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TECHNICAL PAPERS: Combustion and Reactive Flows

A Mathematical Model for Heat and Mass Transfer in Methane-Air Boundary Layers With Catalytic Surface Reactions

[+] Author and Article Information
T. W. Tong

School of Engineering and Applied Science, The George Washington University, Washington, D.C. 20052tong@gwu.edu

M. M. M. Abou-Ellail

School of Engineering and Applied Science, The George Washington University, Washington, D.C. 20052abouellail@hotmail.com

Y. Li

School of Engineering and Applied Science, The George Washington University, Washington, D.C. 20052

J. Heat Transfer 129(8), 939-950 (Jan 09, 2007) (12 pages) doi:10.1115/1.2737479 History: Received September 28, 2006; Revised January 09, 2007

Catalytic combustion of hydrocarbon mixtures involves the adsorption of the fuel and oxidant into a platinum surface, chemical reactions of the adsorbed species, and the desorption of the resulting products. Re-adsorption of some produced gases is also possible. The catalytic reactions can be beneficial in porous burners that use low equivalence ratios. In this case, the porous burner flame can be stabilized at low temperatures to prevent any substantial gas emissions, such as nitric oxide. The present paper is concerned with the numerical computation of heat transfer and chemical reactions in flowing methane-air mixtures over a platinum coated hot plate. Chemical reactions are included in the gas phase and in the solid platinum surface. In the gas phase, 16 species are involved in 49 elementary reactions. On the platinum hot surface, additional surface species are included that are involved in 24 additional surface chemical reactions. The platinum surface temperature is fixed, while the properties of the reacting flow are computed. The flow configuration investigated here is the parallel boundary layer reacting flow. Finite-volume equations are obtained by formal integration over control volumes surrounding each grid node. Up-wind differencing is used to ensure that the influence coefficients are always positive to reflect the physical effect of neighboring nodes on a typical central node. The finite-volume equations are solved iteratively for the reacting gas flow properties. On the platinum surface, surface species balance equations, under steady-state conditions, are solved numerically by an under-relaxed linear algorithm. A non-uniform computational grid is used, concentrating most of the nodes near the catalytic surface. Surface temperatures, 1150 K and 1300 K, caused fast reactions on the catalytic surface, with very slow chemical reactions in the flowing gas. These slow reactions produce mainly intermediate hydrocarbons and unstable species. The computational results for the chemical reaction boundary layer thickness and mass transfer at the gas-surface interface are correlated by non-dimensional relations, taking the Reynolds number as the independent variable. Chemical kinetic relations for the reaction rate are obtained which are dependent on reactants’ concentrations and surface temperature.

Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Layout of the parallel boundary layer flow with surface reactions

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Figure 2

Sherwood number versus Reynolds number for CH4, O2, CO2, and H2O at Ts=1150K

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Figure 3

Surface coverage of surface species versus Reynolds number at Ts=1150K

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Figure 4

Production rate versus Rex for CH4, O2, CO2, and H2O at Ts=1150K

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Figure 5

Combustion boundary layer thickness versus Rex at Ts=1150K

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Figure 6

Transverse profiles of dimensionless axial velocity (u∕U∞) for different axial distances at Ts=1150K, U∞=6cm∕s

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Figure 7

Transverse profiles of reactants mole fraction for different axial locations at Ts=1150K

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Figure 8

Transverse profiles of CO2 mole fraction for different axial locations at Ts=1150K

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Figure 9

Transverse gas temperature profiles for different axial locations at Ts=1150K

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Figure 10

Transverse profiles of selected rare species for different axial locations at Ts=1150K

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Figure 11

Sherwood number versus Reynolds number for CH4, O2, CO2, and H2O at Ts=1300K

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Figure 12

Surface coverage of surface species versus Reynolds number at Ts=1300K

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Figure 13

Production rate versus Rex for CH4, O2, CO2, and H2O at Ts=1300K

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Figure 14

Combustion boundary layer thickness versus Rex at Ts=1300K

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Figure 15

Transverse profiles of dimensionless axial velocity (u∕U∞) for different axial distances at Ts=1300K, U∞=6cm∕s

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Figure 16

Transverse profiles of reactants mole fraction for different axial locations at Ts=1300K

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Figure 17

Transverse profiles of CO2 mole fraction for different axial locations at Ts=1300K

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Figure 18

Transverse gas temperature profiles for different axial locations at Ts=1300K

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Figure 19

Transverse profiles of selected rare species for different axial locations at Ts=1300K

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Figure 20

Surface reaction rate at surface temperature of 1150K

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Figure 21

Surface reaction rate at surface temperature of 1300K

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Figure 22

Heat transfer coefficient versus Reynolds number without chemical reactions

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Figure 23

Nusselt number versus Reynolds number without chemical reactions

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Figure 24

Comparison of heat transfer coefficient

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Figure 25

Nusselt number versus Reynolds number with chemical reactions

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Figure 26

Comparison of heat transfer correlations

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