Conference Year



Microchannel heat exchangers, two-phase flow, header, refrigerant, maldistribution, take-off ratio


Due to the many benefits offered by Microchannel Heat Exchangers (MCHX), such as compactness, high heat transfer coefficients, reduced refrigerant charge, and energy and material cost savings, microchannel condensers and evaporators continue to be increasingly applied and investigated in the HVAC&R fields. One of the practical challenges associated with MCHX is the uniform distribution of two-phase refrigerant in the headers and tubes of the heat exchanger. In MCHX, which typically have port sizes about 1 mm or less, to maintain the pressure drop at reasonable levels while providing fairly uniform two phase flow distribution, an appropriate header size and number of tubes need to be chosen. In this paper, a critical review of experimental and analytical investigations of two-phase flow maldistribution in MCHX is presented. The influence of header and microchannel tube geometry, heat exchanger orientation, flow and operating conditions, fluid properties and flow patterns on the MCHX flow distribution is discussed. Researchers have investigated upward and/or downward two-phase flow in MCHX with horizontal and vertical headers, for which the microchannel tubes/ports are, respectively, vertical and horizontal. Traditionally, compared to investigations in horizontal headers, the studies on vertical headers have been relatively few. However, recently, due to applications involving automotive evaporators, more studies on vertical headers are reported. In all these studies, gravity is seen to profoundly affect the two-phase flow distribution. Various fluids such as R410A, R134a, R245fa, CO2, air-water, etc. have been studied in published works. Fluid thermophysical properties and flow patterns greatly influence the flow distribution in MCHX. Very few investigators have studied the effects of fluid properties on two-phase flow distribution. Zou and Hrnjak (2014)[16] speculated that fluids with high liquid to vapor density ratio would provide better flow distribution. However, this hypothesis needs to be experimentally confirmed.  Most studies agree that in headers, compared to annular flow, churn flow is desirable for better flow distribution. Most of the experimental investigations on flow distribution have been conducted for adiabatic flow. However, the applicability of such investigations to practical situations is dubious as the flow will be accompanied by condensation or boiling heat transfer. Refrigerant mass flux (G) and inlet quality (x) are seen to have a significant impact on flow distribution which is discussed in detail. Tube protrusion into the headers, and tube spacing also greatly affect the flow distribution. Since these interacting factors make the prediction of two-phase flow distribution very complex, a limited number of semi-empirical models/correlations have been proposed to quantify the two-phase flow maldistribution in MCHX. Five correlations for predicting the liquid take-off ratio in MCHX headers were assessed and among these five, Zou and Hrnjak (2013b)[15] correlation for R410A was found to perform reasonably. Based on the current study, recommendations regarding the applicability of these correlations to practical problems have been provided. Having identified and examined the key factors influencing two-phase flow maldistribution in MCHX, recommendations for further study are made.Â