Direct numerical investigations are conducted to study the ultrathin liquid film evaporation of R143a within square nanopillars using an improved thermal pseudo-potential lattice Boltzmann method (LBM) with a multi-block grid. The simulations successfully capture the evaporation-capillary-driven replenishment flow, the self-regulation of the liquid-vapor interface, and the dynamic evolution of the contact angle under high heat fluxes. At an ambient temperature of 311.27 K, the local critical heat flux (CHF) of the nanopillar surface can reach 2.1 kW/cm(2). The results reveal that the CHF is primarily restricted by either the capillary replenishment capability or the thermodynamic limit of homogeneous nucleation, depending on the geometric configurations. With a fixed surface wettability, increasing the nanopillar height or the spatial pitch effectively reduces viscous flow resistance, thereby enhancing the capillary-limited CHF. However, this enhancement eventually reaches a plateau due to the thermodynamic constraint of homogeneous nucleation. This study demonstrates the capability of the LBM in accurately determining the superheat limit of homogeneous nucleation and nucleation site density, providing a theoretical framework for quantifying complex phase-change dynamics in nanostructured surfaces.

Narrow gap channel boiling heat transfer is considered one of the most promising cooling technologies for addressing high heat flux dissipation problems. This paper constructed a pump-driven two-phase experimental loop, using 55 % mass fraction of ethylene glycol-water solution as working fluid. It investigated the effects of sandblasting on the boiling heat transfer performance of the channels. Additionally, microporous-structured copper narrow gap channels were designed and fabricated, impact of the characteristic parameters of the microporous layer on its boiling heat transfer performance was studied. Results show that copper narrow gap channels sandblasted with white corundum and ceramic sand exhibit significantly stronger boiling heat transfer capabilities than those treated with quartz sand. Among different mesh sizes of white corundum-treated surfaces, the 120-mesh white corundum sandblasted surface demonstrates the best boiling heat transfer performance. Microporous layer structure based on copper powder sintering can significantly enhance the boiling heat transfer performance. Compared to the 150-mesh and 250-mesh copper powders, the copper channel with 350-mesh copper powder exhibits the highest boiling heat transfer performance. When the ratio of microporous layer thickness to copper powder particle size is fixed at 4, the copper channel with certain mesh copper powder has the highest boiling heat transfer performance.

Enhancing flow boiling performance within microchannels is crucial for cooling high-power electronic devices. Based on the concept of "equalizing channel dryness", a counter-flow interconnected microchannel structure was proposed to enhance flow boiling. To investigate the flow boiling heat transfer characteristics, enhancement effects, and mechanisms of the combination of microporous layer-modified surfaces formed by copper powder sintering and counter-flow interconnected microchannels, this study employed microporous layers formed on microchannel sidewalls as an enhancement method. A detailed study was conducted on the effects of microporous layer morphology, copper powder particle size, sintering thickness, and sintering position on flow boiling heat transfer characteristics. Additionally, a mechanistic analysis of the capillary wicking process within the sintered copper powder surface was performed. The results show that, compared to smooth surface counter-flow connected microchannels, the microporous layer formed by copper powder sintering significantly enhances flow boiling heat transfer performance, as evidenced by a lower onset boiling superheat, increased critical heat flux (qCHF), and improved heat transfer coefficient (HTC-htp). Furthermore, microchannels with microporous layer sidewalls exhibit a relatively uniform liquid film distribution, which helps maintain annular flow, promotes thin film evaporation, and effectively prevents local dryout caused by film rupture or bubble nucleation. The wicking ability (V') of the microporous layer is found to have a strong linear relationship with the critical heat flux (CHF).

