In some chemistry calculations, it is possible to use the 'Equilibrium' reaction option despite it being physically invalid. The CFX-Solver treatment for this changed in CFX-11.0, viz.:
1) In CFX-10.0 and earlier versions of CFX-5, and also in CFX-4, the recipe for 'Equilibrium' is applied regardless of it being physical invalid. However, this has significant drawbacks:
- The backward rate has wrong physical dimensions! The dimension are wrong by factor [m^3n mol^-n] whenre 'n' is a number of order O(1).
- Forward and backward rates to not balance in chemical equilibrium, i.e. the mixture composition does not converge towards the chemical/thermodynamic equilibrium, but will approach some other composition.
2) Since CFX-11.0 a correction to the 'Equilibrium' option is applied such that the backward rate has the correct dimensions and that the forward/backward rates balance when in equilibrium. The drawbacks in this approach are:
- The stiff chemistry solver may not converge in many cases, and there is no obvious way to fix this.
- The reaction mechanisms may have been developed assuming approach 1), because this is what everybody else is doing.
Unfortunately, users may expect treatment 1) to operate, despite its drawbacks.
This is due to an improvement to the software conflicting with user expectation.
No workaround is currently available.
ANSYS CFX Release 12.0. In CFX-12.0, users will be able to use an expert parameter to choose from the above options, with the default being that the code will stop if the equilibrium reaction is non-physical.