Density

[SaharRasoulpour]

I have a question regarding density in the reaction rate calculation. In Equation 7.39, RHO_S_ALPHA is defined as the bulk density. However, in the source code for the reaction rate, this density is represented as RHO_S. In wall.f90, RHO_S is multiplied by a density adjustment factor to obtain the adjusted density. This suggests that RHO_S in the source code corresponds to the true density. My question is, when defining the density in the MATL line, does that density represent the true density or the bulk density for each component?

[mcgratta]

The DENSITY on the MATL line is the true density; that is, the density of the pure material.

[mcgratta]

I is the index of the interior node. RHO_S is the density of the composite at inner cell I. RHO(I) is the bulk density of material N at inner cell I. In this loop, the densities of the material components are being summed to give the actual density at that inner point.

[SaharRasoulpour]

Is the density in the reaction rate equation (Equation 7.39 of the FDS Technical Reference) defined as bulk density(𝜌𝑆,𝛼)? When looking at the source code in wall.f90, specifically at line 3360:
[REACTION_RATE = ML%A(J)*(RHO_S(N))**ML%N_S(J)EXP(-ML%E(J)/(R0TMP_S))]
the density is written as RHO_S(N). Is this the same as the bulk density?
My main question is: How does FDS use the density defined in the MATL line (which represents the true density of each component) in the reaction rate calculation?

[mcgratta]

If the N refers to a particular material component, then this would not be the total density. This is a subroutine. Look at what variable is being passed into the subroutine.

[SaharRasoulpour]

Can you specifically clarify it? I am trying to understand how FDS uses the input true density for each component of the material and how it relates to the density in the pyrolysis reaction rate equation.

[mcgratta]

RHO_S(N) can be thought of as $\rho_{\rm s,\alpha}$. The subscript $\alpha$ corresponds to the argument N.