Pool boiling performance enhancement has attracted considerable interest on high power electronic cooling. In this paper, we introduce a self-induced jet impingement device for practical boiling enhancement, achieved by integrating the guidance tube with an orifice plate. With R1336mzz(Z) as the working fluid, we conduct visualization and parametric investigations on the pool boiling performance, considering the varying characteristics of this device. Visualization snapshots confirm the liquid-vapor separation and indicate that the two-phase flow pattern in the guidance tube eventually stabilizes as a churn-annular flow. Our findings suggest that self-induced liquid jet impingement has a minimal effect on the nucleate boiling heat transfer coefficient (hNB) under a certain heat flux, around 75 % CHF of the standard pool boiling, as the single-phase heat transfer is weakened due to the low thermal conductivity of R1336mzz(Z). As the heat flux intensifies, we observe substantial enhancements in both the Critical Heat Flux (CHF) and the Maximum Nucleate Boiling Heat transfer coefficient (hMNB) resulted from the delayed onset of boiling crisis, owing to the additional liquid supply and the promotion on vapor exhausting. The parametric studies reveal that the flow rate of the liquid jet, amplified by either extending the length or the inner diameter of the guidance tube, or by increasing the number of jet holes, positively impacts the boiling performance. However, these improvements show diminishing returns. Our study shows enhancements in CHF and hMNB of up to 69.5 % and 36.7 %, respectively, compared to the standard pool boiling.

Microchannel flow boiling is one of the most effective solutions to the problem of heat dissipation caused by high power electronic devices. However, inherent drawbacks such as low critical heat flux due to dry-out of the liquid film near the channel exit, relatively high two-phase flow pressure drop, severe temperature nonuniformity and boiling instability at high heat flux still prevent widespread commercial application. In this study, based on the idea of uniform vapor quality along the flow direction, a copper Counter-Flow Interconnected Microchannel (CFIM) was proposed to address the main issues currently faced by two-phase microchannel heat sink. A comprehensive comparative study of CFIM, Co-Current Microchannel (CCM) and Counter-Flow Microchannel (CFM) was carried out from the aspects of boiling stability, boiling heat transfer characteristics and temperature uniformity. Three slots of 0.2 mm, 0.5 mm and 0.8 mm in width, denoted as IM0.2, IM0.5 and IM0.8, respectively, are considered. Experiments are conducted at mass fluxes of 291-618 kg/m(2)& sdot;s, saturation temperature of 40 degrees C and effective heat fluxes of 3-293 W/cm(2), using the dielectric fluid R1233zd(E) as the working fluid. The results show that the CHF and average two-phase heat transfer coefficient (HTC) of CFIM are increased by 40.6 similar to 100.5 % and 70.4-83.6 %, respectively, while the two-phase pressure drop is decreased by 17-32.6 % compared to CCM. The design of the counter-flow interconnected structure can manipulate the void fraction and two-phase flow pattern, resulting in a near uniform void fraction and flow pattern. The structure of these connecting slots promotes nucleate boiling and fluid mixing of neighboring channels. More importantly, the flow boiling instability and temperature non-uniformity in CFIM are well suppressed. In addition, IM0.2 has higher heat dissipation than IM0.5 and IM0.8 due to enhanced bubble nucleation, reducing superheat requirements and periodic rewetting to the center of the channel. The results of this research reveal that utilizing a microchannel heat sink with a counter-flow interconnected configuration can effectively improve heat transfer performance during flow boiling. Furthermore, it helps to mitigate issues such as flow boiling instability and temperature nonuniformity.

Enhancing pool boiling performance is crucial for cooling high-power electronics. Inspired by the concept of liquid-vapor separation, we have developed a self-induced jet impingement device to enhance pool boiling, achieving notable results when combined with microporous copper surfaces in subsequent studies. This paper focuses on using sandblasted pin-fin surfaces as heating surfaces and explores their pool boiling performance under varied pin-fin and self-induced jet device parameters. Findings indicate that the self-induced jet device effectively mitigates the obstruction caused by nucleating bubbles to liquid replenishment, leading to improved qCHF and hNB@CHF performance compared to standard conditions. The impact of pin-fin sidewall characteristics, determined by the manufacturing process and parameters, is significant, particularly in enhancing boiling heat transfer performance for dielectric liquid cooling processes. Pool boiling performance is negatively affected by too short or too tall pin-fin heights, irrespective of the self-induced jet presence. Simple strategies like increasing guidance tube length or jet holes number are inadequate for enhancing qCHF. However, increasing the number of jet holes with strategically placing it between pin-fins could still improve boiling performance. This study demonstrates qCHF enhancements of up to 145.8%, achieving a qCHF of 61.2 W/cm2, which noticeably surpasses standard pool boiling conditions.

