Heat transfer enhancement using streamlined tubes in polymer heat exchangers.

摘要

Tubes with streamlined outer surface, such as elliptical, lenticular and teardrop shapes, are considered for use in heat exchangers made of polymers. The objective of this study is to determine the effect of tube minor-to-major axis ratio and arrangement on pressure drop and heat transfer of streamlined tubes in crossflow. The two-dimensional flow and temperature fields across individual and banks of streamlined tubes are simulated in the subcritical flow regime. For unsteady flow across individual streamlined tubes, both drag and Nusselt number decrease as the minor-to-major axis ratio is decreased. For banks of streamlined tubes, the pressure drop decreases as the gap-to-diameter ratio is increased due to the decrease in the local pressure gradient in the gap. For banks of streamlined tubes with a gap-to-diameter ratio of 0.25, the pressure drop increases as the minor-to-major axis ratio is decreased. As the gap-to-diameter ratio is increased, banks of streamlined tubes behave like individual tubes; the pressure drop decreases as the minor-to-major axis ratio is decreased. Compared with banks of circular tubes, banks of streamlined tubes always reduce pressure drop. The average heat transfer coefficient of banks of streamlined tubes is reduced as the minor-to-major axis ratio of the tubes is decreased or the gap-to-diameter ratio is increased.; To use the streamlined tubes in a heat exchanger, tube wall thickness must be selected to withstand the internal fluid pressure and minimize the thermal resistance. Mechanical design curves are provided which related to tube deformation and strain with the tube geometry, material properties and internal pressure load. The heat transfer of streamlined tubes with nonuniform wall thickness is evaluated using a tube efficiency, which depends only on the tube geometry and the Biot number. The tube efficiency is calculated based on a two-dimensional conduction analysis and varies from 0 to 1.; Streamlined tubes can be used in polymer heat exchangers to reduce pressure drop. However, the heat transfer of streamlined tubes may be less than circular tubes because of the added conductive resistance. Heat transfer of streamlined tubes can be improved by increasing the material conductivity or modulus.