Browsing by Author "Rowe, Arthur"

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    Analytical modeling of rotating stall and surge
    (Cranfield University, 2014-03) Zoppellari, Serena; Pachidis, Vassilios; Rowe, Arthur; Brown, Steve
    The life and performance of axial compressors are limited by the occurrence of instabilities such as rotating stall and surge. Indeed, in the course of the design phase a great effort is usually devoted to guarantee an adequate safety margin from the region of instabilities’ onset. On the other hand, during its operating life, an axial compressor can be subjected to several conditions that can lead to the inception of stall and its dynamics. A few examples of possible stall causes, for the specific case of an axial compressor embedded in an aircraft engine, are inlet flow distortion, engine wear or shaft failure. The shaft failure case can be seen as an exception, as a matter of fact, after this event surge is a desirable outcome since it can potentially decelerate the over-speeding turbine by reducing the mass flow passing through the engine. The possible occurrence of surge and stall should be predicted and controlled in order to avoid severe damage to the compressor and its surroundings. A lot of research has been carried out in the past years to understand the inception and development of stall to achieve the capability for predicting and controlling this severe phenomenon. Nonetheless, this problem is still not well understood and unpredictable outcomes are still a great concern for many axial compressor’s applications. The lack of knowledge in what concerns inception and development of stall and surge reflects in a lack of tools to investigate, predict and control these unstable phenomena. The tools available to study stall and surge events are still not highly reliable or they are very time consuming as 3D CFD simulations. The doctoral research described herein, aimed at the investigation of the rotating stall phenomenon and the derivation of the compressor characteristic during this unstable condition. Following a detailed analysis of the tools and techniques available in the public domain and the identification of their limitations, the development of a FORTRAN through-flow tool was the methodology chosen. A distinctive feature of the developed tool is the independency from steady state characteristics which is a limitation for the majority of the available tools and its computational efficiency. Particular attention was paid to capture various viscous flow features occuring during rotating stall through the selection and implementation of appropriate semiempirical models and correlations. Different models for pressure loss, stall inceptions and stall cell growth/ speed were implemented and verified along with different triggering techniques to achieve a very close to reality simulation of the overall phenomenon, from stall inception to full development. lel compressors’ technique that allows the correct modeling of asymmetric phenomena. The methodology implemented has proved promising since several simulations were run to test the tool adopting different compressor geometries. Verifications were performed in terms of overall compressor performance, with simulations in all the three possible operating regions (forward, stall and reverse flow), in order to verify the tool’s capability in predicting the compressor characteristics. In terms of flow field, the ability to capture the right circumferential trends of the flow properties was checked through a comparison against 3D CFD simulations. The results obtained have demonstrated the ability of the tool to capture the real behavior of the flow across a compressor subjected to several different unstable conditions that can lead to the onset of phenomena such as rotating stall, classic and deep surge. Indeed, the tool has shown ability to tackle steady and transient phenomena characterized by asymmetric and axis-symmetric flow fields. This document provides several examples of investigations emphasizing the flexibility of the developed methodology. As a matter of fact, within this dissertation, many examples can be found on the effect of the plenum size, on the different transient phenomena experienced by the compressor when subjected to multiple regions of inlet distortion instead of a localized region of low or high flow, on the differences between temporary and stationary inlet disturbances and so on. This document describes in detail the methodology, the implementation of the tool, its verification and possible applications and the recommended future work. The work was funded by Rolls-Royce plc and was carried out within the Rolls-Royce UTC in Performance Engineering at Cranfield as three-year Ph.D. program that started in October 2010.
