FDS User Guide: Issue #14192. Clarify SOLID_PHASE_ONLY

Author: mcgratta

Manuals/FDS_User_Guide/FDS_User_Guide.tex

@@ -3307,8 +3307,8 @@ \subsection{Simulating the Cone Calorimeter}
 
 This section describes how to set up a simple model of the cone calorimeter or similar apparatus. This is not a full 3-D simulation of the apparatus, but rather a 1-D simulation of the solid phase degradation under an imposed external heat flux. While a full 3-D simulation of the cone heater and sample holder can be created in FDS, such a simulation would take some time to complete and would not disentangle issues with the solid phase model from uncertainties in the gas phase. It is worthwhile to perform a quick simulation like the one described here to test the solid phase model only.
 \begin{enumerate}
-\item Create a trivially small mesh, just enough to let FDS run. Since the gas phase calculation is essentially being shut off, you just need nine cells total (\ct{IJK=3,1,3}) for the pressure solver to function properly.
-\item On the \ct{MISC} line, set \ct{SOLID_PHASE_ONLY=T} to turn off all gas phase computation and speed up the simulation.  Note that convective heat transfer to/from the surface is still applied using the ambient temperature.  In this case the heat transfer coefficient is either specified or taken from the gas phase thermal conductivity divided by the wall-normal grid spacing.
+\item Create a trivially small mesh, just enough to allow FDS to run. Since the gas phase calculation is essentially being shut off, you just need nine cells total (\ct{IJK=3,1,3}) for the pressure solver to function properly.
+\item On the \ct{MISC} line, set \ct{SOLID_PHASE_ONLY=T} to turn off the gas phase fluid flow computation. This reduces the CPU time. Note that convective and radiative heat transfer to/from the surface is still applied using the ambient temperature as the assumed gas and far-field temperature. The \ct{HEAT_TRANSFER_COEFFICIENT} is either specified or computed from the gas phase thermal conductivity divided by the wall-normal grid spacing. It is be best to specify it explicitly.
 \item Create \ct{SPEC} lines to list any gas species created in the pyrolysis process. A \ct{REAC} line is not needed, as there is no gas phase combustion allowed.
 \item On the \ct{TIME} line, set \ct{WALL_INCREMENT=1} to force FDS to update the solid phase every time step (normally it does this every other time step), and set \ct{DT} to a value that is appropriate for the solid phase calculation. Since there is no gas phase calculation that will limit the time step, it is best to control this yourself.
 \item Generate \ct{MATL} lines, plus a single \ct{SURF} line, as you normally would, except add \ct{EXTERNAL_FLUX} to the \ct{SURF} line. This is simply a ``virtual'' source that heats the solid. Think of this as a perfect radiant panel or conical heating unit. You can control the \ct{EXTERNAL_FLUX} using either \ct{TAU_EF} or \ct{RAMP_EF}. This is useful if you want to ramp up the heat flux following ignition to account for the additional radiation from the flame. See Sec.~\ref{info:RAMP_Time} for more details about ramps.