Browsing by Author "Amaral Teixeira, Joao"
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Item Open Access Assessment of the power available in a fixed offshore oscillating water column plant(Cranfield University, 2012-12) Holzhauer, Eva; Amaral Teixeira, Joao; Trarieux, F.The early effects of the global warming can be observed and people around the world are beginning to realize the seriousness of the situation. Reducing the CO2 emissions produced by fossil energy seems to be one of the main worldwide technological challenges at the time of writing. Hence, since the oil crisis in the 70s, a growing interest in renewable energies has been noticed. In Europe, the European Commission fixed a target: to produce 20% of the EU energy from renewable sources by 2020. Similar initiatives, in varying degrees, are being considered around the globe. Among all the renewable energy technologies currently on the market, the ocean energy industry is still at an early stage, despite investigations that have been carried out on both tidal and wave energy devices over the past 40 years. The subject of this thesis focuses on one of the wave energy devices: the Oscillating Water Column. The information found in the literature about this type of plants is mainly about onshore and floating offshore OWCs. Very little information about fixed offshore OWC is available. Besides, the availability of large numbers of fixed offshore structures installed in the world oceans suggests that many of these could possibly host an OWC plant. Hence, the present study investigated a fixed offshore OWC. The aim of this thesis is to assess the power available in a fixed offshore OWC plant. To illustrate the procedure of power assessment, the fictional scenario of a platform located in the Santa Maria sea region, off the coast of Californian, is introduced. This work intends to develop a methodology to study the feasibility of such installation and estimate the power extractable through various complementary approaches. From a theoretical approach based on the wave climate of Santa Maria to wave tank experiments with various geometries and shapes of chamber (cylinder and bent duct buoy in frontward and backward position), the viability of a fixed offshore OWC plant is demonstrated for the chosen location. Results highlight the performance of the Backward Bent Duct Buoy (BBDB) for the Santa Maria characteristic sea conditions. With the intention of completing the study with a Computational Fluid Dynamics (CFD) analysis, numerical investigations about the implementation of an alternative method to generate regular waves demonstrates better results of wave propagation than the common wave generation method based on Linear Wave Theory previously used at Cranfield University. In the conclusion, the work achievements and recommendations for future CFD investigations to reproduce the wave tank experiments are discussed.Item Open Access CFD Analyses of Centrifugal Pumps with Emphasis on Factors Affecting Internal Pressure Pulsations(Cranfield University, 2006-10) Spence, R.; Amaral Teixeira, JoaoThe operation of centrifugal pumps can generate instabilities and pressure pulsations that may be detrimental to the integrity and performance of the pump. Until recently these pressure pulsations could only be determined experimentally which resulted in a limited understanding of pressure pulsations around the pump. Industrial pump guarantees are limited to pulsation levels measured at the discharge. However, numerical analysis techniques have advanced to such a stage that they can now be used to explore these effects. The multi-block, structured grid CFD code TASCflow was used to investigate the time variation of pressure within a complete centrifugal pump. A parametric study covered four geometric parameters, namely the cutwater gap, vane arrangement, snubber gap and the sidewall clearance. Taguchi methods allowed the number of transient analyses to be limited to a total of twenty seven. Three flow rates were investigated and the pulsations were extracted at fifteen different locations covering important pump regions. The velocity flow patterns from the transient analyses exhibited important features that were in agreement with two independent sources. The transient flow results compared reasonably with the Weir experimental tests and clearly indicated the pump locations experiencing the largest pulsation levels. It was also noted that monitoring pulsations at the top dead centre of the pump volute casing would provide a better indication of internal pump pulsations than monitoring at the discharge. Taguchi post-processing analysis tools were used to rank the relative importance of the four geometric parameters at each location for each flow rate. The cutwater gap and vane arrangement were found to exert the greatest influence across the various monitored locations and the flow range. However the snubber gap had a dominant influence on the pressure differential across