Дисертації з теми "High-stability"
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1
Kwasnicki, Wieslaw T. "High Speed Transient Stability, multiprocessing solutions." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape17/PQDD_0024/NQ32881.pdf.
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2
Young, David. "Hydrothermal stability of high silica zeolites." Thesis, University of Edinburgh, 1988. http://hdl.handle.net/1842/9864.
Повний текст джерелаАнотація:
This thesis concerns the hydrothermal stability of two zeolite molecular sieves with the MFI structure, ZSM-5 and its 'aluminium free' form silicalite. Silicalite was synthesised from low pH alkali metal free aqueous gels at 95°C and characterised by XRD, SEM, DTA and TG. ZSM-5 was crystallised from similar reaction mixtures but with the addition of aluminium salts. It was found that the incorporation of aluminium into the ZSM-5 framework was favoured by higher reaction temperatures (l50°C) and the use of aluminium nitrate. Solubilities were measured for silicalite, ZSM-5, ZSM-11, ZSM-39, ZSM-48 and EU-4 at temperatures up to 95°C. The solubilities were influenced by framework structure, the presence of template within the zeolite channels and the aluminium content of the lattice. Solubility measurements over a range of liquid/solid ratios showed that calcined silicalite was contaminated with about 1% amorphous silica. Treatment with liquid water over the temperature range 95 to 230°C was used to remove amorphous silica and alnminosilicate species from silicalite and ZSM-5. This hydrothermal treatment had many other effects on the properties of these materials. The water uptake, orthorhombic to monoclinic symmetry transition, framework Si/Al ratio and thermal properties were all affected. SEM showed that the inside of the silicalite crystals was more soluble than the outer surface. The use of hydrothermally treated H-ZSM-5 as a catalyst for but-1-ene isomerisation showed that the treatment could have a marked effect on catalytic behaviour, and particularly on product selectivity.
3
Bonnaure, Laurent Paul. "Modelling high speed multistage compressor stability." Thesis, Massachusetts Institute of Technology, 1991. http://hdl.handle.net/1721.1/13046.
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4
Ebrinc, Ali Aslan. "High Speed Viscous Plane Couette-poiseuille Flow Stability." Phd thesis, METU, 2004. http://etd.lib.metu.edu.tr/upload/3/12604769/index.pdf.
Повний текст джерелаАнотація:
The linear stability of high speed-viscous plane Couette and Couette-Poiseuille flows are investigated numerically. The conservation equations along with Sutherland�
s viscosity law are studied using a second-order finite difference scheme. The basic velocity and temperature distributions are perturbed by a small-amplitude normalmode disturbance. The small-amplitude disturbance equations are solved numerically using a global method using QZ algorithm to find all the eigenvalues at finite Reynolds numbers, and the incompressible limit of these equations is investigated for Couette-Poiseuille flow. It is found that the instabilities occur, although the corresponding growth rates are often small. Two families of wave modes, Mode I (odd modes) and Mode II (even modes), were found to be unstable at finite Reynolds numbers, where Mode II is the dominant instability among the unstable modes for plane Couette flow. The most unstable mode for plane Couette &
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Poiseuille flow is Mode 0, which is not a member of the even modes. Both even and odd modes are acoustic modes created by acoustic reflections between a will and a relative sonic line. The necessary condition for the existence of such acoustic wave modes is that there is a region of locally supersonic mean flow relative to the phase speed of the instability wave. The effects of viscosity and compressibility are also investigated and shown to have a stabilizing role in all cases studied. Couette-Poiseuille flow stability is investigated in case of a choked channel flow, where the maximum velocity in the channel corresponds to sonic velocity. Neutral stability contours were obtained for this flow as a function if the wave number,Reynolds number and the upper wall Mach number. The critical Reynolds number is found as 5718.338 for an upper wall Mach number of 0.0001, corresponding to the fully Poiseuille case.
5
Holmestad, Jon. "High-temperature stability of Al-Mg-Si alloys." Thesis, Norwegian University of Science and Technology, Department of Physics, 2009. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-6347.
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6
Ebrinç, Ali Aslan. "High speed viscous plane couette-poiseuille flow stability." Ankara : METU, 2004. http://etd.lib.metu.edu.tr/upload/12604769/index.pdf.
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7
Ang, Ing Chea. "Stability and quench protection of high-temperature superconductors." Thesis, Massachusetts Institute of Technology, 2006. http://hdl.handle.net/1721.1/35665.
Повний текст джерелаАнотація:
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references (leaves 83-84).
In the design and operation of a superconducting magnet, stability and protection are two key issues that determine the magnet's reliability and safe operation. Although the high-temperature superconductor (HTS) is considered much more stable than the low-temperature superconductor (LTS), it is susceptible to damage caused primarily by three events that can occur in large-scale "real" devices: 1) overheating; 2) high voltage; and, 3) overstressing. In this thesis, we have investigated the first two issues as well acoustic emission (AE) technique as a possible mean for an early detection of a quench. For most of the experimental work reported here, we used "pancake" coils wound with coated YBCO conductor, the HTS of choice by those currently developing HTS-based electric power devices, though, YBCO itself to date is still in the development phase. For protection against overheating, an HTS magnet assembled with pancake coils may be made self-protecting through speedy 2-D or even 3-D normal zone propagation (NZP) within its winding, aided by good thermally-diffusive turn-to-turn spacers.
(cont.) We have found experimentally that good thermal diffusivity alone, however, does not guarantee fast 2-D NZP: thermal contact resistance between winding layers plays a crucial role in NZP in the transverse direction. For high internal voltage, a small test "magnet" consisting of two pancake coils was studied to investigate the internal voltage distributions within the magnet when one of the pancakes was driven normal with a heater. Measured voltage distributions were compared with those of simulation. Finally, to complement standard resistive voltage technique, an acoustic emission (AE) technique was investigated for detection of a quench at an instance earlier than that possible with a resistive voltage technique. With improved understanding of these issues, we should be able to develop protection techniques that ensure reliable and safe operation of HTS devices.
by Ing Chea Ang.
S.M.
8
Xu, Jiapeng. "Stability of high temperature ceramics under corrosive environments." Thesis, Boston University, 2013. https://hdl.handle.net/2144/11090.
Повний текст джерелаАнотація:
Thesis (Ph.D.)--Boston UniversityCurrently, ceramics are being used under increasingly demanding environments. This research involves the study of high-temperature stability of ceramic materials in two diverse applications. The first application involves the use of ceramic materials in gas turbines. SiC/SiC ceramic matrix composites (CMCs) are increasingly being used in the hot-sections of gas turbines; and they are subject to recession of their surface if exposed to a flow of high-velocity water vapor, and to hot-corrosion when exposed to alkali salts. This research involves developing a hybrid system containing an environmental barrier coating (EBC) for protection of the CMC from chemical attack and a thermal barrier coating (TBC) that allows a steep temperature gradient across it to lower the temperature of the CMC for increased lifetimes. The EBC coating is a functionally graded mullite (3Al2O3•2SiO2) deposited by chemical vapor deposition (CVD), the TBC layer is yttria-stabilized zirconia (YSZ) deposited by air plasma spray (APS). The hybrid coating system demonstrated excellent physical and chemical stability under severe thermal shock and exposure to an aggressive hot-corrosion environment. Finite element modeling showed that through-thickness cracks reduce the tensile stresses in the TBC, but also reduce the beneficial compressive stresses in the EBC, and may actually lead to the propagation of the vertical cracks into the EBC. The second application involves the formation of solar-grade silicon by an inexpensive and environmentally friendly electrochemical process using an YSZ solid oxide membrane (SOM) at elevated temperature (~1100°C). The SOM membrane is exposed to a complex fluoride flux with dissolved silica, which is then electrochemically separated into silicon and oxygen. Membrane stability is crucial to ensure high efficiency and long-term performance of the SOM process. A failure model of the SOM membrane by the formation of "inner cracks" was studied, and attributed to yttrium depletion in the YSZ, which leads to phase transformation from the cubic to tetragonal phase. A series of systematic experiments were designed and performed to understand the synergistic roles of silica and YF3 in the flux in membrane degradation. It was shown that silica attacks the SOM membrane, while YF3 in the flux slows down the attack. The mechanism of the yttria depleted layer (YDL) formation was attributed to grain boundary attack by the silica in the flux, which was the rate-controlling step. This led to rapid ingress of the flux into this attacked grain boundaries, and the out diffusion of Y from the cubic YSZ grains to the grain boundary. This depletion of the Y from the cubic grains transformed them into tetragonal. Once all of the cubic grains in the YDL region converted to tetragonal YSZ grains, no further diffusion occurred. Based on the stability test results, a new flux design was proposed and tested. The flux composition did not attack the SOM membrane, and successful separation of silica in the flux to phase pure Si crystals was demonstrated without apparent damage to the SOM membrane, thereby demonstrating the viability of the Si-SOM process.
