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.     

乙烷脉动热管的CFD数值模拟研究及实验对比

为了更好地了解脉动热管内部的内在运行机制,本文在实验的基础上采用VOF模型对乙烷脉动热管的传热特性进行了数值模拟研究.结果表明:充注工质后,脉动热管内部形成了随机的气液分布;在启动过程和稳定运行过程中,温度波动的频率随着加热功率的增大而增大;在稳定运行过程中,脉动热管内的流型也在不停地变化.将模拟结果与前期的实验结果进...     

Gravity effect on spray impact and spray cooling

An experimental study of the film produced by the spray impact on a heated target and of the spray cooling has been performed in normal gravity and in microgravity conditions during parabolic flights. A convex shape of the target allowed visualization of the film evolution and determination of the film characteristics using the image processing. The effects of the spray parameters and of the gravity level on heat transfer have been investigated. It has been found that the spray cooling efficiency depends on the water flow rate in a non-monotonous way. A range of spray parameters at which the effect of gravity level on heat transfer is significant has been determined. It has been found that the spray cooling is less effective in microgravity conditions in comparison with normal gravity and hypergravity.     

单面波浪平板脉动热管的传热性能

对一种单面波浪平板脉动热管的传热性能进行了实验研究,分析在空气强制对流冷却条件下充液率、加热功率、倾角等因素对其传热性能的影响。研究结果表明,除0°倾角外,脉动热管的最佳充液率为20%～30%,倾斜角度对脉动热管传热性能的影响很小,但90°时相对最好。脉动热管在0°放置时其传热性能较差,在低充液率的情况下甚至丧失脉动效果,主要是工质回流不畅的原因,与平板脉动热管的槽道设计有很大关系。此外低加热功率时热管传热性能存在波动,有时甚至不能启动。     

Two-phase bubbly flows in microgravity: Some open questions

Several studies on gas-liquid pipe flows in micro gravity have been performed. They were motivated by the technical problems arising in the design of the thermohydraulic loops for the space applications. Most of the studies were focused on the determination of the flow pattern, wall shear stress, heat transfer and phase fraction and provided many empirical correlations. Unfortunately some basic mechanism are not yet well understood in micro gravity. For example the transition from bubbly to slug flow is well predicted by a critical value of the void fraction depending on an Ohnesorge number, but the criteria of transition cannot take into account the pipe length and the bubble size at the pipe inlet. To improve this criteria, a physical model of bubble coalescence in turbulent flow is used to predict the bubble size evolution along the pipe in micro gravity, but it is still limited to bubble smaller than the pipe diameter and should be extended to larger bubbles to predict the transition to slug flow. Another example concerns the radial distribution of the bubbles in pipe flow, which control the wall heat and momentum transfers. This distribution is very sensitive to gravity. On earth it is mainly controlled by the action of the lift force due to the bubble drift velocity. In micro gravity in absence of bubble drift, the bubbles are dispersed by the turbulence of the liquid and the classical model fails in the prediction of the bubble distribution. The first results of experiments and numerical simulations on isolated bubbles in normal and micro gravity conditions are presented. They should allow in the future improving the modelling of the turbulent bubbly flow in micro gravity but also on earth.     

Pulsating Turbulent Flow of Gas in a Round Pipe

An analysis is performed of the presently available results of experimental and prediction studies into pulsating turbulent flow of liquid in a narrow pipe under conditions when the compressibility is apparent. It is demonstrated that the simulation of such flows in the general case may be performed only numerically, using a model of turbulence that adequately includes the effect of oscillation on turbulent transfer. Use is made of a model of turbulence whose validity is proved by comparing the calculation and experimental results for a wide range of flows. Calculations are performed for a pulsating flow of gas in pipes with isothermal and adiabatic walls, acoustically closed at the outlet, in the frequency range corresponding to the first resonance harmonic. The predicted variations of the heat flux to the wall and of the hydraulic drag, averaged over the oscillation period, as functions of the process parameters such as the Reynolds number of the mean flow and the dimensionless oscillation frequency are discussed.     

