High heat flux flow boiling of R1234yf, R1234ze(E) and R134a in high aspect ratio microchannels

Abstract

Flow boiling of refrigerants in narrow channels has a great potential to cope with future challenges in the thermal management of high heat flux electronics. This study investigated the flow boiling heat transfer of R1234yf, R1234ze(E), and R134a in multi-microchannels as the applied heat flux was varied from boiling incipience to the critical heat flux. A parametric analysis on the two-phase local heat transfer coefficients was conducted for channel mass fluxes from 415 kg/m2s to 1153 kg/m2s and saturation temperatures of 30.5 ∘C and 40.5 ∘C. The test section comprised 17 channels with inlet restrictions. The channels were 298 µm wide, 1176 µm deep and 10 mm long. Three boiling regimes (I, II, III) were identified according to the observed effect of the heat and mass flux. In boiling regime I, the two-phase local heat transfer coefficient was independent of the mass flux and increased with the heat flux. In boiling regime II, the flow departed from this behavior, and a significant effect of the mass flux appeared. Finally, in boiling regime III, the two-phase local heat transfer coefficient decreased steeply with the heat flux. Flow visualization showed that a predominantly bubbly flow was present in the channels during boiling regime I. In boiling regime II, churn flow was the predominant flow regime, but appeared with more frequent vapor slugs as the heat flux was increased, eventually leading to alternating churn and wispy-annular flow. In boiling regime III, the flow regime alternated between churn and wispy-annular, with a dryout incipience leading to the progressive decrease of the two-phase local heat transfer coefficient until the achievement of the critical heat flux. The experimental data for boiling regimes I and II, amounting to 2504 local values, were compared against the prediction of selected literature correlations.