Air-to-refrigerant microchannel heat exchangers are now being used in the HVAC&R industry. Previous research and development of microchannel heat exchangers 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. Variable geometry refers to the use of variable tube and port dimensions, variations in fin type, and various fin density in different 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 article presents studies on the design optimization of variable geometry microchannel heat exchangers based on a validated simulation tool. The optimization study investigates an automotive R134a and R290 condenser and a CO2 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 conventional fixed geometry microchannel heat exchangers used today. The optimization objectives are to maximize capacity and reduce cost. The optimization study shows a 35% reduction in material and 43% savings in envelope volume for a variable geometry gas cooler for the same performance compared to a baseline conventional geometry design. An iterative approach is proposed to calculate the airflow mal-distribution due to the heat exchanger geometry variation. Optimum designs using the calculated airflow distribution are compared with designs that assume uniform air distribution. The comparison showed a 3% difference for the best material saving case found. It is important to consider the impact of geometry variations on airflow distribution and heat exchanger designs. The optimization study reveals the potential of the variable geometry microchannel heat exchanger and motivates engineers to pursue such innovative designs.
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