the impeller shroud and pulses in the pressure differential were evident at reduced flows. Through a rationalisation process reductions in pressure pulsations aimed at increased component life and reduced noise/vibration have resulted in a single recommended geometric arrangement. Further analyses confirmed that the new arrangement did indeed produce lesser pulsations levels. Multiple steady state simulations were analysed to determine if they were a viable substitute for the transient analyses. However it was demonstrated that the steady state pulsations did not adequately capture the magnitude and phase of the pulsations shown by the transient results. Likewise the steady state analyses were unable to predict trends for two differing pump geometries. In order to identify the implications of the CFD data for mechanical integrity, the pressure differential predicted by the transient analyses was compared with the pressure loadings currently utilised in Weir design guidelines; this resulted in a new recommendation for use in future designs. Also finite element analyses were conducted using four pressure loadings taken from the numerical results and a centrifugal loading. These supported the recommendation for an increased loading to be used in the design guidelines. The stress levels at the impeller outlet were found to be extremely sensitive to the snubber gap. The completion of this project has allowed a useful set of recommendations to be made regarding the design of high head double entry pumps.Item Open Access A CFD parametric study of geometrical variations on the pressure pulsations and performance characteristics of a centrifugal pump.(Elsevier Science B.V., Amsterdam., 2009-06-01T00:00:00Z) Spence, R.; Amaral Teixeira, JoaoPressure pulsations may be troublesome during the operation and performance of centrifugal pumps. Such pressure pulsations have traditionally been investigated experimentally but numerical analysis techniques allow these effects to be explored. The multi-block, structured grid CFD code TASCflow has been used to investigate the time variation of pressure within a complete double entry, double volute centrifugal pump. This investigation has taken the form of a parametric study covering four geometric parameters, namely the cutwater gap, vane arrangement, snubber gap and the sidewall clearance. Taguchi methods allowed the number of transient analyses to be limited to a total of 27. Three flow rates were investigated and the pulsations were extracted at 15 different locations covering important pump regions. Taguchi post-processing analysis tools were used to rank the relative importance of the four geometric parameters at each location for each flow rate. The cutwater gap and vane arrangement were found to exert the greatest influence across the various monitored locations and the flow range. A rationalisation process aimed at increased component life and reduced noise/vibration through reductions in pressure pulsations has produced geometric recommendations, which should be useful to designers.Item Open Access Condition monitoring philosophy for tidal turbines(RAMS Consultants, 2014-07-01) Elasha, Faris; Mba, David; Amaral Teixeira, JoaoRenewable energy is currently considered as the main solution to reduce greenhouse gas emission. This has led to great developments in the use of renewable energy for electricity generation. Among many renewable energy resources, tidal energy has the advantage of being predictable, particularly when compared to wind energy. Currently the UK is the world leader in extracting energy from the tide; an estimation shows a potential of 67 TWh per year. In order to ensure safe operation and prolonged life for tidal turbines, condition monitoring is essential. The technology for power generation using tidal turbines is new therefore the condition monitoring concept for these devices is yet to be established. Also, there is a lack of understanding of techniques suitable for health monitoring of the turbine components and support structure given their unique operating environment.In this paper the condition monitoring of a tidal turbine is investigated. The objective is to highlight the need for condition monitoring and establish procedures to decide the condition monitoring techniques required, in addition to highlighting the impact and benefits of applying condition based maintenance. A model for failure analysis is developed to assess the needs for condition monitoring and identify critical components, after which a ‘symptoms analysis’ was performed to decide the appropriate condition monitoring techniques. Finally, the impact of condition monitoring on system reliability is considered.Item Open Access Detailed study on stiffness and load characteristics of film-riding groove types using design of experiments(American Society of Mechanical Engineers (ASME), 2017-04-11) Tibos, S. M.; Georgakis, C.; Harvey, K.; Amaral Teixeira, JoaoIn the application of film-riding