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Numerical modeling based on volume of fluid (VOF) method is conducted and experimentally validated for the two-phase flow and heat transfer in a loop thermosyphon for wide filling ratios. The results show that, under the 87% filling ratio, the loop is dominated by bubbly flow and sub-cooled flow boiling, with sensible heat transfer enhanced by bubble pumping effect, which is beneficial for wobbling, tilting, and mobile applications to avoid evaporator dry-out. Under the 64% filling ratio, geyser boiling instability occurs. A train of bubble-liquid intermittently passes the loop summit under the collaboration of buoyancy, gravity, viscous, and surface tension forces, causing intense flow and temperature oscillations, enhancing both latent and sensible heat transfer. Under the 38% filling ratio, the characteristic flow patterns are churn flow, mist flow, and film condensing, leading to highly efficient latent heat transfer. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, influence of void ratio on thermal performance of heat pipe receiver under microgravity is numerically simulated. Accordingly, mathematical model is set up. Numerical method is offered. The temperature field of Phase Change Material (PCM) canister is shown. Numerical results are compared with numerical ones of National Aeronautics and Space Administration (NASA). Numerical results show that the temperature gradient increases significantly and the utility ratio of PCM decreases obviously as void ratio increases. Void cavity influences the process of phase change greatly. PCM melts slowly during sunlight periods and freezes slowly during eclipse periods as void ratio increases. The thermal resistance of void cavity is much bigger than that of PCM canister wall. Void cavity prevents the heat transfer between PCM zone and canister wall. The temperature gradient of PCM zone is very significant with the effect of void cavity. So the thermal stress of heat pipe receiver may increase, and the lifetime may decrease as void ratio increases. [Copyright &y& Elsevier]

Abstract: In this paper, a natural convection micro cooling system with a capillary microgroove evaporator is proposed. An experimental study on the characteristics of thermal resistance, pressure drop and heat transfer of the cooling system is carried out. Experimental results indicate that the liquid fill ratio has a significant influence on thermal resistance and heat transfer in the cooling system. Increasing system''s cooling capacity at higher input power depends on decreasing the thermal resistance between the outer surfaces of the condenser and ambient environment. Compared with a flat miniature heat pipe (FMHP) and a current fan-cooled radiator for CPU chip of Pentium IV, the present micro cooling system has a stronger heat dissipation capacity. Its best cooling performance at a surface temperature of heat source below 373 K reaches and the maximum heat transportation capacity is 131.8 W. The novel kind of cooling system is suitable for remote cooling of those electronic parts with micro size, high power and thermal sensitivity. [Copyright &y& Elsevier]

Abstract By performing molecular dynamics, the thermal transport properties of seven type graphyne (GY) with different structures (nanosheet, nanotube and GY/graphene (GR) heterojunction) are investigated. It can be observed comparing with other six GY, the thermal conductivity (TC) of (14,14,14)-GY shows unexpectedly significant anisotropy, which is consistent with previous study in Phys. Chem. Chem. Phys. 2016, 18, 24210. This anisotropy is mainly due to the lower contribution of high-frequency and short-wavelength phonons weakened by armchair (14,14,14)-GY. Moreover, when GY is rolled up to GY nanotubes (NTs), it is interesting to discover TC of GYNT, especially (14,14,14)-GYNT, shows a diameter-dependent anisotropy. For large-diameter NT, the TC of zigzag model is higher, however, an opposite phenomenon is obtained with small-diameter model. This phenomenon can be explained by the relative alternation of phonon group velocity and phonon lifetime and the appearance of band gap with the change of diameter. At last, we also notice the GY/GR heterojunctions express much lower thermal resistance (TR) comparing with other carbon based heterojunctions, and the in-plane TR of GY/GR heterojunction is two orders of magnitude lower than that of out-of-plane TR. Further, the alternation of TR can be well explained by the coupling degree of different type phonons. Graphical abstract Comparing with other six type graphyne, the thermal conductivity of (14,14,14)-GY shows unexpected significant anisotropy. And the corresponding nanotube, i.e., (14,14,14)-GY NT, exhibits an interesting diameter-dependent anisotropic thermal conductivity. Image [ABSTRACT FROM AUTHOR]

