Rob
Ansys Employee

As I can't see if the Mixture model is transient it's hard to judge. 

For the transient solver there is a spike in residual at the end of the time step as time is updated and the solver begins to converge the new step. However, if you're seeing the spike at the same rate as the max iterations you need to check each time step is converged. If it's not the results will need to be treated with care as they may not be correct.