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Chapter 12

Thermal Analysis


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Chapter 12 Thermal Analysis

Overview INTR

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INTR In this chapter, we will describe the specifics of a thermal analysis.

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e purpose s wo- o :

To reiterate the general analysis procedure.

To introduce you to thermal loads and boundary conditions

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Chapter 12 A. Preprocessing

Geometry INTR

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INTRGeometry

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an e er e crea e w n or mpor e .

Include details to improve results:

Goal is to sufficientl model the thermal mass of the structure. NT

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Convection loads requires areas be correctly modeled.

Heat generation loads requires the volumes be correctly modeled.

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Chapter 12 A. Preprocessing

Meshing INTR

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Element type

The table below shows commonly used thermal element types.DUCTI

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The nodal DOF is: TEMP.

Commonly used thermal element types

2-D Solid 3-D Solid 3-D Shell Line Elements2-D Solid 3-D Solid 3-D Shell Line Elements NT

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Linear PLANE55 SOLID70 SHELL57 LINK31, 32, 33, 34

SHELL131

Quadratic PLANE77 SOLID90 SHELL132PLANE35 SOLID87

Linear PLANE55 SOLID70 SHELL57 LINK31, 32, 33, 34

SHELL131

Quadratic PLANE77 SOLID90 SHELL132PLANE35 SOLID87SYS

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Material properties

Minimum requirement is Kx, thermal conductivity for steady state

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Setting preferences to thermal limits the Material Model GUI to

display only Thermal properties.


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ea cons an s ec on proper es

Primarily needed for shell and line elements.


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Chapter 12 B. Solution

Overview INTR

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Thermal loading conditions can be:

Temperatures Regions of the model where temperatures are known.

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Heat flow Points where the heat flow rate is known.

Heat flux Surfaces where the heat flow rate per unit area is known.NTOA

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Convections Surfaces where heat is transferred to (or from)

surroundings by means of convection. Input consists of

film coefficient h and bulk temperature of the surrounding

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.

Heat generation Regions where the volumetric heat generation rate is

known.

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Adiabatic surfaces Perfectly insulated surfaces where no heat transfer takes

place.

*


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.

Input consists of emissivity, Stefan-Boltzmann constant,

and optionally, temperature at a space node.

*Not covered in this course


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Nodal Coordinate System INTR

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INTR Unlike the structural analysis displacement and force boundary

conditions, the analogous thermal analysis temperature and heatDUCTI

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ow oun ary con ons are no epen en on e no a

coordinate system.

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Temperature Constraints INTR

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INTRTemperature Constraints

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.

To apply displacement constraints :

Main Menu > Solution > Define Loads > Apply > NTOA

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Thermal > Temperature

Choose where you want to apply the

constraint.

Pick the desired entities in the graphicsSYS

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window.

Then enter the temperature value. Value

defaults to zero.

Or use the D family of commands: DK, DL,art1

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DA, D.


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Concentrated Heat Flow INTR

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INTR To apply a heat flow, the following information is needed:

node or keypoint number (which you can identify by picking)DUCTI

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heat flow magnitude (which should be consistent with the system of

units you are using)

Use: NTOA

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Main Menu > Solution > Define Loads > Apply > Thermal > Heat Flow

Or the commands FK orF

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Heat Flux INTR

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INTRHeat Flux:

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o app y ea ux:

Main Menu > Solution > DefineLoads > Apply > Thermal > Heat flux

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apply the heat flux-- usually on

lines for 2-D models, on areas

for 3-D models.SYS

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graphics window.

Then enter the heat flux

values.art1

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Or use the SF family ofcommands: SFL, SFA, SFE,

SF.


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Convections INTR

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INTRConvective Loads

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o app y a convec on:

Main Menu > Solution > Define Loads > Apply> Thermal > Convection

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convection -- usually on lines for 2-D

models, on areas for 3-D models.

Pick the desired entities in the graphicsSYS

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.

Then enter the film coefficient and bulk

temperature values.

