Investigation of instability and turbulence effects on gas explosions: experiments and modelling

Safe design of industrial facilities requires models that can predict the consequences of accidental gas explosions in complex geometries with sufficient accuracy. Computational fluid dynamics (CFD) models are widely used for consequence assessment in the process industries. The primary mechanism for flame acceleration during gas explosions in congested geometries is the positive feedback between turbulence generated in the reactant mixture, especially in shear and boundary layers from explosion-driven flow past obstacles and walls, and enhanced combustion rates. Additionally, a range of instability phenomena can significantly increase flame acceleration in gas explosions. This doctoral project addresses the following research question: "how can the subgrid representation of flame acceleration mechanisms due to instability effects and flow past obstructed regions be improved in a CFD tool used for consequence assessment of gas explosions? ". The thesis presents and validates new sub-grid models developed for the CFD tool FLACS, focusing on the following flame acceleration mechanisms: (i) the influence of the hydrodynamic and thermal–diffusive instabilities (intrinsic instabilities) on flame acceleration in the initial phase of a gas explosion, (ii) the influence of thermal–diffusive effects on the rate of turbulent combustion for different fuels and mixture concentrations, (iii) the role of the Bénard–von Kármán (BVK) instability downstream of bluff-body obstacles in explosion-induced flow, (iv) how the Rayleigh– Taylor (RT) instability developing on a flame front that is accelerated over an obstacle or a vent opening may enhance the combustion rate, and (v) how flexible obstructions with very small components (in the form of vegetation) induce flame acceleration in gas explosions. There was a lack of available experimental work describing the relative importance of several of the aforementioned effects. Therefore, three experimental campaigns were designed and conducted as part of the doctoral study. Experimental findings thus constitute a significant part of the original scientific contribution of the present work; these can be used to develop sub-grid models for any consequence model system. Three additional campaigns, performed by other research groups, were simulated in order to validate the new sub-grid models. The first experimental campaign presented in the thesis concerned explosion experiments performed in a large-scale, empty, vented enclosure comprising two chambers separated by a doorway. Simulations indicate that model performance might improve by introducing the effect of the RT instability. Furthermore, the results suggest that the model should account for thermal–diffusive instability effects both in the initial phase of a gas explosion and in the regime of turbulent combustion. The onset and growth rate of intrinsic instabilities in gas explosions are closely linked to the value of the Markstein number of the fuel-air mixture. The second experimental campaign presented in the thesis was performed as part of the doctoral study to explore the effect of varying the fuel concentration on overpressures and flame speeds in a series of propane-air explosions. The variations in the fuel concentration effec-