Hybrid turbulence simulation of spray impingement cooling: The effect of vortex motion on turbulent heat flux

Spray impingement has been a major interest of researchers in the areas of spray cooling, internal combustion, fire suppression and spray cooling, etc. for a long time. Numerous studies have been done in the area of spray cooling. Spray cooling with phase change takes advantage of relatively large amounts of latent heat and is capable of removing high heat fluxes from the surface, which has generated the interest of many researchers. In this paper, the turbulent characteristics of vapor formed during the spray impingement are studied. Water and gasoline are used in the numerical analysis of the two‐phase spray impingement on a heated wall. Hybrid turbulence modeling was used for the analysis where the subgrid scale model was employed away from the wall and k–ε model was used near the wall. Gasoline, at 298 K, was sprayed on the heated wall, kept constant at 650 K. The surrounding temperature was maintained at 400 K at the start of the simulation. In case of water and gasoline at Reynolds number 2750, the heated wall was kept constant at 400 K and the surrounding temperature was maintained at 298 K at the start of the simulations. The nozzle diameter of 100µm was used for this study, with the nozzle plate spacing ratio at 10. The spray was impinged on the flat plate at angles of 0, 15, and 30^°. Root mean‐squared velocities and turbulent heat flux were plotted in the water spray impingement for the different angles of impingement. The effect of turbulence on the heat transfer was observed. The effect of vortex motion on the turbulent heat flux values was analyzed using different Reynolds numbers of impingement and at different angles in case of gasoline. The turbulent heat flux attained the maximum values with high vortex formation. Upwash of fluid transported heat away from the wall, producing higher heat flux values in the region. Copyright © 2008 John Wiley & Sons, Ltd.     

Numerical simulations of droplet train and free surface jet impingement

The cooling behavior of the impingement of a droplet train, and free surface jets over a heated and pre-wetted surface is explored employing an Algebraic Volume-of-Fluid methodology. The code is based on a modified version of the two-phase numerical solver interFoam (OpenFOAM) (Trujillo and Lewis, 2012). Two versions of the free surface jet are studied. The first consists of a fully-developed profile exiting the nozzle, and the second is characterized by a uniform velocity distribution. Results show that both jet configurations have higher cooling performance than the droplet train locally and globally, with the fully-developed case being the most effective of the two jet arrangements. Locally, the performance is measured by radial profiles of the boundary-layer-displacement thickness and heat transfer coefficient. Globally, the cooling effectiveness is directly proportional to the surface area that resides within the high-convection region, i.e. before the boundary layer separation point. On a temporal basis, the liquid film within the impingement region of the droplet train exhibits pronounced variations in velocity magnitude and film thickness. This is directly attributed to the nature of continuous droplet impacts affecting the impingement region, and gives rise to an unsteady cooling and heating of the fluid near the wall. In contrast for the jets, the film and the corresponding free surface are nearly steady with only minor perturbations.     

Study on heat transfer performance of spray cooling: model and analysis

In consideration of droplet–film impaction, film formation, film motion, bubble boiling (both wall nucleation bubbles and secondary nucleation bubbles), droplet–bubble interaction, bulk air convection and radiation, a model to predict the heat and mass transfer in spray cooling was presented in this paper. The droplet–film impaction was modeled based on an empirical correlation related with droplet Weber number. The film formation, film motion, bubble growth, and bubble motion were modeled based on dynamics fundamentals. The model was validated by the experimental results provided in this paper, and a favorable comparison was demonstrated with a deviation below 10%. The film thickness, film velocity, and non-uniform surface temperature distribution were obtained numerically, and then analyzed. A parameters sensitivity analysis was made to obtain the influence of spray angle, surface heat flux density, and spray flow rate on the surface temperature distribution, respectively. It can be concluded that the heat transfer induced by droplet–film impaction and film-surface convection is dominant in spray cooling under conditions that the heated surface is not superheated. However, the effect of boiling bubbles increases rapidly while the heated surface becomes superheated.     