sealing technology, there are various groove features that can be used to induce hydrodynamic lift. However, there is little guidance in selecting the relative parameter settings in order to maximize hydrodynamic load and fluid stiffness. In this study, two groove types are investigated—Rayleigh step and inclined groove. The study uses a design of experiments approach and a Reynolds equation solver to explore the design space. Key parameters have been identified that can be used to optimize a seal design. The results indicate that the relationship between parameters is not a simple linear relationship. It was also found that higher pressure drops hinder the hydrodynamic load and stiffness of the seal suggesting an advantage for using hydrostatic load support in such conditions.Item Open Access Development of a Port-Hamiltonian Model for use in oscillating water column control scheme investigations.(Cranfield University, 2014-12) Farman, Judith; Amaral Teixeira, JoaoWith global energy demand estimated to rise considerably and global warming accepted by the majority of scientists, the pressure to reduce fossil fuel usage is increasing. To this end, the UK government has set a target of generating 50% of electricity from renewable energy sources by 2050. It can therefore be deduced that decreasing the cost of renewable energy by increasing the energy capture is critical. Oscillating Water Columns (OWCs) employing bidirectional turbines coupled with generators can be used to capture energy from oceanic waves and convert it to electrical energy. This thesis includes a study to quantify the potential power smoothing that can be achieved from a wave farm of ideal OWC devices and from auxiliary hardware such as flywheel energy storage systems. Also detailed are the upgrades to the OWC test facility at Cranfield University, including the world-first capability to simulate polychromatic waves. This test facility has been employed to validate turbine characteristics derived from Computational Fluid Dynamic (CFD) numerical results. This thesis contains a literature review of the existing control strategies for OWCs that concludes that the optimization of power capture from individual components in the energy chain forces system-level compromises. This conclusion drove the development of an unique energy-based model of the complete wave-to-wire system utilizing port-Hamiltonian mechanics which mandated two modifications to the port-Hamiltonian framework. The first modification to the port-Hamiltonian framework resulted in a new generalized means of modeling systems where the potential energy is dependent on the momentum variables. The second modification expands the port-Hamiltonian framework to allow the modeling of ow source systems in addition to effort source systems. The port-Hamiltonian wave-to-wire OWC model enables the future development of a control approach that optimizes power capture at a system level. As a first step to achieving this goal an Injection Damping Assignment (IDA) Passivity Based Control (PBC) strategy was successfully applied to an OWC system and an energy storage flywheel system. These strategies pave the way for future developments utilizing optimization techniques, such as the use of cost functions to identify the peak efficiency operating condition.Item Open Access Diagnosis of low-speed bearing degradation using acoustic emission techniques(Cranfield University, 2017-01) Alshimmeri, Fiasael; Addali, Abdulmajid; Amaral Teixeira, JoaoIt is widely acknowledged that bearing failures are the primary reason for breakdowns in rotating machinery. These failures are extremely costly, particularly in terms of lost production. Roller bearings are widely used in industrial machinery and need to be maintained in good condition to ensure the continuing efficiency, effectiveness, and profitability of the production process. The research presented here is an investigation of the use of acoustic emission (AE) to monitor bearing conditions at low speeds. Many machines, particularly large, expensive machines operate at speeds below 100 rpm, and such machines are important to the industry. However, the overwhelming proportion of studies have investigated the use of AE techniques for condition monitoring of higher-speed machines (typically several hundred rpm, or even higher). Few researchers have investigated the application of these techniques to low-speed machines (<100 rpm), This PhD addressed this omission and has established which, of the available, AE techniques are suitable for the detection of incipient faults and measurement of fault growth in low-speed bearings. The first objective of this research program was to assess the applicability of AE techniques to monitor low-speed bearings. It was found that the measured statistical parameters successfully monitored bearing conditions at low speeds (10-100 rpm). The second objective was to identify which commonly used statistical parameters derived from the AE