Merge pull request #13909 from mcgratta/master

FDS User Guide: Issue #13516. Clarify LAYER_DIVIDE

Author: mcgratta

Manuals/FDS_User_Guide/FDS_User_Guide.tex

@@ -2426,9 +2426,7 @@ \subsection{Walls with Different Materials Front and Back}
       MATL_ID(1:3,1)     = 'STEEL','INSULATION','PLASTIC'
       THICKNESS(1:3)     = 0.0063,0.036,0.002 /
 \end{lstlisting}
-If, in addition, the insulation material and plastic are combustible, and their burning properties are specified on the appropriate \ct{MATL} lines, then indicating which side of the column would generate the fuel vapor is needed. In this case, the steel is impermeable. Adding the the parameter \ct{LAYER_DIVIDE=2.0} to the \ct{SURF} line labeled \ct{'COLUMN EXTERIOR'} would result in fuel vapors formed by the heating of the two first layers (\ct{'PLASTIC'} and \ct{'INSULATION'}) being driven out of that surface. Similarly, for the \ct{SURF} line labeled \ct{'COLUMN INTERIOR'}, specifying \ct{LAYER_DIVIDE=0.0} would indicate that no fuel vapors are to driven into the interior of the column. In fact, values from 0.0 to 1.0 would work equally because the material \ct{'STEEL'} would not generate any fuel vapors.
-
-By default, \ct{LAYER_DIVIDE} is 0.5 times the number of layers for surfaces with \ct{EXPOSED} backing, and equal to the number of layers for other surfaces.
+If, in addition, the insulation material and plastic are combustible, and their burning properties are specified on the appropriate \ct{MATL} lines, then indicating which side of the column would generate the fuel vapor is needed. In this case, the steel is impermeable. Adding the the parameter \ct{LAYER_DIVIDE=2.0} to the \ct{SURF} line labeled \ct{'COLUMN EXTERIOR'} would result in fuel vapors formed by the heating of the two first layers (\ct{'PLASTIC'} and \ct{'INSULATION'}) being driven out of that surface. Similarly, for the \ct{SURF} line labeled \ct{'COLUMN INTERIOR'}, specifying \ct{LAYER_DIVIDE=0.0} would indicate that no fuel vapors are to driven into the interior of the column. In fact, values from 0.0 to 1.0 would work equally because the material \ct{'STEEL'} would not generate any fuel vapors. By default, \ct{LAYER_DIVIDE} is 0.5 times the number of layers for surfaces with \ct{EXPOSED} backing, and equal to the number of layers for other surfaces. See Section~\ref{info:LAYER_DIVIDE} for more information.
 
 \subsection{Specified Internal Heat Source}
 
@@ -2871,6 +2869,17 @@ \subsection{Reaction Mechanism}
 \end{lstlisting}
 Note that the indices associated with the parameters are not needed {\em in this case}, but they are shown to emphasize that, in general, there can be multiple reactions with corresponding kinetic parameters and products.
 
+\subsection{Solid Phase Gas Transport}\
+\label{info:LAYER_DIVIDE}
+
+The solid phase conduction/reaction algorithm does not have an explicit transport mechanism for pyrolyzed gases. Rather, the pyrolyzates are assumed to appear instantaneously at the surface. Which surface is controlled by the \ct{SURF} line parameter \ct{LAYER_DIVIDE}, a real number that specifies the number of layers whose gaseous pyrolyzates are to be applied to the surface with the given \ct{SURF} label. For example, if \ct{LAYER_DIVIDE=1.5}, the gases generated by the first layer and half of the second layer shall be applied at the given surface. This same partitioning holds whether or not the solid is shrinking or swelling. Be careful to ensure that the specified values of \ct{LAYER_DIVIDE} are consistent for the \ct{SURF} lines that control opposing surfaces. If two different \ct{SURF} lines govern the front and back of a solid obstruction, the specified values of \ct{LAYER_DIVIDE} should sum to the total number of layers. Otherwise, the mass of evolved gases may not be correct.
+
+If \ct{LAYER_DIVIDE} is not set, it is assumed that for a solid with \ct{BACKING='EXPOSED'}, the gases generated within a depth of half the total thickness are to be applied at the front surface and the other half at the back. If the \ct{BACKING} is not \ct{'EXPOSED'}, all of the gases will be applied at the front surface.  
+
+Suppose, for example, that the solid is composed of a layer of insulation on top of a layer of plastic on top of a layer of steel. The steel is impermeable. By setting \ct{LAYER_DIVIDE=2.0} to the \ct{SURF} line whose first layer is the insulation directs the vapors generated by the insulation and plastic to be driven out of the exterior surface of the insulation. Similarly, for the \ct{SURF} line that is applied to the steel, specifying \ct{LAYER_DIVIDE=0.0} would indicate that no fuel vapors are to escape the steel surface. Note that in this instance, the sum of the values of \ct{LAYER_DIVIDE} is not equal to the number of layers, but this is not a problem because the layer of steel does not generate any gases.
+
+
+
 \subsection{Reaction Rates}
 \label{sec:reaction_rates}
 
