Gas foil bearings (GFBs) operating at high temperature rely on thermal management procedures that supply needed cooling flow streams to keep the bearing and rotor from overheating. Poor thermal management not only makes systems inefficient and costly to operate but could also cause bearing seizure and premature system destruction. To date, most of thermal management strategies rely on empirically based "make-and-break" techniques which are often inefficient. This dissertation presents comprehensive measurements of bearing temperatures and shaft dynamics conducted on a hollow rotor supported on two first generation GFBs. The hollow rotor (1.36 kg, 36.51 mm OD and 17.9 mm ID) is heated from inside to reach an outer surface temperature of 120 degrees C. Experiments are conducted with rotor speeds to 30 krpm and with forced streams of air cooling the bearings and rotor. Air pressurization in an enclosure at the rotor mid span forces cooling air through the test GFBs. The cooling effect of the forced external flows is most distinct when the rotor is hottest and operating at the highest speed. The temperature drop per unit cooling flow rate significantly decreases as the cooling flow rate increases. Further measurements at thermal steady state conditions and at constant rotor speeds show that the cooling flows do not affect the amplitude and frequency contents of the rotor motions. Other tests while the rotor decelerates from 30 krpm to rest show that the test system (rigid-mode) critical speeds and modal damping ratio remain nearly invariant for operation with increasing rotor temperatures and with increasing cooling flow rates. Computational model predictions reproduce with accuracy the test data. The work adds to the body of knowledge on GFB performance and operation and provides empirically derived guidance for successful integration of rotor-GFB systems.

Gas bearings enable successful applications in high speed oil-free microturbomachinery. This thesis presents analysis and experiments of the dynamic performance of a rotor supported on Rayleigh step gas bearings. Comprehensive experiments demonstrate that Rayleigh step hybrid gas bearings exhibit adequate stiffness and damping capability in a narrow range of shaft speeds, up to ~ 20 krpm. Rotor coastdown responses were performed for two test bearing sets with nominal radial clearance of 25.4 ?m and 38.1 ?m, respectively. A near-frictionless carbon (NFC) coating was applied on the rotor to reduce friction against its bearings at liftoff and touchdown. However, the rotor still experienced dry friction at low shaft speeds (below ~ 4,000 rpm). Experiments show that the supply pressure raises the rotor critical speed and decreases the system damping ratio. The geometry of the Rayleigh steps distributed on the rotor surface generates a time varying pressure field and results in a sizable 4X super synchronous component of bearing transmitted load. The external supply gas pressure affects slightly the onset speed of instability of the rotor-bearing system. The unstable whirl frequencies are nearly fixed at the system natural frequency (~ 120 Hz). Analysis with a finite element model predicts the stiffness and damping force coefficients for the bearing accounting for a purely hydrodynamic operation condition. Predictions show the synchronous stiffness and damping coefficients decrease with shaft speed. Predicted threshold speeds of instability are lower, ~ 50% or less than the measurement due to the analytical model limitations assuming a grooved stator. The predicted synchronous responses to imbalance correlate well with the measurements, however. The Rayleigh step gas bearing shows similar characteristics to the flexure pivot tilting pad bearing (FPTPB) tested in 2003. However, the test Rayleigh step gas bearings exhibit a much reduced stable operating speed range, below 20 krpm. The maximum speed achieved is much lower to that determined with an identical rotor supported on FPTPBs, i.e. rotor dynamically stable up to 100 krpm. The FPTPB is more reliable in high speed oil-free applications due to its excellent stability characteristics.

No description available.

