Thermal Analysis

1
Thermal Analysis

• Chapter Six

2
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)

3
Basics of Steady-State Heat Transfer

• For a steady-state (static) thermal analysis in
  Simulation, the temperatures T are solved for
  in the matrix belowThis 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

4
Basics of Steady-State Heat Transfer

• Fouriers 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 (Fouriers 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.

5
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 gt Physics Filter, unselect the
  Structural option. Now, the available options
  in the Simulation GUI will only reflect thermal
  analyses.
• This applies to options in theEnvironment and
  Solutionlevels 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.

6
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.

7
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.

8
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.
9
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.
10
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.

11
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 Fouriers 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

12
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.

13
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)

14
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 qwhere
  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.

15
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

16
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.
17
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.

18
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

19
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 conductance

20
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.

21
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.

22
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.

23
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).

24
Thermal Boundary Conditions

• Thermal 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.

25
Thermal Boundary Conditions

• Convection
• Applied to surfaces only.
• Convection relates a ambient temperature with
  the surface temperaturewhere 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)

26
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(TsurfaceTbulk)/2
• Surface temperatureT Tsurface
• Bulk temperatureT Tbulk
• Difference of surface and bulk
  temperaturesT(Tsurface-Tbulk)

27
Thermal Boundary Conditions

• Temperature-Dependent Convection (continued)
• 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.
28
Thermal Boundary Conditions

• Temperature-Dependent Convection (continued)
• The convection data can also be imported from a
  file.

29
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

30
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 gt Options gt Simulation Solution gt
  Solver Type
• The Weak Springs and Large Deflectionoptions
  are meant for structural analyses only,so they
  can be ignored for a thermal analysis.

31
Solution Options

• Informative settings show the user the status of
  the analysis
• For a regular thermal analysis, the Analysis
  Typewill 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 gt Options
  gt Simulation Solution gt Solver Working
  Directory.
• Any solver messages which appear after solution
  can be checked afterwards underSolver Messages

32
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 gt Options gt Simulation Solution gt
  Number of Processors to Use
• Recall that if a Solution Information branch is
  requested, the details of the solution output can
  be examined.

33
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.

34
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.

35
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.

36
Heat Flux

• Heat flux contour or vector plots are available
• Heat flux q is defined asand 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.

37
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.

38
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.

39
F. Workshop 6

• Workshop 6 Thermal Analysis
• Goal
• Analyze the pump housing shown below for its heat
  transfer characteristics.

40
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