Low-Cost Phase-Distribution Enhancement for Two-Phase Heat Exchangers

Military Environmental Control Units (ECUs) are one of the greatest consumers of electric energy in forward operating bases (FOBs). Microchannel condensers have replaced tube-fin coils in both military ECUs and commercial air conditioners to improve system efficiency. Flow into the condenser is single-phase superheated vapor and therefore uniform flow distribution through multiple parallel microchannel arrays is easily achieved. Manufacturers are interested in exploiting the same benefits by replacing the tube-fin evaporator coil with a microchannel heat exchanger. However, flow into the evaporator is a saturated mixture of liquid and vapor refrigerant, typically at a thermodynamic quality of 0.1 0.4, and therefore two-phase flow distribution through a multitude of parallel microchannels can be very non-uniform. In this proposal, Mainstream compares multiple concepts for achieving two-phase distribution in a microchannel evaporator. Pros and cons of each concept regarding cost, manufacturability, size, and weight are summarized. A final concept is selected for investigation in Phase I. A preliminary two-phase finite element model of the phase distribution phenomena for the selected concept is presented and results are discussed. Based on this model we expect to reduce the two-phase maldistribution to less than 10%.