Low-Cost Phase-Distribution Enhancement for Two-Phase Heat Exchangers
Navy SBIR FY2014.1

Sol No.: Navy SBIR FY2014.1
Topic No.: N141-068
Topic Title: Low-Cost Phase-Distribution Enhancement for Two-Phase Heat Exchangers
Proposal No.: N141-068-1046
Firm: Mainstream Engineering Corporation
200 Yellow Place
Pines Industrial Center
Rockledge, Florida 32955-5327
Contact: Joshua Sole
Phone: (321) 631-3550
Web Site: www.mainstream-engr.com
Abstract: 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%.
Benefits: The HVAC industry is heavily driven by cost, size, and EPA regulation. Microchannel heat exchangers have multiple advantages over their tube/fin counterparts in these categories. Microchannel heat exchangers are lower cost than tube/fin heat exchangers due to less expensive materials and less intensive manufacturing processes. They are smaller than tube/fin heat exchangers due to more efficient refrigerant and air-side heat transfer. Finally, EPA regulations are driving the industry toward more environmentally-friendly refrigerants which generally have higher operating pressures. The small passages in a microchannel heat exchanger make it more suitable for high pressure applications than a tube/fin heat exchanger. All of these benefits will lead to rapid adoption of the proposed microchannel evaporator phase-distribution technology.