Visualization and measurement of periodic reverse flow and boiling fluctuations in a microchannel evaporator of an air-conditioning system

This paper presents a simultaneous visualization and measurement on periodic reverse flow and boiling fluctuations of R-134a in a parallel flow microchannel evaporator used for air-conditioning systems. The visualization study shows that the microchannel repeats the transient flow patterns with two stages: liquid rewetting stage, and transient annular evaporating/dewetting stage. Correspondingly, five transient flow patterns are identified along the microchannel: bubbly flow, elongated bubble, liquid lump flow, annular flow, and mist/dryout flow. The reverse flow is observed to begin with the elongated bubble flow at the upstream channel and continue until the transient annular film evaporating/dewetting stage, indicating that the existing immeasurable high pressure within the elongated bubble or at a certain location in the annular flow exceeds the channel inlet and exit pressures. The period of the pressure drop fluctuation and flow pattern alternation are almost identical with that of the reverse flow, and it decreases from 1.49 s to 1.13 s when the average heat flux increases from 14.1 kW/m(2) to 18.3 kW/m(2). The decrease in oscillation period is mainly due to the shortened time interval for the annular film evaporating/dewetting stage. Published by Elsevier Ltd.