This thesis deals with the application of modern identification techniques to model the dynamic relationship between the fuel flow and shaft speeds of a Rolls Royce aircraft gas turbine. It is motivated by the desire to exploit recent advances in modelling linear and nonlinear systems and the need to investigate the suitability of various model representations in nonlinear gas turbine modelling. The first part of the thesis deals with linear gas turbine modelling, with the aim of estimating models which can be used to verify the linearised thermodynamic models derived from the engine physics, at different shaft speeds. A detailed analysis of the engine data is presented and linear engine models are identified at different operating points using time- and frequency-domain techniques. The influence of noise and nonlinearities on the estimated models is studied and it is shown that the use of multisine signals and frequency-domain techniques is particularly suited to this problem, since the derived continuous-time s-domain models can be directly compared with the linearised thermodynamic models. It is also shown that discrete models estimated in the time domain have excellent approximation capabilities but are not suited for the validation of the thermodynamic models since their modes are uncertain and they sometimes result in modes which do not have a continuous-time counterpart. The second part of the thesis deals with the application of several nonlinear system representations to model the nonlinear relationship between the fuel flow and shaft speeds of the gas turbine. Data is analysed in both time- and frequency-domain to gain information about the engine nonlinearity, and several nonlinear model representations are presented along with popular estimation algorithms. Nonlinear models for each shaft were then estimated and the performance of these models was demonstrated by their ability to approximate measured engine data. It is shown that the nonlinear relationship between the fuel flow and shaft speed can be modelled using a Wiener structure, a NARMAX structure or a neural network. Several issues concerning signal design and prior knowledge of the nonlinearity in a system are also discussed and a number of recommendations are made for future gas turbine modelling and testing. This thesis is a contribution to the further application of multifrequency signals and time- and frequency-domain techniques to the identification of linear and nonlinear models for aircraft gas turbines. While this work was applied to a gas turbine, these techniques can be applied to a range of industrial applications that deal with system testing and modelling.

No description available.

Cross flow wind turbines are not unique. The performance of Savonius and Darrieus turbines is well documented. Both share the advantage of being able to accept fluid flow from any direction. The Savonius is drag based and hence has poor power output while the Darrieus is lift based. Due to the fact that the Darrieus has fixed blades the fluid flow through the rotor does not result in optimal lift being generated at all points in the rotation circle. A drawback of the Darrieus system is that it has to operate at a high tip-to wind-speed ratio to obtain reasonable performance with the fixed blades. Deviation from a small optimal range of tip speed ratios results in poor performance. The Darrieus also has poor starting torque. The research conducted in this project focused on overcoming the shortcomings of other turbines and developing an effective cross flow turbine capable of good performance. A number of different concepts were experimented with, however all were based on a symmetrical aerofoil presented to the actual relative airflow at an angle that would produce the highest lift force at all times. The lift force was then utilized to generate movement and to do work on an electrical generator. All concepts contemplated were researched to ascertain their appropriateness for the intended application. During development of the final experimental platform and after lodging of a provisional patent (RSA 2007/00927) it was ascertained that the design shared some similarities with an American patent 5503525 dated 28/4/1994. This patent employed complex electronic sensing and control equipment for control of blade angle. This was thought to be overly complex and costly, particularly for small scale wind energy generation applications and a simpler mechanical solution was sought in the design of the final experimental platform used in this project. The design of the mechanical control system was refined in an attempt to make it simpler, more durable and employ the least number of moving parts. Literature studies and patent searches conducted, suggested that the mechanical control system as developed for the final experimental platform was unique. The enormous variation in the power available from the wind at the different wind speeds likely to be encountered by the device necessitated some means of control. In high wind conditions control of the amount of wind power into the device was deemed to be the preferable means of control. A number of different concepts to achieve this were devised and tested. The final concept employed limited the tail angle deflection and hence the lift produced by the aerofoils. This resulted in a seamless “throttle” control allowing the device to be used in any wind strength by adjusting the control to a position that resulted in the device receiving a suitable amount of power from the wind. The outcome of performance tests conducted indicated that the device has the potential to be developed into a viable wind turbine for both small and large scale applications. The ability to control the power input from the wind to the machine from zero to a maximum is considered to be one of the most beneficial outcomes of this project and together with the quiet operation and low speed, are considered the main advantages of the device over existing wind turbine designs. The possibilities of using the device to compress air for energy storage are exciting avenues that warrant further research.

