Define convection boundary conditions using thermal streams and voids

Learn how to model gas turbine thermal networks using thermal streams, voids, and convective zones.

Download and extract the part files.

Introduction

There are multiple approaches to modeling thermal networks in gas turbine engines. The appropriate method depends on the available input data, overall engine behavior, transient operating conditions, and engineering judgment.

In this tutorial, you will learn how to:

  • Apply Thermal Stream, Thermal Void, and Thermal Convective Zone.
  • Apply wall rotation and swirl ratio corrections to convective boundary conditions.
  • Use the Auto-Connect stream option with a junction.

Load the thermal plugin

Enable the ExpressionsPlugin.dll if not already active. The predefined boundary conditions in the model use heat transfer coefficient (HTC) correlations implemented through custom expressions. These correlations require the plugin to evaluate correctly during the simulation.

  1. Choose File > Utilities > Customer Defaults.
  2. Click Simulation, expand Pre/Post, and scroll to Expressions.
  3. On the Plugin tab, select the Use Custom Plugin check box and in Custom Plugin, type the full path to the ExpressionsPlugin.dll file, as plugin\ExpressionsPlugin.dll.
  4. Click OK, exit Simcenter 3D, and restart the application to activate the plugin.

Define assembly load options

Configure search folders to load a model whose part and FEM files are stored in multiple directories.

  1. On the Home tab, click Assembly Load Options .
  2. From the Load list, select From Search Folders.
  3. In the Add Folder to Search box, browse to the convective_bcs folder.
  4. Append \... to the end of the path so it appears as convective_bcs\....
    Adding \... allows the software to search the selected folder and all its subfolders. However, this option is not recommended for complex or deeply nested directory structures, as it may significantly increase search time.
  5. Click Add to include the path in the search list.
  6. Remove any existing paths that are no longer needed.
  7. Click OK.

Understand flow directions

Review the cooling flow network before applying thermal boundary conditions. The direction and behavior of the flow depends on the data available to you. Typically, this information comes from a 1D secondary air system model and includes mass flow rates, pressures, temperatures, and swirl ratios.

  1. Review the flow schematic and identify: dead cavities, rotor-rotor cavities with leakage, and regions of constant external temperature.
    • A dead cavity with no throughflow.
    • A rotor-rotor cavity with a small amount of leakage passing through it, and there may be recirculating flows due to pumping effects.
    • A constant air temperature at the exterior of the engine.


  2. Apply the thermal boundary conditions as outlined below, based on the identified flow directions and available data. For simplicity in this tutorial, constant values will be used for the convective heat transfer coefficients. In practice, it is recommended to use appropriate correlations such as HTCFORCE, or functions implemented through custom thermal plugins to obtain more realistic results.


Apply the thermal convective zone

Define external convection using condition sequence parameters.

  1. Choose File→Open and open convective_bcs\ASSY\GT1_sim.sim.
  2. Choose Home > Load Type > Thermal Convecting Zone to apply the convective zone.
  3. Select the external 18 edges as shown.


  4. In the Environment group, set:
    • Fluid Materials = Air
    • Temperature = T20 °C
    • Pressure = P20 MPa
    • Heat Transfer Coefficient = 10 W/(m2·°C)

    Note that the convection definition references the condition sequence parameters T20 and P20.

  5. Click OK.

Apply thermal streams

Create one-sided and connected streams, reference upstream conditions, and apply wall rotation corrections.

  1. Choose Home > Load Type > Thermal Stream to apply a one-sided edge stream.
  2. From the Type list, select One-Sided Stream on Edges.
  3. Select the shown 7 edges.


  4. In the Fluid group, from the Fluid Materials, select Air.
  5. In the Stream Conditions group, set:
    • Mass Flow = 0.01*W26 kg/s
    • Inlet Temperature = T26 °C
    • Absolute Pressure = P26 MPa
    • Heat Transfer Coefficient = 100 W/(m2·°C)
    • Heat Pickup = 0
    • Enable Activate Rotating Machinery Options
    • Rotational Effects = Correct for Wall Rotation
    • Swirl Ratio = 1

      A swirl ratio of 1 is assumed, meaning the fluid core rotates at the same angular velocity as the surrounding rotor.

    Because Correct for Wall Rotation is selected, the solver automatically computes the appropriate total relative temperature for convection. This calculation accounts for the applied rotational speed, local radius, and specified swirl ratio.

    A simplified form of the equation used to convert total absolute temperature to total relative temperature assuming constant Cp​ is shown:

    Where:
    • is a swirl ratio.
    • is a rotor speed in rad/s.
    • is a specific heat of the air.
    • is a radius.
    • is a total relative temperature.
    • is a total absolute temperature.
  6. Record the Boundary Condition ID of the created stream, which will be used in the next steps. In this example, it is 178.
  7. Create another thermal stream as shown.


