Transient ignition of a combustible by stationary isothermal bodies

The transient ignition process of a stagnant combustible mixture by a stationary, isothermal flat plate or spherical particle is studied using matched asymptotic technique in the realistic limit of large activation energy. Results show that for a sufficiently-reactive mixture the flow field consists of a locally-similar reactive-diffusive region next to the hot surface and a non-similar transient-diffusive region external to it; that the ignition lag is usually much shorter than the characteristic diffusion time and hence can be considered to be instantaneous; and that the ignition process is only minimally affected by the geometry of the body. An explicit expression for the ignition delay has also been derived, enabling a priori assessment of its magnitude and its functional dependence on the various system parameters of interest.