Impingement cooling of an isoflux flat plate by blockage jet

Abstract

Impingement cooling of an isoflux flat plate by a round jet, generated through a perforated blockage plate, is experimentally investigated at a fixed jet Reynolds number of Rej = 20,000. With the perforation diameter (Dj) fixed, varying the thickness (t) of blockage plate forms three distinct turbulent jets: (1) orifice jet (e.g., t/Dj ≤ 0.5), (2) blockage jet (e.g., 0.5 < t/Dj < 8.0), and (3) tube (or nozzle) jet (e.g., t/Dj ≥ 8.0). Thermofluidic characteristics of these jets in both free exit and impingement were measured and directly compared under consistent conditions. Particular focus was placed upon how these characteristics vary in the intermediate range of jet relative thickness t/Dj, with reference to the two limiting cases, namely, the orifice jet and the tube jet. It was demonstrated that the blockage jet behaves neither as an orifice jet, nor as a tube jet, and its properties cannot be considered interchangeable. Measurements of jet flow properties - exit velocity profile, potential core length and centerline turbulence - highlight the distinguishing features among the three jet types. While the blockage jet exhibits an intermediate thermal performance, the tube jet provides the poorest performance, and the orifice jet performs the best. Since it is not always possible to use an orifice jet in practice (e.g., due to structural constraints), this study provides evidence for the existence of optimal H/Dj for a given jet type (i.e., fixed t/Dj): correspondingly, both primary and secondary thermal peaks shift when t/Dj and H/Dj are varied.