Comprehensive experimental study on critical heat flux inception and sustained dryout in saturated micro-channel boiling of R410A and R454C

Micro-channel evaporators operated under saturated flow boiling are limited by critical heat flux (CHF) inception and sustained dryout. Saturated flow boiling of R410A and the lower-GWP zeotropic blend R454C was investigated in a horizontal flat micro-channel tube using synchronized thermal measurements and high-speed visualization. The tests covered mass fluxes of 297.89–504.49 kg/m 2 s, heat fluxes of 6.44–20.34 kW/m 2 , inlet vapor qualities of 0.03–0.73, and saturation temperatures of 4.94–21.08 °C. A dual-threshold method distinguished CHF inception, defined by the first measurable deterioration in the time-averaged heat transfer coefficient (HTC) supported by visual evidence of intermittent dewetting, from sustained dryout, defined by consecutive HTC decreases accompanied by persistent dryout patterns. The HTC evolution was divided into enhancement, oscillation, and deterioration regions. Increasing heat flux advanced both thresholds, whereas increasing mass flux delayed dryout by strengthening liquid replenishment. Increasing saturation temperature enhanced pre-dryout HTC but promoted earlier deterioration. R410A generally exhibited higher HTC, whereas R454C showed stronger sensitivity to operating conditions because of temperature glide and mixture-related mass-transfer resistance. Existing HTC correlations were most reliable in the pre-dryout region, while quality-dependent CHF correlations systematically overpredicted CHF inception. A threshold-quality-informed Boiling-number correlation achieved a training MAE of 1.98%. However, because it was developed from 15 CHF-inception points for two refrigerants in one fixed geometry, it should be regarded as a present-database semi-empirical model. The results provide a synchronized thermal–visual framework for separately evaluating CHF inception and sustained dryout in refrigerant micro-channel flow boiling.