Fluent 13.0 Lecture07 Heat Transfer

7/21/2019 Fluent 13.0 Lecture07 Heat Transfer

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Customer Training Material

ec ure

Heat Transfer Modelin

ANSYS FLUENT

L7-1ANSYS, Inc. Proprietary

2010 ANSYS, Inc. All rights reserved.Release 13.0

December 2010


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Heat Transfer

Customer Training MaterialIntroduction

This lecture covers how the transport of thermal energy can be computed

using FLUENT:

Convection in the fluid (natural and forced)

Conduction in solid regions

Thermal Radiation

External heat gain/loss from the outer boundaries of the model.



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Heat Transfer

Customer Training MaterialEnergy Equation Introduction

Energy transport equation:

Conduction Species ViscousConvectionUnsteady Enthalpy

Energy E per unit mass is defined as:

Pressure work and kinetic energy are always accounted for with compressibleflows or when using the density-based solvers. For the pressure-based solver,

t ey are om tte an can e a e t roug t e text comman :

The TUI command define/models/energy? will give more options when



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Heat Transfer

Customer Training MaterialWall Boundary Conditions

Five thermal conditions

Heat Flux

Temperature

Convection simulates an external convection environment which is not

modeled (user-prescribed heat transfer coefficient).

Radiation simulates an external radiation environment which is not modeled

user-prescr e

external emissivity and

radiation temperature).

Convection and Radiation

boundary conditions.

Wall material and thickness

can be defined for 1D or

shell conduction calculations. heat transfer calculations.



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Heat Transfer

Customer Training MaterialConjugate Heat Transfer

In this example both fluid and solid zones are being solved for.

Note there is an internal wall boundary condition on the interface, with a coupled

thermal condition. This wall will also have a partner join-shadow. Some

proper es e em ss v y can e g ven eren va ues on eren s es o e wa .

Coolant Flow Past Heated Rods

Grid



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Temperature Contours


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Heat Transfer

Customer Training MaterialConjugate Heat Transfer Example

SymmetryAir outlet

Top wall

Planes

Electronic Component

h = 1.5 W/m2K

T

= 298 K

Air inlet

one a s mo e e

k = 1.0 W/mK

Heat generation rate of 2

watts (each component)

Circuit board (externally cooled)

k = 0.1 W/mK

h = 1.5 W/m2K

.T = 298 K



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=


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Heat Transfer

Customer Training MaterialProblem Setup Heat Source

A volumetric heat source is applied to the solid cell zone of the chip.

This is applied as a source term to the cell zone

Note the units are W/m volume is small so value is hi h.



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Heat Transfer

Customer Training MaterialTemperature Distribution (Front and Top View)

Flow

direction

Convection boundary

1.5 W/m2 K

298 K free stream tempFront View

Air (fluid zone)Temp.

(F)

410

Convection Boundary1.5 W/m2 KElect. Componentsolid zoneBoardsolid zone

378

362298 K free stream temp.

Top View

(image mirrored about symmetry plane)

2 Watts source346

330

Flow

direction

298

314



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Heat Transfer

Customer Training MaterialModelling a Thin Wall

It is often important to model the thermal effects of the wall bounding the

fluid. However, it may not be necessary to mesh it.

Mesh the wall in the pre-processor Assign it as a solid cell zone

Fluid

Solid

Option 2:

Just mesh the fluid region.

Heat can flow in all

directions

Fluid

Specify a wall thickness.

Wall conduction will be accounted for.Heat transfer

normal to wall

Solid

p on : As option 2, but enable shell conduction.

1 layer of virtual cells is created.Fluid



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ese a ec e resu , u canno e

post-processed Heat can flow in all

directions

Solid


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Heat Transfer

Customer Training MaterialModelling a Thin Wall

O tion 3: O tion 2:

For option 2 and option 3 on the last slide (in which it is not necessary to

mesh the solid in the pre-processor), the setup panel looks like this:

Shell conduction enabled Just conduction normal to the solid

FluidFluid

Heat transfer

normal to wall

Solid

Heat can flow in all

directions

Solid

n o cases, a ma er a an

wall thickness are enabled

To add the virtual cells

(Option 3), enable shell

conduction.



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o e ese v r ua ce s canno

be post-processed (or

exported for FSI)


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Heat Transfer

Customer Training MaterialNatural Convection

Many heat transfer problems (especially for ventilation problems) include the

effects of natural convection.

As the fluid warms, some regions become warmer than others, and therefore rise

roug e ac on o uoyancy.