9
Thomas, Dirk. "Lateral Stability of High-Speed Trains at Unsteady Crosswind." Licentiate thesis, KTH, Aeronautical and Vehicle Engineering, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-11377.
Повний текст джерелаАнотація:
Crosswind stability of rail vehicles has been a research area for several decades,mainly motivated by vehicle overturning accidents and higher speeds, but in recenttimes also by issues of lower energy consumption and track maintenance costsdemanding lower vehicle weights. During everyday operation, rail vehicles are subjectedto large lateral influences from track irregularities, track curves and crosswind,leading to large suspension deflections and increased crosswind sensitivity.Also unsteady crosswind like gusts calls for attention. Simulations of possible vehicleoverturning are necessary, but need to take large deflections and high shear inthe suspension into account. If delivering reasonable results, simulations representan important tool for overturning prediction of the vehicle.
In the present work, multibody simulations of a high-speed vehicle at large lateralinfluences from track curves and track irregularities have been carried out, using ahalf-vehicle model in 2D and a model of a whole vehicle in 3D. The vehicle modelsalso include different suspension models. Corresponding field measurements ofthe relative lateral and vertical deflections in the secondary suspension have beenperformed on a fast train and used for validation of the multibody simulations,resulting in good agreement between measurements and simulations.
The 3D vehicle model was further used to study the vehicle response to unsteadycrosswind during curve negotiation where aerodynamic loads obtained by unsteadyComputational Fluid Dynamics, namely Detached Eddy Simulations, representingthree types of gusts were used. In addition, the method of Quasi Transient GustModelling was evaluated in terms of overturning risk. Strong lateral and roll responsesof the vehicle and influences of the gust duration and the relative differencebetween mean and maximum wind speed were observed. Further, variations of suspensionand mass properties of the vehicle were performed to study the influenceon crosswind sensitivity. The position of the centre of mass of the carbody and thelateral bumpstop clearance showed significant influence on the crosswind stability.
10
Payeras, Antoni Mairata i. "Stability and oxidative reactivity of high-valent manganese complexes." Thesis, University of York, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.428448.
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11
Ortega, Gómez Tino. "Stability analysis of the high performance light water reactor." Karlsruhe Forschungszentrum Karlsruhe, 2009. http://d-nb.info/994365624/34.
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12
Virgilio, Gianluca. "Is high-frequency trading a threat to financial stability?" Thesis, University of Hertfordshire, 2017. http://hdl.handle.net/2299/18841.
Повний текст джерелаАнотація:
The purpose of this thesis is: (i) to produce an in-depth data analysis and computer-based simulations of the market environment to investigate whether financial stability is affected by the presence of High-Frequency investors; (ii) to verify how High-Frequency Trading and financial stability interact with each other under non-linear conditions; (iii) whether non-illicit behaviours can still lead to potentially destabilising effects; (iv) to provide quantitative support to the theses, either from the audit trail data or resulting from simulations. Simulations are provided to test whether High-Frequency Trading: (a) has an impact on market volatility, (b) leads to market splitting into two tiers; (c) takes the lion's share of arbitrage opportunities. Audit trail data is analysed to verify some hypotheses on the dynamics of the Flash Crash. The simulation on the impact of High-Frequency Trading on market volatility confirms that when markets are under stress, High-Frequency Trading may cause volatility to significantly increase. However, as the number of ultra-fast participants increases, this phenomenon tends to disappear and volatility realigns to its standard values. The market tiering simulation suggests that High-Frequency traders have some tendency to deal with each other, and that causes Low-Frequency traders also to deal with other slow traders, albeit at a lesser extent. This is also a kind of market instability. High-Frequency Trading potentially allows a few fast traders to grab all the arbitrage-led profits, so falsifying the Efficient Market Hypothesis. This phenomenon may disappear as more High-Frequency traders enter the competition, leading to declining profits. Yet, the whole matter seems a dispute for abnormal gains only between few sub-second traders. All simulations have been carefully designed to provide robust results: the behaviours simulated have been drawn from existing literature and the simplifying assumptions have been kept to a minimum. This maximises the reliability of the results and minimizes the potential of bias. Finally, from the data analysis, the impact of High-Frequency Trading on the Flash Crash seems significant; other sudden crashes occurred since, and more can be expected over the next future. Overall, it can be concluded that High-Frequency Trading shows some controversial aspects impacting on financial stability. The results are at a certain extent confirmed by the audit trail data analysis, although only indirectly, since the details allowing the match between High-Frequency traders and their behaviour are confidential and not publicly available Nevertheless, the findings about HFT-induced volatility, market segmentation and sub-optimal market efficiency, albeit not definitive, suggest that careful monitoring by regulators and policy-makers might be required.
13
Tanvir, Tanvir. "Design and Stability Analysis of a High-Temperature SRAM." University of Akron / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=akron1355516028.
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14
Agulov, A. V., V. A. Turchenko, S. A. Goncharova, U. V. Kudelin, A. A. Goncharov, A. I. Bazhin, and V. A. Stupak. "Thermal stability of film nanostructure of high-melting compounds." Thesis, Видавництво СумДУ, 2011. http://essuir.sumdu.edu.ua/handle/123456789/20656.
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15
Schellenberg, Katja [Verfasser]. "A High-Throughput Analysis of Circadian Protein Stability / Katja Schellenberg." Berlin : Freie Universität Berlin, 2014. http://d-nb.info/1048558614/34.
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16
Erdmann, Natalie. "Identification of high-copy-number inhibitors of P1 plasmid stability." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp01/MQ34055.pdf.
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17
Meegahapola, Lasantha. "Stability analysis and coordinated control strategies during high wind penetration." Thesis, Queen's University Belfast, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.534620.
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18
Mummery, Gavin Thomas. "Developing a high-resolution bioengineering model for slope stability analysis." Thesis, University of Bristol, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.435426.
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19
Nelson, Joseph Richard. "Crystal structure prediction at high pressures : stability, superconductivity and superionicity." Thesis, University of Cambridge, 2017. https://www.repository.cam.ac.uk/handle/1810/268482.
