流动沸腾条件下微通道壁面温度的红外特征

设计并搭建了沸腾换热试验台,采用TH5104红外热像仪测量微通道壁面温度来研究混合制冷工质在微通道内的沸腾换热特性.测量试件是一外径为1.22 mm,内径为0.86 mm,长为200 mm的不锈钢单圆管.实验利用红外热像仪测量并记录下质量流量为1 726～8 635 kg/m2·s,热流密度为65～231 kW/m2时壁面温度的变化情况.实验分析和讨论结果显示:微通道壁面的温度分布沿着轴向变化有明显的规律性;水平微尺度通道内流动沸腾过程中,试件前后段有较大的温差效应,温差的正负与热流密度的大小有关;壁面温度的变化与热流密度、管内工质的流型和换热形式关系密切,流型越复杂,壁面温度变化越剧烈.     

棋盘拓扑矩形通道换热器的流动及换热特性研究

选取矩形换热器的最基本换热单元，通过数值模拟方法对不同截面尺寸的矩形通道换热器的流动和换热特性进行了探究。研究发现：在截面高宽比相同的情况下，随着水力直径的增加，流动阻力逐渐减少，传热性能下降，但换热器的综合换热性能提高；当中心矩形截面面积不变的前提下增大截面高宽比，水力直径不断降低，流动阻力随之增大，换热性能与高宽比并非为线性关系，在研究范围内高宽比为4的结构换热效果最佳。     

微通道中液氮的流动沸腾——换热特性分析

对微通道中液氮流动沸腾换热特性进行试验研究和分析。给出典型的沸腾曲线,分析壁温、干度和换热系数沿微通道管程的变化规律,考察热流密度、质量流量和压力对流动沸腾换热的影响。将126个试验数据点与四个换热关联式比较,并对微通道中流动沸腾换热机理进行分析。结果表明,在多数情况下干度和热流密度对沸腾换热系数的影响较小,换热系数主要决定于质量流量和压力,随两者增加而增加,换热以对流蒸发为主导机理。KLIMENKO关联式预测效果最好,TRAN微通道关联式次之,对常规管道得到广泛使用的CHEN关联式和SHAH关联式都远远高估了试验值。基于两相流压降和换热特性分析,推知微通道中的两相流流型不同于常规管道：在低干度情况下,流型以弥散泡状流为主;而在高干度情况下,流型以由雾状汽芯和不规则液膜组成的环状流为主。     

小尺度圆形通道内流动换热特性试验研究

对圆形细通道流动与换热特性进行了试验测量,着重考察了通道几何参数(直径、长度)对其流阻系数和换热系数的影响规律,试验中雷诺数Re范围为500～5000.研究结果表明:通道直径越小,通道内流体的流阻系数越大,换热系数越大;通道长度越长,通道内流体的流阻系数越小.这将为圆形细通道的应用提供参考.     

立式螺旋管汽水两相流动沸腾干涸点试验研究

为研究螺旋管内汽水两相流干涸特性,在较宽的流动参数范围内(系统压力P=2～7 MPa,质量流速G=150～700 kg/(m~2·s),热流密度q=75～350 k W/m~2)对一立式螺旋管(内径15. 26 mm,螺旋直径350 mm,螺旋升角10°)进行了试验,分析了管内流动沸腾传热系数和壁面温度的变化特征,获得了压力、质量流速、热流密度3个流动参数对临界干度的影响规律,并将现有文献中的临界干度预测关系式与试验结果进行了对比,推荐了适用于该工况范围的关系式。     

竖直矩形窄通道内水流动沸腾换热特性的研究

建立单面加热垂直矩形窄通道流动沸腾换热试验装置,针对截面250mm×3.5mm的窄缝通道,对水流动沸腾换热特性进行试验研究。通过试验分析可知:(1)随着干度的增加,局部换热系数先增加后减小,有一个最大值,此时处于饱和核沸腾区域,其蒸汽干度也接近于0,同时也接近于沸腾起始点。相应地流体从单相流-泡状-块状流-搅拌-环状流转变。(2)在流动沸腾换热中,热流密度对核态沸腾换热有明显影响,而对流动沸腾液膜蒸发的影响甚小,所以可以认为由热流密度的变化而引起的换热变化,主要表现在核态沸腾。(3)入口温度的变化对单相流动的换热系数有影响,而沸腾换热系数与流型及汽泡的产生及扰动有极大关系,入口温度对流动沸腾局部换热系数基本没有影响。     

