Periodical reverse flow and boiling fluctuations in a microchannel evaporator of an air-conditioning system

This paper presents the phenomenon of periodic reverse flow and associated boiling fluctuation found in experiments with a parallel microchannel evaporator used in an R134a air conditioning system. A simultaneous flow visualizations and measurements confirmed the periodic flow reversal. It caused synchronized oscillations of the evaporator inlet pressure and the pressure drop. The magnitude and frequency of oscillations increased with heat flux. Three potential impacts of flow reversal on evaporator performance are identified: 1) moderate liquid maldistribution; 2) reduced heat transfer coefficient; 3) increased refrigerant side pressure drop. Finally, to mitigate impacts of periodic reverse flow, a solution is proposed: to vent and bypass backflow vapor accumulated in the inlet header.

► Visualized flow regime in transparent headers of a microchannel evaporator.
► Characterized R134a periodic reverse flow.
►  Quantified pressure fluctuation and lined them to boiling effects.
► Identified impacts of periodic reverse flow on evaporator performances.
► A solution to mitigate these impacts is proposed.