To evaluate the effects of gravitational conditions on the flow characteristics, temperature field, and pressure distribution of liquid nitrogen, a numerical simulation model has been established for experimental validation. The numerical results demonstrate good agreement with the experimental measurements. On this basis, influences of different gravity environment on the flow behaviors, temperature distribution, and pressure distribution of liquid nitrogen in the National Aeronautics and Space Administration Perfluoro-n-Pentane (C5F12) tank for normal-temperature fluid were analyzed. According to calculation results, under different acceleration of gravity (g) values, the increasing mass of ullage in the tank is the primary driver of increased pressure during tank self-pressurization. The average temperature of the ullage is always higher than that in the liquid zone. The fluctuation amplitude of radial fluid temperature in the ullage exceeds that in the liquid zone. The variation trend of liquid temperature near the gas-liquid interface (GLI) with time is similar to that of gas. When g = 9.81 m/s2, the liquid near the GLI changes from counterclockwise into clockwise movement. As g decreases to 5 & times; 10-3 m/ s2, the liquid near the GLI changes from counterclockwise movement into clockwise movement, whereas the gas in the ullage always moves along the clockwise direction. As g continues to decrease to 5 & times; 10-6 m/s2, the liquid near the GLI always moves along the counterclockwise direction, whereas the gas in the ullage always moves along the clockwise direction. At 100 s after tank self-pressurization, the maximum temperature difference and the maximum pressure difference between the gas and liquid phases all decrease gradually with decreased g, accompanied with all gradually reduced fluid pressure and average temperature of the ullage. Moreover, the thermal stratification in the ullage weakens gradually. However, the difference in average temperature in the liquid zone is relatively small. The numerical simulation results can provide references for the subsequent onorbit pressure control technology of cryogenic-liquid tanks and space cryogenic fluid management.

Based on the demand to enhance static phase change evaporation heat transfer, this research examines the evaporation behavior of sessile droplets exposed to shear flow within a compact wind tunnel. The results indicate that various factors, including substrate materials, substrate structures, relative humidity (RH) , and droplet types, influence the evaporation of droplets affected by shear flow. The evaporation of droplets on copper substrates predominantly aligns with the Constant Contact Radius (CCR) model, exhibiting a higher evaporation rate. Elevated base structures can mitigate the effects of the boundary layer, facilitating faster evaporation of droplets at lower shear flow rates. Increasing relative humidity raises the vapor concentration in the air, thereby inhibiting droplet evaporation. Ethanol droplets under shear flow demonstrate a distinct behavior, with evaporation rates initially increasing before slightly decreasing. At low wind velocities, evaporation is predominantly driven by vapor concentration, while at high wind velocities, it is primarily limited by energy transfer. This research may provide valuable insights for utilizing shear flow to enhance liquid evaporation rates and improve heat transfer efficiency.

Microchannel heat transfer plays an important role in microelectronics technology for heat dissipation, due to its high efficiency and low heat transfer temperature difference and flow resistance. To underpin the fundamental understanding of this technology, the natural evaporation process of absolute ethanol in a capillary tube at inclination angles ranging from 0 degrees to 90 degrees was investigated experimentally by exploring a spectrum of properties, such as Marangoni flow patterns, evaporation rate, heat flux, and temperature distribution. We found that the morphology of the meniscus is similar under different inclination angles, but the liquid and the tube wall slip to varying degrees due to the pressure difference at the liquid-vapor interface during evaporation. Therefore, the force distribution of the meniscus interface is different, and the resultant force is Fmax(60 degrees) > Fmax(0 degrees) > Fmax(30 degrees) > Fmax(90 degrees). We found that the morphology of the meniscus is independent of the inclination angle when absolute ethanol evaporates naturally. And the evaporation rate, heat flux and temperature distribution of meniscus at the initial stage of evaporation follow the law of resultant force distribution. That is, when the inclination angle is 60 degrees, the evaporation rate and heat flux reach the maximum, i.e. 1.64 mu m/s and 10.96 W/cm2, respectively, and the temperature between the center of the meniscus and the wedge region reaches 1.5 degrees C. We used mu-PIV to observe the Marangoni vortex morphology of the vertical section of the meniscus, and found that there are different degrees of deformation at different inclination angles. When the inclination angle is 90 degrees, the Marangoni vortex structure is destroyed.

