Numerical Simulation of Rotating Stall and Surge for the Determination of Dynamic Loads and Blade Response in Aero-engine Core Compressors
Lead Research Organisation:
Imperial College London
Department Name: Mechanical Engineering
Abstract
Aero-engine compressors deliver a large quantity of air at high pressure. From an efficiency viewpoint, it is desirable to operate them at the highest possible pressure ratios but such operating points are inherently unstable because of their close proximity to undesirable aerodynamic phenomena of stall and surge. Rotating stall is a local instability where reduced flow rate gives rise to flow separation and results in the formation of stall cells. These cells begin to rotate around the annulus and hit the blades, thus causing high vibratory loads. Surge is a global instability in which flow reversal occurs throughout the machine, causing high transient stresses in the blading. Deficiencies in understanding the exact mechanisms and a lack of modelling methodology prevent the determination of the dynamic loads and the ensuing blade response. Therefore, current designs are based on safe margins where the bladerow spacing is not optimum. Using an advanced computational method, it is proposed to build a large-scale model of a typical industrial core-compressor which has been the subject of previous studies by the proposers and for which experimental data are available. The aim of the project is not only to understand the rotating stall and surge mechanisms and the links between them, but also to prove the feasibility of the large-scale modelling approach as a design tool. A further objective is the investigation of recovery mechanisms from rotating stall and surge..
Publications
Choi M
(2011)
Effects of Fan Speed on Rotating Stall Inception and Recovery
in Journal of Turbomachinery
Choi M
(2011)
Numerical strategies for capturing rotating stall in fan
in Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy
Choi M
(2011)
Recovery Process from Rotating Stall in a Fan
in Journal of Propulsion and Power
Choi M
(2013)
Validation of Numerical Simulation for Rotating Stall in a Transonic Fan
in Journal of Turbomachinery
| Description | Rotating stall and surge are phenomena that can occur at the extreme operability envelope of an aero engine. The flow at these conditions is characterised by partial reversal total reversal of the flow (known as rotating stall and surge respectively) resulting in partial or total loss of power for the engine. Engines are designed so that the operating point is far from stall, but wear and deterioration and events such as wind gust can move the operating point to stall point. During rotating stall and surge, the stall cells and flow reversal will interact with the structural flexibility of the compressor blades. This causes enormous vibratory stresses on the blades which may result in engine failure or shut down. At the time of writing the proposal, there were no established and validated numerical procedures for obtaining stall and surge. The aim of this research was to develop a validated computer model that can be used for the study of rotating stall and surge and their effects on the blade loading in aero-engine compressors. The in-house unsteady flow and aeroelasticity code AU3D was used in the project. In the first stage of the work, a range of computer models (with different degrees of complexity) for simulating rotating stall in an LP compressor (fan) were tested. The aim was to find the model which can represent the physics as adequately at minimum computational cost. Based on the results of these tests, a computer model for predicting rotating stall was developed. This work was presented at ISUAAAT12 conference and was published in the IMechE journal. The second part of the project focused on a quantitative comparison of rotating stall between experimental and numerical simulation. The test case was a fan rig blade with extensive and reliable experimental data (from Rolls-Royce). The computational results were encouraging and showed good correlation to measured data. The results of this work will be presented at ASME 2011 and have been submitted for publication in the ASME journal. The validated methodology was used to investigate and explain some of the characteristics of rotating stall (such as speed dependency and recovery process) in LP compressors. The results of these calculationsare consistent with the experimental observations and theoretical models. The results were presented in ASME 2010 and were published in ASME and AIAA journals. The evaluation of blade deflection under surge loading is an important design requirement to determine the axial gap between the bladerows. Current methods for determining surge deflections are very crude and result in conservative designs. Surge computations in a core compressor were carried out using both the full model (where every blade in the compressor is modeled) and a single passage model (where only 1 blade is modeled in each bladerow). The blade loadings obtained during deep surge, were similar for both sets of computations, which is important as single passage computations are orders of magnitude faster than the corresponding ones for the whole assembly of blades. The simplified single passage calculation methods are likely to have a big impact on current design procedures, which could lead to more compact and lighter engines. A journal paper showing the above findings is currently under preparation. A major beneficiary of the work will be Rolls-Royce which provided the geometries and other data used in this research work. Technology transfer is already in place through the University Technology Centre (UTC) scheme. Furthermore, other turbomachinery manufacturers, as well as academic researchers will also benefit from this research, as stall and surge are universal problems for both axial and radial-flow machines. |
| Exploitation Route | Now in use at Rolls-Royce |
| Sectors | Aerospace, Defence and Marine |
| Description | Help Rolls-Royce design compressor blades |
| First Year Of Impact | 2011 |
| Sector | Aerospace, Defence and Marine |
| Impact Types | Economic |