AN EXPERIMENTAL INVESTIGATION OF AN ARRAY OF INLINE IMPINGING JETS ON A SURFACE WITH VARYING RIB ORIENTATIONS AND BLOCKAGES

摘要

This investigation focuses on multi-jet impingement configurations for gas turbine geometries in which the objective is to understand the influence of the roughness elements (ribs) on a target surface to the heat transfer and flow field. Existing studies in literature show the implementation of roughness elements for impingement configurations prove to increase heat transfer by 10-30%. Three different surface configurations are chosen for this multi-jet array impingement study: smooth surface (no ribs), small perpendicularly oriented ribs, and large perpendicularly oriented ribs. These roughness elements are non-continuous, broken rib turbulators which are square in cross section and oriented orthogonally to the mean flow direction within the cross flow duct. The roughness elements are oriented perpendicular to the mean flow direction. For each of the ribs tested, the two blockages tested, based on rib-to-channel height, were 20.83% and 41.67%. The jet impingement arrays are of an inline configuration. The Reynolds numbers tested, based on jet diameter, include 4,600, 13,300, 20,600, 30,200. The x/D (streamwise direction), y/D (spanwise direction), z/D (channel height direction) for the impingement array considered are 5 and 10, 8, and 3, respectively. A temperature sensitive paint technique was used to measure the heat transfer at the target surface, in which the local temperature was measured to estimate area averaged heat transfer coefficient (HTC), row averaged HTC, and stagnation region HTC. The spent air is made to exit from one direction only, thus generating a maximum cross flow situation. Keeping the jet diameter fixed at 5.1 mm, the pitch in the streamwise direction is doubled (x/D=10) to study the effect of reducing coolant flow on the Nusselt Number distribution. Direct comparisons for heat transfer augmentation were done for all test nodes, including baseline flat/smooth plate cases. From the local heat transfer distributions of the different array patterns of the roughness elements, this study aims to determine the effect of including these elements on the target surface by the increases seen in heat transfer compared to a flat/smooth target plate.