Control of combustion driven acoustic oscillations using plasma discharges – DRACO

First, fundamental experimental studies of plasma / acoustic and plasma / flame interaction have been performed, at atmospheric pressure, in dedicated set-ups. The non-equilibrium plasmas were obtained by applying nanosecond repetitive high-voltage pulses. The plasma /acoustic interaction has been investigated in a Rijke tube, and in swirled flow, more representative of industrial applications. The flame response to plasma actuation was studied through the combustion of lean methane-air gaseous mixtures.

Advanced optical diagnostics such as Laser Induced Fluorescence (LIF), Optical Emission Spectroscopy (OES), or Coherent Anti-Stokes Raman Spectroscopy (CARS), as well as fast visualization, have been used in order to understand the coupling between plasma and flame, and plasma and acoustic waves.

With the help of numerical simulations of plasma / flame interaction, an empirical model of the effect of NRP plasma discharges on combustion dynamics of lean premixed flames, as a local increase in the burning rate, has been then investigated.

Plasma /Acoustic interaction:
So far, two main results have been obtained. First, the acoustic forcing of the flow has a strong impact on the NRP discharges behavior. For low frequencies, acoustic forcing of the air flow can promote the spark regime of these plasma discharges, when only the NRP corona regime was observed for a steady flow. This result may be explained by the influence of the acoustic forcing on the residence time of air particles in the inter-electrode area. Indeed, under the action of an acoustic forcing, an air particle stays for a longer or a shorter time in the inter-electrode area, and thus is submitted to more or less high-voltage pulses. When an air particle is submitted to enough pulses, the corona to spark transition can occur.
Second, the acoustic source properties of nanosecond repetitively pulsed (NRP) discharges in a ducted configuration has been characterized. Measurements with low-frequency modulated NRP plasma discharges demonstrate the possibility of NRP plasma to act as a controlled acoustic source and show the dependence on gap distance, pulse repetition frequency and duty cycle. The observed acoustic source amplitudes could be confirmed with a theoretical estimate based on compact unsteady heating.

Plasma /Flame interaction:
Since in combustion instabilities, one key factor is the response of the flame to acoustically induced velocity fluctuations, the two types of forcing, acoustic and plasma, have been compared. The effect of NRP glow discharges (non-thermal plasma) and flow perturbations on the flame dynamics are very similar. In order to verify the non-thermal activation of the gas by the plasma, thermometry by CARS has been realized (see illustration). This very interesting result assesses the strong effect of the plasma-activated gas chemistry on the combustion. By comparing this results with numerical simulations, an empirical model of the plasma actuation, as a local increase in the burning rate has been evaluated.

In the next step of the project, the influence of NRP discharges on the dynamics of swirl-stabilized lean premixed flames will be investigated. In particular, the influence of the hydrodynamics generated by the plasma, and its impact on combustion will be characterized. Finally, a plasma actuator will be implemented in a 100 kW test-rig, fed with a mixture of natural gas and air, and active control of combustion oscillations will be studied, using appropriate control schemes. At each step of the project, highly relevant diagnostics will be applied thanks to the know-how of the partners.

In addition to the broad fundamental goals, the project will also reflect many of the stated ANR criteria for broader impact. In particular, the improved understanding of the principles of control of combustion instabilities by non-equilibrium plasma discharges will help developing plasma-assisted combustion research in France and Germany, potentially enabling a wide variety of applications such as control of flame dynamics in high-speed flows or high pressure reactors, noise mitigation, and reduction of fuel consumption and pollutant emissions.

Publications
[1] D.A. Lacoste, J.P. Moeck, Effect of nanosecond glow discharges on a lean premixed V-flame, IEEE Trans. Plasma Sci. 42(12), 4040-4041, 2014.
[2] F. Tholin, A. Bourdon, Influence of the external electrical circuit on the regimes of a nanosecond repetitively pulsed discharge in air at atmospheric pressure, Plasma Phys. Control. Fusion 57, 014016 (12pp), 2015.

