Merge pull request #14156 from mcgratta/master

FDS User Guide: Clarification

Author: mcgratta

Manuals/FDS_User_Guide/FDS_User_Guide.tex

@@ -2381,7 +2381,7 @@ \subsection{Thermal Properties}
 \subsection{Back Side Boundary Conditions}
 \label{info:BACKING}
 
-There are several options for defining the back surface boundary condition of a thermally-thick obstruction. The default, \ct{BACKING='EXPOSED'} on the \ct{SURF} line, assumes that the back side is exposed to the thermal environment behind the solid\footnote{Prior to FDS, the boundary condition \ct{BACKING='EXPOSED'} was limited to zero or one cell thick solids. This restriction no longer applies.}. FDS calculates the heat transfer through the solid into the space behind the wall and vis versa. This heat conduction calculation in the solid is based on the \ct{THICKNESS} of the material layers specified on the \ct{SURF} line, not the dimension of the \ct{OBST} or other solid that the \ct{SURF} is applied to. For example, when modeling a steel plate that is 5~mm thick, if the \ct{OBST} is approximated as a zero-cell thick sheet because 5~mm is less than half the grid dimension of 5~cm, then FDS will still compute the heat transfer through a 5~mm thick plate of steel. If a \ct{SURF} with \ct{BACKING='EXPOSED'} is applied to a solid with a dimension that is much greater than the specified \ct{THICKNESS}, FDS will then ignore \ct{BACKING='EXPOSED'} and apply \ct{BACKING='VOID'} instead. This latter boundary condition is described below.
+There are several options for defining the back surface boundary condition of a thermally-thick obstruction. The default, \ct{BACKING='EXPOSED'} on the \ct{SURF} line, assumes that the back side is exposed to the thermal environment behind the solid\footnote{Prior to FDS, the boundary condition \ct{BACKING='EXPOSED'} was limited to zero or one cell thick solids. This restriction no longer applies.}. FDS calculates the heat transfer through the solid into the space behind the wall and vis versa. This heat conduction calculation in the solid is based on the \ct{THICKNESS} of the material layers specified on the \ct{SURF} line, not the dimension of the \ct{OBST} or other solid that the \ct{SURF} is applied to. For example, when modeling a steel plate that is 5~mm thick, if the \ct{OBST} is approximated as a zero-cell thick sheet because 5~mm is less than half the grid dimension of 5~cm, then FDS will still compute the heat transfer through a 5~mm thick plate of steel. If a \ct{SURF} with \ct{BACKING='EXPOSED'} is applied to the surface of a solid whose dimension in the normal direction is at least one gas phase grid cell length greater than \ct{THICKNESS}, FDS will then ignore \ct{BACKING='EXPOSED'} and apply \ct{BACKING='VOID'} instead. This latter boundary condition is described below.
 
 An alternative back side boundary condition, \ct{BACKING='INSULATED'}, assumes that the solid backs up to a perfectly insulated material, in which case no heat is lost to the back side. Use of this condition means specifying properties of the inner insulating material is not necessary because it is assumed to be perfectly insulated.