Steady-state analyses of the flammability limits of fuel-air mixtures in micro-scale combustion systems with detailed chemical mechanisms

The micro-scale combustion system is modeled as two channels and a catalyst layer with dimensions described in detail above. These dimensions could be increased or decreased as desired for particular applications. This design analysis treats some of these dimensions as variables, and some as constrained by combustion stability considerations. In order for flame holding to occur in the micro-scale combustion system, methane and air must be premixed and provided in an appropriate velocity region to reside. Methane and air are perfectly premixed before they enter the system prior to combustion. The strength of the mixture composition is typically expressed in terms of its equivalence ratio. Platinum is catalytically-active towards promoting the heterogeneous surface reaction. The stoichiometry of a fuel-air mixture contributes to its flammability range. A stoichiometric fuel-air mixture composition contains sufficient oxygen for complete combustion thereby releasing all the latent heat of combustion of the fuel. The strength of a fuel-air mixture composition typically is expressed in terms of its equivalence ratio; the equivalence ratio being the actual fuel-air ratio divided by stoichiometric fuel-air ratio. For example, an equivalence ratio of one represents a stoichiometric fuel-air mixture composition. An equivalence ratio less than one represents a lean mixture and an equivalence ratio less great than one represents a rich mixture. Pressure and temperature contribute to the flammability range of fuel-air mixtures. Typically, with increases in pressure, the rich flammability limit is extended thereby extending the flammability range of the fuel-air mixture. Temperature, on the other hand, partially defines the flammability range of fuel-air mixtures. The lowest temperature at which a flammable fuel-air mixture can be formed, based upon the vapor pressure of the fuel at atmospheric pressure, is the flash point of that fuel-air mixture. Within the flammability range of a fuel-air mixture, at temperatures exceeding the flash point of the fuel-air mixture, auto-ignition of the fuel vapor occurs. Auto-ignition generally occurs at or slightly above the stoichiometric fuel-air mixture composition. The time interval between the mixing of the fuel-air mixture such that it is combustible and the auto-ignition of that fuel-air mixture is known as the auto-ignition delay time. Complex modeling methods and algorithms are required for the micro-scale combustion system due not only to the complex geometry of the system but also the complex physicochemical processes involved. It is therefore essential to reduce the complexity of the model through use of certain simplifying assumptions. Steady-state analyses are performed, variations in system pressure and temperature are determined in accordance with the ideal gas law, and the micro-scale combustion system operates in the laminar flow regime due to the small Reynolds numbers. Detailed chemistry is included in the model. Detailed chemical mechanisms are playing an increasingly important role in developing chemical kinetics models for combustion. Detailed chemical mechanisms are incorporated into the reacting flow for the micro-scale combustion system.