Browsing by Author "Teixeira, Joao Amaral"
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Item Open Access Aerodynamic optimisation of an industrial axial fan blade(Cranfield University, 2006-09) Lotfi, O.; Teixeira, Joao AmaralNumerical optimisation methods have successfully been used for a variety of aerodynamic design problems over quite a few years. However the application of these methods to the aerodynamic blade shape optimisation of industrial axial fans has received much less attention in the literature probably given the fact that the majority of resources available to develop these automated design approaches is to be found in the aerospace field. This work presents the development of an automated design process which was developed to aerodynamically optimise the fan blade geometry. It involves the application of Genetic Algorithm (GA) methods to the aerodynamic shape optimisation of a two-dimensional axial fan cascade as well as the development of a three- dimensional shape optimisation routine. Navier-Stokes CFD codes were used for the 2 and 3-D analyses using steady simulations. The effects of the Variation of the control parameters on the performance of the GA as a optimisation tool is presented. The tournament selection, uniform crossover, creep mutation scheme with elitism appears to work the best of this application. The parallelisation of genetic algorithm was also developed using the Message Passing Interface (MPI) scheme. This essentially reduces the running time for each generation to the amount of time required for performing the genetic operations on just one individual. The aerodynamic optimisation of a low speed fan cascade based on genetic algorithm is presented. A commercial turbo machinery CFD code, CFX-TASCow, was used in the evaluation of the objective function. The optimisation process reduces the total pressure Aerodynamic Optimisation of Industrial Axial Fan Blade O.Lotï ¬ iii Executive Summary Cfae UNIVERSITY fold loss while maintaining the same loading and mass flow rate. This development is related to a significant change in profile curvature in the vicinity of the trailing edge. The flow field inside low speed axial fans characterized by low hub-tip ratio can be highly three-dimensional and particularly complex. As a consequence three-dimensional automated design process was developed to aerodynamically optimise the fan blade geometry taking account of the predicted three dimensional flow. The optimiser employs a genetic algorithm for global optimization purposes and is coupled to the academic Navier-Stokes solver MULTIP. The numerical investigation of the overall performance, efficiency and work-input characteristics of the fan were found to agree well with the previously reported experimental results. The optimization task is accomplished by modifying the blade camber line, lean and sweep while keeping the blade thickness distribution and mass flow rate constant. The optimisation process demonstrated that the fan efficiency can be improved by changing the profile curvature and giving the blade a proper forward sweep. Nevertheless the effect of introducing lean and backward sweep did not improve the fan performance for this particular application. This study demonstrated that the present method offers a promising approach to industrial axial fan designers to help design better machines while contributing to the softening of the design cycle. The results obtained show that the genetic algorithm when coupled to a CFD tool is not only capable of achieving a improvement in the designs of existing axial fan blades effectively but also that they achieve these results with a minimum amount of user expertise.Item Open Access Assessment of surface roughness effects on micro axial turbines(ASME, 2021-01-11) Gamil, Abdelaziz A. A.; Nikolaidis, Theoklis; Teixeira, Joao Amaral; Madani, S. H.; Izadi, AliSurface roughness significantly affects the aerodynamics and heat transfer within micro-scale turbine stages. It results in a considerable increment in the blade profile loss and leads consequently to sizeable performance reductions. The provision of low roughness surfaces in micro gas turbine stages presents challenges on account of the small (mm scale) sizes, manufacturing complexity and associated costs. The axial turbine investigated in this study is fitted to Samad Power’s TwinGen domestic micro combined heat and power unit. The micro gas turbine has a compressor pressure ratio of 3, 1200K turbine inlet temperature and a rotational speed of 170,000 rpm. This paper presents a numerical assessment of the effects of varying the surface roughness on the performance and heat transfer of the micro turbine. The surface roughness was uniformly distributed on the NGV and rotor blades. The results showed that increasing the surface roughness from 3 microns to 6, 20, and 100 microns resulted in a reduction in stage total