Heat transfer and discharge coefficient effects of jet impingement on a surface with hemispherical and teardrop shaped bumps

Abstract

Jet impingement is among the most effective methods of enhancing heat transfer in practical applications, enabling high heat transfer coefficients without the high pressure losses associated with flow through turbulated channels. The gas turbine industry is interested in combining jet impingement with dimpled and bumped plates as a method of improving heat transfer, enabling greater efficiency. This study investigates the average effects on heat transfer and discharge coefficients (Cd) created by arrays of jets impinging on target surfaces with hemispherical or teardrop shaped bumps using heated copper plates with embedded thermocouples. Reynolds number (Re) ranged from 5000 to 20,000 and 5000 to 26,000 for heat transfer and Cd tests, respectively, for all configurations. The jet pitch-to-diameter ratio x/d ranged between 3.3 and 12 and the jet exit-to-target-plate spacing ratio (z/d) ranged from 2 to 5 for all configurations. Additional Re and z/d values were tested for various configurations in order to fully quantify trends. The effects of spent air discharging in multiple directions was also analyzed. Depending on configuration, impinging on the bumped plates produced minor increases, no change, or a reduction in heat transfer compared to the baseline smooth plate depending on configuration. However, when accounting for the increase in surface area associated with the bumped plates, heat transfer was universally improved. Heat transfer was found to be quadratically related to Re and z/d and exponentially related to x/d. Spent air flow had minimal effect on heat transfer except for the x/d=12 configuration. Discharge coefficients were found to be either unaffected or only marginally affected by plate geometry. Cd was found to be independent of z/d for z/d greater than or equal to 2. Although Re and x/d have an effect on Cd, there is no clear relationship between these parameters. Overall, a jet array with x/d=6.6 and z/d=5 impinging directly on every other bump of the hemispherical target plate produced the greatest improvement in heat transfer over the baseline smooth plate, but at the expense of slightly lower Cd values. Overall, jet impingement on bumped surfaces provides an improvement in heat transfer with a small decrease in Cd.