Two-phase structure above hot surfaces in jet impingement boiling

Jet impingement boiling is very efficient in cooling of hot surfaces as a part of the impinging liquid evaporates. Several studies have been carried out to measure and correlate the heat transfer to impinging jets as a function of global parameters such as jet subcooling, jet velocity, nozzle size and distance to the surface, etc. If physically based mechanistic models are to be developed, studies on the fundamentals of two-phase dynamics near the hot surface are required. In the present study the vapor–liquid structures underneath a subcooled (20 K) planar (1 mm × 9 mm) water jet, impinging the heated plate vertically with a velocity of 0.4 m/s, were analyzed by means of a miniaturized optical probe. It has a tip diameter of app. 1.5 μm and is moved toward the plate by a micrometer device. The temperature controlled experimental technique enabled steady-state experiments in all boiling regimes. The optical probe data provides information about the void fraction, the contact frequencies and the distribution of the vapor and liquid contact times as a function of the distance to the surface. The measured contact frequencies range from 40 Hz at the onset of nucleate boiling to nearly 20,000 Hz at the end of the transition boiling regime. Due to condensation in the subcooled jet vapor disappears at a distance to the surface of app. 1.2 mm in nucleate boiling. This vapor layer becomes smaller with increasing wall superheat. In film boiling a vapor film thickness of 8 ± 2 μm was found.     

Heat transfer during transient cooling of high temperature surface with an impinging jet

 An experimental study of transient boiling heat transfer during a cooling of a hot cylindrical block with an impinging water jet has been made at atmospheric pressure. The experimental data were taken for the following conditions: a degree of subcooling of ΔT _sub = 20–80 K, a jet velocity of u _j  = 5–15 m/s, a nozzle diameter of d _j  = 2 mm and three materials of copper, brass and carbon steel. The block was initially and uniformly heated to about 250 °C and the transient temperatures in the block were measured at eight locations in r-direction at two different depths from the surface during the cooling of hot block. The surface heat flux distribution with time was evaluated using a numerical analysis of 2-D heat conduction. Behavior of the wetting front, which is extending the nucleate boiling region outward, is observed with a high-speed video camera. A position of wetting region is measured and it is correlated well with a power function of time. The changes in estimated heat flux and temperature were compared with the position of wetting region to clarify the effects of subcooling, jet velocity and thermal properties of block on the transient cooling. Received on 17 March 2000     

平面液体层碎裂过程实验研究

小宽厚比喷嘴喷射出的平面水膜进入静止空气中,在不同气流流速环境下对水膜碎裂过程进行了实验研究。结果表明,静止空气中的水膜表面波呈现对称波形,射流的碎裂长度随雷诺数的增大而增大,喷射压力对射流碎裂长度没有直接影响。空气助力作用使平面射流表面波的上、下气液交界面出现相位差。水膜的碎裂长度随空气助力气流速度的增大而减小;空气助力对于低雷诺数水膜射流具有很强的促进碎裂作用,所以会极大地改善低雷诺数射流的一次雾化效果。随着水流雷诺数的提高,空气助力作用对水膜碎裂长度的影响大为减弱;即使在高速助力空气的作用下,水膜仍长期保持较稳定的射流流态,没有出现明显的水膜撕裂现象。说明在小宽厚比喷嘴的瑞利(Rayleigh)模式射流中,高雷诺数射流是水膜的稳定因素。与气液流速比、气流马赫数等无量纲参数相比,液体喷射的雷诺数是射流碎裂的主要影响因素。     

Heat transfer characteristics of an annular turbulent impinging jet with a confined wall measured by thermosensitive liquid crystal

The present paper describes the heat transfer characteristics of an annular turbulent impinging jet with a confined wall. The local temperature distribution on the impingement surface was measured using a thermosensitive liquid crystal sheet and an image processor. The net heat flux was evaluated by considering the heat conduction in the heated substrate and the thermal radiation between an upper confining insulated wall and an impingement surface. Distributions of the temperature and Nusselt number on the impingement surface were captured in two-dimensional maps. Effects of the diameter ratio of the annular nozzle, the space between nozzle and impingement surface and the Reynolds number on radial distributions of the local Nusselt number were examined. Experimental formulas of the local Nusselt number were obtained in power-law expressions of r/r_p for the major and minor flow regions.     

Experimental investigation of parameters effect on heat transfer of spray cooling

The effects of spray height, nozzle spray angle, inlet pressure and spray incident angle on heat transfer of spray cooling were studied by an experimental method. Multi-points thermocouples and infrared imaging device were used to measure temperature distribution on heating surface. A Doppler anemometry and a camera were applied to study the spray flow field. The mechanism of heat transfer of spray cooling was concluded on the basis of experimental data and spray characteristics. It is showed that parameters affect heat transfer by way of changing the flow field on the heating surface. Heat transfer performance can be optimized by a smaller spray angle nozzle, which sprays at a smaller spray height and a higher inlet pressure. The effect of incident angle on heat transfer depends on nozzle spray angle and the definition of distance of nozzle to surface.     

