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Numerical Simulation of Hydrogen–Air Boundary Layer Flows Augmented by Catalytic Surface Reactions

[+] Author and Article Information
M. M. M. Abou-Ellail

Department of Mechanical Engineering,  Cairo University, Cairo, Egyptabouellail@hotmail.com

T. W. Tong

 The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kongpptong@polyu.edu.hk

Y. Li

School of Engineering and Applied Science,  The George Washington University, Washington, DC 20052

J. Heat Transfer 133(11), 114501 (Aug 31, 2011) (12 pages) doi:10.1115/1.4004335 History: Received May 07, 2009; Revised May 03, 2011; Published August 31, 2011; Online August 31, 2011

Catalytic combustion of hydrogen–air boundary layers involves the adsorption of hydrogen and oxygen into a platinum coated 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 and catalytic reactors 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 nitrogen oxides. The present paper is concerned with the numerical computation of heat transfer and chemical reactions in hydrogen–air mixture boundary layers that flow over platinum coated hot plates and inside rectangular channels. Chemical reactions are included in the gas-phase as well as on the solid platinum surface. In the gas-phase, eight species are involved in 26 elementary reactions. On the platinum hot surface, additional surface species are included that are involved in 16 additional surface chemical reactions. The platinum surface temperature distribution is prespecified, while the properties of the reacting flow are computed. The flow configurations investigated in the present paper are those of a flat plate boundary layer and a rectangular channel reacting flow. Finite-volume equations are obtained by formal integration over control volumes surrounding each grid node. Hybrid differencing is used to ensure that the finite-difference coefficients are always positive or equal to zero to reflect the real 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. A nonuniform computational grid is used, concentrating most of the nodes in the boundary sub-layer adjoining the catalytic surface. For the flat plate boundary layer flow, the computed OH concentration is compared with experimental and numerical data of similar geometry. The obtained agreement is fairly good, with differences observed for the location of the peak value of OH. Surface temperature of 1170 K caused fast reactions on the catalytic surface in a very small part at the leading edge of the catalytic flat plate. The flat plate computational results for heat and mass transfer and chemical surface reactions at the gas-surface interface are correlated by nondimensional relations. The channel flow computational results are also compared with recent detailed experimental data for similar geometry. In this case, the catalytic surface temperature profile along the x-axis was measured accurately and is used in the present work as the boundary condition for the gas-phase energy equation. The present numerical results for the gas temperature, water vapor mole fraction, and hydrogen mole fraction are compared with the corresponding experimental data. In general, the agreement is very good especially in the first 105 mm. However, some differences are observed in the vicinity of the exit section of the rectangular channel.

Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Layouts of the two flow configurations. (a) Boundary layer flow over a Platinum coated hot flat plate. (b) Rectangular channel hydrogen–air flow with catalytic surface reactions.

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Figure 2

Sherwood number of H2 O versus Reynolds number

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Figure 3

Surface coverage of surface species versus local Rex

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Figure 4

Surface production rate (SPR) of H2 O versus local Reynolds number

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Figure 5

Comparison of present OH concentration with experimental data of Cattolica and Schefer [(1),2]

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Figure 6

Transverse profiles of H2 mole fractions for different axial locations

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Figure 7

Thermal boundary layer thickness versus Reynolds number

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Figure 8

Nusselt number versus local Reynolds number

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Figure 9

Production rates of H2 O, O2 , and H2 versus longitudinal dimensionless distance

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Figure 10

Variations of the product of the concentrations of H2 and O2 , at catalytic surface, along longitudinal distance

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Figure 11

Surface reaction rate versus [H2 ][O2 ], for surface temperatures of 1170 K and 1070 K

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Figure 12

Catalytic surface temperature profile for the channel flow

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Figure 13

Velocity and temperature transverse profiles at streamwise distances of 25 mm, 85 mm, 105 mm, 165 mm, 235 mm, and 265 mm. The dotted line is velocity; the solid line is temperature; the square symbol is temperature experimental data; the horizontal axis is temperature and velocity; the temperature is in Kelvin; the streamwise velocity is in cm s−1 .

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Figure 14

Mole fractions of H2 and H2 O at streamwise distances of 25 mm, 85 mm, 105 mm, 165 mm, 235 mm, and 265 mm. For clarity, the H2 mole fraction has been expanded by a factor of 2. The dotted line is H2 O; the solid line is H2 ; the square symbol is H2 experimental data; the diamond symbol is H2 O experimental data. The horizontal axis is mole fractions.

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Figure 15

Surface coverage of the surface species along for the channel flow

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