Effect of gravity on pool boiling heat transfer on thermal spray coating

This study deals with heat transfer enhancement surface manufactured by thermal spraying. Two thermal spraying methods using copper as a coating material, wire flame spraying (WFS) and vacuum plasma spraying (VPS), were applied to the outside of copper cylinder with 20 mm OD. The surface structure by WFS was denser than that by VPS. The effect of gravity on boiling heat transfer coeffcient and wall superheat at the onset of boiling were experimentally evaluated under micro- and hyper-gravity condition during a parabolic trajectory flight of an airplane. Pool boiling experiments in saturated liquid of HCFC123 were carried out for heat fluxes between 1.0 and 160 kW/m² and saturated temperature of 30 °C. As a result, the surface by VPS produced higher heat transfer coefficient and lower superheat at the onset of boiling under microgravity. For the smooth surface, the effect of gravity on boiling heat transfer coefficient was a little. For the coating, a large difference in heat transfer coefficient to gravity was observed in the moderate heat flux range. The heat transfer coefficinet decreased as gravity changed from the normal to hypergravity, and was improved as gravity changed from the hyperto microgravity. The difference in heat transfer coefficient between the normal and microgravity was a little. Heat transfer enhancement factor was kept over the experimental range of heat flux. It can be said that boiling behavior on thermal spray coating might be influenced by flow convection velocity.     

喷雾冷却临界传热特性试验研究

为了进行极限热工况下的喷雾冷却传热特性研究,设计并搭建采用蒸馏水及乙醇溶液喷雾冷却试验台,分析结构参数、喷雾流量、喷雾腔内压力等对喷雾冷却临界热流密度的影响。试验结果表明:临界热流密度随槽道深度增加先增大后减小,最佳槽道深度为0.8 mm,此时临界热流密度达到326 W/cm^2;随着喷雾流量的增加,临界热流密度始终增大;喷雾腔内压力对临界热流密度基本没有影响。计算结果表明,喷雾冷却效率随槽道深度增加而提升,随喷雾流量的增加而减弱。     

液氢空间贮存过程膜态沸腾数值模拟

为实现液氢在空间中安全高效应用,针对微重力条件下液氢膜态沸腾现象,建立了加热细丝浸没在过冷液氢池中的数值计算模型.采用VOF方法捕捉相界面,相变模型选取Lee模型,利用文献中的实验数据验证了模型的准确性.从气泡运动行为和换热特性两方面开展研究,结果发现液体过冷度和重力水平是影响换热机理的两个重要因素.在高重力水平、低液...     

Pool boiling heat transfer in microgravity

A temperature-controlled pool boiling (TCPB) device has been developed to study the bubble behaviors and heat transfer in pool boiling phenomenon both in normal gravity and in microgravity. The results on heat transfer and bubble dynamic behavior in the experiments aboard the 22^nd Chinese recoverable satellite and those in normal gravity before and after the flight experiment are reported and discussed in the present paper. The onset-boiling temperature is independent, or at least, dependent much weakly on gravity. Heat transfer of nucleate boiling in microgravity is slightly enhanced, while the scale of CHF with gravity is contrary to the traditional viewpoint and can be predicted by LD-Zuber correlation. A forward-and-backward lateral motion of vapor bubbles is observed along the wire before their departure from the wire in microgravity, while three critical bubble diameters divide the observed vapor bubbles into four regions in microgravity. These distinctive bubble behavior can be interpreted by Marangoni effects.     