efficiency of 0.8%, 4% and 12% respectively as well as a comparable decrease in output power (0.7%, 3.6%, and 11% respectively). The turbine temperature was also observed to be very sensitive to surface roughness and a temperature increase of some 5% at the rotor hub and over 4% increment in the blade tip surface was observed for 100 microns when compared to the 3 microns surface roughness case. The findings of this paper highlight the adverse effects of the surface roughness on the micro-turbine performance and temperature distribution as well as the importance of careful consideration of wall roughness during the design and manufacturing stagesItem Open Access Benefits, drawbacks, and future trends of Brayton helium gas turbine cycles for gas-cooled fast reactor and very-high temperature reactor Generation IV nuclear power plants(American Society of Mechanical Engineers, 2020-10-02) Gad-Briggs, Arnold; Osigwe, Emmanuel O.; Pilidis, Pericles; Nikolaidis, Theoklis; Sampath, Suresh; Teixeira, Joao AmaralNumerous studies are on-going on to understand the performance of generation IV (Gen IV) nuclear power plants (NPPs). The objective is to determine optimum operating conditions for efficiency and economic reasons in line with the goals of Gen IV. For Gen IV concepts such as the gas-cooled fast reactors (GFRs) and very-high temperature reactors (VHTRs), the choice of cycle configuration is influenced by component choices, the component configuration and the choice of coolant. The purpose of this paper to present and review current cycles being considered—the simple cycle recuperated (SCR) and the intercooled cycle recuperated (ICR). For both cycles, helium is considered as the coolant in a closed Brayton gas turbine configuration. Comparisons are made for design point (DP) and off-design point (ODP) analyses to emphasize the pros and cons of each cycle. This paper also discusses potential future trends, include higher reactor core outlet temperatures (COT) in excess of 1000 °C and the simplified cycle configurations.Item Open Access Blade roughness effects on compressor and engine performance—a CFD and thermodynamic study(MDPI, 2021-11-04) Alqallaf, Jasem; Teixeira, Joao AmaralDegradation of compressors is a common concern for operators of gas turbine engines (GTEs). Surface roughness, due to erosion or fouling, is considered one of the major factors of the degradation phenomenon in compressors that can negatively affect the designed pressure rise, efficiency, and, therefore, the engine aero/thermodynamic performance. The understanding of the aerodynamic implications of varying the blade surface roughness plays a significant role in establishing the magnitude of performance degradation. The present work investigates the implications due to the degradation of the compressor caused by the operation in eroding environments on the gas turbine cycle performance linking, thereby, the compressor aerodynamics with a thermodynamic cycle. At the core of the present study is the numerical assessment of the effect of surface roughness on compressor performance employing the Computational Fluid Dynamics (CFD) tools. The research engine test case employed in the study comprised a fan, bypass, and two stages of the low pressure compressor (booster). Three operating conditions on the 100% speed-line, including the design point, were investigated. Five roughness cases, in addition to the smooth case, with equivalent sand-grain roughness (ks) of 15, 30, 45, 60, and 150 µm were simulated. Turbomatch the Cranfield in-house gas turbine performance simulation software, was employed to model the degraded engine performance. The study showed that the increase in the uniform roughness is associated with sizable drops in efficiency, booster pressure ratio (PR), non-dimensional mass flow (NDMF), and overall engine pressure ratio (EPR) together with rises in turbine entry temperature (TET) and specific fuel consumption (SFC). The performance degradation evaluation employed variables such as isentropic efficiency (ηis), low pressure compressor (LPC) PR, NDMF, TET, SFC, andEPR. The variation in these quantities showed, for the maximum blade surface degradation case, drops of 7.68%, 2.62% and 3.53%, rises of 1.14% and 0.69%, and a drop of 0.86%, respectively.Item Open Access Deposition of stainless steel thin films: an electron beam physical vapour deposition approach(MDPI, 2019-02-14) Ali, Naser; Teixeira, Joao Amaral; Addali, Abdulmajid; Saeed, Maryam; Al-Zubi, Feras; Sedaghat, Ahmad; Bahzad, HusainThis study demonstrates an electron beam physical vapour deposition approach as an alternative stainless steel thin films fabrication method with controlled layer thickness and uniform particles distribution capability. The films were fabricated at a range of starting electron beam power percentages of 3–10%, and thickness of 50–150 nm. Surface topography and wettability analysis