Integral methods of calculation of heat transfer and drag under conditions of turbulent pipe flow of liquid of variable properties: Steady-state and quasi-steady-state flows in a round pipe with constant density of heat flux to the wall

Integral relations are derived for the calculation of the Nusselt number and coefficients of hydraulic drag and friction drag under conditions of pipe flow of dropping liquid and gas of temperature-dependent physical properties. In the limiting case of steady-state flow of liquid of constant properties, the expression for the Nusselt number transforms to the well-known Lyon integral. The results of calculation of heat transfer and drag by an integral method are compared with more exact results obtained using the numerical solution of the set of differential equations of convective heat transfer. An inference is made about the conditions under which integral methods may be employed. An algorithm is developed for the calculation by an integral method of heat transfer and drag under conditions of quasi-steady-state pulsating flow. It is demonstrated that the flow rate oscillations superposed on the flow in the pipe enhance the effect of the variability of the properties on heat transfer, and for gas on friction drag. For a dropping liquid under conditions of pulsating flow, the friction drag depends less significantly on the variability of the properties (viscosity) than in the case of steady-state flow. The degree of manifestation of the effects identified above is the higher, the higher the oscillation amplitude and the lower the value of the Reynolds number of averaged flow.     

超重力场下球形炸药水下爆炸实验及数值模拟

常重力条件下,Mach数和Froude数无法同时满足相似。采用离心模型实验,研究水下爆炸冲击波特性及气泡脉动规律,并采用数值模拟方法重现了超重力场下的水下爆炸过程。实验及数值模拟的结果表明,重力的改变基本不影响冲击波的峰值,COLE理论在超重力环境下仍然适用,而超重力场下,小当量炸药可以模拟气泡的脉动迁移过程,其结果可以用于预测大当量炸药深水爆炸的特性。此外,为减少计算时间并提高计算精度,采用将二维冲击波作为初始条件映射到三维模型的建模方法。对于冲击波计算,网格尺寸宜取药包半径的1/20¹/10;对于气泡脉动,宜取药包半径的1/21/10;对于气泡脉动,宜取药包半径的1/2¹。     

Two-phase bubbly flows in microgravity: Some open questions

Several studies on gas-liquid pipe flows in micro gravity have been performed. They were motivated by the technical problems arising in the design of the thermohydraulic loops for the space applications. Most of the studies were focused on the determination of the flow pattern, wall shear stress, heat transfer and phase fraction and provided many empirical correlations. Unfortunately some basic mechanism are not yet well understood in micro gravity. For example the transition from bubbly to slug flow is well predicted by a critical value of the void fraction depending on an Ohnesorge number, but the criteria of transition cannot take into account the pipe length and the bubble size at the pipe inlet. To improve this criteria, a physical model of bubble coalescence in turbulent flow is used to predict the bubble size evolution along the pipe in micro gravity, but it is still limited to bubble smaller than the pipe diameter and should be extended to larger bubbles to predict the transition to slug flow.     

分液管对风侧换热器中流量分配的调节

建立分液管和换热器的数学模型，对空气源热泵风侧换热器在有、无分液管的情况下的各盘管中质量流量分配问题分别进行研究。仿真研究分析分液管能够对流量分配不均匀性起到调节作用的原因，同时定量讨论分液管几何尺寸对流量不均匀性的调节作用。分析结果还表明，为了进一步提高换热器性能，除了加装分液管外还应该对换热器各个盘管面积进行优化。     

