Effects of hole arrangements on local heat/mass transfer for impingement/effusion cooling with small hole spacing

The present study investigates the local heat (mass) transfer characteristics of flow through perforated plates. Two parallel perforated plates placed, relative to each other, in either staggered, in-line or shifted in one direction. Hole length to diameter ratio of 1.5, hole pitch to diameter ratio of 3.0, and distance between the perforated plates of 1 to 3 hole diameters are used at hole Reynolds numbers of 3,000 to 14,000. For flows through the staggered layers and the layers shifted in one direction, the mass transfer rates on the windward surface of the second wall increase approximately 50% from impingement cooling alone and are about three to four times that with effusion cooling alone (single perforated plate). The high transfer rate is induced by strong secondary vortices formed between two adjacent impinging jets that are accelerated by the effusion flow. The overall transfer rate is dominated by the target (second) surface (approximately 50%) instead of the inside hole surface that is dominant with a single plate case. The transfer coefficient for the in-line layers is approximately 100% higher on the windward surface of the second wall than that of the single plate case. The transfer coefficient on the leeward surface for the second plate is affected little by upstream flow conditions.