  8. In the Name group, include %%ID in the stream name to automatically reference its BC ID.
  9. In the Fluid group, from the Fluid Materials, select Air.
  10. Reference the outlet conditions of the first stream using its boundary condition ID. During the solve the outlet conditions of the stream with ID 178 will be transferred to the newly created stream.
    • Mass Flow = smo(178) kg/s
    • Inlet Temperature = sto(178) °C
    • Absolute Pressure = sp(178) MPa
  11. Set the following settings:
    • Heat Transfer Coefficient = 100 W/(m2·°C)
    • Heat Pickup = 0
    • Enable Activate Rotating Machinery Options
    • Rotational Effects = Correct for Wall Rotation
    • Swirl Ratio = 1
  12. Create a third stream as shown.


    Make sure that the direction of the stream as shown. If needed, click Reverse Direction.

  13. In the Name group, include %%ID in the stream name to automatically reference its BC ID.
  14. In the Auto-Connect Options group, select Automatically Determine Mass Flow and Automatically Determine Inlet Temperature.
    When you use these options in the thermal streams to automatically connect them, the thermal solver automatically determines the thermal stream mass flow and inlet temperature from other thermal streams to which they are connected creating a flow path.
  15. In the Absolute Pressure box, type sp(179).
  16. For the other settings, use the same as in the previous stream.
  17. Choose Home > Simulation Object Type > Junction to connect thermal stream 179 to the stream 180.
  18. In the Incoming Streams group, from the Stream 1 list, select Thermal Stream 179.
  19. In the Outgoing Streams group, from the Stream 1 list, select Thermal Stream 180.

Apply thermal voids

Define cavity regions and account for heat exchange using void boundary conditions.

  1. Choose Home > Load Type > Thermal Void to create a void region in the first cavity.
  2. In the Regions 1-5 group, click Create Region and select the shown 11 edges.


    If different heat transfer coefficients are required within the void, define multiple regions to assign separate HTC values accordingly.

  3. Set the following settings:
    • Pressure = sp(178)) MPa
    • Heat Transfer Coefficient = 50 W/(m2·°C)
    • Enable Activate Rotating Machinery Options
    • Rotational Effects = Correct for Wall Rotation
    • Swirl Ratio = 1
  4. Use Air as a fluid material, and set Heat Load to 0 W.
  5. Create a second thermal void in the rotor–rotor cavity. Apply a heat load using the pwr() function to transfer energy from the passing stream into the void.
    In this example, it is assumed that 1% of the main cooling mass flow passes through this cavity. Therefore, apply a scaling factor of 0.01 within the pwr() function as 0.01*pwr(180).
  6. For the region, select the following 9 edges and apply the same settings as for the previous void, except set the pressure to sp(180).


Apply stream for the leakage air

Model leakage convection using a two-sided edge stream.

  1. Apply a two sided edge stream to model convection of the leakage air on the highlighted edges.

    Two sided is required here because there are two free edges at this interface. If an internal edge were used by stitching the edges, using a single sided edge stream with the Add thickness option would be appropriate.



  2. In the Path Selection -Side A, select the edge of the rotor stage for disk 3
  3. In the Path Selection -Side B, select the edge of the rotor stage for disk 4.
  4. Make sure that the direction of the stream as shown in the image.


  5. Include %%ID in the stream name to automatically reference its BC ID.
  6. Set the following settings:
    • Mass Flow = 0.001*smo(180) kg/s
    • Inlet Temperature = VT(22) °C
    • Absolute Pressure = VP(22) MPa
    • Heat Transfer Coefficient = 100 W/(m2·°C)
    • Heat Pickup = 0
    • Enable Activate Rotating Machinery Options
    • Rotational Effects = Correct for Wall Rotation
    • Swirl Ratio = 1

Solve and post process

Validate fluid temperatures, pressures, and mass conservation.

  1. Solve the solution.
  2. In the Post Processing navigator, expand the Thermal > Time700.00 > Increment 10,700.00s and double-click Total Absolute Fluid Temperature on Wall - Nodal.


  3. Double-click Fluid Pressure on Walls - Nodal at walls where wall rotation correction is applied.


  4. Double-click Mass Flow Junction Imbalance to verify conservation of mass.


    If this magnitude is non-zero, this means mass has not been conserved at the junction between streams.

Additional notes

  • The Automatically Connect option works for junctions and for streams that geometrically share endpoints.
  • Thermal contacts are typically required in production models but were omitted here to focus on convective boundary conditions.
  • An alternative approach is to mesh 1D ducts and use Thermal Coupling – Convection to connect ducts to component surfaces.