Self-ignition of expanding high-temperature H2-air and CH4-air mixtures

Abstract

The influence of rarefaction waves on the ignition characteristics of H2-and CH4-air mixtures was studiedby solving the corresponding conservation equations for mass, species, energy, and momentum in a onedimensional model. The numerical solution was based on the method of lines and on a dynamic adaptive mesh technique. Because of the stiffness originating from the detailed kinetic mechanism, the resulting system of ordinary differential equations was integrated by a BDF-code. The rarefaction wave running into the initially homogeneous, quiescent, high-temperature gas was initiated by a piston moving with constant velocity. The wave causes a space- and time-dependent temperature profile resulting in space-dependent ignition processes. Because of the complicated interaction between gas dynamics and induction process, different combustion regimes arise. Nearly constant volume explosion regimes were obtained for low piston velocities. An increased piston velocity resulted in a combustion wave with a peak pressure exceeding the constant volume explosion pressure significantly. This is a quite unexpected result because a stronger expansion wave is associated with a temperature and density decrease. The observed peak pressure increase is a consequence of the wave-induced temperature gradient, creating an ignition wave that in turn interacts with its own pressure wave, resulting in a strong coupling between both waves. This effect was first revealed by Zeldovich for hot-spot temperature gradients and could now be shown to be also valid in the presence of rarefaction waves. A further increased piston velocity reduced the arising peak pressure, because the coupling mechanism failed as a result of the stronger temperature gradient.