Inorganic phase-change materials (PCMs) with high melting points have great potential for thermal energy storage systems. Sodium chloride (NaCl) has a high melting point (801 °C) and high latent-heat-storage density (482 kJ/kg). However, it is difficult to encapsulate NaCl using a sintered ceramic shell because of its good wettability against ceramics and high volume-expansion capacity during melting. In this study, a novel NaCl/Al2O3 powder-composite structure was developed as highly stable PCM core material for highly stable encapsulation. The shape-retention performance and the mechanism of NaCl/Al2O3 powder-composite structure during melting were investigated. We have successfully fabricated a NaCl/Al2O3 powder-composite structure, which has a higher NaCl volume ratio of 80 vol% than conventional techniques. The gel-like network structure of Al2O3 particles in molten NaCl was a key structure to keep the shape of the composite ball and to prevent the evaporation of molten NaCl.

A deep understanding of the characteristics and mechanism of geyser boiling and capillary pumping is necessary to optimize a high-temperature sodium heat pipe. In this work, the Volume of Fluid (VOF) two-phase model and the capillary force model in the mesh wick were used to model the complex phase change and fluid flow in the heat pipe. Computational Fluid Dynamics (CFD) simulations successfully predicted the process of bubble nucleation, growth, aggregation, and detachment from the wall in the liquid pool of the evaporation section of the heat pipe in horizontal and tilted states, as well as the reflux phenomenon of capillary suction within the wick. The accuracy and stability of the capillary force model within the wick were verified. In addition, the causes of geyser boiling in heat pipes were analyzed by extracting the oscillation distribution of heat pipe wall temperature. The results show that adding the capillary force model within the wick structure can reasonably simulate the liquid backflow phenomenon at the condensation; Under the horizontal and inclined operating conditions of the heat pipe, the phenomenon of local dry-out will occur, resulting in a sharp increase in local temperature. The speed of bubble detachment and the timely reflux of liquid sodium (condensate) replenishment in the wick play a vital role in the geyser temperature oscillation of the tube wall. The numerical simulation method and the results of this study are anticipated to provide a good reference for the investigation of geyser boiling in high-temperature heat pipes.

Enhancing pool boiling performance is of significant interest for cooling high-power electronics. This paper introduces a self-induced jet impingement device designed for practical boiling enhancement. Using R1336mzz (Z) as the working fluid, we performed an experimental and parametric study on a microporous copper surface, considering the distinct characteristics of this device. Our results demonstrate that, due to the promotion of bubble detachment and the ensuring of sustained liquid absorption, the self-induced jet impingement device augments the Critical Heat Flux (CHF) on the microporous copper surface effectively. Nonetheless, the large predominant size of nucleation sites on this surface highlights the boiling suppression effect from jet impingement, resulting in a minor decrease in the Nucleate Boiling Heat transfer coefficient (hNB). Parametric evaluations reveal that strategies promoting the circulation flow of working fluid have a positive impact on CHF enhancement. Conversely, this device might negatively influence boiling heat transfer on microporous copper surface under conditions of liquid scarcity due to the restricted total flow area of jet holes. Our study indicates CHF and hNB@CHF enhancements of up to 196.0% and 291.0%, culminating in 73.7 W/cm2 and 78.2 kW/(m2 center dot K), respectively, compared to the boiling performances of standard pool boiling on the non-microporous copper surface.

