A theoretical model for annular flow of R1234ze(E) flow boiling in micro-tubes was proposed in this paper. The surface coefficient of heat transfer and pressure drop were calculated using the model. The variation in the liquid film thickness profile due to the gravitational effect

surface tension

and liquid-vapor interfacial stress was considered. The effect of the variation in the liquid film thickness profile on the heat transfer was analyzed. Furthermore

certain existing surface coefficients of heat transfer and empirical correlations of two-phase friction pressure drop were compared with the numerical results

and the comparison revealed deviations within 30%. The numerical results indicated that the variation in the liquid film thickness profile is evidently affected by gravity in the initial flow area

and the gravitational effect starts to weaken with the flow boiling process. The surface coefficient of heat transfer of annular flow with gravity was higher than that without gravity. Hence

the mean surface coefficient of heat transfer increased with gravitational acceleration. Furthermore

surface coefficient of heat transfer and pressure drop increased as the mass flux increased. Conversely

surface coefficient of heat transfer and pressure drop decreased as the diameters increased.