An Experimental Study of Enhanced Dropwise Condensation in Mini Scale Heat Exchanger

Abstract

This study focuses on mini scale heat exchanger, which is composed of mini scale tubes, condensation phase change phenomenon to investigate the effect of condenser tube surface wettability and tube diameter on condensation heat flux and outer tube heat transfer coefficient. Aluminum tubes of outer diameter 3.18mm and 2.38mm with both common aluminum surface and Teflon coated surface owing hydrophobic surface characteristic were tested and compared in the heat exchanger. Test section was designed having five tubes with outer diameter of 3.18mm or six tubes with outer diameter of 2.38mm horizontally arranged at the same height. The effective condensation length is 100mm. Steam was used as working fluid and tap water as coolant. Experiments were performed for test section inlet pressure in the range of 1-2 bar, steam saturation temperature of 100-118°C, steam mass flow rate in the range of 0.02-0.2g/s and heat flux in the range of 15-72kW/m2. The experiment results reveal that the tube surface with hydrophobic characteristic shows dropwise condensation regime. It can remove the liquid condensate faster hence has larger heat flux and higher heat transfer coefficient. The 2.38mm outer diameter tubes show higher heat flux in low inlet pressure region and lower heat flux in high inlet pressure region. And the Reynolds numbers of most experiment data of 2.38mm outer diameter tubes are smaller than those of 3.18mm outer diameter tubes since steam velocities are lower, then the condensation motion is not as fierce as 3.18mm outer diameter tubes. Therefore, the heat transfer coefficients of 2.38mm outer diameter tubes are lower compared with those of 3.18mm outer diameter tubes. More detailed analyses are explained in later chapters.The experiment results were also compared with existing correlations, which are proposed for conventional size external condensation on horizontal tubes. And the correspondence and deviation are also analyzed.