An experimental study on effects of oily content on flow pattern transition over horizontal tubes in a sewage source heat pump system

The spray sewage heat exchanger is better used for sewage source heat pump systems due to its simple structure and good anti-blocking characteristics. To maintain high heat transfer efficiency of using oilfield wastewater, an experimental sewage source heat pump system is built and experimental studies are carried out to investigate the falling film flow patterns of oily wastewater over the horizontal tubes of spray sewage heat exchanger and effects of oily content. The critical spray density for flow pattern transition is clarified based on visualization by high-speed camera, and the influence of spray temperature and oily content are further analyzed. It is found the flow pattern transition is delayed with the critical spray density increased by 3%–12% when the oily content is raised from 0% to 20%, due to the higher temperature reduces the viscosity of the fluid. The critical spray density of flow pattern transition is also influenced by the spray temperature, which reduce the critical spray density from column flow to column-sheet flow from 0.0828 kg m−1 s−1 at 46 °C to 0.0782 kg m−1 s−1 at 65 °C for wastewater of 15% oily content. The tube surface temperature is further analyzed, and different high heat transfer performance regions are specified with a higher range of spray density for oily wastewater.