The Flame Model feature is used to define the heat source using a flame model, typically for the stability analysis in a combustion setup. The heat source depends on the acoustic field and is defined in terms of the so-called n-τ model. Add this node from the More submenu.

In combustion engines, the heat release depends on acoustic oscillations of the fresh fuel supply and the acoustic oscillations are effected by the heat release. This can result in acoustic modes becoming either unstable or damped. The heat release in the Flame Model is modeled with the n-τ model where the heat release in the domain at time t is coupled to the acoustic field at a point or boundary at the time t − τu, see Ref. 75.

The feature adds a monopole domain source Qm to the right-hand side, defined as

where the (isobaric) coefficient of thermal expansion αp is defined in terms of the speed of sound c, heat capacity at constant pressure Cp, ambient temperature T (this value is taken from the Model Input section and can be space dependent), and the ratio of specific heats γ. Qs is the steady heat release, Us is the steady flow speed, nu is the interaction index, τu is the time-lag, xref is the reference position, and nref is the acoustic reference vector.

The Model Input section is always visible in the Flame Model feature as the ambient temperature T is always needed to compute the coefficient of thermal expansion.

Enter the properties defining the flame model. Specify the Interaction index nu, the Time lag τu (SI unit: s), the Steady heat release Qs (SI unit: W/m3), and the Steady flow Us (SI unit: m/s).

Select the acoustic reference point or surface that defines the reference velocity for the flame model. The heat release depends on the velocity at a point or surface, this point or surface has to be selected independently from the domain selection for the flame model. The heat release depends on the reference velocity because of its dependency of the rate of supplied fuel, therefore the acoustic reference point or surface should be upstream from the Flame Model domain.

Select the Acoustic reference as either Reference point or Reference boundary. Depending on the selection, make sure to select either a point in the Reference Point section or a boundary in the Reference Boundary section.

Enter the additional fluid properties necessary to define the heat source. Select the Fluid material from Domain material (the default) or select a material from the list. Enter the Heat capacity at constant pressure Cp (SI unit: J/(kg·K)) and the Ratio of specific heats γ (dimensionless). The default for both is From material. Select User defined to enter a value or expression directly.