雾化特性及系统参数对闭式喷雾冷却系统性能的影响

搭建了以R22为冷却介质的闭式喷雾冷却实验台,通过更换3种不同的喷嘴,分析对比了不同喷嘴类型和喷雾高度下传热性能的变化规律。实验结果表明:选择喷嘴时必须同时考虑喷雾流量和雾化角度的影响;雾化角较小的喷嘴在低高度下适用性较好。同时,研究了系统参数如过冷度和充注压力对喷雾冷却传热性能的影响,得出过冷度对喷雾冷却性能有一定促进作用,但过冷度越大,促进效果越弱;表面传热系数和表面温度均随充注压力的升高而升高,因此高充注压力不适用于需严格控制温度的表面冷却。     

Performance analysis of no-vent fill process for liquid hydrogen tank in terrestrial and on-orbit environments

Two finite difference computer models, aiming at the process predictions of no-vent fill in normal gravity and microgravity environments respectively, are developed to investigate the filling performance in a liquid hydrogen (LH₂) tank. In the normal gravity case model, the tank/fluid system is divided into five control volume including ullage, bulk liquid, gas–liquid interface, ullage-adjacent wall, and liquid-adjacent wall. In the microgravity case model, vapor–liquid thermal equilibrium state is maintained throughout the process, and only two nodes representing fluid and wall regions are applied. To capture the liquid–wall heat transfer accurately, a series of heat transfer mechanisms are considered and modeled successively, including film boiling, transition boiling, nucleate boiling and liquid natural convection. The two models are validated by comparing their prediction with experimental data, which shows good agreement. Then the two models are used to investigate the performance of no-vent fill in different conditions and several conclusions are obtained. It shows that in the normal gravity environment the no-vent fill experiences a continuous pressure rise during the whole process and the maximum pressure occurs at the end of the operation, while the maximum pressure of the microgravity case occurs at the beginning stage of the process. Moreover, it seems that increasing inlet mass flux has an apparent influence on the pressure evolution of no-vent fill process in normal gravity but a little influence in microgravity. The larger initial wall temperature brings about more significant liquid evaporation during the filling operation, and then causes higher pressure evolution, no matter the filling process occurs under normal gravity or microgravity conditions. Reducing inlet liquid temperature can improve the filling performance in normal gravity, but cannot significantly reduce the maximum pressure in microgravity. The presented work benefits the understanding of the no-vent fill performance and may guide the design of on-orbit no-vent fill system.     

Bubble Behavior and Heat Transfer in Quasi-Steady Pool Boiling in Microgravity

Pool boiling of degassed FC-72 on a plane plate heater has been studied experimentally in microgravity. A quasi-steady heating method is adopted, in which the heating voltage is controlled to increase exponentially with time. Compared with terrestrial experiments, bubble behaviors are very different, and have direct effect on heat transfer. Small, primary bubbles attached on the surface seem to be able to suppress the activation of the cavities in the neighborhoods, resulting in a slow increase of the wall temperature with the heat flux. For the high subcooling, the coalesced bubble has a smooth surface and a small size. It is difficult to cover the whole heater surface, resulting in a special region of gradual transitional boiling in which nucleate boiling and local dry area can co-exist. No turning point corresponding to the transition from nucleate boiling to film boiling can be observed. On the contrary, the surface oscillation of the coalesced bubble at low subcooling may cause more activated nucleate sites, and then the surface temperature may keep constant or even fall down with the increasing heat flux. Furthermore, an abrupt transition to film boiling can also be observed. It is shown that heat transfer coefficient and CHF increase with the subcooling or pressure in microgravity, as observed in normal gravity. But the value of CHF is quite lower in microgravity, which may be only about one third of that at the similar pressure and subcooling in terrestrial condition.     

The influence of film structure on the interfacial friction in annular two-phase flow under microgravity and normal gravity conditions

Results for the interfacial friction factor and relative interfacial roughness on the gas-liquid interface are reported for an air-water annular flow in a small inner diameter tube (9.53 mm i.d.). The film structure was obtained through processing the time trace signal of film thickness measurements using conductance probes. The interfacial friction factor and the wave height were altered through changing the gravity level and gas Reynolds number. It was found that the wave height decreased with increasing the gas Reynolds number. The wave height in microgravity is less than half of that in normal gravity, while the friction factor was about 10% smaller in microgravity than that in normal gravity. It was shown that the annular two-phase flow friction factor decreased less dramatically as the relative interfacial roughness decreased compared to the single-phase case. It is interesting to note that the interfacial shear stress values at microgravity were very close (or even larger than) those at normal gravity. This was attributed to the thicker substrate at microgravity.     

