โ Scribed by Y.M. Tang; G. Waldherr; J.I. Jagoda; B.T. Zinn
No coin nor oath required. For personal study only.
This paper discusses the mechanism by which heat release fluctuations drive pressure pulsations in Helmholtz pulse combustors with nonpremixed fuel and air injection, similar to those used in commercialized pulse furnaces. Flow and flame spread in the mixing chamber were mapped using 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 soon as the combustor pressure drops below the reactants' supply pressures. If most of the heat were released at that time, the heat release and pressure fluctuations would be out of phase, which, according to Rayleigh's criterion, would prevent pulse combustion operation. In practice, pulse combustion operation is attained through the interaction of several processes. First, the fuel jet is ignited as soon as it enters the mixing chamber, generating pockets of burning gas. This reacting flow is entrained and convected by the air jet, which follows the fuel jet into the combustor, first downstream and then upstream in the mixing chamber. Simultaneously, fuel and air continue to enter the combustor, but are not immediately ignited, either because of excessive flame 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 sources. 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 ยฐ . This investigation showed that the interaction between complex flow and combustion process within the mixing chamber causes the time delay needed to produce heat release oscillations that are nearly in phase with the pressure oscillations, thus assuring pulse combustion operation.
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