Dryout characteristics of low-GWP working fluids at low mass and heat fluxes in a vertical 4 mm diameter tube

Abstract

The dryout phenomena in flow boiling of low-GWP working fluids at low mass and heat fluxes in a vertical 4 mm diameter tube were examined, and a new correlation of the dryout incipience quality has been proposed. The working fluids examined in the present study included R1233zd(E), R1224yd, R245fa, and HFE347pc-f, under the mass flux, the heat flux, and the reduced pressure ranging in 20–40 kg/(m2⋅s), 6–20 kW/m2, and 0.061–0.138 respectively. The dryout incipience quality was quantified based on the measured local heat transfer coefficients, and the flow boiling behavior near the dryout incipience was visualized through a transparent quartz glass tube with ALD-AZO electrodes on its inner wall. It is found that the dryout incipience quality decreases with increasing the heat flux, decreasing the mass flux, and increasing the reduced pressure. Under the same operating condition, R1233zd(E) shows the smallest value of the dryout incipience quality. From the flow boiling visualization, the dependence of the operating condition and the working fluids upon the dryout incipience is found to be associated with features of the entrained droplets in the vapor core. A new correlation of the dryout incipience quality based on the boiling number, the Weber number, the Reynolds number and the density ratio has been proposed. The new model predicts all the experimental data to within ±10%.