of the samples were investigated to observe the changes in surface microstructure and the contact angle behaviour of 20 °C to 60 °C deionised waters, of pH 4, pH 7, and pH 9, with the as-prepared surfaces. The results indicated that films fabricated at low controlled deposition rates provided uniform particles distribution and had the closest elemental percentages to stainless steel 316L and that increasing the deposition thickness caused the surface roughness to reduce by 38%. Surface wettability behaviour, in general, showed that the surface hydrophobic nature tends to weaken with the increase in temperature of the three examined fluids.Item Open Access Design of a low speed vaneaxial fan(Cranfield University, 2011-10) Andreadis, Eleftherios; Teixeira, Joao AmaralThe ventilation of industrial areas and tunnels is a safety requirement and characterizes the quality of a working environment. Low speed fans are used to achieve the required ventilation level. An attempt to design a low speed vaneaxial fan, meeting the specifications of a given fan that is already in the market takes place in this Thesis. The project was conducted with the support of the Fläkt-Woods Company and the main target is to design a ventilation fan, meeting the requirements for pressure rise, volume flow and size, of an existing model. The efficiency improvement is driven in part by the new national and international legislation concerning the operation of electrical equipment. Companies require higher efficiencies without compromising safety features of the fan and the fan capability to operate at high temperatures. A low speed fan design procedure is established based on the available literature and design tools. The free vortex approach is employed, which provides acceptable efficiency and relatively simpler design. The design procedure can be used to design a fan given a set of customer requirements. Many software tools are used to design the fan. A Matlab code for the blade design is developed and other codes are used to establish the final fan design. The effectiveness of the design procedure is verified with CFD simulations carried out as part of this project. Three new designs that are developed with the established design procedure are presented in this Thesis. The new designs differ in the hub to tip ratio, the rotational speed and the number of the blades and the vanes. The experience acquired from the analysis of the performance of the first new design is used to improve the performance of the following designs in order to achieve the best efficiency possible. The effect of tip clearance is investigated thoroughly in the new designs because the tip clearance has a major impact on the fan performance and safe operation of the fan at high temperatures. The mechanical integrity of the fan is examined last to verify that the fan can operate in high temperature. The target of improved efficiency (higher than 79%) is achieved in one of the fan designs attempted and it was calculated 82%. The off design performance of the new fan is satisfactory as well. This new design can be further optimized, since the modification of minor design features is in itself a methodology that can incrementally improve the efficiency of a low speed fan. The new fan can operate at high temperatures (400°C), however the safety factor at this temperature is 1.25 for combined steady mechanical and thermal loading and it can be further improved either through the use of materials with better resistance in thermal loading or with an increased tip clearance.Item Open Access Detection of natural crack in wind turbine gearbox(Elsevier, 2017-10-30) Shanbr, Suliman; Elasha, Faris; Elforjani, Mohamed; Teixeira, Joao AmaralOne of the most challenging scenarios in bearing diagnosis is the extraction of fault signatures from within other strong components which mask the vibration signal. Usually, the bearing vibration signals are dominated by those of other components such as gears and shafts. A good example of this scenario is the wind turbine gearbox which presents one of the most difficult bearing detection tasks. The non-stationary signal analysis is considered one of the main topics in the field of machinery fault diagnosis. In this paper, a set of signal processing techniques has been studied to investigate their feasibility for bearing fault detection in wind turbine gearbox. These techniques include statistical condition indicators, spectral kurtosis, and envelope analysis. The results of vibration analysis showed the possibility of bearing fault detection in wind turbine high-speed shafts using multiple signal processing techniques. However, among these signal processing techniques, spectral kurtosis followed by envelope analysis provides early fault detection compared to the other techniques employed. In addition, outer race bearing fault indicator provides clear indication of the crack severity