# Influence of Electrode Position on Plasma-Assisted Combustion in a Model Aeronautical Combustor

Jean-Baptiste Perrin-Terrin, Philippe Scouflaire, Sebastien Ducruix, Christophe O Laux

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Jean-Baptiste Perrin-Terrin, Philippe Scouflaire, Sebastien Ducruix, Christophe O Laux. Influence of Electrode Position on Plasma-Assisted Combustion in a Model Aeronautical Combustor. AIAA Scitech 2025, Jan 2025, Orlando, United States. 10.2514/6.2025-0605. hal-04933095

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# Influence of Electrode Position on Plasma-Assisted Combustion in a Model Aeronautical Combustor

Jean-Baptiste Perrin-Terrin<fnref n="1" />, Philippe Scouflaire<fnref n="2" /> Sébastien Ducruix<fnref n="3" /> and Christophe O. Laux<fnref n="4" />

Université Paris-Saclay, CNRS, CentraleSupélec, Laboratoire EM2C, 3, rue Joliot-Curie, 91190 Gif-sur-Yvette

This work presents an experimental study of a gas turbine model combustor stabilized by Nanosecond Repetitively Pulsed (NRP) discharges. The combustor consists of a plenum, a burner, and a combustion chamber with a square section. The injection device, inspired by Lean Premixed technologies, is composed of two stages (pilot and multipoint), both stabilized by swirl. The combustor is operated at atmospheric pressure with air and methane at a flame thermal power of 50 kW, and the power ratio between the stages is varied in this study. The Lean Blow-Out limit is extended using NRP discharges, and the plasma stabilization is enhanced by optimizing the position of the electrode. Three electrode positions are tested: in the pilot stage, in the multipoint stage and in the chamber backplane. The strongest plasma effect at all staging ratios is obtained for the “multipoint position”.

## I. Introduction

Lean Premixed (LPM) combustion is key for future combustion systems as it improves efficiency and reduces the emission of pollutants such as NOₓ, CO, particulates, and unburnt hydrocarbons. LPM combustion relies on efficient atomization and vaporization of the liquid fuel and good mixing of the vaporized gaseous fuel with the surrounding air flow. This is usually achieved by swirling the air flow, thus increasing the turbulence and velocity of the flow. However, the swirled lean flames thereby created are prone to instabilities and lean extinction [1]. Active flame stabilization techniques, in particular, Plasma-assisted Combustion (PAC) [2], can be used to regain stability and harness the full potential of LPM combustion. Among the several technologies used to generate plasmas for PAC applications, Nanosecond Repetitively Pulsed (NRP) discharges stand out for their efficiency [3]. These discharges are formed by pulses of 1–25 kV amplitude lasting a few nanoseconds and repeated at frequencies of 10–100 kHz. NRP discharges can form plasmas in various regimes such as corona, glow or spark [4,5]. For PAC applications, the spark regime has been widely used because it efficiently produces the high density of radicals required to enhance combustion under lean combustion conditions [6]. PAC with NRP has demonstrated excellent results in extending the Lean-Blow-Out (LBO) limit [7,8], controlling and suppressing thermo-acoustic instabilities [9,10], and improving ignition [11,12]. These results were obtained for a wide range of gaseous and liquid fuels, including propane [7,8], methane [13–15], ammonia [16–18], hydrogen [19], n-heptane, dodecane, and kerosene [20,21]. They were also demonstrated on several types of burners: bluff-body burners [7,13], swirl-stabilized flames [20,22], staged injections [8,14], and sequential combustors [15]. PAC can reach excellent energy efficiency with NRP discharges, as notably

<footnote n="1">¹ Ph. D. Student, AIAA Member</footnote>

<footnote n="2">² Research Engineer, AIAA Member</footnote>

<footnote n="3">³ Senior Scientist, AIAA Associate Fellow</footnote>

<footnote n="4">⁴ Professor, AIAA Fellow.</footnote>
shown by Dharmaputra et al. [15] who successfully suppressed thermo-acoustic instabilities in a 74 kW sequential
burner with 1.1 W of plasma power, representing only 0.0015 % of the flame thermal power.

