FDS User Guide: edit text on high MW species to reflect change to EXTINCTION 1.

Author: drjfloyd

Manuals/FDS_User_Guide/FDS_User_Guide.tex

@@ -4621,7 +4621,7 @@ \subsubsection{Enthalpy}
 \be
    c_{p,\alpha} = \frac{\gamma}{\gamma-1} \frac{\R}{W_\alpha}
 \ee
-The ratio of specific heats, \ct{GAMMA}, is 1.4 by default and can be changed on the \ct{MISC} line. If you want all the gas specific heats to follow this relation, set \ct{CONSTANT_SPECIFIC_HEAT_RATIO=T} on the \ct{MISC} line. For high molecular weight species, use of the default gamma will result in very low values of the specific heat which can cause issues with the default extinction model. In this case it is recommended that the specific heat be defined. If the FDS is determining the specific heat using this relation, then it is recommended that you check the \ct{CHID.out} and verify that reasonable specific heat values have been created.
+The ratio of specific heats, \ct{GAMMA}, is 1.4 by default and can be changed on the \ct{MISC} line. If you want all the gas specific heats to follow this relation, set \ct{CONSTANT_SPECIFIC_HEAT_RATIO=T} on the \ct{MISC} line. For high molecular weight species, use of the default gamma will result in very low values of the specific heat. This can cause issues with temperature in regions with high fuel mass fractions and can cause issues with the \ct{EXTINCTION 2} extinction model. For high molecular weight species it is recommended that the specific heat be defined. If the FDS is determining the specific heat using \ct{GAMMA}, then it is recommended that you check the \ct{CHID.out} file and verify that reasonable specific heat values have been created.
 
 Note that species used in chemical reactions must either be predefined or have an explicitly defined $h(T_{\rm ref})$. The exception is the species defined as \ct{FUEL} on the \ct{REAC} input as long as a \ct{HEAT_OF_COMBUSTION} is defined or the reaction is a simple chemistry reaction where \ct{EPUMO2} applies. Specifying either \ct{SPECIFIC_HEAT} or \ct{RAMP_CP} is considered to have explicitly defined $h(T_{\rm ref})$. If you still wish to use the default \ct{EPUMO2} or 13,100~kJ/kg, then you will need to explicitly define it on the \ct{REAC} input.