Microchannel Condensation Heat Transfer

在英国，大型食品零售商大约50%的电力消耗是用于冷却冷藏和冷冻食品柜。他们每年的总电力消耗几乎是10太瓦时。早在1994年，英国就有10%的建筑面积安装了空调。在过去10年建造的建筑物中，这一数字现在超过了25%。冷凝器是蒸汽压缩制冷空调装置的关键部件。设计良好的微通道冷凝器的广泛实施将导致电力需求减少约20%。在英国，这代表着化石燃料消耗和二氧化碳排放的显著减少。改进后的设计也将大大缩小，具有更小的流体库存，并可能降低资本成本。由于表面张力的影响，现有的较大沟道的设计方法在沟道尺寸约1mm时失效。在EPSRC支持的研究项目下，玛丽皇后大学（QM）开发了一个完整的理论模型，适用于微通道中任何流体的冷凝。现有的实验数据在数量和可靠性方面都不足，并且没有涵盖足够广泛的流体性质，以验证和扩展理论。在大多数蒸汽侧研究中，传热系数是从总体冷却剂到蒸汽的测量中推导出来的，因此精度相对较低。代表R134a测量值的四个相关性在该流体中约为30%，而在氨中则相差约4倍。现有的相关性显然不能正确地反映流体性质的影响。QM理论模型不依赖于实验数据。它包括由于表面张力、剪切应力和重力效应引起的横向流动，并已用于生成各种流体、通道形状和尺寸、蒸汽流速和通道倾角的结果。预测结果落在相关性给出的结果范围内。7种流体的理论结果已在会议论文集（9篇论文）和档案期刊（5篇论文）上广泛传播。两个无量纲参数之间的一个简单代数关系最近被开发出来，它非常准确地预测了不同通道截面、流体、蒸汽流速和温差下表面张力主导的流动状态的数值结果。对于表面张力占主导的上游和下游的流态，将得到类似的简化结果，从而为整个冷凝器的设计提供易于使用的公式。一项创新技术将用于测量微通道中的局部温度和热通量。设计了一种新的装置，其中平行微通道（1.5 mm x 1.0 mm）穿过铜块（30 mm x 40 mm x 500 mm长）的中心，在98个精确已知的位置（0.3 mm以内）非常精确地测量温度（在0.05 K以内）。观测结果将用于传导方程的反解，以确定局部蒸汽侧表面温度（0.1 K以内）和热通量（5%以内）。流动可视化研究也将进行，其中测试块的上部由玻璃代替，以便可以使用高速摄影观察通道中的流动。两种流体（蒸汽和FC72）具有非常不同的热物理性质，特别是表面张力，将被使用。这些数据将为目前的理论提供严格的检验，并有助于将其扩展到其他流动型态。最终模型应适用于任何流体、通道几何形状、蒸汽流速和汽壁温差。这将有助于新一代大型制冷和空调设备的设计和优化。

Around 50% of the electricity consumption of large food retailers in the UK is for cooling chilled and frozen food cabinets. Their total annual electrical energy consumption is almost 10 TWh. As early as 1994, 10% of floor space in the UK was serviced by air-conditioning. The figure is now more than 25% for buildings constructed during the past 10 years. The condenser is a key component of vapour compression refrigeration and air conditioning plant. Wide implementation of well-designed microchannel condensers would lead to a reduction of around 20% in power requirement. In the UK this represents a significant decrease in fossil fuel consumption and carbon dioxide emissions. Improved designs would also be significantly smaller, have smaller fluid inventories and possibly lower capital cost. Established design methods for larger channels fail for channel dimensions around 1 mm owing to surface tension effects. A wholly-theoretical model, valid for condensation of any fluid in microchannels, has been developed at Queen Mary (QM) under research programmes supported by EPSRC. Available experimental data are insufficient in number and reliability, and do not cover a sufficiently wide range of fluid properties, to validate and extend the theory. In most investigations vapour-side, heat-transfer coefficients are deduced from overall coolant-to-vapour measurements and are consequently of relatively low accuracy. Four correlations representing measurements for R134a agree to within about 30% for this fluid but differ by a factor of around 4 for ammonia. Existing correlations evidently do not correctly capture fluid property effects.The QM theoretical model has no recourse to experimental data. It includes transverse flow due to surface tension as well as shear stress and gravity effects and has been used to generate results for various fluids, channel shapes and dimensions, vapour flow rates and channel inclinations. The predictions fall within the ranges of the results given by the correlations. Theoretical results for seven fluids have been widely disseminated in conference proceedings (9 papers) and in archival journals (5 papers). A simple algebraic relation between two dimensionless parameters has recently been developed which predicts, very accurately, numerically-obtained results for the surface tension dominated flow regime for different channel cross section, fluids, vapour flow rates and temperature differences. Similar simplified results will be obtained for the flow regimes upstream and downstream of the surface tension dominated region so as to provide readily usable formulae for complete condenser design.An innovative technique will be used to measure local temperatures and heat fluxes in microchannels. A new apparatus has been designed in which parallel microchannels (1.5 mm x 1.0 mm) pass through the centre of a copper block (30 mm x 40 mm x 500 mm long) in which temperatures are very accurately measured (to within 0.05 K) at 98 precisely-known (to within 0.3 mm) locations. The observations will be used in inverse solutions of the conduction equation to determine local vapour-side surface temperatures (to within 0.1 K) and heat fluxes (to within 5%). Flow visualization studies will also be performed in which the upper part of the test block is replaced by glass so that the flow in the channels may be observed using high-speed photography.Two fluids (steam and FC72) with widely different thermophysical properties, notably surface tension, will be used. These data will provide a stringent test of the present theory and facilitate its extension to cover other flow regimes. The final model should be valid for any fluid, channel geometry, vapour flow rate and vapour-to-wall temperature difference. This will facilitate design and optimisation of a new generation of large scale refrigeration and air conditioning equipment.