Experimental investigation of evaporative pulse-spray impingement cooling

This paper describes an experimental facility and methods for measuring the aerodynamic and thermal parameters in the interaction between a pulse spray and a vertical surface. The spray-forming unit contains 16 atomizers with opening time regulation (from 1 to 10 msec) and a frequency of up to 50 Hz. The experiments were performed in the regime of evaporation of the liquid precipitated on the surface in the form of separate drops, rivulets, and a continuously flowing sheet. It has been shown that depending on the time parameters of the pulse spray, the integral heat transfer can effectively be controlled over a wide range.     

微槽道表面喷雾冷却的实验研究

本文建立了以蒸馏水为工质的开放式喷雾冷却系统,研究了工质体积流量、槽道宽度、槽道高度对喷雾冷却系统换热性能的影响。结果表明:保持槽道高度为0.8 mm,喷雾流量为0.45 L/min时,随着槽底宽度从4 mm减小至1 mm,传热系数增加了41%;而当喷雾流量为1.25 L/min时,表面传热系数仅增加了8.5%,因此减小槽底宽度对喷雾冷却效果有一定的促进作用,但大流量时并不明显;保持槽底宽度为2 mm,改变槽道高度,当喷雾流量为0.45 L/min时槽道高度对热沉表面的换热影响较大,存在最优槽道高度(0.8 mm),此时热流密度和表面传热系数分别为198.5 W/cm~2、2.75 W/(cm~2·K),与光滑面相比增加了21.25%和30.95%,且存在最低表面温度;而当喷雾流量增至1.25 L/min时,喷雾冷却效果随着槽道高度的增加而持续增加。在以上基础上推导了微槽表面喷雾冷却强化换热机理,得出反映槽道尺寸对换热影响的微槽群表面无量纲准则方程。     

The influence of gas coflow in a pulse aerosol on evaporation cooling process

Experimental data on the influence of the continuous concurrent air flow on the heat exchange coefficient of a pulse spray and vertical flat heat exchanger are presented. It is shown that continuous air flow not only contributes to cooling but significantly effects the cooling efficiency by including in the heat exchange secondary drops, which originate in the process of bombardment of the cooling film by large drops and their return by the air flow to the surface of dry zones of the heat exchanger.     

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.     

Messung der Spraycharakteristik zur Bestimmung des Wärmeübergangskoeffizienten bei der Spraykühlung

The cooling of parts due to spray cooling can be calculated numerically if the heat transfer coefficient is known. This coefficient can be determined according to an empirical approach of Puschmann [1]. Using this method impact density, drop velocity and drop diameter have to be measured. These were measured using patternators and particle dynamics analysis for the application of surface annealing by spray cooling using forging heat Weber numbers were identified to characterize the drop impact as well as the heat transfer coefficient.     

Impact of selected parameters on refrigerant flow boiling heat transfer and pressure drop in minichannels

This paper presents results concerning flow boiling heat transfer in a rectangular minichannel 1 mm deep, 40 mm wide and 360 mm long. The refrigerant flowing in the minichannel, Fluorinert FC-72, was heated by a thin foil microstructured on the side in contact with the fluid. Two types of microstructured surfaces were used: one with evenly distributed microcavities and the other with non-uniformly distributed minicavities. Liquid crystal thermography was applied to determine the temperature of the smooth side of the foil. The paper analyses mainly the impact of the microstructured heating surface and orientation of the minichanel on the heat transfer coefficient and two phase pressure drop. This required calculating the local values of heat transfer coefficient and measuring the pressure drop for different positions of the minichannel with enhanced heating wall. Moreover, the effects of selected thermal and flow parameters (mass flux density and inlet pressure), the geometric parameters, and the type of cooling liquid on the nucleate boiling heat transfer is studied. From the measurement results it is evident that applying a microstructured surface caused an increase in the heat transfer coefficient, which was approximately twice as high as that reported for the smooth surface. The highest values of the coefficient were observed for position 90° (the vertical minichannel) and position 0° (the horizontal minichannel), whereas the lowest were reported for position 180° (the horizontal minichannel). The experimental data concerning the two-phase flow pressure drop was compared with the calculation results obtained by applying nine correlations known from the literature. It is reported that most of the correlations can be used to predict the two-phase flow pressure drop gradient within an acceptable error limit (±30%) only for positions 90° and 135° (the vertical and inclined minichannels, respectively). The lowest agreement between the experimental data and the theoretical predictions was reported for the horizontal positions of the minichannel.     

