Experimental and Theoretical Investigation of Microchannel Condensation Heat Transfer

本工作是对典型截面尺寸约为1 mm的通道内冷凝的实验和理论研究。冷凝器是发电、制冷、空调等各种工业设备以及流程工业中的关键部件。使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，虽然还没有得到优化，但已经清楚地展示了这种几何结构的有效性，使冷凝器比以前的技术小了四倍，效率提高了10%-20%。汽车设计是基于经验的试错法，这种方法对小单位是可行的。为了优化设计，更重要的是，这项技术可能被用于更大规模的设备，需要对所涉及的工艺有基本的了解。拟议的工作将使更大规模的冷凝器在广泛的应用中得到优化设计，并大大提高目前使用的工厂的性能。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。直到最近才有足够准确的实验数据，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，与这些数据中的大部分很好地吻合，并预测当使用更高表面张力的流体(如氨)时，性能将得到非常显著的改善。这项新工作的目标是获得具有很大不同表面张力的流体的结果，以便对理论进行半经验修正，从而为许多行业的应用提供第一个可靠的工程设计工具。将使用铜微通道冷凝器块进行迄今无与伦比的精度的实验传热和压降测量，其中精确地放置了98个精心校准的热电偶。所需的表面温度和热流密度将通过“逆方法”确定，精度分别为0.1K和5%。我们早期的主要流型(环状、层流)理论(2005)与其他实验室最近(2012)的实验数据在大多数相关流动参数范围内都有很好的预测。到目前为止，唯一可靠的测量方法是合成制冷剂中典型的低表面张力流体。环状层流理论适用于任何流体，并预测高表面张力流体(如氨和蒸汽/水)的性能将大大改善。还将进行可视化测试以确定流动状态。这些数据将与换热和压降数据一起被用来确定环状层流理论的有效性限度，并对该理论进行半经验调整，以涵盖实际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，使更大规模的设备能够更有信心地设计，效率大大提高。

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.