Bubble and Film Dynamics During Condensation of Refrigerants in Minichannels

Condensation of Refrigerant R404a in channels of diameter 0.5–3 mm is investigated. The specific focus of this work is to obtain quantitative information on flow mechanisms using image analysis techniques on high speed video. Experiments are conducted on condensing R404a at mass fluxes ranging from 200 to 800 kg m⁻²s⁻¹ at vapor qualities of 0.05 to 0.95. The experiments are conducted at the high pressures representative of actual operation of air-conditioning and refrigeration equipment, instead of extrapolating findings for adiabatic air-water mixtures at atmospheric pressure to high pressure refrigerant condensation. Pressure drops are also measured as a function of geometry and operating conditions during the flow visualization experiments. Quantitative image analysis enables detailed computation of void fraction and vapor bubble parameters such as frequency, velocity, diameter and length with a high degree of repeatability and with low uncertainties. The effect of operating conditions on the flow patterns, void fractions, and vapor bubble parameters for saturation temperatures ranging from 30 to 60°C is documented. The resulting void fraction models provide closure for pressure drop and heat transfer models, which has thus far not been possible in the reported works in the literature.