Forced convective heat transfer of refrigerant 22 evaporating in upward and downward flow in U-bends

Abstract

This paper presents an experimental study of upward and downward flow of boiling heat transfer characteristics for refrigerant 22 in and downstream of U-bends situated in a vertical plane. Three tubular test sections with 40 mm, 58 mm and 96 mm bend radius respectively have been used. They had an inner tube diameter of 16 mm, and outside diameter of 20 mm. The ranges of heat flux and mass velocity covered in the tests were 5–30 kW m −2 and 120 –420 kg m t-2s −1. Heat transfer coefficients show a strong variation with axial and radial position. The average heat transfer coefficients are higher both in the bends and also downstream of the bends compared with flow in an equivalent horizontal straight tube. Correlations are proposed to predict the average flow of boiling heat transfer coefficients of refrigerant flowing in U-bends.