and progress.Item Open Access Development of polychormatic irregular waves for testing OWC bidirectional turbines at a land-based test facility(European Wave and Tidal Energy Conference, 2013-09) Farman, Judith R.; Teixeira, Joao Amaral; Whidborne, James F.; Mba, David; Natanzi, ShahabThis paper presents the development of a control strategy that allows a dedicated oscillating water column turbine test facility to produce polychromatic waves. This facility allows testing of bi-directional turbines under transient conditions, validation of computational fluid dynamics simulations and testing of turbine control strategies under realistic sea states. The test facility employs a pneumatic wave generator (of unique design) to simulate the air flow of an oscillating water column. This work describes the methodology to create wave time series of various spectra that replicate a range of wave states at various geographical locations employing this rig.Item Open Access Developments for the calculation of heavily loaded journal bearings(Cranfield University, 2010-05) Barrett, D. J. S.; Teixeira, Joao Amaral; McLuckie, I. R. W.This thesis describes the development of an ElastoHydroDynamic (EHD) bearing calculation. The effect of body forces is shown to be important for highly loaded bearings in reciprocating internal combustion engines. Extension of the program to rotating machinery includes an examination of instability in the shaft bearings of a turbocharger. The development of a parameter to predict cavitation damage in a bearing is promising. Several calculation results using the program are shown. These are engine main bearing and connecting rod big-end bearings and full floating bearings for a turbocharger. The calculations on the big-end bearing if a racing engine show why the designers were having difficulty understanding the correct location for the oil feed hole position. Effects of elastic deformation, thermal deformation and manufacturing/assembly deformation all have a significant effect on the extent of the oil film. A novel calculation for a cavitation damage parameter is demonstrated successfully for a heavily loaded diesel engine bearing. The importance of body forces on the oil film due to high accelerations on certain bearings is shown to be theoretically important but not yet demonstrated. The program was written with the intention to be incorporated into the sponsoring company’s range of engine design software. A part of that development process included carrying out calculations to demonstrate to customers and present papers at conferences. The results of some of these calculations have been included in this thesis. Results of a study on the effect of crankshaft geometry on racing engine viscous friction losses were reported in a paper presented at the IDETC conference in Long Beach, 2005. This study used the first version of the software which only included Rigid Hydro Dynamics (RHD) at the time but was usable. Results of a study on stability of shaft motion in high speed turbocharger bearings were reported in a paper at the 8th International Turbocharger conference in London, 2006. At this time the program was still only capable of RHD calculations but could now solve for multiple oil films simultaneously and sweep through the speed range. The studies on the effects of body forces and the development of a cavitation parameter will be presented in papers in the near future.Item Open Access The effect of aluminium nanocoating and water pH value on the wettability behavior of an aluminium surface(Elsevier, 2018-02-27) Ali, Naser; Teixeira, Joao Amaral; Addali, Abdulmajid; Al-Zubi, Feras; Shaban, Ehab; Behbehani, IsmailExperimental investigation was performed to highlight the influence of ionic bounding and surface roughness effects on the surface wettability. Nanocoating technique via e-beam physical vapor deposition process was used to fabricate aluminium (Al) film of 50, 100, and 150 nm on the surface of an Al substrate. Microstructures of the samples before and after deposition were observed using an atomic force microscopy. A goniometer device was later on used to examine the influence of surface topography on deionised water of pH 4, 7 and 9 droplets at a temperature ranging from 10 °C to 60 °C through their contact angles with the substrate surface, for both coated and uncoated samples. It was found that, although the coated layer has reduced the mean surface roughness of the sample from 10.7 nm to 4.23 nm, by filling part of the microstructure gaps with Al nanoparticles, the wettability is believed to be effected by the ionic bounds between the surface and the free anions in the fluid. As the deionised water of pH 4, and 9 gave an increase in the average contact angles with the increase of the coated layer thickness. On the other hand, the deionised water of pH 7 has showed a negative relation with the film thickness, where