Повний текст джерелаАнотація:
The physical and chemical properties of materials are intimately related to their underlying crystal structure: the detailed arrangement of atoms and chemical bonds within. This thesis uses computational methods to predict crystal structure, with a particular focus on structures and stable phases that emerge at high pressure. We explore three distinct systems. We first apply the ab initio random structure searching (AIRSS) technique and density functional theory (DFT) calculations to investigate the high-pressure behaviour of beryllium, magnesium and calcium difluorides. We find that beryllium fluoride is extensively polymorphic at low pressures, and predict two new phases for this compound - the silica moganite and CaCl$_2$ structures - to be stable over the wide pressure range 12-57 GPa. For magnesium fluoride, our results show that the orthorhombic `O-I' TiO$_2$ structure ($Pbca$, $Z=8$) is stable for this compound between 40 and 44 GPa. Our searches find no new phases at the static-lattice level for calcium difluoride between 0 and 70 GPa; however, a phase with $P\overline{6}2m$ symmetry is energetically close to stability over this pressure range, and our calculations predict that this phase is stabilised at high temperature. The $P\overline{6}2m$ structure exhibits an unstable phonon mode at large volumes which may signal a transition to a superionic state at high temperatures. The Group-II difluorides are isoelectronic to a number of other AB$_2$-type compounds such as SiO$_2$ and TiO$_2$, and we discuss our results in light of these similarities. Compressed hydrogen sulfide (H$_2$S) has recently attracted experimental and theoretical interest due to the observation of high-temperature superconductivity in this compound ($T_c$ = 203 K) at high pressure (155 GPa). We use the AIRSS technique and DFT calculations to determine the stable phases and chemical stoichiometries formed in the hydrogen-sulfur system as a function of pressure. We find that this system supports numerous stable compounds: H$_3$S, H$_7$S$_3$, H$_2$S, H$_3$S$_2$, H$_4$S$_3$, H$_2$S$_3$ and HS$_2$, at various pressures. Working as part of a collaboration, our predicted H$_3$S and H$_4$S$_3$ structures are shown to be consistent with XRD data for this system, with H$_4$S$_3$ identified as a major decomposition product of H$_2$S in the lead-up to the superconducting state. Calcium and oxygen are two elements of generally high terrestrial and cosmic abundance, and we explore structures of calcium peroxide (CaO$_2$) in the pressure range 0-200 GPa. Stable structures for CaO$_2$ with $C2/c$, $I4/mcm$ and $P2_1/c$ symmetries emerge at pressures below 40 GPa, which we find are thermodynamically stable against decomposition into CaO and O$_2$. The stability of CaO$_2$ with respect to decomposition increases with pressure, with peak stability occurring at the CaO B1-B2 phase transition at 65 GPa. Phonon calculations using the quasiharmonic approximation show that CaO$_2$ is a stable oxide of calcium at mantle temperatures and pressures, highlighting a possible role for CaO$_2$ in planetary geochemistry, as a mineral redox buffer. We sketch the phase diagram for CaO$_2$, and find at least five new stable phases in the pressure/temperature ranges 0 $\leq P\leq$ 60 GPa, 0 $\leq T\leq$ 600 K, including two new candidates for the zero-pressure ground state structure.
20
Fréchette, Luc G. (Luc Guy). "Implications of stability modeling for high-speed axial compressor design." Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/10721.
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21
Posokhow, Bruno A. (Bruno Andre) 1975. "A study on the stability of a high speed compressor." Thesis, Massachusetts Institute of Technology, 2000. http://hdl.handle.net/1721.1/80618.
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22
Morrison, Sarah J., and Kaitlin M. Kratter. "ORBITAL STABILITY OF MULTI-PLANET SYSTEMS: BEHAVIOR AT HIGH MASSES." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/621257.
Повний текст джерелаАнотація:
In the coming years, high-contrast imaging surveys are expected to reveal the characteristics of the population of wide-orbit, massive, exoplanets. To date, a handful of wide planetary mass companions are known, but only one such multi-planet system has been discovered: HR 8799. For low mass planetary systems, multi-planet interactions play an important role in setting system architecture. In this paper, we explore the stability of these high mass, multi-planet systems. While empirical relationships exist that predict how system stability scales with planet spacing at low masses, we show that extrapolating to super-Jupiter masses can lead to up to an order of magnitude overestimate of stability for massive, tightly packed systems. We show that at both low and high planet masses, overlapping mean-motion resonances trigger chaotic orbital evolution, which leads to system instability. We attribute some of the difference in behavior as a function of mass to the increasing importance of second order resonances at high planet-star mass ratios. We use our tailored high mass planet results to estimate the maximum number of planets that might reside in double component debris disk systems, whose gaps may indicate the presence of massive bodies.
23
Dhorajiwala, Husein, and Agnieszka Owczarczyk. "Global stability of high-rise buildings on foundation on piles." Thesis, KTH, Bro- och stålbyggnad, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-235317.
Повний текст джерелаАнотація:
In Sweden as well as other countries the trend of building higher is more and more popular. The global stability of tall buildings is a very important aspect that has to be taken into account while designing. Foundation on piles, that is common in Sweden, reduces the global stability compared to foundation directly on bedrock. Using inclined piles in the foundation is inevitable for high-rise buildings, because they are essential for transferring the horizontal loads into the bedrock. The aim of this paper is to look into the influence that foundation on piles has on global stability and investigate two simple methods to asses global stability. In this thesis the influence of the stiffness of the substructure (foundation), length and inclination of the piles on the global stability were investigated. It was also looked into how does the pile center affect the rotation and thus global stability. One method that was presented was based on the equivalent stiffness. Displacement at the top of the wall is used in order to calculate the bending stiffness that is reduced due to foundation on piles and further calculate buckling load on the basis of Euler buckling. In the other method that was proposed rotation at the foundation level was taken into account so as to calculate rotational spring stiffness and later buckling load due to combined flexural and rotational buckling. The analysis was conducted on a simple two dimensional problem, namely stabilizing wall as well as a building stabilized by two towers. Three different configurations of piles were investigated for single wall as well as for the structure. The investigation showed that the position of pile center has its effect on the global stability. The closer the pile center is to the foundation on piles the better the global stability of a structure. The length of the piles plays a role in stability as well. The longer the piles are the worse the stability is. With longer piles the overall stiffness of a structure decreases and thus the global stability. The analysis showed that the foundation of piles significantly lowers the stability of high rise building. The investigated methods showed that the one based on rotation at the base gave better results compared to the method based on the equivalent bending stiffness. But to use this first method, the position of the pile center is required to be known in order to get correct results which in a complex structure is hard to estimate. In an analysis of a building stabilized by two towers it was seen that when the inclined piles that are inclined opposite to each other in a pile group and are positioned far from rotation center of a structure it increases the global safety and rotational stiffness as well. It is recommended to use such configuration of piles that the pile center is at the foundation level in order to increase global stability.I Sverige och andra länder är trenden att bygga högre alltmer populärt. Den globala stabiliteten hos höga byggnader är en viktig aspekt som måste beaktas vid byggnadens utformning. Den vanligaste grundläggningsmetoden i Sverige är grundläggning med pålar. Denna typ av grundläggning minskar den globala stabiliteten jämfört med grundläggning direkt på berggrunden. Att använda sneda pålar i grundläggningen är oundviklig för höghus, eftersom de är nödvändiga för att överföra horisontella laster till berggrunden. Syftet med detta examensarbete är att se hur grundläggning på pålar påverkar den globala stabiliteten och undersöka två enkla metoder för global stabilitet. I detta examensarbete undersöktes hur styvheten påverkar grundläggning med pålar med olika längder och lutningar, med hänseende på den globala stabiliteten. Pålcentrumets påverkan av rotation och den globala stabiliteten har även studerats. En metod som presenterades i examensarbetet är baserades på ekvivalent styvhet. Där utböjning på toppen av väggen togs för att beräkna böjstyvheten som reduceras på grund av grundläggning med pålar och ytterligare beräknades knäcklasten baserat på Eulers knäckning. I den andra metoden som föreslogs togs rotationen vid grundläggningsnivån med i beräkningen för att beräkna rotationsfjäderns styvhet och senare knäckningslasten på grund av kombinerad böjnings- och rotationsknäckning. Analysen genomfördes på en enkel tvådimensionell vägg och en tredimensionell byggnad som är stabiliserad av två torn. Tre olika konfigurationer av pålar undersöktes för enkel vägg och även för byggnaden Utredningen av examensarbetet visade att positionen av pålcentrum har en stor påverkan på den globala stabiliteten. Ju närmare pålcentrumet är till grundläggningsnivån desto bättre är den globala stabiliteten hos en konstruktion. Längden på pålarna har även en betydelse när det gäller stabiliteten. Ju längre pålarna är desto värre blir stabiliteten. Med längre pålar minskar den totala styvheten hos hela konstruktionen och därmed minskar även den globala stabiliteten. Utredningen visade även att metoden med rotation vid grundläggningsnivån gav mer noggrannare resultat än metoden för ekvivalent styvhet. Men för att kunna använda den förstnämnda metoden behöver man ha kännedom om vart pålcentrum ligger för konstruktionen och detta kan vara svårt att uppskatta. I en analys av en byggnad stabiliserad av två torn visades det att när pålarna är placerade långt från rotationscentrum av en konstruktion ökar den globala säkerheten och rotationsstyvheten. Det rekommenderas att använda sådan konfiguration av pålar att pålcentrum ligger på grundnivå för att öka den globala stabiliteten.
