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Three-dimensional direct numerical simulations (DNS) of high hydrogen content (HHC) syngas nonpremixed jet flames with a Reynolds number of Re = 6000 have been carried out to study the nitric oxide (NO) pollutant formation. The detailed chemistry employed is the GRI 3.0 updated with the influence of the NCN radical chemistry using flamelet generated manifolds (FGM). Preferential diffusion effects have been considered via FGM tabulation and the reaction progress variable transport equation. The DNS based quantitative results indicate a strong correlation between the flame temperature and NO concentration for the pure hydrogen flame, in which NO formation is mainly characterised by the Zeldovich mechanism. The results also indicate a rapid decrease of maximum NO values in H2/CO syngas mixtures due to lower temperatures associated with the CO-dilution into H2. Results on NO formation routes in H2/CO syngas flames show that while the Zeldovich mechanism dominates the NO formation at low strain rates, the high NO formation rate at high strain rates is entirely caused by the NNH mechanism. We also found that the Fenimore mechanism has a least contribution on NO formation in H2/CO syngas flames due to absence of CH radicals in the oxidation of CO. It is found that, due to preferential diffusion, NO concentration exhibits higher values near the flame base depending on the hydrogen content in H2/CO syngas fuel mixture.

Investigations of the flame-vortex and flame-wall interactions have been performed for hydrogen impinging non-premixed flame at a Reynolds number of 2000 and a nozzle-to-plate distance of 4 jet diameters by direct numerical simulation (DNS) and flamelet generated manifold (FGM) based on detailed chemical kinetics. The results presented in this study were obtained from simulations using a uniform Cartesian grid with 200 x 600 x 600 points. The spatial discretization was carried out using a sixth-order accurate compact finite difference scheme, and the discretized equations were advanced using a third-order accurate fully explicit compact-storage Runge-Kutta scheme. The results show that the inner vortical structures dominate the mixing of the primary jet for the nonbuoyant case, while outer vortical structures dominate over the inner vortical structures in the flow fields of the buoyant cases. The formation of vortical structures due to buoyancy has a direct impact on the flow patterns in both the primary and wall jet streams, which in turn affects the flame temperature and the near-wall heat transfer. It has been found that the buoyancy instability plays a key role in the formation of the much wider and higher value wall heat flux compared with the nonbuoyant case, while external perturbation does not play a significant role. The computational results show an increased wall heat flux with the presence of buoyancy.

A transitional hydrogen air non-premixed impinging jet flame is studied using three-dimensional direct numerical simulation (DNS) and flamelet generated manifolds (FGM) based on detailed chemical kinetics. The simulations are used to investigate the buoyancy instability and the spatial and temporal patterns of the impinging jet flame. The computational domain employed has a size of 4 jet diameters in the streamwise direction and 12 jet diameters in the cross-streamwise direction. The results presented in this study were performed using a uniform Cartesian grid with 200 x 600 x 600 points. Reynolds number used was Re = 2000, based on the inlet reference quantities. The spatial discretisation was carried out using a sixth-order accurate compact finite difference scheme and the discretised equations were advanced in time using a third-order accurate fully explicit compact-storage Runge-Kutta scheme. Results show that the buoyancy and jet shear instability lead to form both inner and outer vortical structures in the primary and wall jet regions, thus complex spatial and temporal variations occur in the mixture fraction, progress variable and temperature fields. Moreover, DNS results suggest that the near-wall vortical structures play an important role in the near-wall heat transfer. These findings may provide useful guidelines for the near-wall combustion modelling using Reynolds-averaged Navier-Stokes modelling or large eddy simulation techniques. Crown Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.