Simulation of H2-Air Non-Premixed Flame Using Combustion Simulation Technique to Reduce Chemical Mechanisms

Abstract

Combustion simulation that uses computational fluid dynamics (CFD) has been widely adopted as the design tool for combustion equipment. Because flow inside such equipment is generally turbulent, turbulence and combustion models are needed to simulate combustion; many combustion simulations have been performed to verify a system’s internal state, such as velocity, pressure, mole fractions of chemical species, and temperature. Combustion simulation of a confined impinging jet reactor has been performed by the large eddy simulation (LES) model serving as the turbulence model and the presumed probability density function (PDF) serving as the combustion model (Daniele, 2009). The analysis shows that a confined impinging jet reactor is indeed an interesting device because of its high mixing efficiency and absence of stagnant and recirculation zones. Under the condition of moderate or intense low-oxygen dilution (MILD), the effect of H2 on H2-CH4 turbulent non-premixed flames was investigated with the improved standard k   model as the turbulence model and with the eddy dissipation concept (EDC) model (Amir et al., 2010). Simulation results show that H2 addition to CH4 leads to improved mixing, increase in turbulent kinetic energy decay along the flame axis, increase in flame entrainment, higher reaction intensities, and increase in mixture ignitability and rate of heat release. Although combustion simulation was considered to be an efficient designing tool, considerable computational time was needed to calculate the chemical reaction. Combustion models that detail chemical mechanisms require reaction calculations involving ndimensional ordinary differential equations (ODEs) that are solved according to the number of chemical species involved. Therefore, reducing computation time for the combustion simulation is a significant problem. If computation time could be easily reduced according to the required prediction accuracy, we would be able to obtain the results more quickly. For example, O is a significant species whose mass fraction is necessary to compute the amount of NO present; therefore, the accuracy of the mass fraction of O cannot be neglected. To determine this mass fraction with sufficient accuracy, it is necessary to build a reduced mechanism including O. Generally, a quasi-steady state or partial equilibrium is assumed when the reduced mechanism is built (Warrants et al., 2006). The chemical equilibrium method (Nagai et al., 2002) does not use reaction equations; instead, the equilibrium composition of a chemical system is determined by minimizing the