24
Moreu, Gamazo José M. (José María). "High-order tuners for convex optimization : stability and accelerated learning." Thesis, Massachusetts Institute of Technology, 2021. https://hdl.handle.net/1721.1/130859.
Повний текст джерелаАнотація:
Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, February, 2021Cataloged from the official PDF version of thesis.
Includes bibliographical references (pages 131-132).
Iterative gradient-based algorithms have been increasingly applied for the training of a broad variety of machine learning models including large neural-nets. In particular, momentum-based methods, with accelerated learning guarantees, have received a lot of attention due to their provable guarantees of fast learning in certain classes of problems and multiple algorithms have been derived. However, properties for these methods hold true only for constant regressors. When time-varying regressors occur, which is commonplace in dynamic systems, many of these momentum-based methods cannot guarantee stability. Recently, a new High-order Tuner (HT) was developed and shown to have 1) stability and asymptotic convergence for time-varying regressors and 2) non-asymptotic accelerated learning guarantees for constant regressors. These results were derived for a linear regression framework producing a quadratic loss function. This thesis extends and discuss the results of this same HT for general smooth convex loss functions. Through the exploitation of convexity and smoothness definitions, we establish similar stability and asymptotic convergence guarantees. Additionally we conjecture that the HT has an accelerated convergence rate. Finally, we provide numerical simulations supporting the satisfactory behavior of the HT algorithm as well as the conjecture of accelerated learning.
by José M. Moreu Gamazo.
S.M.
S.M. Massachusetts Institute of Technology, Department of Mechanical Engineering
25
Gonzalez-Torres, Juan Carlos. "Transient stability of high voltage AC-DC electric transmission systems." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS041.
Повний текст джерелаАнотація:
Les nouvelles politiques adoptées par les autorités nationales ont encouragé pendant les dernières années l'intégration à grande échelle des systèmes d'énergie renouvelable (RES). L'intégration à grande échelle des RES aura inévitablement des conséquences sur le réseau de transport d'électricité tel qu'il est conçu aujourd'hui, car le transport de l'électricité massif sur de longues distances pourrait amener les réseaux de transport à fonctionner près de leurs limites, réduisant ainsi leurs marges de sécurité. Des systèmes de transport d’électricité plus complexes seront donc nécessaires.Dans ce scénario, les systèmes de transmission à Courant Continu Haute Tension (HVDC) constituent la solution la plus intéressante pour le renforcement et l'amélioration des réseaux à Courant Alternatif (AC) existants, non seulement en utilisant des configurations point à point, mais aussi dans des configurations multi-terminales. L'introduction des systèmes HVDC aboutira à terme à un réseau électrique hybride haute tension AC/DC, qui doit être analysé comme un système unique afin de mieux comprendre les interactions entre le réseau AC et le réseau DC.Cette thèse porte sur l'analyse de la stabilité transitoire des systèmes de transmission électrique hybrides AC/DC. Plus particulièrement, deux questions ont été abordées: Quel est l'impact d'un défaut du réseau DC sur la stabilité transitoire du réseau AC? Comment est-il possible de se servir des systèmes de transmission DC en tant qu'actionneurs afin d'améliorer la stabilité transitoire AC ?Dans la première partie de ce travail, les modèles mathématiques du réseau hybride AC/DC sont décrits ainsi que les outils nécessaires à l'analyse du système en tenant compte de sa nature non linéaire. Ensuite, une analyse approfondie de la stabilité transitoire du réseau électrique dans le cas particulier d'un court-circuit dans le réseau DC et l'exécution des stratégies de protection correspondantes sont effectuées. En complément, des indicateurs de stabilité et des outils pour dimensionner les futurs réseaux de la MTDC afin de respecter les contraintes des stratégies de protection existantes sont proposés.La deuxième partie de la thèse porte sur les propositions de commande pour la modulation des références de puissance des systèmes de transmission HVDC dans le but d'améliorer la stabilité transitoire du système AC connecté à ce réseau DC. Tout d'abord, nous axons notre étude sur le contrôle non linéaire des liaisons HVDC point à point dans des liaisons hybrides AC/DC. La compensation rapide des perturbations de puissance, l'injection de puissance d'amortissement et l'injection de puissance de synchronisation sont identifiées comme des mécanismes par lesquels les systèmes HVDC peuvent améliorer les marges de stabilité des réseaux AC.Enfin, une stratégie de contrôle pour l'amélioration de la stabilité transitoire par injection de puissance active dans par un réseau MTDC est proposée. Grâce à la communication entre les stations, la commande décentralisée proposée injecte la puissance d'amortissement et de synchronisation entre chaque paire de convertisseurs en utilisant uniquement des mesures au niveau des convertisseurs. L'implémentation proposée permet d'utiliser au maximum la capacité disponible des convertisseurs en gérant les limites de puissance d'une manière décentraliséeThe new policy frameworks adopted by national authorities has encouraged the large scale-integration of Renewable Energy Systems (RES) into bulk power systems. The large-scale integration of RES will have consequences on the electricity transmission system as it is conceived today, since the transmission of bulk power over long distances could lead the existing transmission systems to work close to their limits, thus decreasing their dynamic security margins. Therefore more complex transmissions systems are needed.Under this scenario, HVDC transmission systems raise as the most attractive solution for the reinforcement and improvement of existing AC networks, not only using point-to-point configurations, but also in a Multi-Terminal configuration. The introduction of HVDC transmission systems will eventually result in a hybrid high voltage AC/DC power system, which requires to be analyzed as a unique system in order to understand the interactions between the AC network and the DC grid.This thesis addresses the transient stability analysis of hybrid AC/DC electric transmission systems. More in particular, two questions sought to be investigated: What is the impact of a DC contingency on AC transient stability? How can we take advantage of the of DC transmission systems as control inputs in order to enhance AC transient stability?In the first part of this work, the mathematical models of the hybrid AC/DC grid are described as well as the necessary tools for the analysis of the system taking into account its nonlinear nature. Then, a thorough analysis of transient stability of the power system in the particular case of a DC fault and the execution of the corresponding protection strategies is done. As a complement, stability indicators and tools for sizing future MTDC grids in order to respect the constraints of existing protection strategies are proposed.The second part of the thesis addresses the control proposals for the modulation of power references of the HVDC transmission systems with the purpose of transient stability enhancement of the surrounding AC system. Firstly, we focus our study in the nonlinear control of point-to-point HVDC links in hybrid corridors. Fast power compensation, injection of damping power and injection of synchronizing power are identified as the mechanisms through which HVDC systems can improve stability margins.Finally, a control strategy for transient stability enhancement via active power injections of an MTDC grid is proposed. Using communication between the stations, the proposed decentralized control injects damping and synchronizing power between each pair of converters using only measurements at the converters level. The proposed implementation allows to fully use the available headroom of the converters by dealing with power limits in a decentralized way
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Moser, David. "Stability of transition metal doped magnesium hydride high-pressure phases." Thesis, University of Salford, 2010. http://usir.salford.ac.uk/26827/.
