A numerical study on non-uniform characteristics of spray falling heat transfer over horizontal tubes in an oily sewage source heat pump

Recently, the spray heat exchanger became an important technology in a sewage source heat pump (SSHP) to recover thermal energy from oily wastewater. Hence, it's necessary to study the heat transfer properties and its enhancements. In this paper, a numerical model with VOF method was established to study the oily wastewater spray falling film over a horizontal tube. The flow field, falling film distribution and temperature distribution and heat transfer coefficient were investigated, and the non-uniformity of flow, temperature and heat transfer were further analyzed. Regions of low level film thickness were found around tube suggesting the non-uniform axial and circumferential distribution, corresponding to low film temperature regions. The non-uniformity of heat transfer coefficient at different circumferential angles was evaluated showing severe non-uniformity in impingement and departure regions due to the intense impinging of falling wastewater at the top and instability of dropping flow at the bottom. Thereafter, the influence of oil content, spray density and tube diameter were studied. The increase in oil content led the flow pattern changed from sheet flow to column-sheet flow due to increase in viscosity, resulting to non-uniformity increase from 0.15 to 0.19. Compared to tube diameter of 25.4 mm, the 12.7 mm tube accelerated flow downstream, therefore, the non-uniformity at the top was weakened to 0.13 and the downstream non-uniformity was strengthened to 0.19. The higher spray density of 0.262 kg/m∙s reduced the negative effect of viscosity, enhanced the flow velocity and weakened the non-uniformity at the top. However, the increase in flow velocity raised the flow instability in downstream suggesting higher non-uniformity.