Comparison of Thermo-Hydraulic Characteristics for Two Types of Dimpled Surfaces

Abstract

Heat transfer and pressure loss data have been obtained for internal flow in confined rectangular channel enhanced with two types of heat transfer augmenters: dimple concavities and combination of dimples on one side with concentrically arranged toroidal banks protruding into the channel on another side. Three different concavity array densities were studied for channel height bulk flow Reynolds numbers based on an equivalent inlet height of a rectangular passage* (foot note) and ranging from 770 to 26,500. Using the average heat transfer coefficient for the fully stabilized region, the overall heat transfer enhancements were compared to baseline smooth channel data. Friction coefficients were also compared to values for a smooth channel. As expected, the surface with banked dimples produces higher heat transfer at the cost of higher friction losses. Initial steps were made to optimize the density of the banked dimple geometry to achieve or exceed heat transfer performance of dimpled surface. Comparison was made for the same depth-to-dimple diameter ratio of 0.2. The results showed that heat transfer enhancements factor in the channel with one side augmented with dimples and another side augmented with banks at the density of 73% can reach 4.3 compared to the enhancement of 3.7 for a high density (67%) double-sided dimpled passage. The corresponding friction factor for studied dimple-bank configuration is reaching 27.0 compared to 2.3 for the dimpled channel. Obtained data indicates that a further improvement in thermo-hydraulic performance of dimpled surfaces is achievable.