Effects of Rotation on Heat Transfer due to Jet Impingement on Cylindrical Dimpled Target Surface

Jet impingement has been extensively used in gas turbine airfoil internal cooling, especially leading edge and mid-chord region, which are subjected to high heat transfer loads. Earlier studies have shown that impingement onto dimpled surface results in higher heat transfer augmentation, particularly in maximum crossflow setting. The present study investigates the effect of Coriolis force and buoyancy in rotating channel featuring round shape jet plate and dimpled target plate, where the spent air is allowed to exit in only one direction, thus imposing maximum crossflow. In order to ensure that the buoyancy acts in a direction similar to actual gas turbine blades, colder air is passed into the feed chamber (also in rotation). Detailed heat transfer measurements are presented using transient liquid crystal thermography, where the target surface is modeled as one-dimensional semi-infinite solid. The jet plate features three (and four) rows of circular holes, where the normalized spanwise and streamwise pitch (x/d, y/d) of jets is kept at 3. The normalized jet-to-target surface distance (z/d) is 4 and the nozzle aspect ratio is 2. The target plate features four rows of dimples, where the dimple print diameter to jet hole diameter ratio is 0.61. The cylindrical dimple arrangement on the target plate has been kept the same for all the configurations tested, where the dimple pitch to dimple print diameter ratio is kept at 1.832. The baseline case for the normalization of Nusselt number is the smooth target surface. Three different configurations of jet plate has been studied. The flow and rotation conditions have been kept the same for all the configurations, where the average Reynolds number (based on jet hole hydraulic diameter: Re_j) has been maintained at 2500 and the rotational speed has been kept at 400 RPM (corresponds to R_oj of 0.00274). Flow experiments have been carried out to determine the variation of discharge coefficient of each hole with the plenum absolute pressure to ambient pressure ratio, and these Cd measurements were used in the determination of the flow distribution in the jet hole plate. Under non-rotating conditions, dimpled target surface have heat transfer enhancements greater than unity. Also, for non-rotating cases, crossflow effects have been seem to be maximum and for long exit lengths, dimpled target surfaces might have lower heat transfer as compared to smooth target. It has been found that rotation has negative effects on heat transfer augmentation on dimpled target surface for all three configurations studied.