Heat transfer in boiling of liquid in a film moving under gravity

Experimental data on heat transfer in boiling in a film of liquid moving under gravity have been processed in dimensionless coordinates. A comparison between the data on heat transfer during boiling in a film with the data on heat transfer in pool boiling is carried out, which makes it possible to more accurately estimate the influence of the film flow velocity and spray entrainment on heat transfer in film boiling.     

Simulation Using Realistic Spray Cooling for the Continuous Casting of Multi—component Steel

A three-dimensional heat transfer model for continuous steel slab casting has been developed with realistic spray cooling patterns and a coupled microsegregation solidification model that calculates the solidification path for multi-component steels.Temperature and composition dependent properties are implemented in a database for 15 chemical species.Considerable effort is made to accurately model the spray cooling heat transfer.Each spray nozzle position and distribution is considered, including variations of the spray patterns with flow rate, and spray overlay .Nozzle type, layout, nozzle-to-slab distance, and spray span and flux are variable.Natural convection,thermal radiation and contact cooling of individual rolls are computed. The present model provides more comprehensive information and realistic slab surface temperatures than results from a model using the “averaged“ treatment of boundary comditions .Cooling operating conditions and parameters of individual spray nozzles can be analzed to optimize muzzle spray distribution ,improve product quality,and troubleshoot issues such as nozzle clogging that may arise during production.One spray cooling correlation is used for the entire machine,achieving as good or better agreement with surface temperatre measurements than was found previously for the model using an “averaged“ treatment of boundary conditions and using three machine-segment-dependent correlations.     

Gravity Influence on Heat Transfer Rate in Flow Boiling

The aim of the present paper is to describe the results of flow boiling heat transfer at low gravity and compare them with those obtained at earth gravity, evaluating possible differences. The experimental campaigns at low gravity have been performed with parabolic flights. The paper will show the analysis of differences between the heat transfer coefficients at normal and at zero gravity, and the study of the effects of mass flux, heat flux, and tube diameter on boiling phenomena at microgravity. Three tube diameters are tested: 6.0, 4.0, and 2.0?mm. With respect to terrestrial gravity, both heat transfer rate enhancement (up to 15?C20%) and deterioration (up to 35%) have been observed. Heat transfer differences for the two gravity conditions may be related to the different bubble size in each of them. The size of a bubble in flow boiling is generally affected by the gravity level, being larger at low gravity, unless inertial forces are largely predominant over buoyancy and other forces acting on the bubble itself when detaching from a heating wall. Heat transfer enhancements at low gravity, are observed in those conditions where the flow pattern is bubbly flow at normal gravity and intermittent flow at low gravity. The results are presented in a flow boiling gravity influence map, which can be considered a useful tool for designing boiling systems for space applications.     

Effect of Gravity on Film Boiling Heat Transfer and Rewetting Temperature during Quenching

This paper describes the experimental strategy developed to improve the modeling of liquid-vapor flows during the chill down of rocket engines by cryogenic fluid in microgravity. A similarity analysis is performed to determine the relevant dimensionless numbers for the design of an experiment similar to engine flows. A literature review on reduced gravity quenching experiments, and on rewetting temperature and film boiling heat transfer shows the lack of validated models for microgravity. Experimental results obtained with the quenching of a glass tube by FC72 during parabolic flight are presented. Especially the impact of gravity and subcooling on rewetting temperature and film boiling heat transfer is investigated. Results show an increase in rewetting temperature, and a decrease in film boiling heat transfer under reduced gravity in agreement with the literature. The comparison of 0 g flow pattern with corresponding tests on ground points out a behavior at 0 g closest to 1 g upflow than 1 g downflow.     