the contact angle reduced as the thickness of the coated layer was increased. The results from the aforementioned approach had showed that nanocoating can endorse the hydrophobicity (unwitting) nature of the surface when associated with free ions hosted by the liquid.Item Open Access Effect of multi-walled carbon nanotubes-based nanofluids on marine gas turbine intercooler performance(MDPI, 2021-09-04) Almurtaji, Salah; Ali, Naser; Teixeira, Joao Amaral; Addali, AbdulmajidCoolants play a major role in the performance of heat exchanging systems. In a marine gas turbine engine, an intercooler is used to reduce the compressed gas temperature between the compressor stages. The thermophysical properties of the coolant running within the intercooler directly influence the level of enhancement in the performance of the unit. Therefore, employing working fluids of exceptional thermal properties is beneficial for improving performance in such applications, compared to conventional fluids. This paper investigates the effect of utilizing nanofluids for enhancing the performance of a marine gas turbine intercooler. Multi-walled carbon nanotubes (MWCNTs)-water with nanofluids at 0.01–0.10 vol % concentration were produced using a two-step controlled-temperature approach ranging from 10 °C to 50 °C. Next, the thermophysical properties of the as-prepared suspensions, such as density, thermal conductivity, specific heat capacity, and viscosity, were characterized. The intercooler performance was then determined by employing the measured data of the MWCNTs-based nanofluids thermophysical properties in theoretical formulae. This includes determining the intercooler effectiveness, heat transfer rate, gas outlet temperature, coolant outlet temperature, and pumping power. Finally, a comparison between a copper-based nanofluid from the literature with the as-prepared MWCNTs-based nanofluid was performed to determine the influence of each of these suspensions on the intercooler performance.Item Open Access Experimental and computational analysis of purge systems for radiation pyrometers(Cranfield University, 2011-07) Taccoli, Cinzia; Teixeira, Joao AmaralMaximizing the turbine entry temperature (TET) is fundamental to increase engine efficiency and reducing fuel consumption. Nonetheless, safety and reliability requirements have to be fulfilled. The life of gas turbine blades is strictly connected to their temperature through the creep deformation process. For this reason temperature monitoring is an essential requirement. Commonly this is achieved by means of devices such as thermocouples which are placed in the bulk flow. The usefulness of these devices as the means of supplying turbine blade temperature information is limited given their slow response time and the fact that the blade temperature is inferred from that of the surrounding gas rather than measured directly. This in turn means that critical blades parts (e.g., trailing edge) or the presence of hot spots are not identified in a discrete manner. These drawbacks can be addressed by using instead a radiation pyrometer, which is characterized by a fast response time, high accuracy, and by being contactless. The pyrometer optical front-end is a lens which collects the radiation emitted by a spot on the turbine blades. However, since the lens is exposed to the harsh engine environment, contaminants entrapped by the turbine flow can therefore be easily deposited on the lens thus filtering the radiation and resulting in an under-estimation of the actual blade temperature. The fouling of the lens is generally tackled by using a purge air system that employs air bled from the compressor to divert those particles whose trajectory is directed towards the lens. Currently the employment of optical pyrometry is often confined to military applications due to the fact that their turbine entry temperatures are higher than in civil applications. Besides, the maintenance schedule established for military engines is far more frequent than what is practiced in airline engines. Therefore, the design of current purge air systems reflects these facts. Before optical pyrometers can be commonly used for civil applications more research is required since some of the fundamentals of the fouling mechanisms remain to be clarified. This is then the knowledge gap the present research sought to fill. Its aim was to conduct a comprehensive investigation of the phenomena that underpins the lens fouling process in order to provide a set of guidelines for optimising the design of purge air systems. The initial part of the research was dedicated to the study of the purge flow inside a given pyrometer configuration. The scope was to identify the main flow structure that determines the fouling process and at the same time to validate the results obtained via computational fluid dynamics (CFD) analyses conducted in a second phase of the