Or use the SF command famil :art1

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SFL, SFA, SFE, SF.

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Heat Generation INTR

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INTRHeat Generation

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o app y ea genera on:

Maine Menu > Solution > Define Loads >Apply > Thermal > Heat Generation

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the heat generation-- usually on

areas for 2-D models, on volumes

for 3-D models.SYS

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graphics window.

Then enter the heat generation

values.art1

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Or use the BF family ofcommands: BFL, BFA, BFE, BF.

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Adiabatic Surfaces INTR

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INTR Adiabatic Surfaces

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This is the default condition, i.e, any surface with no boundaryconditions specified is automatically treated as an adiabatic surface

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Modifying and Deleting Loads INTR

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INTRModifying and Deleting Loads

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o mo y a oa va ue, s mp y reapp y e oa

with the new value.

To delete loads: NTOA

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Main Menu > Solution > Define Loads > Delete

When you delete solid model loads, ANSYS also

automatically deletes all corresponding finite elementSYS

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Solutions Options INTR

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INTRSteady State vs. Transient Analysis

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s ea y s a e ana ys s assumes a e oa ng con ons ave

settled down to a steady level, with little or no time dependency.

A transientanal sis conditions that are chan in with time. NTOA

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NTOA For example, consider the analysis of a clothes iron which takes 1

minute to reach a constant temperatureSYS

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The analysis of the clothes iron for the first 1 minute of operation would be

transient.

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state.

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Chapter 12 C. Postprocessing

Review Results INTR

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INTR Reviewing results of a thermal analysis generally involves:

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thermal gradient distribution thermal flux distribution

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Review Results INTR

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Temperature Distribution:

To plot temperature contours D

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General Postproc > Plot Results > Contour Plot > Nodal Solution > Temperature

Or use the PLNSOL command.

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Review Results INTR

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Thermal Gradients:

To plot thermal gradient contours: D

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General Postproc > Plot Results > Contour Plot > Nodal Solu orPLNSOL command

General Postproc > Plot Results > Contour Plot > Element Solu orPLESOL command

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Review Results INTR

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Thermal Flux:

To lot thermal radient contours: D

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General Postproc > Plot Results > Contour Plot > Nodal Solu orPLNSOL command

General Postproc > Plot Results > Contour Plot > Element Solu orPLESOL command

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Review Results INTR

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INTRReaction Forces

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e sum o e reac on ea ows mus a ance e sum o e

applied heat flows

Best viewed as a listin : NTOA

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General Postprocessor > List Results > Reaction Solution orPRRSOL

command

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Verify Results INTR

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INTRIt is always a good idea to do a sanity check and make sure that

the solution is acceptable. What you need to check depends onD

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e ype o pro em you are so v ng, u ere are some yp ca

questions to ask:

Do FEA results a ree hand calculations or ex erimental data? NTOA

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Is the temperature solution correct? Check the FEA temperature

solution first since FEA heat fluxes are second order results.SYS

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Do the reaction heat flows balance the applied heat flows?

Where is the maximum heat flux located?art1

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If it is at a singularity, such as a point load or a re-entrant corner, thevalue is generally meaningless.

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Verify Results INTR

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INTR Is the mesh adequate?

This is always debatable, but you can gain confidence in the mesh byD

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us ng error es ma on.

Other ways to check mesh adequacy: Plot the element solution (unaveraged stresses) and look for

elements with hi h heat flux radients. These re ions are NTOA

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candidates for mesh refinement.

If there is a significant difference between the nodal (averaged)

and element (unaveraged) heat flux contours, the mesh may be tooSYS

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.

Similarly, if there is a significant difference between

PowerGraphics and full graphics heat flux, the mesh may be too

coarse.

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Re-mesh with twice as many elements, re-solve, and compare theresults. (But this may not always be practical.)

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Chapter 12 D. Workshop

Workshop INTR

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INTR Refer to yourWorkshop Supplementfor instructions on:

W12. Axisymmetric Pipe with FinsD

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intro1 m12 thermal

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