Transient liquid film evolution and diffusion-induced evaporation suppression during flow boiling of R1336mzz(Z)/R1234ze(Z) zeotropic mixtures in a microchannel

Transient liquid film evaporation is a dominant heat-transfer mechanism in microchannel flow boiling, yet its evolution in zeotropic refrigerant mixtures remains poorly quantified because preferential evaporation couples hydrodynamics and species transport. The transient liquid film evolution and bubble dynamics of R1336mzz(Z), R1234ze(Z), and their zeotropic mixtures were experimentally investigated in a 0.6 mm circular microchannel using a laser confocal displacement method (LCDM). Effects of heat flux, mass flux, and mixture composition on film evolution, bubble passage frequency, and flow instability were analyzed. The initial liquid film thickness first increases and then decreases with Ca , reflecting a transition from viscous–capillary deposition to acceleration-dominated thinning. An acceleration-dependent correlation achieved a mean absolute percentage error of 10.86% for independent data at Bo _a ≥ 1.5. Subsequent film evolution was modeled by coupling evaporation and interfacial shear, with acceleration incorporated through the deposited initial film. Under heated, low-bubble-velocity conditions, zeotropic mixtures exhibited slower film depletion than the pure fluids. This behavior was consistent with evaporation suppression caused by preferential evaporation of R1234ze(Z), interfacial enrichment of R1336mzz(Z), and the associated increase in local equilibrium temperature. A time-dependent mass-transfer Péclet number was introduced to couple concentration polarization, temperature glide, and evaporation suppression. The models reproduced most present measurements within ±15%; the pure-fluid model yielded an overall MAPE of 18.47% against independent FC-72 data. Increasing mass flux increased bubble passage frequency and reduced flow instability, while the 41/59 mol% mixture showed moderate frequency variation and lower instability peaks. These findings provide a mechanistic basis for low-GWP two-phase microchannel cooling design.