Effects of various inlet velocity wave patterns on unsteady jet heat transfer on a plane surface are studied experimentally and numerically. The waveforms analyzed include pulse, square, triangular and sinusoidal. The normalized nozzle-to surface-distance Z/D is varied from 2 to 12, the Reynolds number Re from 5100 to 23 000, and the frequency f from 0 to 200 Hz. The performance of unsteady jets is elucidated with the help of the 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 a sufficient quantity of cooling fluid near the plate. At low frequencies, a higher jet Re leads to a much greater decrease in cooling compared with a steady jet. Furthermore, this decrease is particularly pronounced at low Z/D. A performance comparison of unsteady jets shows that pulse jets are superior to square wave jets, which in turn outperform triangular wave jets, while sinusoidal wave jets are the least effective. Although unsteady jets at lower frequencies produce less cooling in the stagnation and transition regions compared with steady jets, they offer improved cooling in the wall jet area. The improved cooling is due to the presence of secondary vortices moving away from the surface. The velocity profiles indicate that pulse jets achieve a maximum velocity that surpasses that of steady and other forms of unsteady jets across all r/D values.
Vivek Mathew Jose (Mon,) studied this question.