Heat transfer characteristics of impinging jets: The influence of unsteadiness with different waveforms

A series of experiments are conducted in which specially designed periodic air jets are applied to a heated surface for the purpose of enhancing heat transfer relative to the corresponding steady air jet. The periodic jets are produced by a mass flow rate controller. The experiments are performed for steady jets and for specially designed periodic jets, including a combination of sinusoidal, triangular and rectangular jets at frequencies from 1.25 to 20 Hz, the triangular signal having a different symmetry (representing the ratio of time to increasing velocity in one cycle to total cycle time) and the rectangular jets having a variable duty cycle (representing the ratio of the pulse cycle on-time to off-time) at frequencies of 10 Hz and 20 Hz. Various unsteady jets show distinguishing frequency dependences and the step change, i.e. the sudden increase or decrease of signal, shows some advantageous influence on heat transfer improvement, especially negative step change. Therefore, the enhancement of combined signals lies between the performances of the individual signals. The enhancement for triangular (or sinusoidal) plus rectangular signals shows some improved performance compared with that of rectangular plus triangular (or sinusoidal) signals. The heat transfers are enhanced as Strouhal number increases from 0.004 to 0.068 on the whole for impinging jet with such combined signals. The signals of triangular jet with symmetry parameter of 0 and 1 have improved heat transfer, and the latter has a slightly better result than the former. The instantaneous changing rate of velocity also has an influence on heat transfer improvements. Thus the duty cycle of 1:2 has the best performance in terms of heat transfer enhancement in this study.