Heat transfer of phase-locked modulated impinging-jet arrays

In order to achieve a more uniform and possibly higher overall heat transfer in arrays of impinging jets, an experiment was conducted aimed at investigating effects of mechanical modulation of a jet assembly for the case that a fixed phase relation exists between the modulated jets. For this experiment a small array consisting of six jets has been constructed. With a rotating shutter disc it was possible to switch the jets on and off with adjustable frequency and with the desired phase relation. Heat transfer of the array of jets has been measured by using an electrically heated stainless steel foil as impingement surface. From the measured temperature distribution the local heat transfer coefficient was evaluated. The set up has been tested with benchmark data for a single jet. The heat transfer has been compared for modulated and unmodulated jets with characteristic time averaged Reynolds number Re=10 000 . The results showed that an improvement in uniformity together with an enhancement in mean heat transfer could only be found around the lowest achievable modulation frequency f≈200 Hz and for dimensionless nozzle to impingement plate spacings H/ D=4 and 6.