### 9.10.1. Fluid Models

The single-mixture fraction approach describes mixing as a two-stream system consisting of fuel and oxidizer. The residual material model extends the mixing model to a three-stream system consisting of fuel, oxidizer and residual material (inert, ballast). The model is available in combination with the partially-premixed combustion models ‘BVM (Partially Premixed)’ and ‘Extended Coherent Flame Model’.

The residual material model accounts for the following effects caused by the presence of ballast material in the mixture:

- Additional degree of freedom for mixture composition (1 extra transport equation)

- Lower peak temperatures because the fuel/air mixture is diluted

- Reduced velocity for flame propagation.

For the Exhaust Gas option the residual material is modeled to consist of products originating from the same fuel and oxidizer as for the combustion. This assumption enables calculating the whole mixture composition from a single-mixture fraction flamelet library, that is, the same library that is also used without a residual material model. The details of the component mass fraction calculation are described in the solver theory documentation, see Mixture Composition in the CFX-Solver Theory Guide.

#### 9.10.1.1. Reinitialization

For transient simulations the residual material can be reinitialized at specified times. Under reinitialization all products currently present in the mixture are marked for being treated as residual material from now on. The Time List may be specified for applying reinitialization once or several times.

A typical example for use of reinitializing the residual material is the simulation of subsequent cycles of an internal combustion engine. By incomplete load exchange a part of the products generated in one cycle will remain in the combustion chamber, hence serving as residual material for the following engine cycle. This can be modeled by specifying the time for reinitialization at some point after the end of combustion in the first cycle and before the fresh mixture enters the chamber for the next cycle.

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