research. Given the reduced dimensions of the pyrometer purge system, it was not possible to gain the appropriate optical access to take flow measurements. Consequently, a large scaled experiment was performed, employing the Particle Image Velocimetry (PIV) technique for the acquisition of experimental data of the flow field. The distortion of the image and light reflection introduced by the presence of curved glass surfaces was investigated by means of a feasibility experiment. The experimental study highlighted the presence of a large recirculation zone that can trap contaminants and direct them towards the pyrometer lens. The experimental data were in agreement with computational fluid dynamics results obtained by using two different turbulence models. In a second instance, attention was focused on the particle deposition as seen from a fluid dynamics perspective. A computational fluid dynamics analysis aimed at reproducing the flow field of an existing pyrometer purge system enabled the identification of those features that can significantly impact on the lens fouling process. It was found that the geometry of the air curtain configuration plays a fundamental role. However, given the high speeds involved, the main force governing the contaminants deposition is the drag. Additionally, particles with high inertia hit the purge tube wall and then bounce towards the pyrometer lens, while contaminants with low inertia can be trapped by a large recirculation zone and subsequently directed towards the lens. In a third phase of the research, the impacts between the contaminant particles and the lens were investigated through a finite element analysis (FEA) aimed at identifying the most important factors that contribute to the lens fouling process. Particles moving at low speed can be deposited on the lens by means of electrostatic and Van der Waals forces. Conversely, particles with very high velocity can be deposited on the lens through the same mechanisms involved in the cold spraying process, which is a technique commonly used for coating deposition. A local melting can occur at the interface between the lens and the contaminants due to the high stresses created by the asperities and high sliding velocity of the particles. As a result, while large particles bounce back, debris remains bonded to the lens surface. Last but not least, the findings of the several steps of the present research have been brought together in order to produce guidelines to be followed by engineers engaged in the redesign of more efficient pyrometer purge systems.Item Open Access Experimental investigation of bubble activity at an early stage using te acoustic emission technique in two-phase flow systems.(2018-01) Alhashan, Taihiret; Addali, Abdulmajid; Teixeira, Joao AmaralThis thesis presents an experimental investigation and identifies the feasibility of the use of AE technology to detect and monitor both early stage bubble occurrence and throughout the boiling process. The research programme also included monitoring of bubble formation/collapse phenomena in ball and globe valves using AE techniques. It was demonstrated that an AE piezoelectric sensor can detect pressure pulses associated with bubble occurrence during pool boiling and cavitation in flow through valves. For the pool boiling test, a dedicated test-rig was used to diagnose and monitor bubble formation. It was concluded that bubble occurrence is detectable with AE techniques and there is a clear relationship between increasing AE levels and bubble formation during the boiling process. For the valve tests, a purpose-built test-rig was used to monitor and detect cavitation phenomena with various flow rates and different valve opening percentages. It was shown that AE will detect incipient cavitation and that there is a clear correlation between AE signal levels and the flow rate through the ball and globe valves at a constant opening percentage. This investigation successfully demonstrated that AE monitoring is capable of early diagnosis and monitoring of bubble formation phenomena in boiling processes and valves. This research developed a methodology and prototype framework for using the AE technique for detection and diagnosis of early bubble formation and collapse, allowing cavitation development to be tracked, and maintenance activity to be planned to maximise equipment life and minimise downtime.Item Open Access Experimental investigation of the influences of different liquid types on acoustic emission energy levels during the bubble formation process(Springer, 2017-09-08) Alhashan, Taihiret; Addali, Abdulmajid; Teixeira, Joao Amaral; Naid, AbdelhamidBubble formation phenomena in a two-phase gas/liquid system occur in many industries that involve boiling; such as desalination stations, nuclear reactors, chemical plants, and fluid piping transportation and processes. Bubble formation phenomena cause problems, such