mech-intro 13.0 l07 thermal

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Release 13.0November 2010

Introduction to ANSYSMechanical

Customer Training Material

Lecture 7

Thermal Analysis

Introduction to ANSYS Mechanical


Release 13.0 November 2010

Customer Training MaterialChapter Overview• In this chapter, performing steady-state thermal analyses in Mechanical will

be covered:A.GeometryB.Assemblies – Solid Body ContactC.Heat LoadsD.Solution OptionsE. Results and PostprocessingF. Workshop 7.1

• The capabilities described in this section are generally applicable to ANSYS DesignSpace licenses and above, except for an ANSYS Structural license.

• Note: advanced topics including thermal transient analyses are covered in the ANSYS Thermal Analysis training course.




Customer Training MaterialBasics of Steady-State Heat Transfer• For a steady-state (static) thermal analysis in Mechanical, the

temperatures {T} are solved for in the matrix below:

• Assumptions:– No transient effects are considered in a steady-state analysis– [K] can be constant or a function of temperature– {Q} can be constant or a function of temperature

TQTTK




Customer Training MaterialBasics of Steady-State Heat Transfer• Fourier’s Law provides the basis of the previous equation:

• Heat flow within a solid (Fourier’s Law) is the basis of [K]• Heat flux, heat flow rate, and convection are treated as boundary conditions on

the system {Q}• Convection is treated as a boundary condition although temperature-dependent

film coefficients are possible• It is important to remember these assumptions related to performing

thermal analyses in Mechanical.




Customer Training MaterialA. Geometry• In thermal analyses all body types are supported:– Solid, surface, and line bodies.

• Line bodies cross-section and orientation is defined within DesignModeler.• The “Point Mass” feature is not available in thermal analyses.

• Shell and line body assumptions:– Shells: no through-thickness temperature gradients. – Line bodies: no through thickness variation. Assumes a constant

temperature across the cross-section.• Temperature variation will still be considered along the line body




Customer Training Material… Material Properties

• Thermal Conductivity is input in the Engineering Data application

• Temperature-dependent thermal conductivity is input as a table

If any temperature-dependent material properties exist, this will result in a nonlinear solution.

• The only required material property for steady state is thermal conductivity.




Customer Training MaterialB. Assemblies – Solid Body Contact• As with structural analyses, contact regions are automatically created to

enable heat transfer between parts of assemblies.




Customer Training Material… Assemblies – Contact Region

– If parts are initially in contact heat transfer can occur between them. – If parts are initially out of contact no heat transfer takes place (see pinball

explanation below).– Summary:

– The pinball region determines when contact occurs and is automatically defined and set to a relatively small value to accommodate small gaps in the model

Initially Touching Inside Pinball Region Outside Pinball RegionBonded Yes Yes NoNo Separation Yes Yes NoRough Yes No NoFrictionless Yes No NoFrictional Yes No No

Contact Type Heat Transfer Between Parts in Contact Region?




Customer Training Material… Assemblies – Contact Region

• If the contact is bonded or no separation, then heat transfer will occur (solid green lines) when the surfaces are within the pinball radius.

Pinball Radius

In this figure on the right, the gap between the two parts is bigger than the pinball region, so no heat transfer will occur between the parts




Customer Training Material… Assemblies – Thermal Conductance

• By default, perfect thermal contact conductance between parts is assumed, meaning no temperature drop occurs at the interface.

• Numerous conditions can contribute to less than perfect contact conductance:– surface flatness– surface finish– oxides– entrapped fluids– contact pressure– surface temperature– use of conductive grease– . . . .

• Continued . . .

DT

T

x




Customer Training Material… Assemblies – Thermal Conductance

– The amount of heat flow across a contact interface is defined by the contact heat flux q:

– where Tcontact is the temperature of a contact “node” and Ttarget is the temperature of the corresponding target “node”.

– By default, TCC is set to a relatively ‘high’ value based on the largest material conductivity defined in the model KXX and the diagonal of the overall geometry bounding box ASMDIAG.

– This essentially provides ‘perfect’ conductance between parts.

contacttarget TTTCCq

ASMDIAGKXXTCC /000,10




Customer Training Material… Assemblies – Thermal Conductance• In ANSYS Professional licenses and above, the user may define a

finite thermal contact conductance (TCC) for Pure Penalty or Augmented Lagrange Formulations.– TCC is input for each contact region in the Details view.– If thermal contact resistance is known, invert this value and divide by the

contacting area to obtain TCC value.

