Micro-channel air cooled condenser performance with two-phase flow of zeotropic refrigerant at high ambient temperatures

A study of the thermal performance of an air-cooled micro-channel condenser using zeotropic refrigerant blend R-410A operating at high reduced pressure and at hot climate was conducted. The investigation of the condensation process at high ambient temperature is worth considering because the condensation saturation temperature should be high enough to be cooled by air at high ambient temperature. In this case a high operating pressure corresponding to the high condensation temperature is required; therefore, the condensation process of R-410A occurs at near-critical pressure and the vapour compression cycle operates in hot weather. In order to achieve a successful condensation process operating at hot climate, micro-channel tubes were suitable because of the high heat transfer coefficient associated with tubes of very small hydraulic diameter. The local heat transfer coefficient of R-410A was determined experimentally during the condensation process across the vapour-liquid dome at 0.7 and 0.8 reduced pressures and at 35 and 45℃ ambient air temperatures, in two different rectangular tubes of Dₕ* =1.26 and 0.52 mm, over a mass flux range of 200≤G*≤800 kg/ m²s. Although, the temperature glide of the refrigerant R-410A was sufficiently small, the measurement of the mass flux and the heat transfer during condensation with other measuring parameters were always difficult to achieve with a high level of accuracy. The latest technology of the micro-foil heat flux sensor technique was used with a bespoke facility to accurately determine the heat duty of condensation along the micro-channel tubes. The behaviour of the heat transfer coefficient with the vapour quality was addressed. In addition, the behaviour of heat flux, vapour quality and wall temperature with the thermal length of the channel were intensively studied. The heat transfer coefficient was found to increase with the mass flux and the vapour quality and to decrease with the ambient temperature. Correlations by other researchers mostly disagreed with the present experimental data. Annular flow regime was adopted due to the cross section of tubes at these diameters. A new correlation in annular flow regime that accounted for the effect of near critical pressure of such refrigerant and the high temperature of the coolant air in the geometry of tubes under consideration was proposed to predict the heat transfer coefficient of condensation for which the available models are insubstantial. The resulting correlation successfully computed the experimental data. The physical comprehension and correlation resulting from this research contribute to enhance the existing knowledge for designing and optimising new equipment that utilise R-410A for air-conditioning and refrigeration applications, particularly in hot climates.