Browsing by Author "Righi, Mauro"
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Item Open Access Body-force and mean-line models for the generation of axial compressor sub-idle characteristics(Cambridge University Press, 2020-07-07) Righi, Mauro; Ferrer-Vidal, Espana-Heredia Luis; Pachidis, VassiliosThis paper describes the application of low-order models to the prediction of the steady performance of axial compressors at sub-idle conditions. An Euler body-force method employing sub-idle performance correlations is developed and presented alongside a mean-line approach employing the same set of correlations. The low-order tools are used to generate the characteristic lines of the compressor in the locked-rotor and zero-torque windmilling conditions. The results are compared against steady-state operating points from three-dimensional (3D) Reynolds-averaged Navier–Stokes (RANS) computational fluid dynamics (CFD) simulations. The accuracy of the low-order tools in reproducing the results from high-fidelity CFD is analysed, and the trade-off with the computational cost of each method is discussed. The low-order tools presented are shown to offer a fast alternative to traditional CFD which can be used to predict the performance in sub-idle conditions of a new compressor design during early development stagesItem Open Access A characteristic-based 1D axial compressor model for stall and surge simulations(American Society of Mechanical Engineers (ASME), 2023-09-09) Kissoon, Sajal; Righi, Mauro; Pawsey, Lucas; Pachidis, Vassilios; Tunstall, Richard; Roumeliotis, IoannisA low-order unsteady one-dimensional axial compressor and combustor model has been developed at Cranfield University as part of a larger unsteady gas turbine engine model, with the ability to simulate compressor stall and surge. The flow is resolved using the 1D unsteady Euler equations and source terms are used to model bleed extraction (and addition), pressure losses, and heat and work exchange. Species tracking is used in the combustor part of the model, using a semi-coupled approach, to keep track of the combustion products and unburnt fuel in the main gas path. The equations are solved using a Roe Approximate Riemann Solver, modified to handle the high magnitude, transient source terms necessary for this simulation. The performance of the compressor during the transient surge event is described by a set of compressor characteristics, including reverse flow and rotating stall regions, obtained from a validated 3D throughflow code, ACRoSS. To replicate the exact response of multi-stage compressors, stage-by-stage characteristics are used during reverse flow. The low-order method presented is successfully verified against ACRoSS for a high-power surge event of a coupled IPC and HPC configuration. The rate at which the total pressure at the outlet of the HPC collapses was calculated to be within 1%. This approach presents a faster alternative to high-fidelity CFD and can be used to investigate the compressor stall behaviour within minutes during the early design phase.Item Open Access Experimental validation of a three-dimensional through-flow model for high-speed compressor surge(Elsevier, 2022-08-09) Righi, Mauro; Pachidis, Vassilios; Könözsy, László Z.; Giersch, Thomas; Schrape, SvenThis paper describes the validation of surge simulations produced using the low-order code ACRoSS with experimental data from a 4. 5-stage high-speed rig, representative of the front stages of a modern high-pressure compressor (HPC). ACRoSS is an unsteady, 3D through-flow code developed at Cranfield University to predict compressor performance during stall events. The experimental data was derived during the Rig250 build 6B test campaign, carried out in 2016 by the DLR Institute of Propulsion Technology. To correctly represent the surge events, the ducting of the testbed upstream and downstream of the compressor is included in the simulation. The results from the low-order model are compared with measurements from unsteady probes for two surge events at design speed with different downstream plenum sizes. The surge frequency and pressure profiles in time are closely reproduced by the low-order model. Analysis of the unsteady pressure measurement and the acoustic waves modelled by ACRoSS indicates that the surge period is likely to be influenced by the reflection of the initial surge wave in the inlet duct. Using the ACRoSS model, surge can be accurately reproduced both in 3D and 2D. Two-dimensional, axisymmetric simulations are shown to be sufficient for the cases investigated, and surge can be simulated with a computational cost of less than one hour per event using just 40 CPUs. This represents over an order of magnitude improvement in computational power and time required to simulate surge, compared to traditional URANS 3D CFD.Item Open Access Low-order models for the calculation of compressor sub-idle characteristics(ISABE, 2019-09-27) Righi, Mauro; Ferrer-Vidal, Luis E.; Allegretti, Alessandro; Pachidis, VassiliosThis paper focuses on the development of low-order models for the generation of compressor sub-idle characteristics via numerical simulation of an axial-flow compressor at sub-idle conditions. A through-flow code using body forces developed by Cranfield University is used as a framework to test three new methods to model blade row performance under sub-idle conditions. The first method is a simplified analytical model of a separated blade passage, originally developed to model reverse flow through the passage. The method consists of a modification to the body forces employed by the code and can be easily adapted to model the sub-idle operating condition. The second method is a set of pressure loss and deviation angle correlations developed at Cranfield University specifically for sub-idle conditions. A third approach makes use of the deviation angle correlations along with the modified body-force method, resulting in a