Addressing Secondary Flow Effects in Turbomachinery with Fidelity CFD

Enhancing turbine performance is crucial to achieving greater efficiency and reliability in turbomachinery. One critical factor that significantly impacts turbine performance is secondary flow, a phenomenon that can account for up to 50% of total aerodynamic losses in axial turbines (Khadim et al., 2017). Understanding and mitigating these losses through advanced simulation techniques is vital for engineers aiming to optimize turbine design and functionality. This blog post explains the complex physics behind secondary flows in the Aachen axial turbine and how to mitigate them using Cadence Fidelity CFD software.

Understanding Secondary Flow

Secondary flow refers to fluid motion that occurs perpendicular to the primary flow direction, a pattern seen in all types of turbomachines in various degrees of complexity. In an axial turbine, the geometry and curvature of the blades induce pressure gradients that lead to the formation of vortices or circulating flow patterns. These secondary flows disrupt the ideal flow field, increasing energy dissipation and reducing overall turbine efficiency.

Secondary flow dynamics are complex and inherently unsteady, arising from intricate three-dimensional (3D) interactions within the flow. As such, accurately simulating these flows presents a significant challenge for engineers and researchers. Advanced computational tools, such as the Cadence Fidelity platform, facilitate in-depth analysis and remediation.

Case Study on Simulating Secondary Flow Dynamics

In this case study, the focus is on the 1.5-stage Aachen axial turbine, leveraging Fidelity Fine Turbo, an end-to-end turbomachinery modeling solution that covers all aspects from 3D meshing to advanced post-processing. The software’s robust capabilities allow for an exhaustive exploration of secondary flow phenomena, providing engineers with the tools necessary to enhance turbine performance.

Illustration of a 1.5-stage Aachen turbine with an enlarged view of the blade passages

The design of the Aachen turbine features an unshrouded rotor armed with cylindrical, untwisted, airfoil-shaped blades. This configuration is crucial as the blade curvature generates a pressure differential, creating high pressure on the pressure side (PS) and low pressure on the suction side (SS). This differential drives the flow from the PS to the SS, resulting in transverse secondary flows that deviate from the primary axial direction.

Visualization of secondary flow structures in the turbine blade passage (Khadim et al., 2017)

Understanding the mechanisms behind secondary flow formation requires an appreciation for boundary layer effects. Low-momentum fluid accumulates near the walls, which is then swept away by the pressure-driven cross-flow. This process generates secondary vortex systems, including horseshoe vortices (HSVs) near the blade’s leading edge. These vortices split into two legs at the stagnation point, ultimately contributing to the formation of a passage vortex (PV) and inducing wall vortices. Additionally, corner vortices arise at the blade-wall junctions due to the interaction between the boundary layer and crossflow.

 Simulation Setup with Fidelity CFD Platform

The first step in addressing secondary flow challenges is setting up a robust simulation environment. The Fidelity CFD includes high-fidelity mesh generation, with automated grid generation that transforms complex geometries into high-quality meshes for CFD analysis. It adeptly handles both unstructured and structured multi-block meshing, allowing for precise capture of flow dynamics. In this case, a structured, multi-block mesh is generated for one passage per row using pre-defined turbomachinery templates that allow for minimal user input.

Simulation begins with geometry definition, based on meanline design parameters. Parameters include blade profiles, hub and shroud contours, and dictate the flow characteristics throughout the turbine. Next, meridional flow paths specify the trajectory of the working fluid (in this case, perfect air), guiding it from the turbine inlet to the outlet.

A 2D mesh is first generated in the blade-to-blade plane along meridional flow paths, capturing essential details of the blade geometry and flow passage. This forms the basis for constructing the 3D volume mesh. High-resolution meshes are critical in regions of complex flow dynamics, particularly near the blade surfaces where intricate interactions occur. For the Aachen turbine, around 1.6 million mesh cells were created, and they were efficiently processed in less than 30 seconds using eight CPU cores.

Contour plot of velocity magnitude at (a) 1%, (b) 5%, and (c) 25% of blade span

Fine-Tuned Mesh Settings for Enhanced Resolution

The different rows within the turbine require tailored mesh settings. Row 2, experiencing a rotational speed at 3,500 RPM, necessitates a finer grid to ensure accurate resolution of critical flow phenomena such as secondary flows and potential flow separation. In contrast, rows 1 and 3, characterized by steadier aerodynamic flow, allowed for coarser grids, optimizing computational resources without sacrificing accuracy. Meticulous attention to each domain's individual operational characteristics is essential for accurately reflecting real-world conditions within the model.

Mitigating Secondary Flow Effects: Path Forward

By employing advanced CFD tools, engineers can conduct thorough analyses of secondary flow interactions and their impact on turbine efficiency. Understanding the underlying physics of these secondary flows illuminates pathways for mitigation and offers insights into optimizing turbine design for improved performance. Mitigating secondary flow effects may involve design changes, such as revising blade profiles to minimize pressure differentials or employing advanced flow control techniques.

Engineers can also explore altering the arrangement of blades or utilizing adaptive blade design concepts to further reduce energy losses. Integrating CFD analyses with design and optimization cycles fosters a proactive approach to performance enhancement. By simulating potential design variations and their impacts on secondary flow, engineers can make informed decisions that lead to more efficient turbine systems.

With advanced CFD platforms such as Fidelity CFD, engineers are better equipped to analyze complex fluid dynamics and mitigate losses. As the industry continues to evolve, the ability to harness these technologies will play a vital role in shaping the future of turbomachinery design and efficiency. Embracing these advancements will ultimately lead to performance gains by reducing secondary flow losses, driving the next generation of high-performance turbines.

Reference

1. T. K. Kadhim, A. Rona, H. M. B. Obaida, and K. Leschke, “The performance of a 1.5 stage axial turbine with a non-axisymmetric casing at off-design conditions,” presented at the 9th International Conference on Applied Energy (ICAE2017), Cardiff, UK, Aug. 21-24, 2017

For detailed information on how to mitigate secondary flows in turbomachinery using Fidelity CFD, read the technical brief From Losses to Gains: Mitigating Secondary Flow Effects to Maximize Turbine Performance.