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Fuel cells are a clean, high-efficiency source for power generation. This innovative technology is going to penetrate all aspects in our life, from utility distributed power, transportation applications, down to power sources for portable devices such as laptop computer and cell phones. To enable the usage of fuel cell, developing power converters dedicated for fuel cells becomes imminent. Currently, the full-bridge converter is the dominating topology in high power dc/dc applications. Although multiphase converters have been proposed, most of them are dealing with high input-voltage systems, and their device characteristic is not suitable for a low voltage source such as a fuel cell. For a high power fuel cell system, high voltage conversion ratios and high input currents are the major obstacles to achieving high-efficiency power conversions. This dissertation proposes a novel 3-phase 6-leg dc/dc power converter with transformer isolation to overcome these obstacles. Major features of the proposed converter include: (1) Increase converter power rating by paralleling phases, not by paralleling multiple devices; (2) Double the output voltage by transformer delta-wye connection, thus lowering the turns-ratio; (3) Reduce the size of output filter and input dc bus capacitor with interleaved control; (4) Achieve Zero-Voltage Zero-Current Switching (ZVZCS) over a wide load range without auxiliary circuitry. High conversion efficiency above 96% is verified with different measurement approaches in experiments. This dissertation also presents the power stage and control design for the proposed converter. Control design guideline is provided and the design result is confirmed with both simulation and hardware experiments. When using the fuel cell for stationary utility power applications, a low-frequency ripple interaction was identified among fuel cell, dc/dc converter and dc/ac inverter. This low frequency ripple tends to not only damage the fuel cell, but also reduce the source capability. This dissertation also investigates the mechanism of ripple current propagation and exploits the solutions. A linearized ac model is derived and used to explain the ripple propagation. An active ripple reduction technique by the use of the current loop control is proposed. This active current loop control does not add extra converters or expensive energy storage components. Rather, it allows a reduction in capacitance because the ripple current flowing into the capacitor is substantially reduced, and less capacitance can be used while maintaining a clean dc bus voltage. The design process and guideline for the proposed control is suggested, and the effectiveness of this active control is validated by both simulation and experimental results. / Ph. D.

The phase behavior and phase separation kinetics in polymer solutions in binary mixtures of supercritical carbon dioxide (CO2) and organic solvents were studied for two systems. Solutions of polyethylene (PE) in CO2 + n-pentane were selected as one model system to study both the solid-fluid (S-F) and liquid-liquid (L-L) phase transitions as well as the interplay of these two types of phase separations on the final morphological and thermal properties of PE crystals. Solutions of polysulfone (PSF) in CO2 + tetrahydrofuran (THF) were selected as another model system because of the technological importance of this membrane forming polymer and because of the broad interest in developing new solvent/non-solvent systems for forming microporous materials. These phase boundaries were determined using a high-pressure view-cell and optical techniques over a temperature range of 90-165 oC and pressures up to 55 MPa for PE/n-pentane/CO2 system, and over a temperature range of 25 to 155 oC and pressures up to 70 MPa for PSF/THF/CO2 system. For PE solutions, it has been found that the addition of CO2 to the PE/n-pentane system shifts the L-L phase boundary to significantly higher pressures, but moves the S-F phase boundary only slightly to higher temperatures. The S-F phase boundary which represents the crystallization/melting process in the polymer solution was about 10 oC lower than the crystallization/melting temperatures of the neat polyethylene samples determined by differential scanning calorimetry (DSC). It was further found that the S-F phase boundary in n-pentane displays a unique sensitivity to the pressure-temperature conditions and moves to lower temperatures in the pressure range from 38 to 42 MPa. This effect even though not as augmented remains also for the S-F boundary in the solutions in CO2 + n-pentane mixtures. The miscibility of PSF in THF + CO2 was investigated at CO2 levels up to 14 wt %. This system shows lower critical solution temperature (LCST)-type phase behavior at low CO2 content, which is shifted to upper critical solution temperature (UCST)-type at higher CO2 levels along with an increase in the miscibility pressures. In contrast to the PE system, this system was found to display multiple miscibility windows. A "U"-shaped phase boundary in 92 % THF + 8 % CO2 mixture was observed to transfer to a "W"-shaped phase boundary at 10 wt % CO2, which was further separated into a double "U"-shaped phase boundary at 13 wt % CO2. The specific volume of the polysulfone solutions were found to display a variation parallel to this changing pattern in the phase boundaries, with reduced miscibility being accompanied with an increase in the specific volume. The phase separation kinetics in these two polymer solutions were investigated using time- and angle-resolved light scattering techniques. With the PE solutions, the focus was on the kinetics of S-F phase separation (crystallization) and miscibility and (melting) in n-pentane. Experiments were conducted with relatively dilute solutions at concentrations up to 2.3 wt %. The results show that the crystallization which was induced by cooling at constant pressure is dominated by a nucleation and growth process. In the majority of the experiments the particle growth process was observed to last for about 1 minute with a slight dependence on the crystallization pressure. The phase separation kinetics in PSF solutions were conducted only in a solvent mixture containing 90 wt % THF and 10 wt % CO2. Polymer concentrations were varied up to 3.3 wt %. This system was also observed to undergo phase separation by only nucleation and growth mechanism under these conditions upon reducing the pressure at constant temperature. Several experiments were conducted using a multiple rapid pressure drop technique to identify the depth of the metastable region. PE crystals that were produced by crossing the S-F boundary by different paths were collected and characterized by field emission scanning electron microscopy (FESEM) and DSC. Crystallization was carried out either by cooling at constant pressure, or by cooling without pressure adjustment, or by first crossing the L-L boundary via pressure reduction at a constant temperature followed by cooling. For crystal recovery, the system was depressurized to ambient conditions irrespective of the path. It was found that all of the crystals formed from these solutions show multiple melting peaks in their first DSC heating scans, which however collapse into one crystallization peak in the cooling scans and one melting peak in the second heating scans. The temperatures corresponding to the multiple melting peaks were lower than the single melting temperature of the original PE sample and the melting temperature observed in the second heating scans for all samples. The multiple melting peaks were attributed to the presence of different lamellar thickness that are formed in the crystallization, final depressurization and sample collection stages. Depending upon the crystallization path some differences were noted. The crystals formed by first going through L-L phase separation displayed predominately double melting peaks in the first DSC scan. It was observed that the overall crystallinity is increased by more than 10 % to about 75 % compared to the crystallinity of the original PE sample, which is about 63 %. FESEM characterization showed that the prevailing morphology is composed of plate-like lamellae that show different level of agglomeration depending on the crystallization conditions. The overall structures of the particles were ellipsoid for crystals formed from dilute solutions. For crystals formed from the 1% PE solution, crystal sizes ranged from 4 mm ´ 10 mm for crystals formed at 14 MPa to 30 mm ´ 45 mm at 45 MPa. The crystals formed from 5 wt % solutions in n-pentane at pressures in the range of 38-54 MPa showed different morphologies with features of shish-kebab like structures which were however absent in crystals formed from n-pentane + CO2 solutions. The crystals that were formed from first crossing the L-L phase boundary followed by cooling showed two distinct particle size ranges that were attributed to crystals formed from the polymer-rich and polymer-lean phases that evolve when the L-L phase boundary is crossed. / Ph. D.

