CHEMICAL KINETICS ANALYSIS FOR LIFTED TURBULENT JET FLAME USING MULTI-FLAMELET GENERATED MANIFOLD APPROACH
The present paper reports on the numerical investigation of
lifted turbulent jet flames with H2/N2 fuel issuing into a vitiated coflow. The hot vitiated co-flow containing oxygen as well as combustion products, stabilizes the lifted turbulent flame by providing an autoignition source. A 2D axisymmetric formulation has been used for the predictions of fluid flow, while multidimensional Flamelet Generated Manifold (multi-FGM) approach has been used for turbulence-chemistry interactions in conjunction with RANS approach. Standard k − ε model is used to model the turbulent nature of the jet. The chemical kinetics in H2-O2 combustion is followed by
Muller et al.  and Li et al.  mechanisms. The major difference between the two mechanisms is the value of rate constants contributing towards source of the autoignition and the corresponding enthalpy of formation of OH radicals. The lift-off height is determined from the axial distance (from the burner exit) at which the auto-ignition occurs, and is located through local concentration of OH radical equivalent to 2*10-4. In order to understand the impact of chemical kinetics on the autoignition, speeding up (Set A) and delaying (Set B) auto-ignition controlled reaction rates are augmented and corresponding changes in lift-off height are observed. The reaction which speeds-up the autoignition phenomena underpredicts the lift-off height, while those which are responsible for delaying the autoignition phenomena over-predict the liftoff height as compared to the experimental measurements. The six most dominant reactions in Mueller as well as Li mechanism that causes a delay or speed-up in auto ignition are discussed. A comprehensive chemical kinetics sensitivity analysis is carried out in understanding the underlying behavior of HO2 radicals as autoignition precursor and OH radicals as reaction rate determinant. It can be concluded that the reactions in Set A are most sensitive to auto-ignition and thus plays a vital role in determining the lift-off height. In order to further investigate the sensitivity of reactions in Set A on auto-ignition, rates of some dominant reactions in Set A are changed separately and corresponding lift-off heights are observed. The results obtained in the current study elucidates that the flame is largely controlled by chemical kinetics.