Numerical investigation of hydrogen flame structure and NOx formation in a coaxial dual swirl burner

A numerical study of swirling hydrogen flame structure and nitrogen oxides (NOx) formation routes in a coaxial dual swirl burner is presented. Turbulent combustion modeling is performed by using partially stirred reactor (PaSR) model and flamelet generated manifold (FGM) model. NO prediction is conducted by transport model and chemistry tabulation methods, in combination with detailed chemical kinetic scheme. The numerical approaches are firstly validated against the experimental measurements, and overall good agreements are obtained by both PaSR and FGM for swirling velocity field and fundamental flame structure. The hydrogen flame takes a M-shape reaction layer consisting of two distinct flame branches with different stabilization regime. The shear reaction layer evolves in a relatively high velocity region between hydrogen and air streams, while the central reaction layer evolves in a relatively low velocity region and lies in central recirculation zone. FGM-NO model provides a fairly good prediction for NO concentration at combustor outlet with a relative error of 9.5%. The way the flame branches are stabilized has noticeable impacts on NO formation as the NO reactions in two flame branches are determined by different reaction routes. Thermal route dominates in central reaction region, whereas the N2O and NNH routes are found to be important for the preliminary conversion of NO in the vicinity of shear flame front.