signal (RMS, kurtosis, amplitude and counts) could identify the onset of a fault in either race. It was found that the change in AE amplitude and AE RMS could identify the presence of a small fault seeded into either the inner or the outer races. However, the severe attenuation of the signal from the inner race meant that, while AE amplitude and RMS could readily identify the incipient fault, kurtosis and the AE counts could not. Thus, more attention needs to be given to analysing the signal from the inner race. The third objective was to identify a measure that would assess the degree of severity of the fault. However, once the defect was established, it was found that of the parameters used only AE RMS was sensitive to defect size. The fourth objective was to assess whether the AE signal is able to detect defects located at either the centre or edge of the outer race of a bearing rotating at low speeds. It is found that all the measured AE parameters had higher values when the defect was seeded in the middle of the outer race, possibly due to the shorter path traversed by the signal between source and sensor which gave a lower attenuation than when the defect was on the edge of the outer race. Moreover, AE can detect the defect at both locations, which confirmed the applicability of the AE to monitor the defects at any location on the outer race.Item Open Access The effects of wear on abradable honeycomb labyrinth seals(Cranfield University, 2007-03) Collins, Dermot; Amaral Teixeira, JoaoThis thesis reports on work undertaken to understand the effects, due to wear, on the performance of abradable honeycomb labyrinth seals. The phenomena studied are aerodynamic in nature and include compressible flow, turbulent flow, recirculation and separation at a range of pressure ratios from 1.20 up to 3.50. Four primary methods of investigation were used: experimental, numerical using CFD, numerical using theoretical derivations and numerical using established labyrinth seal specific computer codes. Effects of seal clearance, pressure ratio and tooth to groove location have been investigated with overall performance and inter-seal pressure distribution recorded experimentally and numerically for comparison. Worn experimental results, when compared to their unworn equivalent, recorded large increases in mass flow of up to 50% when the labyrinth teeth are located centrally in the groove. Significant performance enhancements were achieved through offsetting the teeth with respect to the groove, particularly in an upstream sense. There was a marked deterioration with the labyrinth teeth located at the groove exit. Inter-seal pressure distributions showed that the first and final teeth did most of the work achieving significantly larger pressure drops which goes against current seal understanding of increasing pressure drop through the seal. Numerical work was undertaken to further investigate these effects. However, due to the complex 3-D geometry of an abradable honeycomb labyrinth seal a 2-D simplification technique was developed to speed up the investigative process. Using this technique CFD was found capable of replicating the experimental data regarding overall seal performance and inter-seal pressure distributions. The pressure on the final tooth proved to be the hardest experimental data to recreate using CFD, particularly at high pressure ratios when shocks are likely to form. Further numerical work was undertaken using computer codes and theoretical derivations. This work proved that the understanding of the seal loss coefficients used by both methods was not adequate for the current study with the experimental data recreated least successfully. Suggestions are given for enhancement of seal design, including axial location and seal computational routines, which will limit the impact of a 1.5% increase in operational cost that is likely to accrue from seal deterioration.Item Open Access Exploration of the possibility of acoustic emission technique in detection and diagnosis of bubble formation and collapse in valves(International Organization of Scientific Research, 2016-11-30) Alhashan, Taihiret; Addali, Abdulmajid; Amaral Teixeira, JoaoThe application of acoustic emission (AE) technique in detection and monitoring of bubble formation and collapse in valves are presented in this review. The generation of AE signals and the basic compositions of AE detection system are briefly explained. The applications of AE technique in valves are focused on condition monitoring and detection bubble formation (bubble cavitation), and leakage of water through valves. All results prove that the AE technique works well for detection and diagnosis of failures during valves.Item Open Access Fault diagnosis and health management of bearings in rotating equipment based on vibration analysis – a review(JVE International, 2021-11-26) Althubaiti, Adnan; Elasha, Faris; Amaral Teixeira, JoaoThere is an ever-increasing need to optimise bearing lifetime