What Is a Combustion Model?

The purpose of combustion models is to calculate the reaction state space, i.e. the concentrations of the various species present in a chemical reaction, and the quantities they influence, viz., density, viscosity, and temperature. To this end, STAR-CCM+ provides several models based on various physical and chemical approximations. The general principles underlying these models are described below.

A large chemical reaction set, such as found in hydrocarbon combustion, can span a wide range of time scales. In addition to these time scales, the turbulent flow field imposes its own limits on length and time scales, ranging from the Kolmogorov and Batchelor time scales at the low end to the large, energy-containing eddy time scales at the high end. Resolving all the length and time scales affecting the grid-mean properties in a reacting flow system demands computational resources beyond those currently available. We therefore need combustion models to account for the processes that occur at length and time scales below what we can resolve on a numerical simulation grid.

Eddy break-up (EBU) models.

The Coherent Flame model (CFM).

Presumed Probability Distribution (PPDF) models.

The choice of combustion model is decided by knowing the Damkohler number, defined as:

When the Damkohler number is very large, the reaction rate is controlled by the turbulent mixing that brings reactants together at the molecular scale. In this limit, the standard EBU and the equilibrium PPDF models are fairly accurate because they assume that the reaction occurs instantaneously upon micromixing.

The PPDF model parametrization depends on whether the flow is assumed to be locally adiabatic or non-adiabatic:

Under the adiabatic assumption, chemical equilibrium within a control volume is affected only by the scalar mixing process and the initial temperature of the control volume.

Under the non-adiabatic assumption, chemical equilibrium within a control volume can also change due to heat losses or gains. This requires the solution of a transport equation for enthalpy. Typically, the most important contributor to this loss or gain is radiative heat transfer in participating media.

When the Damkohler number is of order 1, finite rate kinetics must be considered. The EBU model can be extended to account for these effects using reaction rates from finite-rate kinetics based on mean species concentrations and mean temperatures. The adiabatic PPDF model can be extended to account for finite-rate kinetic effects by using the laminar flamelet approach instead of the equilibrium approach. At present, only the adiabatic flamelet method has been implemented.