Electrical and thermal management in nanodevices by means of carbon nanotube is highly promising. One main challenge toward CNT-based nanoscale electrical and thermal management devices is the development of effective strategies for reducing the bundle–bundle interface resistance. Here we report a novel strategy, based on the densification of CNT bundles and the functionalization of inter-bundle interfaces for effectively enhanced interfacial electrical and thermal transport. The densification is realized by utilizing the local electrostatic cohesion; and the functionalization is realized by the interface-decorated functional groups. Experiments and theoretical analysis demonstrated obviously enhanced interfacial electrical and thermal conductance originates from: (1) local Coulomb electrostatic cohesion between CNT bundles due to surface-induced dipole moments. This effect can promote both electrical and thermal conductance nearly 2.8 times higher than non-functionalized counterpart. (2) Increased interfacial electron transport channels and thermal vibrations due to surface-decorated functional groups. This effect can bring about up to 75% and 95% improvement for thermal and electrical conductance, respectively. This study provides a new methodology for tunable operation of electrical and thermal properties at inter-bundle interfaces and guidance for design of CNT-based electrical and thermal management devices. [ABSTRACT FROM AUTHOR]

To avoid the high flow frictional loss associated with conventional wire mesh Stirling regenerators, a micro-channel type stacked porous-sheets Stirling regenerator is investigated. An analytical solution is derived for the transient heat transfer characteristics of the fully developed reciprocating laminar flow under prescribed wall temperature profiles. The complex Nusselt number (Nu) is expressed as a function of kinetic Reynolds number (Re ω ) and Prandtl number (Pr). At low Re ω of less than 10, the real part of Nu has an almost constant value of 6.0, approximately equal to the known real-valued Nu for the fully developed unidirectional laminar flow under constant wall heat flux, while the imaginary part is negligible, thus “scaling effect” can be utilized to enhance heat transfer. At higher Re ω , both the real and imaginary parts of Nu increase with the increase of Re ω and Pr, and the phase shift between the temperature difference and the heat flux gradually increases and approaches 45°. Approximate analytical solutions are also deduced for the entrance region from the integral boundary layer equations in both cases of “Thermally developing flow” and “Simultaneously developing flow”. The heat transfer is enhanced in the entrance region and the local Nu in the flow direction approaches the corresponding values of fully developed flow. The analytical results are confirmed by dynamic mesh CFD results, and the obtained Nu∼ Re ω data and patterns generally agree with available analytical and experimental data from published literatures. Application of the analytical results to the design and optimization of Stirling regenerator are also shown. [ABSTRACT FROM AUTHOR]

Highlights: [•] A new type of solar collector with a large integrated heat pipe is designed. [•] An analysis of heat transfer inside new collector is presented. [•] A simulation of the heat transfer model is conducted. [•] The new collector can contribute to the application of more solar energy. [Copyright &y& Elsevier]

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

Thermal transport properties are a significant criterion of performance evaluation for various fascinating nanoscale materials. In this review, we summarize the recent research progresses from China in the field of nanoscale thermal transport properties. Both experimental advances and atomic-level simulation development are reviewed for typical categories of nanoscale materials and structures, i.e. , nanotubes and nanowires, nanofilms, nano-interfaces, nano-functional materials and those involving in near field radiation. Some fascinating aspects about the frontier issues in nanoscale heat transfer are also highlighted. In particular, researches have witnessed a remarkable growth in the interface-dominated microscopic thermal transport from both molecular dynamics simulation and experimental methods. In addition, challenges and opportunities will be touched on for the emerging field of near-field radiation. Which is dominated by simulation predictions but with encouraging experimental advances. [ABSTRACT FROM AUTHOR]

• Thermal characteristics of a loop thermosyphon under anti-gravity are studied. • The operation principle under anti-gravity is discussed. • The influence of inclination angles and cooling water temperature are investigated. In various fields of thermal management and protection, such as vehicles, aircraft, and mobile electronic devices, it is crucial to transfer heat effectively from the top to the bottom. The use of a simple structure for this transfer can reduce the weight of the heat transfer system. Theoretical studies have demonstrated that the two-phase thermosyphon loop (TPTL) with a high filling ratio has the potential for excellent performance in anti-gravity applications. In this study, we investigate the thermal performance of a high filling ratio TPTL under anti-gravity conditions. Our results reveal that, contrary to previous understanding, a TPTL with a high filling ratio can effectively transfer heat in anti-gravity. However, we also found that the loop thermal resistance of TPTL under anti-gravity is greater than under gravity, and the heating surface temperature is higher. Additionally, we experimentally study the effect of inclination angles on anti-gravity performance, and our results indicate that smaller angles result in better heat transfer. Finally, we investigate the impact of cooling water temperature on thermal performance. Overall, considering the energy efficiency of the heat dissipation system and the heat transfer capacity of the device, it is not recommended to use cooling water at a lower temperature. This research sheds light on the potential for TPTLs to operate in anti-gravity conditions and supports their practical applications. [ABSTRACT FROM AUTHOR]

