The impact of fin deformation on flowboiling heat transfer and pressure drop inmicrofin tubes

© 2018 by Begell House, Inc. In air-conditioning and refrigeration (HVACR) equipment, microfin or internally grooved tubes are commonly used to enhance the thermal performance of evaporators and heat pumps. Such tubes are mechanically expanded by a mandrel into a fin pack to minimize the thermal contact resistance between the tube and the air side fins. However, tube expansion also deforms the inner enhancements to varying degrees, which degrades the in-tube thermal performance. Extensive published data on boiling heat transfer coefficients and pressure drop in pristine microfin tubes exist. However, there is lack of knowledge about the influence of microfin deformation on the thermal-hydraulic performance of microfin tubes. This brings into question the use of pristine tube data for designing HVACR heat exchanger equipment. In this work, we first present an analysis of the changes in the internal surface area of microfin tubes arising from tube expansion. A computational model of a coaxial tube heat exchanger is then used to estimate the changes in thermal-hydraulic performance of the expanded microfin tube. In-tube flow boiling of R-410A at 300 kg/m.s2 and 0°C saturation temperature, a condition typically encountered in HVACR applications is simulated. The tube heat transfer rate is degraded with increasing fin deformation, and the loss can be up to 5% for a tube with 10% fin deformation. The refrigerant pressure drop decreases by up to about 6% due to the increase in tube cross-sectional area and the loss in tube surface area caused by mechanical expansion of the tube