as a decrease in equipment efficiency, vibration, noise, and solid surface erosion. Applications of the acoustic emission (AE) technique for monitoring bubble formation and burst stages in boiling processes are marginal in terms of extension in comparison to other applications of the AE technique. The use of the AE technique in this experimental investigation covers the frequency range between 100 and 1000 kHz, showing that the AE sensor can detect acoustic emissions from an occurrence of bubble formation. Statistically, it was found that the best AE parameter indicator for bubble formation was AE-RMS (root mean square).Item Open Access Feasibility study for a hydroelectric installation on the Arno River (Italy)(Cranfield University, 2011-05) Micheli, Lorenzo; Mba, David; Teixeira, Joao AmaralEnergy need is a primary requirement of our society. Many interests and concerns turn around this matter. It includes policy, economy, environment, etc... Renewable energy is considered to be a good alternative from fossil sources and nuclear power. Renewable sources are known as “green” because of the low impact on the Earth equilibriums. Furthermore they are not exhaustible because they utilize the ecosystem cycles. Hydroelectricity is an established technology. In most of the industrialized countries large scale hydropower has been widely exploited, but there are possibilities of growth for mini-hydro schemes. In developing countries the unexploited potential is considered to be bigger. This thesis details an analysis into various aspects of hydropower, in particular it deals with micro-hydro and pico-hydro applications. A literature review about existing plants is presented; a few cases are shown in which pico hydro plants are used for the electrification of remote communities in developing countries. A feasibility study has been carried out for a hydroelectric installation on the Arno river (Italy). Three different solutions have been proposed for the realization of the scheme. One of them is a pico hydro installation. Hypothetical benefits from the plant realization have been evaluated, together with the scenario in which this plant would operate.Item Open Access Identifying bubble occurance during pool boiling employing acoustic emission technique(Elsevier, 2017-12-11) Alhashan, Taihiret; Addali, Abdulmajid; Teixeira, Joao Amaral; Elhashan, SaidThis paper reports the results of a study for the early detection of bubble formation during the boiling process using acoustic emission. The feasibility of using AE technology to detect and monitor early bubble formation during pool boiling is assessed, and the results show that AE technology is an affective tool for this purpose. There is a clear correlation between the AE signal levels and height of the water level above the heated surface during the boiling process. The different types of heated fluid influence AE energy levels during the bubble formation process. Statistically, it was found that the best AE parameters to indicate bubble formation were AE-RMS, AE-Energy and AE-Amplitude.Item Open Access Inflow turbulent effects of tidal turbines, experimental and numerical study.(2018-04) Camosi, Luca; Teixeira, Joao AmaralHydrokinetic tidal turbines are devices for which the relative importance of the incoming flow on fatigue life is higher when compared to wind generators. This is due to the difference in density between the two fluids and because of the more turbulent nature of marine channel flows [50] compared to winds. This work is part of an effort to produce a more accurate description of the impact of turbulence on open-rotor turbines in general. In order to reach the described scope the author conducted an experimental campaign and developed numerical tools within the OpenFOAM [5] CFD libraries. The experimental campaign conducted in IFREMER (Boulogne-sur-Mer France) provides data used for the numerical model validation. The developed models consists in two main parts: 1. an actuator line turbine model similar to that presented by Churchfield in [18] but more integrated with the OpenFOAM libraries 2. two novel generators of turbulence for Large Eddy Simulations (LES) inlet boundary conditions • one that combines the state of the art generator philosophy for CFD [60] with a more general statistical representation approach by Shinozuka and al. [57]. This method is designed to accurately match the first order statistics of a flow given the turbulence spectra in a position of interest. • a second approach has been developed in order to reproduce the typical flow structures particular of a specific site. It makes use of Proper Orthogonal Decomposition (POD) in order to describe the intrinsically non homogeneous nature of environmental turbulence. Such is the case in tidal channel flows. The combination of the developed libraries constitutes an enabling simulation toolbox for the study of tidal