Abstract: An experimental investigation is performed to study the effect of unsteady impinging jet on convective heat transfer. Periodical unsteady flows with different shapes and frequencies are generated by a mass flow rate control system. Experiments are carried out for three typical periodical flows, namely sinusoidal, triangular and rectangular flows, at frequencies from 1.25 to 40 Hz. By means of an enhancement coefficient (Nusselt number of the unsteady flow over that for the steady flow at same time-averaged flow rate) the heat transfer performance is characterized. Although convective heat transfer rate is less effective by imposing a lower frequency unsteady jet, heat transfer augmentation occurs above a threshold frequency. Experiments show that the enhancement in heat transfer is also highly dependent on the signal wave shape. The most effective unsteadiness of the three signals is the rectangular flow which has a step change in signal function itself, and the maximum increase in heat transfer is of the order over 30% at a frequency of 40 Hz. Unfortunately, for the sinusoidal and triangular flows without step change in signals, the heat transfer enhancement is very limited even at a frequency of 40 Hz.

Key words: Frequency, Heat transfer augmentation, Unsteady impinging jet, Wave shape