Define convection boundary conditions using ducts

Learn how to model gas turbine thermal networks using a 1D duct approach.

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 build a physics-based thermal model of a gas turbine using a 1D duct approach. You will:

  • Set up a duct cooling network and model convection between ducts and components.
  • Apply wall rotation and swirl ratio corrections to convective boundary conditions.
  • Apply a heat load to account for windage heating.
  • Reference duct outlet conditions within other boundary conditions.
  • Post-process and validate key thermal results.

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.

Understand flow directions

Review the cooling flow network and determine how thermal boundary conditions will be applied.

The first step in defining thermal boundary conditions is to clearly understand the direction and nature of the flow. This depends on the information available. Typically, data comes from a 1D secondary air system model and includes mass flow rates, pressures, temperatures, and swirl ratios.

Understanding these flow features is essential before selecting and applying appropriate thermal boundary conditions.

  1. Review the provided flow schematic and identify blade cooling path (1) flow, disk cavity purge flow (2), vane cooling flow (3), co-rotating cavities (4) with no throughflow with unknown flow directions, and external constant temperature regions (5).


  2. Based on the identified flow directions and the available input data, the following thermal boundary conditions are applied to represent the physics of the system:
    • An external Thermal Convecting Zone is defined to model heat exchange between the engine exterior and the surrounding environment.
    • On the 1D ducts, the primary flow quantities are prescribed such as mass flow, total pressure, and inlet temperature.
    • Thermal Coupling – Convection to connect the 1D fluid elements to the surrounding wall surfaces.
    • Duct Node Convective Coupling in regions such as cavities where flow is assumed to be well mixed to connect a single duct node to multiple surrounding surfaces.


Apply the convective zone

Define the external engine convection using condition sequence parameters.

  1. Choose File→Open and open thermal_bcs_ducts\HPT_sim.sim.
  2. Choose File→Preferences→User Interface and on the Dialog and Precision page, reset the dialog box memory.
  3. Click OK.
  4. Choose Home > Load Type > Thermal Convecting Zone to apply the convective zone.
  5. Select the external 32 edges as shown.


  6. In the Environment group, set:
    • Fluid Materials = Air
    • Temperature = T20 °C
    • Pressure = P20 MPa
    • Heat Transfer Coefficient = 20 W/(m2·°C)

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

  7. Click OK.

Setup the 1D duct network for secondary cooling flow

Define duct boundary conditions and convective couplings for the secondary cooling flow.

  1. Press Ctrl+W and click Mesh Points to inspect node locations.


    Mesh points are used in the 1D duct network to maintain associativity when the fluid mesh is regenerated. If boundary conditions are applied directly to selected nodes and the model is re-meshed, those node selections may be lost, requiring all node-based boundary conditions to be redefined. Mesh points prevent this issue by forcing a node to be created at a specified location during meshing, ensuring consistent and robust boundary condition assignment.

  2. In the Simulation Navigator, expand the Groups node, click SAS_Network to display the curves associated to the 1D fluid network.
  3. Choose Home > Loads and Conditions > Simulation Object Type > Duct Flow Boundary Conditions to define pressure for the secondary cooling flow.
  4. From the Type list, select Duct Total Pressure.
  5. Select the following mesh point.


  6. In the Total Pressure - Gauge box, type 0.6*P26 MPa.
  7. Click Apply.
  8. Apply the same duct boundary condition to the indicated mesh point, using a pressure of 0.5*P26 MPa.


  9. In the Simulation Navigator, click next to boundary conditions to hide their display in the graphics window.
  10. Choose Home > Properties > More > Condition Sequences to inspect the defined conditions..
    Because this is a transient analysis, certain quantities may need to be scaled using condition sequence parameters.
  11. On the Parameters tab, press Ctrl, click NH and SCALER, right-click the selection and select Plot XY.


    The SCALER parameter is a dimensionless quantity that scales from 0 to 1 based on speed. It can be used to scale quantities that are defined at full load or full power and need to be scaled through the transient cycle.
  12. Apply the Duct Fan/Pump type of the duct boundary condition to the selected curves, and set the mass flow rate of 1*SCALER kg/s.


    Note:
    The 1D element type Duct with Mass Flow Axisymmetric is used. This is the recommended duct type when importing results from an external 1D solver. It does not compute mass flow from pressure; the mass flow must be defined directly.

    For radiation and convection calculations, the axisymmetric elements are expanded circumferentially according to the Number of Axisymmetric Segments specified in Thermal Solution Parameters solver settings.

    Increasing the number of segments can improve accuracy, but it also increases solution time.

  13. Choose Home > Loads and Conditions > Constraint Type > Temperature to define the inlet temperature.
  14. From the Selection Method list, select Duct Fluid Nodes and select the shown node.