The evaporation of droplets containing nanoparticles is commonplace in industrial processes, while there is a lack of research on the instability-related characteristics of evaporation convection in nanofluid droplets, which differ from those of pure fluid droplets. Here we investigate the convection instability patterns and transition phenomenon in an Al2O3-ethanol nanofluid sessile droplet evaporation. Three different convection flow patterns are observed under the influence of both Marangoni effect and buoyancy during the evaporation: Two macroscopic convection cells appear at first, followed by the periodic generation and propagation of hydrothermal waves (HTWs) near the contact line. Then, the Bénard-Marangoni (BM) convection cells gradually emerge and eventually assume a dominant role. The deposition patterns, partly different from the classic coffee-ring pattern, are closely related to the flow patterns of HTWs and BM convection cells during the pinning stage of droplet evaporation. The critical Marangoni (Ma) and Rayleigh (Ra) numbers for the onset of convection flow instability increase with the increase of substrate heating temperature.

In order to study the effects of residual gravitational acceleration g on the flow,phase dis-tribution,temperature distribution,and pressure distribution of liquid nitrogen tank during self-pressur-ization,the self-pressurization process of liquid nitrogen tank under different g was numerically simulat-ed by the Volume-of-Fluid(VOF)method.The results show that under the condition of large g,the flu-id pressure in the tank increases gradually along the direction of residual gravity,and the temperature of the ullage in the tank increases with the continuous heat leakage of the tank wall,and the gas tempera-ture near the wall is the highest,and the gas temperature near the liquid is the lowest,while the temper-ature in the liquid bulk zone of the tank changes little with time.With the decrease of g,the liquid in the tank is more likely to climb along the wall of the tank with better infiltration,and the temperature difference of the fluid in the tank is gradually reduced.In the case of small g,after the fluid flow in the tank is stable,the ullage will be wrapped in the middle of the tank,forming a spherical bubble.The dif-ference of the fluid temperature in the tank gradually increases and then decreases with time.In zero gravity environment,the presence or absence of heat leakage(qw=0.5 W·m-2)on the tank wall has no significant influence on the fluid movement and phase distribution in the tank,and within the initial time interval △tf(0≤△tf≤40 s),the influence of the presence or absence of qw on the temperature distribution of the fluid in the tank also is not significant except near the wall of the tank.Numerical simulation results are expected to provide references to further study the on-orbit pressure control tech-nique of cryogenic liquid tanks and space cryogenic fluid management.

The evaporation of binary mixture drops has shown significant potential for industrial applications in the past decades. While the majority of scientific research was devoted to water-based mixtures In this study, we aimed to investigate the impact of component concentration and volatility on binary drop evaporation. Specifically, drop evaporation experiments were conducted on Ethanol-HFE7100 and Isopropanol-HFE7100 mixtures, and Water-Ethanol and Isopropanol-Ethanol mixture are selected as the comparison. A strong selective evaporation effect was observed. The evaporation behavior of each mixture was found to occur in three distinct stages. Notably, the HFE7100 component was found to evaporate during the entire lifetime of the Ethanol-based mixture, with domination in the first stage. In contrast, the Isopropanol-HFE7100 mixture exhibited as two independent fluids at moderate concentrations of HFE7100, with the HFE-7100 component evaporating only during the first stage and pure Isopropanol evaporating during the last stage. These findings hold potential for various industrial applications.

To extend the on-orbit coasting time of cryogenic upper stages and meet the requirements of deep space exploration missions, the effects of the heat leakage rate qw of the tank wall, settling acceleration and liquid fill level on the reorientation of liquid nitrogen were analysed. Simulation results showed that the reorientation flow pattern was not considerably influenced by the absence/presence of qw duo to the rapid reorientation process, but the enhancement of buoyancy convection effect would lead to the significant influence of heat leakage on the fluid temperature distribution. The strength of the buoyancy convection effect and the quality of the ullage directly affected the heat transfer rate between the tank wall and ullage, and then determined the size of the temperature difference between the gas and liquid phase in the tank. An increase in the liquid fill level would enhance the chaotic motion of bubbles within the liquid, consequently increasing the difficulty of bubble discharge.