Fluctuating flow in a liquid layer and secondary spray created by an impacting spray

A spray impacting onto a wall produces a flow of secondary droplets. For relatively sparse spray these secondary droplets are produced by the splashing of the impacting drops and their interactions. For dense sprays, like Diesel injection sprays, these secondary droplets are created by the fluctuating liquid film created on the wall. In the present paper hydrodynamic models are presented for these two extreme cases. The velocities of the secondary droplets produced by the crown splash in a sparse spray are described theoretically. Next, the fluctuations in the motion of the liquid film created by a dense impacting spray are analyzed statistically. This motion yields the formation of finger-like jets, as observed in experiments of a Diesel spray impacting onto a rigid wall. The characteristic size and velocity of the film fluctuations are estimated. These two theoretical models are validated by comparison with the experimental data.     

Investigation of two plane paralleltiinven ilated jets

Measurements of mean velocity components, mean flow direction, turbulent intensities and Reynolds shear stress were made with a split film probe of hot wire anemometer to investigate the flow field generated by two identical jets of air issuing from plane parallel nozzles in a common end wall and mixing with the ambient room air. Due to the sensitivity of the split film probe to the flow direction, the reverse flow in the converging region was detected by the split film probe and observed by flow visualization. The mean velocity approaches self-preservation in both the converging and the combined regions, while the turbulent intensities and Reynolds shear stress approach self-preservation in the combined region only. The trajectory of the maximum velocity is almost unchanged by variance of nozzle spacing in the converging region. The distance of the merging point from the nozzle exit increases linearly with nozzle spacing. The spread of the converging jet increases more rapidly than that of the combined jet.     

The effects of countercurrent single and two-phase flows on the quenching rate of hot surfaces

A theoretical and experimental study of liquid film flow and quench front propagation on hot vertical surfaces in the presence of single and two-phase upflows is reported. Experiments in which a water jet was used to cool a single rod set in a transparent tube showed that the downward progress of a quench front was significantly retarded or even stopped by an air or steam upflow. Climbing quench fronts were observed which sometimes coexisted with the falling quench fronts, but at surface temperatures above 900°C and flow rates below 0.9 g/s/cm both quench fronts became stationary. Conventional flooding correlations have been shown to overestimate the upflow necessary to cause flow reversal particularly at high surface temperatures. The analysis of film flow shows that a simple interfacial shear model, although incapable of describing the observed wave dynamics, can be used to correlate trends in the available data on rewetting and downcomer penetration. A preliminary theoretical examination of the wave dynamics at the onset of flow reversal is presented based on the Jeffreys theory of growth of finite-amplitude waves.     

Wall jet similarity of impinging planar underexpanded jets

Velocity profiles and wall shear stress values in the wall jet region of planar underexpanded impinging jets are parameterized based on nozzle parameters (stand-off height, jet hydraulic diameter, and nozzle pressure ratio). Computational fluid dynamics is used to calculate the velocity fields of impinging jets with height-to-diameter ratios in the range of 15–30 and nozzle pressure ratio in the range of 1.2–3.0. The wall jet has an incomplete self-similar profile with a typical triple-layer structure as in traditional wall jets. The effects of compressibility are found to be insignificant for wall jets with Ma < 0.8. Wall jet analysis yielded power-law relationships with source dependent coefficients describing maximum velocity, friction velocity, and wall distances for maximum and half-maximum velocities. Source dependency is determined using the conjugate gradient method. These power-law relationships can be used for mapping wall shear stress as a function of nozzle parameters.     

Rewetting of a solid cylinder with precursory cooling

A model for the rewetting of a solid cylinder is solved by the Wiener-Hopf technique. A constant heat transfer coefficient is assumed in the wetted part of the cylindrical rod, whereas an exponentially decaying heat flux is assumed in that part of the solid which is cooled by a mixture of vapor and liquid droplets (the precursory cooling). Accurate predictions of the rewetting velocity are obtained for a wide range of model parameters. The results of the present solution are found to compare favorably with a separation of variables solution obtained by Olek (1987). Values of the rewetting velocity where precursory cooling is neglected are recovered as a particular case. The decomposition in the form of infinite products presented here is shown to yield results which agree well with those derived from a decomposition by Evans (1984), employing the Cauchy theorem.Work done at Paul-Scherrer Institute PSI (formerly EIR), CH-5303 Würenlingen, Switzerland.     

