微尺度水平单管内流动沸腾特性实验研究

对非共沸混合工质R134a/R32(75/25)在水平微尺度管道内的流动沸腾换热实验结果进行了分析和讨论,以探究微细通道内流动沸腾换热的主导机制。对影响其换热的多种因素(热流密度、质量流量和质量干度)进行了分析,实验得出,当质量干度较低时,热流密度和质量流量共同控制着微尺度管内的换热方式,当热流密度的影响占主导地位时,管道内的换热以核态沸腾为主;当质量流量的影响占主导地位时,管道内的换热以强制对流为主。     

微尺度水平管内沸腾换热特性研究

对不同质量分数下非共沸混合工质(R134a/R32)在微尺度管道内的流动沸腾换热特性进行了比较和分析,阐述了热流密度、质量流量和质量干度对换热的影响。结果表明:热流密度对换热的影响随着质量流量的增加而愈加明显;在质量分数为75%/25%和65%/35%时,换热系数随着质量流量的增大而增大;而质量分数为85%/15%时,换热系数随质量流量的变化先增加后减小;随着质量干度的增加,换热系数在各质量分数下基本上都呈上升趋势。     

R290/R134a在水平微肋管中沸腾传热的数值模拟

为探究混合制冷剂R290/R134a(4/6)在水平微肋管中的沸腾传热特性,采用了CFD软件数值模拟的方法,对混合制冷剂R290/R134a(4/6)分别在外径为7 mm,长为500 mm的水平光滑管和微肋管中进行数值模拟与理论分析。分析了质量流量、热流密度,以及干度对混合制冷剂在水平微肋管中换热特性的影响。结果表明：两种管型的沸腾换热系数随质量流量、热流密度和干度的增大出现先增大后减小的趋势;热流密度对制冷剂沸腾换热系数的影响最大,在质量流量保持不变,改变热流密度的条件下,微肋管最大传热系数分别为光滑管的1.30、1.31、1.26倍;质量流量的增加提高了制冷剂的临界干度,光滑管与微肋管最大临界干度分别为0.57、0.63。     

微细通道内非共沸工质流动沸腾换热实验研究

本文实验研究了非共沸混合制冷剂R134a/R245fa(质量比为0.7/0.3)在长度为45 mm、65 mm的平行微通道内的流动沸腾特性。微通道由30个截面为0.5 mm×0.5 mm矩形微通道组成。得到了质量流速在542.22～995.56kg·m~(-2)·s~(-1)、热流密度在4.9～100.2 W·cm~(-2)时的流动沸腾传热系数,并对R134a/R245fa和R134a的换热系数进行对比,结果表明热流密度、质量流速、通道长度对换热系数均有影响。考虑R134a/R245fa相变时的附加传质阻力对换热的影响下,在纯工质关联式中引入混合物影响因子,得到了适用于本实验工况下R134a/R245fa的关联式。     

R410A与R22在水平微翅管内流动沸腾传热特性研究

建立了水平管流动沸腾试验台,采用恒热流加热方法,对 R410A 在水平微翅管内流动沸腾特性进行了实验研究,分析了影响 R410A 在水平微翅管内换热系数的因素,考察了工质质量流量、热流密度、质量干度以及微翅管的几何参数对工质的流动沸腾换热性能的影响关系.通过对比 R410A 与 R22 的实验数据,分析比较二者的换热系数,结果表明R410A 与 R22 相差不大,R22 比 R410A 的换热系数大约高 7.5%.     

R134a/R245fa非共沸混合工质流动沸腾液膜蒸发特性研究

本文针对水平矩形通道内非共沸混合工质的流动沸腾分层流状态,同时考虑靠近其气液界面处气相与液相浓度边界层的存在,对液相浓度边界层的传质系数进行了修正,构建了对应的流动沸腾液膜蒸发模型,以R134a/R245fa混合工质为研究对象,探讨了不同入口组分质量、质量流速及热流密度等条件下液膜蒸发过程的热质传递规律,以气液相浓度边界层内的组分质量差作为气液相传质阻力的表征,界面温度和主流饱和温度之差为传热阻力的表征,深入分析了传热阻力、传质阻力与混合工质传热特性之间的内在联系.     