Heat and mass transfer characteristics in a spray chamber

This paper presents a one-dimensional mathematical model for heat and mass transfer of water droplets in a spray chamber. The model includes drop size distribution and velocity of the droplets generated by a nozzle of inlet diameter 3.2 mm. By using the conservation of mass and energy, the changes in water temperature, air temperature and humidity along the spray cone in the spray chamber can be calculated. This model is tested with two different water mass flows. The results look reasonable from practical point of view and they also show that higher water mass flow results in a higher air temperature drop and higher humidity.     

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.     

The study of entropy generation during flow boiling in a micro-fin tube

Increasing in the heat transfer rate in flow boiling is a common and key issue for engineers. Generally, the heat transfer coefficient augmentation methods are divided into two main categories (active and passive methods). In passive methods the increase in heat transfer rate causes the increase in pressure drop. In order to evaluate the contribution of heat transfer and pressure drop mechanisms, the entropy generation analysis is used. In this paper, the entropy generation in micro-fin tube is investigated under flow boiling condition. The effect of different geometrical parameters and flow conditions is discussed on pressure drop contribution and heat transfer one in entropy generation, irreversibility distribution ratio (IDR) and Bejan number (Be). The frictional pressure drop and heat transfer coefficient in the micro-fin tube and the helically coiled one are compared as two enhancements passive heat transfer methods with the smooth straight tube in the literatures. Therefore, by introducing entropy generation number (N_s), the favorable geometry between the micro-fin tube and the helically coiled one with respect to the smooth straight tube is recognizable at equivalent boundary conditions.     

采用新制冷剂R134a和混合制冷剂R410A的板式蒸发器性能研究

采用分布参数法对波纹型通道板式蒸发器建立数学模型,并进行了数值模拟.通过计算板内局部蒸发传热系数和压降可以简化板式蒸发器内复杂三维网状流动的传热特性.针对应用较广的R134a和R410A制冷剂来比较和分析板式蒸发器在小的温差下的传热性能.在3种不同的计算工况下简要分析了各种热力参数的变化对蒸发器整体传热性能的影响.不同的制冷剂,其传热系数和压降差别较大,相同工况下采用R410A替代R22,板式蒸发器的传热性能可提高8.5%～10.0%,且压降可大幅降低.     

椭圆形套管-管翅式蒸发式冷凝器传热性能实验研究

本文搭建了蒸发式冷凝器性能测试系统,采用控制变量法实验研究了迎面风速、喷淋密度、湿球温度、循环水温度、冷却水流量各参数变化对椭圆形套管-管翅式蒸发式冷凝器传热性能的影响。实验结果表明：该冷凝器实验系统的最佳迎面风速和喷淋密度分别为3.1 m/s和0.005 6 kg/(m·s),冷凝器管外空气压降随迎面风速的增大而迅速增加;随着空气湿球温度升高,冷凝器外传热过程的热流密度(即外热流密度)降低67.5%,而内传热过程的热流密度(即内热流密度)增大47.5%,依靠内传热过程的增强,冷凝器性能良好;随着循环水温度升高,冷凝器的内热流密度降低率高达64.6%,传热性能急剧下降;随着冷却水流量增大,冷凝器的内热流密度大幅提高2.92倍,总热流密度增大21.1%,传热性能显著增强;该冷凝器在低湿球温度、低循环水温度、大冷却水流量的工况下传热性能较优。     

Expansion efficiency of pulse tube in pulse tube refrigerator including shuttle heat transfer effect

This paper describes simple analysis of the pulse tube expansion efficiency. Four dimensionless operating parameters of pulse tube refrigerator are needed to express the enthalpy flow at the cold end of the pulse tube. In this analysis, the expansion efficiency is calculated from the ratio of the diathermic enthalpy flow (non-zero gas-to-wall heat transfer) to the adiabatic enthalpy flow (zero gas-to-wall heat transfer). The analytic procedure is carried out under several simplified assumptions, and the resultant expression is remarkably simple and useful. The optimal design of pulse tube refrigerator can be greatly assisted by the enthalpy flow calculation with four dimensionless parameters introduced in this paper.     