and maintenance cost through detecting faults at earlier stages. This can be achieved through improving diagnosis and prognosis of bearing faults to better determine bearing remaining useful life (RUL). Until now there has been limited research into the prognosis of bearing life in rotating machines. Towards the development of improved approaches to prognosis of bearing faults a review of fault diagnosis and health management systems research is presented. Traditional time and frequency domain extraction techniques together with machine learning algorithms, both traditional and deep learning, are considered as novel approaches for the development of new prognosis techniques. Different approaches make use of the advantages of each technique while overcoming the disadvantages towards the development of intelligent systems to determine the RUL of bearings. The review shows that while there are numerous approaches to diagnosis and prognosis, they are suitable for certain cases or are domain specific and cannot be generalised.Item Open Access Heat removal in axial flow high pressure gas turbine(Cranfield University, 2016) Alhajeri, Hamad; Amaral Teixeira, Joao; Addali, AbdulmajidThe demand for high power in aircraft gas turbine engines as well as industrial gas turbine prime mover promotes increasing the turbine entry temperature, the mass flow rate and the overall pressure ratio. High turbine entry temperature is however the most convenient way to increase the thrust without requiring a large change in the engine size. This research is focused on improving the internal cooling of high pressure turbine blade by investigating a range of solutions that can contribute to the more effective removal of heat when compared with existing configuration. The role played by the shape of the internal blade passages is investigated with numerical methods. In addition, the application of mist air as a means of enhanced heat removal is studied. The research covers three main area of investigation. The first one is concerned with the supply of mist on to the coolant flow as a mean to enhancing heat transfer. The second area of investigation is the manipulation of the secondary flow through cross-section variation as a means to augment heat transfer. Lastly a combination of a number of geometrical features in the passage is investigated. A promising technique to significantly improve heat transfer is to inject liquid droplets into the coolant flow. The droplets which will evaporate after travelling a certain distance, act as a cooling sink which consequently promote added heat removal. Due to the promising results of mist cooling in the literature, this research investigated its effect on a roughened cooling passage with five levels of mist mass percentages. In order to validate the numerical model, two stages were carried out. First, one single-phase flow case was validated against experimental results available in the open literature. Analysing the effect of the rotational force, on both flow physics and heat transfer, on the ribbed channel was the main concern of this investigation. Furthermore, the computational results using mist injection were also validated against the experimental results available in the literature. Injection of mist in the coolant flow helped achieve up to a 300% increase in the average flow temperature of the stream, therefore in extracting significantly more heat from the wall. The Nusselt number increased by 97% for the rotating leading edge at 5% mist injection. In the case of air only, the heat transfers decrease in the second passage, while in the mist case, the heat transfer tends to increase in the second passage. Heat transfer increases quasi linearly with the increase of the mist percentage when there is no rotation. However, in the presence of rotation, the heat transfers increase with an increase in mist content up to 4%, thereafter the heat transfer whilst still rising does so more gradually. The second part of this research studies the effect of non-uniform cross- section on the secondary flow and heat transfer in order to identify a preferential design for the blade cooling internal passage. Four different cross-sections were investigated. All cases start with square cross-section which then change all the way until it reaches the 180 degree turn before it changes back to square cross-section at the outlet. All cases were simulated at four different speeds. At low speeds the rectangle and trapezoidal cross-section achieved high heat transfer. At high speed the pentagonal and rectangular cross-sections achieved high heat transfer. Pressure loss is accounted for while making use of the thermal performance factor parameter which accounts for both heat transfer and pressure loss. The pentagonal cross-section showed high potential in terms of the thermal performance factor with a value over 0.8 and higher by 33% when compared to the rectangular case. In the final section multiple enhancement techniques