Повний текст джерелаАнотація:
MgH2 is hydrogen dense but normally has a rutile structure, which is too stable with too low a hydrogen diffusion rate for practical applications. At pressures of several GPa a CaF2 structure phase has been predicted for MgH2 . With a small fraction of the magnesium atoms substituted by transition metal atoms such as Ti, this cubic phase remains stable when the high GPa synthetic pressure is released with typical composition MgyTiHie- How the metal-hydrogen bonding differs from that in the conventional MgH2 and TiH2 has been investigated in this work. Using Electron Localisation Function topological analysis, the site preference for hydrogen entering the host lattice can be predicted. Vibrational spectra have been calculated with Density Functional Theory (DFT) with the aim of modelling the incoherent inelastic neutron scattering spectra, which have been measured at the TOSCA spectrometer at ISIS. DFT calculations show a weakening in the force constants of the Ti-H and Mg-H bond which can be directly related to the FCC structures of TiH2 and MgH2 . The structural similarities of the three FCC systems lead to a better understanding of the formation process of the new ternary compounds. The presence of two non-equivalent types of tetrahedral site with different force constants resulting from the normal modes of vibration suggests a twostep hydrogenation and dehydrogenation process. Comparison with the TiHx system is also interesting in terms of hydrogen diffusion inside the host lattice and a quasi-elastic neutron scattering experiment has been carried out to investigate this. Gravimetric measurements allow us to investigate the reversibility of incompletely dehydrogenated samples and results showing the fast kinetics are presented. If the partial reversibility could be optimised with respect to TM selection and addition, it would mean a breakthrough for storing hydrogen in magnesium hydrides.
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Munro, Keith Alistair. "High-pressure high-temperature behaviour of the lanthanide metals." Thesis, University of Edinburgh, 2017. http://hdl.handle.net/1842/28881.
Повний текст джерелаАнотація:
The high-pressure behaviour of the lanthanide series of metals has been the subject of study since the work of Percy Bridgman in the 1940s. Differences in said behaviour between the different lanthanide metals are attributed to the increasing occupation of the 4f electron shell as Z increases. Upon compression, or as Z decreases, the trivalent lanthanides (La to Lu, excluding Eu and Yb) undergo a common phase transformation sequence through various close packed structures: hcp → Sm-type (the structure adopted by samarium at ambient conditions) → dhcp → fcc → distorted fcc (d-fcc). Upon further compression, the lanthanide metals experience a first order transition to a "volume collapsed" phase. Many studies have focused on the low-Z members of the series, since the various phase transitions occur at much lower pressure where it is comparatively easy to collect high quality data. By contrast, the other members of the series have received comparability little attention, and there are even fewer reports of the structural behaviour of the lanthanide metals at high pressure and high temperature. This thesis contains the results of angle-dispersive x-ray powder diffraction experiments at high pressure and high temperature of the various members of the lanthanide metals. Ce has been the subject of many previous studies, but a systematic x-ray diffraction study of the fcc/d-fcc phase boundary has never been attempted. Furthermore, the location in P-T space of the high temperature fcc/bct/d-fcc triple point has only been inferred, due to the lack of data on the fcc/bct phase boundary at high temperature. The high-pressure high-temperature phase diagram of Ce is presented and discussed. La is unique amongst the lanthanide metals due to its empty 4f shell at ambient conditions. Despite this, La undergoes the common lanthanide transformation sequence up to the d-fcc phase, after which it undergoes a re-entrant transition back to the fcc phase at 60 GPa. The diffraction peaks of d-fcc La are shown in this thesis to undergo changes in intensity upon compression, indicating a transformation to the oI 16 structure found in Pr. La is one of the few elements whose behaviour has been unknown above 100 GPa, and results of La's structural behaviour upon compression to 280 GPa are presented and discussed. At 76 GPa, La begins a transition from the fcc phase to a new phase with the bct structure. Finally, the d-fcc→fcc re-entrant phase transition has been determined at various temperatures, and the d-fcc stability region has been mapped out. Finally, x-ray diffraction experiments were performed on Gd up to 100 GPa and ~700 K, to determine the structure of the d-fcc phase and the "volume collapsed" phase. While d-fcc Gd does not undergo pressure-induced changes similar to its low Z brethren, the d-fcc Gd remains stable up to 41 GPa at 700 K, putting a constraint on the d-fcc stability region. The data collected on Gd's "volume collapsed" phase cannot be fitted to the currently accepted mC4 structure. This has implications for our understanding of the lanthanide series as a whole, since most of of the heavier members, and some of the lighter lanthanides, are reported to adopt the mC4 structure.
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Coleman, Simon Anthony. "The aerodynamics of ground vehicles in cross winds." Thesis, University of Nottingham, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.278349.
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Kinloch, Stephen Adam. "Interphase modification in TATB filled polymer bonded explosives." Thesis, Cranfield University, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.303329.
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Hanan, Nathan, and David Nix. "Extending the Stability of Intravenous Ampicillin." The University of Arizona, 2012. http://hdl.handle.net/10150/614496.
Повний текст джерелаАнотація:
Class of 2012 AbstractSpecific Aims: To assess the chemical stability of ampicillin for injection in normal saline at pH values ranging from 5 to 6. Methods: A stability-indicating high performance liquid chromatography (HPLC) method was developed and used to determine the stability of ampicillin for injection in normal saline following buffering with sodium acetate and acid adjustment with HCl at pH values of 5, 5.5, and 6. To confirm that the assay was stability-indicating, ampicillin trihydrate reference standard (1 mg/mL) was exposed to alkali, acid, and oxidative stress conditions and analyzed by HPLC for the presence of degradation products. Analysis was performed on a reverse-phase (C-18) column with a mobile phase consisting of water, acetonitrile, 1 M monobasic potassium phosphate, and 1 N acetic acid (909:80:10:1). Other HPLC parameters were: flow rate 1 mL/min; detection wavelength 254 nm; injection volume 20 µL; column temperature 30˚C. The method was evaluated for linearity, precision, and accuracy. The chemical stability of ampicillin for injection (18 mg/mL) in normal saline and sodium acetate (pH adjusted at values of 5, 5.5, and 6) was assessed at baseline (t=0), 7, 11, 17, 31, and 44 hours and compared to a control solution (no pH adjustment). Measurements at each time interval were performed in triplicate. Main Results: Ampicillin trihydrate reference standard (1 mg/mL) was adequately separated from degradation products following exposure to alkali, acid, and oxidative stress conditions. After 16 hours, a precipitate was observed in the solution at pH 6, and therefore stability is not reported. All other solutions (pH 5, pH 5.5, and control) were stable for at least 24 hours at room temperature and yielded t90 values of 110, 64.2, and 27.5 hours, respectively. Conclusions: Adjustment of intravenous ampicillin solutions to pH values of 5 or 5.5 significantly increased stability. Ampicillin appears to be most stable at a pH near its isoelectric point (pH 5).
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Liu, Qiongxi. "An investigation of the stability of advanced carbon-based coatings under high temperature and high pressure." Thesis, University of Birmingham, 2017. http://etheses.bham.ac.uk//id/eprint/7794/.
Повний текст джерелаАнотація:
In the thesis, four DLC coatings were selected: two hydrogen free DLC coatings (GCr and GCrSi) and two hydrogenated DLC coatings (DCr and DCrSi), which are the two main groups in the DLC family. Three annealing conditions (argon annealing, air annealing and HIPping/high pressure argon annealing) were designed to evaluate the stability and performance of the four DLC coatings. The microstructure of the DLC coatings was characterised by Raman spectrum, GIXRD, SEM/Due-beam SEM, EDX and TEM. Scratch tests were performed to assess the adhesion of the coatings. Both the mechanical and tribological properties of the DLC coatings were evaluated including nanohardness, coefficient of friction and wear factor. The influence of high pressure and oxidation was discussed, and the comparison of four different coatings was made.