In recent work, Blanchard et al. [14] applied NRP discharges in the BIMER facility, a high-power (20–150 kW) staged combustor representative of aeronautical combustion chambers. BIMER features a swirl-stabilized two-stage injector comprised of a central pilot stage and an annular multipoint stage. When more than 20% of the fuel was injected through the pilot stage, the LBO limit was extended by at least 50% with a plasma power of less than 0.5% of the flame thermal power. However, when all fuel was injected through the multipoint stage, the effect of the plasma was much lower. The authors suggested that the LBO extension at low staging factor (see Eq. (1)) might be improved by changing the location of the plasma in the combustion chamber. Indeed, at low staging factors, the pilot flow comprises mostly air and the OH-radicals produced by the NRP discharges recombine well before reaching the multipoint region where fuel is present. As a result, the combustion kinetics are not enhanced by the discharge, and the LBO limit is not extended.

In the present work, we assess the effect of the plasma location on the extension of the LBO limit by changing the
position of the electrode in the combustion chamber. To this end, the experimental setup of Ref. [14] is adapted to
explore several electrode locations.

II. Experimental setup

A. BIMER-PAC: a gas turbine-model combustor for plasma-assisted combustion

The BIMER-PAC combustor is described in detail by Blanchard et al. [14]. This facility is an adapted version of the one presented in Ref. [23–25] and is used to study the effect of NRP discharges in a swirled staged combustor. The combustor is illustrated in Fig. 1, with a schematic and a picture of the experimental facility. It comprises an air plenum, a two-stage injector, and a combustion chamber with a square cross-section of 15 cm and a length of 50 cm. The chamber is made of two water-cooled steel walls and two silica windows for optical access. Two smaller silica windows are added to the water-cooled wall on the injector side for laser diagnostic purposes. The backplane of the chamber is also water-cooled. The NRP discharges are applied using a water-cooled electrode as in Ref. [14].

As shown in Fig. 1, two K-type thermocouples are inserted into the lower wall through blind holes located on the centerline at 25 cm and 42 cm from the chamber backplane. They are used to monitor the wall surface temperature to determine when thermal equilibrium is reached. Additional thermocouples are mounted in the water-cooling system to measure the extracted heat flux.

<figure><img src="image_2.png" /><figcaption>Fig. 1 Left: Schematic of the BIMER-PAC combustor (adapted from [14,25]). Right: picture of the assisted flame.</figcaption></figure>

Fig. 2a shows the two-stage injector. In the pilot stage, fuel is injected axially through a 4-mm diameter tube. In the multipoint stage, the fuel is injected in a cross-flow configuration through the multipoint crown (15 holes of diameter 700 µm). The design of this injector is representative of staged LPM combustors found in aircraft engines such as the Twin Annular Premixing Swirled or Lean Direct Injection combustors [26]. The pilot stage creates a fuel-rich flame that stabilizes the lean and well-premixed multipoint flame that produces most of the power. The methane
mass flow rates of both stages can be independently controlled via two mass flow controllers (Bronkhorst EL-Flow F-203AC-AAD-44-V for the pilot stage and Bronkhorst Cori-Flow M15-RAD-44-0-S for the multipoint stage). We define the staging factor $\alpha_p$ as follows:

$$ \alpha_p = \frac{\dot{m}_{CH_4,pilot}}{\dot{m}_{CH_4,pilot} + \dot{m}_{CH_4,multi}} = \frac{\dot{m}_{CH_4,pilot}}{\dot{m}_{CH_4,tot}} \quad (1) $$

where $\dot{m}_{CH_4,pilot}$ and $\dot{m}_{CH_4,multi}$ are the mass flow rates of the pilot and multipoint stage, respectively. When $\alpha_p = 0\%$, all fuel is injected into the multipoint stage, and when $\alpha_p = 100\%$ all fuel goes through the pilot stage. The air mass flow rate is controlled by a single mass flow controller (Bronkhorst EL-Flow F-206BI-FA-00-V). The air is fed into the plenum and the head losses naturally split the flow: 20% goes through the pilot swirler and 80% through the multipoint swirler [23]. Three fuel equivalence ratios can be defined: global $\Phi_g$, pilot $\Phi_p$, and multipoint $\Phi_m$. They are given by the following formulas:

$$ \begin{aligned} \Phi_g &= s \times \dot{m}_{CH_4,tot} / \dot{m}_{air,tot} \\ \Phi_p &= 5\alpha_p \times \Phi_g \\ \Phi_m &= 1.25(1 - \alpha_p) \times \Phi_g \end{aligned} \quad (2) $$

where s is the mass stoichiometric ratio.

