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FDS Source: Issue #14427. Loosen rules for backside surface properties #14429

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mcgratta merged 1 commit into from
Mar 26, 2025
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FDS Source: Issue #14427. Loosen rules for backside surface properties #14429

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6 changes: 3 additions & 3 deletions Manuals/FDS_User_Guide/FDS_User_Guide.tex
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Expand Up @@ -2428,9 +2428,9 @@ \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. 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.
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. 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 to which the \ct{SURF} is applied. 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.

If a \ct{VENT} is applied to a thermally-thick obstruction, the \ct{SURF_ID} associated with this \ct{VENT} must describe a thermally-thick solid as well; that is, the \ct{SURF} line must have a \ct{MATL_ID}.
If a \ct{VENT} is applied to a thermally-thick obstruction with 3-D heat conduction, the \ct{SURF_ID} associated with this \ct{VENT} must describe a thermally-thick, 3-D solid as well; that is, the \ct{SURF} line must have a \ct{MATL_ID} and \ct{HT3D=T}.

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.

Expand Down Expand Up @@ -14090,7 +14090,7 @@ \chapter{Error Codes}
375 \> \ct{OBST ... is VARIABLE_THICKNESS or HT3D and needs a MATL_ID.} \> Section~\ref{info:HT3D_Limitations} \\
376 \> \ct{Meshes must have the same y/z bounds for TUNNEL_PRECONDITIONER.} \> Section~\ref{tunnel_solver} \\
377 \> \ct{SURF ... IMPINGING JET model requires ...} \> Section~\ref{info:impinging_jet} \\
378 \> \ct{SURF ... cannot be applied to a thermally-thick solid.} \> Section~\ref{info:BACKING} \\
378 \> \ct{SURF ... cannot be applied to a 3-D conducting solid.} \> Section~\ref{info:BACKING} \\
\> \> \\
381 \> \ct{Need more spectral band limits.} \> Section~\ref{info:RADI_Wide_Band} \\
382 \> \ct{Spectral band limits should be given in ascending order.} \> Section~\ref{info:RADI_Wide_Band} \\
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6 changes: 3 additions & 3 deletions Source/init.f90
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Expand Up @@ -4048,10 +4048,10 @@ SUBROUTINE FIND_WALL_BACK_INDEX(NM,IW)
ONE_D%BACK_INDEX = OM%CELL(IC)%WALL_INDEX(IOR)
ONE_D%BACK_MESH = NOM
ONE_D%BACK_SURF = OM%CELL(IC)%SURF_INDEX(IOR)
SF_BACK => SURFACE(ONE_D%BACK_SURF)
IF (NOM>0) THEN
IF (SURFACE(ONE_D%BACK_SURF)%THERMAL_BC_INDEX/=THERMALLY_THICK) THEN
WRITE(MESSAGE,'(3A)') 'ERROR(378): SURF ',TRIM(SURFACE(ONE_D%BACK_SURF)%ID),&
' cannot be applied to a thermally-thick solid.'
IF (SF%HT_DIM>1 .AND. SF_BACK%HT_DIM==1) THEN
WRITE(MESSAGE,'(3A)') 'ERROR(378): SURF ',TRIM(SF_BACK%ID),' cannot be applied to 3-D conducting solid.'
CALL SHUTDOWN(MESSAGE,PROCESS_0_ONLY=.FALSE.) ; RETURN
ENDIF
OS => M%OMESH(NOM)%WALL_RECV_BUFFER
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