Numerical study on heat transfer of oily wastewater spray falling film over a horizontal tube in a sewage source heat pump

Sewage heat exchanger is the key equipment of oily sewage source heat pump system, which has great influence on its performance. Among many kinds of heat exchangers, the spray heat exchanger has the advantages of simple structure, high heat transfer efficiency, good operability and anti-blocking ability. Therefore, it can be used in a sewage source heat pump to recover energy from the oily wastewater. However, the heat transfer and flow characteristics of oily wastewater spraying in the heat exchanger are different from the ordinary sewage heat pump system. Hence, it's necessary to investigate the heat transfer of oily wastewater spray falling film on horizontal tube for better application. In this paper, a three-dimensional numerical model was established for a spraying heat exchanger in the oily sewage source heat pump system, and a mixture of water and glycerin was used to simulate the oily wastewater. The VOF model was used to track the liquid-gas interface of oily wastewater spray falling film over a horizontal tube in the heat exchanger, and the effects of different glycerin content, heat flux, spray density, tube diameter, liquid distributor height and spray temperature on the heat transfer coefficient were investigated. To further understand the mechanism of heat transfer inside the film, the temperature distribution inside the liquid film was obtained and analyzed. The results show that: (1) the heat transfer characteristic can be divided into three regions around the tube; (2) the local heat transfer coefficient decreased with increase of glycerin content in the thermal developing region; (3) the local heat transfer coefficient increased with the increase of the spray density and spray temperature, decreased with the increase of the tube diameter and was little affected by heat flux in the thermal developing region, and the increase in the liquid distributor height caused increase of local heat transfer coefficient in the upper part of the tube.