相变喷雾冷却技术的研究进展

论述了中国国内外近年来喷雾冷却技术在沸腾区相变换热的研究进展,并对影响沸腾区喷雾冷却换热性能的喷雾特性及外部特性作了总结.沸腾区的研究包括实验研究和理论及数值研究两部分内容.实验研究主要分为两方面:一是对整体换热性能的研究;二是针对临界热流密度(CHF)的研究;理论及数值研究主要集中在对液膜分布、气泡生长及二次成核等过...     

The effects of gravity on the features of the interfacial waves in annular two-phase flow

A physical model of interfacial waves in annular two-phase flow was studied in both microgravity and normal gravity. The wave structure was obtained for local film thickness and velocity measurements using a conductance probe technique. It was found that the wave height, and not its width, is strongly affected by changing the gravity level. In fact, the wave height in normal gravity is more than twice that in microgravity. Using an analogous approach to a turbulent, single-phase flow in a rough tube, a preliminary mathematical model was proposed to calculate the wave amplitude. The model fits well with the experimental data and shows that the wave height in normal gravity is approximately 1.7 times the combined thickness of the viscous sublayer and transition zones in the turbulent gas stream. The wave height in microgravity was estimated to be approximately 80% of the total thickness.     

Numerical Simulation of Bubble Dynamics in Microgravity

A numerical method for the simulation of two-phase flows under microgravity conditions is presented in this paper. The level set method is combined with the moving mesh method in a collocated grid to capture the moving interfaces of the two-phase flow, and a SIMPLER-based method is employed to numerically solve the complete incompressible Navier-Stokes equations, and the surface tension force is modeled by a continuum surface force approximation. Based on the numerical results, the coalescence process of two bubbles under microgravity conditions (10^???2×g) is compared to that under normal gravity, and the effect of gravities on the bubbles coalescence dynamics is analyzed. It is showed that the velocity fields inside and around the bubbles under different gravity conditions are quite similar, but the strength of vortices behind the bubbles in the normal gravity is much stronger than that under microgravity conditions. It is also found that under microgravity conditions, the time for two bubbles coalescence is much longer, and the deformation of bubbles is much less, than that under the normal gravity.     

以乙醇溶液为冷却工质的开式喷雾冷却系统实验研究

在开式循环喷雾冷却系统上研究质量分数分别为10%、20%、30%、50%、70%、99%的乙醇溶液对喷雾冷却效果影响,结果表明:热沉表面热流密度和传热系数先随乙醇水溶液浓度的增大先增大,当乙醇质量分数为50%时达到最大值,随后随乙醇浓度的增大而减小。体积流量0.944 46L/min,乙醇质量分数为50%时,喷雾冷却传热系数为6.41 W/(cm~2·K),热流密度为216.04W/cm~2,较之纯水传热系数增加了34.95%,热流密度增加了24.9%。结果表明使用乙醇溶液作为冷却剂的喷雾冷却系统能同时满足高热流密度和低换热表面温度的要求,具有良好、稳定的换热冷却能力。推导了反映介质喷雾特性和蒸发强度对换热影响的光滑表面量纲一换热准则方程,涉及的物理参量较少,因此可方便用于工程设计。     

Cryogenic Boiling and Two-Phase Flow during Pipe Chilldown in Earth and Reduced Gravity

For many industrial, medical and space technologies, cryogenic fluids play indispensable roles. An integral part of the cryogenic transport processes is the chilldown of the system components during initial applications. In this paper, we report experimental results for a chilldown process that is involved with the unsteady two-phase vapor-liquid flow and boiling heat transfer of the cryogen coupled with the transient heat conduction inside pipe walls. We have provided fundamental understanding on the physics of the two-phase flow and boiling heat transfer during cryogenic quenching through experimental observation, measurement and analysis. Based on the temperature measurement of the tube wall, the terrestrial cryogenic chilldown process is divided into three stages of film boiling, nucleate boiling and single-phase convection that bears a close similarity to the conventional pool boiling process. In earth gravity, cooling rate is non-uniform circumferentially due to a stratified flow pattern that gives rise to more cooling on the bottom wall by liquid filaments. In microgravity, there is no stratified flow and the absence of the gravitational force sends liquid filaments to the central core and replaces them by low thermal conductivity vapor that significantly reduces the heat transfer from the wall. Thus, the chilldown process is axisymmetric, but longer in microgravity.      

