Thermo-chemical characteristics of R134a flow boiling in helically coiled tubes at low mass flux and low pressure

The characteristics of R134a heat transfer coefficients and wall temperature distribution were investigated under low mass flux and low pressure conditions in a helically coiled tube with heated length of 7070 mm, outer diameter of 10 mm, inner diameter of 7.6 mm, coil diameter of 300 mm and helical pitch of 40 mm. System pressures, mass fluxes and inlet qualities range from 0.20 to 0.75 MPa, 50 to 260 kg/m² s and ?0.18 to 0.40, respectively. It was found that the wall temperatures in descending segments of coiled tube were higher than those of climbing ones, while the heat transfer coefficients varied inversely. Around the section circumference, the outside temperature was lower than the inside one; this is more apparent at very low mass flux and pressure conditions. The heat transfer coefficient increases with increasing mass flux, vapor quality and heat flux. However, the pressure has an indeterminate effect. New heat transfer coefficient correlations for current conditions were developed comparing with existing correlations.