Reversing flow in microchannel evaporations

Abstract

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.

The paper presents the phenomenon of periodically reversing flow in a microchannel evaporators used in air conditioning systems which will be illustrated with high speed videos in the presentation. These evaporators have multiple parallel channels between 0.5 and 1mm and headers (manifolds) that have to accommodate tubes of typically 15 to 25 mm major (width). A simultaneous flow visualizations and pressure measurements confirmed the periodic flow reversal and associated fluctuations. The oscillatory behavior of the flow is linked and related with oscillations of the evaporator pressure in the inlet header and the pressure drop. Visualization of the flow in headers and tubes will be presented. Paper will present experiments that have revealed significant ratio of reversed vapor flow – in the range of 3-7% of the mail flow, in addition to liquid that is reversed alongside. The magnitude and frequency of oscillations are directly proportional to heat flux. Three potential impacts of flow reversal on evaporator performance are discussed: 1) moderate liquid maldistribution; 2) reduced heat transfer; 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.