Comparison of Reactive Flow Equilibrium Closure Assumptions in CTH

An equilibrium closure assumption is typically made in hydrocodes for reactive flow models between the unreacted and product equations of state. In the CTH (not an acronym) shock physics code the assumption of density and temperature equilibrium is made by default, whereas most other codes make a pressure and temperature equilibrium assumption. The main reason for this difference is the computational efficiency in making the density and temperature assumption over the pressure and temperature one. With parameters fit to data, either assumption can be used accurately predict reactive flow response using the various models. However, the model parameters fit based on one assumption cannot necessarily be used directly in the same model with a different closure assumption. A new framework is introduced to allow this assumption to be changed independently within CTH for each reactive material. Comparisons of the response and computational cost of the reactive flow models with the different equilibrium assumption will be presented for two of the most heavily used models - History Variable Reactive Burn (HVRB) and Ignition and Growth (IG). The comparison will be conducted for PBX9501 for 1D plate impact, and simple 3D problems.