Heat transfer and pressure drop characteristics of R1234yf during evaporation in a plate heat exchanger with offset strip fins: An experimental study

Plate heat exchangers (PHEs) have been suggested for use in secondary loop air conditioning systems in automobiles operated with flammable refrigerants, such as R1234yf. In this study, the evaporation heat transfer coefficient (HTC) and two-phase frictional pressure drop (FPD) of R1234yf in a brazed PHE with offset strip fins were investigated. The mass flux in the experiment was 40–80 kg m⁻² s⁻¹, heat flux was 4–10 kW m⁻², vapor quality at the inlet was 0.1–0.8, and saturation temperature was 5–15°C. Prior to the primary analysis, a coolant-to-coolant test was performed using a modified Wilson plot approach to estimate the multiplier (C) and the Reynolds number exponential (n). The observed C and n values were 0.868 and -0.496, respectively, with an R² value of 0.977. The main results demonstrated an increase in the evaporation HTC with an increase in vapor quality in the initial stages, followed by a subsequent decrease, forming a peak at a mean vapor quality of 0.35–0.41. The nucleate and convective heat transfer regimes were dominant during the evaporation of R1234yf at lower and higher mean vapor qualities, respectively. Evidently, the evaporation HTC was significantly influenced by the mass flux and vapor quality compared with the heat flux and saturation temperature. Furthermore, the two-phase FPD continuously increased with the mass flux, vapor quality, and heat flux, although it decreased with the saturation temperature. As a result, new correlations are proposed herein for forecasting the Nusselt number and friction factor of R1234yf with average absolute errors of ±12% and ±15%, respectively.