The combustion characteristics and stable limit of a novel combustor with gradient porous media for hydrogen-enriched natural gas

Injecting hydrogen into natural gas pipelines for delivery to end users is currently considered as one of the potential solutions to the challenges of hydrogen transportation and utilization. However, the blending of hydrogen accelerates the flame propagation speed of the mixture, leading to flame instability and other issues. In this paper, a gradient porous structural burner was proposed, then the effects of pore size gradient and porosity gradient on the combustion characteristics and stability limits of hydrogen-enriched natural gas flames were investigated numerically. This paper proposed a spatial gradient-based porous structure, and the results showed that the axial gradient porous structure with increasing pore size enhances the maximum temperature by approximately 26 %. The radial gradient structures with decreasing porosity and increasing pore size increased the maximum temperature by approximately 25.8 % and 24.8 %, respectively, and improved flame uniformity. Compared to the conventional two-stage uniform structure, the axial pore size increment structure improved the stability limit by approximately 117 %, while the radial pore size increment structure increased the stability limit by 47 %. The results provide reference for the design and optimization of new porous media burner structures that are applicable under wide power ranges and hydrogen blending conditions.