Structures of multiple combustion waves formed under filtration of lean hydrogen-air mixtures in a packed bed

Combustion of lean hydrogen-air mixtures under filtration in an inert porous medium is studied experimentally. With the low hydrogen concentration (3–6%), superadiabatic wave propagation as a localized, stable temperature zone is observed with the combustion temperature reaching 800°C, which is much in excess of the adiabatic reaction temperature (300–500°C). The superadiabatic temperature rise due to effective heat regeneration in the system of “porous body-filtrating gas”, results in complete combustion of even very lean reacting mixtures. It is found that with increasing hydrogen concentration, the localized waves become thermodiffusionally unstable, forming multidimensional cellular structures. Depending on experimental parameters, mainly the hydrogen concentration, two types of cellular structures are observed: three-dimensional spatial and planar. The cellular structures are characterized by partial combustion of hydrogen. This results in the formation of double wave structures, with the cellular structure followed by a stable or decaying superadiabatic wave. The formation of cellular structures is interpreted based on thermodiffusive instability of pore-level-driven combustion. A computational model incorporating detailed hydrogen-oxygen chemistry is used to analyze kinetic profiles over experimental temperature distributions.