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El, Chehaly Mohamed. "Power system stability analysis with a high penetration of distributed generation." Thesis, McGill University, 2010. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=92399.
Повний текст джерелаАнотація:
The operation and structure of distribution system is changing with the integration of distributed generation, based on alternative energy sources, including renewable energy sources (wind, solar). Among the new issues, there is the question of stability of distribution systems in the presence of a large penetration of distributed generation (DG).With power systems operating near their loadability limits, voltage stability becomes an important issue. Many utilities have implemented long-term solutions to counter this problem, such as adding transmission lines and new power plants in order to improve the reliability of the power system. An alternative solution consists in increase of generation at the distribution level. This is also a way of meeting growing economical, technical and environmental constraints. The size, the technology and the placement of DG play an important role in the operation of distribution systems.
For long-term voltage stability analysis, bus-based voltage indices are implemented using Power-Voltage curves (PV curves) to analyze the contribution of DG. Large-penetration of DG also has an impact on the short-term stability (voltage stability and transient stability) of the system. Indices are developed to rank contingencies and show how different DG interconnections affect the system during faults. Time-domain simulations are used to perform those studies. For short-term voltage stability, voltage dips at load buses are monitored following a large disturbance. The maximum rotor speed deviations of centralized synchronous generators are used to study the transient stability of the overall system.
All of the indices defined above are tested on three commonly used test systems: the IEEE RTS-96 24-bus system, the IEEE 39-bus New England system and the IEEE 118 bus system. The studies conclude that, regardless of its technology, DG mostly improves the long-term voltage stability particularly when located in areas near large loads. With DG injecting reactive power, the voltage security margin is increased. Synchronous machine based DG has the greatest impact on the short-term stability when located near large centralized generators. For short-term voltage stability, the contribution of DG is most noticeable when it absorbs reactive power, thus lowering the voltage dips.
La pénétration de la production décentralisée (DG) ne cesse de croître dans les réseaux électriques. La structure des réseaux conventionnels est modifiée à cause de la présence des sources de puissance alternatives localisées au niveau de la distribution. La stabilité du système est donc étudiée afin de déterminer les conséquences de la grande pénétration de DG.
La stabilité de tension devient une préoccupation majeure de la plupart des utilités dont leur capacité de charge approche les limites. Plusieurs utilités ont mis en uvre des solutions à long terme pour contrer ce problème, telles que l'implémentation de nouvelles lignes de transmission et de nouvelles centrales électriques afin de garantir la fiabilité du réseau électrique. D'autres utilités ont opté à des solutions plus modernes telles que la production décentralisée qui satisfait les contraintes économiques, techniques et environnementales.
Concernant l'analyse de la stabilité de tension à long terme, des indices basés sur la tension sont implémentés à l'aide des courbes PV pour analyser la contribution de la DG. La haute pénétration de la DG a également un impact sur la stabilité à court terme du système. Les indices sont mis au point pour classifier les défauts et exposer les effets des différentes interconnexions de la DG sur le réseau. Ces études ont été performées à l'aide de plusieurs simulations. Concernant la stabilité de tension à court terme, les creux de tension qui apparaissent à la charge lors d'un défaut sont utilisés pour déterminer la stabilité du système. Les plus grands écarts de vitesse du rotor des générateurs synchrones centralisés aident à déterminer la stabilité transitoire du réseau.
Ces indices sont testés en utilisant trois réseaux électriques. Indépendamment de la technologie, les DGs améliorent la stabilité de tension à long terme quand elles sont situées proches des grosses charges. La stabilité de tension est augmentée lorsque les DG opèrent avec un facteur de puissance inductif. Les DGs basées sur des machines synchrones améliorent le plus la stabilité à court terme, lorsqu'elles sont situées proche des grands générateurs centralisés. Pour la stabilité de tension à court terme, la contribution de la DG est plus visible quand elle opère avec un facteur de puissance capacitif.
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YANG, Demao. "Compression Stability of High Strength Steel Sections with Low Strain-Hardening." University of Sydney. School of Civil Engineering, 2003. http://hdl.handle.net/2123/561.
Повний текст джерелаАнотація:
Thin-walled steel sections made from high strength thin cold-reduced G550 steel to Australian Standard AS 1397-1993 under compression are investigated experimentally and theoretically in this thesis. This thesis describes three series of compression tests performed on box-section stub columns, box-section long columns and lipped channel section columns cold-formed from high strength steel plates in 0.42 mm or 0.60 mm thickness with nominal yield stress of 550 MPa. The tests presented in this thesis formed part of an Australian Research Council research project entitled: Compression Stability of High Strength Steel Sections with Low Strain-Hardening. For the fix-ended stub column tests, a total of 94 lipped-square and hexagonal section stub columns were tested to study the influence of low strain hardening of G550 steel on the compressive section capacities of the column members. For the pin-ended long column tests, a total of 28 box-section columns were tested to study the stability of members with sections which undergo local instability at loads significantly less than the ultimate loads. For the fix-ended lipped channel section columns, a total of 21 stub and long columns were tested to study the failure resulting from local and distortional buckling with interaction between the modes. A numerical simulation on the three series of tests using the commercial finite element computer program ABAQUS is also presented as part of this thesis. The post-buckling behaviour of thin-walled compression members is investigated. The effect of changing variables, such as geometric imperfections and end boundary conditions is also investigated. The ABAQUS analysis gives accurate simulations of the tests and is in good agreement to the experimental results. Theoretical studies using finite strip methods are presented in this thesis to investigate the buckling behaviour of cold-formed members in compression. The theoretical studies provide valuable information on the local and distortional buckling stresses for use in the interaction buckling studies. The finite strip models used are the semi-analytical and spline models. As expected for the stub columns tests, the greatest effect of low strain hardening was for the stockier sections where material properties play an important role. For the more slender sections where elastic local buckling and post-local buckling are more important, the effect of low strain hardening does not appear to be as significant. The pin-ended and fix-ended long column tests show that interaction, which is between local and overall buckling in the box sections, and between local and distortional buckling in the open channel sections, has a significant effect on their member capacities. The results of the successful column tests and ABAQUS simulation have been compared with the design procedures in the Australian & New Zealand Standard for Cold-Formed Steel Structures AS&NZS 4600 and the North American Specification for Cold-Formed Steel Structural Members prepared by the American Iron and Steel Institute. The stub column tests show that the current design rules give too conservative predictions on the compressive section capacities of the column members; whereas the long column tests show that the current column design rules are unconservative if used in their current form for G550 steel. Three design proposals are presented in this thesis to account for the effects of high strength thin steels on the section and member capacities.
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Lau, Ka. "Effects of biopolymer on stability mechanisms of aerated high-sugar confectionery." Thesis, University of Leeds, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.426843.
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McDowell, John Christopher. "Preparation characterisation and stability of selected high critical temperature ceramic superconductors." Thesis, University of Ulster, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.260854.
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Norcio, Lawrence P. "Stability studies of coal liquid products using high performance liquid chromatography." Morgantown, W. Va. : [West Virginia University Libraries], 1999. http://etd.wvu.edu/templates/showETD.cfm?recnum=984.
Повний текст джерелаАнотація:
Thesis (Ph. D.)--West Virginia University, 1999.Title from document title page. Document formatted into pages; contains xi, 152 p. : ill. (some col.) Vita. Includes abstract. Includes bibliographical references (p. 127-133).
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Hu, Rui Ph D. Massachusetts Institute of Technology. "Stability analysis of natural circulation in BWRs at high pressure conditions." Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1721.1/46431.
