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Freely Propagating Hydrogen-Methane Flames


Introduction

Recent interest in alternative fuels such as hydrogen or syngas, obtained from coal gasification, has sparked the development of burners that can operate over a broad range of fuels. There is also an increasing interest in lean-premixed systems that have the potential for the high efficiency and low pollutant emissions that result from lower burnt-gas temperatures. The feasibility of lean premixed fuel-flexible systems depends on the development of robust flame stabilization techniques and the thorough understanding of flame propagation and emissions physics. One issue that must be addressed when dealing with a variety of fuels is the possible impact of thermo-diffusive instabilities on the overall flame dynamics.

Simulations

We have run simulations of a range of mixed (hydrogen and methane) flames. The basic simulation methodology for low-speed reacting flows, the feedback control procedure, and the flow configuration are the same as described in Freely Propagating Hydrogen Flames.

In this study, we focus primarily on the structure of the heat release and carry out analysis of reaction path (diagram) of carbon chemistry for the mixed fuels, which provides insight on understanding flame dynamics.

The figure above shows a subregion of quasi-steady premixed lean hydrogen-methane (0.875:0.125) flame indicating hydrogen fuel consumption rate (the finest spatial resolution of 39 microns); label A represents a cell that is burning strongly, and label B shows a cusp.

The figures above show chemical path diagrams for regions A and D, respectively.

Tracing through the flame surface locally along integral curves of the temperature gradient, one finds that profiles of various chemical species and heat release resemble those of flat steady flames across a broad range inlet fuel mixtures.

Figures on the left and right compare the heat release between freely propagating and flat flames for mixtures with 87.5% and 50% hydrogen, respectively.

First, the local flame speeds have strong linear correlations with positive curvature for flames with hydrogen content. Second, the slope increases with increasing hydrogen addition, indicating the hydrogen consumption rate is more sensitive to curvature or the positive curvature has stronger effect on hydrogen consumption. Third, increasing hydrogen fraction in the fuel mixture results in higher methane consumption rate. The figures below on the left and right are for mixture fuels with hydrogen fraction of 87.5% and 50%, respectively.

The distribution of heat deposition into the gas due to convection, conduction, diffusion (heat transfer accompanying differential species diffusion) and chemical reactions are evaluated for six mixture flames. More details will appear in an upcoming paper by Xinfeng Gao, Marc S. Day and John B. Bell, "Heat Release in Freely-Propagating Lean Premixed Hydrogen-Methane Mixtures"

Any questions should be directed to Marc Day, John Bell or Xinfeng Gao.

References

John B. Bell, Robert K. Cheng, Marcus S. Day and Ian G. Shepherd, "Numerical Simulation of Lewis Number Effects on Lean Premixed Turbulent Flames". [pdf]