In recent years, the thermal pseudo-potential lattice Boltzmann method (LBM) has been widely adopted in numerical simulations of liquid-vapor phase transition systems. However, the unit conversion for thermal pseudo-potential LBM remains incomplete and elusive, and the numerical simulation efficiency is limited by the uniformly single-block grid. In this paper, the dimensionless evolution equations of thermal pseudo-potential LBM with multi-relaxation time operators are derived for the convenience of adopting real physical parameters and improvement of computational efficiency. The energy equation is re-derived and improved for enhanced accuracy and convenience of numerical calculation. Additionally, a more accurate Martin-Hou equation of state for cryogen is adopted and a modified term for surface tension coefficient is improved to confirm that the surface tension coefficient is grid independent. Moreover, a three-layer boundary structure for the coarse grid is proposed to introduce the multi-block grid into the thermal pseudo-potential LBM for taking into account the intermolecular force and internal heat source term. The aforementioned works improve the thermal pseudo-potential LBM and enable efficient and accurate simulation of the liquid-vapor phase transition within the three-dimensional structure with real physical parameters of a specific working fluid. Finally, numerical simulations are adopted to validate the efficiency and accuracy of the proposed improvements for simulating liquid-vapor phase transition.

The objective of this research is to develop encapsulated phase change materials with applications exceeding 1000 degrees C that can be applied to concentrated solar power systems and industrial waste heat recovery. In this study, a macro-encapsulation phase change storage material with a copper-based core and an alumina-based shell was developed by slip-casting method and filling mixture of copper beads and Cu-40 %Al powder. The capsule was able to achieve self-sealing by local oxidation reaction at the entrance without leakage. After aging tests at 1100 degrees C for up to 1000 h, no leakage or damage was found and mass increase of the encapsulate phase change material was only 4.5 %, confirming the superior durability and oxidation resistance of the encapsulated phase change material. The heat storage density of the copper-based phase change material was evaluated as high as 147 J/g. The heat storage capacity did not decrease after 1000 h and ten cycles of aging. The mechanism of its high durability was explored by morphological observations and Raman spectroscopy. In the original material, a small amount of aluminum was oxidized with priority to alumina which significantly enhanced the durability of the capsules. This macro-encapsulated phase change material exhibits excellent thermal storage performance, as well as self-sealing and durability properties, which are of great significance for improving the efficiency of thermal energy recovery above 1000 degrees C.

Two-phase mini/micro-channel heat sink is one of the most effective solutions to the heat dissipation of high-power electronic devices in a narrow space. Flow boiling instability and wall temperature nonuni-formity are however the two notorious problems of two-phase mini/micro-channel heat sink that have hindered their practical applications to a large degree. This study proposed a counter-flow interconnected minichannel (CFIM) to improve the flow boiling stability and the temperature uniformity of a mechan-ically machined copper minichannel heat sink. Firstly, a comprehensive comparative study of CFIM and co-current minichannel (CCM) was carried out concerning two-phase flow patterns, boiling heat transfer characteristics, and wall temperature uniformity. Secondly, the effects of mass flux, subcooling and satu-ration temperature on the boiling heat transfer performance of the two kinds of heat sinks were further investigated. Experiments were carried out at a saturated temperature of 40 -50 & DEG;C, mass flux of 291 -618 kg/(m2.s), and subcooling of 15 -30 & DEG;C, using a new generation of environment-friendly coolant with a low boiling point, R1233zd(E). In particular, no backflow or partial dry-out is observed in the channel throughout all the tested conditions. Due to the mixing of fluid between adjacent channels with counter flow caused by interconnected slots in CFIM, low upstream vapor quality and high downstream vapor quality were avoided. Therefore, this counter-flow interconnected mechanism in the current design can be significantly enhanced in flow boiling stability and temperature uniformity, up to an effective heat flux of 230.4 W/cm2 with 51.2% increment of heat transfer coefficient, 56% increment of coefficient of performance (COP) and 48.5% reduction of pressure drop when compared with those of traditional CCM design. Overall, the research results have good guidance and reference for the application and research of two-phase minichannel heat sinks. & COPY; 2023 Published by Elsevier Ltd.

