogenized medium. The second one is to be able to directly compute stress and strain on the most critical channels located in this area. This paper assesses the method’s effectiveness for thermal loadings in order to conduct thermal stress analyses. First, the equivalent elastic constants of the EHM are obtained with a numerical Finite Elements Method. Then, two 2D cases using EHM are compared against a 2D explicit model (i.e. explicit geometry for the core channels). These cases are chosen to validate the EHM itself and its effectiveness for a real section of the heat exchanger. Results show very good agreement with a relative difference lower than 1%. In addition, the sensitivity to the number of layers added between the EHM and the cover plates is analysed. It is recommended interposing at least two layers of patterns to obtain converged results for the considered configuration. Finally, a triangular mesh is considered to reduce the size of the model. No difference with a regular quadrangular mesh can be observed whereas the computing time is reduced. This method can be used to perform the design of any CHE under thermal loading as long as the channel arrangement shows periodicity.

Air-Gap Diffusion Distillation (AGDD) is a new technology aiming at solving the problem of the safety of drinking water for residents in remote areas that uses a super hydrophilic porous medium as the hot channel and evaporation surface. In the experiment, it was found that the parameters of porous media have a significant influence on the desalination (evaporation) efficiency of AGDD. Although porous media are widely used as evaporation components, the factors affecting their evaporation efficiency are not fully understood. The evaporation process in super hydrophilic porous media is rarely discussed. A large number of experiments have been carried out based on AGDD. The introduction of statistical methods solves the problem that experiments cannot distinguish the contribution of complex parameters of porous media to evaporation efficiency. Stepwise regression analysis is used to reduce the dimensionality of the independent variables and construct regression equations (coefficient of determination R² reached 81.3%–96.8%). Evaporation flux correlations and dimensionless mass transfer correlations are established based on porous media parameters. We found that the surface evaporation of super hydrophilic porous media can be divided into three stages: diffusion evaporation, capillary evaporation, and thermal evaporation. The evaporation efficiency of these three stages is controlled by the vapor diffusion process resistance, capillary force, and energy supply. At low saturation, evaporation efficiency is limited by the resistance of the vapor diffusion process. The evaporation efficiency of the porous media is affected predominantly by the pore size, the specific surface area, porosity and the characteristic length. At high saturation, the evaporation efficiency becomes influenced primarily by the permeability. A small thickness and a high hydrophilicity also improve the evaporation efficiency.

Ultra-thin flattened heat pipe (UTHP) is an effective solution to solve the problem of high-power density heat dissipation in narrow space. The key factors that determine its thermal performance include: the shapes and sizes of the UTHP, the wick structure, the type of working fluid and its filling ratio. The change in the filling ratio means not only a change in the amount of the working fluid, but also a change in the space distribution of the gas and liquid phases inside the heat pipe. Therefore, it is important to explore the effect of liquid filling ratio on the thermal performance of UTHP. It can provide effective guidance for the production of UTHP. In this work, experiments were conducted on four groups of UTHPs with different mesh wicks under a series of liquid filling ratios. The results demonstrate that the volume of the filling working fluid should account for 22%–37% of the total internal volume of the UTHP to avoid deterioration of heat transfer during the operation of the UTHP. In addition, a prediction model of the evaporator temperature has been established to provide guidance for the application of UTHPs.

This paper discusses an improved thermal management system to ameliorate the performance of lithium-ion battery storage systems for electric vehicles (EVs) applications. A compact and lightweight cold plate is designed and fabricated to fit 18650-type lithium-ion batteries, using aluminum-finned copper tubes. A dynamic temperature PID (proportional, integral, differential) control algorithm for electronic expansion valves is developed to study using EV air conditioning refrigerant, R134a, to control battery modules’ temperature with this compact and lightweight thermal management system. The experimental results show that the proposed battery thermal management system can effectively control the battery module’s temperature. In addition, during 1C discharge, when the PID temperature algorithm control scheme is used, the maximum temperature difference across the battery module peaks at less than 4°C, and the maximum temperature within the battery module is less than 36°C.

The aim of the present work is to evaluate proton exchange membrane (PEM) fuel cell performance with a modified serpentine flow field with right angle turn by numerical modeling. A 3-D PEM fuel cell model of size 50 cm2 active area is developed. A conventional serpentine flow field is modified and the same is considered for the supply of reactants. Computational fluid dynamics (CFD) based simulations were conducted to analyse the pressure drop, distribution of reactants (H2 and O2), liquid water activity, current flux density and water content in the membrane. From the simulation results, polarization curve is drawn to validate the literature data of PEMFC with the conventional serpentine flow field. Comparison of simulated polarization curve with literature data revealed that modified serpentine flow field performance is better than conventional serpentine flow field as it offers better water exclusion and uniform sharing of reactants. From this study, it is concluded that model of flow field pattern influences the functioning of fuel cell and utmost care must take while selecting a pattern for flow field of PEM fuel cell.

