Session: 37-05 Radial Turbomachinery Modelling

Paper Number: 153790

A New Body Force-Based Modelling Approach for Centrifugal Compressor Post-Stall Aerodynamics 

Simulation of turbomachinery aerodynamics is typically carried out using conventional CFD approaches (most commonly (U)RANS), whereby the flow path and blade geometry is modelled in detail. These methods over the years, have proven their capability of accurately replicating complex steady-state and transient flows, but the associated computational cost is often excessive. This is particularly the case when considering coupled systems, e.g. fan-intake interaction or transient simulation of full annulus, non-axisymmetric flows in multi-stage turbomachinery components, e.g. inlet flow distortions, rotating stall or surge in multi-stage compressors. Body force modelling is a computationally efficient alternative method, replacing the bladed components with a body force field which introduces losses and flow turning. This allows the entire turbomachinery component to be simulated as an empty duct. Since losses are typically introduced by the body force model, the solution is commonly obtained by solving the inviscid, compressible flow equations (Euler). These dramatically reduce the computational cost and grid complexity as it removes the need for highly refined end wall grids and the governing equations are drastically simplified. This results in at least one order of magnitude reduction in computational time. Such methods have successfully been implemented in the past for axial compressor post-stall aerodynamic simulations but no successful approach has been reported for centrifugal compressor post-stall aerodynamics modelling.

In the present work, a new body-force based approach, focused on centrifugal compressor stall and surge modelling is presented. The proposed method is based on a previously developed (by thew same author) custom body force model comprising a base turning force, a correction force and a viscous force coupled with distribution coefficients. An analytical, lower-order modelling approach to estimate aerodynamic blockage, losses due to flow separation as well as change in flow direction and additional losses due to flow mixing is developed. These are numerically implemented within a custom axisymmetric Euler solver in the relative FoR with blockage. The solver utilises the Godunov scheme coupled with the HLLC Riemann solver reformulated in the relative FoR. Steady-state and transient simulations are performed with the custom code, while additional steady-state operating points are obtained with conventional 3D CFD (RANS) model. The NASA CC3 compressor is selected as it constitutes the only open-source geometry with available experimental surge data.

The development and assumptions of the blockage-mixing model are described. These are formulated to allow their applicability in axial (constant area mixing) and radial (variable area mixing) bladed components and consequently are applicable to vaned diffuser blades, axial rotors/stators and impellers. The method to model the highly separated flow when the flow enters the passage from the trailing edge is also presented. These methods are then compared against conventional 2D and 3D test cases to assess their validity and accuracy. After the initial verification of the proposed methodology, reverse flow characteristics of the NASA CC3 compressor are obtained with the body force code and compared against the corresponding ones derived from 3D CFD. Finally, a transient surge simulation is carried out in the custom code by attaching a plenum, the volume of which, is varied as the size of the experimental one is not provided. The pressure measurements during the surge cycles are then compared against the experimental ones, indicating great agreement, replicating key flow-features recorded by the performed PIV experimental measurements. To the authors’ best knowledge this is the world’s first, successful and validated body-force modelling approach for steady-state and transient centrifugal compressor post-stall aerodynamics.

Presenting Author: Dimitrios Lamprakis Cranfield University

Presenting Author Biography: Dr. Dimitrios Lamprakis was born in 1991, in Athens, Greece. He obtained his 5-year mechanical engineering diploma in Greece, specialising in energy, aeronautics and environment. In 2019 he obtained his MSc in aerospace propulsion from Cranfield university UK (1st class). Shortly after the completion of his MSc, Dimitrios started a PhD programme, in collaboration with Rolls-Royce US (Indianapolis) on axi-centrifugal compressor surge and stall modelling using body forces and reduced-order methods. After the submission of his PhD in 2022, he joined the Rolls-Royce UTC in Cranfield university as a research fellow. He is currently working on the LH2GT project, conducting CFD analysis on a LH2 pump for aircraft applications. In parallel he is working on turbine subidle aerodynamics and compressor surge and stall modelling. His main area of expertise is turbomachinery aerodynamics, with focus on numerical methods for CFD applications, body force modelling for internal flows and surge and stall modelling.

Authors:

Dimitrios Lamprakis Cranfield University
Mauro Righi Cranfield University
Vassilios Pachidis Cranfield University