Impingement cooling flow structure and heat transfer along rib-roughened walls

Abstract

Slot-air-jet impingement cooling flow structure and heat transfer along rib-roughened walls is studied experimentally. The flow structure along the ribbed wall is visualized with smoke that is generated by vaporizing the oil coated over a heated resistance wire. The effect of different rib protrusions (heights) on the impinging flow and heat transfer along the wall is studied, which is achieved by using different sizes of nozzles. Two different ribbed walls with different rib pitches are selected which have a rib pitch-to-height ratio of 3 and 4, respectively. During the experiments, the Reynolds number varies from 2500 to 11 000, the slot width-to-rib height ratio from 1.17 to 6.67, and nozzle-to-plate spacing from 2 to 16. Due to the protrusion of the rib, the formation of an air bubble enclosing the cavity occurs which can prevent the jet from impinging on the wall and reduce the heat transfer. However, some portion of the jet flow in the downstream region, especially when it becomes turbulent, can penetrate the air bubble and impinge, and recirculate inside the cavity, which significantly increases the heat transfer. In general, the flow structure and the heat transfer observed are significantly different from the results of a flat plate. A comparison and correlations of the stagnation point Nusselt number under different conditions are presented and discussed.