• Nano-groove structures were conducive to evaporation at low superheat. The structure hindered the heat convection of the liquid atoms near the structure wall, thus heat accumulated in the central region on the heat source, causing the liquid in this region to evaporate faster, which will ultimately contribute to nucleation. • The evaporation rate of structured surface was larger than the smooth surface, which was due to the higher heat flux even though the areas of heating source were the same. The main reason for this phenomenon was owing to the mass transport induced by the structure. • A verification model for the mass transport was constructed. The velocity of liquid transport on the structured surface was faster than the smooth surface significantly. Bubble behaviors in nucleate boiling have been continuously concerned, traditional experiment believes that micro/nano structures can improve the bubble behaviors by increasing the capillary force to enhance the heat efficiency of nucleate boiling. However, due to the scale limitation of the macro experiment, the evolution of the solid-liquid-gas interface (three-phase contact line) during the bubble formation and expansion on the structured surface is still not clear. Therefore, a three-phase contact line evolution model with a groove structure was constructed in this work. Results showed that the heat transfer performance of the structured surface was better than the smooth surface even though the heating area was the same and the structure was adiabatic. The three-phase contact line appeared on the structured surface but did not appear on the smooth surface when the superheat was low, which was due to the structure hindered the heat convection inner the liquid and enhanced the heat accumulation in a smaller region, then enhanced the evaporation. It was also found that structures were conducive to the liquid transport from the unheated area to the heating area, which hindered the expansion of the three-phase contact line and improved the evaporation of the microlayer that surrounded the three-phase contact line. These phenomena and mechanisms will contribute to explaining why some micro/nano structures can improve the expansion bubbles and reduce the departure radius, and can provide a more effective theoretical basis for designing the surface with the nanostructure. [ABSTRACT FROM AUTHOR]

• Heat transfer mechanism of TPLT with different filling ratios are revealed. • The parameter of subcooling degree is introduced to Lee model. • The heat transfer mechanism is subcooled flow boiling at high filling ratio. • The heat transfer mechanism is saturated pool boiling at low filling ratio. • For high filling ratios, the bubble size decreases with heat input increases. As an efficient heat transfer device, two-phase loop thermosyphon (TPLT) has a broad application prospect in many engineering fields, and its internal gas-liquid phase change and flow characteristics are the key factors affecting the heat transfer performance. In order to get a better thermal design and performance, it is necessary to clarify the heat transfer mechanism of the TPLT, especially under the condition of high filling ratio. In this study, the visualization experiment was combined with the CFD method to study the heat transfer and flow characteristics of TPLT. Due to the strong coupling relationship among the boiling process and the condensate process, the Lee model is optimized, and the relationship between saturation temperature and local pressure, and the parameter of subcooling degree in the Lee model were both introduced to accurately simulate the phase change process. Results shows that the optimized model has good prediction performance for the heat and mass transfer simulation of the loop thermosyphon. The two-phase flow in the loop thermosyphon changes from slug flow to churn flow with the increase of heating power at low filling ratio (50%), while the bubble size decreases and the number of bubbles increases with the increase of heating power (20W-80W) at high filling ratio (70%). The heat transfer mechanism before and after model optimization has changed, from saturated flow boiling to subcooled flow boiling at high filling ratios. At low filling ratio (50%), the fluid in heating section is saturated state, and the flow velocity at the bottom of the loop is low (0.018m/s). At high filling ratio (70%), the fluid in the heating section is supercooled state, and the fluid flow velocity at the bottom of the loop is high (0.171m/s). This indicates that the heat transfer mechanism of the loop thermosyphon at low filling ratio was saturated pool boiling, and the filmwise condensation occurs in the condensation section. However, the heat transfer mechanism is subcooled flow boiling at high filling ratio, and the gas-liquid two phases in the condensation section are in a mixed state. [ABSTRACT FROM AUTHOR]