矩形通道内沸腾换热特性及可视化研究

考虑到内燃机缸盖结构的复杂性,其内部冷却通道的换热特性及沸腾状态难以用试验手段进行准确的分析与监测。以简化的矩形流道作为有效的研究对象,针对于流速、压力、温度等流动参数对沸腾换热的影响展开试验研究,并利用高速相机监测不同过热度下的沸腾现象。研究表明:适当的降低流速可以在不影响换热的情况下优化冷却系统的结构尺寸,而提高冷却介质的温度和压力反而会降低沸腾换热效率。气泡的数量、尺寸均可反应出不同的沸腾换热状态,同时气泡的演化行为(生成、滑移、聚合、脱离、消失)可以在冷却介质中造成扰动,从而促进换热的发生。为内燃机缸盖沸腾换热方式的设计提供一定的理论参考。     

微尺度通道内混合物流动沸腾特性研究

对非共沸混合工质R32／R134a(25％／75％)在微尺度管内的流动沸腾换热特性进行了试验研究。试验结果表明,在较高热流密度下,微尺度管内流动沸腾换热与质量干度和质量流量基本无关,热流密度对换热有着很大的影响,在较宽的热流密度范围内,核态沸腾在换热过程中占据主导地位。和细小管道相比,在相同条件下,微尺度管道内的流动沸腾表面传热系数高于细小管道。     

微通道内壁面粗糙度对电渗流影响有限元模拟

Lab-on-a-Chip(芯片实验室)系统迅速发展对微流体的研究提出更高的要求,微通道壁面粗糙度对微尺度下的流体存在显著影响。采用有限元模拟研究了壁面机糙度对圆形截面微通道内电渗流的影响机理。壁面粗糙度几何模型采用三角形波形并考察了微通道两端存在阻碍压力作用下的情况。结果表明,壁面粗糙度宽度和间隔增加时微通道中截面流速先减小后增加。相对粗糙度值在0.01～6%之间时截面流速随着相对粗糙度的增加非线性减小,但减小趋势变缓。相对粗糙度增加时压力与截面流速线的斜率减小,截面流速不易受压力变化的影响。     

非均匀加热条件下微矩形槽道内的滑移流动换热特性

利用正交函数法对定热流密度加热、壁面温度在周向可任意变化条件下,气体在微矩形槽道内的热充分发展滑移流动的换热特性进行理论分析,获得相应条件下的Nu数计算方法及换热特性,并与大尺度槽道的换热特性进行比较,探讨了Kn数、槽道高宽比及不同加热条件对微矩形槽道内滑移流动换热性能的影响。结果表明,在任何加热条件下,微矩形槽道内的平均Nu数均低于相同加热条件下大尺度矩形槽道中的Nu数,且随Kn数的增加而减小。高宽比越小,平均Nu数下降越大。在相同的高宽比和Kn数下,单边加热条件下的换热性能相比相同加热条件的常规大槽道内的换热性能下降最小。     

Effect on boiling heat transfer of horizontal smooth minichannel for R-410A and R-407C

An experimental study of boiling heat transfer with refrigerants R-410A and R-407C is presented. The present paper is focused on pressure drop and boiling heat transfer coefficient of the refrigerants inside a horizontal smooth minichannel. To evaluate the diameter size effect on pressure and heat transfer characteristics, minichannels with inner diameters of 1.5 mm and 3.0 mm and with lengths of 1500 mm and 3000 mm respectively are used. The pressure drop increases with mass flux and heat flux for both inner tube diameters and for both the refrigerants. The pressure drop of R-407C is higher than that of R-410A, but the heat transfer coefficient of R-410A is higher than of R-407C at the low quality region. The heat transfer coefficient in the tube with an inner diameter of 1.5 mm is higher than that of 3.0 mm diameter tube at the low quality region. The comparison of present heat transfer coefficient with the predictions of some previous correlations shows a large deviation. Therefore, there is a necessity to develop a new correlation.     

Two-phase flow heat transfer of propane vaporization in horizontal minichannels

Experiments were performed on the convective boiling heat transfer in horizontal minichannels using propane. The test section was made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm and lengths of 1000 mm and 2000 mm, respectively, and it was uniformly heated by applying an electric current directly to the tubes. Local heat transfer coefficients were obtained for a heat flux range of 5–20 kW m⁻², a mass flux range of 50–400 kg m⁻² s⁻¹, saturation temperatures of 10, 5, and 0°C and quality ranges of up to 1.0. The nucleate boiling heat transfer contribution was predominant, particularly at the low quality region. Decreases in the heat transfer coefficient occurred at a lower vapor quality with a rise of heat flux and mass flux, and with a lower saturation temperature and inner tube diameter. Laminar flow appeared in the minichannel flows. A new boiling heat transfer coefficient correlation that is based on the superposition model for propane was developed with 8.27% mean deviation. This paper was recommended for publication in revised form by Associate Editor Jae Young Lee Jong-Taek Oh received his B.S., M.S. and Ph.D. degrees in Refrigeration Engineering from Pukyong National University, Korea. Dr. Oh is currently a Professor at Department of Refrigeration and Air Conditioning Engineering, Chonnam National University at Yeosu, Korea. Dr. Oh’s research interests are in the area of boiling and condensation heat transfer and pressure drop of refrigerants with small tubes, heat pump and transportation refrigeration.     

