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Experimental characterization of plasma-assisted flame stabilization mechanisms for aircraft engines and gas turbines

Abstract : The reduction of pollutant emissions in aircraft engines and gas turbines has become a major issue for industry because of increased environmental regulations. An efficient solution to reduce NOx formation is to maintain a relatively low flame temperature, which can be achieved by using lean premixed combustion systems. However, too low a flame temperature may affect the combustion efficiency and cause an increase in CO and unburned hydrocarbon (CHx) emissions. In addition, flame instabilities or even extinction may occur when the operating regime becomes near the lean flammability limit. Combining flame stability, combustion efficiency and pollutant reduction issues is then very challenging when designing these emerging new combustion technologies. Previous work at the EM2C laboratory has shown that lean premixed flames can be effectively stabilized by a local addition of energy with Nanosecond Repetitively Pulsed (NRP) discharges (Pilla et al. 2006). These plasma discharges produce a local increase in active species concentrations and heat (Pilla et al. 2006, Rusterholtz et al., 2013). Figure 1 shows the spontaneous emission of OH radicals in a lean premixed propane/air flame. Without plasma (Figure1 left), the reaction zone is very small, confined to the recirculation zone above the bluff-body, and the flame exhibits instabilities. By applying NRP discharges in the recirculation zone, the chemical activity the flame is enhanced (Figure1 right). Although the flame temperature is relatively low, the combustion efficiency is improved and therefore less production of CO and CHx is expected. Plasma-assisted stabilization of lean flames was also recently obtained in larger scale burners, allowing reductions by a factor of up to 4 of the lean extinction limit of a 75 kW premixed propane-air flame at atmospheric pressure (Barbosa et al. 2015), and of a 200 kW kerosene-air turbojet aerodynamic injector operating at 3 bar (Heid et al. 2009). In all cases, plasma-assisted stabilization of lean flames was obtained with plasma powers of less than 1% of the power released by the flame. The high efficiency of plasma-stabilization is due to the fact that the energy of the electric discharge is spent on ionization, excitation and dissociation of molecules rather than just increasing the gas temperature. The relaxation of discharge-excited nitrogen molecules by collisional quenching reactions with oxygen molecules results in an ultrafast (time scales of nanoseconds) increase of radicals and gas temperature inside the discharge channel. In typical conditions, near-full dissociation of molecular oxygen can be obtained in the inter-electrode region (Rusterholtz et al. 2013, Lo et al. 2014,). This high concentration of radicals and increase of heat have a positive effect on the flame stabilization (Pilla et al. 2006, Castela et al. 2016), but it is not clear which is the dominant effect. Thesis objectives The objective of the thesis is to better understand the mechanisms of flame stabilization by nanosecond plasma discharges, and in particular to understand the relative importance of the thermal and chemical effects induced by the plasma. This will be achieved by performing advanced diagnostics (laser diagnostics and spectroscopy) on two burner configurations, and by comparing the measurements with the results of numerical simulations developed in parallel by another Ph.D. student at EM2C, in collaboration with CERFACS and TURBOMECA, for the following laboratory burners: • The MINI-PAC burner (15 kW), which promotes a low-turbulence, propane/air flame: experiments performed by Pilla et al. (2006) have shown an efficient stabilization of the flame when a plasma discharge is produced in the flow recirculation zone, partially composed of burnt gases, located above the flame-stabilizing bluff-body. • The BIMER combustor (50 kW), which is a confined highly turbulent swirled combustor representative of aeronautical engines, fed either with propane (Barbosa et al. 2009) or with liquid dodecane to mimic kerosene (Renaud et al. 2015). Previous experiments demonstrated that the plasma improves the propane/air flame stability, with a decrease of the lean blow-off limit by more than a factor of 4 (Barbosa et al. 2015). These burners are representative of real turbulent combustion systems and their understanding will open the way to applications of plasma discharges on large scale industrial burners. The work will benefit from our collaboration with numerical simulation groups (EM2C, CERFACS and TURBOMECA) who will simulate the two burner configurations in parallel of this thesis, building on previous numerical work at EM2C (Castela et al. 2016). Comparing the results of the measurements and the simulations will provide useful insights into the mechanisms of plasma-flame interactions. An important part of the Ph.D. thesis will be to implement and develop optical diagnostics to characterize the plasma-stabilized flames in terms of species concentrations (radical species, combustion products, unburned gases, pollutants,…), temperatures, and velocity fields. This task will require advanced optical diagnostics such as absolute emission and absorption spectroscopy, laser-induced fluorescence (one and two photons), diode or quantum-cascade laser measurements, Planar Imaging Velocimetry, or Schlieren, for which our laboratory has developed a strong expertise.
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https://hal-centralesupelec.archives-ouvertes.fr/hal-02024814
Contributor : Victorien Blanchard <>
Submitted on : Tuesday, February 19, 2019 - 1:20:59 PM
Last modification on : Friday, July 3, 2020 - 9:28:04 AM

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  • HAL Id : hal-02024814, version 1

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Victorien Blanchard. Experimental characterization of plasma-assisted flame stabilization mechanisms for aircraft engines and gas turbines. 2021. ⟨hal-02024814⟩

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