are combined in the sudden expansion case, such as, ribs, slots and ribbed slot. The maximum heat enhancement is achieved once all previous techniques are used together. Under these circumstances the Nusselt number increased by 60% in the proposed new design.Item Open Access Heat removal in high pressure turbine seal segments(Cranfield University, 2017-02) Alenezi, Abdulrahman H.; Amaral Teixeira, Joao; Addali, AbdulmajidAn important parameter for turbomachinery designers is “clearance control”, because the clearances between interfaces must be set to optimum values to maximize power output, operational life and efficiency. Leakage of hot gas result- ing from excessive clearance, can lead to flow instabilities, components overheat- ing, lower cycle efficiency and a dramatic increase in specific fuel consumption (SFC). Seal segments are used to reduce blade tip leakage, maintain coolant air flow and the stability of rotor-dynamic systems, helping to maximize blade perfor- mance. Seal segments in the High-Pressure Turbine (HPT) stages are one of the hottest components as they face the hot gases coming from the combustion chamber with temperatures which can reach 1700 0 C and which makes them sub- ject to oxidation, erosion, and creep. Thus, seal segments need to be protected. They are currently cooled using jet impingement techniques, passing cooling air (supplied by the high-pressure stage of the compressor) through channels to di- rectly impinge on the hot surfaces. The focus of this research was to improve the jet impingement cooling of the seal segments in HPTs by investigating methods that provide more effective heat removal. The role played by configurations of ribs (surface roughness using be- spoke turbulators), custom-made seal-segments, and surface features such as contouring, both in isolation and combination, were investigated using numerical methods. A set of 174 simulations were carried including the use of uniform and non-uniform roughness elements with different shapes and heights. Firstly, three different uniform roughness elements were tested, a square cross-sectional continuous rib, a hemi-spherical pin-fin and a cubical pin-fin for three jet impingement angles of α=90°, 60° and 45°. Each roughness element was also tested for six different heights (e) between 0.25 mm and 1.5 mm in increments of 0.25 mm. Results are presented in the form of average Nusselt number within and beyond the stagnation region. Secondly, the effect of using a roughness element with a square cross section in the shape of a circle, on the average Nu was investigated for four different radial locations (R), three jet angles (α) and six rib heights (e). Finally, the roughness element used was continuous, of square cross-sec- tion, in the shape of tear drops and reversed tear drops. This meant the rib did not act as a total barrier to flow in either the uphill or downhill direction.Item Open Access A hybrid prognostic methodology for tidal turbine gearboxes(Elsevier, 2017-07-24) Elasha, Faris; Mba, David; Togneri, Michael; Masters, Ian; Amaral Teixeira, JoaoTidal energy is one of promising solutions for reducing greenhouse gas emissions and it is estimated that 100 TWh of electricity could be produced every year from suitable sites around the world. Although premature gearbox failures have plagued the wind turbine industry, and considerable research efforts continue to address this challenge, tidal turbine gearboxes are expected to experience higher mechanical failure rates given they will experience higher torque and thrust forces. In order to minimize the maintenance cost and prevent unexpected failures there exists a fundamental need for prognostic tools that can reliably estimate the current health and predict the future condition of the gearbox. This paper presents a life assessment methodology for tidal turbine gearboxes which was developed with synthetic data generated using a blade element momentum theory (BEMT) model. The latter has been used extensively for performance and load modelling of tidal turbines. The prognostic model developed was validated using experimental data.Item Open Access New pH correlations for stainless steel 316L, alumina, and copper(I) oxide nanofluids fabricated at controlled sonication temperatures(Trans Tech Publications, 2019-06-19) Ali, Naser; Amaral Teixeira, Joao; Addali, AbdulmajidThis research investigates the pH value of stainless steel (SS) 316L/ deionised water (DIW), alumina (Al2O3)/DIW, and copper (I) oxide (Cu2O)/DIW nanofluids prepared using a two-step controlled sonication temperature approach of 10°C to 60°C. The nanoparticles volumetric concentration of each family of as-prepared nanofluid ranged from 0.1 to 1.0 vol%, using as-received nanopowders, of 18 – 80 nm average particles size. Furthermore, the pH measuring apparatus and the measurement procedure were validated by determining the pH of commercially supplied calibration fluids, of pH 4, 7, and 10. Following the validation, pH