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Flame Surface Density (FSD) models for Large Eddy Simulation (LES) are implemented and tested for a canonical configuration and a practical bluff body stabilised burner, comparing common algebraic closures with a transport equation closure in the context of turbulent premixed combustion. The transported method is expected to yield advantages over algebraic closures, as the equilibrium of subgrid production and destruction of FSD is no longer enforced and resolved processes of strain, propagation and curvature are explicitly accounted for. These advantages might have the potential to improve the ability to capture large-scale unsteady flame propagation in situations with combustion instabilities or situations where the flame encounters progressive wrinkling with time. The initial study of a propagating turbulent flame in wind-tunnel turbulence shows that the Algebraic Flame Surface Density (FSDA) method can predict an excessively wrinkled flame under fine grid conditions, potentially increasing the consumption rate of reactants to artificially higher levels. In contrast, the Flame Surface Density Transport (FSDT) closure predicts a smooth flame front and avoids the formation of artificial flame cusps when the grid is refined. Five FSDA models and the FSDT approach are then applied to the LES of the Volvo Rig. The predicted mean velocities are found to be relatively insensitive to the use of the FSDT and FSDA approaches, whereas temperature predictions exhibit appreciable differences for different formulations. The FSDT approach yields very similar temperature predictions to two of the tested FSDA models, quantitatively capturing the mean temperature. Grid refinement is found to improve the FSDT predictions of the mean flame spread. Overall, the paper demonstrates that the apparently complicated FSD transport equation approach can be implemented and applied to realistic, strongly wrinkled flames with good success, and opens up the field for further work to improve the models and the overall FSDT approach.

Structural steel has poor fire resistance properties and often requires thermal protection. Passive thermal protection systems such as insulation for steel members by application of spray-applied fire-resistance materials (SFRM) on surfaces of structural steel members are expensive and represent a significant portion of building costs for steel structures. Also, design codes around the world are progressing towards performance-based approaches rather than prescriptive-based approaches in design for fire safety. However, it is difficult to account realistically for all probable fire scenarios, and the actual fire resistance of a structure may vary significantly depending on the nature of the fire, location of origin and characteristics of the building. A means to define and identify the maximum and minimum fire loading scenarios causing instability of a given structure is therefore desirable. Presented in this paper is a novel global optimization approach for determining the highest temperature, lowest temperature, most localized, and most distributed fire scenarios causing instability for an unbraced structural steel frame. The investigation assumes that the columns are fire protected but the beams are unprotected, and may be extended to apply in other configurations and framing materials.