Experimental study on flow boiling heat transfer characteristics of R134a, R245fa and R134a/R245fa mixture at high saturation temperatures

Abstract

The flow boiling heat transfer characteristics of R245fa, R134a and non-azeotropic mixture R134a/R245fa (0.33/0.67 by mass) at high saturation temperatures were investigated in a horizontal tube with 10 mm I.D. Effects of high saturation temperatures on heat transfer coefficients of R134a, R245fa and the mixture were investigated from 55 °C to 95 °C. Heat flux was controlled from 6 to 24 kW/m2 and mass flux ranged from 100 to 300 kg/m2.s. The results show the heat transfer coefficient of the mixture (R134a/R245fa with 33%/67% by mass) is much lower than that of pure R134a and is close to that of pure R245fa. At high saturation temperatures, mass flow rate has less influence on heat transfer coefficient of R245fa and the mixture at a large heat flux. The heat transfer coefficients of three working fluids increase slightly when the saturated temperatures change within 10 °C. The pressure drops of pure R245fa and the binary mixture were studied and the pressure drop of the mixture (R134a/R245fa with 33%/67% by mass) is a little higher than that of R245fa.A modified heat transfer correlation considering the mixing effect of binary mixture is proposed for the mixture of R245fa/R134a, and the mean absolute deviation is within 20%. A modified two phase frictional factor for the mixture is proposed and the deviation of the mixture using modified two phase frictional factor is between −6% and +22%.