Effect of Equivalence Ratio and Inlet Velocity

Calculations were performed for both cordierite and FeCr alloy wall materials, at increased inlet velocities of = 2.50 and 0.50 m/s and pressures p = 1 and 5 bar, respectively, for p = 0.4. For time integration up to t = 150 s, ignition could not be achieved for both investigated materials. This necessitated an increase in the equivalence ratio to p = 0.6 to enhance the catalytic reactivity and thus ignite the microreactor. 

Cases 11-14 in Table 8.2 pertain to simulations at p = 0.6 and p — 5 bar, for inlet velocities /in = 0.30, and 0.50 m/s. By comparing ignition and steady-state times of Cases 11 with 5 and 12 with 10, a clear benefit is evident for the higher p, with fig reduced to less than 50% of its corresponding value at (p — 0.4, for both materials. 

As a result of the increased catalytic reactivity of methane on Pt with rising equivalence ratio (see also [16]), the differences in characteristic times between cordierite and FeCr alloy, and in particular the ignition times, diminish. Direct comparisons for both materials and equivalence ratios are provided in Fig. 8.12, where the exhaust gas temperature of the microreactor is plotted versus the elapsed 

Characteristically, at steady state and cp — 0.4 radiation losses amount to 25.7% of the enthalpic inlet content for FeCr alloy (Case 10) as opposed to 19.9% for cordierite (Case 5). 

The observed changes in the slope of the rising exhaust gas temperature (marked with arrows in Fig. 8.12) occur approximately at the times when the propagating reaction zone reaches the beginning of the coated section (jc = 1 mm), which locally increases the wall temperature and consequently the radiation losses. The impact of radiation, an important effect neglected in microreactor studies, will be further elaborated in the coming section. 

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