7.3.1 General Description

The relationships for calculating char particle burning rates are presented and discussed in detail by Smith [ 324]. The particle reaction rate, ${\cal R}$ (kg/m $^2$-s), can be expressed as

where

$D_0$	=	bulk diffusion coefficient (m/s)
$C_g$	=	mean reacting gas species concentration in the bulk (kg/m $^3$)
$C_s$	=	mean reacting gas species concentration at the particle surface (kg/m $^3$)
$R_c$	=	chemical reaction rate coefficient (units vary)
$N$	=	apparent reaction order (dimensionless)

In Equation  7.3-1, the concentration at the particle surface, $C_s$, is not known, so it should be eliminated, and the expression is recast as follows:

This equation has to be solved by an iterative procedure, with the exception of the cases when $N=1$ or $N=0$. When $N=1$, Equation  7.3-2 can be written as

In the case of $N=0$, if there is a finite concentration of reactant at the particle surface, the solid depletion rate is equal to the chemical reaction rate. If there is no reactant at the surface, the solid depletion rate changes abruptly to the diffusion-controlled rate. In this case, however, ANSYS FLUENT will always use the chemical reaction rate for stability reasons.