correlations were obtained from the experimental measurements of the 0.1, 0.5, and 1.0 vol% nanofluids in terms of varied sonication bath temperatures and volumetric concentrations. Those correlations were then combined into one robust pHnf correlation and validated using the pH data of the 0.3 and 0.7 vol% nanofluids. The new proposed correlation was found to have a 2.18%, 0.92%, and 0.63%, average deviation from the experimental pH measurements of SS 316L, Al2O3, and Cu2O nanofluids, respectively, with an overall prediction accuracy of ~ 92%.Item Open Access Optimisation of bidrectional impulse turbines for wve power generation(Cranfield University, 2009-03) Banks, K; Amaral Teixeira, JoaoThe generation of electricity from ocean waves using oscillating water column (OWC) wave energy converters is currently uneconomic due to the high capital cost and low efficiencies of such devices. The bidirectional air turbines utilised in OWCS are one of the principal sources of inefficiency and a significant increase in their performance would improve the prospects of commercial scale wave power generation. The ability of computational fluid dynamics (CFD) to predict the performance of both Wells and impulse type bidirectional turbines for use in OWCS was examined by comparison with experimental results taken from the literature. A design process was then undertaken for a datum impulse turbine and a novel high-efficiency impulse turbine arrangement. Numerical performance predictions are presented with a comparison against experimental data from a large-scale oscillating-flow test rig. An automated design and aerodynamic optimisation system was subsequently developed for application to this novel impulse turbine design. The optimiser employs a hybridised genetic algorithm along with Kriging meta¬models to significantly decrease the number of expensive calls to the 3D-CFD code used to evaluate the objective function. Comparisons to a number of state of the art optimisation algorithms from the literature on some mathematical test functions indicated that the optimiser had equivalent or better performance for most problems. A parameter study was carried out to investigate the effect of various turbine in design variables, before undertaking a 14-variable global design optimisation. A 5-variable optimisation exercise was then performed to investigate the effi- ciency gains that could be achieved by using three-dimensional rotor blades. Substantial gains in performance were attained and the predicted levels of efficiency are significantly higher than those previously reported in the literature for other bidirectional impulse turbine designs.Item Open Access Tidal turbine modelling from the perspective of design and operation(Cranfield University, 2016-06) Corsar, Michael; Amaral Teixeira, JoaoThe aim of this thesis is to study the effects of turbulent flow on a fixed pitch tidal current turbine from the perspective of turbine design and operation. A prototype turbine, Deltastream as it is known, is being developed by Tidal Energy Ltd for deployment in Ramsey Sound, Wales. It is well known that turbulence plays an important role in the fatigue life of marine turbines. Field measurements of tidal flow at the turbine site were analysed to establish the velocity spectra and turbulence intensity. This revealed a wide range of anisotropic turbulence which is dependent upon the tidal direction with intensities ranging from 5-20%. A numerical turbine model based on momentum theory was constructed in a time marching formulation that accounts for the effects of dynamic inflow and rotationally augmented airfoil stall delay properties. The turbine rotor design allows for load alleviation by regulation of the turbine tip speed ratio. At flow velocities above the rated velocity the tip speed ratio can be increased to reduce turbine loads. The model has been combined with a novel rotor speed control algorithm that estimates unsteady turbine inflow velocity from turbine loading without the requirement for external sensing of flow speed. When the turbine is subjected to three dimensional turbulent inflow the rotor speed controller has been shown to significantly reduce the fatigue effect of unsteady, turbulent flow. The turbine blade design has been developed using the model established. Experimental validation studies were carried out at 1/16th scale in turbulent conditions. Studies using the model have; identified the relationship between turbulence intensity and turbine fatigue load, established a controller schedule to significantly reduce fatigue loading and determined the blading fatigue life in realistic turbulent flows.Item Open Access Transition Modelling for Axial Compressor Flows(Cranfield University, 2008-11-03) Beevers, A.; Wells, Roger; Amaral Teixeira, Joao; Smart, Palie; Engineering and Physical Sciences (EPSRC)Abstract The application of Menter's transition model (Menter et al. (2004a), here-after known as the ץ - θ model) available