Specified Burn Rate Model Reference

The default specified burn rate function is the Wiebe model, although you can apply any user-defined burn rate.

Specified Burn Rate Model Properties

Convection

Sets the discretization scheme that Simcenter STAR-CCM+ uses for computing the convection flux on a cell face in appropriate transport equations. More information is given in the related topic for the Convection Term:

1st-Order

First-order upwind scheme. This scheme scales the transported quantity by the upstream or downstream mass flowrate depending on flow direction. Only use when a higher-order scheme fails to give convergence, or in order to obtain an initial solution before switching to a higher-order scheme.

2nd-Order

Second-order upwind scheme. This scheme introduces linear interpolation of cell values on either side of the upstream or downstream face. Using this scheme can lead to poorer convergence properties, but gives accuracy as good as or better than the first-order scheme.

MUSCL 3rd-order/CD

Hybrid MUSCL third-order/central-differencing. Adds the Bounded Differencing sub-node.

Combustion Reset Time

For a multi-cycle run, after the completion of combustion and before the exhaust valve opening (EVO), you reset the progress variable from 1.0 (burnt state) to 0.0 (unburnt/EGR) in the in-cylinder region. Set as 5 degrees before EVO by default.

Specified Burn Rate Method

Method	Corresponding Method Node
Wiebe Function Specified Burn Rate Defines the specified burn rate using the Wiebe function Eqn. (4053).	Wiebe Function Specified Burn Rate Wiebe Function Exponential Constant, N $n$ in Eqn. (4053). Timing at 50% Fuel Burned Time at which 50% of the fuel is burned. Duration of 10%-90% Fuel Burn Time duration from the time at which 10% of the fuel is burnt until the time at which 90% of the fuel is burnt.
Table Method Allows you to define the specified burn rate using a custom table that you import.	Table Method Input Table Specifies the table from which the crank angle degree and mean progress variable are interpolated. Do not specify units with column headers for the table. Table: Crank Angle Degree Specifies the column within the table which represents the crank angle degree, specified in the following format: <start angle in deg> ... <start angle + cycle length in deg> Table: Mean Progress Variable Specifies the column within the table which represents the mean progress variable.

Secondary Gradients

There are two sources of secondary gradients in Simcenter STAR-CCM+ flow solvers:

• boundary secondary gradients for diffusion
• interior secondary gradients for diffusion at cell faces

Use this property to control which gradients are included in the solver. On gives both gradients while Off excludes them. Interior Only and Boundaries Only select the corresponding gradients.

Flow Boundary Diffusion

When activated, this property includes the flow-boundary diffusion fluxes (or viscous fluxes for flow models) as given by Eqn. (899). This property is activated by default.

Initial Conditions

Mixture Fraction

The mixture fraction is the atomic mass fraction that originated from the fuel stream. 1.0 is equal to 100%. To initialize with pure oxidizer, maintain the default of 0.0. For inflows where the mixture is unburnt, the Mixture Fraction Profile is the mass fraction of the fuel.

Boundary Settings

Inflow or Outflow Boundaries

Mixture Fraction

See Initial Conditions: Mixture Fraction.

Region Settings

Applies to fluid regions.

Specified Burn Rate Option

Specified Burn Rate	Corresponding Physics Value Nodes
Activated The Specified Burn Rate combustion model is used to determine the burn rate.	Engine Offset Timing Sets the profile for the engine offset timing. The engine offset timing is added to the absolute crank angle to determine the ignition timing. You can use this option for multiple cylinders, and also to de-activate non-cylinder parts—in particular, the intake and exhaust ports. Spark Plug Center You define the coordinates of the spark plug center using a vector Value and Coordinate System within which the vector value is defined.
Deactivated The progress variable is not solved.

Specified Burn Rate Solver Properties

Under-Relaxation Factor

In order to promote convergence, this property is used to under-relax changes of the solution during the iterative process. If residuals show solution divergence or do not decrease, reduce the under-relaxation factor.

Solver Frozen

When On, the solver does not update any quantity during an iteration. It is Off by default. This is a debugging option that can result in non-recoverable errors and wrong solutions due to missing storage. See Finite Volume Solvers Reference for details.

Reconstruction Frozen

When On, Simcenter STAR-CCM+ does not update reconstruction gradients with each iteration, but rather uses gradients from the last iteration in which they were updated. Activate Temporary Storage Retained in conjunction with this property. This property is Off by default.

Reconstruction Zeroed

When On, the solver sets reconstruction gradients to zero at the next iteration. This action means that face values used for upwinding (Eqn. (905)) and for computing cell gradients (Eqn. (917) and Eqn. (918)) become first-order estimates. This property is Off by default. If you turn this property Off after having it On, the solver recomputes the gradients on the next iteration.

Temporary Storage Retained

When On, Simcenter STAR-CCM+ retains additional field data that the solver generates during an iteration. The particular data retained depends on the solver, and becomes available as field functions during subsequent iterations. Off by default.

Field Functions

Turbulent Length Scale

Turbulent length scale $L$ that the Specified Burn Rate model uses when propagating the 3D combustion solution outward from the spherical spark solution.

Turbulent Time Scale

Turbulent time scale $\tau$ that the Specified Burn Rate model uses when propagating the 3D combustion solution outward from the spherical spark solution.

Fluid Stream Manager

You can right-click the Specified Burn Rate > Fluid Stream Manager node and create up to 3 streams for Fuel, Oxidizer, and EGR. You set the Fluid Stream Components for each fluid stream.

Fluid Stream Components

Allows you to select the fluid stream components for the selected fluid stream. You can select the fluid stream components from a list of the components that are already specified within the Multi-Component Gas or Multi-Component Liquid models.

Note	Only single component fuel is allowed.