KNOWN ISSUE: Use of particle integration time step in a user defined particle reaction (Class 3 Error)
If the particle reaction rate is user defined via a user Fortran routine and the variable 'Particle Integration Timestep' is passed as an argument to the user routine, its value will not be that being used for that step, and will often be large and negative. However, depending upon how the value is used in the user routine, this may not always be noticeable by the user.
The problem occurs because the time step is not initialized correctly before it is passed to the user routine.
No workaround is currently available.
ANSYS CFX Release 13.0. A custom executable for ANSYS CFX Release 12.1 is available on request.