Experimental investigation of irradiance from combustion in porous media with different geometries

Abstract

Radiant porous burners with a high power density, emitting intense radiation were investigated. Different geometrical structures were applied as porous inlay in a state-of-the-art, two-layer porous burner with a rectangular shape. Structures were made of SiSiC and manufactured by the replica method using foaming and hybrid additive manufacturing. Subject to the study were foam structures as well as lattice structures with random strut distribution based on the Voronoi tessellation and with regular distribution based on the Kelvin and Hendecahedron cell geometry. The structures were designed with the intention of enhancing the irradiation by optimising the specific surface area distribution along the flow direction. The additive manufacturing method enables this through a local increase in pore density and the implementation of additional surfaces. Image processing was used to demonstrate the effectiveness of this approach and to characterise the structures in specific surface areas. Volume averaged values and distribution along the thickness were analysed. The radiation efficiency was derived from measurements of the radiation intensity on discrete points in parallel to the radiating surface using a radiometer. The burner was operated with methane as fuel at a specific burner power in the range of 600 kW m⁻² to 1000 kW m⁻² and an equivalence ratio of $\varphi =0.7$. Measured radiation efficiency was compared to a limiting radiation efficiency obtained from theoretical calculations. Highest radiation efficiency was obtained for a foam structure with a specific surface area of 622 m⁻¹. Structures based on the geometry of a hendecahedron and a kelvin cell achieved comparable efficiencies. The lowest values were observed for a randomly distributed lattice structure with nominal pore density of 10 PPI. A relation between volume averaged specific surface area values and measured radiation efficiency is derived and proven. Additionally, efficiency could be improved by targeted surface area increase applying closed windows or a gradient in pore size respectively.