There is a need to improve the capabilities of cooling technology to dissipate increasingly high heat fluxes (approaching 200 W/cm²) from electronic components in order to maintain acceptable operating temperatures. The linear compressor with clearance seal and flexure spring which has been used for many years for space application, is an attractive proposition for vapour compression refrigeration (VCR) systems in electronics cooling applications because it offers several advantages over traditional compressor technology. This thesis introduces a novel design of oil-free linear compressor and investigates the performance of the linear compressor system and the key issues with respect to the operations of the compressor. A comprehensive system analysis has been carried out for the linear compressor regarding the moving magnet motor characteristics, spring stiffness, damping, imbalance of current in coils, DC offset and leakage across the clearance seal. To measure the performance of the linear compressor, an experimental apparatus and a data acquistion system have been built. A harmonic fitting approach has been proposed to reconstruct current, voltage and displacement signals using fast Fourier transform (FFT) and minimisation algorithms. The piston dynamics have been modelled to infer pressure-volume (P-V) diagrams. Preliminary measurements of the linear compressor at resonance using nitrogen show that an adiabatic efficiency of 42% - 60% and a motor efficiency of 71% - 89% are maintained over a wide range of operating parameters, indicating that systems using this type of compressor could have good part load efficiency. At the design point (a pressure ratio of 3.0 and a stroke of 13 mm), pressure drop loss is responsible for 27% of the total losses and the most significant loss is the copper loss (34%). A commercial crank-drive compressor with comparable properties has also been evaluated for comparison, showing that the prototype linear motor has a much higher efficiency than the rotary induction motor, particularly at low electrical power inputs while the volumetric efficiencies in the crank-drive compressor are about 20% higher. The linear compressor in a refrigeration system using R134a has been measured for different strokes under each pressure ratio with a fixed condenser outlet temperature of 50°C and evaporator temperature ranging from 6°C to 27°C. A model of the resonant frequency using discharge and suction pressures and the stroke has been validated. A coefficient of performance (COP) of 3.2 has been achieved at a cooling capacity of 384 W and an evaporator temperature of 20°C. This is assumed to be a typical CPU cooling condition. A Proportional-Integral-Derivative (PID)/Pulse-Width-Modulation (PWM) control system using a solenoid operated valve has been developed for DC offset and clearance control. Operation of the linear compressor with a fixed clearance of 0.8 mm using nitrogen has been carried out in comparison with the fixed zero DC offset operation. When operated with a fixed clearance, the volumetric efficiency increases with a decreasing stroke but the fixed clearance operation requires a higher power input. This provides evidence that the piston control in a refrigeration system with capacity control should be for a zero DC offset.

No description available.

No description available.

