Chemical kinetic reduction

One of the most important points for the chemical kinetic reduction it is the incorporation in turbulent flame calculations, where the use of detailed chemistry can make the problem intractable due to the enormous computational cost. Although tabulation methods (in PDF models) have demonstrated their potential, the use of reduced kinetic schemes is the simpler alternative method for solving reactive DNS and LES simulations. The reduced kinetics is also useful in studies of flame stability and dynamics. One of the Group's research line is the chemical kinetic reduction, in particular for hydrogen and syngas (H2/CO mixtures), based on steady-state and partial equilibrium hypothesis.

Reduced mechanism for hydrogen combustion

From the 21 elementary reactions involved in the hydrogen combustion, we already know that a one-step kinetics, 2H2 + O2 ⇒ 2H2O, suffices to describe lean deflagrations (Fernández-Galisteo et al. 2009) and that a three-step mechanism, 3H2 + O2 ⇔ 2H2O + 2H,  H + H + M ⇔ H2 + M and H2 + O2 ⇔ HO2 + H, can characterize premixed, non-premixed and auto-ignition flames (Boivin et al. 2011). The figure below shows the planar flame propagation velocity of hydrogen-air mixtures with the equivalence ratio in normal conditions. In the figure we include the results with the 21st elementary reactions of the detailed San Diego Mechanism (solid curve), the corresponding results with the one-step reduced mechanism (dashed curve) and experimental measurements (symbols).


In the figure below we plot the instant isocontour of temperature in a LES-DNS simulation of a supersonic hydrogen-air flame stabilized by autoignition. The results with detailed mechanism (left) and with the three-step mechanism (right) are plotted. The objetive of the simulation is to test that the reduced mechanism correctly describes the flame structure even when autoignition, diffusion and premixed combustion processes are involved under intense fluctuating flow conditions.


h2_difusion_jet    h2_difusion_jet_reduced


Reduced mechanism for syngas combustion

The syngas combustion can be properly described with a four-step mechanism even under conditions of interest for gas-turbine operation (Boivin et al. 2011). The fourth reaction involves the CO oxidation to give CO2 through the path CO + H2O ⇔ CO+ H2, where the reaction rate depends on two elementary reactions that involves CO and the intermediate species H, OH and HO2. This mechanism reduces to the former three-steps kinetics described before for the hydrogen combustion. The figure below shows the flame propagation velocity with the equivalence ratio for four pressures and for two different H2/CO ratios. In the figure we include the results with the 30 elementary reactions of the detailed San Diego Mechanism (solid curve), with a skeletal mechanism of 16 elementary reactions (dashed curve) and with the four-step mechanism (dot-dashed curve). In the figure we also include experimental measurements (symbol).


syngas laminar propagation velocity