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    Gas turbine shaft over-speed / failure modelling: aero/thermodynamics modelling and overall engine system response
    (Cranfield University, 2014-04) Soria, Carlos; Pachidis, Vassilios; Rowe, Arthur; Brown, Steve
    Gas turbine design needs of high-speed turbomachinery whose layout is organised in compressor-turbine pairs mechanically linked by concentric shafts. The mechanical failure of a shaft leads to compressor-turbine decoupling provoking the acceleration of the free-running turbine. In view of such scenario, it is of paramount importance to guaranty the mechanical integrity of the turbine, in terms of high energy debris release. Certification authorities require proof that any possible failure will be contained; admitting the reliable simulation capability of the event as certification strategy. The objectives of this research activity have aimed at the development of reliable simulation tools based on analytical and semi-empirical models. The integration of all the different models/modules together in an “all-in-one” tool provides the sponsor company with the capability to simulate and assess various shaft over-speed scenarios during the early stages of an engine's design and development program. Shaft failure event cannot be understood unless engine components interaction and fast transient effects are taken into account in a global manner. The high vibration level consequence of the breakage, or the thermodynamic mismatch due to the rapid free-running compressor deceleration, trigger the surge of the compression system which affects to the performance of every engine component. Fully-transient simulation capability to model compression system post-stall performance and secondary air system behaviour has been developed. Component map prediction tools have been created for compressor reverse flow performance and turbines affected by inlet distorted flows. The development of the so-called “all-in-one” simulation tool has been completed and it has been applied to the modelling of a real case of shaft failure. Reliable prediction of thermodynamic properties evolution and over-speeding turbine terminal speed have been shown. The robustness and flexibility of the simulation tool have been demonstrated by its application to different theoretical scenarios.
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    Gas turbine shaft over-speed/failure performance modelling
    (Cranfield University, 2010-10) Gallar, Luis; Pachidis, Vassilios; Singh, R.; Rowe, Arthur; Brown, Steve
    A gas turbine engine can over-speed due to various reasons, including shaft failure, variable geometry mal-schedule or fuel system malfunction. In any case, engine manufacturers are required to demonstrate that a shaft over-speed event will not result in an uncontained failure with high energy debris being released from the engine. Although the certification authority can be satisfied that the engine is shaft failure safe by conducting large scale tests, a purely experimental approach would be very complex and expensive. Moreover, today’s poor understanding of the event leads to conservative designs that exert unavoidable penalties on the engine performance and weight. It is in this context that the need for an analytical approach and small scale testing arises to model the progression of the event. This work is part of a wider long term research collaboration between Cranfield University and Rolls-Royce that attempts to enhance today’s modelling capability of gas turbine shaft overspeed/ failure events. The final aim of the project is the development of a generic advanced performance prediction tool able to account for all the complex and heavily interrelated phenomena to ascertain the terminal speed of the over-speeding turbine. This multidisciplinary “all in one” tool will allow to include the shaft failure scenario early into the design process and eliminate current conservative design approaches while maintaining the high standard of airworthiness required for certification. This thesis focuses on the aerothermal performance modelling of turbomachinery components at the extreme off-design conditions experienced during a shaft over-speed event. In particular, novel modelling techniques and methodologies at the forefront of knowledge have been developed to simulate the performance of turbine vanes at high negative incidence angles, derive the extended compressor characteristics in reverse flow and calculate the response of the air system during rapid transient among others. These component models are ready to be integrated into a generic single computational tool that, once validated against engine data available from the sponsor, can be applied to different engines and scenarios. The collaboration with Rolls-Royce provided the opportunity to conduct research on other areas related to performance engineering apart from the shaft failure modelling. The present study makes several noteworthy contributions on compressor variable geometry loss, deviation and stall modelling, compressor variable geometry schedule optimisation and on the effect of using real gas models instead of the perfect gas assumption in engine performance simulation codes.
