Numerical analysis on the characteristic chemical time scale and combustion regime of natural gas MILD combustion

Moderate or intense low-oxygen dilution (MILD) combustion is regarded as a novel combustion technology due to high thermal efficiency and low pollution emissions. Understanding the interaction between turbulence and chemistry can provide a deep insight into the chemical reaction zone structure and MILD combustion regime. Numerical simulation of natural gas MILD combustion in the International Flame Research Foundation (IFRF) test furnace was carried out by using standard k-e model with Eddy Dissipation Concept (EDC) model. Then a modified Jacobian matrix method with a simplified selection method was developed to calculate the characteristic chemical time scale and the Damkohler number and subsequently to identify the MILD combustion regime of natural gas. All chemical time scales were calculated from the eigenvalues of the chemical source term Jacobian matrix, and the characteristic chemical time scale was chosen out through the simplified and effective selection of major species and chemical time scales. The results show that the characteristic chemical time scale is 10(-6)-10(-4) sin the main combustion zone and about 10(-2) sin the post flame zone. The calculated Damkohler number is close to unity and the combustion regime of natural gas combustion is determined as the distributed reaction zone with a higher air jet velocity, which reflects the MILD combustion characteristics. The numerical results confirm that the natural gas MILD combustion in the IFRF furnace is in the finite-rate chemistry regime and disperses throughout the whole furnace volume which is significantly different from the conventional combustion mode.