Abstract
The flow characteristics of sewage directly affect the technical and economic performance of sewage-source heat pump systems in heat extraction from sewage. To prove there is significant improvement in heat extraction from sewage with the spray heat exchanger for sewage heat pump system (SHPS): (1) The flow states of the falling film on and between the multiple horizontal tubes were investigated experimentally. The correlations of Reynolds number (range from 100 to 600) and the tube distances to the flow were derived; (2) A model was developed for the falling film flow state analysis of the horizontal elliptical tube of the spray heat exchanger using the ellipse plotting method, which was constructed using approximately three circular arcs. The laser-induced fluorescence technology was used to measure the horizontal tube falling film thickness of water. This experiment focused on the film thickness distribution characteristics of Al-brass tubes with outside diameters of 19 mm and 25.4 mm in column flow with the tube spacing ranging from 20 mm to 40 mm and the Reynolds number (Re) ranging from 184 to 368. The images of the gas-liquid interface were captured with a digital camera. The image processing technology was used to quantitatively describe the liquid film thickness around the horizontal tube. The comparisons between the falling film thickness calculation results and experimental data were in good agreement for the variation of the angle of the elliptical surface and the specific liquid rate. Furthermore, a novel correlation was developed to describe the Nusselt number which was subjected to the examined tube shapes; (3) The heat extraction performance of SHPS with spray heat exchanger was experimentally investigated. The temperature of the sewage ranged from 10 to 30℃. The heat supply, input power, COP, and output water temperature of the condenser of the SHPS were discussed. The results of the proposed calculation method were in conformity with the experimental data and gave good predictions with maximum deviation being lower than 8.6%. Thus, the proposed method is helpful in improving the design and the performance of the SHPS.
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