This work concerns the prediction of potentially damaging thermoacoustic oscillations in gas turbine combustion systems by computational means. A framework is laid out to predict numerically the frequency and stability of thermoacoustic oscillations, with focus on the high frequency screech instability of afterburners. A hybrid numerical method is used that includes separate calculations of the mean flow and of the perturbed field due to the acoustic oscillations. This modularity supports the choice of models that are the most appropriate for combustion and for acoustic wave propagation, which are the processes that make up the feedback mechanism that can lead to the establishment of an instability. This gives flexibility, improved accuracy and more insight into the physics of the thermoacoustic system at a potentially reduced computational cost. The mechanism leading to screech involves the formation of vortices induced by acoustic transverse modes at the afterburner flameholder. These vortices trap fresh reactants that burn after a certain time delay, therefore feeding energy into the oscillation. Within a linear approximation, the effect of small amplitude acoustic fluctuations on the flame is studied by perturbing harmonically the transverse velocity at the flameholder lip over a range of frequencies using forced combustion CFD calculations. The response in heat release rate, which is a thermoacoustic source of sound, is represented by a flame transfer function (FTF). It is argued that for the investigation of screech oscillations, this FTF must be multi-dimensional because of the transverse nature of the acoustic oscillation. For fully premixed flames, the main contributor to heat release rate fluctuations is the variation in flame surface area. This information is used to develop a novel flame model that represents the multi-dimensional, frequency dependent response of the flame to velocity perturbations. Compared to FTFs, which require computationally expensive forced calculations, this model has the advantage of providing the frequency dependent flame response as part of the acoustic calculation. After verification and validation of each of the tools used for the acoustic and combustion simulations, this flame model is used in the analysis of a simplified afterburner, where a high frequency, radial and longitudinal resonant mode was computed. Convective modes, which are important in the prediction of the frequency of thermoacoustic oscillations are predicted as a result of the interaction between the acoustic wave and the flame.

This thesis describes an experimental investigation of the flame structure close to the extinction and the blow-off events of non-premixed and spray flames stabilized on an axisymmetric bluff body in a confined swirl configuration. The comparison of flames of different canonical types in the same basic aerodynamic field allows insights on the relative blow-off behaviour. The first part of the thesis describes several velocity measurements in non-reacting and reacting flows. The main usefulness of this data is to provide the aerodynamic flow pattern and some discussion on the velocity field and the related recirculation zones. The velocity and turbulence information obtained are particularly useful for providing data, which is crucial for validation of computational models. The second part describes an experimental investigation of non-premixed stable flames very close to the blow-off condition. The measurements included visualisation of the blow-off transient with 5 kHz OH* chemiluminescence, which allowed a quantification of the average duration of the blow-off transient. OH-PLIF images at 5 kHz for flames far from and close to extinction showed that the non-premixed flame intermittently lifts-off the bluff body, with increasing probability as the fuel velocity increases. The flame sheet shows evidence of localised extinctions, which are more pronounced as approaching blow-off. The measurements include blow-off limits and their attempted correlation. It was found that a correlation based on a Damkohler number does a reasonable job at collapsing the dataset. The final part examines the blow-off behaviour of swirling spray flames for two different fuels: n-heptane and n-decane. The measurements include blow-off limits and their att~mpted correlation, visualisation of the blow-off transient with 5 kHz OH* chemiluminescence, and the quantification of the average duration of the blow-off transient. It was found that the average duration of the blow-off event is in order of the tens of ms for both spray flames (10-16 ms). The blow-off event is therefore a relatively slow process for the spray ~ames using n-heptane and decane fuels. This suggests that control measures, such as fast fuel injection, coupled with appropriate detection, such as with chemiluminescence monitoring, may have a reasonable chance of success in keeping the flame alight very close to the blow-off limit. These results, together with those obtained for the non-premixed gaseous case form a wide body of experimental data available for the validation of turbulent flame models. The quantification of some properties during the blow-off transient can assist studies of extinction based on large-eddy simulation that have a promise of capturing combustion transients.

