
The Energy Equation
FLUENT solves the energy equation in the following form:
where is the effective conductivity ( , where is the turbulent thermal conductivity, defined according to the turbulence model being used), and is the diffusion flux of species . The first three terms on the righthand side of Equation 13.21 represent energy transfer due to conduction, species diffusion, and viscous dissipation, respectively. includes the heat of chemical reaction, and any other volumetric heat sources you have defined.
In Equation 13.21,
(13.22) 
where sensible enthalpy is defined for ideal gases as
and for incompressible flows as
In Equations 13.23 and 13.24, is the mass fraction of species and
where is 298.15 K.
The Energy Equation for the NonPremixed Combustion Model
When the nonadiabatic nonpremixed combustion model is enabled, FLUENT solves the total enthalpy form of the energy equation:
Under the assumption that the Lewis number (Le) = 1, the conduction and species diffusion terms combine to give the first term on the righthand side of the above equation while the contribution from viscous dissipation appears in the nonconservative form as the second term. The total enthalpy is defined as
where is the mass fraction of species and
(13.28) 
is the formation enthalpy of species at the reference temperature .
Inclusion of Pressure Work and Kinetic Energy Terms
Equation 13.21 includes pressure work and kinetic energy terms which are often negligible in incompressible flows. For this reason, the pressurebased solver by default does not include the pressure work or kinetic energy when you are solving incompressible flow. If you wish to include these terms, use the define/models/energy? text command to turn them on.
Pressure work and kinetic energy are always accounted for when you are modeling compressible flow or using the densitybased solver.
Inclusion of the Viscous Dissipation Terms
Equations 13.21 and 13.26 include viscous dissipation terms, which describe the thermal energy created by viscous shear in the flow.
When the pressurebased solver is used, FLUENT's default form of the energy equation does not include them (because viscous heating is often negligible). Viscous heating will be important when the Brinkman number, Br, approaches or exceeds unity, where
and represents the temperature difference in the system.
When your problem requires inclusion of the viscous dissipation terms and you are using the pressurebased solver, you should activate the terms using the Viscous Heating option in the Viscous Model panel. Compressible flows typically have . Note, however, that when the pressurebased solver is used, FLUENT does not automatically activate the viscous dissipation if you have defined a compressible flow model.
When the densitybased solver is used, the viscous dissipation terms are always included when the energy equation is solved.
Inclusion of the Species Diffusion Term
Equations 13.21 and 13.26 both include the effect of enthalpy transport due to species diffusion.
When the pressurebased solver is used, the term
is included in Equation 13.21 by default. If you do not want to include it, you can turn off the Diffusion Energy Source option in the Species Model panel.
When the nonadiabatic nonpremixed combustion model is being used, this term does not explicitly appear in the energy equation, because it is included in the first term on the righthand side of Equation 13.26.
When the densitybased solver is used, this term is always included in the energy equation.
Energy Sources Due to Reaction
Sources of energy , , in Equation 13.21 include the source of energy due to chemical reaction:
where is the enthalpy of formation of species and is the volumetric rate of creation of species .
In the energy equation used for nonadiabatic nonpremixed combustion (Equation 13.26), the heat of formation is included in the definition of enthalpy (see Equation 13.27), so reaction sources of energy are not included in .
Energy Sources Due To Radiation
When one of the radiation models is being used, in Equation 13.21 or 13.26 also includes radiation source terms. See Section 13.3 for details.
Interphase Energy Sources
It should be noted that the energy sources, , also include heat transfer between the continuous and the discrete phase. This is discussed further in Section 22.9.1.
Energy Equation in Solid Regions
In solid regions, the energy transport equation used by FLUENT has the following form:
where  =  density  
=  sensible enthalpy,  
=  conductivity  
=  temperature  
=  volumetric heat source 
The second term on the lefthand side of Equation 13.211 represents convective energy transfer due to rotational or translational motion of the solids. The velocity field is computed from the motion specified for the solid zone (see Section 7.18). The terms on the righthand side of Equation 13.211 are the heat flux due to conduction and volumetric heat sources within the solid, respectively.
Anisotropic Conductivity in Solids
When you use the pressurebased solver, FLUENT allows you to specify anisotropic conductivity for solid materials. The conduction term for an anisotropic solid has the form
(13.212) 
where is the conductivity matrix. See Section 8.5.5 for details on specifying anisotropic conductivity for solid materials.
Diffusion at Inlets
The net transport of energy at inlets consists of both the convection and diffusion components. The convection component is fixed by the inlet temperature specified by you. The diffusion component, however, depends on the gradient of the computed temperature field. Thus the diffusion component (and therefore the net inlet transport) is not specified a priori.
In some cases, you may wish to specify the net inlet transport of energy rather than the inlet temperature. If you are using the pressurebased solver, you can do this by disabling inlet energy diffusion. By default, FLUENT includes the diffusion flux of energy at inlets. To turn off inlet diffusion, use the define/models/energy? text command.
Inlet diffusion cannot be turned off if you are using the densitybased solver.