Combustion stability and hetero-/homogeneous chemistry interactions for fuel-lean hydrogen/air mixtures in palladium-coated microchannels

Abstract

Heterogeneous catalytic chemistry was considered in combination with homogeneous gas-phase chemistry and detailed transport to resolve overall combustion conditions in a palladium-coated microchannel for a fuel-lean mixture of hydrogen and air with an equivalence ratio of 0.4. Numerical simulations were run on a two-dimensional model of the channel having a height of 1 mm and a length of 10 mm. the channel walls were modeled with a thickness of 0.1mm, and conduction through the solid wall was analyzed at two different solid thermal conductivities of $k_s$ = 1 and 16 W/mK. Radiation among various surfaces was accounted for, while the outer (upper) horizontal wall of the channel was modeled as a convective boundary with a heat loss coefficient $h$. Pressures of 1 and 5 bar were considered. Inlet velocity $U_{in}$, along with $h$, were altered separately to generate stability maps based on the critical heat loss coefficient $h_{cr}$ ($h_{cr}$ denotes the greatest value of the heat loss coefficient along the upper surface before extinction is realized). Stability envelopes were determined to be universally broadened at lower pressure and thermal conductivity, and such stability envelopes were facilitated solely by catalytic surface reactions. For low values of $h$, away from critical points, catalytic surface interactions led to superadiabatic wall temperatures, while homogeneous combustion suppressed such interactions. Additionally, critical heat transfer coefficients were one to three magnitudes lower for palladium than for a similar platinum coated microchannel with hydrogen fuel.