The combined inclined rib pair (CIRP) is the first time proposed to improve the sensor performance of particle velocity sensor (PVS) by using a three-dimensional numerical method. The method is verified by the experiment results in the literature. The optimal plain channel parameters are determined as the basic sensor structure. In comparison of plain channel, both heat transfer characteristics and sensor performance are enhanced effectively by arranging the CIRP. The reason is that the high flow rate region caused by the CIRP can maintain strongly in the whole fluid field if there are enough rib pairs. Furthermore, the produced longitudinal vortex pair can get a better fluid mix, which is more conductive to heat transfer. The increasing height and number of the CIRP can improve the heat transfer characteristics, but the flow resistance will increase as well. For the purpose of finding the best overall performance, the effects of the parameters including the geometric sizes and the position of the CIRP have been investigated. The results show that PVS will get the best sensitivity when the rib length and width are 0.2 mm and 0.03 mm respectively, and the distance between rib pair and between ribs in the same pair are 0.15 mm and 0.3 mm respectively. Besides, the most suitable crossing angle is 45°. Thus, the performance of PVS can be significantly improved by this novel structure.

The aim of this current work is to investigate the effects of operating parameters on heat transfer characteristics between gas and in-cylinder solid surfaces in a reciprocating compressor. A numerical model has been implemented in MATLAB based on mass balance, the first law of thermodynamics, and NIST REFPROP database for gas properties and thermodynamic relationships. The impacts of key parameters such as rotational speed, suction temperature, pressure ratio and suction pressure are particularly evaluated on heat transfer coefficient (HTC), heat flux (HF) and gas temperature. Results demonstrate that HTC increases markedly with the increase of rotational speed, pressure ratio and suction pressure, and decreases slightly with the increase of suction temperature; the instantaneous HF strengthens significantly with the increase of rotational speed, pressure ratio and suction pressure, whereas the mean HF magnitude transferred from gas to the wall decreases with the increase of rotational speed; the maximum gas temperature is more sensitive to suction temperature and pressure ratio.

Battery thermal management is very crucial for the safe and long-term operation of electric vehicles or hybrid electric vehicles. In this study, numerical simulation method is adopted to simulate the temperature field of Li-ion battery cell and module. It is proved that the maximum temperature and maximum temperature difference of battery cell and module increase with the increase of charge/discharge rate (C-rate) of the battery. For battery module, it can reach a maximum temperature of 61.1°C at a C-rate of 2 under natural convection condition with the ambient temperature of 20.0°C. A battery thermal management system based on micro heat pipe array (BTMS-MHPA) is deeply investigated. Experiments are conducted to compare the cooling effect on the battery module with different cooling methods, which include natural cooling, only MHPA, MHPA with fan. The maximum temperature of battery module which is cooled by MHPA with a fan is 43.4°C at a C-rate of 2, which is lower than that in the condition of natural cooling. Meanwhile, the maximum temperature difference was also greatly reduced by the application of MHPA cooling. The experimental results confirm that the feasibility and superiority of the BTMS-MHPA for guaranteeing the working temperature range and temperature uniformity of the battery.

Given the complexity of the thermo-hydro-chemically coupled phase transition process of hydrates, real-time in-situ observations are required. Thermometry maps are particularly essential in analyzing the heat transfer process during the growth and dissociation of crystal hydrates. In this study, we present the temporally and spatially resolved thermometry of the formation of tetrahydrofuran hydrates based on the temperature dependence of the chemical shift of the water proton. Images of temperature changes were synchronously obtained using a 9.4 T 1H magnetic resonance imaging (MRI) system to predict the saturation level of the aqueous solution, phases of the solid hydrates, and the positive temperature anomaly of the exothermic reaction. It was observed that variations in the MRI signal decreased while the temperature rise differed significantly in space and time. The results predicted in this study could have significant implications in optimizing the phase transition process of gas hydrates.

The Active Antenna Unit (AAU) on the outdoor tower is the key equipment to support the mobile communication of 5G. To suppress the overheating of AAU in summer, effective cooling measures are essential. In the present study, a numerical model of an AAU device with two chips in the outdoor environment was established to explore the surface temperature distribution under a coupling heat transfer process with natural convection and solar radiation was obtained. Moreover, the effects of the fin number, the fin height and the heat flux were discussed on the cooling performance. The results proved that the fins with a number of 12 presented the best cooling performance in this paper. By contrast, increasing the height of fins was still an effective way to improve the cooling performance of fins in outdoor conditions and to resist the thermal shock of chips. Besides, punching through holes on the fins, adding graphite heat spreader and reducing surface emissivity are effective ways to improve the cooling performance. After the optimizations, the maximum temperature decreased by 3.5°C in total. In other words, the contribution of these optimizations to the cooling performance was equivalent to an increase of fin height in 9 mm.

