How to extract and track clearances in gas turbine engines?

This article describes methods on how to extract and track clearances across the whole engine modeling (WEM).

Introduction

One of the primary purposes of a whole engine model is to calculate clearances for the entire engine, including blade tips, labyrinth seals, rim cavities, and axial gaps in the flow path. Understanding these clearances in gas turbine engines is important for predicting performance, improving efficiency, and identifying potential contact between parts. Gas turbine design teams often optimize clearances by conducting sensitivity analyses. These analyses involve varying engine geometry, materials, cooling networks, quantities, and transient engine behavior to achieve the best possible performance.

Understanding the evolution of clearances in a gas turbine engine allows you to:​
  • Maximize the performance of both the turbine and compressor by maximizing the energy extracted from the flow in the turbine​ and the pressure ratio in the compressor​.
  • Maximize gas turbine efficiency.
  • Understand and mitigate the risk of rubbing.​
  • Define rubbing allowance.
  • Define cold build clearances and recommend safety margins​.
  • Optimize any active clearance control technology​.
  • Ensure aerodynamic stability in the compressor.
  • Anticipate engine behavior under various operating conditions.

From WEM, you can compute radial tip, axial, and labyrinth seals clearances as shown.

Partial WEM cross section showing radial tip, axial, and labyrinth seals clearances.

These clearances are critical for maintaining engine efficiency and preventing damage. For instance, large tip clearances allow primary flow path air to leak around the airfoil, which reduces compressor efficiency. Similarly, large seal clearances enable flow to recirculate back to the previous stage, further decreasing compressor efficiency. On the other hand, excessively tight clearances can lead to blade loss, causing vibration issues and potential destruction of downstream components. Thus, clearance analysis involves finding the optimal balance between engine efficiency and reliability.

Extracting and tracking clearance data

You can extract clearance data using one of the following methods.

Extracting displacements manually
You can display the displacement results and create graphs in the Post Processing Navigator.
  1. Plot the displacements.
  2. Choose the Create Graph command and select, for example the leading and trailing edge nodes on the blade and static side as shown.
    Graph dialog box with four nodes selected representing the blade's leading and trailing edge nodes and their corresponding nodes on the static side that are displayed in the graphics window next to the dialog box.
  3. Right-click the curves in the graph and select Export to export the data to the CSV format.
    Time history curves showing the X displacement of the four nodes previously selected and the context menu displaying the selected Export command.

This manual method of selecting nodes during post-processing is quick and requires no advanced setup. However, it cannot be easily repeated between solves, and the data series names do not reflect the spatial location of the data. As a result, further post-processing becomes more tedious, as node numbers must be visually tracked—introducing inefficiencies and increasing the risk of errors in the analysis.

Using result variables and result probes
A result variable is used to reference and extract data from the solver results file. It is then used as a definition input in a result probe formula.
  1. Create a result variable, for example, for Cartesian X displacements.
    Result Variable dialog box with selected options for creating Cartesian X displacement result variable.
  2. Create a result probe, for example, on the points of the leading and trailing edges of the compressor blade.
    Displayed stage compressor blade with leading and trailing edge nodes selected and opened Result Probe: Baseline dialog box.

    You can also create a single result probe on multiple points to extract all data at once.

  3. In the Simulation Navigator, right-click the result probe, and choose Create Graph to plot the graph.
  4. Export the graphs to the CSV format for further post processing.
Using an expression in the result probes
You can create an expression that calculates clearances at specific locations and under certain conditions, for example, transient scenarios, and reference it in the result probe.
  1. Use the Point command to define a point in space or on the model where you want to extract specific results before setting up result probes.
  2. Specify the point name by right-clicking the point and selecting Properties. For example, STATOR_1, ROTOR_1, STATOR_2, ROTOR_2, STATOR_3, ROTOR_3, STATOR_4, and ROTOR_4.
  3. Create a result probe and define it as a formula to extract clearance.
    • For example, to extract the cold build clearance at the location with the minimum gap between the labyrinth seal teeth, use the following expression: 1.2954+MINIMUM((DX("STATOR_1")-DX("ROTOR_1")),(DX(STATOR_2")-DX("ROTOR_2")),(DX("STATOR_3")-DX("ROTOR_3")),(DX("STATOR_4")-DX("ROTOR_4")))
      Result Probe dialog box showing the expression in the formula field and the labyrinth seal in the graphics window.
    • To extract the change in the displacements in the radial and axial directions, use the built-in functions: DR("POINT_1")-DR("POINT_2") and DZ("POINT_1")-DZ("POINT_2") respectively.

      This result probe calculates the relative movement between the rotor and stator points. You can also include the cold gap based on the initial position of points, using the X() function with point names as arguments.

  4. In the Simulation Navigator, right-click the result probe, and choose Create Graph to generate a graph of the gap values at each transient time step to determine the clearance gap distance.
  5. Export the graph to the CSV format for further post processing.
Extracting gap distance from contact results
The Final Contact Separation result set displays the gap distance between the two sides of the contact boundary condition throughout the transient cycle. To enable this request, in the Structural Output Request, on the Contact Result page, check Enable BCRESULTS Request check box, and select SEPDIS from the Separation and Slide Distance list before solving the solution. This helps identify potential contact points and rubbing issues.

Final Contact Separation result set shown in the Post-Processing Navigator and displayed in the graphics window.

  1. Create a result variable. Make sure to select Final Contact Separation from the Result Type list.

    Result Variable dialog box with Final Contact Separation selected in the Result Type list.

  2. Create a result probe, referencing the defined result variable.
  3. In the Simulation Navigator, right-click the result probe, and choose Create Graph.
  4. Export the graph to the CSV format for further post processing.

When creating a contact between two edges to obtain the gap distance, set the Interface Behavior (INTRFC) to Inactive in the Contact Parameters -Local Pair Overrides modeling object. This allows the Nastran solver to compute gap distance results without influencing the structural response of the model, such as displacements or stresses. In this scenario, the inactive joint is used solely to obtain gap distance results during post-processing.

Modeling clearance effects in a secondary air flow system using co-simulation

Further, you can use the extracted displacements to model clearance effects in a secondary airflow system (SAS) of the blade cooling supply network using co-simulation. You can send transient clearances from WEM to SAS and calculate off-design mass flows that may not be captured in a standard design process. For example, during a transient setup, some clearances may become extremely tight, significantly restricting mass flow and leading to elevated metal temperatures due to inappropriate cooling flow.

To define an interface between the 1D external solver and Simcenter 3D WEM:
  1. Open the Co-simulation Manager dialog box to create an External Solver modeling object and all the necessary co-simulation interfaces.
  2. Create the External Solver modeling object to specify the path to the external SAS solver file.
  3. Create a Co-simulation Interface simulation object to define a coupling interface between your 3D thermal analysis model and the external model. You can use:
    • The Custom Expression type to define a custom expression to compute a length in your model and transfer it to the external solver. You can define the custom expression using named points, expression operators, and functions.
    • The Gap Distance type to transfer the gap distance between two edges or two surfaces of a 3D, 2D, or 2D axisymmetric model to the external solver. This co-simulation interface type is only available in multiphysics coupled thermal-structural solutions. You compute the gap distance using automatic or manual edge-to-edge or surface-to-surface contacts and requesting thermo-mechanical contacts.
  4. Activate the co-simulation solve in the Solution dialog box and launch the solve.
  5. (Optional) In the Solution Monitor, plot the co-simulation variables evolution. You can see the evolution of the clearance computed using the expression defined in the co-simulation interface.