The shedding kinematics of water droplets in a condensation environment when exposed to aerodynamic forces in microgravity was studied. Understanding the shedding of droplets from a surface is a critical part of the dropwise condensation process for improving heat transfer. Because gravity as a droplet removal technique is not available in space, the use of airflow to shed droplets is considered for condensing heat exchangers in environmental control and life support systems. Surface coatings affect drop adhesion, and here, four different surfaces (PMMA, PS, PTFE, and SHS) and various droplet sizes (80, 60, and 40 μL) were used to understand the above phenomenon. It was found that the critical velocity to shed a droplet in microgravity was up to 8% lower than that in normal gravity. Also, the effect of the droplet size was investigated for both microgravity and normal gravity; the shedding velocity was lower for microgravity, and it decreased as droplet size increased. Increasing the hydrophobicity of the coating decreased the critical velocity for shedding. Finally, the droplet was found to detach from superhydrophobic surfaces in microgravity. The detachment of droplets from the substrate will hamper the condensation process that can produce a larger fresh area; also, detachment of droplets and entrainment in airflow counter the concept of removing moisture from the air in a dehumidification process.

To preliminarily investigate the liquid atomization (LA) process of a close-coupled vortical loop slit atomizer under realistic conditions, a numerical simulation of the flying trajectory of atomized droplets which were formed after the breaking of a metal melt and would not break anymore in the swirling flow field was carried out with the discrete phase model (DPM) based on analysis of the single-phase flow field characteristics of the gas. The results demonstrate that the radial paving scope of atomized droplets that are far away from the central axis and enter into the flow field from the source position expands at the front end of the melt delivery tube (MDT). The radial pavement scope of these atomized droplets at the front end of MDT is negatively correlated with their diameter. The recirculation zone (RZ) in the flow field of the vortical atomizer and the kinetic energy of the atomized droplets can significantly influence their flight trajectories. Moreover, two breakup models can be applied to a metal melt in a vortical atomizer-a fluctuating breakup model and a sheet breakup model.

Thermocapillary convection of nanofluid with evaporating phase change interface occurs in a variety of industrial processes such as micro/nano fabrication, ink-jet printing, thin film coatings, etc. Previous studies have mostly focused on the phenomena of thermocapillary convection in pure fluids without phase change. This paper reports the first fundamental experimental work on the thermocapillary flow of a thin nanofluid layer under the effect of evaporation. This research focuses on the behavior of a volatile thin nanofluid layer in a rectangular test cell under the effects of horizontal temperature gradient. The buoyancy effect can be neglected inside this thin liquid layer as in microgravity conditions. HEE7200 and HFE7200-Al 2 O 3 nanofluid are used as working fluids to analyze the effect of nanoparticle addition. The results indicate that the linear relationship between the thickness of the liquid layer and the duration of evaporation is not changed by nanoparticles. HFE7200-Al 2 O 3 nanofluid always has a higher evaporation rate than its base fluid with the temperature ranging from 2.98 °C to 13.92 °C. The critical Marangoni number for the nanofluid is lower than that of the pure fluid, which indicates that the addition of nanoparticles promotes the flow pattern transition.

Evaporation of nanofluid droplets whose size is larger than capillary length is common in industrial processes, but there is currently a lack of research on the evaporation kinetics and more complex deposition patterns under the combined effects of buoyancy and Marangoni convection of large evaporating droplet. Experimental investigations on evaporation of Al2O3-H2O nanofluid sessile droplet with initial contact line diameter of 4 mm on heating PTFE coating substrate are reported. The variation in contact angle, contact radius, and droplet volume over time under different conditions were obtained through experiments. The average evaporation rate is calculated. Internal flow characteristics are inferred through the surface temperature distribution detected by the top view infrared camera. The constant contact radius mode takes place at the beginning of evaporation. Then mixed evaporation mode and stick slip mode may occur for different substrate temperature, nanoparticle mass concentration and nanoparticle size. Increasing nanoparticle size causes a decrease in average evaporation rate. Whether the use of nanofluids can improve evaporation rate and heat transfer or not depends on multiple factors. Convection cells exist, and three kinds of deposition patterns are formed due to the underlying coupled transport phenomena. Under the combined effect of buoyancy and Marangoni convection in this article, the most common coffee-ring pattern does not appear. The deposition patterns are directly affected by the pinning stability, which decrease at higher temperature.