Printed circuit heat exchangers (PCHEs) are considered as the most promising heat exchangers for use of the supercritical carbon dioxide (S-CO₂) Brayton cycle. As crucial components operating at high pressure and thermal load at the same time, PCHE structural integrity evaluations are essential. In this study, to assess the structural strength of PCHEs serving as recuperators and precoolers in the S-CO₂ Brayton cycle as a waste heat recovery system for marine engines, the finite element method (FEM) is used and compared with a currently used method from ASME codes. The effects of temperature and pressure on the hot and cold sides are studied in terms of the temperature and pressure differences between the two sides and the main factors affecting its strength discussed. Then, detailed stress intensities of a PCHE under design conditions are investigated, and the results indicate that the highest stress appears at the middle of the semicircular arc of the channel, except for a concentration near the channel tip regions. Stresses of the PCHE are mainly caused by both pressure and temperature differences, with the minimum effect from temperature. The synthesis of the temperature and pressure fields exhibits a complicated action on the total stress under the design conditions. FEM was a more comprehensive means for structural assessment than the method from ASME codes. Further structural optimization of PCHE is conducted to ensure a maximum life span. This research work can provide theoretical guidance for structural integrity assessment of PCHE for the S-CO₂ Brayton cycle.

U-shaped channel is a common unit structure in heat exchanger. The pressure loss and heat transfer characteristics of U-shaped channel have important influence on the comprehensive performance of heat exchanger. In this paper, combined with the thermo-physical property of supercritical CO₂ (SCO₂), the influence of the guide vanes on the pressure loss and heat transfer characteristics of SCO₂ in U-shaped channel was studied numerically. The four different guide vanes included integral guide vane, two-stage symmetric guide vane, two-stage asymmetric guide vane and discrete guide vane. CO₂ Real Gas Property file (RGP) was imported into ANSYS CFX commercial software for calculation, and the turbulence model was SST k-ω. At the same time, the influence of different inlet Reynolds numbers and heating conditions of channel wall were also studied.

In this paper, a one-dimensional thermodynamic model was developed to evaluate the device-level performance of thermoelectric cooler (TEC) with the Thomson effect, contact resistance, gap heat leakage, heat sink, and heat load taken into account. The model was generalized and simplified by introducing dimensionless parameters. Experimental measurements showed good agreement with analytical results. The parametric analysis indicated that the influence of the Thomson effect on cooling capacity continued to expand with increasing current, while the effect on COP hardly changed with current. Low thermal contact resistance was beneficial to obtain lower hot-junction temperature, which can even reduce 2 K compared with the electrical contact resistance in the case study. The gap heat leakage was a negative factor affecting the cooling performance. When the thermal resistance of the heat sink was small, the negative effect of heat leakage on performance would be further enlarged. The enhancement of heat load temperature would increase the cooling power of the TEC. For example, an increase of 5 K in heat load can increase the cooling capacity by about 4%. However, once the current exceeded the optimum value, the raising effect on the cooling power would be weakened. The research can provide an analytical approach for the designer to perform trade studies to optimize the TEC system.

Based on the actual operation parameters and temperature-dependent material properties of a gas turbine unit, composite cooling blade model and corresponding reliable boundary conditions were established. Transient thermal-fluid-solid coupling simulations were then comprehensively conducted to analyze the transient flow and the temperature field of the blade under startup, shutdown, and variable loads condition. Combined with the obtained transient temperature data, the non-linear finite element method was exploited to examine the effect of these transient operations on the turbine blade thermal stress characteristics. Results show that the temperature and pressure on the blade surface increase with the load level and vice versa. As the startup process progresses, the film cooling effectiveness and the heat convection of airflows inside the blade continuously grow; high-temperature areas on the pressure surface and along the trailing edge of the blade tip gradually disappear. Locally high-temperature zones with the maximum of 1280 K are generated at the air inlet and outlet of the blade platform and the leading edge of the blade tip. The high thermal stresses detected on the higher temperature side of the temperature gradient are commonly generated in places with large temperature gradients and significant geometry variations. For the startup/shutdown process, the rate of increase/decrease of the thermal stress is positively correlated with the load variation rate. A slight variation rate of the load (1.52%/min) can lead to an apparent alteration (41%) to the thermal stress. In operations under action of the variable load, although thermal stress is less sensitive to the load variation, the rising or falling rate of the exerted load still needs to be carefully controlled due to the highly leveled thermal stresses.

Loop heat pipes (LHPs) are attractive two-phase thermal control devices for satellites, electronics and many other applications. They are capable of transporting heat efficiently for long distances up to several meters at any orientation. This paper investigated the heat transfer characteristics of loop heat pipes with long distances and small diameter transport lines. Small stainless steel tubes of 2 mm and 3 mm in inner diameters were chosen as liquid lines and vapor lines of the LHPs. The local thermal resistances in the evaporator of the 6 m-LHP were researched and analyzed, which indicated that the thermal resistance between the aluminum block and the vapor in the vapor channel accounted for a major proportion of the total thermal resistance. The effect of heat sink temperatures on the performance of the 6 m-LHP were compared with 10°C, 15°C, 20°C and 25°C cooling water temperatures. Moreover, the thermal characteristics of LHPs with transport distances of 2 m and 16 m were also experimentally investigated. The 16 m-LHP could achieve a heat transfer capacity of 100 W and the 2 m-LHP could reach more than 339 W, on the premise of the evaporator temperature below 100°C. The thermal resistance of the 2 m-LHP could achieve 0.125°C/W.