@@ -5512,19 +5521,19 @@ \subsection{Chemical Time Integration}
 \end{lstlisting}
 Internally, CVODE assigns default values to several key parameters, as listed below:
 \begin{itemize}
-\setlength{\itemsep}{-4pt}   
-\item \ct{FINITE_RATE_MIN_TEMP}=300~$^\circ$C 
-\item \ct{ODE_MIN_ATOL}=1E-10 
-\item \ct{ODE_REL_ERROR}=1E-6 
-\item \ct{ZZ_MIN_GLOBAL}=1E-10 
-\item \ct{EQUIV_RATIO_CHECK}=F 
-\item \ct{MIN_EQUIV_RATIO}=0.0 
-\item \ct{MAX_EQUIV_RATIO}=20.0 
-\item \ct{DO_CHEM_LOAD_BALANCE}=T 
+\setlength{\itemsep}{-4pt}
+\item \ct{FINITE_RATE_MIN_TEMP}=300~$^\circ$C
+\item \ct{ODE_MIN_ATOL}=1E-10
+\item \ct{ODE_REL_ERROR}=1E-6
+\item \ct{ZZ_MIN_GLOBAL}=1E-10
+\item \ct{EQUIV_RATIO_CHECK}=F
+\item \ct{MIN_EQUIV_RATIO}=0.0
+\item \ct{MAX_EQUIV_RATIO}=20.0
+\item \ct{DO_CHEM_LOAD_BALANCE}=T
 \end{itemize}
-The parameter \ct{FINITE_RATE_MIN_TEMP} defines the minimum temperature (in ~$^\circ$C) above which chemistry calculation will be performed. CVODE allows specification of relative and absolute tolerances at the species-level using the \ct{ODE_REL_ERROR} and \ct{ODE_ABS_ERROR} parameters in the \ct{SPEC} input line. These tolerances can also be set globally in the \ct{COMB} input line, with species-level settings taking precedence. Currently, CVODE does not allow relative tolerance at the species level. The minimum concentration of a species is determined as the product of \ct{ODE_REL_ERROR} and \ct{ZZ_MIN_GLOBAL}. Concentrations below this threshold are treated as zero. 
+The parameter \ct{FINITE_RATE_MIN_TEMP} defines the minimum temperature (in ~$^\circ$C) above which chemistry calculation will be performed. CVODE allows specification of relative and absolute tolerances at the species-level using the \ct{ODE_REL_ERROR} and \ct{ODE_ABS_ERROR} parameters in the \ct{SPEC} input line. These tolerances can also be set globally in the \ct{COMB} input line, with species-level settings taking precedence. Currently, CVODE does not allow relative tolerance at the species level. The minimum concentration of a species is determined as the product of \ct{ODE_REL_ERROR} and \ct{ZZ_MIN_GLOBAL}. Concentrations below this threshold are treated as zero.
 
-Additional optional parameters include \ct{EQUIV_RATIO_CHECK}, \ct{MIN_EQUIV_RATIO}, and \ct{MAX_EQUIV_RATIO}. When \ct{EQUIV_RATIO_CHECK} is enabled (set to true), the chemistry calculation is performed only for those cells for which equivalence ratio is within the specified \ct{MIN_EQUIV_RATIO} and \ct{MAX_EQUIV_RATIO} limits, reducing computational time. Enabling \ct{DO_CHEM_LOAD_BALANCE} significantly accelerates chemistry calculations by distributing the computational load evenly across all MPI processes. 
+Additional optional parameters include \ct{EQUIV_RATIO_CHECK}, \ct{MIN_EQUIV_RATIO}, and \ct{MAX_EQUIV_RATIO}. When \ct{EQUIV_RATIO_CHECK} is enabled (set to true), the chemistry calculation is performed only for those cells for which equivalence ratio is within the specified \ct{MIN_EQUIV_RATIO} and \ct{MAX_EQUIV_RATIO} limits, reducing computational time. Enabling \ct{DO_CHEM_LOAD_BALANCE} significantly accelerates chemistry calculations by distributing the computational load evenly across all MPI processes.
 User can modify the default values of any or all of these parameters as needed using the following line in the FDS input file:
 \begin{lstlisting}
 &COMB
@@ -13388,7 +13397,7 @@ \section{\texorpdfstring{{\tt SURF}}{SURF} (Surface Properties)}
 \ct{INIT_PER_AREA}                                     & Real            & Section~\ref{info:trees}                  & m$^{-2}$            &                         \\ \hline
 \ct{INNER_RADIUS}                                       & Real            & Section~\ref{info:PART_GEOMETRY}          & m                   &                         \\ \hline
 \ct{INTERNAL_HEAT_SOURCE}                              & Real Array      & Section~\ref{info:INTERNAL_HEAT_SOURCE}   & kW/m$^3$            &                         \\ \hline
-\ct{LAYER_DIVIDE}                                       & Real            & Section~\ref{info:EXPOSED}                &                     & \ct{N_LAYERS}/2       \\ \hline
+\ct{LAYER_DIVIDE}                                       & Real            & Section~\ref{info:LAYER_DIVIDE}                &                     & \ct{N_LAYERS}/2       \\ \hline
 \ct{LEAK_PATH}                                          & Int.~Pair       & Section~\ref{info:Leaks}                  &                     &                         \\ \hline
 \ct{LEAK_PATH_ID}                                      & Character~Pair  & Section~\ref{info:Leaks}                  &                     &                         \\ \hline
 \ct{LENGTH}                                              & Real            & Section~\ref{info:PART_GEOMETRY}          & m                   &                         \\ \hline