This project aims to provide an in-depth understanding on pin-fin based cooling structure for gas turbine heat transfer applications, particularly for turbine hot-path airfoil blade trailing edge cooling. Based on past researches, new cooling structures are then proposed and investigated. This thesis is comprised by four primary studies including three experimental studies and one numerical study. Transient thermochromic liquid crystal method was used to measure endwall heat transfer coefficient of the channel while lumped capacitance method was used to measure the average heat transfer coefficient of cooling structure surface in corresponding studies. The Reynolds number was evaluated and pressure drop of flow across test channel was measured by the pressure taps. Parameters, such as Nusselt number, friction factor and thermal performance index, were evaluated based on experimental results. A scaled realistic NGV (Nozzle Guide Vane) hub platform model was tested. The local heat transfer distribution of its endwall was measured by liquid crystal method. The test has been carried out at Reynolds number range from 10,000 to 40,000. Two impinging nozzle plate with nozzle diameter of 5.5mm and 11.0mm were used. General findings include low heat transfer at acute angle corner and imbalance heat transfer distribution between upstream jet impingement region and downstream pin-fin region. The heat transfer rate at pin-fin region is only 44% of that at jet impingement region. Additionally, the existence of film hole (extraction hole) upstream of pin-fin region has insignificant influence on downstream pin-fin heat transfer in this test. It is also found that the heat transfer has been enhanced by 40% when the impinging nozzle diameter was doubled. Furthermore, the buoyancy effect at inlet flow has certain impact on magnitude and distribution of heat transfer at jet impinging target surface. The new elongated pedestal structure was proposed and investigated experimentally and numerically. Four elongated pedestal test sections with D/d=5.0 and 8.0, X/d=0.8 to 1.2, S/d=1.175 to 1.5 were designed and have been tested at Reynolds number range from 6,000 to 25,000. The average heat transfer coefficient at pedestal surface has been measured by lumped capacitance method. Revealed by the results, the heat transfer coefficient of pedestal surface could be at most 70% higher than that of endwall. Meanwhile, the pedestal surface could account for 50% of overall heat transfer at specific cases. The elongated pedestal structure enhanced the endwall heat transfer up to 9 times compare to reference data. Moreover, the elongated pedestal structure achieved similar heat transfer level comparing with perforated blockage structure but obtained 3 times higher heat transfer enhancement comparing to circular pin-fin structure. Generally, the tightly spaced structure obtained higher overall heat transfer than that of widely spaced structure which is same as circular pin-fin array. Via the numerical study, the flow behavior of elongated pedestal array is more like the turning flow inside the bending duct instead of flow around pin-fin structure. An extra structure, known as split elongated pedestal, has been studied numerically. However, the split elongated pedestal did not show significant improvement as expected in heat transfer enhancement as well as overall thermal performance. Currently split opening did not lead to significant flow interaction between two split parts. But it is recommended to further investigate this structure with much smaller split opening. Furthermore, three test sections with multiply cooling structure implemented were studied at Reynolds number range from 9,000 to 30,000. In addition, the test sections were modified in order to generate non-uniform inlet flow. One key finding is that the non-uniform inlet flow generated in this study leads to 25%-30% reduction in endwall heat transfer. Compare to circular pin-fin structure, cooling structure with high duct cross-section area block ratio, such as elongated pedestal and perforated blockage, provided more desired heat transfer distribution and higher heat transfer rate. Benefited from turbulence promotion by upstream pin-fin array, the heat transfer of downstream cooling configurations have been improved by 51%, 42% and 73% for pin-fin array, elongated pedestal array and perforated blockage array, respectively.

