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.