Flow boiling heat transfer, pressure drop and flow patterns of the environmentally friendly refrigerant R1234yf for cooling avionics

Abstract

• Slug, throat-annular, wavy-annular, annular, and annular-mist flow are identified. • HTC and FPD are affected by mass flux, heat flux, saturation pressure and vapor quality. • Flow boiling HTC and FPD of R1234yf are smaller than that of R134a within 20%. • Nucleate and convective boiling dominate regions split by a vapor quality of 0.4. • New HTC and FPD correlations show mean absolute deviations of 9.3% and 6.3%. Flow boiling technology is an effective way to solve the heat dissipation problem of avionics. However, studies on the flow boiling characteristics of environmentally friendly refrigerants are still insufficient. To expand the related global database containing both experimental conditions and results, new experimental data are needed. Here, horizontal flow boiling experiments were conducted using R1234yf, and the heat transfer coefficients (HTCs), frictional pressure drops (FPDs) and flow patterns in a 1.88 mm circular minichannel were obtained under mass fluxes, saturation pressures and heat fluxes of 400–870 kg m −2 s −1 , 0.6–0.8 MPa and 40–65 kW m −2 , respectively. For comparison, experiments using R134a were carried out under the same conditions. The results show that for both R1234yf and R134a, the HTC increased with increasing mass flux, heat flux and saturation pressure, while the FPD decreased with increasing saturation pressure and increased with increasing mass flux but remained almost constant with heat flux. The heat transfer was dominated by nucleate boiling at lower vapor quality and transformed into convective boiling at higher vapor quality, with a threshold vapor quality of around 0.4. Typical flow patterns, including slug flow, throat-annular flow, wavy-annular flow, annular flow, and annular-mist flow, were identified through visualization. Through comparison, it was found that the HTC and FPD of R1234yf are smaller than those of R134a under the same conditions. In addition, based on the experimental HTC and FPD, several correlations yielding smaller predicted deviations are suggested, and new correlations having mean absolute deviations of 9.3 % and 6.3 % for flow boiling HTC and FPD of R1234yf are proposed.