Transcript Heat Transfer and Thermal Boundary Conditions

Fluent Software Training
TRN-98-006
Heat Transfer and Thermal Boundary
Conditions
Headlamp modeled with
Discrete Ordinates
Radiation Model
F1
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Outline







Introduction
Thermal Boundary Conditions
Fluid Properties
Conjugate Heat Transfer
Natural Convection
Radiation
Periodic Heat Transfer
F2
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Introduction



Heat transfer in Fluent solvers allows inclusion of heat transfer within
fluid and solid regions in your model.
Handles problems ranging from thermal mixing within a fluid to
conduction in composite solids.
Energy transport equation is solved, subject to a wide range of thermal
boundary conditions.
F3
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Options

Inclusion of species diffusion term


Energy equation includes effect of enthalpy transport due to species
diffusion, which contributes to energy balance.
This term is included in the energy equation by default.



Energy equation in conducting solids

In conducting solid regions, simple conduction equation solved



You can turn off the Diffusion Energy Source option in the Species
Model panel.
Term always included in the coupled solver.
Includes heat flux due to conduction and volumetric heat sources within
solid.
Convective term also included for moving solids.
Energy sources due to chemical reaction are included for reacting flow
cases.
F4
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
User Inputs for Heat Transfer (1)
1. Activate calculation of heat transfer.


Select the Enable Energy option in the Energy
panel.
Define  Models  Energy...
Enabling reacting flow or radiation will toggle
Enable Energy on without visiting this panel.
F5
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
User Inputs for Heat Transfer (2)
2. To include viscous heating terms in energy equation, turn on Viscous
Heating in Viscous Model panel.



Describes thermal energy created by viscous shear in the flow.
Often negligible; not included in default form of energy equation.
Enable when shear stress in fluid is large (e.g., in lubrication problems)
and/or in high-velocity, compressible flows.
3. Define thermal boundary conditions.
Define  Boundary Conditions...
4. Define material properties for heat transfer.
Define  Materials...


Heat capacity and thermal conductivity must be defined.
You can specify many properties as functions of temperature.
F6
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Solution Process for Heat Transfer


Many simple heat transfer problems can be successfully solved using
default solution parameters.
However, you may accelerate convergence and/or improve the stability
of the solution process by changing the options below:



Underrelaxation of energy equation.
Solve  Controls  Solution...
Disabling species diffusion term.
Define  Models  Species...
Compute isothermal flow first, then add calculation of energy equation.
Solve  Controls  Solution...
F7
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Theoretical Basis of Wall Heat Transfer

For laminar flows, fluid side heat transfer is approximated as:
q   k
T
n
k
wall
T
n
n = local coordinate normal to wall



For turbulent flows, law of the wall is extended to treat wall heat flux.
The wall-function approach implicitly accounts for viscous sublayer.
The near-wall treatment is extended to account for viscous dissipation
which occurs in the boundary layer of high-speed flows.
F8
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Thermal Boundary Conditions at Flow Inlets
and Exits



At flow inlets, must supply
fluid temperature.
At flow exits, fluid
temperature extrapolated
from upstream value.
At pressure outlets, where
flow reversal may occur,
“backflow” temperature is
required.
F9
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Thermal Conditions for Fluids and Solids

Can specify an energy source
using Source Terms option.
F10
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Thermal Boundary Conditions at Walls

Use any of following thermal
conditions at walls:





Specified heat flux
Specified temperature
Convective heat transfer
External radiation
Combined external radiation
and external convective heat
transfer
F11
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Fluid Properties

Fluid properties such as heat capacity, conductivity, and viscosity can
be defined as:







Constant
Temperature-dependent
Composition-dependent
Computed by kinetic theory
Computed by user-defined functions
Density can be computed by ideal gas law.
Alternately, density can be treated as:



Constant (with optional Boussinesq modeling)
Temperature-dependent
Composition-dependent
F12
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Conjugate Heat Transfer


Ability to compute conduction of heat through solids, coupled with
convective heat transfer in fluid.
In 2D Cartesian coordinates:

 wcwT     kw T     kw T   q
t
x  x  y  y 

Solid properties may vary with location, e.g.,





Density, w
Specific heat, cw
Conductivity, kw
Solid conductivity, kw, may also be function of temperature.
q is a uniformly distributed volumetric heat source.

May be function of time and space (using profiles or user-defined functions).
F13
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Conjugate Heat Transfer in Fuel-Rod
Assembly



Fluid flow equations not
solved within solid regions.
Energy equation solved
simultaneously in full domain.
Convective terms dropped in
stationary solid regions.
Grid
Velocity vectors
Temperature contours
F14
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Natural Convection - Introduction


Natural convection occurs
when heat is added to fluid
and fluid density varies with
temperature.
Flow is induced by force of
gravity acting on density
variation.
F15
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Natural Convection - Boussinesq Model

Makes simplifying assumption that density is uniform.

Except for body force term in momentum equation, which is replaced by:
(    0 ) g    0  (T  T0 ) g


Valid when density variations are small.
When to use Boussinesq model:


Essential to calculate time-dependent natural convection inside closed
domains.
Can also be used for steady-state problems.



