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Show that the chemist formulates an applicable reaction mechanism ( with rate constants ) and that he formulates and solves an appropiate system of governing equations. Once the mechanism and governing equations are defined, CHEMKIN provides a means to describe symbolically the mechanism and a means to describe computationally the system of governing equations ( a 200 Gas-phase subroutines Library). CHEMKIN does not provide a means to solve the governing equations, therefore selection and implementation of a solution method is left to the user. The reaction mechanism used for this study has been developed by AIR LIQUIDE in colaboration with a french laboratory [4]. The reactions of this mechanism has been selected to give an as good as possible description of the methane combustion [5][6], of the thermal NOx formation [7] and the prompt NO formation [8]. Ethane and propane combustion are also taken into account [6]. The complex and detailed mechanism contains 195 reactions and 53 species. The governing equations are those of an isothermal and isobaric reactor. 3. Combining the two codes ATHENA and CHEMKIN Combustion modelers agree that for diffusion flames, the mIxIng laws govern the main species chemistry. It means that kinetics is much faster than diffusion. It is not any more the case when studying the low concentration species as NO for instance. A second point is that 3D fluid mechanics code as ATHENA can not today deal with kinetics, especially when looking at the pollutant chemistry. Studying NO formation means at least few tens of reaction, when taking into account both thermal NOx ( Zeldovitch mechanism) [7] and promt NOx formation [8]. Advanced laboratories are working on reducing those complex reaction mechanism in order to include a five to ten reactions system in a 3D fluid mechanics code. But nothing has been published yet. AIR LIQUID researchers decided then to apprehend the problem by splitting it in two steps. First running ATHENA we can calculate Temperature profile on an average trajectory of the flame as well as the gas velocity profiles. Then splitting the flame volume in N cells as indicated on figure 1, we can calculate for each cell an average temperature T and an average residence time t r • Each cell is then treated as an isothermal reactor at the atmospheric pressure by running the code CHEMKIN. N must be large enough in order to have short residence time in each cell and a small temperature difference from cell to cell. In order to feet with the diffusion model predictions (ATHENA) regarding the global reaction rate ( methane and oxygen consumption ), only a fraction of the still not consumed comburent and fuel is "auth~rized to burn" in the cell n (figure 2). This "authorized to burn" fraction is add to the already burned amount of gas, and the chemical evolution of that mixture is determined by running CHEHKIN as indicated above. III Experimental validation An example of the results we got with this two steps NOx formation modeling are the curves showed on figures 4,5 and 6. Ye investigated the effect of the oxygen ratio in the comburent on the NOx level for a 130 kY stochiometric natural gas flame. Natural gas was 4% nitogen and the furnace geometry was as shown on figure 3. The experimental combustion chamber is cylindrical, 2 m long and 0.60 m large, with a vertical wall at the two third of the combustion chamber. The reason |