高效、紧凑的换热方式需求日益增大,具有高度方向速度梯度大的窄缝通道成为最有前景的方式之一.本文以质量分数为55%的乙二醇水溶液为工质,针对钛窄缝通道在负压工况进行流动沸腾换热实验.实验在质量流率750～2000kg/(m2·s)、饱和温度为80～90℃、入口温度60～70℃的条件下进行.结果表明,钛需要更高的热流密度激活大量成核点,从而其过冷沸腾起始点(ONB)前后平均换热系数h基本不变;质量流量对于ONB和沸腾充分发展阶段的平均换热系数影响很大;在高过冷度时,沸腾充分发展阶段,钛窄缝通道换热性能对于入口温度不敏感;提高进口温度降低过冷度可以极大提高平均换热系数,70℃条件下平均换热系数在沸腾充分发展阶段可以提高65%;背压对于换热性能的影响主要在沸腾充分发展阶段,背压越低平均换热系数越大.

Wetting states for droplets have been extensively investigated in the past. As the counter phase of the droplets, bubbles' wetting states have rarely been systematically explored. The wetting state of a bubble is closely related to its departure diameter, which plays significant roles in bubble-generated processes in boiling heat transfer and gas-evolving reactions. Based on the principle of minimum surface energy, we explicitly define three equilibrium wetting states (hemi-wicking state, Wenzel state, and Cassie-Baxter state) for bubbles on micro-/nanostructured surfaces in this paper. We analyze the three-phase contact line profiles for bubbles under these wetting states and propose theoretical models for predicting departure diameters of hemi-wicking-state bubble and Wenzel-state bubble on micro-/nanostructured surfaces. We identify competing effects of bubble departure in Wenzel state: the augmentation of contact line length due to the roughness, which would delay bubble departure, and the decrease of contact line length due to the reduced apparent contact angle, which would facilitate bubble departure. We demonstrate that hemi-wicking-state bubble exhibits a much smaller departure diameter on the textured surfaces. These findings are supported by numerical simulations by the three-dimensional (3D) multiple-relaxation-time lattice Boltzmann method. It is found that the length of the outermost contact lines instead of all contact lines determines the departure diameter of hemi-wicking-state bubble based on bubble detachment processes captured by our 3D numerical simulations. This work offers an avenue for the accurate prediction and control of bubble departure behaviors from micro-/nanostructured surfaces, and therefore can guide optimal designs of micro-/nanostructured surfaces in a variety of applications in boiling, desalination, and hydrogen production by electrolysis.

通过改进后的QSGS方法构造了三维复杂多孔吸液芯结构,采用格子Boltzmann方法和GPU并行算法,数值模拟了孔隙尺度下多孔吸液芯毛细抽吸两相传输动态特性,并探讨了表面润湿性和孔隙率对两相界面分布和毛细性能的影响.研究结果表明,对于三维复杂结构多孔吸液芯,毛细抽吸过程中两相界面不规则性、分布不均匀性程度较大,孔尺度效应显著.在一定范围内,随着多孔表面润湿性的增强,吸液芯孔隙率的降低,毛细作用相应增强,吸液芯内液体抽吸速率增加,最大吸液量逐步增大,毛细性能也随之提高.

本文建立了三维堆积床储热系统数值模型,对高温梯级相变胶囊堆积床系统的储热性能进行了数值研究,采用等效导热系数模型对相变胶囊内自然对流传热进行计算,并通过与文献实验研究结果对比验证了堆积床数值模型的可靠性.研究结果表明,当换热流体入口温度由673.15 K增加到723.15 K时,梯级系统储热量和储?量分别增加了19％和22％,储热过程所需时间减少,平均储热功率得到提高,但储热过程平均?效率受入口温度的影响较小.换热流体入口流量的增大并不会导致相变胶囊内所储存的总热量和总?量增加,然而入口流量的增大意味着系统输入?的增加,当入口流量由0.00250 kg/s增加到0.00500 kg/s时,平均?效率由30％下降到19％.同时,储热时间大大缩短,平均储热功率提升了68.5％.

