Why is the temperature for my combustion simulation too low, ie: lower than the inlet gas temperature? This happens in CFX 10.0 and CFX 11.0, but does not happen when I run CFX 5.7.1!
The reason for the observed temperature undershoots in CFX 10.0 and 11.0 is in the way the shared high resolution beta factor for energy and mass fractions is calculated:
1) The "Freestream Damping Option" for the High Resolution advection scheme was changed from 1st order (5.7.1) to 2nd order (10.0 and 11.0). To see the difference, look at the "Energy and Mass Fractions.Beta" variable in Post
2) By default energy is excluded from the calculation of the common Beta for energy and mass fractions.
Because of 2) the HR Beta for energy is determined from the component mass fractions alone.
Because of 1) in 10.0/11.0 the advection scheme will be second order in regions of constant composition (in 5.7.1 it used to be blended to 1st order). However, if temperature gradients are present in such a region, then the second order scheme may cause oscillations/over-/under-shoots as observed in the current case. In 5.7.1, such regions were blended to 1st order, which did avoid the oscillations, but at the cost of reduced accuracy in regions where we could afford the higher order.
With the original testing on the combined energy and mass fractions beta, including energy in the calculation caused robustness/convergence issues for some of the test cases. Therefore it was decided to disable it by default.
Some improvements are expected due to other numerics changes. Therefore, this setting will be reconsidered for release 12.0
There are two ways to solve the issue (each sufficient on its own):
a) Include energy in mcf beta calculation (try this first). Set the following expert parameter in order to include energy in the Beta calculation:
energy in mcf beta = T
b) A different workaround (less recommended) is to revert the freestream blending to 1st order. This can be achieved by adding the following CCL parameter:
Freestream Damping Option = First Order