Effects of various inlet waveforms on jet impingement heat transfer

Effects of various inlet velocity wave patterns on the unsteady jet heat transfer that occurs on a plane surface are studied experimentally and numerically. The waveforms analysed include pulse, square, triangular and sinusoidal. The normalised nozzle to surface distance(Z/D) is varied from 2 to 12, Reynolds number(Re) from 5100 to 23000 and frequency(f) from 0 to 200 Hz. The performance of unsteady jets is elucidated with the help of velocity profile and boundary layer. At lower Re, unsteady jets with frequencies exceeding 100 Hz are more effective than steady jets; however, at lower frequencies, the opposite holds true. The frequency should be such that it should disrupt the boundary layer and also maintain sufficient quantity of cooling fluid near the plate. At low frequencies, a higher jet Re leads to a much greater decrease in cooling compared to steady jet. Furthermore, this decrease is particularly pronounced at low Z/D. At frequencies and Re considered, performance comparison of unsteady jets shows that pulse jets are superior to square wave jets, which in turn outperform triangular jets, while sinusoidal jets are the least effective. In unsteady jets, improved cooling in the wall jet area is due to the presence of secondary vortices.