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Show / I MA TH[MA TICAL MOD[L L i l'JG 1.4 TOMJ:tl T ION I TL,T _- I I I _..-1---_ BUPN[P D [ ~ IGf\J --- A[ROD YNAMIC [ HARtle T[ RIZ or 5 MI..It T[ S TS C-4u< -r I I j ~ ,.. 1-4 C(,'C I ''( 0.5 MV BURN[R ruRNAC[ ;~ COMBUST/ON // T[ S TS // 5 MIJt BURN[R CAUX-C COMBUST ION T[~TS 50 MVt ..... BURN[R S. GILLA #1 COMBUST ION T[STS Figllre 1 : Infrastrllctllres IIsed f or bllrllers development. Reliable prediction methods can be used for the development of burners. The numerical prediction with finite difference methods of the flow field including mean velocities, temperature and concentrations of major species, has been currently used as a tool in the design and development of furnaces. The calculation of emissions of NO x, in particular in highly complex swirling flows as those encountered in low NOx burners, is still problematic. Though detailed kinetic schemes are available for nitrogen chemistry in flames, they cannot be used in the calculation of the flow field, as they include a staggering amount of species and reactions. The calculation of formation rates at several thousands nodes and the solution of a number of differential equations equal to the number of species present is very time consuming. In order to simulate and predict the behaviour of 10w-NOx burners, two alternative ways can be followed: • use of a simplified NO nlechanism, which includes only a small number of species; • development of a 'hybrid model". In the first step the mean velocities and the temperature distributions are predicted with the a high density grid, conventional CFD code, which takes into account a limited number of reactions for the fuel oxidation. On the basis of this flow field the concentration of minor species, in particular nitrogen species, are evaluated using a detailed mechanism. The separation into two steps is possible as minor species have only a small influence on the flow fields. This calculation uses a reduced chemical mechanism for the fuel oxidation, just detailed enough to predict the effect of chemical reaction on the flow field . To evaluate the performance of low NOx burners, the code FLUENT has been used to calculate the flow field, temperature and major species distributions and a kinetic program has been written to evaluate the concentrations of minor species, based on subroutines from the CHE!vfKIN software package. The experiments carried out on fuel atomization (or pulverized coal distribution), near flow field in cold conditions, and finally in combustion tests gave the required pieces of information for defining the reactor network (simplified flow field) and the operating conditions (temperature and concentration in each reactor of the network) on which the detailed kinetic model had been run. 3 |