Time-Resolved Thermal Surface Flow Structures in Impinging Slot Jet Flows

This paper presents low-frequency time-resolved measurements of impingement surface temperature in a turbulent submerged impinging slot jet flow at an exit Reynolds number of 22,500 using infrared (IR) thermography. The nozzle-to-surface spacing was varied from 0.5 to 5 nozzle hydraulic diameters. Time-traces of temperature at specific locations of interest, identified from mean temperature maps, are presented. At a nozzle spacing of 5 hydraulic diameters corresponding to transitional jet impingement, the time traces of surface temperature show mild, but periodically repeating hot and cold streaks, suggesting the presence of near-wall streamwise counter-rotating vortex pairs along the impingement line. At a closer nozzle spacing of 0.5 hydraulic diameter corresponding to impingement of the jet potential core, the temperature streaks at the impingement line could not be detected. Previously unreported distinct thermal streaks are observed at locations corresponding to the local minimum and secondary maximum in heat transfer. Data of rms-averaged temperature fluctuations for the two spacings corroborate well with time trace observations.