Experimental and Theoretical Investigation of Microchannel Condensation Heat Transfer

这项工作是对典型截面尺寸约为1mm的通道中凝结的实验和理论研究。冷凝器是发电、制冷、空调等各种工业装置以及过程工业中的关键部件。采用微通道管的冷凝器已经在汽车空调中成功使用了大约25年，虽然尚未优化，但已经清楚地证明了这种几何形状的有效性，使冷凝器体积减小了四倍，效率比早期技术提高了10-20%。汽车设计是基于经验的试错方法，是可行的小单位。为了优化设计，更重要的是，该技术可以用于更大规模的设备，需要对所涉及的过程有基本的了解。所提出的工作将使更大规模的冷凝器优化设计，适用于广泛的应用，大大提高了目前正在使用的工厂的性能。在制冷和空调方面，如果在英国广泛使用，改进后的技术可以节省高达10%的能源需求，并相应减少二氧化碳排放。最近才有足够精确的实验数据，而且这些数据仅适用于低表面张力流体（合成制冷剂）。我们之前的理论适用于任何流体，与这些数据非常吻合，并预测当使用氨等表面张力较高的流体时，性能会得到显著改善。新工作的目标是获得具有广泛不同表面张力的流体的结果，以便对理论进行半经验修正，从而为众多行业的应用提供第一个可靠的工程设计工具。实验传热和压降测量迄今为止无与伦比的精度将使用铜微通道冷凝器块，其中98个精心校准的热电偶精确定位。所需的表面温度和热通量将由“逆法”确定，精度分别为0.1 K和5%。我们早期的理论（2005年）主要的流动形式（环形，层流）紧密地预测了最近（2012年）来自其他实验室的实验数据在大多数相关流动参数范围内。迄今为止，唯一可靠的可用测量是低表面张力流体的典型合成制冷剂。环空层流理论适用于任何流体，并预测氨和蒸汽/水等表面张力较高的流体的性能将大大提高。还将进行可视化试验以确定流动状态。这些数据将与传热和压降数据一起用于确定环形层流理论的有效性极限，并对该理论进行半经验调整，以涵盖实践中可能发生的所有情况。因此，该项目将提供第一批可靠的、广泛适用的工具，使人们能够更有信心地设计效率大大提高的大型设备。

This work is an experimental and theoretical investigation of condensation in channels having a typical cross section dimension around 1 mm. The condenser is a key component in a wide range of industrial plant such as power generation, refrigeration and air conditioning and in the process industries. Condensers employing microchannel tubes have been used successfully in automotive air conditioners for around 25 years and, while not yet optimized, have clearly demonstrated the effectiveness of this geometry resulting in condensers four times smaller and with efficiencies 10-20% higher than earlier technologies. Automotive designs are based on empirical trial-and-error methods that are feasible for small units. In order that designs may be optimised, and more importantly, that the technology may be taken up for larger scale equipment, fundamental understanding of the processes involved is needed. The proposed work will enable optimized design of larger scale condensers for a wide range of applications with vastly improved performance over plant currently in use. In refrigeration and air conditioning the improved technology could save up to 10% of the energy demand, with corresponding reduction carbon dioxide emissions, if widely used in the UK.Experimental data of sufficient accuracy have only recently become available and these are only for low surface tension fluids (synthetic refrigerants). Our earlier theory, applicable to any fluid, is in good agreement with much of these data and predicts very significantly improved performance when using higher surface tension fluids such as ammonia. The objective of the new work is to obtain results for fluids having widely different surface tensions to enable semi empirical modification of the theory and thus to provide the first reliable engineering design tools for application by numerous industries.Experimental heat transfer and pressure drop measurements of hitherto unexcelled accuracy will be made using a copper microchannel condenser block in which 98 carefully calibrated thermocouples are precisely located. The required surface temperatures and heat fluxes will be determined by the "inverse method" with accuracy 0.1 K and 5% respectively.Our earlier theory (2005) for the predominant flow regime (annular, laminar) closely predicts the most recent (2012) experimental data from other laboratories over most of the ranges of the relevant flow parameters. To date the only reliable available measurements are for low surface tension fluids typical of synthetic refrigerants. The annular laminar flow theory is valid for any fluid and predicts greatly improved performance for higher surface tension fluids such as ammonia and steam/water.Visualization tests will also be done to establish the flow regimes. These will be used, together with the heat transfer and pressure drop data, to establish the limits of validity of the annular laminar flow theory and to develop semi-empirical adjustments to the theory to cover all circumstances which may occur in practice. The project will thus provide the first reliable, widely applicable tools which will enable more confident design of the larger scale devices of greatly improved efficiency.