Effect of mainstream flow oscillations on the spatial and temporal variations of film cooling*

The influence of mainstream oscillations on the spatio-temporal variation of film cooling were quantified extensively by the fast-response pressure-sensitive paint (fast-PSP) technique. During the experiment, coolant was discharged from a 35°-inclined round hole and injected into the cross flow.

While the steady state (i.e., f = 0 Hz) was selected as the baseline, the freestream oscillating frequency were increased from 5 Hz to as high as 20 Hz. Measured instantaneous effectiveness results were analyzed in terms of the ensemble-averaged and phase-averaged methods, where the latter one was acquired through triple decomposition. Compared to the steady state, a reduction of 30–35% was observed for the ensemble-averaged results at f = 20 Hz cases. As for the phase-averaged results, a relatively weak dependence was observed at 5 Hz with respect to the phase angles. However, as the frequency increased to 20 Hz, obtained effectiveness exhibited a strong correlation with the velocity phase.

In general, mainstream oscillations have a significant impact on the film cooling behavior. They not only degrade the cooling effectiveness, but also increase the unsteadiness in film cooling.