Flame stability and structure of opposed methane/air jet in coaxial tubes

By using the skeletal reaction mechanism, the combustion in an opposed methane jet in coaxial narrow air stream tubes is studied. The temperature, species distribution, heat flux, flame stretch rate and flame curvature are calculated. When the fuel flow rate (Q_F)=120 ml·min^-1, with increasing Q_A, the flame changes from flat-disk shape to curved one and covers the inner tube exit, and the flame is forced to move toward the exit of inner pipe. The flame is compressed and the distributions of temperature and species are compact. When equivalence ratio (ER)>1.00, with increasing Q_A, the peak temperature and the flame length along the flame surface rise and reach the maximum, while when ER≤1.00, the peak temperature and the flame length decrease. The after burning gas preheats the inlet methane through the inner pipe and the total heat flux is influenced by the flame temperature and gas flow. With the increase of Q_A, the heat flux is much stronger and the preheating increases, reaching the maximum when Q_A=2450 ml·min^-1. When Q_A>2450 ml·min^-1, the preheating goes down. When ER>1.00, the stretch rate of flame κ is small and changes slowly at the beginning, and then it rises sharply along the flame surface but finally it is no more than 65 s^-1. When ER≤1.00, along the flame surface, κ increases first and then decreases, and finally become negative with the minimum value of -262 s^-1. The increase of Q_A makes κ change seriously. The maximum of κ is 638 s^-1.