HEAT RELEASE TIMING IN A NONPREMIXED HELMHOLTZ PULSE COMBUSTOR

This paper discusses the mechanism by which heat release fluctuations drive pressure pulsations in Helmholtz pulse combustors with nonpremix ed fuel and air injection, similar to those used in commercialized pul se furnaces. Flow and flame spread in the mixing chamber were mapped u sing high-speed shadowgraphy, extensive laser Doppler velocimetry, and radical imaging. Flow visualization and velocity measurements showed that a fuel jet followed by an air jet enter the pulse combustor as so on as the combustor pressure drops below the reactants' supply pressur es. If most of the heat were released at that time, the heat release a nd pressure fluctuations would be out of phase, which, according to Ra yleigh's criterion, would prevent pulse combustion operation. In pract ice, pulse combustion operation is attained through the interaction of several processes. First, the fuel jet is ignited as soon as it enter s the mixing chamber, generating pockets of burning gas. This reacting flow is entrained and convected by the air jet, which follows the fue l jet into the combustor, first downstream and then upstream in the mi xing chamber. Simultaneously, fuel and air continue to enter the combu stor, but are not immediately ignited, either because of excessive fla me stretch caused by the fast moving fuel and air jets or because the air stream has displaced any hot gases that could act as ignition sour ces. Once the reacting gas pockets return to the upstream half of the mixing chamber, they ignite the combustible mixture that has collected there. This causes a rapid increase in heat release rate, which leads the pressure oscillation by around 30 degrees. This investigation sho wed that the interaction between complex flow and combustion processes within the mixing chamber causes the time delay needed to produce hea t release oscillations that are nearly in phase with the pressure osci llations, thus assuring pulse combustion operation.