Fig. 2b shows the detail of the electrode positioning system. The design of this system must meet three stringent constraints: (1) it must be air tight, (2) it must allow electrode mobility, and (3) it must sustain high heat fluxes (direct contact with the flame) and strong electric fields (up to 15 kV in the electrode) as the water cooled-wall is kept around 100°C and electrically grounded. The retained system is as follows: the electrode is inserted through the upper wall via a small silica window. Silica offers strong thermal resistance and good electrical insulation. An oblong hole in the window enables translating the electrode along the x-axis and another silica part is added to cover the hole. To ensure air tightness, the two silica parts are clamped with a brass flange. Two additional degrees of freedom are obtained via translation and rotation along or around the vertical y-axis. The electrode position is represented by the coordinates $(x, y, \theta)$, and the reference point $O_{x,y,\theta}$, shown in Fig. 2b. Note that $O_{x,y,\theta}$ corresponds to the intersection of the centerline of the combustor and the backplane. This system allows for a precise measurement of the electrode position. There are two geometrical differences between the present setup and the one used in Ref. [14]: first, in our system, the electrode is bent by 90° whereas the one of Blanchard et al. had a wider bend of 120°; second, they inserted the electrode in a circular hole 3.2 cm off the centerline of the chamber axis whereas here the whole system is placed on centerline.

<figure><img src="image_3.png" /><figcaption>Fig. 2 (a) Schematic of the two-stage injector of the BIMER-PAC combustor and of the high-voltage electrode (adapted from [14]). (b) Detailed schematic of the electrode positioning system.</figcaption></figure>

## B. Diagnostics

The NRP discharges are produced by an FID-FPG 15-100NM10 pulser with negative polarity. The applied voltage V can be adjusted between 3 and 15 kV. The Pulse Repetition Frequency (PRF) is adjusted by a gate-and-delay generator (BNC model 575) used to trigger the pulser. The PRF is between 30 and 77 kHz. Voltage and current waveforms are measured with probes (a Lecroy PPE-20 kV, 100-MHz bandwidth voltage probe, and a 200-MHz bandwidth current
monitor, Pearson model 6585) connected to an oscilloscope (TeledyneLecroy HDO 6104B). The probes are placed 2-m away from the pulser in a 7-m coaxial cable corresponding to the “remote configuration” defined in the appendix of Ref. [13]. Typical waveforms are shown in Fig. 3. The deposited energy is obtained by integrating the product of the voltage and the current. The current and voltage traces are synchronized by superimposing the first peaks of voltage and current, assuming a resistive load at the measurement location.

<figure><img src="image_4.png" /><figcaption>Fig. 3 Example of current and voltage waveforms (top) and measured deposited energy (bottom).</figcaption></figure>

An ICCD camera (Princeton Instruments, PI-MAX 4) equipped with a 50-mm lens (Nikon, AF Nikkor f/1.4) and a bandpass filter (Asahi spectra, F0101, Center wavelength = 430 nm, spectral FWHM = 10 nm) centered on the CH* chemiluminescent emission is used for flame imaging. Images are acquired through one of the quartz windows with an exposure time of 200 µs.

In some of the conditions investigated in this work, the plasma emission was found to be much more intense than the CH chemiluminescent emission. In these cases, the camera is synchronized with the plasma pulses, and the exposure time is reduced to 5 µs in order to exclude the temporal window where most plasma emission is produced (note that 5 µs is much less than the pulse repetition period). The filter is removed for all imaging measurements performed with the 5-µs exposure time because the signal is very low. The images correspond to the full visible spectral range (200-900 nm).

### III. Results

#### A. Positions of the Electrode

In this work, we investigate three electrode positions, called the “pilot position”, the “backplane position”, and the “multipoint position”. Fig. 4 shows a top schematic view of each position as well as the $(x, y, \theta)$ coordinates defining that position.

The pilot position is the closest case to the previous study of Blanchard *et al.* [14] with the plasma formed in the pilot flow region, between the electrode and the inner divergent of the injector. However, we could not reproduce the 7-mm gap of Ref. [14] because it caused strong parasitic discharges in the adapted silica window. To prevent these parasitic discharges, the gap distance is reduced to 5 mm at that position.

