Modélisation et simulation de la combustion turbulente d’un biogaz à l’aide d’une cinétique réduite, dans le code Openfoam

Abstract

In general, nowadays, detailed mechanisms of chemical combustion reactions still require high processing times during simulations for computers. However, time saving in calculation, while keeping the precision during digital design of new combustion chambers is a major challenge. In order to accelerate the treatment of chemical kinetics, this thesis is part of the contribution to the improvement of a reduction mechanisms method. In this work, the reduction model developed is applied to the modeling of turbulent combustion of methane, then biogas, proposed as a substitute for natural gas in "Computational Fluid Dynamics" (CFD), "Open Fields Operation And Manipulation" (OpenFOAM) codes. The method used is a multi-stage reduction algorithm applied to the coupling of the "Directed Relation Graph" (DRG) and "DRG-Aided Sensitivity Analysis" (DRGASA) methods in four steps. This approach consists in obtaining simplified and compact reaction mechanisms ; that is, cut according to the initial conditions and to the limits of the flame. Three experimental flames were chosen to validate the simplified mechanisms of the reduction method, namely : the flames DLR-A, D from Sandia and OXYFLAM-2 A from the International Flame Research Foundation (IFRF). The detailed chemical kinetic mechanism used is that of the "Gas Research Institute" version 3.0 for the combustion of natural gas, called GRI-Mech 3.0 (51 species and 325 reactions). Going from GRI 3.0, the reduction method developed allow to obtain : (i) for the flame DLR-A, a type C2 mechanism, of 21 species and 51 reactions, for the flame D, a C2 type mechanism, 20 species and 47 reactions and (iii) for the flame OXYFLAM-2 A, a C2 type mechanism, 21 species and 39 reactions. Thus, the average percentage of the reduction is 79.14%. Cou- pling between chemistry and turbulence is made using combustion models such as : "Partialy Stired Reactor" (PaSR), "Eddy Dissipation Concept" (EDC) and "Steady Laminar Flamelet Model" (SLFM). The models k − ε and P -1 were used to take into account turbulence and radiation. The LES (Smagorinsky) turbulence model was tested for the case of flame D. The performances of the chemical mechanisms with different species and elementary reactions derived from GRI-Mech 3.0 are evaluated and show a good agreement with the experimental results. Their implementation during the simulations allows fairly reasonable calculation times. Once the C2 mechanisms of the multi-stage method have been validated , their applications to the turbulent combustion of biogas are carried out in experimental flame configurations. These biogas, made up of CH4 and CO2 in different proportions, come from the anaerobic digestion of cow dung, sheep slurry and pig slurry. As the proportion of CH4 in the biogas decreases, this leads to a reduction in the residence time, mixing time and reaction rate of the fuel during combustion. We therefore observe a reduction in the length of the flame and a rapid drop in temperature. Due to the slightly lower temperatures compared to those of natural gas, the CO2-rich biogas flames have the advantage to produce few pollutants namely CO and NO.