turbines dynamic response to turbulence. The author could test the qualitative behaviour of the second approach compared to the current state of the art showing promising results. Results from the first approach intended for the dynamic validation of the IFREMER turbine model are not shown in this thesis report being analysis not completed yet. The comparison of the torque and thrust signatures on the turbine from different inflow generators shows equivocally the importance of accurate modelling of turbulence when assessing the effects of dynamic load on hydrokinetic tidal turbines. Furthermore the POD methodology is used to analyse flow data from the Ramsey Sound site where the company sponsor of this work installed a full-size device. These show the high meaningfulness of the POD methodology in the description of turbulent flows.Item Open Access Investigation of a radial turbine design for a utility-scale supercritical CO2 power cycle(MDPI, 2020-06-17) El Samad, Tala; Teixeira, Joao Amaral; Oakey, JohnThis paper presents the design procedure and analysis of a radial turbine design for a mid-scale supercritical CO2 power cycle. Firstly, thermodynamic analysis of a mid-range utility-scale cycle, similar to that proposed by NET Power, is established while lowering the turbine inlet temperature to 900 ◦C in order to remove cooling complexities within the radial turbine passages. The cycle conditions are then considered for the design of a 100 MWth power scale turbine by using lower and higher fidelity methods. A 510 mm diameter radial turbine, running at 21,409 rpm, capable of operating within a 5% range of the required cycle conditions, is designed and presented. Results from computational fluid dynamics simulations indicate the loss mechanisms responsible for the low-end value of the turbine total-to-total efficiency which is 69.87%. Those include shock losses at stator outlet, incidence losses at rotor inlet, and various mixing zones within the passage. Mechanical stress calculations show that the current blade design flow path of the rotor experiences tolerable stress values, however a more detailed re-visitation of disc design is necessitated to ensure an adequate safety margin for given materials. A discussion of the enabling technologies needed for the adoption of a mid-size radial turbine is given based on current advancements in seals, bearings, and materials for supercritical CO2 cyclesItem Open Access Investigation of effective groove types for a film riding seal(ASME, 2017-02-14) Teixeira, Joao Amaral; Tibos, S. M.; Georgakis, C.Over the past two decades, significant efforts have been made to introduce film riding sealing technology on large industrial or aerospace gas turbines. The main challenge comes from the high surface speeds and high temperatures, which lead to large thermal distortions. One approach to tackle the effect of thermally induced distortion is to design a seal to operate at a large film to limit the viscous heat generation. To design a seal pad that maximizes force at relatively high film heights, it is important to select the seal groove type that looks the most promising to deliver this characteristic. Several groove types have been assessed as part of this study. The most promising groove type is the Rayleigh step, which gives the strongest level of combined hydrostatic and hydrodynamic load support while also being easier to tessellate on individual seal segments. The results generated using a uniform grid Reynolds equation method show reasonable agreement with computational fluid dynamics (CFD) calculations. This provides confidence in the validity of the method, approach, and results.Item Open Access Investigation of hydrostatic fluid forces in varying clearance turbomachinery seals(ASME, 2017-11-21) Tibos, S. M.; Georgakis, C.; Teixeira, Joao Amaral; Hogg, S.Varying clearance, rotor-following seals are a key technology for meeting the demands of increased machine flexibility for conventional power units. These seals follow the rotor through hydrodynamic or hydrostatic mechanisms. Forward-facing step (FFS) and Rayleigh step designs are known to produce positive fluid stiffness. However, there is very limited modeling or experimental data available on the hydrostatic fluid forces generated from either design. A quasi-one-dimensional (1D) method has been developed to describe both designs and validated using test data. Tests have shown that the FFS and the Rayleigh step design are both capable of producing positive film stiffness and there is little difference in hydrostatic force generation between the two designs. This means any additional hydrodynamic features in the Rayleigh step design should have a limited effect on hydrostatic fluid stiffness. The analytical model is capable of modeling both the inertial fluid forces and the viscous fluid losses, and the predictions are in good agreement with the test data.