Control of combustion driven acoustic oscillations using plasma discharges

• Applicant: Professor Dr.-Ing. Jonas Moeck
• Subject Area: Fluid Mechanics
• Term: from 2013 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 242643022

• Final Report Year: 2018

Final Report Abstract

Thermoacoustic instabilities are a major challenge in the development of modern combustor technology for low-emission power and propulsion systems. Active control of these undesirable dynamics has been well advanced; however, suitable actuator technology is still missing. Nonequilibrium plasma discharges are promising, but their effect on the combustor dynamics is difficult to understand due to multiple interaction mechanisms between sound, flow, acoustics, and flame. In the present project, we have conducted detailed experimental investigations of these interaction mechanisms and significantly advanced the concept of combustion instability control by plasma discharges. Sound wave generation by low-frequency modulated non-equilibrium discharges was analyzed for broad ranges of relevant parameters. The dominant sound generation mechanism could be linked to unsteady heating. Using a model for compact acoustic sources, the sound source amplitude can be well estimated based on the electrical power evaluated at the modulation frequency. The sound source mechanism associated with modulated nanosecond pulsed discharges was then successfully used to control self-excited oscillations in a prototypical thermoacoustic system, the Rijke tube. The interaction of repetitively pulsed nanosecond glow discharges with laminar premixed flames was investigated in a set of experiments involving generic flames and different forcing mechanisms. It was demonstrated that modulated plasma forcing generates approximately harmonic fluctuations of the flame front and the heat release rate. The bandwidth is very similar to that obtained with acoustic forcing; however, the flame response to modulated plasma forcing exhibits bandpass characteristics. This property is entirely consistent with the physics of the forcing mechanism, as a continuous application of repetitively pulsed discharges does not affect the heat release rate. The key mechanism in the dynamic flame response to plasma forcing is found to be an increased local reactivity associated with thermal and chemical effects. A numerical model reflecting this mechanism reproduces the experimentally observed step response well. Another important interaction mechanism is the effect of unsteady flow on discharge formation. This aspect was investigated in a relevant pin–annular electrode configuration in reacting and non-reacting conditions, without and with acoustic forcing. Higher temperature and ionized species promote discharge formation. In a quiescent gas, subsequent discharges would therefore preferentially propagate along the same path. In a non-quiescent medium, the pretreated gas is advected in the time between two pulses so that a transient evolution of subsequent discharges is observed. In a turbulent swirl flame, a repeating sequence of downstream movement, elongation, break-off, and new ignition at a random location occurs. In addition, acoustic oscillation frequency and amplitude can have a strong impact on the plasma regime; a model derived based on the residence time of a fluid parcel in the inter-electrode region shows good agreement with the data. Turbulent swirl flames forced by low-frequency modulated nanosecond pulsed discharges exhibit clear harmonic fluctuations over a relevant frequency range. The bandpass characteristic – a fundamental property associated with this forcing type – is again evidenced. A plasma power of the order of 1% of the thermal power of the flame is sufficient to generate significant fluctuations in the coherent response of the flame. The perturbation patterns observed in experiments are very similar to swirl flames forced by acoustic waves or equivalence ratio fluctuations, which underlines the kinematic nature of the response to plasma forcing. The close similarity of the flame’s response to acoustic and plasma forcing facilitate effective suppression of self-excited thermoacoustic oscillations by means of feedback control, which is successfully implemented experimentally.

Publications

• Effect of nanosecond glow discharges on a lean premixed V-flame. IEEE Trans. Plasma Sci. 42(12):4040–4041, 2014
  Lacoste D., Moeck J.
  
• Stabilization of a methane–air swirl flame by rotating nanosecond spark discharges. IEEE Trans. Plasma Sci. 42(10):2412–2413, 2014.
  Moeck J., Lacoste D., Durox D., Guiberti T., Schuller T., Laux C.
  
• Influence of air flow parameters on nanosecond repetitively pulsed spark discharges in a pin-annular electrode configuration. J. Phys. D: Appl. Phys. 49:155205 (15 pages), 2016
  Heitz S., Moeck J., Schuller T., Veynante D., Lacoste D.
  
• Sound generation and control of thermoacoustic instabilities by nanosecond plasma discharges. In: Proceedings of the 23rd International Congress on Sound and Vibration. July 10–14, Athens, Greece, 2016
  Bölke O., Moeck J.P., Lacoste D.A.
  
• Analysis of the step responses of laminar premixed flames to forcing by non-thermal plasma. Proc. Combust. Inst. 36:4145–4153, 2017
  Lacoste D., Moeck J., Roberts W., Chung S., Cha M.
  
• Effect of nanosecond plasma discharges on methane-air premixed stagnation flames. In: Proceedings of the 8th European Combustion Meeting. April 18– 21, Dubrovnik, Croatia, 2017
  Heitz S., Lacoste D., Moeck J.
  
• Transfer functions of laminar premixed flames subjected to forcing by acoustic waves, AC electric fields, and non-thermal plasma discharges. Proc. Combust. Inst. 36:4183–4192, 2017
  Lacoste D., Xiong Y., Moeck J., Chung S., Roberts W., Cha M.
  
• Unsteady response of a swirl-stabilized premixed flame to low-frequency modulated nanosecond-plasma forcing. In: Proceedings of the 8th European Combustion Meeting. April 18–21, Dubrovnik, Croatia, 2017
  Bölke O., Reumschüssel M., Lacoste D., Moeck J.