Oscillatory thermal structures induced by unconfined slot jet impingement

Low-frequency temporal measurements of impingement surface temperature in a nominally steady turbulent submerged impinging slot jet flow using infrared (IR) thermography are presented and analyzed. Data of surface temperature oscillations were recorded at an exit Reynolds number of 22,500 and at nozzle spacings (Y) of 0.5, 1.0, 2.0, 4.5 and 5 nozzle hydraulic diameters (Dh) from the impingement surface. Mean temperature maps were used to identify specific locations (lines) of interest on the impingement surface for transient experiments. Low-magnitude, periodically repeating thermal streaks were observed for the Y/Dh = 5.0 jet impingement, suggesting the presence of near-wall streamwise counter-rotating vortex pairs along the impingement line. For the Y/Dh = 0.50 jet impingement, temperature streaks at the impingement line were not detected; however, distinct thermal streaks were observed at locations corresponding to the local minimum and secondary maximum in heat transfer. To better understand the trends in the temperature oscillations, the time series were analyzed using a proper orthogonal decomposition (POD) method. Approximately 80% of the variance in temperature fluctuations at the impingement line could be reconstructed from five modes for the Y/Dh = 5.0 jet impingement. For the near-wall (Y/Dh = 0.50) jet impingement, the dominant two modes accounted for 80% of the variance in temperature fluctuations at the locations of minimum and secondary peak in heat transfer.