In turbomachinery, impellers are critical components responsible for transferring energy from a motor to a fluid by accelerating it outwards from the center of rotation. Designing high-efficiency closed centrifugal impellers presents severe geometric challenges: the blades must follow highly optimized, three-dimensional curved paths to minimize aerodynamic losses, prevent cavitation, and maximize pressure recovery.
"By implementing a hybrid modeling workflow in KOMPAS-3D—where high-precision coordinates are automatically translated into 3D curves, surface patches, and seamless solid bodies—designers can construct production-ready turbomachinery with mathematical precision."
Core Technical Concepts in Impeller Design
Before reviewing the CAD steps, it is essential to understand the aerodynamic and structural logic incorporated in this impeller:
- Backward-Curved Blades: Bending the blades opposite to the direction of rotation ensures higher operational efficiency, stable head-capacity behavior, and lower surge susceptibility.
- Splitter Blades: Shorter, intermediate blades sit at the outer diameter of the impeller. They provide optimal fluid guidance at the outlet, preventing jet-wake flow separation while maintaining a large throat area at the inlet to avoid choking.
- Closed Impeller Structure: The blades are fully enclosed between the backplate (hub) and the front cover plate (shroud). This watertight sealing prevents fluid recirculation losses, drastically increasing volumetric efficiency compared to open impellers.
Step-by-Step CAD Modeling Timeline
The following sequential timeline documents the exact, step-by-step CAD workflow recorded in our high-resolution design video, illustrating the automated B-spline reconstruction, hybrid surfacing, direct modeling calibration, circular array patterning, and stress-concentration filleting.
The modeling workflow begins by launching the native KOMPAS-Macro utility within the Part workbench. This interface serves as the entry point for selecting and running custom automation scripts.
Active Tasks
- Launch Macro selection dialog
- Initialize Part (.m3d) environment
- Prepare script execution path
Engineering Insight
Executing automation directly through the native Macro Manager ensures a clean workflow execution integrated into the CAD tree.
Rather than using a generic turbomachinery importer utility, the custom Python script is loaded. The designer selects the generated coordinate files in the TurboGrid .curve format, which contain coordinates for the impeller hub and shroud flow paths.
Active Tasks
- Select impeller-stage1_hub.curve file
- Select impeller-stage1_shroud.curve file
- Configure coordinates for script input
Engineering Insight
Using .curve data generated by fluid dynamics programs directly connects the hydrodynamic design coordinates to the CAD environment.
The custom Python script parses the coordinate data from the TurboGrid curve files and directly draws/constructs the 3D spline curves in the KOMPAS-3D viewport, establishing the precise blade paths in the tree.
Active Tasks
- Script reads 322 points for the hub curve
- Script reads 242 points for the shroud curve
- Directly draws 3D spline curves in viewport
Engineering Insight
Because the script itself draws the spline curves, manual sketching is completely bypassed, eliminating transcription errors and guaranteeing absolute curve accuracy.
Using the Surface by grid of curves feature in KOMPAS-3D, the designer selects the generated splines to loft and construct the complex three-dimensional suction and pressure surfaces of the impeller blade.
Active Tasks
- Select Surface by Grid of Curves feature
- Map Direction U and V curves from splines
- Loft aerodynamic blade surfaces
Engineering Insight
The Surface by Grid of Curves feature allows for perfect geometric control of the three-dimensional blade profiles across varying span-wise layers.
To transition from open surface grids to a sealed watertight shell, the designer executes the Patch tool, selecting the edge boundaries of the leading and trailing edges to form clean end-cap surfaces.
Active Tasks
- Capping boundary edge loops
- Prepare surface sheet assemblies for stitching
- Create watertight zero-thickness boundaries
Engineering Insight
Applying exact surface boundaries at the leading and trailing edges establishes the sharp, aerodynamically optimized stagnation boundaries required for fluid entry.
The individual surface grids and cap patches are knitted together. The resulting watertight envelopes are solidified into dense 3D volumes. This yields two separate solids in the tree: the Main Blade and the Splitter Blade.
Active Tasks
- Stitch surfaces into a solid envelope
- Generate distinct 'Main Blade' and 'Splitter Blade' solids
- Organize part hierarchy under Part (Solids-2)
Engineering Insight
Converting surfaces to solid structures is mandatory to perform downstream physical calculations such as material mass, structural integrity (FEA), and Boolean merges.
Instead of direct modeling face deletions, this step is simply joining the faces (stitching surfaces). The individual surface sheets are stitched together to ensure a watertight sheet assembly before converting them into solid volumes.
Active Tasks
- Select adjoining blade boundary faces
- Stitch and join surfaces into unified sheet
- Ensure airtight boundary geometry
Engineering Insight
Joining and sewing the individual surface boundaries is a critical prerequisite to ensure that the sheet can be successfully thickened or solidified without geometric gaps.
The selected blade faces are moved/offset specifically to align and match perfectly with the hub and shroud surfaces. This alignment is critical to prevent gaps and geometric mismatch errors, ensuring a flawless Boolean union later.
