1. Introduction to NX Thermal Analysis
Student Guide
March 2015
MT15032-s-NX 10

2. Pre and post processing thermal models

3. Thermal solver algorithm with control volumes
Step 1:
Elements and nodes from the mesh are used to define the model's solid geometry.
Step 2:
During the solve, calculation points are established at each element's center of gravity (CG).
Step 3:
Capacitances and heat flows for the control volume are “lumped” at calculation points.
Step 4:
The conductance algorithm constrains a piecewise-linear element temperature function to satisfy governing Partial Differential Equations (PDEs).
Step 5:
Thermal loads and boundary conditions are applied to the calculation points or the element's CGs.

4. Thermal solver algorithm with control volumes
Step 6:
Heat transfer is computed at the centroidal calculation point.
Step 7:
Heat flow results in the CGs are distributed by interpolating results to the nodes.
Element
CG and calculation point Conductances
Node
Boundary calculation points

5. Thermal results and the CG method
Two sets of temperature results are obtained from a thermal analysis:
Temperature — Nodal
Temperature — Element
Nodal temperatures are interpolated values from the results obtained at the calculation points.
Element temperatures are the values at the element's center of gravity without any interpolation.
Node
CG calculation point Boundary calculation point

6. Element types supported by the thermal solver
Hexahedral or brick, tetrahedral, and wedge 2D
Triangular or quadrilateral thin, multi-layer, or null shells
1D and 0D
Beams, ducts, or ducts with wall
Parabolic elements
A parabolic element incorporates mid-side nodes, in addition to nodes at the vertices.
The thermal solver uses a control volume formulation, with variables calculated at the element's centroid. Changing an element's order from linear to parabolic will have no effect on the formulation.
Temperature results are calculated for all nodes (including mid-side nodes) by interpolating the temperatures at the element's centroid.

7. Material Properties — Solid
Material property (SI units)
Defined in
Notes
Basic structural properties
Mass Density (kg/m³)
NX Materials
Required for transient analysis
Thermal properties
Thermal Conductivity (W/m-K)
Required for steady state and transient analysis
May be defined as isotropic or orthotropic
Specific Heat (J/kg-K)
Diffuse and opaque surface properties
Emissivity (unitless)
Thermo-Optical Property modeling object
Required for all thermal radiation simulations

8. Material Creation
For thermal models the material creation forms are shown.

9. Meshing Thermal & Flow Models

10. Common Mesh Issues
Poor element quality.
Lack of transition between mesh types.
Insufficient resolution to correctly capture physics.
Abrupt changes in element size.
Missing mesh mating conditions.

11. Pyramid Transitions
Without pyramid transitions
With pyramid transitions

12. Tet vs Hex
Tet Mesh Conforms better to the geometry but requires more resources to solve. Hex Mesh Can generate fewer elements but requires significant effort to generate in complex geometries.
Tet
Hex
Tet (regular)
Elements
33,792
8,000
21,828
Nodes
7,097
9,261
4,875

13. Tet vs Hex
NX Flow: Use number of nodes to judge model size NX Thermal: Use number of elements to judge model size
NX Flow: To model the flow, at least 5 to 8 elements thru the flow channel NX Thermal: To model thermal gradients, at least 3 to 5 thru the thickness

14. Improper thermal element mesh
The following cases are not common and they will not conduct heat under default meshing conditions:
1D element connected to the edge of a 3D solid element. 1D elements must be connected to an edge of a 2D element or to another 1D element.
Hingedoor: 2D element connected along an edge to a 3D solid element. All nodes on a 2D element must be connected to all nodes on the face of a solid or connected along their edge to another 2D element or 1D element.

15. Improper thermal element mesh
Flagpole: 1D element connected at one end to a 2D element. The 1D element must be connected to an edge of a 2D element or to another 1D element.
Mass element connected to the corner of a 2D or 3D element. Mass elements must be placed at the end of 1D elements.

16. Activity: Model inquiry — capacitor Optional
In this activity, you will:
Find, tag, and display selected nodes and elements.
Create groups.
Post process information about group of elements and nodes.

17. Activity - Optional
Geometry Preparation:
Surface wrapping complex models
Simplify polygon geometry before meshing — In this activity, you will mesh a simple part to familiarize yourself with the geometry abstraction tools.

18. Thermo-optical properties
Thermo-optical properties are necessary for radiation analyses. You must specify:
Emissivity
Absorptivity
Specular reflectivity
Transmissivity
Index of refraction
Use the Modeling Objects Manager to define Thermo-Optical Properties in the FEM or Simulation files.

