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A new method of dual-frequency excited plasma (DFEP) is proposed to enhance ignition for supersonic combustion chamber. It is demonstrated experimentally that this method successfully achieves the acceleration of flame establishment and achieves the purpose of intensified ignition for supersonic flame in a real axisymmetric supersonic combustion chamber. The differences between the DFEP method and the traditional DC excited plasma (DCEP) method in the stage of local-flame establishment, the state of local-flame holding, the state of global-flame backpropagation, and the state of global-flame downstream transport are compared. It is found that the DFEP method can not only shorten the time of the state of the local-flame holding from 292.2 ms to 49.3 ms, but also prevent the suspension of the global-flame when the global-flame is in backpropagation, but this method cannot increase the local-flame propagation velocity and the global-flame backpropagation velocity. Detailed analysis and speculate together indicate that the effect of DFEP on flame comes from its ability to reduce the suspension time of the flame. The reason why DFEP has such a ability is discussed, and a new view never projected before is speculated: ➀ The flame (and shockwave) itself is in an unstable equilibrium between the positive feedback of backpropagation, the negative feedback of flow and the constraint of the expanding section, and the suspension time of flame (as well as shockwave) is positively related to the probability of the shockwave leaving the expanding section by self-excited oscillation. ➁ The extra active O radicals generated by the DFEP can increase the combustion heat release rate and the shockwave intensity, thereby providing a new path to help heat accumulation and making the flame easier to escape from the expanding section under the action of self-excited oscillation. [ABSTRACT FROM AUTHOR]

Facing the requirement of flameholding in a supersonic combustion chamber, a flameholding experiment under dual frequency excited plasma (DF excited plasma) was carried out in an axisymmetric supersonic combustion chamber. The flame establishment process was observed by using a high-speed-camera, and the flameholding characteristics with or without DF excited plasma were analyzed under different equivalence ratio(ER). The experimental results show that the flame will be enhanced by using dual-frequency excitation plasma in the supersonic combustion chamber: the flameholding mode was transformed from a global flame to a local flame, as well as the pressure of the combustion chamber was decreased significantly, when the DF excited plasma is switched off. In the ER range of 0.51 (0.18+0.33)∼0.60 (0.18+0.42), the DF excited plasma could induce a transformation from a local to a global flame, while if the ER is too high (ER>0.69 (0.18+0.51)) or too low (ER<0.18), the action of the DF excited plasma always couldn't. A basic feedback mechanism of the flameholding was proposed, with the consideration of the experimental results. The O radicals generated by the DF excited plasma was one important factors for flame reverse propagation, and the existence of the global flame was another necessary factor. Under the combined action of the O radicals of DF excited plasma and the global flame, the positive feedback process in the combustion chamber will be intensified, and the flame will propagate reversely. [ABSTRACT FROM AUTHOR]