Active Tasks
- Select blade boundary faces
- Move selected faces to overlap hub/shroud
- Eliminate gaps to prevent Boolean errors
Engineering Insight
Moving faces to achieve a perfect physical overlap with the hub and shroud prevents mesh self-intersections and zero-thickness geometry errors during Boolean operations.
The revolved solid hub is generated. The designer initiates the Array by concentric grid command, selecting the thickened main and splitter blade solids to pattern around the central Z-axis.
Active Tasks
- Define rotation axis: Central Z-Axis
- Select Main & Splitter solids as patterned inputs
- Configure array parameters and concentric grid
Engineering Insight
Circular patterning ensures absolute geometrical symmetry, which is vital for preventing rotor imbalance and mechanical vibration.
The pattern generates an alternating array of blades. The designer runs a Boolean operation (Merging) to fuse the patterned blades and the revolved hub backplate solid into a single, seamless monolithic solid.
Active Tasks
- Concentric array patterns 7 main and 7 splitter blades
- Perform Boolean Merging (Union) of all blade solids
- Preview single contiguous impeller solid
Engineering Insight
Fusing the components into a single solid simulates monolithic casting or multi-axis CNC milling, forming a robust, continuous load path.
This step is specifically cutting the face and matching it with the impeller outer diameter, rather than checking a symmetric pattern or sketch alignment. This trims the blade edges to align perfectly with the target casing diameter.
Active Tasks
- Cut and trim outer blade boundary faces
- Match outer blade profile with target impeller diameter
- Ensure clean geometric matching
Engineering Insight
Trimming the blade boundaries to perfectly match the target impeller outer diameter ensures the rotor satisfies exact operational clearance tolerances inside the casing.
The designer applies constant radius fillets along the intersections where the blades meet the hub disk backplate, smoothing the sharp root junctions to distribute rotational bending stresses.
Active Tasks
- Select blade-to-hub edge intersections
- Apply constant radius joint filleting
- Distribute stress concentrations at the backplate
Engineering Insight
Rotational and hydrodynamic loads create high bending stresses at the base of the blades. Applying fillets along the blade-hub attachments significantly increases dynamic fatigue life.
Symmetrically, the designer applies constant radius fillets along the intersections where the blades meet the integrated front shroud cover disk, securing the blade-shroud attachment.
Active Tasks
- Select blade-to-shroud edge intersections
- Apply constant radius joint filleting
- Distribute stress concentrations at the cover plate
Engineering Insight
Filleting the blades where they meet the front shroud ensures structural unity of the enclosed rotor, protecting the blade-to-disk joints against vibration and high shear forces.
The 350D closed centrifugal impeller is complete. The solid body is fully verified, displaying the clean, rendered 3D part rotated to showcase the backward-swept blade curvature and closed cover plate.
Active Tasks
- Fully enclosed closed-shrouded design validated
- Backward-curved high-performance blades
- Clean, production-ready 3D CAD model
Engineering Insight
The finished geometry showcases how hybrid modeling (B-spline curves lofted to grid surfaces, converted to solid features, and patterned concentric grids) can yield a production-ready aerospace-grade asset.
Key Takeaways & CAD Best Practices
Summarizing the advanced modeling operations in KOMPAS-3D reveals several invaluable best practices for complex turbomachinery design:
| Modeling Challenge | Conventional Approach | KOMPAS-3D Best Practice | Engineering Benefit |
|---|---|---|---|
| Blade Shape Definition | Manual 3D coordinate sketching | Custom Python Macro Script drawing splines from TurboGrid .curve data | 100% coordinates accuracy, bypassing manual sketching to eliminate human transcription errors. |
| Blade Surface Creation | Thickening flat sketches | Surface by Grid of Curves Feature | Enables high-precision creation of complex 3D suction and pressure surfaces with varying span-wise layers. |
| Boolean Union Errors | Leaving gaps or zero-thickness interfaces | Joining/stitching faces & Moving faces to overlap hub/shroud surfaces | Prevents self-intersection and zero-thickness geometry errors during Boolean operations. |
| Rotor Clearance & Diameter | Manual dimension matching | Trimming and Cutting the outer faces to match impeller diameter | Ensures the finished rotor satisfies exact casing design clearance tolerances. |
| Stress Concentrations | Leaving sharp blade-to-disk joints | Blade-to-Hub & Blade-to-Shroud Joint Filleting | Ensures solid structural unity of the enclosed rotor, protecting the attachment joints from vibration and dynamic shear stresses. |
Conclusion
Building the 350D closed centrifugal impeller demonstrates the massive benefits of a robust CAD modeling workflow. By utilizing Python macro scripting to import TurboGrid .curve files and automatically draw splines, creating surfaces using the Surface by Grid of Curves feature, joining the boundary faces, moving faces to align perfectly with the hub and shroud, applying fillets to the blade-to-hub and blade-to-shroud attachment joints, and leveraging circular concentric arrays and Boolean unions, engineers can construct highly durable, ultra-high-efficiency turbomachinery components ready for CNC manufacturing and structural simulation.