Loop heat pipe (LHP), which is the star-rated way to solve high heat flux heat transfer, is a promising heat dissipation technology for electronic devices. The optimized design of porous wick in LHP has become the key to increasing the heat transfer capacity and reducing costs. In this study, a polymer material-polyethersulfone (PES) film was chosen as the porous wick. Theoretically, the PES wick has a special structure with dual pore size, which achieves a synergistic enhancement of capillary force and permeability. The experimental investigation compared the heat transfer capacity of LHP with the PES wick, copper mesh wick, and sintered nickel powder wick. The PES wick shows a most significant performance than the other two wicks. The heating surface temperature remains 62 °C in a 55% filling ratio when the LHP is under 300 W heat load. The LHP system and evaporator have respectively the thermal resistance of 0.161 °C/W and 0.056 °C/W. In addition, PES wick shows great potential for application in the LHP field with the advantages of light weight, simple preparation, and low cost. This paper sheds some light on the LHP wick choice to enhance heat transfer. • A simple, low-cost PES film wick for LHP was prepared and characterized. • Polyethersulfone porous wick has a unique dual pore size structure. • PES wick shows better performance than sintered nickel wick and copper mesh wick. • The weight of the PES wick is only 7.9% as compared to the weight of the sintered nickel wick. [ABSTRACT FROM AUTHOR]

[Display omitted] • An ACM was developed for the water gas shift reaction in a high-temperature shift reactor. • The simulation results of the ACM were compared with those obtained by the PCM and PRCFD. • The ACM exhibited equivalent accuracy as the PRCFD but comparative efficiency as the PCM. • The ACM is suitable for the simulation of high-temperature shift with internal mass transfer. The packed bed reactor with the high-temperature shift reaction has been a competitive industrial device for the production of hydrogen fuel. While the temperature distribution in the packed bed plays an important role in improving the reaction efficiency, and numerical simulation is a powerful way to recover the heat transfer inside the reactor. Nevertheless, the current simulation methods are either high computational cost or low accuracy for the high-temperature shift. To accomplish the effective computational cost and accuracy simultaneously, we developed a two-dimensional anisotropic continuum model with modified space-dependent effectiveness factor and anisotropic thermal conduction based on the pseudo continuum model. Meanwhile, the simulation results were compared with those by the particle-resolved 3D computational fluid dynamics to validate the model accuracy. Due to the overestimation of the radial thermal conductivity by the pseudo continuum model, the axial temperature rise in the packed bed was only 10 K, but a significant temperature rise of 50 K was presented by the other two methods, suggesting the coordinate computational accuracy of the anisotropic continuum model. However, the computational time consumed by the particle-resolved 3D computational fluid dynamics was up to 1320 min, which exceeds a thousand times more than that in current advanced model. All the comparisons indicate the anisotropic continuum model can predict the heat transfer through the packed bed with sufficient computational accuracy and efficiency, making a powerful and time-saving method to conduct the effective heat management in packed bed reactors for actual industrial applications. [ABSTRACT FROM AUTHOR]

The Kelvin cells are normally applied to represent the open-cell metal foams for simulation due to the similar pore structure. However, the experimental comparison between Kelvin cells and open-cell metal foams is rarely reported. This study compares the forced convective heat transfer properties of four Ni70Cr30 metal foams and one GH4169 Kevin cells manufactured from additive manufacturing. The results indicate that the pressure drop and overall heat transfer coefficient (HTC) increase with the increase of PPI, while decreasing with the increase of porosity. Moreover, the comparisons between present data and data from the literature prove the reasonability of the experiments. The pressure drop and overall HTC of Kelvin cells are 64.4% and 12.3% higher than open-cell metal foams, while showing 26.2% lower overall heat transfer performance at 6 m/s. The pore structure, thicker skeleton, higher porosity, and surface roughness of Kelvin cells contribute to the higher pressure drop than open-cell metal foams. The comparison among experimental correlations and present data indicates that the correlations for predicting the behavior of open-cell metal foams could not be used directly to predict Kelvin cells. [ABSTRACT FROM AUTHOR]

