Merge pull request #13118 from drjfloyd/master

FDS User Guide: Clarify that RADIATIVE_FRACTION is combustion and not…

Manuals/FDS_User_Guide/FDS_User_Guide.tex

@@ -5508,7 +5508,7 @@ \section{Basic Radiation Parameters: The \texorpdfstring{{\tt RADI}}{RADI} Namel
 
 {\ct RADI} is the namelist group that contains parameters related to the radiation solver. There can be only one {\ct RADI} line in the input file.
 
-An important quantity in fire science is the fraction of the fire's heat release rate released in the form of thermal radiation, commonly referred to as the {\em radiative fraction}, symbolically denoted $\chi_{\rm r}$. It is a function of the fire size, flame temperature, and the chemical composition of the fuel and combustion products. The flame temperature, as opposed to the average cell temperature, is not reliably calculated in a large scale fire simulation because the flame sheet is not well-resolved on a relatively coarse numerical grid. Thus, the source term in the radiation transport equation (RTE), because of its $T^4$ dependence, cannot be reliably calculated. As a practical alternative, the parameter {\ct RADIATIVE\_FRACTION} on the {\ct REAC} line allows you to specify explicitly the fraction of the total combustion energy that is released in the form of thermal radiation.  By default, the {\ct RADIATIVE\_FRACTION} is set to a specific value that is based on the reaction's {\ct FUEL} for an LES calculation, and it is set to zero for DNS, in which case the amount of energy radiated by the fire is predicted rather than prescribed. Table~\ref{tab:chi_r} lists the default radiative fraction for some common pure fuels. Many of these values are based on measurements performed on relatively small flames by Tewarson~\cite{SFPE:Tewarson}. These values may change with increasing fire size; thus, the measurements cited by Beyler~\cite{Beyler2:SFPE} may be more appropriate for larger fires. If in doubt, select the value that is appropriate for your fire. There is no single value of radiative fraction for a given fuel.
+An important quantity in fire science is the fraction of the fire's heat release rate released in the form of thermal radiation, commonly referred to as the {\em radiative fraction}, symbolically denoted $\chi_{\rm r}$. It is a function of the fire size, flame temperature, and the chemical composition of the fuel and combustion products. The flame temperature, as opposed to the average cell temperature, is not reliably calculated in a large scale fire simulation because the flame sheet is not well-resolved on a relatively coarse numerical grid. Thus, the source term in the radiation transport equation (RTE), because of its $T^4$ dependence, cannot be reliably calculated. As a practical alternative, the parameter {\ct RADIATIVE\_FRACTION} on the {\ct REAC} line allows you to set a lower bound on the fraction of the total energy due to combustion that is released in the form of thermal radiation, i.e. {\ct RADIATIVE\_FRACTION} only applies to gas cells with combustion and not to gas in a hot upper layer where the cell temperature can reliably be used in the RTE. By default, the {\ct RADIATIVE\_FRACTION} is set to a specific value that is based on the reaction's {\ct FUEL} for an LES calculation, and it is set to zero for DNS, in which case the amount of energy radiated by the fire is predicted rather than prescribed. Table~\ref{tab:chi_r} lists the default radiative fraction for some common pure fuels. Many of these values are based on measurements performed on relatively small flames by Tewarson~\cite{SFPE:Tewarson}. These values may change with increasing fire size; thus, the measurements cited by Beyler~\cite{Beyler2:SFPE} may be more appropriate for larger fires. If in doubt, select the value that is appropriate for your fire. There is no single value of radiative fraction for a given fuel.
 
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