OPTICAL-ACOUSTIC FEEDBACK APPLIED TO A TURBULENT-DIFFUSION FLAME

A laser schlieren sensing system has been coupled to provide feedback acoustic excitation of a co-annular turbulent diffusion flame of propa ne and air. It is found that positive feedback oscillation was induced at a level of feedback gain that was consistent with open loop broad band response measurements. The frequency of feedback induced oscillat ion was consistent with the disturbances moving at the speed of the fl ow from the fuel nozzle, and the frequency showed regular cyclical var iation as the sensing beam was moved slowly along the nozzle axis. Thi s corresponded to regular physical stretching or compression of the in duced structure so that an integral number of disturbances lay between an apparent origin (just outside the nozzle) and the sensing beam. Wi thout filtering the feedback system tended predominantly to lock into higher frequency, smaller structures associated with the inner mixing region. However, with appropriate band pass filtering in the feedback amplifier it was possible to induce either inner or outer structures b y feedback. Temperature increases of between 40 degrees C and 60 degre es C were induced in the flame centre line temperatures, these increas es extending far downstream of the location of the sensing beam. It ap peared that feedback moved the apparent origin of the flame towards th e nozzle as a consequence of enhanced mixing. (C) 1998 Elsevier Scienc e Inc. All rights reserved.