Numerical investigation of wall heat conduction effects on catalytic combustion in split and continuous monolith tubes

The optimum length of a monolith tube is one for which near-hundred percent conversion is attained, and at the same time, the catalyst over the entire length of the tube is utilized. In practice, the length is adjusted by stacking monolith plugs end-to-end. In this study, the repercussions of such a practice are investigated numerically with the goal to determine if a tube of length 2L demonstrates the same behavior as two tubes of length L each, stacked end-to-end. Catalytic combustion of methane–air mixture on a platinum catalyst is considered. The studies are conducted using a multi-step reaction mechanism involving 24 surface reactions between 19 species. Two different materials are considered for the walls of the monolith tube, namely silicon carbide and cordierite. Both steady state and transient simulations are performed. Results indicate that the ignition and blowout limits can be significantly different between split and continuous tubes when the wall is made up of a high thermal conductivity material, such as silicon carbide. For steady state combustion, for both wall materials, the point of attachment of the flame to the wall is altered by splitting the tube—the effect being more pronounced for silicon carbide and at relatively high Reynolds numbers. These results imply that axial heat conduction, or lack thereof due to thermal contact resistance, through the walls of the monolith results in thermal non-equilibrium between the solid and fluid phase, and subsequently affects ignition and flame stability in catalytic combustion.