微通道过冷沸腾实验研究

设计并搭建了以R134a为工质的微通道散热及可视化实验台。对R134a在不同饱和温度、流速、热流密度条件下流经微通道产生过冷沸腾状态时的换热性能进行了实验研究。研究了壁面过热度,质量流速对热沉换热系数和压降的影响,通过结果分析,将整得换热区域分为单相对流、过冷沸腾、饱和沸腾三个区域。并采用基于高速摄像机的可视化技术对微通道内气泡运动状态进行了分析。     

R32在水平细管内的流动沸腾实验研究

本文对R32在水平管内的流动沸腾进行了实验研究。实验测试段为内径2 mm的水平光滑不锈钢管,实验的蒸发温度为15℃,流量密度为100 kg/(m~2·s),热流密度为6～24 kW/m~2。通过试验获得R32的流动沸腾换热系数,同时与R134a和HFO1234yf进行比较。结果发现R32的传热系数是HFO1234yf换热系数的1～2倍。同时利用现有的公式对R32和HFO1234yf的换热系数进行了预测,发现这些公式还存在欠缺,需要进一步的改进。     

R11/R134a混合工质管束外沸腾传热的EHD强化试验研究

采用EHD强化技术对R11/R134a混合工质进行管束外沸腾传热的试验研究,以试验所获得的大量数据为基础,分析了电场电压、热流密度与换热系数、强化系数之间的关系,并重点分析了不同工质组分对EHD强化效果的影响,为探索EHD强化沸腾换热的机理以及将其推广到工程应用提供了一定依据。     

矩形微槽道内R134a流动沸腾换热特性的实验研究

本文主要研究了制冷剂R134a.在水平矩形(截面为1 mm×1 mm)微槽道内的流动沸腾换热特性。通过可视化手段观察到流动沸腾过程中的流型变化。同时得到了质量流速在60~1100 kg/(m~2s)、热流密度在33~120 kW/m~2时的流动沸腾换热系数,并对R134a的沸腾曲线作了讨论。通过可视化结果,发现了从泡状流到干涸流的7种流型。换热系数随着热流密度的增加而增加,干涸流的出现会导致换热系数迅速减小。核态沸腾传热在受限气泡到弹状流阶段得到增强。在搅混-环状流到环状流阶段,R134a的传热系数稳定在一个较高的值。此外,质量流速越大,CHF值越高。     

Evaporative Heat Transfer Characteristics of R404a and R134a under Varied Heat Flux Conditions

The two-phase heat transfer coefficients of R404A and R134a in a smooth tube of 7.49-mm inner diameter were experimentally investigated at low heat and mass flux conditions. The test section is a 10-m-long counter-flow horizontal double-tube heat exchanger with refrigerant flow inside the tube and hot fluid in the annulus. The heat transfer coefficients along the length of the test section were measured experimentally under varied heat flux conditions between 4 and 18 kW m^?2 and mass flux ranging between 57 and 102 kg m^?2 s^?1 (2.5 to 4.5 g s^?1) for saturation temperatures of ?10°C, ?5°C, and 0°C. The saturation temperatures correspond to pressures of 4.4, 5.2, and 6.1 bar for R404A and 2.0, 2.4, and 3.0 bar for R134a, respectively. The results showed that under the tested conditions, the contribution of the nucleate boiling mechanism is predominant in the heat transfer coefficient throughout the flow boiling process. The Kattan–Thome–Favrat flow pattern maps confirm the occurrence of stratified and stratified-wavy flow patterns for all of the tested conditions. The average heat transfer coefficient of R404A is estimated to be 26 to 30% higher than that of R134a for the same saturation temperature.     

