Model radiation on 3D components

Model radiative heat transfer on 3D components and compare full radiation solutions with simplified approaches and evaluate solve time trade-offs.

Download and extract the part files.

Introduction

Include radiation in a gas turbine model when radiative heat transfer significantly affects component temperatures. The decision to model radiation depends on engine design, engineering judgment, test data, and available analysis tools. Radiation becomes important when:
  • Components have large temperature differences and they have a view factor with each other.
  • Convection effects diminish, for example during shutdown when fluid flow throughout the engine decreases.

Typical regions in gas turbines include:

  • The exhaust assembly, where hot diffusor walls radiate to cooler outer casings.
  • The turbine inlet, where flowpath components are exposed to combustion gases or flame.

When modeling radiation, evaluate the following:

  • Can you represent components with 2D elements?
  • Can you apply cyclic symmetry?
  • Can you divide the model into multiple enclosures to reduce solve time?
  • Can you accept reduced accuracy to achieve significantly faster solve times?

In this tutorial, you will:

  • Set up a radiation enclosure on 3D components.
  • Evaluate methods to reduce radiation solve time.
  • Inspect results and reports from a cyclic symmetry model with periodic radiation.
  • Inspect results and reports from a cyclic symmetry model without periodic radiation.
  • Build a simplified radiation model that solves quickly.
  • Compare the results of different modeling approaches.

Load and inspect the model

Inspect components, identify radiating regions, and review boundary conditions.

  1. Choose File→Open and open radiation/strut_sim1.sim.
  2. Inspect the geometry and identify major components of the 1/5 cyclically symmetric sector.


  3. Determine which components will radiate with each other.
    The most critical radiation region is between the strut shield and strut due to a high view factor and large temperature difference. The next most important region is between the diffusor and casing, due to large surface area, high view factor, and large temperature difference.
  4. Inspect the applied boundary conditions.
    Temperature constraints are applied on:
    • The outer casing
    • The inner casing
    • The diffuser
    • The strut shield
  5. Select the temperature constraints, right-click the selection and select Plot Contour to visualize the applied temperatures.


Create a Radiation Enclosure on 3D surfaces

Define a radiation enclosure using Monte Carlo view factor calculation.

In this analysis, you can choose from several modeling approaches. For example, you could divide the radiation enclosure into three separate regions as shown to reduce solve time, but this approach would reduce accuracy.

In this configuration, some surfaces in enclosure 1 would physically see surfaces in enclosures 2 and 3. However, dividing the model into separate enclosures eliminates many shadowing and visibility checks, which reduces computational cost but decreases physical accuracy.
  1. Activate the Radiation solution.
  2. Choose Home > Loads and Conditions > Simulation Object Type > Radiation to define a single radiation enclosure for maximum accuracy.
    Use this approach when you need to compute view factors only once, extract them, and reuse them in subsequent analyses.
  3. For the Top Side Region, select the inside surfaces of the casing, the outer surfaces of the diffusor, the strut, and the inside of the strut shield.


    34 surfaces are selected.

  4. Make sure that the Monte Carlo method is selected.
  5. Next to Monte Carlo Settings, click Edit and make sure that Number of Rays is set to 2000.
    You can increase the ray count if required until results converge within the required accuracy.
  6. Click OK.

Create a Cyclic Symmetry simulation object

Enable periodic radiation in a cyclic symmetry model.

  1. Choose Home > Loads and Conditions > Simulation Object Type > Cyclic Symmetry .
  2. For the Source Region, select the shown faces.


  3. For the Target Region, select the shown faces.


  4. In the Direction group, select the Dynamic coordinate system.
  5. Click Calculate Segment.
    Notice that he number of calculated segments is 5.
  6. In the Stages group, select 2 solid bodies and set the Stage Number (STAGE) to 1 to ensure that the solver scales mass, conductivity, and surface area correctly.
  7. In the Additional Thermal Parameters group, select the Enable Radiative Thermal Rotational Periodicity check box.
    The solver copies these elements across all cyclic sectors and calculates view factors between all surfaces.
  8. Click OK.
    View factor calculations for the enclosure defined in the previous step can take a long time to solve. Use the following approaches to reduce solve time:
    • Subdivide solid bodies and apply swept meshes to reduce the number of surface elements.
    • Reduce the number of heat flow reports, as they require additional iterations.
    • Run the thermal solver in parallel.
    • Use radiation patches. You can define them either in Advanced Parameters or in the solver options under Radiation; note that these methods behave differently.

