### 7.16.2. Knock Model

The knock model is applicable when the principal combustion model would not ignite otherwise. This is the case for combustion models for premixed or partially-premixed combustion:

- Burning Velocity Model (BVM)

- Extended Coherent Flame Model (ECFM)

When autoignition (knock) occurs, a source term is added for reaction progress (BVM) or for flame surface density (ECFM), respectively. The magnitude of the source is defined by the Knock Reaction Rate, which by default is a first order Arrhenius reaction. For the BVM the following source is added locally to the reaction progress equation:

| (7–105)

where denotes a step function that is zero for and one for , is the Pre Exponential Factor with a default value of and is the Activation Energy with a default value of 15078 J/mol.

In order to improve the numerical robustness of the model, the knock reaction rate is limited to consume only a certain fraction of the available fuel per timestep. The specified fraction is obeyed accurately when using the first order time discretization scheme, and it is approximate for the second order scheme. The default limit is , that is no more than 60% of the fuel locally available may burnt by knock per timestep.

For the ECFM, the flame surface density is locally pulled towards a value equivalent to the knock reaction rate by adding a corresponding ‘Dirichlet’ source term:

| (7–106)

where

and, where is a large coefficient (for example, ).

Release 18.2 - © ANSYS, Inc. All rights reserved.