Flow characteristics of a large-size pressure-atomized spray using DTV

The purpose of this study is to characterize the atomization of a jet of water sprayed into the air at high velocity through a commercial nozzle widely used for sprinkler irrigation. The typical diameter of the droplets present in the spray is in the range of several tens of micrometers to several millimeters. They are visualized by ombroscopy. A specific Droplet Tracking Velocimetry (DTV) technique is developed to estimate the size and velocity of these highly polydispersed droplets that are distinctly non spherical. This analysis is performed from the rupture of the liquid core region (about a distance of 550 nozzle diameters) to the dispersed zone (about a distance of 900 nozzle diameters). With this technique, we obtain joint size-velocity measurements that are rarely produced. Especially two velocity components and also a large diameter range are characterized at the same time; while with other techniques, such as Particle Doppler Anemometry (PDA), the diameter range is quite reduced and requires specific settings. Additional measurements of the liquid volume fraction are performed using a single mode fiber-optic probe. In the light of our experimental data, it appears that the turbulent droplet motion in the spray is strongly anisotropic. This anisotropy is quite unexpected because other studies on sprays (generally concerned with engine applications) show a relatively low anisotropy. We attribute this increase of anisotropy to the fact that, for this type of spray, the droplet relaxation time is long in comparison to the characteristic time of the turbulence and that biggest droplets are still submitted to atomization process. This strong anisotropy is responsible for the poor radial dispersion of the spray.     

Ultra fast cooling of hot steel plate by air atomized spray with salt solution

In the present study, the applicability of air atomized spray with the salt added water has been studied for ultra fast cooling (UFC) of a 6 mm thick AISI-304 hot steel plate. The investigation includes the effect of salt (NaCl and MgSO₄) concentration and spray mass flux on the cooling rate. The initial temperature of the steel plate before the commencement of cooling is kept at 900 °C or above, which is usually observed as the “finish rolling temperature” in the hot strip mill of a steel plant. The heat transfer analysis shows that air atomized spray with the MgSO₄ salt produces 1.5 times higher cooling rate than atomized spray with the pure water, whereas air atomized spray with NaCl produces only 1.2 times higher cooling rate. In transition boiling regime, the salt deposition occurs which causes enhancement in heat transfer rate by conduction. Moreover, surface tension is the governing parameter behind the vapour film instability and this length scale increases with increase in surface tension of coolant. Overall, the achieved cooling rates produced by both types of salt added air atomized spray are found to be in the UFC regime.     

基于Fluent的超高压撞击流乳化喷嘴的优化分析

高压液体通过喷嘴加速,形成高速射流,与相反方向的另一股射流相互撞击,发生强烈的相互作用,产生强烈的径向和轴向湍流速度分量以及狭窄的高压高速湍流区,在此区域内,相间或液滴间的碰撞互磨产生的挤压力和剪切力使流体被细化。本文从液体连续相撞击流的两个特征：微观混合和压力波动入手,逐一分析了撞击速度与微观混合、压力波动的关系,得出了压力波动与撞击流速度乱U0成正比关系,微观混合与U^3 0成正比的规律。同时,用流体模拟软件Fluent对喷嘴的结构和尺寸进行优化,并得出最合理的喷嘴结构和尺寸。模拟认为：在相同压力下,采用矩形槽,出口孔径为0．2mm,槽的深度为0．27mm的结构时撞击速度达到最大,并通过实验验证了这一结论。     

高超声速边界层液膜演化过程和冷却机理研究

高超声速液膜冷却技术是通过一系列狭缝或孔洞压出冷却工质,在飞行器表面边界层形成一层低温冷却膜,阻止高超声速气流对飞行器的气动加热.其作为一种主动冷却方式在高超声速飞行器表面热防护有着巨大的应用潜力.文章采用数值方法,结合VOF模型,研究25 km飞行高度和Ma=5气流条件下的液膜铺展情况,并通过不同冷却工质的入射速度、角度、表面张力和黏性系数条件,讨论了液膜在平板上的演化过程和冷却机理.结果表明,在气流作用下,液膜向壁面下游发展,液膜的存在导致边界层分离,连续液膜会在一定位置断裂为液块,然后进一步破碎为液滴.入射条件和液体性质的改变,会影响液膜沿流向的发展,具体表现在连续液膜断裂点的位置和连续液膜的厚度.在所设定的计算域内,壁面热流降低了80%～95%,液膜对壁面的冷却效率随着液膜形态的变化而变化.     

Dynamic pressure based prediction of spray cooling heat transfer coefficients

An important goal of spray cooling research is the ability to predict local heat transfer from the spray hydrodynamics. It is postulated that the local normal pressure exerted by the spray onto the heated surface can be used to obtain the local heat transfer coefficient. This hypothesis was tested using data obtained from hollow cone, full cone, and linear sprays at four nozzle pressures and three stand-off distances. A correlation between the pressure and heat transfer coefficient was determined from the data, then used to “predict” the heat transfer coefficient to verify the accuracy of the correlation. The area averaged heat transfer coefficient could be predicted within 25%, indicating that pressure can be used to predict the local heat transfer coefficient in the single-phase regime.