Fluent-Intro 16.0 L08 HeatTransfer PDF
Fluent-Intro 16.0 L08 HeatTransfer PDF
Fluent-Intro 16.0 L08 HeatTransfer PDF
Heat Transfer
16.0 Release
Learning Objectives:
You will be familiar with Fluents heat transfer modeling capabilities and be able to
set up and solve problems involving all modes of heat transfer
Intro. Overview Wall BCs Applications 1-way Thermal FSI Summary
2 2016 ANSYS, Inc. February 23, 2016
Heat Transfer Modeling in Fluent
All modes of heat transfer can be taken into account
with CFD simulation :
Conduction
Convection (forced and natural)
Radiation
Numerous processes can be included as appropriate
Interphase energy source (phase change)
Fluid-solid conjugate heat transfer
Viscous dissipation
Species diffusion
To model heat transfer, activate the energy equation
Expand the models branch in the Tree, right click on Energy
and choose "On"
Intro. Overview Wall BCs Applications 1-way Thermal FSI Summary
3 2016 ANSYS, Inc. February 23, 2016
Convection Heat Transfer
As a fluid moves, it carries heat with it this is called convection
Thus, heat transfer is coupled to the fluid flow solution
Energy + Fluid flow equations activated means Convection is computed
Conduction also solved in fluid when Energy activated
Additionally:
T
The rate of heat transfer depends
strongly on the fluid velocity
q
Tbody
Fluid properties may vary significantly
with temperature (e.g., air)
At walls, the heat transfer coefficient q = h (Tbody T ) = h T
is computed by the turbulent thermal
wall functions h = average heat transfer coefficient (W/m2-K)
Mathematically, qconduction = k T
Option 2: Fluid
Just mesh the fluid region
Specify a wall thickness Solid
Heat transfer
Wall conduction will be accounted for normal to wall
Fluid
Option 3:
As option 2, but enable Shell Conduction
1 or more layers of virtual cells will be created Heat can flow in all Solid
directions
Intro. Overview Wall BCs Applications 1-way Thermal FSI Summary
7 2016 ANSYS, Inc. February 23, 2016
Managing Shell Conduction Walls
From Define > Shell Conduction
Manager, all shell conduction
boundaries can be managed in one
panel
It is still possible to define shell
conduction in the boundary conditions
panel for individual walls
Select more than one zone in Shell
Conduction Zones to efficiently apply
identical settings to different walls
Also possible to read and write shell
conduction settings in .csv format
Especially useful for models with a
large number of shell conduction walls
Intro. Overview Wall BCs Applications 1-way Thermal FSI Summary
8 2016 ANSYS, Inc. February 23, 2016
Conjugate Heat Transfer (CHT)
At a wall between a fluid and a solid zone or a wall with fluid on both sides, a wall / wall_shadow
is created automatically by Fluent while reading the mesh file
By default, the Coupled boundary condition automatically balances energy on the two sides of the walls
Possible, but uncommon, to uncouple and to specify different thermal conditions on each side
Coolant Flow Past Heated Rods
Grid
Velocity Vectors
Temperature Contours
Intro. Overview Wall BCs Applications 1-way Thermal FSI Summary
9 2016 ANSYS, Inc. February 23, 2016
Natural Convection
Natural convection occurs when fluid density is
temperature dependent and heat is added to fluid
Flow is induced by gravitational force acting on density
differences
When gravity is activated in Fluent, the pressure gradient
and body force terms in the momentum equation are
rewritten as
with
Visible
Ultraviolet Infrared
-5 -4 -3 -2 -1 0 1 2 3 4 5
log10 (Wavelength), m
For semi-transparent bodies (e.g., glass, combustion product gases), radiation is a volumetric
phenomenon since emissions can escape from within bodies
For opaque bodies, radiation is essentially a surface phenomena since nearly all internal emissions are
absorbed within the body
Intro. Overview Wall BCs Applications 1-way Thermal FSI Summary
12 2016 ANSYS, Inc. February 23, 2016
When to Include Radiation?
Radiation effects should be accounted for if
qrad = (Tmax
4
Tmin
4
)
Stefan-Boltzmann constant
5.670410-8 W/(m2K4)
is of the same order or magnitude than the convective and
conductive heat transfer rates. This is usually true at high
temperatures but can also be true at lower temperatures, depending
on the application
Estimate the magnitude of conduction or convection heat transfer in
the system as q =h T conv T ( wall bulk )
Compare qrad with qconv
Intro. Overview Wall BCs Applications 1-way Thermal FSI Summary
13 2016 ANSYS, Inc. February 23, 2016
Optical Thickness and Radiation Modeling
The optical thickness should be determined
before choosing a radiation model
Optically thick/dense means that the fluid absorbs and re-emits the radiation
Intro. Overview Wall BCs Applications 1-way Thermal FSI Summary
14 2016 ANSYS, Inc. February 23, 2016
Choosing a Radiation Model
The radiation model selected must be appropriate for the optical thickness of
the system being simulated
Model Optical Thickness Computational Expense
When optical thickness = 0, S2S has comparable
Surface to surface model (S2S) 0
accuracy with DO at less computational expense
Very low computational expense for solar
Solar load model 0 (except window panes)
radiation problems compared to the DO model
Rosseland >5 Very inexpensive but very limited in applicability
P-1 >1 Reasonable accuracy for moderate cost
The most computationally expensive model but
Discrete ordinates model (DO) All
also the most comprehensive and accurate
Discrete Transfer Method Cheaper than DO but not available in parallel so
All
(DTRM) rarely used
In terms of accuracy, DO and DTRM are most accurate (S2S is accurate for optical thickness = 0)
Intro. Overview Wall BCs Applications 1-way Thermal FSI Summary
15 2016 ANSYS, Inc. February 23, 2016
Phase Change
Heat released or absorbed when matter changes state
There are many different forms of phase change
Condensation Tracks from evaporating liquid pentane
Evaporation droplets and temperature contours for
Boiling pentane combustion with the non-
Melting/Solidification premixed combustion model
Pressure work and kinetic energy are always accounted for with compressible
flows or when using the density-based solvers. For the pressure-based solver, they
are omitted and can be added through a text command:
Thot 10 75,000
Thot
( W ) P
+ ( U W ) = 2W + ( 0 ) g
t z
where P' is the static gauge pressure used by Fluent for
boundary conditions and post-processing
This pressure transformation avoids round off error and
simplifies the setup of pressure boundary conditions
The pressure profile on boundaries is dependent on the value of o, because the value entered
in the boundary conditions panel corresponds to the modified pressure, P (= P o g z)
If the computational domain contains pressure inlets and outlets connected to the same
external environment, o should be set equal to the ambient density and a constant pressure of
0 Pa specified for inlets and outlets
Use pressure based pseudo transient approach for High Rayleigh number (turbulent flow)
L
t
g T
Use k-epsilon for buoyant stratified flows
Fluid
Fluid
Resulting radiation dI
I + ds
ds ds
For optically thin media the DOM or DTM models may be used
DTM can be less accurate in models with long/thin geometries
DOM uses the most computational resources,
Both models can be used in optically thick media, but the P1 model uses far less
computational resources
S2S is only for non-participating media such as air (Optical Thickness = 0)
Ra x = g..
Rayleigh number : Ra = Buoyancy force / Losses due to viscosity T .x 3