  15. In the Temperature box, type T26 °C.
  16. Click OK.
  17. Choose Home > Loads and Conditions > Simulation Object Type > Thermal Coupling - Convection to model convection between the fluid duct elements and the rotor surface elements.
  18. Select the shown edges.


  19. Set the following settings:
    • Heat Transfer Coefficient = 100 W/m2·°C
    • Rotational Effects = Correct for Wall Rotation
    • Swirl Velocity = nh*radius mm/s to account for swirl velocity and rotational total temperature effects.

    Since Correct for Wall Rotation is specified, the solver will correctly calculate the relative temperature to use for convection based on applied rotation, swirl velocity, and radius. The simplified equation to convert total absolute to total relative temperature considering 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.
  20. Apply convective coupling with the same settings to the shown edges:


  21. In the Thermal Coupling - Convection dialog box, from the Type list, select Duct Node Convective Coupling to define a duct node convection coupling for the large cavity with zero radial through flow.
    This cavity flow is expected to be complex due to disk pumping effects and net axial throughflow. For simplicity, we will assume it is well mixed by applying a duct node convective coupling, which connects a single node (mesh point) to multiple surfaces.
  22. Select the shown 31 edges as convecting region, and shown node as a fluid duct node.


  23. Set the following settings:
    • Heat Transfer Coefficient = 100 W/m2·°C
    • Rotational Effects = Correct for Wall Rotation
    • Swirl Velocity = nh*radius mm/s to account for swirl velocity and rotational total temperature effects.
  24. Select all the created duct and convective couplings boundary conditions and move under the HPT_1D_Disk_ID folder.

Setup the 1D duct network for blade cooling flow

Review the flow splits, define duct labels, and convective couplings for the blade cooling system.

  1. Inspect the applied pressures, duct temperature constraints, and prescribed mass flows on the duct elements as shown below. Note that a flow split exists where the mass flow has not yet been defined.


  2. Inspect the flow direction for the shown ducts.


  3. In the Simulation Navigator, display the 1D mesh.
  4. Choose Home > Checks and Information > More > 1D Element Directions to display the directions of 1D elements.
  5. From the list, select Displayed.


    Notice that the displayed flow direction is incorrect. If this occurs, reverse the element direction in the FEM file to ensure the flow is oriented properly.

  6. In the Simulation Navigator, right-click the FEM file and select Open in Window.
  7. Expand 1D Collectors > Duct , right-click 1d_mesh(2) and select Unlock.
  8. Click Update.
  9. Right-click Duct and select Check All > Duplicate Nodes and click List Nodes.
    Notice the duplicated nodes.
  10. Select Displayed and click Merge Nodes.
  11. Display the element direction again for 1d_mesh(2) and click Reverse Directions.
  12. Switch to the Simulation file window.
  13. Apply a Duct Fan/Pump type of the duct boundary condition to define a mass flow of 0.8*0.05*W26 kg/s at the flow split as shown.


    When the mass flow is defined in one branch, the mass flow in the other branch is automatically determined by conservation of mass.

  14. In the Simulation Navigator, under Loads, in the HPT_StreamOnFace folder, right-click the HPT_Stream 21 node and select Edit to examine the flow split where part of the supply air goes to the blade and part of the air purges into the flowpath. Notice the use of DMO(), DTO(), and DPO() which allows you to connect boundary conditions to ducts.
    The following schematic shows flow directions.

    For this connection to work, a Duct Label type boundary condition is created on the 1D fluid mesh at the shown location, where data is to be extracted. The labeled location must be at a free end.

  15. Inspect the Duct_Label_at_Blade boundary condition. Observe that its Boundary Condition ID = 573 is referenced in the DMO(), DTO(), and DPO() expressions within HPT_Stream 21.
  16. Choose Home > Loads and Conditions > Simulation Object Type > Thermal Coupling - Convection to apply the following convective coupling for the blade cooling.
    These may use built-in correlations such as HTCFORCE or user-defined expressions. The inputs to these correlations do not always accurately represent the true geometry and often rely on simplified estimates of hydraulic diameter. The details of the built-in expressions can be reviewed in the Expression dialog box. In practice, users commonly develop their own custom expressions and reference them in the convective boundary conditions to better reflect the specific geometry and flow conditions.
    • For the blade_supply_cc1 coupling, select:
      Convecting Region (4 edges) Fluid Ducts (3 curves)




      Use the following settings:
      • Heat Transfer Coefficient = HTCFORCE(30[mm],"DUCT_FULL") W/m2·°C
      • Only Connect Overlapping Elements is cleared
    • For the blade_supply_cc2 coupling, select:
      Convecting Region (22 edges) Fluid Ducts (1 curve)




