A numerical investigation of coupled geometric effects on methane-air deflagration dynamics

• Validated LES resolves coupled effects of obstacle shape, blockage ratio, and thickness on methane-air deflagration. • Obstacle shape alters flame structure: from fragmented fronts to smoother propagation. • Blockage ratio is key: higher BR shortens flame transit and increases speed. • At low BR, thicker obstacles stabilize shear layers and reduce pressure rise. Obstacle-driven flame acceleration in confined methane–air deflagrations poses a persistent hazard to industrial and utility corridors. Internal geometry-shape, blockage ratio (BR), and thickness-regulates turbulence–flame coupling, yet their combined influence remains incompletely resolved. This study employs a validated large-eddy simulation (LES) framework with adaptive mesh refinement and a partially premixed combustion model to examine these interactions in a two-dimensional straight duct. The matrix sweeps rectangular, elliptical, and triangular obstacles across BR = 0.3, 0.5, 0.7, while thickness (10, 20, 30 mm) is varied at BR = 0.3 to isolate thickness effects under comparable confinement, yielding 15 total cases (five cases per obstacle shape). Under the present conditions, BR appears to be the dominant accelerator: increasing BR from 0.3 to 0.7 raises the maximum pressure-rise rate, (d P /d t ) max , by as much as 637% in this dataset. A second trend emerges at low BR, hereafter referred to as a thickness-stabilization effect. Increasing thickness from 10 to 30 mm consistently lowers the assessed hazard; in the rectangular configuration, (d P /d t ) max decreases by approximately 74%. Flow-field diagnostics offer a coherent explanation. Thin plates generate highly unstable shear layers that roll up almost immediately, promote fine-scale wrinkling, and accelerate the front. With added thickness, the obstacle behaves in a splitter-plate-like manner: the onset of shear-layer roll-up is delayed, the near wake is stabilized, and shedding organizes into more coherent vortical patterns-behavior that is consistent with slower propagation. Shape continues to organize the wake and flame brush. Rectangular obstacles tend to fragment the front and bias it toward asymmetry, whereas elliptical profiles support smoother, more continuous propagation under otherwise similar conditions. Taken together, the results suggest that obstacle geometry maps systematically onto deflagration dynamics and may inform evidence-based guidance for passive safety design in confined infrastructure.