Microgravity performance of micro pulsating heat pipes

A micro pulsating heat pipe made of a thin clear Teflon tube of 1.6 mm ID was used to observe the pulsating flow inside a heat pipe under different gravity levels using parabolic flights. More vigorous pulsating flow was observed under microgravity, compared to the depressed movements under hypergravity. Two metallic micro pulsating heat pipes made of an aluminum plate with small internal channels were also tested to investigate the effect of gravity on their heat transfer characteristics. Reduced gravity experiments were performed aboard Falcon 20 aircraft flying parabolic trajectories. Under normal and hypergravity conditions, both the orientation of the pulsating heat pipe and locations of the heated and cooled sections affected the heat transfer performance. Under reduced gravity, however, the heat pipes showed better operating and heat transfer performance than that under normal and hypergravity. These experiments have for the first time confirmed that pulsating heat pipes are capable of operating under reduced gravity and thus are suitable for deployment in space applications such as satellites.     

Measurement of spray boiling refrigerant coefficients in an integral-fin tube bundle segment simulating a full bundle

Outside (refrigerant) boiling coefficients for a combination of spray and drip boiling for a low pressure refrigerant have been obtained from overall heat transfer coefficients in a 1024 fins per meter tube bundle segment. The tubes were heated by water on the inside; liquid refrigerant was sprayed and/or dripped on the outside. Also, refrigerant vapor was supplied at the bottom of the bundle segment. This configuration simulates an actual flooded evaporator under spray boiling conditions. The dripping corresponds to liquid film falling from upper rows while the inlet vapor is equivalent to the vaporized refrigerant rising from lower tubes; the refrigerant vapor can influence heat transfer performance by the combined effects of gas convection and liquid shear on the tubes. For a nominal heat flux of 23,975 W/m², a bundle average outside heat transfer coefficient of 8522 W/m² °C, based on nominal tube outer diameter, was found at an average bundle vapor mass flux equal to 12.4 kg/s m². The distributor plate below the bundle enhanced the heat transfer, especially at lower vapor mass fluxes, by providing a level of liquid hold-up just below the bottom tube row.     

Experimental Study of Nucleate Pool Boiling of FC-72 on Smooth Surface under Microgravity

Experiments of highly subcooled nucleate pool boiling of FC-72 with dissolved air were studied both in short-term microgravity condition utilizing the drop tower Beijing and in normal gravity conditions. The bubble behavior and heat transfer of air-dissolved FC-72 on a small scale silicon chip (10 × 10 × 0.5 mm³) were obtained at the bulk liquid subcooling of 41 K and nominal pressure of 102 kPa. The boiling heat transfer performance in low heat flux region in microgravity is similar to that in normal gravity condition, while vapor bubbles increase in size but little coalescence occurs among bubbles, and then forms a large bubble remains attached to the heater surface during the whole microgravity period. Thermocapillary convection may be an important mechanism of boiling heat transfer in this case. With further increasing in heat flux to the fully developed nucleate boiling region, the vapor bubbles number as well as their size significantly increase in microgravity. Rapid coalescence occurs among adjacent bubbles and then the coalesced large bubble can depart from the heating surface during the microgravity period. The reason of the large bubble departure is mainly attributed to the momentum effects caused by the coalescence of small bubbles with the large one. Hence, the steady-state pool boiling can still be obtained in microgravity. In the high heat flux regime near the critical heat flux, significant deterioration of heat transfer was observed, and a large coalesced bubble forms quickly and almost covers the whole heater surface, leading to the occurrence of the critical heat flux in microgravity condition.