The backplane position, in which the plasma is formed between the electrode and the backplane, is interesting as the plasma can interact with the gases from the Outer Recirculation Zone (ORZ). This configuration has been found to be effective in a swirl-stabilized partially premixed combustor [20].

The multipoint position, where the plasma is formed between the electrode and the outer divergent of the injector, seems promising to improve stabilization at low $\alpha_p$ because it allows direct interaction between the plasma and the multipoint flow.
<figure><img src="image_5.png" /><figcaption>Fig. 4 Top view of the electrode positions studied in this work. The corresponding (x, y, θ) coordinates are given for each position. d is the gap distance between the tip of the electrode and the injector. The plasma location is also indicated in the graph.</figcaption></figure>

## B. Study of the Pilot Position and comparison with previous work

The LBO limit with and without plasma at 50 kW for several staging factors is measured in the pilot position. Fig. 5 compares these results with those obtained by Blanchard *et al.* [14].

Without plasma, significant differences in the LBO limit are observed. In the present work, the stability domain of the flame without plasma is wider than in the experiments of Ref. [14]. This difference can be attributed to the fact that the previous divergent nozzle had suffered from years of erosion due to experiments and was replaced for this work. Because the divergent nozzle is very thin, erosion can strongly affect the geometry of the injector and the flow topology, which greatly affects the stability of the flame. In future work, the divergent nozzle could be advantageously replaced with a thicker wall. Nevertheless, in the present experiments, the replacement nozzle has the same thickness as the original one.

With plasma, the LBO extension is very similar to the one obtained in the experiments of Ref. [14]. It is interesting to note that the LBO limit with plasma of Ref. [14], obtained in deteriorated flow conditions, is quasi-similar to the LBO limit without plasma obtained in the ideal flow conditions of this work: the application of the plasma not only compensates for the degraded conditions but also extends the LBO limit. Additionally, we probe an intermediate staging factor $\alpha_p = 10\%$, and a significant extension of 17% is obtained at this low staging factor.

<figure><img src="image_6.png" /><figcaption>Fig. 5: Equivalence ratio at LBO limit without plasma (black symbols) and with plasma (red symbols). Square symbols are the results from Blanchard *et al.* [14] and triangle symbols from this work in the pilot position. In this work, for $\alpha_p = 80\%$ and 100%, the plotted value is an upper bound as the air mass flow controller reached full capacity and no extinction was observed.</figcaption></figure>
The average plasma power is another difference found between the present work and the results of Ref. [14]. For the same applied voltage and PRF, we obtain an average power of 235 W versus 125 W in Ref. [14]. This difference is due to a correction of the compensation of the voltage probe. The same Lecroy PPE 20 kV probe is used in both studies, but in Ref. [14] the non-compensated probe underestimated the peak voltage by approximately a factor of 2.

### C. Effect at $\alpha_p = 0\%$

The case with $\alpha_p = 0\%$ corresponds to high power regimes such as during take-off. In the *pilot position*, as shown in Fig. 5, we observe no effect of the plasma assistance on the LBO limit for $\alpha_p = 0\%$. This is consistent with the previous findings of Ref. [14]: a plasma formed in the pilot region has little effect on the flame when all the fuel flows through the multipoint stage. At this staging factor, the extinction of the flame is slow and progressive: as the equivalence ratio is decreased, the flame starts to lift, then slowly fades, oscillates, and finally extinguishes [14]. For this reason, at $\alpha_p = 0\%$, Blanchard *et al.* [14] defined the LBO limit as the flame lift-off. Fig. 6 shows the 50-kW multipoint flame at the lift-off equivalence ratio, without plasma and with plasma. It appears clearly that the application of the plasma in the pilot position has little effect on the flame anchoring. In the *multipoint and backplane positions*, the effect of the plasma is difficult to measure because the bright plasma emission overshadows the CH\* chemiluminescence of the flame. To circumvent this limitation, we synchronize gated camera images with the period following the discharge pulse. In order to assess the effect of the plasma with the electrodes placed at the various positions, 200 frames are acquired at each electrode position. To compare the effect of the plasma on the flame shape, the flame contour of each frame is extracted using Otsu thresholding. The results are shown in Fig. 7. On the first row, no plasma is applied and the flame is well lifted. Without plasma, the position of the electrode has no effect on the flame shape. When the plasma is applied in the *pilot position* (left column), even with a PRF of 77 kHz, the flame is not well re-anchored. The flame front near the injector fluctuates over a wide spatial range. Conversely, in the *backplane and multipoint positions* (center and right columns) at 30-kHz PRF a small anchoring effect of the plasma is observed with a bending of the branches toward the injector. This effect is more significant at 77 kHz with flame branches reaching the injector and a more pronounced global V-shape.

