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In this paper, with both void cavity and phase change considered, influence of void ratio on phase change in thermal storage canister of heat pipe receiver under microgravity is numerically simulated. Accordingly, physical and mathematical models are built. A solidification–melting model upon the enthalpy–porosity method is specially provided to deal with phase changes. The change of liquid fraction with respect to void ratio and the liquid fraction distribution of different void ratios in a thermal storage canister of a heat pipe receiver are shown. Numerical results are compared with experimental ones. Research results indicate that the void cavity prevents the process of phase change significantly. Phase-change material (PCM) melts slowly during sunlight periods and freezes slowly during eclipse periods as void ratio increases. The utility ratio of PCM during both sunlight periods and eclipse periods decreases obviously as the void ratio increases. The void cavity prevents the heat transfer between the PCM zone and canister wall. The void cavity blocks the processes of both melting and solidification during cycle orbital periods. [ABSTRACT FROM AUTHOR]

In this article, a unit heat pipe receiver in an advanced solar dynamic system is numerically simulated. Accordingly, a mathematical model is set up, a numerical method is offered, and numerical results are compared with numerical results of the National Aeronautics and Space Administration (NASA). With both void cavity and phase change considered, influence of radial thickness of phase change material (PCM) on thermal performance of heat pipe receiver is numerically analyzed under microgravity. Numerical results indicate that decreasing the radial thickness of PCM in each canister has an effect of decreasing the thermal resistance of PCM. The variation of heat pipe temperature decreases as the thermal resistance between the heat pipe and the melt front decreases. The variation in heat pipe temperature is seen to be substantially reduced as the radial thickness of PCM is decreased to between 14.5 and 17 mm. Beyond this point, the impact of further reductions appears limited. The effect can be achieved with far fewer canisters with some degree of increasing thermal enhancement. When the radial thickness of PCM for unit heat pipe receiver is between 14.5 and 17 mm, the thermal performance of the heat pipe receiver is stable, and thermal spot during sunlight periods and thermal ratcheting during eclipse periods may be alleviated. The research results can be used to guide the designing and optimization of a PCM canister for a heat pipe receiver. [ABSTRACT FROM AUTHOR]

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]

An electrified reactor driven by renewable electricity, substituting conventionally fired reactors, will reduce CO 2 and provide a compact manner for hydrogen generation. Herein, we proposed a high-efficiency electrified reactor based on a high-temperature heat pipe (HTPR). The start-up of the HTPR powered by electricity was firstly evaluated, showing a start-up duration of 250 s and maintaining significant temperature uniformity of 96.7%. Then, the feasibility and performance of the HTPR were validated by experiments. The methane conversion and hydrogen mole fraction were 80.0% and 70.5% at a steam to carbon ratio of three, respectively. Finally, the reaction performance under different operating conditions was simulated. The methane conversion and the hydrogen mole fraction were attenuated with the increase in flow rate, and improving catalyst porosity is beneficial for the heat transfer between catalysts and gases. This study serves as a framework for designing an electrified reactor for endothermic reactions. • An electrified high temperature heat pipe reactor (HTPR) was proposed. • The HTPR shows high temperature uniformity about 96.7%. • The HTPR achieved methane conversion of 80% and hydrogen purity of 70.5%. [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]

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]

• Solar-thermochemical conversion mechanism for the reactor was revealed. • The maximum solar-thermochemical conversion efficiency is 41% • The heat conduction is the prime heat transfer mode in the reactor cavity. The high temperature heat pipe reactor (HTPR) driven by solar energy via steam methane reforming opens up a new avenue to produce hydrogen as aviation fuel with low carbon emission. Herein, the energy conversion and transfer process of steam methane reforming in the HTPR was disclosed by experiments and simulations. Firstly, the solar to thermal conversion and the heat transfer process in the start-up stage of the HTPR were investigated. The solar thermal response was immediate, and the reactor temperature difference was merely about 30 °C from top to bottom, providing a uniform thermal boundary for the steam methane reforming reaction. Then, the thermochemical conversion performance of the HTPR was unveiled. Despite the uneven energy flux on the HTPR outwall, it still provided a uniform thermal boundary with a temperature of up to 750 °C because of its high thermal conductivity. Furthermore, the solar-thermochemical efficiency of the HTPR was about 41.0% after optimizing the operating parameters, surpassing that of the state-of-the-art solar thermochemical reactor. Finally, the heat loss of the HTPR and the heat transfer mode in the reactor cavity were analyzed. The radiation of the reactor outwall accounted for 44.5% of the total heat loss, and the thermal conduction was the major heat transfer mode in the cavity. This work deepens our understanding of the thermochemical conversion mechanism of the HTPR, paving the way for improving hydrogen yields. [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]

• 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]

• A concentric annular high temperature heat pipe reactor (HTPR) was proposed. • The achieved the hybrid energy supply for stable hydrogen production. • A uniform thermal boundary was provided by the HTPR for steam methane reforming. • The solar to thermochemical efficiency is 45%. Solar-powered steam methane reforming could offer an efficient avenue for hydrogen generation, therefore reducing carbon dioxide emissions. However, the current solar thermochemical reactor (STR) is subjected to the mismatch between the continuous chemical process and the intermittent solar irradiance. In this work, we proposed a novel heat pipe tubular reactor based on the high-temperature heat pipe, i.e., a high-temperature heat pipe reactor (HTPR), achieving a highly effective heat transfer and a hybrid energy supply, i.e., concentrated solar energy and photovoltaic electricity. These two heating modes enable the HTPR to achieve a round-the-clock reforming process. Firstly, the HTPR start-up performance was measured by a concentrated solar simulator and a high-frequency induction device, and the start-up time for both heating modes was only 9 min. The methane conversion and the hydrogen purity were respectively 90% and 74%, approaching the thermodynamic equilibrium of SMR. Then, the continuous operation stability of the HTPR was conducted by a hybrid energy supply, i.e., solar and electricity. The methane conversion and hydrogen purity retained 91% and 72% for 24 h, respectively. Finally, the heat and mass transfer in the reaction cavity of the HTPR was simulated. The results show that the high thermal conductivity of the HTPR is capable of supplying energy to the strongly endothermic SMR, therefore promoting H 2 yields. This work provides a new way, in the aspect of reactor design, for resolving the unstable hydrogen production by renewable energy. [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]