Numerical Investigation of Radiation Effects in Monolithic Catalytic Combustion Reactors

In modeling catalytic combustion in a monolithic catalytic converter, it is generally assumed that the gas within the individual monolith channels does not interfere with thermal radiation. To date, no quantitative study has been undertaken to validate this assumption. Past studies for carbon monoxide combustion also appear to indicate that the emissivity of the washcoat has little effect on the thermal radiation field. In order to investigate these two issues, methane-air combustion on platinum is modeled inside a single channel of a monolith using a detailed surface reaction mechanism comprised of 24 reactions between 19 species. Radiation transport is modeled using the Discrete Ordinates Method and a gray formulation. Planck-mean absorption coefficients of the gases, calculated from the HITEMP and HITRAN databases, are used to investigate participating medium effects. All calculations were performed using the commercial CFD code, CFD-ACE+™, supplemented by user-subroutines for calculating the radiative properties of the gas mixture. Results show that the conversion percentages and temperature distributions are unaltered by the inclusion of participating medium radiation effects, verifying the commonly held belief, stated earlier. However, in strong contrast with carbon monoxide combustion, the emissivity of the washcoat was found to significantly affect flammability limits in the case of methane combustion—the flame being hotter and more stable for smaller values of emissivity.

Numerical Investigation of Radiation Effects in Catalytic Combustion

In modeling catalytic combustion in a monolithic catalytic converter, it is generally assumed that the gas within the individual monolith channels does not interfere with thermal radiation. To date, no quantitative study has been undertaken to validate this assumption. Past studies for carbon monoxide combustion also appear to indicate that the emissivity of the washcoat has little effect on the thermal radiation field. In order to investigate these two issues, methane-air combustion on platinum is modeled inside a single channel of a monolith using a detailed surface reaction mechanism comprised of 24 reactions between 19 species. Radiation transport is modeled using the Discrete Ordinates Method and a gray formulation. Planck-mean absorption coefficients of the gases, calculated from the HITEMP and HITRAN databases, are used to investigate participating medium effects. All calculations were performed using the commercial CFD code, CFD-ACE+™, supplemented by user-subroutines for calculating the absorption coefficient of the gas mixture. Results show that the conversion percentages and temperature distributions are unaltered by the inclusion of participating medium radiation effects, verifying the commonly held belief, stated earlier. However, in strong contrast with carbon monoxide combustion, the emissivity of the washcoat was found to significantly affect flammability limits in the case of methane combustion—the flame being hotter and more stable for smaller values of emissivity.

Gaseous combustion at high pressures. Part III.-The energy absorbing function and activation of nitrogen in the combustion of carbon monoxide

In the previous paper of the series* on the explosion of isothermic hydrogen-air and carbon monoxide-air mixtures in the theoretical proportions for complete combustion, at an initial pressure of 50 atmosphere it was shown- (1) that whereas in the case of hydrogen-air mixtures, the maximum pressure was always attained in about 0·005 second after the commencement of combustion, and the cooling set in almost immediately thereafter, in the case of the corresponding carbon monoxide-air mixtures, the time similarly taken for the attainment of maximum pressure was about forty times longer (namely, between 0·18 and 0·24 second), and cooling was delayed for quite an appreciable interval, showing that heat energy was still being liberated long after the maximum temperature had been reached; and (2) that the replacement, even in very small properties, of carbon monoxide bu its equivalent of hydrogen in the mixture 2CO+O 2 +4N 2 had an altogether disproportionately large influence in accelerating the rise of pressure on explosion; indeed, it seemed as though the hydrogen had imposed its own character upon the whole course of the Carbon monoxide combustion, even to the extract of suppressing the aforesaid marked evolution of heat after the attainment of maximum pressure.