Повний текст джерелаАнотація:
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2007.Includes bibliographical references (leaves 112-115).
At rated conditions, a natural circulation boiling water reactor (NCBWR) depends completely on buoyancy to remove heat from the reactor core. This raises the issue of potential unstable flow. oscillations. The objective of this work is to assess the characteristics of stability in a NCBWR at rated conditions, and the sensitivity to design and operating conditions in comparison to previous BWRs. Two kinds of instabilities, namely Ledinegg flow excursion and Density Wave Oscillations (DWO), have been studied. The DWO analyses were conducted for three oscillation modes: Single Channel thermal-hydraulic stability, coupled neutronics region-wide out-of-phase stability and core-wide in-phase stability. Using frequency domain methods, the three types of DWO stability characteristics of the NCBWR and their sensitivity to the operating parameters and design features have been determined. The characteristic equations are constructed from linearized equations, which are derived for small deviations around steady operating conditions. The Economic Simplified Boiling Water Reactor (ESBWR) is used in our analysis as a reference NCBWR design. It is found that the ESBWR can be stable with a large margin around the operating conditions by proper choice of the core inlet orifice scheme, and for appropriate power to flow ratios. In single channel stability analysis, neutronic feedback is neglected. Design features of the ESBWR, including shorter fuel bundle and use of part-length rods in the assemblies, tend to improve the thermal-hydraulic stability performance. However, the thermal-hydraulic stability margin is still lower than that of a typical BWR at rated conditions. In neutronic-coupled out-of-phase as well as in-phase stability analysis, the perturbation decay ratios for ESBWR at our assumed conditions are higher than that of a typical BWR (Peach Bottom 2) at rated conditions, due to its lower thermal-hydraulic stability margin and higher neutronic feedback.
(cont.) Nevertheless, the stability criteria are satisfied. To evaluate the NCBWR stability performance, comparison with BWR/Peach Bottom 2 at both the rated condition and maximum natural circulation condition has been conducted. Sensitivity studies are performed on the effects of design features and operating parameters, including chimney length, inlet orifice coefficient, power, flow rate, and axial power distribution, reactivity coefficients, fuel pellet-clad gap conductance. It can be concluded that the NCBWR and BWR stabilities are similarly sensitive to operating parameters.
by Rui Hu.
S.M.
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Nelson, Weston McClain. "Spin Stability of Sounding Rocket Secondary Payloads Following High Velocity Ejections." DigitalCommons@USU, 2013. https://digitalcommons.usu.edu/etd/1961.
Повний текст джерелаАнотація:
The Auroral Spatial Structures Probe (ASSP) mission is a sounding rocket mission studying solar energy input to space weather. ASSP requires the high velocity ejection (up to 50 m/s) of 6 secondary payloads, spin stabilized perpendicular to the ejection velocity. The proposed scientific instrumentation depends on a high degree of spin stability, requiring a maximum coning angle of less than 5º. It also requires that the spin axis be aligned within 25º of the local magnetic field lines. The maximum velocities of current ejection methods are typically less than 10m/s, and often produce coning angles in excess of 20º. Because of this they do not meet the ASSP mission requirements. To meet these requirements a new ejection method is being developed by NASA Wallops Flight Facility. Success of the technique in meeting coning angle and B-field alignment requirements is evaluated herein by modeling secondary payload dynamic behavior using a 6-DOF dynamic simulation employing state space integration written in MATLAB. Simulation results showed that secondary payload mass balancing is the most important factor in meeting stability requirements. Secondary mass payload properties will be measured using an inverted torsion pendulum. If moment of inertia measurement errors can be reduced to 0.5%, it is possible to achieve mean coning and B-field alignment angles of 2.16º and 2.71º, respectively.
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Sharma, Abhinav. "ESTIMATING THE EFFECTS OF BLASTING VIBRATIONS ON THE HIGH-WALL STABILITY." UKnowledge, 2017. https://uknowledge.uky.edu/mng_etds/38.
Повний текст джерелаАнотація:
The stability of the high-walls is one of the major concerns for open pit mines. Among the various factors affecting the stability of high-walls, blast vibrations can be an important one. In general, worldwide the established respective government regulations and industry standards are used as guidance to determine the maximum recommended levels of the peak particle velocity and frequency from the blast to avoid any effects on the structures around the mining project. However, most of the regulations are meant for buildings or houses and do not concern high-walls.
This thesis investigates the response of high-walls under the effects of vibrations from mine blasting. In this research, the relationship between the high-wall response, the geometry of the slope, the frequency and the amplitude, of the ground vibration produced by blasting, is explored using numerical models in 3DEC. The numerical models were calibrated initially with data collected using seismographs installed in a surface mine operation and recording vibrations produced by an underground mine drill and blast operation. Once the calibration was accomplished, a parametric study was developed to explore the relationships between various parameters under study and its impact on the stability of high-walls.
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Yao, Yiying. "Thermal Stability of Al₂O₃/Silicone Composites as High-Temperature Encapsulants." Diss., Virginia Tech, 2014. http://hdl.handle.net/10919/50593.
Повний текст джерелаАнотація:
Conventional microelectronic and power electronic packages based on Si devices usually work below 150°C. The emergence of wide-bandgap devices, which potentially operate above a junction temperature of 250°C, results in growing research interest in high-density and high-temperature packaging. There are high-temperature materials such as encapsulants on the market that are claimed for capability of continuous operation at or above 250°C. With an objective of identifying encapsulants suitable for packaging wide-bandgap devices, some of commercial high-temperature encapsulants were obtained and evaluated at the beginning of this study.
The evaluation revealed that silicone elastomers are processable for various types of package structure and exhibit excellent dielectric performance in a wide temperature range (25 - 250°C) but are insufficiently stable against long-term aging (used by some manufacturers, e.g., P²SI, to evaluate polymer stability) at 250°C. These materials cracked during aging, causing their dielectric strength to decrease quickly (as soon as 3 days) and significantly (60 - 70%) to approximately 5 kV/mm, which is below the value required by semiconductor packaging. The results of this evaluation clearly suggested that silicone needs higher thermal stability to reliably encapsulate wide-bandgap devices.
Literature survey then investigated possible methods to improve silicone stability. Adding fillers is reported to be effective possibly due to the interaction between filler surface and polymer chains. However, the interaction mechanism is not clearly documented. In this study, the effect of Al₂O₃ filler on thermal stability was first investigated by comparing the performance of unfilled and Al₂O₃-filled silicones in weight-loss measurements and dielectric characterization. All test results on composites filed with Al₂O₃ micro-rods indicated that thermal stability increased with increasing filler loading. Thermogravimetric analysis (TGA) test demonstrated that the temperature of degradation onset increased from 330 to 379°C with a 30 wt% loading of Al₂O₃ rods. In isothermal soak test, unfilled and 30-wt%-filled silicones lost 10% of polymer weight in 700 and 1800 hours, respectively. The dielectric characterization found that both Weibull parameters, characteristic dielectric strength (E₀, representing the electric field at which 62.3% of samples are electrically broken down) and shape parameter (β, representing the spread of data. The larger the β, the narrower the distribution) can reflect the thermal stability of polymers. Both of them were influenced by microstructure evolution, to which β was found to be more sensitive than E₀. The characteristic dielectric strength of unfilled silicone decreased significantly after 240 hours of aging at 250°C, whereas that of Al₂O₃/silicone composites exhibited no significant change within 560 hours. The shape parameter of Al₂O₃-filled silicone decreased slower than that of unfilled silicone, also indicating the positive effect of Al₂O₃ micro-rods on thermal stability.