This example shows a generic LNG liquefaction site, several hundred metresacross. Large amounts of waste heat are dissipated by the air coolers (rows of

.

cleanly away from the site.

o sc argesRed surface shows

where air is more than

5C above ambient

temperature

Note transparent regions.

These contain objects too



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,

cell zone condition is used

Ambient

Wind

where hot cloud

fails to clear site


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Heat Transfer

Customer Training MaterialNatural Convection [2]

The underlying term for the buoyant force in the momentum equations is

where is the local density and o a reference density( )g0

The reference density, o is set on the Operating Conditions panel.

Note that mathematically, whatever is set foro will cancel itself out when

integrated across the domain. However careful choice ofo will make a big

difference to the rate of convergence (in some cases whether the model will even

converge or not).

For enclosed problems, pick value ofo that represents a typical mean density

in the flow. For external (dispersion) problems select o for the ambient flow,

Remember to define gravity vector



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Heat Transfer

Customer Training MaterialNatural Convection [3]

E.g. consider the forces acting on this flow between a hot and cold wall

Well posed simulationflow

o set to a value in the middle of the cavity

Near the hot wall, the buoyant force term will be upwards, whilst atthe cold wall this term will be downwards.

,

converge easily.

flow

Badly posed simulation

o set too high (equivalent to a temperature colder than at the cold

wall)

flow

e source erms ere ore pro uce: A very high upwards force at the hot wall

A lesser, but still upwards, force at the cold wall.



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as the top case, but convergence will be difficult.flow


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Heat Transfer

Customer Training MaterialNatural Convection the Boussinesq Model

A simplification can be made in some cases where the variation in density is small.

Recall the solver must compute velocity, temperature, and pressure

Rather than introducing anothervariable density (which adds an extra unknown

thus intensifying computational effort)

Instead for fluid density select Boussinesq.

And define a thermal expansion coefficient ,

(value in standard engineering texts)

Buoyant force is computed from

The same comments as on the previous slides (for setting the reference density o)

apply here for setting the reference temperature To - set in the Operating



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Heat Transfer

Customer Training MaterialRadiation

a a on e ec s s ou e accoun e or w en s o

comparable magnitude as the convection and conduction heat transfer rates.

is the Stefan-Boltzmann constant, 5.6710-8 W/(m2K4)

To account for radiation, radiative intensity transport equations (RTEs) are solved.

Local absorption by fluid and at boundaries couples these RTEs with the energyequation.

,can be coupled to the flow.

Radiation intensity, I(r,s), is directionally and spatially dependent.

Five radiation models are available in FLUENT(see the Appendix for details on each model).

Discrete Transfer Radiation Model (DTRM)

P1 Radiation Model

Rosseland Model



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ur ace- o- ur ace


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Heat Transfer

Customer Training MaterialSelecting a Radiation Model

Some general guidelines for

radiation model selection:

Computational effort

with the least amount of effort.

Accuracy

DTRM and DOM are the most

.

Optical thickness

Use DTRM/DOM for optically thin

media (L


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Heat Transfer

Customer Training MaterialPerforming a 1-way Thermal FSI Simulation

The results of the FLUENT model can be transferred to another FE code

for further analysis (for example to compute thermal stresses).

Using Workbench, it is very easy to map the FLUENT data over to an

ANSYS Mechanical

simulation.

Just right click on theo u on ce , en

Transfer Data To

New Static Structural



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Heat Transfer

Customer Training MaterialPerforming a 1-way Thermal FSI Simulation

Within the ANSYS Mechanical application (see image), the solution data from

FLUENT is available as an Imported Load.

Note an enhancement at

R13 is that volumetric (notjust surface) quantities can be

.

In this heat-exchanger example,

FLUENT solved the temperature

of the pipe, as well as the thermal

conduction in the solid.

The solid i e tem erature is

interpolated from FLUENT tothe Mechanical application

which then performs a thermal

stress anal sis.



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Heat Transfer

Customer Training MaterialExporting Data from FLUENT

FLUENT solution data can

also be exported in many

other formats for use in

applications outside of the

Workbench environment.

These are available in the

File > Export menu in

FLUENT.

Note that in this case, the

data is exported at the same

grid locations as the FLUENT

mesh.