      Use the following settings:
      • Heat Transfer Coefficient = HTCFORCE(30[mm],"DUCT_FULL")
      • Only Connect Overlapping Elements is cleared
      • Rotational Effects = Correct for Wall Rotation
      • Swirl Ratio = 0.55
    • For the blade_supply_cc3 coupling, select:
      Convecting Region (4 edges) Fluid Ducts (3 curves)




      Use the following settings:
      • Heat Transfer Coefficient = HTCFORCE(30[mm],"DUCT_FULL")
      • Only Connect Overlapping Elements is cleared
      • Rotational Effects = Correct for Wall Rotation
      • Swirl Velocity = 0.55*nh*radius
    • For the blade_supply_cc4 coupling, select:
      Convecting Region (7 edges) Fluid Ducts (3 curves)




      Use the following settings:
      • Heat Transfer Coefficient = 100 W/m2·°C
      • Only Connect Overlapping Elements is cleared
    • For the blade_supply_cc5 coupling, select:
      Convecting Region (1 face) Fluid Ducts (1 curve)




      Use the following settings:
      • Heat Transfer Coefficient = 6.35*PI()*10.99*20/6.35/11*100 W/m2·°C
      • Only Connect Overlapping Elements is selected

      When applying convective couplings to faces that represent holes or other plane stress regions, you must apply appropriate scaling factors to the heat transfer coefficient to ensure the correct convective heat transfer.

      In this example, the HTC is defines as 6.35*PI()*10.99*20/6.35/11*100. This represents , where:

      • HTC = 100
    • For the blade_supply_cc6 coupling, select:
      Convecting Region (11 edges) Fluid Ducts (2 curves)




      Use the following settings:
      • Heat Transfer Coefficient = HTCFORCE(100[mm],"DUCT_FULL")
      • Only Connect Overlapping Elements is cleared
      • Rotational Effects = Correct for Wall Rotation
      • Swirl Velocity = 0.5*nh*radius
    • For the blade_supply_cc7 coupling, select:
      Convecting Region (19 edges) Fluid Ducts (1 curve)




      Use the following settings:
      • Heat Transfer Coefficient = HTCFORCE(30[mm],"DUCT_FULL")
      • Only Connect Overlapping Elements is cleared
      • Rotational Effects = Correct for Wall Rotation
      • Swirl Velocity = 0.5*nh*radius
    • For the purge_cc8 coupling, select:
      Convecting Region (16 edges) Fluid Ducts (1curve)




      Use the following settings:
      • Heat Transfer Coefficient = HTCFORCE(30[mm],"DUCT_FULL")
      • Only Connect Overlapping Elements is cleared
  17. Move all created boundary conditions under the HPT_1D_Blade_Cooling folder.
  18. Choose Home > Loads and Conditions > Load Type > Thermal Loads to apply a heat load directly to the duct which will account for windage rise in a rotor stator cavity.


  19. In the Heat Load box, type 1*SCALER W.

Solve and post process

This model is configured for both thermal and structural analysis. However, since this activity focuses on the thermal setup, we will run the thermal solver only to reduce computational time. Solve and verify fluid temperatures, pressures, mass conservation, and applied heat loads.

  1. In the Simulation Navigator, right-click Mission1 and select Edit.
  2. On the General page, in the Solve Options group, clear the Solve Structural check box.
  3. Right-click the Mission 1 node and select Solve.
  4. In the Post Processing navigator, double-click Thermal and expand Thermal > Time700.00 > Increment 5,700.00s and double-click Total Absolute Fluid Temperature on Wall - Nodal at walls where wall rotation correction is applied.


    The temperature ranges from approximately 15 °C to 1626 °C.

  5. Double-click Total Relative Fluid Temperature on Walls - Nodal to display relative fluid temperature.


  6. Double-click Fluid Pressure on Walls - Nodal to display pressure on walls.


  7. Compare 1D Flow Temperature with Total Absolute Fluid Temperature on Walls.
    Enable the 1D solver elements via the post view and verify that the total absolute temperature at the wall matches the corresponding 1D flow temperature for the specified connections.
  8. Inspect Mass Flow Junction Imbalance to confirm conservation of mass.
    If this value is non-zero, it indicates that mass is not conserved at a junction (for example, at a duct node connected to three elements).

  9. Verify applied heat load magnitude using Identify Results and confirm the total heat equals 1[W] when SCALER=1.
    Tip: Select all the elements on that fluid duct portion where the thermal load is applied.

Additional notes

  • Thermal contacts are typically required in production models, though omitted here because this activity focuses on convective thermal boundary conditions.
  • An alternative modeling approach uses streams and voids instead of explicit 1D ducts. In this approach, the thermal solver automatically generates the fluid elements used for convection, and the user connects the fluid network through junctions and expressions.
  • When including radiation in axisymmetric models, use the Monte Carlo view factor method for improved accuracy.