Improved thermal stability can be explained by restrained chain mobility caused by interfacial hydrogen bonds, which are formed between hydroxyl groups on Al₂O₃ surface and silicone backbone. In this study, the effect of hydrogen bonds was investigated by dehydrating Al₂O₃ micro-rods at high temperature in N₂ to partially destroy the bonds. Removal of hydrogen bonds impaired thermal stability by increasing the initial weight-loss rate from 0.025 to 0.036 wt%/hour. The results explained the importance of interfacial hydrogen bond, which effectively reduced the average chain mobility, hindered the formation of degradation products, and led to higher thermal stability.
The main discoveries of this study are listed below:
1. Al₂O₃ micro-rods were found to efficiently improve the thermal stability of silicone elastomer used for high-temperature encapsulation.
2. Characteristic dielectric strength and shape parameter obtained from Weibull distribution reflected the change of material microstructure caused by thermal aging. The shape parameter was found to be more sensitive to microscale defects, which were responsible for dielectric breakdown at low electric field.
3. Hydrogen bonds existing at filler/matrix interface were proven to be responsible for the improvement of thermal stability because they effectively restrained the average chain mobility of the silicone matrix.Ph. D.
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Yakusheva, Anastasia A. "Devitrification Kinetics and Optical Stability of Optical Fibers at High Temperatures." Thesis, Virginia Tech, 2018. http://hdl.handle.net/10919/83487.
Повний текст джерелаАнотація:
Reliable sensing and monitoring systems based on optical fibers operating at high temperatures and in harsh environments are of high demand. One of the limitations of such systems is the devitrification of the fused silica based core and cladding glass at elevated temperatures. Crystallites can nucleate on the surface of the cladding and grow into the core. The formation of these crystalline flaws in the optical fiber causes stress concentration and extrinsic optical scattering and in addition leads to decreased mechanical properties and reduced optical stability. Commercial optical fibers of different compositions and core-cladding design were characterized in this study with respect to crystallization rate under various conditions. The optical stability was monitored with an optical spectrum analyzer. The crystallites were characterized with SEM and optical microscopy. The activation energies of crystallization for High OH and Low OH multimode fibers were estimated by measuring the crystal growth rate at different temperatures. The residual stress resulting from the formation of the crystals, which can lead to decreased mechanical performance of the fibers, was characterized with polarized light optical microscopy. The influence of water vapor in the atmosphere on the crystallization rate was determined. The features induced in the attenuation spectra were consistent with hydroxyl (OH) absorption peak. Spectral features such as thermal emission and hydroxyl absorption bands are discussed.
The results obtained in this study can be used for selecting optical fibers for high temperature applications.Master of Science
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Sen, Shiladitya. "Engineering Proteins for Enhanced Stability using High-throughput and Combinatorial methods." The Ohio State University, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=osu1385987653.
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Hamza, Haval Rawf. "The impacts of high-frequency trading on the financial markets’ stability." Kent State University / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=kent1428416050.
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Tamimi, Ala. "Voltage stability limits for weak power systems with high wind penetration." Diss., Kansas State University, 2011. http://hdl.handle.net/2097/13100.
Повний текст джерелаАнотація:
Doctor of PhilosophyDepartment of Electrical and Computer Engineering
Anil Pahwa
Shelli K. Starrett
Analysis of power system voltage stability has practical value in increasing wind penetration levels. As wind penetration levels increase in power systems, voltage stability challenges arise due to locating wind resources far away from load centers. This dissertation presents several different voltage stability methods for sizing new wind farms. Power system wind penetration levels depend on the available voltage stability margins (VSMs) of the existing power system and system load characteristics. Three new iterative methods have been developed to maximize wind penetration level in weak power systems based on systems’ VSMs. The first two methods use an iterative approach for increasing the size of each wind farm until reaching the collapse point. Wind farms with less negative impact on system VSMs are sized larger than others. A third wind farm sizing method has been developed using modal analysis in conjunction with the traditional voltage stability method (Q-V method). Wind farms are placed at buses in the power system which have the lowest negative impact on voltage instability modes (strong wind injection buses). By placing the wind farms at the strongest wind injection buses, higher amounts of wind power can be injected into the power system. To further increase wind penetration in weak power systems, two additional techniques are introduced and applied to the western Kansas power system. The first technique uses modes of voltage instability to place voltage support equipment like static var compensators at locations in the power system where they provide the needed reactive power support for increasing levels of wind penetration. The second technique uses the fact that wind patterns at a wind farm site may rarely allow the wind farm to produce its maximum capacity during the peak loading hours. Wind farm maximum sizes can be increased above their maximum voltage stable size limit without driving the power system into becoming voltage unstable. Preventing voltage collapse for the additional increases in wind farm sizes is accomplished by disconnecting some wind turbines inside the wind farm during critical times to reduce its power output to a voltage stable level.
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Garby, Romain. "Simulations of flame stabilization and stability in high-pressure propulsion systems." Phd thesis, Toulouse, INPT, 2013. http://oatao.univ-toulouse.fr/9706/1/garby.pdf.
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Andersson, Joel, and Ludvig Hagberg. "Stability Analysis of High Concrete Dams : Longtan Dam - a case study." Thesis, Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-67719.
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Grizio, Miranda. "Optimizing emulsion stability of high-oil pourable dressings using different stabilizers." Kansas State University, 2011. http://hdl.handle.net/2097/9202.
Повний текст джерелаАнотація:
Master of ScienceDepartment of Food Science Institute
Fadi M. Aramouni
To find an optimal formulation of oil-in-water (O/W) emulsion pourable dressings containing 60% and 70% soybean oil, nine stabilizers common to the dressing industry were tested, each at three concentrations. The stabilizers tested were xanthan gum, propylene glycol alginate (PGA), a xanthan gum/guar gum/sodium alginate blend, a viscosifying modified corn starch, an emulsifying modified corn starch, microcrystalline cellulose (MCC), liquid salted egg yolks, liquid salted whole eggs, and dried egg whites (DEW). The emulsions were made with a laboratory high shear disperser and evaluated over 8 weeks by measurement of creaming in 100- mL graduated cylinders and by change in viscosity as measured by a Brookfield viscometer. The stabilizers and concentrations most effective at preventing separation and maintaining viscosity were MCC at 1%, 2%, and 3% in the 60% oil emulsion and 0.25% xanthan gum in the 70% oil emulsion. Average viscosities ranged from 3300 - 23,400 centipoise (cP). The emulsifying starch and dried egg whites were also effective at preventing creaming, but failed to maintain viscosity. The other ingredients showed some initial emulsion stability followed by either a gradual or sudden decrease in stabilization, as seen by decreasing viscosity and eventual separation.
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Blank, V. D., A. N. Kirichenko, A. K. Aseev, V. N. Denisov, I. A. Perezhogin, B. A. Kulnitskiy, and A. A. Nuzhdin. "Influence of Shear Deformation on Carbon Onions Stability under High Pressure." Thesis, Sumy State University, 2012. http://essuir.sumdu.edu.ua/handle/123456789/35018.
Повний текст джерелаАнотація:
The influence of shear deformation on carbon onions stability under high pressure up to 45 GPa was
investigated in a Shear Diamond Anvil Cell (SDAC) by the Raman spectroscopy and the Transmission
Electron Microscopy (TEM). At shear less then 40 degrees the carbon onions are stable up to 30 GPa. Biger shear deformation leads to increasing of size and destruction of the onions and to formation of sp3 C-C
bonds. At pressure exceeded 45 GPa shear deformation leads to diamond-like carbon (DLC) formation.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35018
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Ortega, Gómez Tino [Verfasser]. "Stability analysis of the high performance light water reactor / Tino Ortega Gómez." Karlsruhe : Forschungszentrum Karlsruhe, 2009. http://d-nb.info/994365624/34.
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Rojko, Roman. "New concepts for transverse beam stability in high-current heavy-ion synchrotrons." Phd thesis, Berlin : Dissertation.de, 2003. http://elib.tu-darmstadt.de/diss/000382.
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