Combustion Instabilities in Plasma Driven Flame Holders for Afterburners

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Conventional flame holders in afterburners cause non-negligible stagnation pressure losses throughout a mission even though the afterburner is only in use for less than a minute at a time. In addition, the hot metal of the flame holder becomes a source of IR radiation that an enemy missile can hone in on throughout the flight. This makes the plasma based Radical Jet Generator (RJG) an attractive alternative, because it is only deployed while the afterburner is in use. The RJG consists of a cavity, which is supplied with a combustible gas mixture that is ignited with a high frequency (HF) spark. Hot combustion products carrying radicals that are generated by partial combustion in the cavity are injected through an orifice into a combustible main flow. There the hot, reacting jet stabilizes the main combustion process. To improve flame holding the jet must penetrate well into the main flow. This can be achieved by accelerating the flow through a small nozzle. Unfortunately, this causes a combustion instability in the cavity, which significantly affects the radical concentration and, hence, the flame holding capability of the jet. Careful examination of the processes in the RJG indicates that when a spark is first originated it creates a partially conducting channel that is convected downstream by the flow. As the spark path grows so does the breakdown voltage until the energy required to form as new, more direct spark path is lower than that for the extended channel. At that instant, a new spark channel is established and the process repeats. Direct observations have shown that each HF spark channel ignites a flame kernel that grows as it propagates downstream. This produces pockets of reactants separated by products, which propagate downstream. Once the velocity in the RJG exceeds the threshold required for the combustion process to become unstable its frequency depends only on the equivalence ratio in the RJG, and not on further changes in the flow velocity or in the frequency of the HF spark. While the flow speed determines the residence time in the reactor, the equivalence ratio affects the ignition delay time and the flame speed. The interplay of these reactant properties affects the frequency of the combustion instabilities. This paper discusses the relative importance of the above mechanisms and points the way to a successful application of a RJG based flame holder.