steady-state thermal - lawrence berkeley national …als/fea/ansys_v9_inf… · ppt file · web...

Thermal Analysis

Chapter Six

March 29, 2005Inventory

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Steady-State Thermal Analysis

Chapter Overview• In this chapter, performing steady-state thermal analyses in

Simulation will be covered:– Geometry and Elements– Contact and Types of Supported Assemblies– Environment, including Loads and Supports– Solving Models– Results and Postprocessing

• The capabilities described in this section are generally applicable to ANSYS DesignSpace Entra licenses and above, except for an ANSYS Structural license.– Some options discussed in this chapter may require more

advanced licenses, but these are noted accordingly.– It is assumed that the user has reviewed Chapters 1-3 prior to

this chapter. (Chapters 4-5 are optional)


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Basics of Steady-State Heat Transfer• For a steady-state (static) thermal analysis in Simulation,

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

This results in the following assumptions:– No transient effects are considered in a steady-state analysis– [K] can be constant or a function of temperature

• Temperature-dependent thermal conductivity can be input for each material property

– {Q} can be constant or a function of temperature• Temperature-dependent film coefficients can be input for

convective boundary conditions

TQTTK


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

– This means that the thermal analysis Simulation solves for is a conduction-based 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}• No radiation is currently considered• No time-dependent effects are currently considered

– Heat transfer analysis is different from CFD (Computational Fluid Dynamics)

• Convection is treated as a simple boundary condition, although temperature-dependent film coefficients are possible.

• If a conjugate heat transfer/fluid problem needs to be analyzed, one must use ANSYS CFD tools instead.

• It is important to remember these assumptions related to performing thermal analyses in Simulation.


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Physics Filters• Before proceeding to a detailed discussion on performing

thermal analyses in Simulation, it is useful to point out that if a thermal-only solution is to be performed, the Physics Filter can be useful to filter the GUI.– Under “View menu > Physics Filter,” unselect the “Structural”

option. Now, the available options in the Simulation GUI will only reflect thermal analyses.

– This applies to options in the“Environment” and “Solution”levels only.

– If a thermal-stress simulation is to be performed, do not turn off any physics filters since both structural and thermal options may be required.

ANSYS License AvailabilityDesignSpace Entra xDesignSpace xProfessional xStructuralMechanical/Multiphysics x


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A. Geometry• In thermal analyses, all types of bodies supported by

Simulation may be used.– Solid, surface, and line bodies are supported by all products

which support thermal analyses.• For surface bodies, thickness must be input in the Details view of

the Geometry branch• The cross-section and orientation of line bodies is defined within

DesignModeler and is imported into Simulation automatically. Although the cross-section and orientation is defined, this information is meant for structural analyses, and the actual thermal link element will have an ‘effective’ cross-section based on the input properties.

• No heat flux or vector heat flux output is available with line bodies. Only temperature results are available for line bodies.

– The “Point Mass” feature is not applicable in thermal analyses• Point Mass is described in Chapter 4, Linear Structural Analysis.



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… Geometry• It is important to understand assumptions related to using

shell and line bodies:– For shell bodies, through-thickness temperature gradients are

not considered. A shell body should be used for thin structures when it can be safe to assume temperatures on top and bottom of surface are the same.

• Temperature variation will still be considered across the surface, just not through the thickness, which is not explicitly modeled.

– For line bodies, thickness variation in the cross-section is not considered. A line body should be used for beam- or truss-like structures, where the temperature can be assumed to be constant across the cross-section.

• Temperature variation will still be considered along the line body, just not through the cross-section, which is not explicitly modeled.



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… Material Properties• The only required material property is thermal conductivity.

– Material input is under the “Engineering Data” tab, and material assignment is per part under the “Geometry” branch

– Thermal Conductivity is input under the Engineering Data tab. Temperature-dependent thermal conductivity can be input as a table.

– Other material inputis not used in thermal.


If any temperature-dependent material properties exist, this will result in a nonlinear solution. This is because the temperatures are solved for, but the materials are dependent on the temperatures, so it is not linear.


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B. Assemblies – Solid Body Contact• When importing assemblies of solid parts, contact regions

are automatically created between the solid bodies.– Surface-to-surface contact allows non-matching meshes at

boundaries between solid parts– Contact enables heat transfer between parts in an assembly

Model shown is from a sample Inventor assembly.

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… Assemblies – Contact Region• In Simulation, the concept of contact and target surfaces

are used for each contact region.– One side of the contact region is comprised of “contact”

face(s), the other side of the region is made of “target” face(s).– Heat flow is allowed between contact and target faces (based

on the contact normal direction)• When one side is the contact and the other side is the target, this is

called asymmetric contact. On the other hand, if both sides are made to be contact & target, this is called symmetric contact. However, the designation of which side is contact or target is unimportant in thermal analysis.

