Assessment of ignition hazard in turbulent flammable gas mixers combining a Lagrangian approach and large eddy simulation

In this article, a Lagrangian approach is adopted to assess both mixing efficiency and ignition hazard of fuel/oxygen flammable gas mixing in a jets-in-cross-flow configuration. Large eddy simulation is used to provide the unsteady fields of turbulent gas mixing. To achieve statistical reliability of the fluid dynamics field, such as velocity, pressure, temperature, and chemical composition, a large number of Lagrangian markers are dumped into the inlet streams of the mixer and their trajectories are determined during the numerical simulation. Those trajectories are stored in a file processed afterward to build up the probability density functions of the fuel molar fraction of Lagrangian markers gauging both mixer efficiency and safety. Flammability limits provided by H.F. Coward and G.W. Jones [U.S. Bur. Mines Bull. 503 (1952)] and autoignition delays from a detailed kinetics scheme are required to define the hazard range. Processing the marker trajectory file allows the calculation of relevant Lagrangian properties such as (i) the critical residence time in flammability areas for the ignition case and (ii) the marker Damköhler number for the autoignition case, both including the subgrid turbulence effects. The latter are accounted for by assuming a presumed beta shape for the large eddy probability density function of subgrid fluctuations of mixture fraction. Analysis of all marker trajectories leads to a computation of global probability density functions of Lagrangian properties that characterize mixer safety.