Effects of Hydrogen Addition on Lean Combustion. 3rd Report, Computational Study of Reaction Scheme in Tubular Flames.

Abstract

To stabilize a flame and enlarge its reaction rate in lean combustion, we have focused on hydrogen addition and rotational flow. We present results of numerical simulations of premixed flames in a stretched, rotating flow field for methane/hydrogen/air mixtures to investigate effects of hydrogen addition on the lean combustion. The governing equations with a similarity solution are conservation equations of mass, momentum, chemical species and energy. Equation of state is also used and a reduced 4-step mechanism is adopted. Detailed transport properties are taken into account. Results show that with increasing the concentration of added hydrogen, (1) the flame temperature monotonically increases, (2) the flame diameter which is determined by the maximum heat release rate is extended, (3) the reaction rate is accelerated and burning velocity is increased. These results are in accordance with those of previous experiments. To make clear the mechanism, we discuss the flame characteristics based on the chemical scheme and radical transport. Particularly, we examine the non-equilibrium (super-equilibrium) concentration in the reaction zone, and compare it with equilibrium value obtained by chemical equilibrium calculation. It could be concluded that effects of hydrogen addition are explained by the fact that the non-equilibrium radical concentration in the reaction zone is increased by hydrogen addition, which plays an important role in chain-reactions in lean combustion.