Study on Flow Boiling Heat Transfer in Segmented Internally-Threaded Tube

The flow boiling heat transfer characteristics of R245fa in segmented internally-threaded tubes are studied experimentally. The heat transfer performances of smooth tube, front-threaded tube, rear-threaded tube, and full-threaded tube are compared. The experiments are carried out with heat flux ranging from 14.01 to 48.79 kW·m-2 and mass flux ranging from 125 to 375 kg·m-2·s-1. The experimental results show that the internal thread structure facilitates the heat transfer. At low mass fluxes and low heat fluxes, the heat transfer performance of the full-threaded tube is the best, while at high heat fluxes and high mass fluxes, that of the front-threaded tube is the best. Convective boiling is dominant in the smooth tube and the front-threaded tube, and corresponding heat transfer coefficients increase significantly with increasing mass flux. In contrast, nucleate boiling dominates in the rear- and full-threaded tube, and the heat transfer coefficients almost keep unchanged with increasing mass flux. The front- and rear-threaded tubes exhibit significantly different heat transfer characteristics, although they have the same heat transfer area. The effect of convective perturbation and bubble nucleation caused by the internal thread structure varies with the zone and intensity, leading to the change of the dominant mechanism of boiling heat transfer.