77th Annual Meeting of the APS Division of Fluid Dynamics (November 24, 2024 — November 26, 2024)

P2690492: Flame-vortex interaction behind a bluff body obstacle

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Authors

• Kevin Cheevers, University of Ottawa
• Matei Radulescu, University of Ottawa

DOI: https://doi.org/10.1103/APS.DFD.2024.GFM.P2690492

Schlieren photographs of an atmospheric hydrogen-air flame propagating through a shock tube with an open end in which a rectangular obstacle is placed. The flame, ignited using a long wire located near the closed end of the shock tube which spans its height, forms an interface between low density product gases and higher density fresh gases. Due to the volumetric expansion of the hot burnt gases, a flow is driven ahead of the flame towards the open end of the shock tube. As a consequence of the obstacle in the flow field, the flow locally accelerates until reaching the vena contracta and decelerates behind the obstacle due to the change in cross-section of the tube. Additionally, an irrotational vortex forms downstream of the bluff body. Shortly after its ignition near the solid wall, the flame surface wrinkles as a result of flame intrinsic instabilities, increasing the flame surface area, which in turn drives a stronger upstream flow. As the flame approaches the obstacle, the misalignment between the density gradient across the flame and the acceleration gradient in the flow results in Rayleigh-Taylor instabilities which deform the flame surface, prompting larger amplitude cells near the top of the channel and smaller amplitude cells near the bottom of the channel. After passing the obstacle, the flame surface is disrupted as it convects through a turbulent boundary layer. The flame is then entrained by the vortex which has grown since the ignition of the flame, leading to a strong increase in the flame surface area and thus the consumption rate of fresh gases. The pressure downstream of the obstacle sufficiently increases from this enhancement of the flame to reverse the flow passing over the obstacle, forming another vortex which remains above the obstacle.