This setting controls the accuracy and time step of the particle tracking integration. A time step is chosen locally as the element length scale divided by the particle speed divided by the number of integration steps per element. The integration accuracy will be increased by using a higher number of integration steps at the expense of computational time. The default value of 10 is usually suitable.

This parameter can override the automatic time step obtained from the element size and the particle velocity. You might need to use it in bubbly flows with the virtual mass force being used.

When doing a calculation with multiphase reactions, the values of reactants decrease with time. If too big a time step is used, the values of reactants can become negative. To prevent this, a bound is placed on the particle integration time step. The Chemistry Time Step Multiplier is a factor (default of 1.0) that is bounded by the time it takes for a mass fraction to reach zero. If a smaller value is used, then the mass fraction will go towards zero. The need for this parameter is uncommon for most applications.