Stability Maps Pressure, Gas-phase Chemistry and Fuel Transport Properties Effects

1 Stability Maps Pressure, Gas-Phase Chemistry and Fuel Transport Properties Effects 

In a fashion similar to earlier studies for methane [4], Fig. 7.4 indicates that by increasing the operating pressure of the propane-fueled microreactor from 1 bar (diamonds) to 5 bar (squares) a significant, almost two-fold increase on the maximum allowable external heat transfer coefficient h is achieved for the same mass throughput. At the same time, the upper mass inflow limit before blowout is extended substantially ( 124%). Recent kinetic studies revealed a positive dependence of propane reactivity on platinum [8], while earlier studies suggested a corresponding positive dependence for gas-phase reactions [20]. The 

The stable combustion regime at p — bar is essentially a subset of that at p = 5 bar, and the same holds true for methane. 

The most profound observation in Fig. 7.4 is the significant difference between propane and methane stability limits. At the extinction branch of the stability limits (f/jN 0.2 m/s at 5 bar), the propane-fueled catalytic reactor is substantially more robust against external heat losses. Despite the higher catalytic [22] and gas-phase reactivity of propane when compared to methane, at the blowout branch this trend is reversed. Methane has much broader limits, in terms of maximum allowable critical heat transfer coefficient h and of inlet velocity Um- This behavior of catalytic microreactors comes in stark contrast to pure gas-phase microreactors [23], whereby the stability diagrams of methane are substantially narrower than those of propane. 

Recent studies on propane-fiieled catalytic microcombustors [25] at atmospheric pressure hinted towards the importance of moleeular transport effects in reactors of sub-millimeter scales. Since in lean methane/air combustion the fuel has Lewis number Lcch4 0.97, the methane transverse transport towards the catalytic channel surface will be considerably higher than that of propane, which is highly diffusionally imbalanced with LecjHs 1-82. It can thus be expected that when the residence time in the channel has the same order of magnitude as the transverse fuel diffusion times, fuel conversion and combustion stability will be impacted primarily by the transport rather than the chemical properties of the fuel. 

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