Wind turbines are the largest contributor to renewable energy both in Britain and the rest of Europe. With a rise in the installed capacity and an increase in offshore wind energy due to governments green targets by 2020, there has been a large development in new wind turbines for optimized performance. The present thesis deals with the uncertainties in regards to the lightning phenomenon on wind turbines with emphasis on the rotor blades. Rotor blades are the most expensive part to replace in the event of lightning related damage. The research presents results based on lightning data analysis on wind turbines, backed up by finite element analysis testing of wind turbine systems. The final chapters include the testing and improving of lightning protection systems installed on modern day rotor blades. The first part of the thesis deals with the theoretical understanding of the lightning phenomenon and its effect on wind turbine systems. The core work of the research begins with the analysis of lightning data collected over Nysted wind farm and different wind turbines installed over the world. The data analysis helps in identifying the parts of the wind turbine that are at high risk to lightning attachment and related damage. The peak current levels of the lightning strikes seen on the wind turbine are compared with those in modern day lightning standards, and show that historic data in the standards are not an exact match to the real case scenarios. The lightning data analysis also sheds light into the importance of upward initiated lightning, which will become important for large wind turbines, especially in their new offshore environment. A full scale 3D FEA model of a wind turbine, with lightning protection systems installed in its rotor blades, is subjected to electrical stresses to find likely attachment points in regards to upward initiated lightning, and these results are later compared to those found in the data analysis. The second half of the thesis deals with the testing of new materials and prototype blades, to be introduced to reduce their radar cross section. The new materials include a large amount of carbon content which affects the efficiency of the lightning protection system. High voltage and high current tests backed up with finite element analysis have been performed to find how these new materials affect the performance of the lightning protection system. The results indicate that further work needs to be done before these new materials can be integrated into the blade, as they increase the risk of lightning related damage to the blade.

No description available.

No description available.

Titre de l'écran-titre (visionné le 17 novembre 2020) / Les turbines à axe vertical longtemps gardées dans l'ombre des turbines à axe horizontal commencent depuis quelques années à prendre davantage de place sur le marché des énergies renouvelables. Ce nouvel engouement pour cette technologie est notamment dû aux récentes avancées quant à leur efficacité les rendant de plus en plus concurrentielles. Ce présent mémoire s'inscrit dans la recherche sur l'amélioration de cette technologie en considérant l'utilisation de pales flexibles agissant comme un système passif de variation d'angle d'attaque. Il sera question dans un premier temps d'une étude 2D employant une turbine à pales droites munie d'un bord de fuite flexible. Cette étude est réalisée dans un premier temps afin de sélectionner les meilleures caractéristiques pour une étude 3D qui est menée par la suite. Un modèle employant la mécanique des fluides numériques couplé à un modèle d'élément fini est employé et validé dans ce mémoire. Le logiciel OpenFOAM utilisant une librairie maison pour le couplage solide est utilisé en 2D. La résolution du domaine 3D est quant à elle réalisée en employant le logiciel StarCCM+ ainsi qu'un couplage fluide structure intégrée à même le logiciel. Également, la modélisation de la turbulence est réalisée en employant le modèle de turbulence RANS k-ω SST, dans sa forme instationnaire. Les résultats de la première étude 2D montrent une augmentation de l'efficacité pour des conditions d'opération en dessous et au-delà du point de meilleur rendement. Cependant, il est également démontré que l'utilisation de pales flexibles diminue l'efficacité de la turbine à son point de meilleur rendement. Enfin, l'impact des effets 3D sur le comportement dynamique des pales flexibles vient changer les observations réalisées en 2D au point de meilleur rendement. En effet, la modélisation tridimensionnelle du problème permet de mettre en lumière une augmentation globale de l'efficacité de la turbine en réduisant considérablement la traînée des pales en agissant directement sur la formation des tourbillons de bout de pale / Vertical-axis turbines, long kept in the shadow of horizontal-axis turbines, are starting to gain more importance in the renewable energy market nowadays. This new trend comes from recent advances in the efficiency, making them more competitive. This thesis takes part in the actual research by considering a flexible blade as a possible passive system to improve the efficiency of this technology. At first, a 2D study using a straight blade turbine with a flexible trailing edge is done in order to select the best characteristics for a 3D study which is carried out subsequently. A model employing computational fluid dynamic coupled to a finite element model is used and validated in this thesis. OpenFOAM software is used with an in-house library which allows fluid-solid couplings in 2D. The resolution of the 3D domain is conducted by using StarCCM+ software as well as a fluid-solid coupling integrated into the software. Also, the turbulence modeling is performed using the unsteady form of the RANS k-ω SST turbulence model. The result of the 2D study shows an increase in efficiency for the operating conditions below and above the optimal efficiency point compared with a turbine with rigid blade. It is also shown that the flexible blades decrease the efficiency of the turbine at the optimal efficiency point. However, the impact of 3D effects on the dynamic behaviour of flexible blades changes the observations made in 2D at the optimal efficiency point. Indeed, the threedimensional modeling of the problem makes it possible to highlight an overall increase in the efficiency of the turbine by considerably reducing the vortex drag of the blades by acting directly on the formation of the tip vortices.