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 propane 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 oscillation was consistent with the disturbances moving at the speed of the flow from the fuel nozzle, and the frequency showed regular cyclical variation as the sensing beam was moved slowly along the nozzle axis. This corresponded to regular physical stretching or compression of the induced structure so that an integral number of disturbances lay between an apparent origin (just outside the nozzle) and the sensing beam. Without 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 by feedback. Temperatures increases of between 40°C and 60°C were induced in the flame centre line temperatures, these increases extending far downstream of the location of the sensing beam. It appeared that feedback moved the apparent origin of the flame towards the nozzle as a consequence of enhanced mixing. | A laser schlieren sensing system has been coupled to provide feedback acoustic excitation of a co-annular turbulent diffusion flame of propane 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 oscillation was consistent with the disturbances moving at the speed of the flow from the fuel nozzle, and the frequency showed regular cyclical variation as the sensing beam was moved slowly along the nozzle axis. This corresponded to regular physical stretching or compression of the induced structure so that an integral number of disturbances lay between an apparent origin (just outside the nozzle) and the sensing beam. Without 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 by feedback. Temperature increases of between 40 °C and 60 °C were induced in the flame centre line temperatures, these increases extending far downstream of the location of the sensing beam. It appeared that feedback moved the apparent origin of the flame towards the nozzle as a consequence of enhanced mixing.