• By default, Simulation uses symmetric contact for solid assemblies.

• For ANSYS Professional licenses and above, the user may change to asymmetric contact, as desired.



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… Assemblies – Contact Region• As noted in the previous slide, heat flows within a contact

region in the contact normal direction– No heat spreading is considered in the contact/target interface

• Heat spreading is considered within shell or solid elements at the contact or target surfaces because of Fourier’s Law

• Heat flow within the contact region is in the contact normal direction only

• This means that, regardless of the definition of the contact region, heat flows only if a target element is present in the normal direction

In the figure on the left, the solid green double-arrows indicate heat flow within the contact region. Heat flow only occurs if a target surface is normal to a contact surface.

The light, dotted green arrows indicate that no heat transfer will occur between parts.



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… Assemblies – Contact Region• In Simulation, various contact behaviors exist

– The contact “Type” is meant for structural applications– If the parts are initially in contact, heat transfer will occur

between the parts. If the parts are initially out of contact, the parts will not transfer heat between each other.

– Based on the contact type, whether heat will be transferred between contact and target surfaces is outlined below:

– The pinball region is automatically defined and set to a relatively small value to accommodate small gaps which may present in the model. The pinball region will be discussed next.

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

Contact Type Heat Transfer Between Parts in Contact Region?



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… Assemblies – Contact Region• The pinball region may be input and

visualized in ANSYS Professional licenses and above.

– If the target nodes lie within the pinball region and the contact is bonded or no separation, then heat transfer will occur (solid green lines)

– Otherwise, no heat transfer will occur between nodes (dotted green lines)

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

Pinball Radius

ANSYS License AvailabilityDesignSpace EntraDesignSpaceProfessional xStructuralMechanical/Multiphysics x


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… Assemblies – Thermal Conductance• By default, a high thermal contact conductance (TCC) is

defined between parts of an assembly– The amount of heat flow between two parts 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” located in the contact normal direction.

– 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



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… Assemblies – Thermal Conductance• Perfect thermal contact conductance between parts means

that no temperature drop is assumed at the interface.

• One may want to include finite thermal conductance instead– Two surfaces (at different temperatures) in contact experience a

temperature drop across the interface. The drop is due to imperfect contact between the two surfaces. The imperfect contact, and hence the finite contact conductance, can be influenced by many factors such as:

• surface flatness• surface finish• oxides• entrapped fluids• contact pressure• surface temperature• use of conductive grease

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… Assemblies – Thermal Conductance• In ANSYS Professional licenses and above, the user may

define a finite thermal contact conductance (TCC) if the Pure Penalty or Augmented Lagrange Formulation is used.– The thermal contact conductance per unit area is input for

each contact region in the Details view, as shown below.– If thermal contact resistance is known, invert this value and

divide by the contacting area to obtain TCC value.– When this is done, there will now be a temperature drop

between the contact and target surfaces for a contact region.

If “Thermal Conductance” is left at “Program Chosen,” near-perfect thermal contact conductance will be defined.

The user can change this to “Manual” to input finite thermal contact conductance instead, which is the same as including thermal contact resistance at a contact interface.



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… Assemblies – Thermal Conductance• If using symmetric contact, the user

does not need to account for a ‘double’ thermal contact resistance.– Input values as normal

• MPC bonded contact allows for perfect thermal contact conductance.– In this case, no thermal contact

conductance is used nor defined because ‘contact’ is related via constraint equations.

– The contact “node” and corresponding target “node” will have the same temperature because of perfect contact conductance.


Do not use “Normal Lagrange” formulation for thermal analyses. If selected, the ANSYS solver will actually use “Augmented Lagrange” with a ‘perfect’ thermal contact conductance.


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… Assemblies – Surface Body Contact• For ANSYS Professional licenses and above, mixed

assemblies of shells and solids are supported– Allows for more complex modeling of assemblies, taking

advantage of the benefits of shells, when applicable



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… Assemblies – Surface Body Contact• Edge contact is a subset of general contact

– For contact including shell faces or solid edges, only bonded or no separation behavior is allowed.

– For contact involving shell edges, only bonded behavior using MPC formulation is allowed.

• For MPC-based bonded contact, user can set the search direction (the way in which the multi-point constraints are written) as eitherthe target normal or pinball region.

• If a gap exists (as is often the case with shell assemblies), the pinball region can beused for the search direction to detect contact beyond a gap.

• MPC results in perfect contact conductanceANSYS License AvailabilityDesignSpace EntraDesignSpaceProfessional xStructuralMechanical/Multiphysics x


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… Assemblies – Spot Weld• Spot welds provide a means of connecting shell

assemblies at discrete points for heat transfer– Spotweld definition is done in the CAD software. Currently,

only DesignModeler and Unigraphics define spotwelds in a manner that Simulation supports.

