Flow boiling heat transfer of low-GWP working fluids at low mass and heat fluxes in a 4 mm diameter tube

Abstract

With particular focus on applications to small scale binary cycle systems, the flow boiling heat transfer performances of low-GWP working fluids in a 4 mm diameter tube with vertical upward flow configuration have been investigated based on local heat transfer measurements and visualization of diabatic flow. The working fluids examined in the present study were R245fa, R1233zd(E), R1224yd(Z), and HFE347pc-f. The local heat transfer coefficients (HTC) were measured in a stainless tube with Joule heating while the inlet quality was regulated by preheating. The ranges of the mass and heat fluxes covered 20-40 kg/(m2⋅s) and 6-20 kW/m2, respectively, and the reduced pressure was varied from 0.061 to 0.138. Using a 200 nm-thick layer of transparent aluminum-zinc-oxide (AZO) electrodes deposited by atomic layer deposition (ALD) onto the inner wall of a quartz glass tube, the flow boiling behavior was visualized under diabatic condition. It is shown that the measured HTC of all working fluids examined exhibit strong sensitivity to the heat flux and weak sensitivity to the mass flux. The dominance of the nucleate boiling effect is suggested for the quality of less than 0.5, and the convection boiling effect plays an increasing role for the quality of larger than 0.5 until the dryout incipience. It is quantified that the averaged HTC for different working fluids highly depend on the dryout incipience qualities. The measured HTC profiles were compared to conventional heat transfer correlations, and a possible way to improve the correlation suited in the present low mass and heat fluxes condition has been proposed.