非共沸工质R134a /R245fa的管内冷凝换热特性研究

为研究非共沸工质的冷凝换热特性,本文基于Nusselt理论,建立了竖直圆管内非共沸混合蒸汽的冷凝模型,研究了不同质量比例的R134a/R245fa在不同条件下的冷凝换热特性,结果表明:混合蒸汽质量比例不同,两种组分的露点温度不同,混合物的冷凝特性不同,低沸点组分的气-液相份额差是表征传质阻力的关键因素;混合蒸汽质量比例、质量流速、壁面温度、压力是影响非共沸混合工质冷凝换热的重要因素。     

泡沫金属对含油制冷剂管内流动沸腾换热特性的影响

实验研究了制冷剂-润滑油混合流体在内嵌泡沫金属圆管内流动沸腾的换热特性。泡沫金属为10ppi、90%孔隙率;制冷剂为R410A,润滑油为VG68,油浓度为0～5%。实验结果表明:纯制冷剂工况下,泡沫金属强化流动沸腾换热系数,换热系数提高30%～120%;含油工况下,泡沫金属只强化流动沸腾换热系数20%以下,在低质流密度或者高质流密度的高干度情况下出现恶化换热的情况。润滑油总是恶化制冷剂在内嵌泡沫金属圆管内流动沸腾的换热系数,换热系数最多恶化71%,且在低质流密度下对换热的恶化比在高质流密度工况下严重。     

Non-Uniform Onset of Nucleate Flow Boiling of R-134a Inside a Glass Minichannel

The present work is an experimental investigation of the incipient boiling of R134a inside a circular glass minichannel mounted horizontally and equipped with a series of transparent indium tin oxide heaters. The effects of heat flux input levels and refrigerant mass fluxes on the onset nucleate boiling process and on the saturated boiling heat transfer rate are quantitatively explored. The flow pattern visualizations, carried on by means of a high-speed camera, show that the nucleation process is oddly non-uniform: the first vapor bubbles are always generated on the upper side of the tube and lead to a first wall temperature drop. A further increase in the heat flux values results in an increased wall superheat until bubble nucleation also originates on the lower side of the tube, causing a second wall temperature drop. Finally, at higher heat input levels, the boiling process becomes uniformly distributed on the inner tube surface. This phenomenon occurred also after a 180° rotation of the glass tube, and, after a critical analysis of the potential origins, it remains presently unexplained. An evaluation of heat transfer coefficients for low vapor quality regimes is finally presented.     

Influence of nucleation on the flow boiling heat transfer coefficient of a refrigerant mixture under varied heat flux conditions

The influence of nucleation on the flow boiling heat transfer coefficient of R-134a/R-290/R-600a refrigerant mixture is experimentally studied in a smooth horizontal tube of 12.7 mm diameter. The heat transfer coefficients are experimentally measured for stratified flow patterns under a varied heat flux condition; a condition found in the evaporator of refrigerators and deep freezers. The experiments are conducted in a counter-current heat exchanger test section. By regulating the flow rate and inlet temperature of acetone, which is the heating fluid flowing in the outer tube, a varied heat flux is provided to the refrigerant flowing in the inner tube. The refrigerant mass flow rate is fixed between 3 and 5 g s⁻¹ and its inlet temperature between −8.59 and 5.33°C, which corresponds to a pressure of 3.2 to 5 bar. The significance of nucleate boiling prevailing in the above-mentioned evaporators is highlighted. The experimental heat transfer coefficients are also compared with well known heat transfer correlations.     

三维微肋螺旋管内流动沸腾流型与传热性能

采用三维微肋螺旋管进行了制冷剂R134a在管内的流动沸腾传热与流型可视化实验。随着流量和干度的变化,流型可划分为泡状流、塞状流、分层波状流、间歇流以及环状流。在Taitel-Dukler流型图上给出了流型的分区及其转变曲线,讨论了螺旋管内两相流动流型转变的特性。传热实验揭示了质量流量、热流密度及蒸汽干度对传热性能的影响,三维微肋螺旋管的强化因子为1.5-2.1。     

超临界R134a水平圆管内冷却换热模拟研究

采用增强壁面函数的标准k-ε模型对超临界R134a水平圆管内冷却换热进行了模拟研究.分析了管内不同截面上流体温度、速度和湍动能的分布情况及对应关系。研究了质量流量和浮升力对换热系数的影响。结果表明,流体速度随着温度的降低而减小,并且最大速度处对应着最高温度和最小湍动能.换热系数随着质量流量的增加而增大,其峰值出现在准临界温度附近。浮升力在似液体区的影响较大,对流体换热起到增强的效果。