Impingement Heat Transfer on Dimpled Surfaces Using a Transient Liquid Crystal Technique

Abstract

Jet impingement heat transfer measurements are done for an array of inline air jets (four rows with each row having 12 jet holes) impinging orthogonally on dimpled (rough) target surfaces. Dimples 0.635 cm in diameter (equal to the jet diameter) and 0.3175 cm deep on the target surfaces are made in two different patterns (23 x 9 = 207 dimples and 11 × 5 = 55 dimples) to make it a dimpled surface. Detailed measurements are done using a transient liquid crystal technique. Three different spent air crossflow orientations are studied. Measurements are done at four jet Reynolds numbers ranging from 4.85 x 103 to 1.83 x 10 4 . The results show that the Nusselt numbers for a dimpled and a smooth surface are about the same. However, the dimpled surface provides a higher heat transfer due to an increased surface area when compared with a smooth surface. Similarly, the dimpled surface produces Nusselt numbers comparable to those of a pinned surface (a surface having pins of same size and number instead of dimples) for spent air crossflow orientations 2 and 3 but generates lower Nusselt numbers for orientation 1