NUMERICAL SIMULATIONS OF A LIFTED HYDROGEN JET FLAME USING FLAMELET GENERATED MANIFOLD APPROACH

A turbulent lifted H-2/N-2 jet flame in a vitiating co-flow environment is numerically investigated, using the Flamelet Generated Manifold (FGM) combustion model with Large Eddy Simulations (LES). Due to the hot vitiated H-2/air co-flow, the primary stabilization mechanism is the autoignition followed by a premixed flame. In addition to using H-2 as a fuel, this flame poses two other modeling challenges: (i) the autoignition, which is a transient chemistry-driven phenomenon; (ii) the existence of multiple combustion regimes, e.g., diffusion at autoignition location but premixed in the post flame. A series of LES/FGM simulations are completed in this work by reducing the co-flow temperature from 1045 K to 1000 K. The FGM model can predict the characteristics of the flame by showing a lifted flame. It also accurately predicts the trend in the flame lift-off distance with a change in the co-flow temperature. The current results are compared for mixture fraction, temperature, and OH mass fraction at multiple locations, which have also been correctly captured. It is noted that for a high co-flow temperature (and hence a low lift-off distance), the flame's lift-off is highly sensitive to the inlet boundary conditions and the mesh resolution near the jet entry. A relatively coarse mesh is used for all the simulations, which is generated using a careful strategy that not only resolves the jet instabilities near the fuel inlet, but also keeps the overall mesh count low and allows for a large computational time step. A systematic sensitivity analysis on the computational speed is also performed. This work provides some useful guidelines in simulating the H2 diluted flames using the FGM model, which may be valuable to the gas turbine industry.