Low flame temperature considerably limits the pollutant production but causes flame stabilization issues. An emerging solution to enhance flame stabilization is to generate high-voltage electrical discharges between two electrodes, localized near the flame reaction zone. A plasma is locally generated, which interacts with the combustion. Despite this proven efficiency, the fundamental mechanisms of plasma-assisted combustion are not well understood, and important questions are still open.
The objective of this project is to elaborate and validate against experiments a modeling route suitable to perform simulations of realistic turbulent combustion systems accounting for plasma-flame interactions. Such simulations have, to the best of our knowledge, never been realized. This study will give insights into the plasma flame interaction mechanisms. A modeling route will be designed to perform the first LES of plasma-assisted turbulent combustion. <br /><br />The following issues will be addressed to achieve this objective:<br /><br />1) The NRP discharge properties have been identified in heated air, with a full time and space resolved characterization of the main radicals produced by the discharge (O atoms, electrons, excited molecules), all synchronized with current, voltage and energy measurements. However, the species produced in a mixture representative of a combustor, which may contain fuel and recirculating burnt gases have not been quantified yet. The temporal and spatial distributions of species and temperature within and around the discharge have to be characterized in such vitiated environment. In this project, both experimental and numerical characterizations of the discharge in recirculating gases will be performed. It will give insight on the local interaction of the NRP discharges with the reactive flow dynamics.<br /><br />2) An NRP discharge model must be coupled with a turbulent combustion LES model. The main issue is to account for the excited species generated by the plasma field in the combustion chemistry.<br /><br />3) Detailed experimental characterizations of plasma-assisted turbulent combustion at both laboratory and semi-industrial scales will be performed. It will improve our understanding of the flame stabilization mechanism and will provide data for LES model validations.<br /><br />4) To demonstrate the efficiency of the developed LES approach, we will perform the simulation of an aeronautical combustor stabilized by NRP discharges a final applicative challenge.
The objectives of the program are to develop and validate a modeling strategy able to perform LES of plasma-assisted combustion from academic to industrial environment. To achieve this goal, the project is divided in 5 tasks.
Task 1 is the fundamental experiments on the bluff-body Mini-PAC configuration. The first objective of this task, led by CORIA, is to characterize the discharge properties in an environment representative of a practical combustor, i.e. in a mixture composed of air, fuel and also burnt gases. This experimental characterization will serve to validate the detailed simulation of plasma-flame interactions. The second objective is to provide data at the flame scale to validate the LES plasma-assisted combustion model.
Task 2, led by CERFACS is dedicated to detailed numerical simulations of plasma-assisted combustion. A first objective is to capture the impact of the plasma on the combustion chemistry, by developing an analytical chemical scheme taking into account O atoms created during the plasma phase. A second task, in collaboration with LPP, is the development of a discharge code capable of simulating NRP.
The discharge code combined with the new combustion chemistry will be applied to the Mini-PAC configuration, to compare with measurements and results of task 4 on the same configuration, where relaxation of excited N2 is added.
Task 3, led by the numerical team of EM2C, aims to develop and validate the LES plasma-assisted combustion model. Simulations of the academic Mini-PAC and semi-industrial BIMER configuration will be performed during this task.
Task 4, led by the experimental team of EM2C, is dedicated to the experiments on the BIMER configuration.
Task 5, led by the industrial partner SAFRAN Turbomeca, aims to perform the simulation of a practical helicopter combustion chamber with NRP discharges.
A premixed Bluff-Body burner (called «miniPAC«) was designed and manufactured in two strictly identical copies, for experiments conducted in parallel at CORIA and EM2C.
A first series of experiments was carried out at CORIA on this configuration. Simultaneous measurements of velocity fields by PIV, resolved in time, and a follow-up of turbulent reaction zones by direct high rate imaging were performed, and supplemented by Spontaneous Raman Diffusion measurements.
Experimental work at the EM2C laboratory focused on the characterization of NRP discharges. The experimental team measured the diameter of the discharge for different equivalence ratio and quantities of energy deposited as a function of the average temperature during the discharge.
CERFACS and LPP have adapted the CFD code AVBP to the NRP discharge calculation. An essential element is the choice of a chemical kinetics for air, today based on N2+, O2+, O2-, O2-, O- and e-. The system of equations to be solved therefore contains a total of 8 equations: 4 for electrons and one for each ion. In addition, there is a Poisson equation for the electric field. The first tests are encouraging and reproduce standard streamer behaviors in accordance with the literature.