Provided changes in temperature are small
You can get faster convergence for many natural-convection flows than by
using fluid density as function of temperature.
Cannot be used with species calculations or reacting flows.
F16
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
User Inputs for Natural Convection (1)
1. Set gravitational acceleration.
Define  Operating Conditions...
2. Fluid density
(a) If using Boussinesq model:
Select boussinesq as the Density method and assign a
constant value.
 Set the Thermal Expansion Coefficient.
Define  Materials…


Set the Operating Temperature in the Operating
Conditions panel.
Define  Operating Conditions...
(b) Otherwise, define fluid density as function of
temperature.
F17
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
User Inputs for Natural Convection (2)
3. Optionally, specify Operating Density.

Does not apply for Boussinesq model.
4. Set boundary conditions.
Define  Boundary Conditions...
F18
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Radiation

Radiation intensity along any
direction entering medium
is reduced by:



Radiation intensity along any
direction entering medium is
augmented by:



Local absorption
Out-scattering (scattering away
from the direction)
Local emission
In-scattering (scattering into the direction)
Four radiation models are provided in FLUENT:




Discrete Ordinates Model (DOM)
Discrete Transfer Radiation Model (DTRM)
P-1 Radiation Model
Rosseland Model (limited applicability)
F19
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Discrete Ordinates Model

The radiative transfer equation is solved for a discrete number of finite
solid angles:
I i
 absorption em m ision scatteringi
xi

Advantages:





Conservative method leads to heat balance for coarse discretization.
Accuracy can be increased by using a finer discretization.
Accounts for scattering, semi-transparent media, specular surfaces.
Banded-gray option for wavelength-dependent transmission.
Limitations:

Solving a problem with a large number of ordinates is CPU-intensive.
F20
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Discrete Transfer Radiation Model (DTRM)


Main assumption: radiation leaving surface element in a specific range of
solid angles can be approximated by a single ray.
Uses ray-tracing technique to integrate radiant intensity along each ray:
dI
T 4
  I  
ds


Advantages:




Relatively simple model.
Can increase accuracy by increasing number of rays.
Applies to wide range of optical thicknesses.
Limitations:



Assumes all surfaces are diffuse.
Effect of scattering not included.
Solving a problem with a large number of rays is CPU-intensive.
F21
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
P-1 Model

Main assumption: radiation intensity can be decomposed into series of
spherical harmonics.



Advantages:





Only first term in this (rapidly converging) series used in P-1 model.
Effects of particles, droplets, and soot can be included.
Radiative transfer equation easy to solve with little CPU demand.
Includes effect of scattering.
Works reasonably well for combustion applications where optical
thickness is large.
Easily applied to complicated geometries with curvilinear coordinates.
Limitations:



Assumes all surfaces are diffuse.
May result in loss of accuracy, depending on complexity of geometry, if
optical thickness is small.
Tends to overpredict radiative fluxes from localized heat sources or sinks.
F22
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Choosing a Radiation Model

For certain problems, one radiation model may be more
appropriate in general.
Define  Models  Radiation...






Computational effort: P-1 gives reasonable accuracy with
less effort.
Accuracy: DTRM and DOM more accurate.
Optical thickness: DTRM/DOM for optically thin media
(optical thickness << 1); P-1 better for optically thick media.
Scattering: P-1 and DOM account for scattering.
Particulate effects: P-1 and DOM account for radiation exchange between gas
and particulates.
Localized heat sources: DTRM/DOM with sufficiently large number of rays/
ordinates is more appropriate.
F23
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Periodic Heat Transfer (1)


Also known as streamwise-periodic or fully-developed flow.
Used when flow and heat transfer patterns are repeated, e.g.,



Compact heat exchangers
Flow across tube banks
Geometry and boundary conditions repeat in streamwise direction.
inflow
outflow
Outflow at one periodic boundary
is inflow at the other
F24
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Periodic Heat Transfer (2)



Temperature (and pressure) vary in streamwise direction.
Scaled temperature (and periodic pressure) is same at periodic
boundaries.
For fixed wall temperature problems, scaled temperature defined as:
T  Twall

Tb  Twall
Tb = suitably defined bulk temperature

Can also model flows with specified wall heat flux.
F25
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Periodic Heat Transfer (3)

Periodic heat transfer is subject to the following constraints:






Either constant temperature or fixed flux bounds.
Conducting regions cannot straddle periodic plane.
Properties cannot be functions of temperature.
Radiative heat transfer cannot be modeled.
Viscous heating only available with heat flux wall boundaries.
Flow must be specified by pressure jump in coupled solvers.
Contours of Scaled Temperature
F26
© Fluent Inc. 7/17/2015
Fluent Software Training
TRN-98-006
Summary


Heat transfer modeling is available in all Fluent solvers.
After activating heat transfer, you must provide:



Thermal conditions at walls and flow boundaries
Fluid properties for energy equation
Available heat transfer modeling options include:






Species diffusion heat source
Combustion heat source
Conjugate heat transfer
Natural convection
Radiation
Periodic heat transfer
F27
© Fluent Inc. 7/17/2015