This work experimentally investigated the effects of different factors, including nanoparticle size and type, volume fraction, and base fluid, on the thermal conductivity enhancement of nanofluids. The experimental results indicate that the thermal conductivity enhancement of nanofluids is proportional to the thermal conductivity of the nanoparticles, with the enhancement being more pronounced for fluids with lower thermal conductivity. Meanwhile, the thermal conductivity of nanofluids decreases with increasing particle size and increases with increasing volume fraction. In addition, elongated particles are superior to spherical ones for thermal conductivity enhancement. This paper also proposes a thermal conductivity model by introducing the effect of nanoparticle size based on the previous classical thermal conductivity model via the method of dimensional analysis. This model analyzes the magnitude of influencing factors on the thermal conductivity of nanofluid and proposes suggestions for an improvement in thermal conductivity enhancement.

The dynamics of droplet under shear flow was investigated in normal gravity and microgravity. Drop under shear flow has many applications in aerospace such as aircraft de-icing and environmental control and life support system in space station. Shape analysis of the droplet showed that drop in microgravity have a greater contact angle and height, but a smaller contact radius, than that in normal gravity. The difference between advancing and receding contact angle θ a -θ r is greater in microgravity than that in normal gravity under the same wind speed. The effect of airflow on the droplet is mainly dependent on the We number and the dimensionless number k′ , which characterizes the ratio of wind force and adhesion force. the k’ in microgravity is larger than that in normal gravity. The droplet exposed to airflow from static to motion state can be divided into three regimes.

Droplet dynamics and condensation phenomena are widespread in nature and industrial applications, and the fundamentals of various technological applications. Currently, with the rapid development of interfacial materials, microfluidics, micro/nano fabrication technology, as well as the intersection of fluid mechanics, interfacial mechanics, heat and mass transfer, thermodynamics and reaction kinetics and other disciplines, the preparation and design of various novel functional surfaces have contributed to the local modulation of droplets (including nucleation, jumping and directional migration) and the improvement of condensation heat transfer, further deepening the understanding of relevant mechanisms. The wetting and dynamic characteristics of droplets involve complex solid-liquid interfacial interactions, so that the local modulation of microdroplets and the extension of enhanced condensation heat transfer by means of complex micro/nano structures and hydrophilic/hydrophobic properties is one of the current hot topics in heat and mass transfer research. This work presents a detailed review of several scientific issues related to the droplet dynamics and dropwise condensation heat transfer under the influence of multiple factors (including fluid property, surface structure, wettability, temperature external field, etc.). Firstly, the basic theory of droplet wetting on the solid wall is introduced, and the mechanism of solid-liquid interfacial interaction involving droplet jumping and directional migration on the functional surfaces under the various influencing factors is discussed. Optimizing the surface structure for the local modulation of droplets is of guidance for condensation heat transfer. Secondly, we summarize the existing theoretical models of dropwise condensation applicable to various functional surfaces and briefly outline the current numerical models for simulating dropwise condensation at different scales, as well as the fabricating techniques of coatings and functional surfaces for enhancing heat transfer. Finally, the relevant problems and challenges are summarized and future research is discussed.

The effects of phase change on the stability of a horizontally heated liquid layer are studied experimentally in this paper. Results are obtained for two volatile liquids with similar Prandtl numbers in a rectangular geometry with different temperature differences. Three different flow states occur with the variation of the liquid depth, namely oscillating multicellular convection, hydrothermal waves and steady flow. The critical conditions for the transition between the different flow states are identified and discussed. In addition, the presence of evaporation at the interface plays an essential role in the flow instabilities. The results show that evaporation at the surface and associated surface deformation tend to inhibit the development of a hydrothermal wave but conversely promote the transition of oscillating multicellular convection. Furthermore, the transient nature of HTWs is shown to be little affected by the phase change.

利用实践十号返回式科学实验卫星蒸发对流箱,开展了三相线处于钉扎状态且接触半径大于毛细长度的无水乙醇大滴在加热PTFE表面蒸发的地基科学实验.实验发现,液滴体积随时间线性递减,但钉扎大液滴蒸发过程中没有出现恒定接触角(CCA)阶段.与小液滴蒸发的恒定接触半径(CCR)阶段相同,大液滴的平均蒸发速率也与初始体积无关,表明受重力影响明显的大液滴蒸发主要发生在三相线附近区域.瞬时蒸发速率经历了迅速上升和之后的长时间内保持稳定两个阶段.与数值模拟结果对比分析发现,准静态扩散模型低估了三相线处于钉扎状态的大液滴瞬时蒸发速率,而同时考虑蒸气扩散与空气中自然对流经验模型的准确性取决于实验所用工质.