Elsevier

Combustion and Flame

Volume 128, Issue 4, March 2002, Pages 340-368
Combustion and Flame

An experimental and numerical investigation of homogeneous ignition in catalytically stabilized combustion of hydrogen/air mixtures over platinum

https://doi.org/10.1016/S0010-2180(01)00363-7Get rights and content

Abstract

The gas-phase ignition of fuel-lean hydrogen/air mixtures over platinum was investigated experimentally and numerically in laminar channel-flow configurations. Experiments were performed at atmospheric pressure in an optically accessible catalytic channel combustor established by two Pt-coated parallel plates, 300 mm long (streamwise direction) and placed 7 mm apart (transverse direction). Planar laser induced fluorescence (PLIF) of the OH radical along the streamwise plane of symmetry was used to monitor the onset of homogeneous ignition, one-dimensional Raman measurements (across the 7-mm transverse direction) provided the boundary layer profiles of the major species and temperature, and thermocouples embedded beneath the catalyst yielded the surface temperature distribution. Computations were carried out using a two-dimensional elliptic fluid mechanical model that included multicomponent transport and elementary homogeneous (gas-phase) and heterogeneous (catalytic) chemical reaction schemes. Four homogeneous and three heterogeneous reaction schemes were tested in the model against measured homogeneous ignition characteristics. The differences between measured and predicted homogeneous ignition distances could be substantial (ranging from 8% to 66%, depending on the particular hetero/homogeneous schemes) and were ascribed primarily to the homogeneous reaction pathway. Sensitivity analysis indicated that the discrepancies induced by the gas-phase schemes originated either from the presence of heterogeneously-produced water due to its effectiveness as collision partner in the chain terminating reaction H + O2 + M = HO2 + M, or from an overall overprediction of the radical pool in the preignition zone. The heterogeneous schemes had significant differences in their surface coverage and radical fluxes, but these variations had practically no impact on homogeneous ignition. Sensitivity and reaction flux analyses have shown that this was attributed to the ability of all heterogeneous schemes to capture the measured mass-transport-limited fuel conversion and to the relative insensitivity of homogeneous ignition on the magnitude of the heterogeneous radical fluxes, provided that all radical adsorption reactions (OH, H, and O) were included in the heterogeneous schemes.

Section snippets

Nomenclature

    b

    Channel half-height, Fig. 1

    cp

    Specific heat at constant pressure

    Ck

    Concentration of gas-phase species

    Dkl

    Multicomponent diffusion coefficient, Eq. 7

    DkT

    Species thermal diffusion coefficient, Eq. 7

    Dkm

    Mixture-average species diffusion coefficient, Eq. 8

    Dk

    Effective species diffusion coefficient, Eq. 12

    E

    Activation energy

    h

    Total enthalpy, Eq. 4

    hko

    Chemical enthalpy of gas-phase species, Eq. 9

    Jk

    Heterogeneous molar flux of gas-phase species

    k

    Reaction rate coefficient

    Kc

    Reaction equilibrium constant

    Kg

    Total

Burner geometry and flow conditions

A catalytic burner especially suited for H2/air CST has been built, using design concepts based on our earlier, smaller-size CH4/air reactor [12]. The combustor (see Fig. 1) consisted of two horizontal Si[SiC] ceramic plates, 300 mm long (L), 110-mm wide and 10-mm thick. Four rectangular (4 × 4 × 7 mm3) ceramic spacers were positioned at the plate corners to maintain a constant plate separation of 7 mm (2b). Both Si[SiC] plates were bevelled at their edges to accommodate two quartz windows

Governing equations and boundary conditions

The governing equations for a steady, laminar reactive flow with homogeneous and heterogeneous chemical reactions are, in their elliptic 2-D Cartesian form, as follows:

Continuity equation: ∂(ρu)∂x+∂(ρv)∂y=0

Momentum equations: ∂(ρuu)∂x+∂(ρvu)∂y+∂p∂x∂x∂u∂x23·μ∂u∂x+∂v∂y∂yμ∂u∂y+∂v∂x=0∂(ρuv)∂x+∂(ρvv)∂y+∂p∂y∂xμ∂v∂x+∂u∂y∂y∂v∂y23 μ∂u∂x+∂v∂y=0

Energy equation: ∂(ρuh)∂x+∂(ρvh)∂y+∂xρ k=1KgYkhkVk,x−λ ∂T∂x+∂yρk=1KgYkhkVk,y−λ ∂T∂y=0

Gas phase species equations: ∂(ρuYk)∂x+∂(ρvYk)∂y+∂x (ρYkV

Experimental results

The measured surface temperature distributions are depicted in Fig. 4. The profiles of Fig. 4 extend up to 250 mm (the range of interest for the present computations) and have been constructed by curve-fitting through the individual thermocouple measurements; the particular type of curve-fit did not influence the predicted homogeneous ignition characteristics. The suppression of the high entry temperatures (compare with Fig. 2) was evident: all profiles peaked at distances x ≥ 100 mm. In

Conclusions

The catalytically stabilized combustion of lean H2/air mixtures was investigated numerically and experimentally at atmospheric pressure in laminar channel-flow configurations. Measured homogeneous ignition distances (xig) were compared against numerical predictions using a 2-D elliptic fluid mechanical model with multicomponent transport and elementary hetero/homogeneous reaction schemes; four different homogeneous and three heterogeneous schemes were tested. The following are the key

Acknowledgements

Support for this work was provided by the Swiss Federal Office of Energy (BFE) under contract No. 59048 and Alstom Power Technology of Switzerland.

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