Di Sarli, Valeria (2008) Study of unsteady premixed flame propagation during an explosion: interaction between combustion and turbulence. [Tesi di dottorato] (Unpublished)

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Gas explosions almost always occur in presence of obstacles that disturb the flat flame propagation. The unsteady coupling of the moving flame front and the turbulent vortices generated by the local blockage intensifies the flame acceleration and the subsequent overpressure rise. In the present research activity, in order to get insights about the nature of obstacles-induced explosions a fundamental study is performed based on the combined use of advanced experimental and numerical tools. The experiments have been focused on the unsteady flame/vortex interaction. A novel twin section combustion chamber has been utilised to allow the controlled formation of stable and repeatable vortical structures into which a flame front can propagate. High-Speed Laser Sheet Flow Visualisation (HSLSFV) has been employed to record the time evolutions of both, flame shape and scales of flame front wrinkling. Time-Resolved Particle Image Velocimetry (TRPIV) has been used to measure the velocity vectors fields ahead of the propagating flame fronts. The results obtained have allowed identifying and quantifying the various combustion regimes that establish during the unsteady flame propagation. Depending on the dimension and rotational velocity of the vortical structures encountered and, then, on the strength of the flame/vortex interaction, the propagating flame is driven to burn from the initial laminar−noeffect regime up to the wrinkled flame regime or pockets formation regime. These regimes differ for the effects produced by the vortices on the flame surface and structure and, consequently, on the way and rate of burning. The experimental results have been used as a guide for the development of a Computational Fluid Dynamics (CFD) model of unsteady premixed flame propagation through obstacles. The model is based on the Large Eddy Simulation (LES) approach. An assessment has been performed of different literature combustion sub-models proposed for LES of fully turbulent premixed flame (Colin et al., 2000; Flohr & Pitsch, 2000; Kim & Menon, 2000; Charlette et al., 2002; Pitsch & Duchamp De Lageneste, 2002) to select the sub-model that best grasps the flame evolution through the different combustion regimes identified. The model results have been compared against the experimental data by Patel et al. (2002) on the flame propagation through repeated obstacles in an open-end explosion chamber. III It has been found that the sub-model by Charlette et al. (2002) gives a very satisfactory agreement with the experimental results in terms of shape and structure of the flame, flame arrival times and locations, flame speed profile along the chamber and overpressure time history. Further validation of the LES model implementing this combustion sub-model versus the experimental data on the unsteady flame/vortex interaction has deonstrated its excellent ability to reproduce besides the flame shapes and propagation times, also the velocity vectors maps ahead of the front and then the details of the coupling between flame and flow field. On the other hand, changes of the constants and parameters values of the sub-models other than Charlette et al. (2002) have also allowed the correct simulation of the unsteady flame propagation through obstacles. Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations of the experiment by Patel et al. (2002) have also been run and the results compared to the LES ones. It has been shown that the LES modelling outperforms the conventional statistical approach, which is indeed not able to correctly simulate the time and spatial development of the vortices and their effects on the flame structure and speed. With the validated LES model ad hoc simulations have been carried out and the obtained results examined to understand the mechanisms and phenomena correlating flame structure, speed and resulting overpressure during the unsteady flame propagation in a vented obstructed chamber. The competition between combustion rate and venting rate, that establishes in the chamber zones upstream and downstream of the obstacle, has been identified as the mechanism responsible for the overpressure peaks observed. In particular, the blocking effects of the obstacle-side combustion on the propagation upstream of the obstruction, and of the external explosion on the turbulent combustion of the mixture trapped downstream of the obstacle (over-combustion) have been highlighted. It has been found that both obstacle-side combustion and overcombustion are strongly affected by the combustion/turbulence interaction and obstacle blockage. Furthermore, the simulations results have shown that the effects of the operating and geometrical parameters (fuel/air mixture composition, obstacle blockage ratio and shape, obstacles number) on the overpressure peaks can be explained through the effects on the obstacle-side combustion and overcombustion themselves. When the parameters changes are able to increase the amounts of fresh mixture involved in the combustion upstream and downstream of the obstacle, and the intensity of the turbulent flow field induced around the IV obstruction, obstacle-side combustion and overcombustion become more violent and the related overpressure peaks stronger.

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