– Spotwelds can also be created in Simulation manually, but only at discrete vertices.

ANSYS License AvailabilityDesignSpace EntraDesignSpace xProfessional xStructuralMechanical/Multiphysics xDesignModeler xPro/ENGINEERUnigraphics xSolidWorksInventorSolid EdgeMechanical DesktopCATIA V4CATIA V5ACIS (SAT)ParasolidIGES


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C. Loads• There are three types of loads in thermal analyses:

– Heat Loads:• These loads pump heat into the system.• Heat loads can be input as a known heat flow rate or heat flow rate

per unit area or unit volume.– Adiabatic Condition:

• This is the naturally-occurring boundary condition, where there is not heat flow through the surface.

– Thermal Boundary Conditions:• These boundary conditions act as heat sources or heat sinks with

a known temperature condition.• These can be either a prescribed temperature or a convection

boundary condition with a known bulk temperature.



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… Heat Loads• Heat Flow:

– A heat flow rate can be applied to a vertex, edge, or surface. The load gets distributed for multiple selections.

– Heat flow has units of energy/time (i.e., power).

• Heat Flux:– A heat flux can be applied to surfaces only.– Heat flux has units of energy/time/area (i.e., power/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. Also, if multiple loads are present, the effect is cumulative.



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… Adiabatic Conditions• Perfectly Insulated:

– Perfectly insulated condition is applied to surfaces– Can be thought of as a zero heat flow rate loading– This is actually the naturally-occurring condition in thermal

analyses, when no load is applied.• Usually, one does not need to apply a perfectly insulated condition

on surfaces since that is the natural behavior for a regular surface.• Hence, this loading is meant to be used as a way to remove loading

on specified surfaces. For example, it may be easier for a user to apply heat flux or convection on all surfaces, then use the perfectly insulated condition to selectively ‘remove’ the loading on some surfaces (such as those in contact with other parts).



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… Thermal Boundary ConditionsThermal boundary conditions present a known local or ‘remote’ temperature condition.

• At least one type of thermal boundary condition must be present. Otherwise, the steady-state temperature will be infinite if only heat is pumped into a system!

• Also, Given Temperature or Convection load should not be applied on surfaces that already have another heat load or thermal boundary condition applied to it.

– If applied on an entity which also has a heat load, the temperature boundary condition will override.

– Perfect insulation will override thermal boundary conditions.

• Given Temperature:– This imposes a temperature on vertices, edges, or surfaces.– Temperature is the degree of freedom solved for, but this fixes

the temperature on selected entities to a given value.ANSYS License AvailabilityDesignSpace Entra xDesignSpace xProfessional xStructuralMechanical/Multiphysics x


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… Thermal Boundary Conditions• Convection:

– Applied to surfaces only.– Convection relates a ‘ambient temperature’ with the surface

temperature:

where the convective heat flux q is related to a film coefficient h, the surface area A, and the difference in the surface temperature Tsurface & ambient temperature Tbulk.

– Meant to provide a simplified way of accounting for heat transport from a fluid. “h” and “Tbulk” are user-input values.

– The film coefficient h can be constant or input from a file (next)

ambientsurface TThAq



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… Thermal Boundary Conditions• Temperature-Dependent Convection (continued):

– If film coefficient h is input from a file, this can be a constant or temperature-dependent value h(T).

• Define a convection boundary condition under the Environment branch and define the Type to be “Temperature-Dependent”. Next, select “New Convection…” for the Correlation. The “Engineering Data” tab will open and the Coefficient Type can then be defined for the new convection load.

• Determine what temperature is used for h(T) first, for temperature-dependent film coefficients. Temperature can be:

– Average film temperatureT=(Tsurface+Tbulk)/2

– Surface temperatureT= Tsurface

– Bulk temperatureT= Tbulk

– Difference of surface and bulk temperaturesT=(Tsurface-Tbulk)


Select the temperature-dependency from the pull-down menu


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• After the type of temperature-dependency is selected, the user may input the film coefficients and temperatures in a table. The values are plotted on a graph, as shown below.


If any temperature-dependent convection load is applied, this will result in a nonlinear solution since the surface temperature is solved for, but the film coefficient h is based on a function of the surface temperature.

The only exception is if the film coefficient h is based on a function of the bulk temperature only. In Simulation, the bulk temperature is constant and input by the user, so this load will not be nonlinear.

Right mouse click on the table to add or delete values.


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• The convection data can also be imported from a file.