19. Optics Properties
Created as a modeling object.
Assigned at the collector level.

20. Multi-layer shells
You can model objects made of multiple layers with different properties using a single mesh with multi-layer physical property.
Model single or multiple material properties, thicknesses, and layers.
Calculate individual temperatures for each layer.
Multi layer shells provide a detailed picture of the thermal exchange without using 3D solid elements.
Multi-layer shell types
Multi-Layer Shell Uniform
Multi-Layer Shell — Non-Uniform

21. Treatment of multi-layer shells and heat transfer phenomena
Thermal boundary conditions
Radiation
Thermal loads and temperature constraints can be applied to any or all layers.
Only the top and bottom layers of a multi-layer shell are used to calculate radiative fluxes. Elements radiate from their top or bottom sides.
Couplings connecting a multi-layer mesh to another mesh are established either from the top or the bottom layer, depending on which layer is closer to the connected mesh.
Thermal couplings
Conduction Intermediate layers model conduction between the top and bottom sides. In-plane conduction is also calculated.
Convection Flow surfaces convect from top or bottom layer of multi-layer shells.
Post View layer results under the nodes labeled Ply1, Ply2, until PlyN. processing

22. Treatment of multi-layer shells and heat transfer phenomena

23. Multi-layer shells types
Uniform layer shell
Each layer has the same material and thickness.
Requires a odd number of layers.
Layers are modeled with a through-plane conduction.
For materials with a constant specific heat, each layer has the same capacitance.
The layer thickness is used to calculate the volume of the elements and the surface area of the element edges.
For isotropic materials, the material thermal conductivity is used to calculate a conductive heat transfer coefficient between each pair of adjacent layers.
For orthotropic materials, the Z-axis thermal conductivity property is used to create the conductive thermal couplings between layers.

24. Activities
NX Help / CAE
Advanced Simulation Tutorials:
Thermal analysis of a PCB with a chip

25. Solution Data
Available results include:
Temperature
Conductive Heat Flux
Temperature Gradient
Total Flux and Total Load
Residuals
Convection Coefficients
View Factor Sums, Net Radiative Flux, Radiosity Flux, and Irradiance Flux

26. Thermal Report
Report
Use a Report simulation object to output result values in text or HTML format.
Data for elements and regions are available depending on the solver used and the report type selected.
The Track During Flow Solve report type generates a text report in the Analysis Job Information dialog box.

27. Edit Post View
Color Display
Smooth
Banded
Iso-Lines
Spheres
Color Spectrum
Structural
Thermal
Thermal Inverted
Grayscale
Spotlight

28. Iso-surface plots
An isosurface represents points of a constant value (e.g constant pressures) within a volume of space.

29. Activity: Introduction to transient modeling
In this activity, you will:
Review simple concepts of heat transfer such as conduction and convection.
Set up a transient analysis and use a heater control.

30. Connecting meshes

31. What is a thermal coupling?
Is a conductance between unconnected, possibly non-aligned elements.
Is an additional heat path through which heat may flow.
Simulates a conductance based on values you specify and the area of overlap between the selected element sets.
Is typically intended to model conductance between parallel surfaces or edges, without explicitly modeling the heat transfer medium between them, example bolted joints, welds, pins, etc.

32. Types
Thermal Coupling
Interface Resistance
Surface-to-Surface Contact
Thermal Coupling — Radiation

33. Primary element selection and conduction modeling
In thermal couplings, primary element selection depends on the model you are analyzing:
Select the smaller segment as a primary region.
Only the nearest secondary elements will be coupled.
Each primary element is connected to one or more secondary elements, depending on proximity and overlap.
There may not be a connection to every secondary element.
The primary element selection does not control the direction of heat flow.

34. Primary element selection and conduction modeling
Correct Incorrect Primary region
Secondary region

35. Thermal couplings between overlapping or intersecting entities
The Only Connect Overlapping Elements option can be useful when the selected primary and secondary areas are different, at an angle, or not aligned.
This option controls how the thermal coupling connection is found and created between primary sub-elements and secondary elements.
Primary element with sub-divison
Secondary element
Element normal
Thermal coupling

36. Activity: Heat transfer analysis between a chip, PCB, and casing
In this activity, you will:
Learn basic concepts of thermal couplings.
Troubleshoot the most common errors faced when using thermal couplings.

37. Activity: Thermal couplings in electronic systems
In this activity, you will:
Calculate thermal couplings and apply an interface resistance.
Create thermal perfect contacts.

38. Activity: PCB heat transfer analysis
In this activity, you will:
Review concepts of part idealization, heat transfer, and thermal couplings.
Simulate a fan failure scenario using a transient thermal analysis.

39. Boundary Conditions

40. Thermal Loads
Thermal Loads
Use the Thermal Loads command to apply known heat sources that can be constant, time varying, and/or spatially distributed.
Types
Use Heat Load if you know the load in units of power, for example on a chip or stove plate model. (power)
Use Heat Flux if you know the value of heat entering through an area, for example, solar heat flux. (power/area)
Use Heat Generation if you know the value of heat generated in a volume, for example, burning coal. (power/volume)

41. Thermal Loads
Thermal Load options vary according to the Type selected:
Defining a Heater Control lets you control how the load is activated by a temperature sensor.
When using Multi–Layer Shells, you can define to which layer you apply the thermal load.

