Suppression of combustion instabilities of premixed hydrogen/air flames in microchannels using heterogeneous reactions

The dynamics of fuel-lean (φ = 0.5) premixed hydrogen/air atmospheric pressure flames are investigated numerically in a 1-mm-height planar channel with platinum-coated walls, as a function of the inlet velocity and the catalytic reactivity. Simulations are carried out with a fully elliptic 2D transient code that includes detailed heterogeneous (catalytic) and homogeneous (gas-phase) chemical reaction schemes. The channel wall temperature is prescribed and the inlet properties are uniform, while the catalytic reactivity is controlled by varying the parameter As, which is the ratio of the catalytically-active area to the geometrical channel surface area. It is shown that the rich flame dynamics of the non-catalytic case (As = 0), which include oscillating and asymmetric flame modes, can be suppressed by appropriate selection of the catalytic reactivity. The oscillating flames disappear at As = 6 × 10−3 and the asymmetric ones at As = 1.15 × 10−2, while for higher values of As only stationary and symmetric V-shaped flames are obtained. The suppression of the flame dynamics can be attributed to the theoretically-predicted diminishing sensitivity of the homogeneous ignition distance to small perturbations of the gaseous reactivity with increasing catalytic reactivity. The results indicate that a feasible way to eliminate undesirable unsteady combustion modes in practical microcombustors is to apply a pre-determined catalyst loading on the channel walls.