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… Thermal Loads Summary• For some structural users, it may be useful to provide an

analogy of structural and thermal analyses:

– There are some types of loads that do not have any analogy• There is no thermal equivalent for inertial loads such as rotational

velocity or acceleration• The analogy of convective boundary condition is a ‘foundation

stiffness’ support in structural terms, similar to a grounded spring

Structural ThermalNatural Condition No external force Perfectly Insulated

(No heat flow rate)Direct Given Displacement Given TemperatureIndirect ConvectionDirect Force Heat FlowPer Area Pressure Heat FluxPer Volume Thermal Expansion Internal Heat Generation

Inertial Loads Acceleration

Boundary ConditionsLoad


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D. Solution Options• Solution options can be set under the “Solutions” branch:

– The ANSYS database can be saved if “SaveANSYS db” is set

• Useful if you want to open a database in ANSYS– Two solvers are available in Simulation

• The default solver is automatically chosen and does not usually need to be changed.

• The “Iterative” solver can be efficient for solvinglarge models whereas the “Direct” solver is a robust solver and handles any situation.

• The ability to change the default solver is under “Tools > Options… > Simulation: Solution > Solver Type”

– The “Weak Springs” and “Large Deflection”options are meant for structural analyses only,so they can be ignored for a thermal analysis.



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… Solution Options– Informative settings show the user the status of the analysis:

• For a regular thermal analysis, the “Analysis Type”will be set to “Static Thermal.” If structuralsupports and results are present, then theanalysis type will be “Thermal Stress.”

• A nonlinear solution will be required if temperature-dependent (a) material properties or(b) convection film coefficients are present. This means that several internal iterations will be run to achieve heat equilibrium.

• The solver working directory is where scratch filesare saved during the solution of the equations.By default, the TEMP directory of your Windowssystem environment variable is used, although thiscan be changed in “Tools > Options… > Simulation: Solution > Solver Working Directory”.

– Any solver messages which appear after solution can be checked afterwards under“Solver Messages”



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… Solving the Model• To solve the model, request results first (covered next) and

click on the “Solve” button on the Standard Toolbar– By default, two processors (if present) will be used for parallel

processing. To change this, use “Tools > Options… > Simulation: Solution > Number of Processors to Use”

– Recall that if a “Solution Information” branch is requested, the details of the solution output can be examined.



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… Solving the Model• To perform a thermal-stress solution, simply add structural

support(s) and request structural results, then solve the model.– Structural loads are optional but can also be added.– Simulation will know that a thermal-stress analysis is to be

performed (under Details view of the Solution branch). The following will be performed automatically:

• A steady-state thermal analysis will be performed• The temperature field will be mapped back onto the structural

model• A structural analysis will be performed

– See Chapter 4 for details on Structural Analyses– Simulation automates this type of coupled-field solution, so

the user does not have to worry about the above details.



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E. Results and Postprocessing• Various results are available for postprocessing:

– Temperature– Heat Flux– “Reaction” Heat Flow Rate

• In Simulation, results are usually requested before solving, but they can be requested afterwards, too.– If you solve a model then request results afterwards, click on

the “Solve” button , and the results will be retrieved. A new solution is not required for retrieving output of a solved model.


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… Temperature• Temperature contour plots can be requested:

– Temperature is the degree of freedom solved for,and it is the most basic output request.

– Temperature is a scalar quantity and, therefore,has no direction associated with it.



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

– Heat flux q is defined as

and is related to the thermal gradient T. The heat flux output has three components and can aid the user in seeing how the heat is flowing.

– The magnitude plotted as contours: “Total Heat Flux”– The magnitude & direction as vectors: “Vector Heat Flux”

• Recall that wireframe is best for viewing vectors– Components of heat flux

can be requested with “Directional Heat Flux” and can be mapped on any coordinate system.

TKXXq



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… Reaction Heat Flow Rate• Reaction heat flow rates is available for any Given

Temperature or Convection boundary condition– Recall that both given temperature and convection supply a

known temperature, either directly or indirectly. Hence, this acts as a heat source/sink, and the amount of heat flowing in (positive) or out (negative) of the support can be output.

– For each individual Given Temperature orConvection load, the Reaction heat flow rateis printed in the Details view after a solution.



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… Reaction Heat Flow Rate• The “Worksheet” tab for “Environment” branch has a

tabular summary of reaction heat flow rates.– If a thermal support shares a vertex, edge, or surface with

another thermal support or load, the reported reaction heat flow rate may be incorrect. This is due to the fact that the underlying mesh will have multiple supports applied to the same nodes. The solution will still be valid, but the reported values may not be accurate because of this.



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F. Workshop 6• Workshop 6 – Thermal Analysis

• Goal:– Analyze the pump housing shown below for its heat transfer

characteristics.

steady-state thermal - lawrence berkeley national …als/fea/ansys_v9_inf… · ppt file · web...

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