42. Thermal Constraints
Temperature
Use the Temperature constraint to apply known temperatures to a heat source or sink on the model.
You can define temperature values as:
Constant
Varying with time, heat flow rate, or thermal capacity spatially distributed
Varying with time, heat flow rate, or thermal capacity, and be spatially distributed
When using Multi–Layer Shells you can define to which layer you apply the temperature constraint.

43. Thermal Constraints
Convection to Environment
Use the Convection to Environment constraint to model free or forced convection without modeling the fluid. You can use different types according to the phenomena modeled.
Use this option when you know either the Convection Coefficient or
Parameter and Exponent and the fluid temperature.
Free Convection to Environment
Use this option when you want to use a specific free convection correlation.
Forced Convection to Environment
Use this option when you want to use a specific forced convection correlation.
Both in transient and steady state solves, the solver calculates a single convection coefficient value for the entire convecting surface based on the characteristic information you specify.

44. Thermal Constraints
Simple Radiation to Environment
Use the Simple Environment Radiation constraint to define radiation on surfaces for which you know emissivity and view factors. To define thermo-optical properties and the view factor, two options are available from the Type list:
Gray Body View Factor
The emissivity of the faces is obtained from the Thermo-optical properties
modeling objects you define in the mesh collectors.
You define the gray body view factor.
Effective emissivity
You define the emissivity of the faces you select.
The radiative environment temperature can be set to ambient or a value you specify.

45. Modifying global initial conditions
The options on the Initial Conditions tab of the Solution dialog box define initial values for the variables on the whole model.
Automatic sets the initial temperatures to the values specified on the
Ambient Conditions tab.
Uniform sets the initial values to the ones you specify. Use Uniform if the conditions differ from ambient conditions.
From Results In Other Directory sets the initial values to the thermal results from previous analysis using the same mesh. This option overrides any initial conditions constraint. You must specify the folder containing the results. The solver file (TEMPF format) in this folder defines initial conditions.
Perform Steady-State Solution sets the initial conditions of a transient run to the temperature results of an automatically run steady-state analysis (applicable only for transient analyses).

46. Activity: Heat distribution on a brake assembly
In this activity, you will:
Apply basic thermal boundary conditions.
Use basic thermal couplings to conduct heat between two components.

47. Solving

48. Solving a thermal analysis
Apply all thermal loads, initial, and boundary conditions on the meshed model.
In the Simulation Navigator, right-click on the Solution node. Select Edit.
In the Solution dialog box, on the Solution Details tab, select a Run Directory option to specify a location for the result files.
Select Steady State or Transient.
On the Transient Setup tab, review and modify the transient parameters.
On the Results Options tab, select the desired output result types.
(Optional) To modify the solver parameters, right-click the Solution node and select Solver Parameters.
In the Simulation Navigator, right-click on the Solution node and select
Solve.

49. Adaptive time stepping method
When you specify Automatic from the Time Step Option list, the thermal solver uses the adaptive time stepping method to automatically determine the time step sizes in a transient thermal simulation. This method handles the sharp changes in temperature at boundary conditions. When there are no abrupt changes in boundary conditions, it accelerates the simulation without losing accuracy. The time step size calculation is based on the estimated error between a quadratic fit and a linear fit of the solution in time.
You specify the temperature error tolerance, the minimum time step size, and the maximum time step size.

50. Thermal solver parameters
In the Solver Parameters dialog box on the Thermal Solver tab, multiple options are available to adjust thermal solution convergence:
Steady State Convergence Control
The convergence criteria between two steady state iterations can be set to either a ΔT, an iteration limit, or left to automatic.
Transient Convergence Control
For transient analyses, you can adjust the maximum temperature change between two iterations of a given time step. When the temperature change of all elements is below this number, the solution at that iteration has converged.

51. Radiation solver parameters
You can modify the accuracy and preferences to use in radiation calculations on the Solver Parameters tab.
You can control the accuracy and calculation of view factor residuals by modifying View Factor Adjustment and Radiation patches.
Activating On Screen Rendering can accelerate view factor calculation, but it will tie up the monitor. This option only improves performance when the run uses the same workstation for both processing (CPU) and display (graphics card and monitor).

52. NX Thermal Files
INPF primary input file GTEMP nodal temperatures TEMPF element centroid temperatures MSGF program messages REPF details of calculations VUFF view factors tmggeom.dat contains model and parameters tmgrslt.dat can be processed to create a universal file Recommend: set the results directory to solution name

53. Solution Monitor
The Solution Monitor provides you with information from the solver during the analysis.
Displayed information
Depending on the solver environment, the following information is displayed:
Solver version, running time, and run directory folder name.
Model summary. For example in the NX Thermal/Flow solver environment, this summary includes the number of entities, ambient conditions, internal mesh check, and boundary conditions.
Status of the solution and current module being executed by the thermal solver.
Convergence residuals at each iteration during the analysis.
Warnings and errors.

54. Activities
Radiation
Heat transfer analysis of an oven

55. Activities
Meshing and Material Properties:
Meshing and assembly FEMs review

56. Smarter decisions, better products.
SIEMENS PLM Software
GTAC
Smarter decisions, better products.
Student Guide November, 2013
MT15032-s-NX 9