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Appendix

7-21ANSYS, Inc. Proprietary

2009 ANSYS, Inc. All rights reserved.April 28, 2009

Inventory #002600


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Heat Transfer

Customer Training MaterialEnergy Equation for Solid Regions

Ability to compute conduction of heat through solids

Ener e uation:

enthalpy:

Anisotropic conductivityin solids (pressure-based

solver only)



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Heat Transfer

Customer Training MaterialSolar Load Model

Solar load model

Ray tracing algorithm for solarradiant energy transport: Compatiblewith all radiation models

Available with parallel solver (but raytracing algorithm is not parallelized)

3D only

Specifications

Sun direction vector

Solar intensity (direct, diffuse)

direction and direct intensity usingtheoretical maximum or fair weatherconditions

Transient cases

When direction vector is specified withsolar calculator, sun direction vectorwill change accordingly in transient



December 2010

s mu a on

Specify time steps per solar load

update


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Heat Transfer

Customer Training MaterialEnergy Equation Terms Viscous Dissipation

Energy source due to viscous

dissipation:

Also called viscous heating.

Im ortant when viscous shear in

fluid is large (e.g. lubrication) and/or

in high-velocity compressible flows.

Often negligible Not included by default in the

pressure-based solver.

Always included in the density-

based solver.

Important when the Brinkmannumber approaches or exceeds

unity:



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Heat Transfer

Customer Training MaterialEnergy Equation Terms Species Diffusion

Energy source due to species

diffusion included for multiple

species flows.

Includes the effect of enthalpy

transportu u

Always included in the density-

based solver.

-

based solver.



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Heat Transfer

Customer Training MaterialEnergy Equation Terms Source Terms

Energy source due to chemical reaction is included for reacting flows.

Enthalpy of formation of all species.

Volumetric rate of creation of all species.

Energy source due to radiation includes radiation source terms.

Interphase energy source:

DPM, spray, particles



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Heat Transfer

Customer Training MaterialThin and Two-Sided Walls

In the Thin Wall approach, the wall thickness is not explicitly meshed.

Model thin layer of material between two zones

Thermal resistance x/kis artificiall a lied b the solver.

Boundary conditions specified on the outside surface.

Exterior wall Interior wall Interior wall shadow

(user-specified

thickness)

Outer surface

(calculated)

-

thickness)

-

thickness)

Inner surface

(thermal boundary

condition specified

here)22 orTq

11 orTq

Thermal boundar conditions are

Fluid orsolid

cellsx 1k 2k

Thermal boundar conditions are

Fluid orsolid

cells

Fluid orsolid

cells



December 2010

supplied on the inner surface of a thin

wall

supplied on the inner surfaces of

uncoupled wall/shadow pairs


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Heat Transfer

Customer Training MaterialDiscrete Ordinates Model

The radiative transfer equation is solved for a discrete number of finitesolid angles, s:

Absor tion

Advantages:

Conservative method leads to heat balance for coarse discretization. Accuracy can be increased by using a finer discretization.

Most comprehensive radiation model:

Accounts for scattering, semi-transparent media, specular surfaces, and wavelength-dependent transmission using banded-gray option.

Limitations:

Solving a problem with a large number of ordinates is CPU-intensive.



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Heat Transfer

Customer Training MaterialDiscrete Transfer Radiation Model (DTRM)

Main assumption Radiation leaving a surface element within a specified range of

solid angles can be approximated by a single ray.

Uses a ray-tracing technique to integrate radiant intensity along each ray:

Advantages:

Relatively simple model. an ncrease accuracy y ncreas ng num er o rays.

Applies to wide range of optical thicknesses.

Assumes all surfaces are diffuse.

Effect of scattering not included.

Solvin a roblem with a lar e number of ra s is CPU-intensive.



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Heat Transfer

Customer Training MaterialP-1 Model

Main assumption The directional dependence in RTE is integrated out, resultingin a diffusion equation for incident radiation.

Radiative transfer equation easy to solve with little CPU demand.

Includes effect of scattering.

Effects of particles, droplets, and soot can be included.

or s reasona y we or app ca ons w ere e op ca c ness s arge e.g.combustion).

Limitations: Assumes all surfaces are diffuse.

May result in loss of accuracy (depending on the complexity of the geometry) if theoptical thickness is small.

.



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Heat Transfer

Customer Training MaterialReporting Heat Flux

Heat flux report:

It is recommended that you

perform a heat balance check

so o ensure a your so u on

is truly converged.

Exporting Heat Flux Data:

It is possible to export heat

flux data on wall zones

generic file.

Use the text interface:

file ex ort custom-heat-flux

File format for each selected face zone:

zone-name nfaces



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x_ y_ z_ _w _c

f


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Heat Transfer

Customer Training MaterialReporting Heat Transfer Coefficient

Wall-function-based heat transfer coefficient

where cP is the specific heat, kP is the turbulence kinetic energy at point P, and

T* is the dimensionless temperature:

Available only when the flow is turbulent and Energy equation is enabled.

Alternative for cases with adiabatic walls.



December 2010

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Fluent 13.0 Lecture07 Heat Transfer

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