Heat transfer and flow field measurements of a pulsating round jet impinging on a flat heated surface

Heat transfer and flow field characteristics of a pulsating round air jet, impinging on a flat heated target surface were measured and compared to those of a steady impinging jet. The jet was partially confined and the target surface stand-off distance equaled two nozzle diameters, D. Investigated Reynolds numbers ranged from Re = 4,606 to 13,513, while pulsating jet velocities varied nearly sinusoidal with Strouhal numbers ranging between 2×10⁻³< Sr<15.6×10⁻³. Results indicated that besides the well-known secondary Nusselt number peak at approximately 2D from the stagnation point, an additional distinct increase in Nusselt number was detected at approximately 4D from the stagnation point as a result of partial jet confinement. For the studied Re and Sr-number range, jet pulsation resulted in both enhanced and slightly reduced overall heat transfer. Our results indicated that the physical mechanism responsible for overall heat transfer enhancement or reduction due to jet pulsation is increased or reduced primary vortex generation Strouhal numbers (Sr_v), respectively. The latter were extracted from phase-locked particle image velocimetry measurements. Overall heat transfer modulation linearly depended on the ratio of the ensemble averaged pulsating jet and steady jet primary vortex generation Strouhal numbers, Sr¯_v/Sr¯_v,0. Overall heat transfer enhancement and reduction was obtained when Sr¯_v/Sr¯_v,0>1 and Sr¯_v/Sr¯_v,0<1, respectively.