hybrid approach. These three methods are implemented in the through-flow code to obtain low-order models that are then used to generate compressor characteristics under locked rotor and windmilling conditions. The code created is able to generate compressor characteristics throughout the sub-idle operating regime in a few minutes. The low-order model results are compared against experimental data from a sub-idle compressor rig and CFD RANS simulations of the same compressor. The generated characteristics show promising results, with only minor calibration required for the numerically calculated characteristics to match those generated via experiment.Item Open Access On the prediction of the reverse flow and rotating stall characteristics of high-speed axial compressors using a three-dimensional through-flow code(Elsevier, 2019-11-22) Righi, Mauro; Pachidis, Vassilios; Könözsy, László Z.This paper presents the development of a low-order three-dimensional through-flow code created at Cranfield University in the UK, named ACROSS (Axial Compressor Rotating Stall and Surge simulator), and its application to create the reverse flow and rotating stall characteristics of a modern high-speed compressor. The compressor modelled is a six-stage axial-flow machine, representative of a modern aero-engine high pressure compressor. The tool has been previously validated using experimental data from two low-speed compressor rigs. This article describes how the tool’s robustness and computational speed have been im proved by introducing higher-order schemes to model the circumferential fluxes and the rigid movement of the flow in the rotating blade rows. Further improvements include variable axial discretization, an algorithm to introduce random flow perturbations in the flow field and an improved plenum model. The compressor is first modelled in reverse flow conditions to create its reverse-flow characteristics and these are then compared against results from high-fidelity 3D CFD simulations. Results obtained suggest that despite the presence of three-dimensional flow features, 2D axi-symmetric simulations are adequate to generate the full range of reverse flow characteristics of the compressor. The rotating stall characteristics at 77% and 100% corrected rotational speed are created by modelling several steady rotating stall cases in 3D. Using the code ACROSS, the complete map of the compressor modelled, comprising of forward flow, reverse flow and rotating stall characteristics, was created in only 5 days using 3 desktop workstations. For comparison, state-of-the-art high-fidelity 3D CFD requires several days to simulate a single rotating stall case on a high performance computing facilityItem Open Access Three-dimensional low-order surge model for high-speed axial compressors(Global Power & Propulsion Society, 2020-12-18) Righi, Mauro; Pachidis, Vassilios; Könözsy, László Z.; Zhao, Fanzhou; Vahdati, MehdiSurge in modern aero-engines can lead to violent disruption of the flow, damage to the blade structures and eventually engine shutdown. Knowledge of unsteady performance and loading during surge is crucial for compressor design, however, the understanding and prediction capability for this phenomenon is still very limited. While useful for the investigation of specific cases, costly experimental tests and high-fidelity CFD simulations cannot be used routinely in the design process of compressor systems. There is therefore an interest in developing a low-order model which can predict compressor performance during surge with sufficient accuracy at significantly reduced computational cost. This paper describes the validation of an unsteady 3D through-flow code developed at Cranfield University for the low-order modelling of surge in axial compressors. The geometry investigated is an 8-stage rig representative of a modern aero-engine IP compressor. Two deep surge events are modelled at part speed and full speed respectively. The results are compared against high-fidelity, full annulus, URANS simulations conducted at Imperial College. Comparison of massflow, pressure and temperature time histories shows a close match between the low-order and the higher-fidelity methods. The low-order model is shown capable of predicting many transient flow features which were observed in the high-fidelity simulations, while reducing the computational cost by up to two orders of magnitudeItem Open Access Three-dimensional through-flow modelling of axial flow compressor rotating stall and surge(Elsevier, 2018-04-17) Righi, Mauro; Pachidis, Vassilios; Könözsy, László Z.; Pawsey, LucasThis paper presents a three-dimensional through-flow approach based on the cylindrical Euler equations incorporating a body force method. Blade performance is captured through a mixture of empirical correlations and a novel reverse flow treatment. The code is the first application of a physically correct Godunov solver to three-dimensional rotating stall and surge modelling. This solver ensures the accurate calculation of inter-cell fluxes unlike in typical modern CFD codes in which the non-linear convective terms are linearised. Validation consists of modelling a low speed three-stage axial compressor in all operating regions, recreating the reverse flow, rotating stall and forward flow characteristics with good agreement to experimental data. Additional comparisons are made against rotating stall cell size and speed, to which good agreement is also shown. The paper ends with some full surge cycle simulations modifying both the tank volume after the compressor and the level of inlet distortion applied. Both tank volume and level of distortion have been found to affect the type of instability developed. The development of this code is a step forward in compressor rotating stall and reverse flow modelling and allows recreation of a full compressor map at a significantly low computational cost when compared to commercially available 3D CFD codes.