Refrigerant film flow and heat transfer characteristics on the elliptical tube under constant wall temperature

Abstract

• CFD models of falling film on horizontal tubes are verified by experimental data. • Effects of wall heating conditions on heat transfer of falling film are explored. • 11 refrigerants film flow and heat transfer under different temperature is studied. • Correlations of Nusselt number and film thickness for elliptical tubes are fitted. In this paper, the 2D model of the elliptical tube is established to simulate the film hydrodynamics and heat transfer performance. The volume of fluid (VOF) is used and the model is verified by the experimental data in the previous literature. The effects of the heating condition, fluid medium, inlet temperature ( T in ) on the film thickness and heat transfer coefficients are explored. The studied 11 refrigerants include ethane, propane, R123, R1234yf, R1234ze(E), R125, R134a, R143a, R152a, R227ea, R245fa at T in = –30, –10 and 10 °C. The results show at Re = 2000, the local heat transfer coefficients of the propane at T in = 10 °C under fixed heat flux are 19% greater than those under fixed wall temperature. Both the local film thickness and local heat transfer coefficients increase with Reynolds number ( Re ). The overall external heat transfer coefficients increase with Re , but the growth rate slows down at Re ≥ 1500. Fluids have different results of film thickness and heat transfer coefficients even they have a similar Kapitza number ( Ka ). Different from the water and seawater, the heat transfer coefficients reduce as the inlet temperature increases for the refrigerants. A correlation is proposed to predict local film thickness on both circular and elliptical tubes. A correlation is acquired by the fitting method to predict the overall external heat transfer coefficients under the heating condition of fixed wall temperature, which can capture 89% of 132 data within the deviations of ±10%.