Chapter 6 Thermal Analysis

Chapter Overview In this chapter, performing steady-state thermal analyses in Simulation will be covered: Geometry Assemblies – Solid Body Contact Heat Loads Solution Options Results and Postprocessing Workshop 6.1 The capabilities described in this section are generally applicable to ANSYS DesignSpace Entra 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.

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: 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

Basics 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 Simulation.

A. 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

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

B. Assemblies – Solid Body Contact As with structural analyses, contact regions are automatically created to enable heat transfer between parts of assemblies.

… 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

… 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

… 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

… 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.

… 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.

… Assemblies – Spot Weld Spot welds provide discreet heat transfer points: Spotweld definition is done in the CAD software (currently only DesignModeler and Unigraphics). T2 T1

C. Heat Loads Heat Flow: Perfectly insulated (heat flow = 0): 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.

… 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

… 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

… Thermal Boundary Conditions Temperature-Dependent Convection: Select “Tabular (Temperature)” for the coefficient type. Enter coefficient vs temperature tablular data. In the details, specify how temperature is to be handled for h(T).

… Thermal Boundary Conditions Several common convection correlations can be imported from a sample library. New correlations can be stored in libraries.

… Thermal Boundary Conditions Radiation: Applied to surfaces (edges in 2D analyses) Where: σ = Stefan-Boltzman constant ε = Emmisivity A = Area of radiating surface F = Form factor (1) Provides for radiation to ambient only, not between surfaces (form factor assumed to be 1). Stefan Boltzman constant is set automatically based on the active working unit system.

D. 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.

… 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.

E. Results and Postprocessing Various results are available for postprocessing: Temperature Heat Flux “Reaction” Heat Flow Rate User defined results In Simulation, 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.

… Temperature Temperature: Temperature is a scalar quantity and has no direction associated with it.

… 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

… 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. Select from Probe menu OR Drag and drop boundary condition

F. Workshop 6 – Steady State Thermal Analysis Goal: Analyze the pump housing shown below for its heat transfer characteristics.