    In this tutorial the analysis uses the unmodified setup, which results in the longest solve time compared to the approaches listed above. The goal is to compute the view factors once with radiation reports enabled and then reuse them in a simplified analysis.

Apply Heat Map reports

Apply Heat Map reports to several regions where we are interested in view factors.

  1. Choose Home > Loads and Conditions > Simulation Object Type > Report .
  2. From the type list, select Heat Maps.
  3. In the Name box, type outer_case.
  4. Select the shown faces.


  5. Apply the following Heat Map reports.
    Name Selection
    outer_diff

    outer_strut

    inner_case

    inner_diff

    inner_strut

    strut_shield

    mid_strut

Solve a solution and inspect results

Inspect ther esults..

  1. Solve a solution or skip that step and access the report data and results from the radiation folder.
  2. Open and inspect GroupReport.csv from the results directory.
    The report lists heat flows between all groups for which heat map reports were defined, along with the corresponding view factors (see line 3680). The file contains 36 view factors, and 16 of these entries have a black body view factor equal to 0.

    To simplify the model, select and apply only the largest view factors, as shown below.

    From To Black Body View Factor (ij) Gray Body View Factor (ij) ScriptF(ij)
    Outer
    outer_case outer_diff 3.9996762 3.3517566 2.6814077
    outer_case outer_strut 0.52274847 0.43345985 0.3467682
    outer_diff outer_strut 0.18255374 0.22132197 0.17705737
    Middle
    strut_shield mid_strut 2.7622998 2.2735267 1.8188225
    Inner
    inner_case inner_diff 4.2297301 3.5060833 2.8048646
    inner_case inner_strut 0.48406899 0.51408845 0.41127047
    inner_diff inner_strut 0.99462914 0.84458834 0.67567039
    Note:
    Divide the reported values by the number of cyclic sectors when applying them in a Thermal Coupling – Radiation simulation object.

    Use the Gray Body View Factor option for thermal coupling radiation objects, as it accounts for diffuse reflections.

Apply radiation thermal couplings and solve the simplified radiation solution

Replace enclosure radiation with gray-body thermal couplings.

  1. Activate the Radiation_Simplify solution.
  2. Choose Home > Loads and Conditions > Simulation Object Type > Thermal Coupling - Radiation.
  3. In the Primary Region group, select the shown faces.
  4. In the Secondary Region group, select the shown faces.
  5. From the Type list, select Gray Body View Factor.
  6. In the Gray Body View Factor box, type 3.352/5.
  7. Clear the Only Connect Overlapping Elements check box to couple the entire primary region to the nearest secondary region.
  8. Create another six radiation couplings based on dominant view factors.
    Primary Region Secondary Region Gray Body View Factor
    0.433/5
    0.221/5
    2.274/5




    		3.506/5




    		0.514/5




    		0.845/5
    Note:
    The emissivity of the mesh collector will be used in the radiation calculation along with grey body view factor to calculation radiative heat flow.
  9. Edit the Cyclic Symmetry and make sure that the Enable Radiative Thermal Rotational Periodicity is cleared.
  10. Solve the Radiation_Simplify solution.

Use Result Probes to compare results

Compare full radiation, simplified radiation, and non-periodic radiation cases.

  1. Choose Results > Manipulation > Result Probe to extract the averaged nodal temperature of the strut region which is most influenced by radiation.
  2. Set the following:
    • Load Case = All
    • Selection Type = Nodes
    • Select the strut region which is most influenced by radiation.
    • Select the Combine Across Entities check box.
    • Combined Value = Average
    • Unit = °C
    • Output Options = List
    • Clear the Create Output check box
  3. Right click the result probe in each of the three solutions available, and select Create Graph.
  4. In the Post Processing Navigator, with one of the graphs already displayed, press CTRL to select the two other graph items, and choose Overlay.

    From these curves, the following conclusions are:

    • The simplified radiation model predicts an average temperature within approximately 6 °C of the full radiation model at steady-state conditions.
    • The close agreement between the full radiation model and the simplified model validates the use of gray body view factors and radiative conductances in the simplified approach.
    • Disabling periodic radiation in the enclosure produces significantly different results, demonstrating that this approach does not adequately capture radiative behavior.

Additional Notes

  • Avoid excessive heat map reports to reduce solve time.
  • Review solve times:
    • Radiation Enclosure with cyclic radiation: 26 hr 11 min
    • Simplified radiation model: 4 min
    • Radiation enclosure without cyclic radiation: 43 min