Multiscale tomographic diagnostics of methane/air flame propagation through gradient porous media: From laminar wrinkling to turbulent transition in confined spaces

<title>Abstract</title> <p>This study presents a comprehensive multiscale investigation of premixed methane/air flame propagation through gradient porous media (PM) in a confined rectangular duct, employing synchronized high-speed schlieren tomography, particle image velocimetry (PIV), and dynamic pressure measurements. Five configurations were examined: empty tube, uniform 10 PPI, 20 PPI, 40 PPI alumina foams, and a novel gradient arrangement (10-20-40 PPI). Results reveal that the gradient PM architecture effectively modulates flame acceleration by sequentially increasing flow resistance, reducing the maximum flame speed from 387 m/s (empty tube) to 268 m/s (gradient), corresponding to a 30.7% suppression. The fractal dimension analysis of flame front wrinkling demonstrates that gradient PM delays the transition from corrugated flamelets to broken reaction zones by approximately 8-10 ms, providing a critical time window for active suppression systems. Spectral analysis of pressure oscillations identifies dominant acoustic modes at 1.2 kHz and 3.5 kHz, with gradient PM attenuating pressure wave amplitude by 25.3%. Quantitative validation against the Bychkov flame acceleration model and laminar burning velocity correlations from the literature confirms the reliability of the experimental data, with deviations within 8.5%. These findings provide fundamental insights into flame-porous structure interactions and offer practical design criteria for gradient suppression barriers in industrial explosion protection systems.</p>