EXPERIMENTAL STUDY OF HFC/HFO MIXTURES FLOW BOILING INSIDE A CIRCULAR MINICHANNEL

In recent years hydrofluoroolefins (HFOs) have emerged as low Global Warming Potential (GWP) substitutes for R134a in heat transfer applications. As a drawback, HFOs are classified as mildly flammable (A2L class). In the search for non-flammable alternatives to R134a, mixtures of hydrofluoroolefins and hydrofluorocarbons can be adopted for better trade-offs among GWP, flammability, volumetric capacity, and cycle performance. Among R134a low-GWP substitutes that belong to ASHRAE A1 classification, R513A (R134a/R1234yf at 44/56% by mass) and R450A (R134a/R1234ze(E) at 42/58% by mass) can be considered as promising alternatives. The aim of the present work is to investigate the heat transfer performance of R513A and R450A during flow boiling inside a 0.96 mm inner diameter channel. R513A is an azeotropic mixture while R450A is a near-azeotropic blend (temperature glide 0.6 K at 30 °C). Flow boiling tests are performed at 30 °C saturation temperature and mass flux from 300 kg m-2 s-1 to 500 kg m-2 s-1. The effect of vapor quality, mass flux and heat flux on the flow boiling heat transfer coefficient of the blends is evaluated. The flow boiling heat transfer database is then compared with predictions from available correlations in the literature