A numerical study has been made to examine the thermal fields of rectangular impinging jets with different duty cycle(DC), which is defined as the ratio of pulse cycle on-time to off cycle time in one period, inlet Reynolds number and frequency. The Nusselt numbers show best enhancement with DC=1:3, Re=22000 and f=40Hz. The possible reason for this is that the instantaneous velocity change in this case from off to on and on to off, especially when the velocity changes from on to off, is more severe than other cases.

An experimental investigation has been carried out to study the effect of unsteady periodically impinging jets on the flow field and heat transfer characteristics. The experiments are performed for steady jets and for typical periodical jets (i.e., sinusoidal and rectangular jets) at frequencies from 1.25 to 40Hz. The periodical jets are produced by a special mass flow rate controller. The investigation shows that the stagnation point heat transfer does not show any enhancement for the periodically impinging jets when the frequency is lower. Various signals of unsteady jets show distinguishing frequency dependences and the rectangular jet, which has a step change in signal function itself, is the most effective one for heat transfer improvement and the degree of enhancement is in the range 30–40% at frequency of 40 Hz. This increase is believed to be caused by higher oscillations and strong entrainments to the ambient fluid. The hotwire anemometry is used to measure the velocity at centerline of the nozzle and PIV is used to measure the phase-locked flow field of the periodically impinging jet. The flow field is also obtained by numerical simulation with CFD.

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