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The general conservation equations from which the equations solved by ANSYS FLUENT are derived are presented in this section, followed by the solved equations themselves.
Equations in General Form
Conservation of Mass
The continuity equation for phase
is
where
is the velocity of phase
and
characterizes the mass transfer from the
to
phase, and
characterizes the mass transfer from phase
to phase
, and you are able to specify these mechanisms separately.
By default, the source term
on the right-hand side of Equation
16.5-4 is zero, but you can specify a constant or user-defined mass source for each phase. A similar term appears in the momentum and enthalpy equations. See Section
16.7 for more information on the modeling of mass transfer in
ANSYS FLUENT's general multiphase models.
Conservation of Momentum
The momentum balance for phase
yields
where
is the
phase stress-strain tensor
Here
and
are the shear and bulk viscosity of phase
,
is an external body force,
is a lift force,
is a virtual mass force,
is an interaction force between phases, and
is the pressure shared by all phases.
is the interphase velocity, defined as follows. If
(i.e., phase
mass is being transferred to phase
),
; if
(i.e., phase
mass is being transferred to phase
),
. Likewise, if
then
, if
then
.
Equation
16.5-5 must be closed with appropriate expressions for the interphase force
. This force depends on the friction, pressure, cohesion, and other effects, and is subject to the conditions that
and
.
ANSYS FLUENT uses a simple interaction term of the following form:
where
(
) is the interphase momentum exchange coefficient (described in Section
16.5.4).
Lift Forces
For multiphase flows, ANSYS FLUENT can include the effect of lift forces on the secondary phase particles (or droplets or bubbles). These lift forces act on a particle mainly due to velocity gradients in the primary-phase flow field. The lift force will be more significant for larger particles, but the ANSYS FLUENT model assumes that the particle diameter is much smaller than the interparticle spacing. Thus, the inclusion of lift forces is not appropriate for closely packed particles or for very small particles.
The lift force acting on a secondary phase
in a primary phase
is computed from [
80]
The lift force
will be added to the right-hand side of the momentum equation for both phases (
).
In most cases, the lift force is insignificant compared to the drag force, so there is no reason to include this extra term. If the lift force is significant (e.g., if the phases separate quickly), it may be appropriate to include this term. By default,
is not included. The lift force and lift coefficient can be specified for each pair of phases, if desired.
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It is important that if you include the lift force in your calculation, you need not include it everywhere in the computational domain since it is computationally expensive to converge. For example, in the wall boundary layer for turbulent bubbly flows in channels, the lift force is significant when the slip velocity is large in the vicinity of high strain rates for the primary phase.
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Virtual Mass Force
For multiphase flows,
ANSYS FLUENT includes the "virtual mass effect'' that occurs when a secondary phase
accelerates relative to the primary phase
. The inertia of the primary-phase mass encountered by the accelerating particles (or droplets or bubbles) exerts a "virtual mass force'' on the particles [
80]:
The term
denotes the phase material time derivative of the form
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(16.5-10) |
The virtual mass force
will be added to the right-hand side of the momentum equation for both phases (
).
The virtual mass effect is significant when the secondary phase density is much smaller than the primary phase density (e.g., for a transient bubble column). By default,
is not included.
Conservation of Energy
To describe the conservation of energy in Eulerian multiphase applications, a separate enthalpy equation can be written for each phase:
where
is the specific enthalpy of the
phase,
is the heat flux,
is a source term that includes sources of enthalpy (e.g., due to chemical reaction or radiation),
is the intensity of heat exchange between the
and
phases, and
is the interphase enthalpy (e.g., the enthalpy of the vapor at the temperature of the droplets, in the case of evaporation). The heat exchange between phases must comply with the local balance conditions
and
.
Equations Solved by
ANSYS FLUENT
The equations for fluid-fluid and granular multiphase flows, as solved by
ANSYS FLUENT, are presented here for the general case of an
-phase flow.
Continuity Equation
The volume fraction of each phase is calculated from a continuity equation:
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(16.5-12) |
where
is the phase reference density, or the volume averaged density of the
phase in the solution domain.
The solution of this equation for each secondary phase, along with the condition that the volume fractions sum to one (given by Equation 16.5-2), allows for the calculation of the primary-phase volume fraction. This treatment is common to fluid-fluid and granular flows.
Fluid-Fluid Momentum Equations
The conservation of momentum for a fluid phase
is
Here
is the acceleration due to gravity and
,
,
, and
are as defined for Equation
16.5-5.
Fluid-Solid Momentum Equations
Following the work of [ 3, 49, 71, 110, 183, 208, 254, 343], ANSYS FLUENT uses a multi-fluid granular model to describe the flow behavior of a fluid-solid mixture. The solid-phase stresses are derived by making an analogy between the random particle motion arising from particle-particle collisions and the thermal motion of molecules in a gas, taking into account the inelasticity of the granular phase. As is the case for a gas, the intensity of the particle velocity fluctuations determines the stresses, viscosity, and pressure of the solid phase. The kinetic energy associated with the particle velocity fluctuations is represented by a "pseudothermal'' or granular temperature which is proportional to the mean square of the random motion of particles.
The conservation of momentum for the fluid phases is similar to Equation
16.5-13, and that for the
solid phase is
where
is the
solids pressure,
is the momentum exchange coefficient between fluid or solid phase
and solid phase
,
is the total number of phases, and
,
, and
are as defined for Equation
16.5-5.
Conservation of Energy
The equation solved by ANSYS FLUENT for the conservation of energy is Equation 16.5-11.