Effect of graded structure on CH₄/air combustion in porous media burner

Porous media combustion is an efficient method for utilizing CH₄, and optimizing the design of porous media burner can further enhance combustion performance. This study experimentally investigates the combustion characteristics of porous media with step and graded variation in PPI (pores per inch) parameter. In addition to conventional temperature distribution and emission characteristics, we conducted diagnosis and analysis of the flame structure during combustion using the steady-state and transient distributions of OH*(excited OH radicals). Based on this analysis, we explained the reasons for low lean combustion limit. Super-adiabatic combustion was achieved under most operating conditions, and this performance was characterized using the recirculation efficiency and speed-up ratio. The experimental results show that the graded PPI structure exhibits a lower lean combustion limit, reaching as low as 0.38 at u = 0.6 m/s. Additionally, it produces lower pollutant emissions, with CO emissions of approximately 58 ppm at Φ = 0.65 and NO emissions below 11 ppm under all conditions. However, its super-adiabatic characteristic is inferior to that of the step PPI structure, with lower recirculation efficiency and speed-up ratio across all conditions. The OH* distribution characteristics finds that the lower lean combustion limit in porous media combustion is related to flame re-ignition under discontinuous OH* distribution. Statistical analysis of OH* distribution clarifies the influence of PPI design variation on the lean combustion limit and establishes correlations among temperature distribution, super-adiabatic characteristics, and emission properties.