in the CFX CFD code, for use within an axial compressor design group was studied. Simulations of a range of turbomachinery applicable test cases were undertaken, including a range of transitional flat plates and a 2D compressor cascade. Results were com¬pared to experimental data and the results of simulations performed with standard turbulence models. The ץ - θ model significantly improved the prediction of the boundary layer development, compared to the turbulence models. Comparisons with ex¬perimental data were also good. Features such as mid-chord transitional separation bubbles were predicted with the ץ - θ model, but not with the turbulence models. The ץ - θ model offered no consistent improved accuracy over the κ - ω SST turbulence model when predicting leading edge separa¬tion bubbles. The more accurate simulation of the boundary layer enables a closer prediction of viscous losses. 2D and 3D unsteady simulations of a low-speed axial compressor stator blade boundary layer, subject to impinging rotor wakes, were conducted. The pur¬pose was to determine the performance of the ץ - θ model in this environment, as there is no available literature for this. For both simulations, the model gave a good qualitative agreement to experimental data in the prediction of passing rotor wake effects on the suction surface. The effects on the pressure surface transition region due to wake passing were poorly predicted. All models were simulated on low and high-speed axial compressor stages. Results showed no improvement over the turbulence models of the ץ - θ model to predict blade exit parameters. The ץ - θ model does not present a significant enough improvement in the prediction of the flow to warrant its regular use in the design of axial compressor blading. However, it presents a useful tool in the development of high lift compressor blading.Item Open Access Unsteadiness In An Embedded Axial Compressor Stage(Cranfield University, 2008-02) Naylor, Edward; Ivey, Paul C.; Amaral Teixeira, JoaoPrevious research on blade boundary layers in turbomachinery have been recognised to crucially influence the stability and performance of the gas turbine components. The interactions between rotating and stationary blade rows inevitably make the flow environment within a multistage axial compressor unsteady. Research conducted at midspan in Low Pressure turbines has shown that patches of transitional flow can withstand higher levels of deceleration, helping the boundary layer stay attached. An experimental investigation into unsteadiness in a embedded stage was conducted in the third-stage of the Cranfield four-stage Low Speed Research Compressor at two operating points: peak efficiency and near stall. This build of the Cranfield Rig was equipped with three-dimensional blading. A three-hole pressure probe was traversed at the exit of Rotor 3 in the rotating frame of reference and at the exit of Stator 3 in the stationary frame of reference. In addition measurements were made at the exit of both Rotor and Stator 3 using a slanted hotwire rotated about its axis. This measurement technique gave time-resolved three-dimensional velocities. Coupled to the exit traverses a series of boundary layer traverses were performed along Stator 3 suction surface covering the midchord region at midspan and close to the casing endwall. To aid in the understanding and interpretation of the experimental campaign, three-dimensional computations of Stator 3 were made at the two operating points using the commercial Computational Fluid Dynamics code ANSYS-CFX . A two-dimensional unsteady calculation of Rotor 3, Stator 3 and Rotator 4 at midspan and peak efficiency was also performed. The time-resolved measurements downstream of Rotor 3 showed that the rotor wake was characterised by high levels of random unsteadiness and increased incidence onto the stator row. The increase in incidence across the wake was two to three times that experienced with change in flow coefficient. Therefore the increased incidence and turbulence in a rotor wake will have a significant influence on the unsteady development of a downstream boundary layer. Measurements of the boundary layer at design condition at midspan show evidence of laminar and transitional flow up to 50% of the suction surface length. The boundary layer flow periodically undergoes transition due to the convection of the wake-induced strip that was generated close to the leading edge. Towards the casing the picture is altered slightly due to the stator-casing separation region. Boundary layer transition is completed farther forward and the transition length reduced. At off-design the picture is completely altered. Transition is completed upstream of 25% suction surface length and the flow shows only a modulating variation with blade passing. The stator-casing separation region grows in spanwise extent and the boundary layer flow on the stator surface is completely separated aft of 50% of suction surface length.