Transient temperature field and heat transfer measurement of oblique jet impingement by thermographic phosphor

The transient heat transfer characteristics of a hot plate cooled by an oblique jet were investigated by thermographic phosphor thermometry. The initial surface temperature was 360 °C, and 2D instantaneous temperature fields were measured with 0.1-s time intervals for a jet Reynolds number of 3500. The distance from the nozzle to the surface and the angle of impingement were varied for measurements. Manganese-activated magnesium fluorogermanate (Mg4FGeO6:Mn) was used as a thermographic phosphor, and a pulsed UV-LED with a 385-nm wavelength was used for the light source. A CMOS high-speed camera acquired phosphorescence images at 4000 frames per second. The decay-slope method was used for calibration, and the uncertainty in the temperature measurement was less than ±3% for the wide temperature range of 130-530 °C. A 1D semi-infinite solid model was used to obtain the local heat transfer coefficient. The transient heat transfer is almost two times greater than the steady-state value. The maximum heat transfer coefficient occurred at the stagnation point, and a secondary peak appeared at high impinging angle. When the distance from the nozzle to plate is fixed, the air jet with high impinging angle shows better cooling performance. Flow visualization and time-resolved PIV measurements reveal that the secondary heat transfer peak is associated with unsteady vortex at the beginning of the wall jet.