Direct numerical simulation of transient ignition of diluted hydrogen versus heated air in axisymmetric counterflow

摘要

The sensitivity of super-equilibrium OH to the initial width and amplitude of O-atom deposition used to trigger ignition in a mixing layer of heated air versus ambient hydrogenitrogen is numerically investigated in an axisymmetric counterflow configuration. This represents an extension of a previous study that compared one-dimensional opposed jet computations with an axisymmetric counterflow ignition experiment. The previous one-dimensional computations did not capture the degree of super-equilibrium OH that was measured during the transition from thermal runaway to the formation of a steady flame. The present two-dimensional simulations show that the spatial distribution and the magnitude of the OH overshoot are governed by multidimensional effects. The degree of OH overshoot increases as the diameter of the initial O-atom deposition region decreases. This result is attributed to preferential diffusion of hydrogen in the highly curved leading portion of the edge flame that is established following thermal runaway at the ignition kernel. The simulations show that the ignition delay decreases as the amplitude of the initial O-atom deposition increases as expected. It is also found that the structure of the resulting diffusion flame corresponds to Linan's 'premixed flame regime' in which only the oxidizer leaks through the reaction zone. The flame exists under fuel lean rather than stoichiometric mixture fraction conditions. The edge flame structure resulting from thermal runaway in the present nonpremixed counterflow system resembles edge flames in a homogenous mixture flowing against hot inert in counterflow.