A hybrid BML-fractal approach for the mean reaction rate modelling of accidental gas explosions in partially confined obstructed geometries

Abstract

We propose a new approach to model the mean reaction rate of premixed turbulent flames when simulating accidental gas explosions with computational fluid dynamics (CFD). The combination between Bray–Moss–Libby (BML) approach and the fractal concept was evaluated. The fractal concept is used for closing the integral length scale of wrinkling in the BML formulation replacing the utilisation of empirical functions. The proposed expression accounts for the effect of local length scales of turbulence on the flame front via the fractal outer and inner cut-offs, which were taken respectively as the integral length scale of turbulence and the Gibson length scale. The initial laminar phase of flame propagation is described by a laminar combustion model and the onset of the transition from the laminar to turbulent combustion is assumed to take place at a threshold value of the turbulent Reynolds number. The approach is implemented and evaluated within an in-house developed code called STOKES (Shock Towards Kinetic Explosion Simulator) that solves the full set of Navier–Stokes equations. The equations are parameterised according to the porosity distributed resistance (PDR) method in which the porous mesh accounts for the presence of small obstacles that are responsible for turbulence generation during an explosion. Simulations are carried out for vented explosions of both methane–air and propane–air mixtures in partially obstructed chambers. Results are compared with experimental data, FLACS simulations and a typical model for the length of wrinkling which uses an empirical function instead of the fractal approach. Comparisons lead to overall good agreement, indicating that the proposed model can be used in numerical simulations of accidental explosions to support consequence modelling in the framework of risk analysis.