Turbulent catalytically stabilized combustion of hydrogen/air mixtures in entry channel flows

The turbulent catalytically stabilized combustion (CST) of fuel-lean hydrogen/air premixtures over platinum was investigated experimentally and numerically in the entrance region of a channel. Experiments were carried out in an optically accessible catalytic channel reactor with incoming Reynolds numbers up to 3 x 10(4) and involved particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) for the characterization of the in-channel and the inlet flow, respectively, 1-D spontaneous Raman measurements of major species concentrations over the catalyst boundary layer for the assessment of turbulent scalar transport, and planar laser-induced fluorescence (LIF) of the OH radical for the determination of gaseous combustion. Additional PIV measurements were carried out with room-temperature air flows having Reynolds numbers comparable to those of the reacting cases. The numerical predictions included a Favre-averaged moment closure approach with different low-Reynolds number (LR) near-wall turbulence models. The reacting cases exhibited a strong relaminarization of the turbulent flow due to the intense heating from the catalytic surfaces and this was captured only by a recent LR heat-transfer model. Standard LR turbulence models performed excellently in the nonreacting flows but they overpredicted substantially the turbulence levels of the reacting cases. The suppression of turbulence led to a reduction of the catalytic hydrogen conversion and to a promotion of homogeneous ignition. Parametric numerical studies have delineated the regimes of flow laminarization in CST. It was shown that for a certain range of inlet Reynolds numbers, inlet turbulent kinetic energies and channel wall temperatures, a simpler laminar model was sufficient to capture key CST characteristics such as catalytic fuel conversion and onset of homogeneous ignition, while standard LR turbulence models overpredicted substantially the former and could not reproduce the latter. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.