• It was found that the position of the nucleate site varies with the spacing of structures. Then the evolutions of the three-phase contact line were also different between the cases. The mechanism of this phenomenon was analyzed by kinetic-potential energy theory, which was mainly related to the spacing of structures. • It was found that the relationship between liquid-solid contact area and the heat transfer enhancement of bubble expansion was not monotonically increasing. The evaporation of the microlayer around the three-phase contact line was more critical to CHF, which was not necessarily related to the liquid-solid contact area. • Liquid retention induced by the various height of structures also had significant effects on heat transfer of bubble expansion. The increased rate of CHF with the increase of liquid retention gradually slowed down, which was due to the limited effect of larger liquid retention on the liquid at bottom of structures. As the main factor limiting the performance of microprocessors, how to improve its heat dissipation capacity is critical. Nucleate boiling on the structured surface is a method that can improve heat transfer significantly due to two-phase flow, and bubble behaviors have been attracted much attention because of its capability of enhancement on critical heat flux(CHF). Although a large number of studies have proved that the structured surface can improve CHF by increasing the number of nucleation sites, bubble departure frequency and reducing bubble departure radius, the mechanism of solid-liquid-gas interface evolution on the structured surface is still unclear, which has significant effects on nucleate boiling. Therefore, a model of nucleate boiling of liquid on the structured surface was constructed, the processes of nucleate generated and bubble growth were observed directly by using molecular dynamics simulation. A novel phenomenon was observed in which the position and number of interfaces are mainly dependent on the restriction of potential energy generated by structures. This phenomenon affects CHF significantly due to the area of the microlayer beneath the bubble was different. In addition, the effects of liquid retention on interface and CHF were also explored, the result showed that CHF increased with the increase of liquid retention, and the increase rate of CHF was slowed down due to the more retained liquid in the structures. We believe that this research can improve the existent theory of nucleate boiling and provide a more effective theoretical basis for the design, processing and application of surface with the nanostructure. [ABSTRACT FROM AUTHOR]

A theoretical model for axial capillary flow in rectangular microgrooves was presented with the assumptions that the meniscus radius in the cross section is approximately constant, and the height of the centerline film recedes along the axial direction with a convex form. Experimental data obtained by Micro-PIV verify the assumptions. Comparison with the experimental data shows that the model is accurate to predict the axial profile of the liquid film for inclined rectangular microgrooves. [ABSTRACT FROM AUTHOR]

The mini-diameter loop thermosyphon, which has an excellent heat transfer performance with simple, compact structure and pump-free advantages, can effectively solve the problem of heat transfer under high heat load in a confined space. The mini-diameter loop thermosyphon exhibits different gas-liquid transport and heat transfer patterns than the conventional-diameter one due to various gas-liquid phase transformation properties and a strong friction force in the mini-diameter channel. Furthermore, data on the visual analysis of gas-liquid transport and heat transfer properties of the mini-diameter loop thermosyphon under various filling ratios is still lacking. In this paper, a visible experimental research on a mini-diameter loop thermosyphon (3 mm) in a wide filling ratio range(44% -97%) is undertaken. The maximal heat transfer capacity increases initially and subsequently falls with the filling ratio, according to the testing data. At a low filling ratio, the flow pattern changes as slug flow - churn flow - annular flow with the heat load increase, leading to a low loop thermal resistance, and dry out occurs at 185 W heat load under 50% filling ratio. The gas-liquid flow shifts to slug flow-geyser boiling-bubble flow at high filling ratios (73% and 82%), and the bubble departure diameter steadily decreases with increasing heat load. Consequently, the heat transfer performance (thermal resistance) has been enhanced, and the maximum heat transfer capacity has been increased to 280 W. Single-phase flow is plainly seen in the loop when the filling ratio reaches 91% or even 97%, thus reduced heat transfer capacity. Therefore, the high-filling-ratio mini-diameter loop thermosyphon is better for high-load heat dissipation. However, a low thermal resistance shows at the low filling ratio, but the heat load is also low. As a result, a loop thermosyphon with a low filling ratio should be selected for the low heat load situation. [ABSTRACT FROM AUTHOR]

Highlights: [•] The influences of configurations of heat transport pipeline are studied. [•] At all operating conditions the droplet flow is observed. [•] The effects of geometrical parameters on the robustness of system are considered. [•] Heat transfer for condensation is enhanced with increasing inner diameter. [•] The lowest evaporator temperature is achieved with the longest tube length. [ABSTRACT FROM AUTHOR]

