Experienmental and theoretical combustion rates of graphite plates in hot parallel flows of air and O2/N2 mixtures

Authors

  • Herbert Wilhelmi,

    1. Institute of High Temperature Technology and Industrial Furnaces, Rheinisch-Westfälische-Technische Hochschule Aachen (RWTH), Germany
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  • Rudi Waibel,

    1. Institute of Thermodynamics, Dept. of Aeronautical and Aerospace Engineering, University of the Federal Armed Forces of Germany, Munich
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  • Volker Lippig,

    1. Institute of Thermodynamics, Dept. of Aeronautical and Aerospace Engineering, University of the Federal Armed Forces of Germany, Munich
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  • Dieter Straub

    1. Institute of Thermodynamics, Dept. of Aeronautical and Aerospace Engineering, University of the Federal Armed Forces of Germany, Munich
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  • Paper presented by H. Wilhelmi at the “Jahrestreffen der Verfahrens-Ingenieure”, Hannover, September 21 to 23, 1988.

Abstract

Combustion rate of flat graphite plates in oxidizing flows with velocities of up to 80 m/s and stagnation temperatures between 1600 and 2500 K were investigated. The flow field and its local properties were recorded in addition to surface temperature of the solid sample and change in surface structure. The experiments, in which the surface temperature of graphite was adjusted and carefully controlled by radiation shields, indicated that, under these experimental conditions, between 15 and 20 wt-% of carbon was lost as particles of up to 100 μm which burnt rapidly in the boundary layer region. Subtraction of erosion rates from the recorded global combustion rates yield the heterogeneous chemical reaction rates, which are approximated by an Arrhenius type function for the mathematical modelling. Since steep temperature and concentration gradients occur in the boundary layer, the local values of the multicomponent transport coefficients were evaluated in order to design a more realistic computational procedure which, in addition, also includes radiative heat transfer. Computational checks revealed that, in the past, the assumption of mean constant values for the transport coefficients has led to errors of up to 100% in the evaluation of the chemical reaction and global combustion rates.

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