### 3.2.15. Component

When a fluid is a variable composition mixture, set the initial mass fractions for each of the components in the fluid. If modeling combustion, setting a mass fraction of 0.232 for Oxygen, and then setting all other components to `Automatic` is a good initial guess. For details, see Combustion Modeling.

For non-combusting reactions, `Automatic` is a sensible initial guess method if you do not know the initial concentrations of components at the start of the simulation.

Note: When restarting a calculation, you can rename a defined material component, or add (define) a new material component in the solver input file. For the renamed and/or added material components, there is no initial condition information stored in the initial values file; therefore, these component mass fractions are initialized using user-specified values and/or automatically generated default values.

Since a combination of initialization options may be used to initialize all of the component mass fractions, it is likely that the component mass fractions (including the `Constraint` mass fraction) will no longer sum to 1. If this is the case, the solver scales the defined mass fractions and/or recalculates the `Constraint` mass fraction to enforce a component mass fraction sum of 1. The solver also issues a warning explaining that the initial component mass fractions that were produced may not be precisely those defined by the initial condition information in the initial values file.

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