Abstract: The thermal boundary conductance between multilayer structures including Al film, semiconductors with high Debye temperatures (GaN, AlN, Si, diamond) and dielectric substrates (sapphire) has been measured using a two-color femtosecond laser pump-probe system (a variation of transient time-domain thermoreflectance, TDTR). The thermal boundary conductance for the combinations of semiconductors and dielectrics falls within a relatively narrow range, 10–20 MW m−2 K−1, at room temperature. The measured thermal boundary conductance between Al film and semiconductor or dielectric substrates is one order of magnitude larger than that between semiconductor and dielectric substrates. A modified diffuse mismatch model (DMM) is used to interpret the data and extract the phonon transmissivity at the interface. The predicted results of the DMM corrected by attenuation constant agree well with the experimental values. Over a wide phonon velocity, both the measured and predicted results decrease with the increasing average phonon velocity. Both the vibration mismatch and changes in the localized phonon transport near the interface contribute to the reduction in thermal boundary conductance. Other scattering mechanisms are discussed which may explain the failure of the DMM at room temperature. [Copyright &y& Elsevier]

Abstract: A high performance model Stirling engine, in which the heater, regenerator and cooler as a whole is formed by hundreds of porous metal sheets, is identified for theoretical analysis to facilitate the future scale-up design. The reciprocating flow and heat transfer both in the heat exchanger and in the full engine is simulated by a dynamic mesh Computational Fluid Dynamics (CFD) method, and is validated by analytical solutions and experimental data. An optimization method is also developed to incorporate the entropy generation caused by flow friction and irreversible heat transfer. The results show that relatively high indicated power of 33.4 W is obtained, corresponding to a specific power of 1.88 W/cm3 and a thermal efficiency of 43.9%, which are attributable to the extremely small flow friction loss and excellent heat transfer characteristics in the regular shaped microchannels, as well as to the compact heat exchanger design that significantly reduces the dead volume. Given the same operating conditions, the optimized porous-sheets regenerator has a significantly lower total loss of available work while maintaining even higher thermal effectiveness in comparison with the optimized conventional wire mesh regenerator. [Copyright &y& Elsevier]

• Heat transfer mechanism of TPLT with high filling ratio is revealed. • Pool boiling is changed into flow boiling with filling ratio increases. • Heat transfer performance positively correlates with the circulation velocity. • For high filling ratios, thermal resistance decreases with heat input increases. The two-phase loop thermosyphon (TPLT) with a high filling ratio is considered an efficient heat transfer device and therefore has a good application prospect in the field of high heat flux dissipation engineering. However, the heat transfer mechanism of the TPLT with high filling ratios remains elusive. In this study, we establish a computational fluid dynamics (CFD) model of the loop thermosyphon. In order to accurately simulate boiling and condensation processes, we take into account the relationship between saturation temperature and pressure in the phase change model. There were two flow patterns, i.e., the bubbly flow and the slug flow, under low filling ratios in the heating section, in which the pool boiling dominates the main heat transfer mechanism, and dropwise condensation occurred in the condensation section. The driving force to propel the liquid during the vapor bubbles rise cannot be transferred to the liquid in the right tube, resulting in lower flow velocity. In stark contrast, the single-phase flow and the bubbly flow occurred periodically in the loop thermosyphon with high filling ratios, in which the dominant heat transfer mechanism is the flow boiling. The rising bubbles promote the clockwise flow of liquid in the loop like a pump. Because of the switch of the heat transfer mechanism, the average flow velocity increased, and the thermal resistance decreased. The minimum thermal resistance was 0.15 K/W under a filling ratio of 75%, and the maximum average velocity of the fluid was 0.19 m/s. Furthermore, we found a strong coupling relationship between the heat transfer performance and the flow characteristics of the loop thermosyphon. For low filling ratios, local dry-out occurred in the loop thermosyphon under a high input heat flux, leading to an increase in the thermal resistance. For high filling ratios, the average velocity of the fluid was augmented with the increase in heating power, while the liquid was capable of being replenished quickly in the heating section to avoid the dry-out. Therefore, the loop thermosyphon with high filling ratios enables a better heat transfer performance under a high heat flux than those with low filling ratios. [ABSTRACT FROM AUTHOR]

