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There are two methods of modeling a chemically reacting, surface thermochemistry boundary condition in Hero.  These methods utilize the same underlying theory to compute a thermochemical heat flux and a surface recession rate, but use different approaches.  This is a complex boundary condition model and care must be taken to apply it correctly.  The user specifies the enthalpy conductance on element faces and must also provide additional information to interpolate into tables containing boundary gas enthalpy data and surface thermochemistry data.

The original implemented in Hero the user applies a thermochemistry face boundary condition to define the enthalpy conductance, as well as a pressure face boundary condition to represent the static pressure at the edge of the boundary layer.   Two model properties are also required: Total Pressure and Boundary Gas Enthalpy.  The Total Pressure model property defined the total pressure as a function of time.  The Total Pressure is used with the static pressure boundary condition to interpolate into the Boundary Gas Enthalpy table to get static enthalpy, total enthalpy, Prandlt number, and Schmidt number.  The ablating material that the thermochemical (enthalpy conductance) boundary condition is applied to must have a Thermochemistry material property.  This data is typically obtained from several ACE output file and processed by FEM Builder (or possibly Chemics if you jump through a few hoops).  The static and total pressure, along with temperature and B'g, are used to interpolate to get B'c and a composite thermochemical enthalpy value.

The awkwardness of the this method is apparent when you consider that the total pressure is being used to interpolate for local conditions when computing the thermochemical heat flux and a surface recession rate for each element face.  

The new and recommended way of applying thermochemical boundary conditions is different from the original implementation because it uses static temperature and static pressure as interpolation variables -- the dependency on total pressure had been eliminated.  So, instead of adding a pressure boundary condition to define static pressure, create Thermochemistry T & P boundary condition to define the static temperature and static pressure on element faces.  Also define Total Enthalpy and Boundary Gas Enthalpy Static table model properties, instead of total pressure and boundary gas enthalpy table.  The Thermochemistry material property is created by reading surface thermochemistry output file from Chemics.  This form of the Thermochemistry material property is a function of static temperature and static pressure.

The following table summarizes these two methods.