Influence of the Pulse Shape on Heat Transfer at the Stagnation Point of an Unsteady Axisymmetric Impinging Jet

A numerical study of the effect of the pulse shape (rectangular, triangular, and sinusoidal) on heat transfer at the stagnation point of a turbulent pulsed jet was carried out. It is shown that both an increase and a suppression of heat transfer are possible in a nonstationary impinging jet in comparison with a stationary jet for all studied pulse shapes. In the area of small distances between the pipe outlet cross-section and the target surface (H/D <= 6) in a pulsed jet, the heat transfer at the stagnation point increases with an increase in the pulse frequency, while an increase in frequency causes a decrease in heat transfer for H/D > 8. An increase in the Reynolds number leads to a decrease in the heat transfer intensification ratio, and the data for all frequencies approach the steady-state impinging jet regime. The predicted results are compared with the experimental data available in the literature. Satisfactory agreement was obtained on the effect of the shape and frequency of pulses on heat transfer at the stagnation point by the target surface.