微通道中液氮的流动沸腾——两相流动压降分析

对液氮在直径为0.531 mm,加热长度为250mm的圆管中的流动沸腾压降和传热特性进行研究.作为第一部分,主要对微通道中液氮的两相流动压降进行试验研究与分析.结果表明:在核态沸腾起始时,质量流量迅速降低,而压降突然增大,并伴随着明显的温度滞后,幅度约为4.0～5.0 K.由于压降很大,在微通道内液氮的两相流动中会出现闪蒸,从而对质量干度产生重要影响.最后,利用均相模型和三个两相流动模型(L-M模型,Chisholm B系数模型和Friedel模型)对微通道沿程压降进行分析和比较.不同于常规通道的是,均相模型可以很好地预测压降试验结果,而三个两相流动模型的预测偏差较大,这是由于在微小通道中的高速流动情况下,汽相和液相混合比较均匀;同时液氮的液汽密度比很小,这也有利于均相模型的预测.     

Measurement and correlation of boiling heat transfer coefficient of R-1234yf in horizontal small tubes

We report experimental data of boiling heat transfer of R-1234yf in horizontal small tubes. The experimental data obtained in the horizontal circular small tubes of 1.5 and 3.0 mm inner diameter, the lengths of 1000 and 2000 mm, the mass flux range from 200–650 kg/m²s, the heat flux range from 5–40 kW/m² and saturation temperature of 10 and 15°C, was used to develop a modified correlation for the heat transfer coefficient. The flow pattern of the experimental data was mapped and analyzed with existing flow pattern maps. The heat transfer coefficient was also compared with some well-known correlations.     

Cavitating nozzle flows in micro- and minichannels under the effect of turbulence

The cavitation phenomenon inside micro- and minichannel configurations was numerically investigated. The simulations for each channel were performed at different upstream pressures varying from 1 to 15 MPa. Two microchannel configurations with inner diameters of 152 and 254 μm and two minichannel configurations with inner diameters of 504 and 762 μm were simulated. To validate the numerical approach, micro-jet impingement from a microchannel with an inner diameter of 152 μm was first simulated at different Reynolds numbers. Then, the mixture model was used to model the multiphase flow inside the channels. The results of this study present major differences in the cavitating flows between the micro- and miniscale channels and show that the pressure profile and vapor phase distribution exhibit different features. The static pressure drops to negative values (tensile stress) in microchannels, while the minimum static pressure in minichannels is found to be equal to vapor saturation pressure, and higher velocity magnitudes especially at the outlet are visible in the microchannels. It is shown that for higher upstream pressures, the cavitating flow extends over the length of the micro/minichannel, thereby increasing the possibility of collapse at the outlet. The effect of energy associated with turbulence was investigated at high Reynolds numbers for both micro/minichannels and its impact was analyzed using wall shear stress, turbulence kinetic energy and mean velocity at various locations of the micro/minichannels.     

A study on performance analysis of the helically coiled evaporator with circular minichannels

In order to develop a compact evaporator, experiments that show characteristics of evaporating heat transfer and pressure drop in the helically coiled minichannel were performed in our previous research. This study was focused on the performance analysis of helically coiled heat exchangers with circular minichannels with an inner diameter=1.0 mm. The working fluid was R-22, and the properties of R-22 were estimated using the REFPROP program. Numerical simulation was performed to compare results with the experimental results of the helically coiled heat exchanger. As the heat transfer rate and pressure drop were calculated at the micro segment of the branch channels, the performance of the evaporator was evaluated. The following conclusions were obtained through the numerical simulations of the helically coiled heat exchanger. It showed good performance when the flow rate of each branch channels was suitable to heat load of air-side. The numerical simulation value agreed with experimental results within ± 15%. In this study, a numerical simulation program was developed to estimate the performance of a helically coiled evaporator. And, an optimum helically coiled minichannels evaporator was designed.     

内螺旋管两相流沸腾传热和摩擦阻力特性的试验研究

本文阐述国产300MW直流锅炉采用的ψ22×5.5mm四头碳钢内螺纹管高压汽水两相流沸腾传热特性和摩擦阻力特性的试验研究结果。沸腾传热试验参数范围为：压力P＝9.8～22.6MPa,质量流速G＝650～1750kg/m 2·s,内壁热负荷q＝200～610kW/m 2。摩擦阻力试验参数范围为：压力P＝9.8～20.6MPa,质量流速G＝1530～3560kg/m 2·s,蒸汽干度x＝0～1.0。通过试验,得出了内螺纹管的壁温变化规律、发生传热恶化的条件、抑制沸腾传热恶化的效果及两相流摩擦阻力的计算式。还提出了一个反映内螺纹管的结构参数对其传热特怀影响的无因次数,并对几种头数相同的内螺纹管的传热特性进行了比较和评价。