Conferences and workshops
[3] J.P. Moeck, D.A. Lacoste, Spectroscopic analysis of the response of a laminar premixed flame to excitation by nanosecond glow discharges, 35th International Symposium on Combustion, San Fransisco, CA, USA, August 3-8, 2014.
[4] D.A. Lacoste, S.A. Heitz, J.P. Moeck, Temperature measurement of plasma-assisted flames: comparison between optical emission spectroscopy and 2-color laser induced fluorescence techniques, 7th European Combustion Meeting, Budapest, Hungary, March 30 – April 2, 2015.
[5] S. A. Heitz, J.P. Moeck, T. Schuller, D.A. Lacoste, Experimental investigation of the effect of inter-electrode air flow on the corona-spark transition for nanosecond repetitively pulsed plasma discharges, 5th ATW Fundamentals of Aerodynamic Flow and Combustion Control by Plasmas, Les Houches, France, April 12-17, 2015.
[6] O.B. Bölke, D.A. Lacoste, J.P. Moeck, Acoustic characterization of NRP discharges using multi-microphone-method (MMM), 5th ATW Fundamentals of Aerodynamic Flow and Combustion Control by Plasmas, Les Houches, France, April 12-17, 2015.

Combustion instabilities are a serious problem for the design of modern combustion chambers in aero-engines and land-based gas turbines for power generation. One promissing approach to suppress the high-amplitude pressure pulsations associated with these thermoacoustic instabilities is based on active control of the system dynamics. Previous studies have demonstrated the capability of numerous methods, including acoustic forcing and fuel injection modulation. However, the applicability of these methods in full-scale engines remains limited, mainly because of restrictions in available actuator technology and performance.
In 2012, the two partners involved in the project DRACO, namely EM2C (France) and TUB (Germany), have successfully tested the use of non-equilibrium plasma discharges to control flame dynamics [Lacoste2013, Moeck2013]. The proof-of-concept has been done that Nanosecond Repetitively Pulsed (NRP) discharges can be used as an actuator for active control of combustion instabilities, without the drawbacks of traditional actuators such as loudspeakers or fuel valves. However, while this preliminary study was promising, it appears that the coupling between plasma and flame dynamics remains unclear.

The project DRACO focuses on (1) the fundamental understanding of the coupling of combustion dynamics and non-equilibrium plasma discharges, (2) the experimental evaluation of the potential of non-equilibrium plasma discharges for the active control of combustion instabilities. Experiments will be carried out in France and in Germany, to benefit from the complementary expertise of the two partners. First, fundamental studies of plasma / acoustic and plasma / flame interaction will be performed, at atmospheric pressure, in dedicated set-ups. With the help of numerical simulations of plasma / flame interaction, an empirical model of the effect of NRP plasma discharges on combustion dynamics of lean premixed flames will be proposed. In a second step, the influence of NRP discharges on the dynamics of swirl-stabilized lean premixed flames will be investigated. In particular, the influence of the hydrodynamics generated by the plasma, and its impact on combustion will be characterized. Finally, a plasma actuator will be implemented in a 100 kW test-rig, fed with a premixture of natural gas and air, and active control of combustion oscillations will be studied, using appropriate control schemes. At each step of the project, highly relevant diagnostics will be applied thanks to the know-how of the partners.

In addition to the broad fundamental goals, the proposed project will also reflect many of the stated ANR criteria for broader impact. In particular, the improved understanding of the principles of control of combustion instabilities by non-equilibrium plasma discharges will help developing plasma-assisted combustion research in France and Germany, potentially enabling a wide variety of applications such as control of flame dynamics in high-speed flows or high pressure reactors, noise mitigation, and reduction of fuel consumption and pollutant emissions. By maintaining regular participant meetings, by students, researchers and Pis, participation in French, German, and international scientific conferences, and by publishing the results in well-recognized scientific journals, the obtained progress will be rapidly disseminated to the scientific community on a world-wide scale.