In parallel, a plasma-assisted combustion model was implemented by the EM2C laboratory's numerical team in the LES Low-Mach code YALES2 and in the compressible LES code AVBP. The model has been validated by comparing the results of different 0D, 1D and 2D test cases simulated with the two codes (YALES2, AVBP) with experimental and numerical results. First LES simulations of plasma ignition and flame stabilization, in the case of the Mini-PAC configuration, were performed.
The measurements planned on the Mini-PAc burner concern the mapping of temperature and species concentration by Spontaneous Raman Diffusion, with and without discharge, on an operating point at the limit of stability. In parallel, measurements will be carried out on the BIMER experimental bench, representative of an aeronautical combustion chamber
Detailed numerical simulations of plasma discharges will be conducted. The results of these calculations will validate the turbulent plasma-assisted combustion model. The first simulations of turbulent flames stabilized by a plasma discharge will then be possible. The numerical solutions will be compared against the experimental results at the end of the project. Finally, the NRP technology will be numerically tested on a real SAFRAN aeronautical engine.
Collin-Bastiani F, Vermorel O, Lacour C, Lecordier B, Cuenot B. DNS of spark ignition using Analytically Reduced Chemistry including plasma kinetics. Proceedings of the Combustion Institute. 2018.
Yacine Bechane, Nasser Darabiha, Vincent Moureau, Christophe Laux, Benoît Fiorina.Large Eddy Simulations of turbulent flame stabilization by pulsed plasma discharges. Scitech AIAA (2019)
Nelson VALDEZ, David HONORE, Corine LACOUR, Bertrand LECORDIER, Armelle CESSOU. Turbulent Bluff-Body flames close to stability limits revealed by coupling of high speed optical diagnostics. Poster présenté à la Journée des Doctorants en Combustion, Orléans, 19 janvier 2018
The reduction of pollutant emissions in aircraft engines and power plants has become a major issue for gas turbine manufacturers as a result of more stringent environmental regulations and increased environmental concern. An efficient solution to reduce pollutant formation is to maintain a relatively low temperature in the combustor primary zone, by decreasing for instance the mixture equivalence ratio. The issue is that a low flame temperature induces slower chemical reaction rates often resulting in an increase of flame instabilities and extinctions.
An emerging solution to enable flame stabilization in leaner regimes, suitable to a wide range of combustion applications, is to generate electrical discharges at the flame basis. Among these various types of discharges, the Nanosecond Repetitively Pulsed discharges have shown to beparticularly efficient. Despite this proven efficiency, the fundamental mechanisms of plasma-assisted combustion are not well understood. Also, the numerical tools needed by engineers to assess the performance of NRP discharge in practical configurations and optimize their design do not exist.
The objective of this project is to elaborate and validate against experimental data a modeling route suitable to perform simulations of realistic turbulent combustion systems accounting for plasma-flame interactions.
The discharge properties will be first characterized numerically and experimentally in a mixture representative of a combustor, which may contain fuel and recirculating burnt gases. These temporal and spatial distributions of species and temperature within the discharge will serve to calibrate a semi-empirical plasma-assisted combustion model recently developed. This model will be formulated in a LES context and implemented in an unstructured CFD solver.
To build the validation database, experiments of plasma-assisted combustion will be conducted on two configurations:
- The first one is a bluff body flame on which it has been already shown that the NRP discharge enables the flame stabilization over very lean regimes. The NRP discharge is efficient when located in small gases recirculation zone. We will perform complementary measurements on this configuration to characterize the composition and flow properties within the recirculation zone.
- The second configuration is a swirled combustor representative of aeronautical combustion chambers. It is composed of a two-stage swirled injector and a rectangular combustion chamber with optical access ports. We will perform measurements to characterize the effect of the plasma on the flame stabilization process. It will be interesting to show that the plasma impacts significantly the stabilization mechanisms.
The LES plasma-assisted combustion model will be applied to both bluff-body flames and swirled combustor configuration. Finally, we will perform the LES of a practical Lean Premixed Turbomeca combustor stabilized by NRP discharges as a project final big challenge.
Monsieur Benoit Fiorina (EM2C)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
STE CIVILE CERFACS
SAFRAN - TURBOMECA SAFRAN - TURBOMECA
LPP Laboratoire de Physique des Plasmas
Help of the ANR 755,820 euros
Beginning and duration of the scientific project: September 2016 - 48 Months