<figure><img src="image_7.png" /><figcaption>Fig. 6 CH\* chemiluminescence average images at $P_{th} = 50$ kW, $\alpha_p = 0\%$, and lift-off equivalence ratio $\Phi = 0.67$: (left) without plasma, (right) with plasma. The electrode is in the pilot position, PRF = 30 kHz and $V = 13$ kV (right). The injection system is shown in the left image.</figcaption></figure>

These results demonstrate that applying the plasma in the ORZ or in the multipoint flow is more effective for flame stabilization at low staging factors. However, because of plasma emission, it is still challenging to quantitatively measure the influence of the plasma on the flame. New diagnostics are under development to better assess the increase in the LBO limit at these electrode positions.
<figure><img src="image_8.png" /><figcaption>Fig. 7 Flame contours at 50 kW, $\alpha_p$ = 0% and $\phi$ = 0.67. Each picture corresponds to an acquisition of 200 frames. The contour of each frame (obtained with Otsu thresholding method) is plotted. The colormap corresponds to the probability of presence of the contour normalized by 25% (i.e. a pixel is 1 if it is contained in at least 25% of the contours and 0 if contained in no contour). Top row: flame alone. Middle row: plasma at 30 kHz and 13 kV. Bottom row: plasma at 77 kHz and 13 kV. Left column: electrode placed in the pilot position. Center column: in the backplane position. Right column: in the multipoint position. In the backplane and multipoint positions, a mask (shown in gray) is used to obscure the bright emission from the plasma. For the backplane position at 77 kHz, the plasma emission is too intense to analyze the left part of the picture.</figcaption></figure>

### D. Effect at $\alpha_p \ge 20\%$

The multipoint and backplane positions have shown positive results at $\alpha_p$ = 0%. Their effects on the LBO limit at higher $\alpha_p$ without plasma are presented in Fig. 8 and compared to those obtained in the pilot position. The backplane position has no effect on the LBO limit at all staging factors. On the contrary, the multipoint position extends the LBO in a similar way as the pilot position except for $\alpha_p$ = 20% where the LBO extension is still slightly higher in the pilot position.
<figure><img src="image_9.png" /><figcaption>Fig. 8 Equivalence ratios at LBO limit without plasma (black squares) and with plasma: pilot position (red triangles), backplane position (orange triangles), and multipoint position (green triangles). The mean plasma power is 235 W for all cases.</figcaption></figure>

## IV. Conclusions and Future Work

This study improves the previous findings on the BIMER-PAC experiment by optimizing the placement of the electrode in the system. Positioning the electrode in the multipoint flow region is found to improve flame stability at all staging factors. In comparison with the pilot position, placing the electrode in the multipoint flow shows a stronger effect of the anchoring of the flame at low equivalence ratios and similar LBO extension for higher values. This work is a first step toward integrating plasma-assisted combustion in real combustion systems.

In future work, a new method to quantitatively measure the plasma influence on the LBO extension at low equivalence ratios will be developed. Additionally, the plasma parameters will be optimized to minimize the plasma power while increasing the effect on the combustion. Finally, the plasma discharge will be characterized with optical emission spectroscopy measurements and fast imaging of the discharge. The information collected on the plasma will be useful to better understand the stabilizing effects of PAC and will be used in large eddy simulations for modeling the plasma characteristics.

## V. Acknowledgments

This work has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (ERC GreenBlue grant agreement No.101021538). The authors would like to thank Dr. Victorien Blanchard for useful discussions about the previous studies, Nicolas Vaysse for discussions on the imaging processing routines, and Erika Jean-Bart, Jérôme Bonnety, Yannick Le Teno, Hubert Jubeau, and Koro Sokhona for technical assistance with the experimental facility.

A CC-BY-4.0 public copyright license has been applied by the authors to the present document and will be applied to all subsequent versions up to the Author Accepted Manuscript arising from this submission, in accordance with the grant's open access conditions.

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