Drag Coefficient of a Slowly Moving Carbon Particle Undergoing Combustion

Abstract

The drag coeflcient of a burning carbon particle in compressible low Reynolds number flow is studied with the matched asymptotic (inner-and-outer) expansion method, related to the pulverized coal combustion. Chemical reactions considered comprise the surface C-0₂and C-C0₂reactions. Frozen mode with no gas-phase reaction is considered because ignition of CO-flame over the burning carbon does not occur for the present situation. It is shown that the friction drag coefficient is reduced by the mass transfer from the carbon surface, that the pressure drag coefficient is enhanced, that the thrust drag coefficient is two order of magnitude smaller than the others. It has turned out that the total drag coefficient is nearly the same as that for nonreactive particle. Results imply that not the mass transfer but the heat transfer mainly governs the total drag coefficient. An approximate expression of the total drag coefficient is obtained, which is expected to be useful for practical applications. Temporal variation of the drag coefficients is also studied, related to the pulverized coal combution in a combustor. It is shown that the calculated drag coefficient considerably deviates from the Stokes drag coefficient and that at high temperatures both the radiative heat loss and the endothermic surface C-C0₂reaction exert great influence to reduce the drag coefficient due to a decrease in the particle temperature.

Key words:

• Drag coefficient
• carbon combustion
• surface reaction.