Heterogeneous reaction characteristics and their effects on homogeneous combustion of methane/air mixture in micro channels I. Thermal analysis

Catalytic combustion of methane/air mixture in various channels was numerically investigated to illuminate the heat characteristics of heterogeneous reactions (HTRs) and their effects on homogeneous reactions (HRs). A two-dimensional (2D) simulation model of channels with a width (d) of 2.00-2.82 mm was employed, which included detailed gas-phase and surface reaction mechanisms. Further, the heat transfer mechanisms were elucidated. HRs always occupied the main part of over 80% of the total released heat in all the cases. HTR is relatively amplified in narrower channels, and S (the ratio of heat released by HTR to that of all reactions) rises by several percentage as d decreases (S = 17.61% when d = 2.10 mm), and it is more effective to use a catalyst in the microcombustor with a smaller gap size to enhance combustion reaction rate and stability. The onsite-S showed a trend of stable-decrease-peak-rapid increase-slow increase along x, corresponding to preheating, homogeneous ignition, heterogeneous ignition (conversion of CO to CO2 and H-2 to H2O), intensive HTR stage (consumption of O, H, and OH and production of H2O and H-2), and complete consumption of reactants in the gas phase. The analysis about the heat sources for homogeneous ignition shows that homogeneous ignition was dominated by the preheating heat from hot gas downstream, and affected by transverse preheating heat. As d decreased, the preheating heat obtained from the transverse preheating increased by several percentages, even reaching to a similar value to preheating heat from hot gas downstream when d = 2.10 mm. The heat released by weak HTR only takes a small part of preheating heat of inlet mixture and homogeneous ignition. After homogeneous ignition, HR promoted the HTR to the maximum. Generally, HTR would delay the homogeneous ignition, enlarge the flame thickness, increase the peak of flame temperature and the exit gas temperature, and improve the completeness of combustion.