The optimization of pore structure for metal foam is considered a feasible approach for improving the overall heat transfer performance. Thus, we numerically investigated Kelvin cells with different throat areas and structures (elliptical Kelvin cell (EKC)) to characterize the influence on pressure drop and heat transfer coefficient using FLUENT 18.0. The standard k – ε model exhibited a better agreement with experimental data and required less time to achieve convergence. The results revealed that the throat area could not feasibly optimize the overall heat transfer performance. Moreover, the area goodness factor j / f that considered the influences of both heat transfer coefficient and pressure drop on the overall heat transfer performance of EKC with the higher than conventional Kelvin cell. Based on comparative analysis between pressure, velocity, turbulence kinetic energy, and temperature distribution, increasing the space and decreasing the angle between the skeleton and flow direction caused a significant pressure drop in the EKC samples. Owing to the existence of a lower temperature area at the leeward of skeletons and a small difference of impingement cooling on windward skeletons, the reduction of HTC was acceptable. Therefore, the EKC exhibited immense potential for enhancing the design of heat transfer devices. • Kelvin cells with varying long and short throat diagonal and axes ratios. • Pressure drop and heat transfer coefficient were investigated comprehensively. • Pressure, velocity, and turbulence kinetic energy distributions were compared. • Increasing the elliptical axes ratio can optimize the overall heat transfer. [ABSTRACT FROM AUTHOR]

Abstract: Thermal conductivities of [mmim]DMP/CH3OH with mole fraction of [mmim]DMP (x =0.00, 0.10, 0.20, 0.30, 0.50, 0.70, 0.85, 1.00) and thermal conductivities of [mmim]DMP/H2O with mole fraction of [mmim]DMP (x =0.025, 0.05, 0.10, 0.20, 0.30, 0.50, 0.70, 0.90, 1.00) at T =298.15K were measured by freestanding sensor-based 3ω technique. The zigzag temperature response curves of the sensor were observed and used to extract the specimen thermal conductivity. The total uncertainty in thermal conductivity measurement and the maximum fitting error were estimated to be within 7.7% and 8.0%, respectively. Intuitively, as increasing the mole fraction of [mmim]DMP, the thermal conductivities of both [mmim]DMP/CH3OH and [mmim]DMP/H2O vary exponentially. Through the comparison with the assuming solutions, the heat transfer performances of [mmim]DMP/CH3OH and [mmim]DMP/H2O solutions were evaluated. [Copyright &y& Elsevier]

To further enhance the melting rate of the metal foam composite phase change material (MFCPCM), we took partial and gradient optimizations on the pore densities of metal foams. The partially optimized models, including Partial-80-5-5 and Partial-5-5-80, were compared with the Uniform-5 model. Results show that the Partial-80-5-5 model has the most developed melting among the three models. It illustrates that enlarging the pore density in the top region is conducive to accelerating the whole melting process. Besides, through a further comparison of the Partial-80-5-5, Partial-40-5-5, and Partial-20-5-5 models, we concluded that the larger the pore-density in the top region is, the faster the melting is. Subsequently, the gradient optimizations, including Gradient-80-20-5 and Gradient-80-40-5 models, were experimented with and analyzed. It was obtained that the Gradient-80-40-5 model has the fastest melting rate among all models. The inhibition of the large pore density on natural convection at the top and middle regions causes a strong vortex at the bottom region, so the melting process is significantly reinforced. Through the optimizations on the metal foam's pore density, the energy storage rate can achieve a prominent enhancement. [Display omitted] • The metal foam composite PCM is visually studied. • The partial and gradient optimization on the pore density is conducted. • The inner heat transfer mechanism is clarified. • The enhancement of the energy storage performance is achieved. [ABSTRACT FROM AUTHOR]

