Alternative approaches to the reaction rate modelling in gas explosion simulation using STOKES

Abstract

Computational fluid dynamics (CFD) has been increasingly applied to process safety studies involving scenarios of accidental gas explosions. By offering a deeper understanding of the underlying physics of flame propagation, CFD can improve existing consequence analysis and risk assessment, particularly when confinement or obstructions are present. In the numerical modelling of gas explosions, the reaction rate is a quantity of fundamental importance because it ultimately leads to the values of flame speed and peaks of overpressure. In this work, we investigate the reaction rate models implemented in the CFD tool STOKES, which has been designed for the simulation of gas explosions in complex geometries. STOKES counts on the Bray–Moss–Libby (BML) reaction rate model, where the effects of flame stretch are neglected in its original version, which may lead to an inaccurate representation of the flame behaviour. Also, the length scale of wrinkling is often calculated as a function of the velocity fluctuations via empirical correlations that depend on adjustable constants. In a first approach, we propose an expression for calculating the stretch factor dynamically, based on the local divergence of velocity. In a second approach, a hybrid reaction rate model is presented, incorporating the well-known fractal approach into the BML model. The hybrid approach also counts on the calibration of parameters of the initial laminar burning model. Despite some improvement, the proposed dynamic stretch factor leads to unexpectedly enlarged flame contours. Results using the hybrid approach on the other hand improved agreement with experimental data, particularly in the early stages of flame acceleration when combustion chambers of relatively small sizes are considered. However, applications in real-scale semi-confined geometries indicate that the initial stages of flame development require further work.