超重力场下球形炸药水下爆炸实验及数值模拟

常重力条件下,Mach数和Froude数无法同时满足相似。采用离心模型实验,研究水下爆炸冲击波特性及气泡脉动规律,并采用数值模拟方法重现了超重力场下的水下爆炸过程。实验及数值模拟的结果表明,重力的改变基本不影响冲击波的峰值,COLE理论在超重力环境下仍然适用,而超重力场下,小当量炸药可以模拟气泡的脉动迁移过程,其结果可以用于预测大当量炸药深水爆炸的特性。此外,为减少计算时间并提高计算精度,采用将二维冲击波作为初始条件映射到三维模型的建模方法。对于冲击波计算,网格尺寸宜取药包半径的1/20~1/10;对于气泡脉动,宜取药包半径的1/2~1。     

N-甲基吡咯烷酮热管启动性能和等温性能实验研究

复现锡凝固点的中温热管固定点炉,需要选择合适工质的中温热管。研制了2根以N-甲基吡咯烷酮(NMP)为工质的重力热管(1#,2#),并在同等定温加热条件下对其启动特性和等温特性进行实验研究。结果表明:2#热管启动时间快于1#热管,且启动后稳定性优于1#热管;初步证实NMP热管在170~320℃温度范围内可运行;此外,制作工艺可影响热管的启动特性和等温特性,在相同的运行条件下,热管制作工艺越合理,热管冷凝段上部温度相对越高。     

Experimental study on pressure drop and heat transfer in pipelines for brine based ice slurry. Part I: Operational parameters correlations

Ice slurry flow through horizontal pipes is studied experimentally in order to find out its heat transfer and isothermal friction properties. Using 9% NaCl brine as the carrier fluid, different flow conditions are discussed with a view to analyse each involved variable. In this Part I, experimental data are directly correlated and easy-to-use expressions for the Darcy friction factor and the Nusselt number were obtained as functions of non-dimensional parameters expressing flow and geometrical properties. The most remarkable conclusion obtained is a clear influence of the ice particle–pipe diameter ratio in the pressure drop parameters, not previously taken into account, which must be confirmed in heat transfer process. The thermal and hydraulic performance of ice slurry flowing through corrugated pipes is also analysed showing, with regard to smooth pipe, the same behaviour in pressure drop and opposite behaviour in heat transfer.     

Flow boiling in microgravity condition investigation using inverse techniques

The objective is to provide a method to obtain local heat transfer coefficients in small channels when flow boiling occurs. The experimental device has been developed to perform investigations in parabolic flights campaigns on board A300-ZéroG. Simultaneously flow visualization and thermo-hydraulic measurements are carried out to investigate the two phase flow and heat transfer in minichannels. The experiments are conducted with HFE-7100 in several operating conditions for three hydraulic diameters. The investigations concern flow pattern and the associated heat transfer coefficient during convective for several gravity levels. We mainly on the thermal measurements which consists in inversing experimental temperature measurements (thermocouples) to derive the local surface temperature and heat flux. For the investigated operating conditions, the heat transfer coefficient is found to vary along the flow axis especially at the channel entrance zone.     

板式单环路脉动热管的流动和传热特性

以制冷剂R245fa为工质对常规以及带连通通道的板式单环路脉动热管流动特性进行了可视化研究,并对传热性能进行了测试,考察了充液率和加热功率的影响规律。结果表明:对于常规环路,充液率一定时,随加热功率增加,工作方式依次经历振荡流、有方向改变的循环流、定向循环流,且充液率越高,出现定向循环的加热功率越低;连通通道提供了流动的额外通路,工作方式更具振荡特性,定向循环在高充液率、高功率下才能出现。在合适的充液率下,连通通道不但能降低热阻,而且能提高传热极限。     

三维脉动热管耦合相变材料的传热与节能特性研究

为研究三维脉动热管与相变材料耦合系统的传热与节能特性,本文搭建了三维脉动热管/相变材料耦合模块实验台,使用甲醇、乙醇和丙酮3种工质对充液率为34％、44％的三维脉动热管进行研究,建立了耦合模块的热阻模型,定义了耦合模块的热阻变化率;研究了在变送风温度(3～9 m/s)和速度(20～26℃)条件下耦合模块的热阻变化规律和...     