微圆管内R1234ze(E)流动沸腾的环状流特性模拟研究

本文建立了制冷剂R1234ze(E)在微圆管内流动沸腾过程中的环状流模型,对传热和气液两相流动压降进行了模拟研究。综合考虑重力、表面张力及气液界面剪切力的影响,模拟分析了周向液膜不均匀分布特性及该特性对流动与换热的影响,经验证,计算结果与已有实验结果吻合较好。本文还研究了不同因素对环状流区域表面传热系数与压降的影响。模拟结果表明:在流动起始区域,截面液膜厚度的分布受重力作用影响,随着流动沸腾过程的进行,该影响作用开始减弱,且有重力作用时的环状流平均表面传热系数高于无重力作用时的环状流平均表面传热系数,随着重力加速度的增加,环状流的平均表面传热系数不断增大;随着质量流速的增大,表面传热系数与压降均随之增大;随着管径增大,表面传热系数与压降均随之减小。     

水平内螺纹管内R410A流动凝结换热的实验研究

本文实验研究了R410A在水平内螺纹管内的流动凝结换热特性,分析了水力工况、测试管结构参数对管内制冷剂侧表面传热系数、压降的影响。结果表明:表面传热系数、压降均随着质量流速的增加、冷凝温度的降低而增大;虽然表面传热系数随着测试水Re的增加而减小,但测试水Re对压降的影响很小。利用单位压降表面传热系数对换热进行综合性能评价时发现,单位压降表面传热系数随着质量流速的增加而减小,随着冷凝温度的增大而增大。将实验数据与经典关联式的预测值进行对比,对于光滑管,除了Akers et al.关联式低估了实验数据,Shah关联式与Thome et al.关联式均高估了实验数据,并且Thome et al.关联式表现出最高的预测精度。而对于内螺纹强化管,Cavallini et al.关联式展现出最高的预测精度,而Koyama et al.关联式与Miyara et al.关联式均低估了实验数据。     

空气侧结构对多元微通道平行流冷凝器传热流动性能的影响

建立了多元微通道平行流冷凝器的稳态分布参数模型,与实验对比验证了模型的正确性。利用所建立的模型,研究了翅片高度、翅片间距、百叶窗开窗间距、百叶窗开窗角度变化对多元微通道平行流冷凝器传热和流动性能的影响。结果表明,随着翅片高度的增大,换热量逐渐增大,空气侧压降逐渐减小;随着翅片间距或者百叶窗开窗间距的增大,换热量和空气侧压降都是逐渐减小;随着百叶窗开窗角度的增大,换热量和空气侧压降都是逐渐增大。     

Condensation of refrigerants in a multiport tube with rectangular minichannels

This study investigated the condensation heat transfer and pressure drop characteristics of refrigerants R134a, R32, R1234ze(E), and R410A in a horizontal multiport tube with rectangular minichannels, in the mass velocity range of 100–400 kg m⁻² s⁻¹ and saturation temperature set at 40 and 60 °C. The effect of mass velocity, vapor quality, saturation temperature, refrigerant properties, and hydraulic diameter of rectangular channels on condensation characteristics is clarified. A new correlation is proposed for predicting the frictional pressure drop for condensation flow in minichannels. A heat transfer model for condensation heat transfer in rectangular minichannels is developed considering the flow patterns and effects of vapor shear stress and surface tension. Then, based on this model, a new heat transfer correlation is proposed. The proposed correlations successfully predict the experimental frictional pressure drop and heat transfer coefficients of the test refrigerants in horizontal rectangular minichannels.     

不同结构布置空调用翅片管冷凝器性能的仿真研究

在空调用翅片管冷凝器的几何结构尺寸相同,空气的进口状态和流量相同的条件下,采用计算机仿真技术,研究了支路数、管排数对翅片管冷凝器的传热与流动特性的影响,结果表明:随着支路数的增大,压降随之减小,最大值为2个支路时的33.8kPa,最小值为6个支路时,仅为0.9kPa;空气与制冷剂间传热温差增大,总传热系数减小,冷凝器的换热量递减,最大值比最小值大32.1%。随着排数的增多,压降增大,4排管的压降是1排管的4.3倍;空气侧换热系数与制冷剂侧换热系数的变化呈相反趋势,但传热温差增大,换热量也增大,4排管的换热量是1排管的2.45倍。     

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.     

Effect of pulse frequency on heat transfer at the stagnation point of an impinging turbulent jet

Heat transfer in an impact pulsed air jet is numerically studied using the Reynolds stress model. It is shown that both enhancement and suppression in the heat transfer are possible in an impinging pulsed jet as compared with a steady flow. The heat transfer intensifies with the pulse frequency at a stagnation point in the region of small distances between a pipe exit cross section and an obstacle (H/D ≥ 6), while an increase in the pulse frequency causes a decrease in the heat transfer for H/D > 8. An increase in the Reynolds number causes a deintensification of the heat transfer, and the data for all frequencies approach the single-phase flow mode. A comparison with available data by other authors is made, and satisfactory agreement is obtained with respect to the pulse frequency effect on the heat transfer between a gas jet and the impact surface.     

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。