Flow boiling heat transfer of R134a and R404A in a microfin tube at low mass fluxes and low heat fluxes

An experimental investigation of flow boiling heat transfer in a commercially available microfin tube with 9.52 mm outer diameter has been carried out. The microfin tube is made of copper with a total fin number of 55 and a helix angle of 15A degrees. The fin height is 0.24 mm and the inner tube diameter at fin root is 8.95 mm. The test tube is 1 m long and is electrically heated. The experiments have been performed at saturation temperatures between 0 and -20A degrees C. The mass flux was varied between 25 and 150 kg/m(2)s, the heat flux from 15,000 W/m(2) down to 1,000 W/m(2). All measurements have been performed at constant inlet vapour quality ranging from 0.1 to 0.7. The measured heat transfer coefficients range from 1,300 to 15,700 W/m(2)K for R134a and from 912 to 11,451 W/m(2)K for R404A. The mean heat transfer coefficient of R134a is in average 1.5 times higher than for R404A. The mean heat transfer coefficient has been compared with the correlations by Koyama et al. and by Kandlikar. The deviations are within +/- 30% and +/- 15%, respectively. The influence of the mass flux on the heat transfer is most significant between 25 and 62.5 kg/m(2)s, where the flow pattern changes from stratified wavy flow to almost annular flow. This flow pattern transition is shifted to lower mass fluxes for the microfin tube compared to the smooth tube.