Experimental investigation of flame-holding system for the suppression of ramjet rumble

Combustion instabilities often occur in the liquid-fueled ramjet combustors using bluff body or sudden expansion for stabilization. From a practical point of view, the most severe oscillations are at the 100–500-Hz range. This low-frequency rumble is generally characterized by longitudinal acoustic oscillations. It has been shown that combustion oscillations can be stabilized by controlled periodic addition of secondary fuel, usually in the form of premixed gas fuel. The main idea of the present study is to use the air from the oscillating flow of the unstable combustor for the atomization and distribution of part of the main liquid fuel to obtain the required oscillating phase-shifted heat addition for stabilization. The effervescent spray injection, at relatively low operating pressure, was investigated as a model for pressure-dependent atomizer. A special laser light sheet system was used to obtain an integral indication of spray oscillation. Phase Doppler anemometry measurements were performed to determine the droplet velocity and droplet diameter oscillations with the different phase shifts respect to the oscillations of the atomizing air. A special diagnostic system based on the chemiluminescence of CH radicals is used for direct determination of heat addition oscillations.

The measurements revealed very fast response of the combined effervescent atomizer and flame-holding configuration. Response time between pressure perturbations and the heat release, of the order of 1–3 ms, as well as the limited spatial distribution of the control heat release was achieved. These characteristics proved the system's ability to serve as a passive control unit for suppressing low-frequency combustion oscillations in unstable combustors.