Saturated/subcooled flow boiling heat transfer characteristics for R134a, HFE-7100 and Deionized water inside micro/mini-channels: Part I - Visualization of flow patterns and new diabatic two-phase flow pattern transition regimes

Abstract

An experimental investigation on flow saturated/subcooled flow boiling heat transfer for R134a, HFE-7100 and Deionized water inside micro/mini-channels (L/dh = 40/53/70) was presented. Flow patterns and their transitions of three working fluids at various liquid subcoolings (0 ∼ 70 K) were visualized and analyzed. Experiments were carried out at a saturated pressure of 770 kPa for R134a and at atmospheric pressure for HFE-7100 and Deionized water. The mass flux varied from 400 to 800 kg/m2s, and heat fluxes up to 270.1 W/cm2 over the entire range of vapor quality (0 ∼ 1). The effects of mass flux, heat flux, liquid subcoolings and geometry parameters on flow pattern evaluation and transition were discussed. Bubble (isolated and confined bubbles), slug, churn, annular and dry-out flows were observed for the micro/mini-channels. The results indicated that the flow patterns have a significant difference on flow boiling under different liquid subcoolings. There is obvious flow pattern transition from slug flow to annular flow for nearly saturated boiling (ΔTsub ≤ 10 K). However, with the increase of inlet liquid subcooling, only obvious bubble flow and slug flow were observed during the entire boiling process, and the annular flow was not obvious before triggering the CHF. Besides, the flow pattern transition criterion is modified and the new flow pattern transition regimes including saturated/subcooled flow boiling for different working fluids are developed based on present experimental results.