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    Gas turbine sub-idle performance modelling : groundstart altitude relight, and windmilling
    (Cranfield University, 2013-01) Grech, Nicholas; Pachidis, Vassilios; Zachos, Pavlos K.; Rowe, Arthur; Brown, Steve; Tunstall, Richard
    Engine performance modelling is a major part of the engine design process, in which specialist solvers are employed to predict, understand and analyse the engine’s behaviour at various operating conditions. Sub-idle whole engine performance synthesis solvers are not as reliable and accurate as design point solvers. Lack of knowledge and data result in component characteristics being reverse-engineered or extrapolated from above-idle data. More stringent requirements on groundstart and relight capabilities, has prompted the need to advance the knowledge on low-speed engine performance, thereby requiring more robust sub-idle performance synthesis solvers. The objective of this study, was to improve the accuracy and reliability of a current aero gas turbine sub-idle performance solver by studying each component in isolation through numerical simulations. Areas researched were: low-speed and locked-rotor com- pressor characteristics, low-power combustion efficiency, air blast atomizer and combustor performance at sub-idle, torque-based whole engine sub-idle performance synthesis, and mixer performance at far off-design conditions. The observations and results from the numerical simulations form the contribution to knowledge of this research. Numerical simulations of compressor blades under highly negative incidence angles show the complex nature of the flow, with the results used to determine a suitable flow deviation model, a method to extract blade aerodynamic char- acteristics in highly separated flows, and measure the blockage caused by highly separated flow with operating condition and blade geometry. The study also concluded that the use of Blade Element Theory is not accurate enough to be used at such far off-design con- ditions. The linearised parameter-based whole engine performance solver was converted to used torque-based parameters, which validated against engine test data, shows that it is suitable for low-power simulations with the advantage of having the potential to start engine simulations from static conditions. A study of air-blast atomization at windmilling relight conditions has shown that current established correlations used to predict spray characteristics are not suitable for altitude relight studies, tending to overestimate the atomization quality. Also discovered is the highly influential interaction of compressor wakes with the combustor and atomizer under altitude relight conditions, resulting in more favourable lighting conditions than previous assumptions and models have shown. This is a completely new discovery which will result in a change in the way combustors are designed and sized for relight conditions, and the way combustion rig tests are conducted. The study also has valuable industrial contributions. The locked-rotor numerical data was used within a stage-stacking compressible flow code to estimate the compressor sub- idle map, of which results were used within a whole engine performance solver and results validated against actual engine test data. The atomization studies at relight were used to factor in the insensitivity of current spray correlations, which together with a newly de- veloped sub-idle combustion efficiency sub-routine, are used to determine the combustion efficiency at low-power settings. The interaction of compressor wakes with the atomizer showed that atomizer performance at relight is underestimated, resulting in oversized combustors. By using the knowledge gained within this research, combustor size can be reduced, resulting in lower NOx at take-off and a smaller and lighter core, with a com- bustor requiring less cooling air. The component research has advanced the knowledge and modelling capability of sub-idle performance solvers, increasing their reliability and encouraging their use for future aero gas turbine engines.
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    Sub-idle modelling of gas turbines: Altitude relight and windmilling
    (Cranfield University, 2007-10-05) Howard, Jason; Rowe, Arthur; Pilidis, Pericles; Clark, G.; Naylor, P.
    Gas turbine sub-idle performance modelling is still in an early development stage and this research aims to provide and improve present techniques, for modelling of windmilling and transient windmilling relights, through to groundstart simulations. Engine ATF data was studied and used to align models created within this research for low and high bypass engines, and compare these models simulation results. Methods for the extrapolation of component characteristics are improved and performed in linearised parameter form, and the most efficient approach discussed. The mixer behaviour is analysed and recommendations of off-design mixer behaviour representation in a sub-idle model are proposed and performed within the modelling. Combustion at sub-idle conditions is investigated with regards to the loading parameter definition, and also its representation for the influence of evaporation rate being limiting to overall combustion efficiency. A method is proposed on extrapolating and representation of the combustion characteristic. Compressor behaviour and the blade torques at locked rotor and windmilling conditions are studied using 3D CFD, producing insight and discussion on CFD suitability and what it can offer at these operating conditions. From the CFD studies generic loss coefficients were created for all compressor blades, from which a zero speed is created for the whole compressor, from a theoretical stage stacking calculation. This zero-speed curve is shown to allow interpolation of component characteristics to the sub-idle region, improving the definition and a predictive approach. A windmilling conditions cascade test rig is proposed, designed and built for validating the CFD loss coefficients. The findings and discussions within this thesis provide useful reference material on this complicated and little documented area of research. The modelling and methods proposed, provide great advancement of the research area, along with further integration of the Cranfield UTC in performance with Rolls-Royce.