Nanoscale liquid-vapor interfacial transport phenomena are of great significance to a variety of applications including evaporation, condescension, boiling and micro/nano-fluidics. In this work, we propose a mesoscopic approach to describe the nanoscale liquid-vapor interfacial statics and dynamics by combining the pseudopotential multiphase lattice Boltzmann method, the theorem of corresponding states and the principle of dynamic similarity. We demonstrate that our mesoscopic predictions of density profile, interfacial thicknesses and surface tensions for planar liquid-vapor interfaces of various real fluids agree very well with the NIST recommended data and molecular theories of capillarity in a very wide temperature range. We quantify the size effects of nano-bubbles and nano-droplets on the surface tension of water under a saturation temperature of 100 degrees C. We show that the surface tensions of nano-bubbles decrease while the surface tensions of nano-droplets increase with increasing size, and the predictions from static cases and dynamic cases are consistent. The mesoscopic approach proposed in this paper could well resolve nanoscale liquid-vapor interfacial phenomena for various real fluids with high resolution, high accuracy and affordable computation cost in a very wide temperature range, which paves the way for quantitative investigations of nanoscale liquid-vapor interfacial transport for real fluids in practical applications. (C) 2022 Elsevier Ltd. All rights reserved.

In order to analyze the geyser boiling phenomenon in high temperature wick sodium heat pipe, it is used as a numerical simulation method for safety analysis of high temperature heat pipe system.The geyser boiling phenomenon of high temperature sodium heat pipe was analyzed by using Volume of fluid (VOF) model and capillary force model.The process of bubble nucleation, growth, polymerization, and separation from the wall in the liquid pool of heat pipe evaporation section, as well as the backflow phenomenon of capillary suction in the wick were obtained by numerical calculation, which verified the accuracy and stability of capillary force model in the wick.In addition, the causes of geyser boiling of heat pipes were analyzed by extracting oscillating temperature distribution of heat pipe wall.The results show that the speed of bubble separation and the timely reflow of liquid sodium (condensate) in the wick play a key role in the periodic oscillation of tube wall temperature.Therefore, the numerical simulation method established in this study can provide reference for transient calculation of geyser boiling of sodium heat pipes.

Quartet structure generation set (QSGS) method is improved to numerically reconstruct three-dimensional heterogeneous porous wicks with uniform pore size distribution. The capillary pumping processes of the reconstructed random porous wicks are simulated at pore scale by using a three-dimensional two-phase lattice Boltzmann model. The evolutions of two-phase interface and the variations of the imbibed liquid volume fraction with time are analyzed under the conditions of different porosity, pore structure, and surface wettability. The comparisons between the LBM results and those predicted by a macroscopic scale homogenous model are also conducted. It is found that due to the pore scale effects, the two-phase interface in a random porous media is very irregular, especially at the earlier stages of capillary pumping process when the liquid penetration is faster, and the imbibed liquid does not increase exponentially with time as predicted by the macroscopic scale model. Meanwhile, the liquid penetration rate does not decrease monotonously, but exhibits different degrees of fluctuations. The pore scale effects are more prominent in the cases of lower porosity, smaller pore size and better surface wettability. In the parametric range of the present study, the capillary performance increases with the decreasing of porosity, average pore radius (when porosity is fixed) and contact angle.

搭建了镓铟锡合金低温液态金属接触热阻实验平台,验证了实验系统的可靠性,对比了常规导热硅脂和液态金属的热阻特性,研究了压力、温度和界面特性对接触热阻的影响规律.实验结果表明,特征测点温度稳定变化且在稳态下具有很小的线性偏差(ε＜6％),表明该实验系统具有较高的可靠性.与导热硅脂相比,在74 kPa的界面压力下液态金属的接触热阻降低约71％,且界面接触热阻随着压力的增加而降低.此外,界面温度的升高造成试件界面处液态金属导热系数降低,进而增加界面接触热阻.通过对铜表面镀镍处理能够有效防止液态金属对铜表面发生腐蚀,但镍层存在造成接触热阻增大.