Measurement of detailed heat transfer on a surface under arrays of impinging elliptic jets by a transient liquid crystal technique

Abstract

Detailed heat transfer characteristics on a flat surface under arrays of impinging elliptic jets were measured by a transient liquid crystal technique. The elliptic jet holes of five different aspect ratios, AR = 4, 2, 1, 0.25, and 0.5, jet Reynolds numbers Re = 1500, 3000, and 4500, and three exit flow conditions are considered to investigate impingement heat transfer performance and the associated flow structure at various conditions. Results show that effects of the aspect ratio and crossflow have significant influences on the axial shift of the impingement/touchdown locations. The present thermographs with the high- Nu spots are very useful for understanding of the flow characteristics and jet structure deformation at various conditions. The axis-switchover phenomenon is found with the elliptic jets of aspect ratio AR > 1, but it does not occur in the cases of AR ⩽ 1. Among the five aspect ratios considered, the mean heat transfer rates with the elliptic jets of AR = 0.5 are the highest at Re = 3000 and 4500; while, in the low- Re case of Re = 1500, the jet array with jet arrays with AR = 2 and 1 perform better than that with AR = 0.5. In addition, among the three, the two-way exit flow condition is most beneficial to the heat transfer characteristics of an impinging jet array.