Thermal contact conductance can be input which is the same as including thermal contact resistance at a contact interface.




Customer Training Material… Assemblies – Spot Weld• Spot welds provide discreet heat transfer points:– Spotweld definition is done in the CAD software (currently only

DesignModeler and Unigraphics).

T1

T2




Customer Training MaterialC. Heat Loads

• Heat Flow:– A heat flow rate can be applied to a vertex, edge, or surface. The load is

distributed for multiple selections.– Heat flow has units of energy/time.

• Perfectly insulated (heat flow = 0):– Available to remove surfaces from previously applied boundary conditions.

• Heat Flux:– Heat flux can be applied to surfaces only (edges in 2D).– Heat flux has units of energy/time/area.

• Internal Heat Generation:– An internal heat generation rate can be applied to bodies only.– Heat generation has units of energy/time/volume.

A positive value for heat load will add energy to the system.




Customer Training Material… Thermal Boundary Conditions

Temperature, Convection and Radiation:• At least one type of thermal boundary condition must be present to prevent the

thermal equivalent of rigid body motion.• Given Temperature or Convection load should not be applied on surfaces that

already have another heat load or thermal boundary condition applied to it.• Perfect insulation will override thermal boundary conditions.

• Given Temperature:– Imposes a temperature on vertices, edges, surfaces or bodies– Temperature is the degree of freedom solved for




Customer Training Material… Thermal Boundary Conditions• Convection:– Applied to surfaces only (edges in 2D analyses).– Convection q is defined by a film coefficient h, the surface area A, and

the difference in the surface temperature Tsurface & ambient temperature Tambient

– “h” and “Tambient” are user input values.– The film coefficient h can be constant or temperature dependent

ambientsurface TThAq




Customer Training Material… Thermal Boundary Conditions• Temperature-Dependent Convection:

– Select “Tabular (Temperature)” for the coefficient type.

– Enter coefficient vs temperature tabular data.

– In the details, specify how temperature is to be handled for h(T).




Customer Training Material… Thermal Boundary Conditions• Several common convection correlations can be imported from a

sample library. New correlations can be stored in libraries.




Customer Training Material. . . Thermal Boundary Conditions• Radiation:– Applied to surfaces (edges in 2D analyses)

– Where:• σ = Stefan-Boltzman constant• ε = Emissivity• A = Area of radiating surface• F = Form factor

– Correlations:– To ambient (form factor assumed to be 1) OR – Surface to surface (view factors calculated).

– Stefan Boltzman constant is set automatically based on the active working unit system

44ambientsurfaceR TTFAQ




Customer Training MaterialD. Solution Options• Inserting the “Steady-State Thermal” from the

Workbench toolbox will set up a SS Thermal system in the project schematic.

• In Mechanical the “Analysis Settings” can be used to set solution options for the thermal analysis.– Note, the same Analysis Data Management

options discussed in chapter 4 regarding static analyses are available here.




Customer Training Material… Solving the Model• To perform a thermal-stress solution link a structural analysis to

the thermal model at the Solution level.• An “imported load” branch is inserted in the Static Structural

branch along with any applied structural loads and supports.– Solve the Structural branch.




Customer Training MaterialE. Results and Postprocessing• Various results are available for postprocessing:– Temperature– Heat Flux– “Reaction” Heat Flow Rate– User defined results

• In Mechanical, results are usually requested before solving, but they can be requested afterwards, too.– A new solution is not required for retrieving output of a solved model.




Customer Training Material… Temperature• Temperature:– Temperature is a scalar quantity and has no

direction associated with it.




Customer Training Material… Heat Flux• Heat flux contour or vector plots are available:

– Heat flux q is defined as

– “Total Heat Flux” and “Directional Heat Flux” can be requested

• The magnitude & direction can be plotted as vectors by activating vector mode

TKXXq




Customer Training Material… Reaction Heat Flow Rate• Reaction heat flow rates are available for Given Temperature,

convection or radiation boundary conditions:– Reaction heat flow rate is requested by inserting a probe - OR– Alternately users can drag and drop a boundary condition onto the

Solution branch to retrieve the reaction.

OR

Select from Probe menu

Drag and drop boundary condition




Customer Training MaterialF. Workshop 7 – Steady State Thermal Analysis

• Workshop 7.1 – Steady State Thermal Analysis• Goal:– Analyze the pump housing shown below for its heat transfer

characteristics.

mech-intro 13.0 l07 thermal

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