Investigation of flow boiling visualization and heat transfer characteristics for low-GWP zeotropic mixture R1234ze(E)/R1336mzz(Z) in transparent annular heat exchangers

For the substitution of high-GWP working fluids in high-temperature heat pumps and organic Rankine cycle systems, this study presents an in-depth experimental study of the flow-boiling heat transfer mechanism of R1234ze(E)/R1336mzz(Z). A sapphire-quartz composite annular heat exchanger was implemented to achieve simultaneous visualization of two-phase flow patterns and synchronized measurement of heat transfer parameters. Systematic comparisons were performed between macro- (8 mm) and mini-channel (2 mm) tubes under varying operational conditions, including mass fluxes (100–600 kg/m²·s), vapor qualities (0.1–0.9), and bubble point temperatures (45/55°C). Experimental results revealed distinct flow pattern transitions: macro-channels exhibited bubble, stratified, and annular flows, whereas mini-channels predominantly displayed slug, plug, and annular flows. A dimensionless parameter integrating inertial, gravitational, and surface tension forces was proposed to demarcate flow pattern boundaries. Heat transfer analysis demonstrated significant enhancement in heat transfer coefficients (HTCs) with increasing mass flux and vapor quality, with mini-channels outperforming macro-channels. To address the systematic overprediction of HTCs by existing flow boiling correlations in non-annular flow regimes, a modified Liu-Winterton model incorporating vapor-liquid composition differentials and Bond number effects was developed, reducing the mean absolute deviation to 11.65%. This work provides critical experimental foundations for designing and optimizing heat exchangers using large-temperature-glide zeotropic mixtures.