Submitted by Marilene Donadel (marilene.donadel@unioeste.br) on 2018-11-05T20:12:01Z No. of bitstreams: 1 Fernanda_Tavares_2018.pdf: 3497938 bytes, checksum: 33976a24dbef1bda196a346bd634eade (MD5) / Made available in DSpace on 2018-11-05T20:12:01Z (GMT). No. of bitstreams: 1 Fernanda_Tavares_2018.pdf: 3497938 bytes, checksum: 33976a24dbef1bda196a346bd634eade (MD5) Previous issue date: 2018-05-16 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Lipases are a class of enzymes responsible for breaking down the ester bonds of triglycerides; they can derivate from microbial, animal or vegetal origin, the last ones can be found mainly from the germination processes of oilseeds. Castor bean seeds have the advantage of having lipases in its composition even in non-germinated state. In this sense, the present work aims to evaluate the use of castor bean as a source of lipase in the hydrolysis of triglycerides to obtain free fatty acids, employing sonication and orbital agitation system, investigating the use of in natura seeds and seeds after oil extraction process, to enable its commercial use. In addition, the crambe oil is employed, an oil that is rich in monosaturated free fatty acids, easy obtainment and it is has no use in the food chain. Using CCRD experimental designs an optimum point was found for in natura seeds in the condition of temperature of 33 °C, 0.87 buffer/oil (m/m) and 0.31 catalyst/substrate (m/m) reaching a yield of 91% after 4 hours of reaction; for the incubated seeds the optimum condition was in the temperature of 43.8 °C, 1.8 buffer/oil (m/m) and 0.07 catalyst/substrate (m/m) in which approximately 88% yield was achieved after 4 hours of reaction. In order to optimize the reaction conditions, the use of ultrasound in the enzymatic hydrolysis was evaluated, as optimal conditions were approximately 70% of ultrasound power, 1.79 buffer/oil (m/m), 0.25 catalyst/substrate (m/m) for the in natura seeds, achieving approximately 86% of yield after 5 minutes of ultrasonic reaction and 91% after more 30 minutes of reaction with mechanical stirring and for incubated seeds approximately 68% of ultrasound power, 1.67 buffer/oil (m/m), 0.06 catalyst/substrate (m/m), reaching 73% of yield after 5 minutes of ultrasonic reaction and 77% after more 30 minutes of reaction with mechanical stirring. The best yield, therefore, was verified when in natura seeds were employed; these seeds don’t go through any preparation process and have emulsifying components that can help in the triglycerides hydrolysis. Besides that, a mathematical model was proposed and it was satisfactory to predict the experimental data of crambe oil enzymatic hydrolysis when in natura and incubated seeds were employed in mechanical-orbital agitation reactions and with ultrasonic emulsification. In this sense, the contribution of the present work is to give a better understanding for the enzymatic hydrolysis of triglycerides with the utilization of vegetal lipases from castor seeds, and employing an oil that has not been yet well studied and utilized. / Lipases são uma classe de enzimas responsáveis pela quebra de ligações éster de triglicerídeos e podem ser de origem microbiana, animal ou vegetal, sendo que as ultimas podem ser encontradas principalmente a partir do processo de germinação de sementes oleaginosas. As sementes da mamona possuem a vantagem de conterem lipase em sua composição mesmo em fase não germinativa. Dessa forma, o presente trabalho tem como objetivo avaliar o uso da semente de mamona como fonte de lipase na hidrólise de triglicerídeos para obtenção de ácidos graxos livres empregando ultrassom e sistema de agitação orbital, investigando o uso das sementes in natura e sementes após o processo de extração de óleo, para viabilizar seu uso comercial. Além disso, busca-se empregar o óleo de crambe, um óleo rico em ácidos graxos monoinsaturados, de fácil obtenção e que não é usado na cadeia alimentar. Empregando planejamentos experimentais DCCR verificou-se ponto ótimo para sementes in natura na condição de 33°C de temperatura, 0,87 solução tampão/óleo (m/m) e 0,31 catalisador/substrato (m/m) no qual atingiu-se 91% de rendimento após 4 horas de reação; para as sementes incubadas o ponto ótimo foi nas condições de 43,8°C de temperatura, 1,8 solução tampão/óleo (m/m) e 0,07 catalisador/substrato (m/m) no qual atingiu-se aproximadamente 88% de rendimento após 4 horas de reação. Visando otimizar as condições reacionais, foi avaliado o uso de ultrassom na hidrólise enzimática, as condições ótimas foram de aproximadamente 70% de potência do ultrassom, 1,79 tampão/óleo (m/m), 0,25 catalisador/substrato (m/m) para as sementes in natura, atingindo se aproximadamente 86% de rendimento após 5 minutos de reação em ultrassom e 91% após mais 30 minutos de reação com agitação mecânica-orbital e para as sementes incubadas aproximadamente 68% de potência do ultrassom, 1,67 tampão/óleo (m/m), 0,06 catalisador/substrato (m/m), atingindo se aproximadamente 73% de rendimento após 5 minutos de reação em ultrassom e 77% após mais 30 minutos de reação com agitação mecânica. Verificou-se, portanto, um melhor rendimento com a utilização das sementes in natura, que não passam por nenhum processo de preparo, e possuem componentes emulsificantes que auxiliam na hidrólise de triglicerídeos. Além disso, um modelo matemático foi proposto e foi satisfatório para predição dos dados experimentais das hidrólises enzimáticas do óleo de crambe empregando sementes in natura e incubadas em reações com agitação mecânica-orbital e em ultrassom. Dessa forma, o presente trabalho contribui de forma a melhorar o entendimento do mecanismo da reação de hidrólise enzimática de triglicerídeos com utilização de lipases vegetais a partir da semente de mamona, empregando um óleo ainda pouco utilizado e estudado, o crambe.

The compressor for air supply to the fuel cell stack is a critical component of the balance of plant, especially for mobile applications. The main requirements of the compressor are the performance regarding pressure ratio, mass flow and efficiency at minimal size and weight. The turbo compressor technology is ideally suited to cope with these requirements. The lifetime requirement and the necessity of oil and particle free air supply advised to employ air bearings. The fuel cell air supply requirements are in conflict with the turbo compressor pressure ratio and mass flow characteristics. Possible solutions and their impact onto compressor design and fuel cell operation are described in this paper. The chosen system design approach considering all design aspects and its interactions during the design phase is beneficial in order to achieve the most lightweight and efficient air supply system for fuel cells. Experimental validation of an air bearing turbo compressor for a 100 kW fuel cell stack on an aerodynamic test rig verifies the predicted performance.

Air bearing radial turbo (also called centrifugal) compressors prevail in most mobile fuel cell air supply applications due to the small size and weight, the high efficiency and the oil- and maintenance free operation. An important aspect in mobile fuel cell applications is the vibration resistance of all system components, including the compressor, with vibration requirements up to 20 g in automotive applications. This paper gives the background of the air bearing vibration characteristics, depicting the dependencies of vibration resistance on inlet conditions and operating points. The critical operating conditions concerning vibration resistance are identified, and it is outlined how vibration requirements can be included in the design process of an air bearing turbo compressor. A visualization of vibration resistance in the commonly used compressor map is presented, allowing the fuel cell system integrator to take qualified decisions for the mechanical integration of the compressor concerning vibrations.