An experimental study on influence of oily content on spray falling film flow and heat transfer properties over horizontal tubes in a sewage source heat pump

In the sewage source heat pump (SSHP) system, which is used to recover the low-level thermal energy in oily sewage, the sewage heat exchanger is an important equipment to determine the heat exchange efficiency of the system. In this paper, an oily sewage source heat pump (OSHP) system experimental setup with a spray-type sewage heat exchanger as the evaporator was established. The effects of oil content, spray density and spray temperature on the flow patterns between horizontal tubes and heat flux of tube were investigated and analyzed. It was found that the critical spray density for transformation from droplet flow to column flow was increased to 0.109 kg m(-1) s(-1) for oily sewage, which was much higher than 0.082 kg m(-1) s(-1) of pure water for pure water, indicating that a much higher spray density was needed to maintain a higher heat transfer for oily sewage. With the increase in spray temperature of oily sewage, the critical spray density of flow pattern transformation from droplet flow to column flow was decreased from 0.137 kg m(-1) s(-1) at 46 degrees C to 0.082 kg m(-1) s(-1) at 65 degrees C. Besides, oily sewage had obvious lower heat flux than pure water, and the difference was enlarged with the increase in spray density. New heat transfer correlations obtained depending on Re and Pr numbers were proposed, showing more accurate predictions than previously reported models. Corresponding to the falling film flow pattern, a clear dividing between 0.055 and 0.082 kg m(-1) s(-1) for pure water and between 0.082 and 0.109 kg m(-1) s(-1) for oily sewage, for the tube surface temperature distribution for droplet flow and column flow was observed, indicating the significantly high heat transfer for column flow pattern compared with droplet flow.

Automated summary

This paper investigates the effects of oil content, spray density, and spray temperature on the flow patterns and heat flux of a spray-type sewage heat exchanger in an oily sewage source heat pump system. The study reveals the need for higher spray density to maintain heat transfer in oily sewage compared to pure water, and proposes new heat transfer correlations that provide more accurate predictions.