• In this paper, several high temperature heat pipes with good heat transfer performance are designed and prepared. • The test-bed for frozen startup and heat transfer performance of HTHP is set up. • The experimental study on the adaptive heat transfer characteristics of HTHP are systematically carried out. • The key influencing factors of adaptive heat transfer characteristics are summarized. High temperature heat pipe (HTHP) employs alkali metal or its alloy as working fluid. Due to the large latent heat of its working fluid during phase change heat transfer, HTHP has the advantages of high temperature uniformity, high working temperature and high intensity heat transfer capacity, which has broad application prospects in the field of HTHP cooled nuclear reactors and the thermal protection of hypersonic vehicles. In order to simplify the structure of heat transfer system in nuclear reactors, so that it can safely and reliably realize passive heat transfer without additional pump power under various operating conditions, HTHP must have adaptive heat transfer capacity. In this paper, several HTHPs with excellent heat transfer performance are designed and prepared. The test-bed for frozen startup and heat transfer performance of HTHP is set up, and the electromagnetic induction high frequency heater is used as high temperature heat source. The thermal performance of HTHP with different heating power, heating position and the length of evaporation section (adaptive heat transfer characteristics) is experimentally studied, and the theoretical model is used to validate the experimental result. The experimental and theoretical results show that high vacuum degree is the prerequisite for excellent heat transfer performance of HTHP; high capillary driving force and suction force, reasonable filling ratio of working fluid are the key factors for HTHP to have adaptive heat transfer capacity, and the diameter of HTHP also has important influence on adaptive heat transfer capacity. [ABSTRACT FROM AUTHOR]

• Kelvin cell structure was numerically analyzed to evaluate heat transfer performance. • Optimized Kelvin cell skeletons improved heat transfer performance at high velocity. • Elliptical skeletons reduced pressure drop and turbulence at the throat. • Elliptical skeleton achieved uniform velocity distributions. The optimization of pore structure for metal foam is a feasible method for reducing the pressure drop and improving the overall heat transfer performance, especially at high velocity. In this regard, the optimized Kelvin cell with elliptical skeletons with a cross-section ratio of the long axis to the short axis (b/a) of 1.0, 1.4, and 2.0 are considered to evaluate the effects of b/a on the pressure drop and the heat transfer coefficient (HTC). The results indicate that both the pressure drop and HTC decrease with an increase in b/a. However, the pressure drop reduces more significantly. For instance, as b/a increases from 1.0 to 2.0, the pressure drop and the volumetric HTC at 90 m/s decrease by 98.5% and 6.3%, respectively. Therefore, the value of the volumetric area goodness factor (which considers both the effect of heat transfer coefficient and pressure drop on the overall heat transfer performance) j v / f of the sample with b/a = 2.0 is 86.7% higher than the sample with b/a = 1.0 at 90 m/s. As the velocity increases, the effect of b/a on the overall heat transfer performance increases. Compared with the cylindrical skeleton velocity distribution and pressure drop, the elliptical skeletons lead to a more uniform velocity distribution and reduce pressure drop significantly; nevertheless, the temperature distributions barely change. Moreover, the elliptical skeleton increases the specific surface area. Therefore, the j v / f of the elliptical skeleton significantly improves. This study provides a new direction for the design of novel metal foams with excellent overall heat transfer performance for heat transfer devices. [ABSTRACT FROM AUTHOR]

• Forced convective heat transfer characteristics of metal foam were numerically investigated. • 3D models of the metal foam were obtained using X-ray imaging. • Metal foam with higher porosity and PPI had better heat transfer performance. • The effect of closed pores on heat transfer performance was investigated. Previously, extensive efforts have been made in investigating heat transfer in open-cell metal foams in order to better understand and apply them in engineering applications. As a step in this direction, this study prepared five three-dimensional models of copper foam with various porosities (0.82, 0.87, and 0.90) and pores per inch (PPI) (10, 20, and 40) using X-ray computed tomography and several software applications to simulate forced convection heat transfer in a metal foam at a high velocity. The effects of porosity, PPI, and closed pores on the pressure drop and heat transfer properties were investigated. The results indicated that the pressure drop and heat transfer coefficient were more sensitive to porosity than PPI. However, the specific surface area of the metal foam significantly affected the volumetric heat transfer performance. Additionally, the area goodness factors (j/f) were used to evaluate the comprehensive heat transfer performance (CHTP) of each sample. The value of j/f decreased rapidly in the velocity range of 0–20 m/s but slowly in the range of 20–70 m/s, and sample 40–0.87 exhibited a better CHTP. Moreover, the closed pores in the metal foam had a significant effect on the pressure drop, heat transfer properties, and CHTP due to the obstructions posed by the closed pores. Our work helps provide a better understanding for the application of metal foam in heat transfer enhancement at high velocities. [ABSTRACT FROM AUTHOR]