Ignition of plane laminar premixed jets in a hot inert environment

Abstract

Ignition of a premixed combustible gas at a normal temperature discharged from a long narrow slit of width d 0 with velocity u 0 into an inert environment at a uniformly high temperature is investigated by the method of large activation energy asymptotics. A critical Damköhler number analytically depending on conditions at the jet exit, the ambient conditions, the equivalence ratio, and thermophysical properties and the overall chemical kinetic mechanism of the premixed combustible gas is defined as an ignition criterion. A minimum streamwise distance downstream of the jet exit to achieve ignition, called an ignition distance, is determined according to this criterion. The results reveal that there is an optimal equivalence ratio at which the ignition distance x 1 is minimum. The ignition distance is linearly proportional to the velocity at the jet exit. For fixed u 0 , the ignition distance is proportional to d 0 1 2 . However, the ignition distance x 1 drops off with d 0 1 2 when the mass flow rate flowing through the jet exit remains constant. For a constant mass flow rate, by increasing the initial jet temperature, the ignition distance increases first and then decreases after the initial jet temperature exceeds a critical value. This critical value of initial jet temperature increases with the ambient temperature. The ignition distance decreases monotonically with the initial jet temperature for fixed u 0 . The ignition distance decreases exponentially with the ambient temperature. The effect of ambient pressure P ξ on x 1 is entirely governed by the total reaction order n. The dependence of x 1 on P ξ is found to be x 1 ∼ P ∞ 1 − 3n 4 .