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.     

Hydrodynamics and heat transfer in pulsating turbulent flow of gas in a heated pipe

The paper deals with the investigation of the effect produced by the dependence of the physical properties on temperature and flow rate fluctuations on heat transfer and drag under conditions of turbulent pipe flow of gas. The method of finite differences is used to solve numerically a set of equations of motion, continuity, and energy written in a narrow channel approximation. A model of turbulence is used which takes into account the effect of the variability of the properties and of the nonstationarity of flow on turbulent transfer. In the particular case of steady-state flow of gas being heated, the calculation results fit well the available experimental data. It is found that the heat transfer depends on the heating rate more significantly than the friction drag. In the case of pulsating flow, the part of hydraulic drag is estimated which is spent for the variation of longitudinal velocity along the pipe and is due to the thermal acceleration of gas. It is demonstrated that the main features of pulsating flow, which were previously investigated for a liquid of constant properties and for a dropping liquid of variable viscosity, are retained for the gas being heated as well. Comparison is made for the gas and dropping liquid of the effect made by various process parameters such as the Reynolds, Stokes, and Prandtl numbers, the heating rate, and the form of thermal condition on the wall on the period average Nusselt number and coefficient of friction drag.     

Effects of Reduced Gravity Conditions on Bubble Dispersion Characteristics in the Bubble Column

Bubble-liquid turbulent flow has an excellent heat and mass transfer behaviors than single gas or liquid flow. In order to analyze the effects of normal and reduced gravity on cold bubble-liquid two-phase turbulent flow in bubble column a second-order moment cold bubble-liquid two-phase turbulent model was developed to disclose the bubble dispersion characteristics. Under the reduced gravity condition, volume fraction caused by the decrease of buoyance force is larger than normal gravity level due to bigger bubble solid volume. In addition, bubble frequency is also decreased by in decrease of buoyance force. Normal and shear stresses have strongly anisotropic characteristics at every directions and have larger values under normal gravity than reduced gravity. The liquid turbulent kinetic energy has the two-peak bimodal distribution and weaker than bubble turbulent kinetic energy with one peak unimodal, which is caused by vigorous wake fluctuations. The correlation of fluctuation velocities between bubble and liquid has clearly anisotropic behaviors Under reduced gravity, the bubble motion has a little impact on liquid turbulent flow caused by slight buoyancy force, however, it will greatly reduce the liquid turbulent intensity due to energy cascade transport, which was transformed into bubbles or dissipated by interface friction. Bubble formation and detachment mechanisms affected by gravity conditions lead to the different levels of bubble dispersion distributions.     

Thermal management with self-rewetting fluids

The present paper describes the results of a series of microgravity experiments on thermal management device, actually wickless heat pipes, with using the so-called “self t-rewetting fluids” (dilute aqueous solutions of high carbon alcohols) as a working fluid. Although most of liquids show a decrease in the surface tension with increasing temperature, self-rewetting fluids show exceptionally an increase in the surface tension with increasing temperature. This particular characteristic allows for a spontaneous liquid supply to hotter interface by the thermocapillary flow. When liquid/vapor phase change takes place, furthermore, additional Marangoni effect due to concentration gradient by the preferential evaporation of alcohol-rich composition in the aqueous solutions is induced. A considerably strong liquid inflow to dry patch or thin film is therefore expected at three-phase interline or liquid/vapor interface. One of the most promising applications of the self-rewetting fluids in space is wickless heat pipes in which condensate spontaneously returns to evaporation region by enhanced Marangoni effect. Demonstrational experiments on the fluid behavior in a transparent glass tube wickless heat pipe were conducted in JAMIC, and spontaneous liquid return velocities were measured. The present authors then performed parabolic flight experiments on heat transfer characteristics of prototype wickless copper heat pipes, and the performance was compared with ordinary heat pipe having wick structure and with other working fluid.