Air-to-refrigerant microchannel heat exchangers (MCHXs) are now being used in the heating, ventilation, air-conditioning and refrigeration (HVAC&R) industry. The previous research and development of MCHXs has reached a plateau, in that, the optimum designs cannot be further improved with the limited number of geometry related design variables currently available. The ever-evolving simulation and manufacturing capabilities have given engineers new opportunities in pursuing complex and cost-efficient novel heat exchanger designs. Recently, microchannel heat exchanger designs with variable tubes, ports and fins have been proposed. Such designs adopt variable tube and fin geometry within the heat exchanger core. Adaptive geometry refers to the changes in tube and port dimensions, changes in fin type and fin density in various sections of the heat exchanger core. The locations of individual tubes and fins can also vary, especially in multi-slab configurations. The goals of this new concept are heat transfer enhancement, material savings and fulfilling special design and application requirements. This paper presents studies on the design optimization of variable geometry MCHXs based on a validated simulation tool. The optimization study investigates an R134a condenser a CO_2 gas cooler in air-conditioning systems. The objective of the study is to evaluate the potential cost and performance benefits of variable geometry microchannel heat exchangers compared to traditional fixed geometry microchannel heat exchangers used today. The optimization objectives are performance enhancement and cost reduction. Condenser designs generally consist of two sections, a main section and a sub-cooler section. The majority of the condensing capacity is contributed by the main section whereas the sub-cooler section ensures that the outlet refrigerant is fully sub-cooled. Conventional sub-cooler section has excess tubes and fins due to pressure drop and manufacturing constraints. The new variable geometry design can significantly lower the material cost in the sub-cooler section while maintaining certain refrigerant pressure drop. The optimization study shows a 30 percent reduction in material and 40 percent savings in envelope volume for a variable geometry gas cooler for the same performance compared to a baseline standard geometry design. The optimization study reveals the potential of the variable geometry MCHX and will motivate engineers to pursue such innovative designs.
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