This thesis proposes one comparison methodology for single and three-phase high power high frequency transformers in power conversion systems. The objective is to compare the volume of the transformers. And single and three-phase Dual Active Bridge Converter (DAB1 and DAB3) topologies with single and three-phase isolating transformers are selected for the transformer comparison. Design optimization of power transformer has been studied and simplified models have been built for the single and three-phase transformer design optimization in this work, including assumptions for core shapes, materials, winding structures and thermal model. Two design methods have been proposed according to different design constraints, named T – B Method and J – B Method separately. T – B Method is based on feature of the core, which has the major limits of maximum flux density and temperature rise. The flux density should not reach the saturation value of the core, and temperature rise should meet specifications in different applications to assure the performance of the core (permeability, saturation flux density, and core loss) and the insulation of the wire. And J – B Method starts from the comparison of area product in conventional design method. The relationship between area product of transformer cores and the flux and current of the transformer in design is analyzed. There is specified relationship between area product of single and three-phase transformers if flux and current densities are specified for both. Thus J – B Method is proposed with the design constraints of specified current and flux density. Both design methods include both single and three-phase transformer design. One example case for single and three-phase transformer comparison is selected as high power high frequency DAB conversion system. Operation principles are studied for both DAB1 and DAB3 based on previous work. And transformer design based on the T – B and J – B Methods are carried out and transformer volumes are compared. And results show that three-phase transformer has little benefit in volume or thermal than single-phase transformer, when they are utilized in single-phase DAB and three-phase DAB converters separately. Scaled-down single and three-phase DAB systems have been built and volume and thermal tests have been carried out. / Master of Science

Radio frequency (RF) receivers are inherently non-linear due to non-linear components contained within the RF front-end such as the low noise amplifier (LNA) and mixer. When receivers operate in the non-linear region, this will affect the system performance due to intermodulation products, and cross-modulation, to name a few. Intermodulation products are the result of adjacent channel signals that combine and create intermodulation distortion of the received signal. We call these adjacent channel signals blockers. Receiving blockers are unavoidable in wideband receivers and their effect must be analyzed and properly addressed. This M.S. Thesis studies the effect of blockers on system performance, specifically the symbol error rate (SER), as a function of the receiver non-linearity figure and the blocking signal power and modulation format. There have been numerous studies on the effect of non-linearity in the probability of true and false detections in spectrum sensing when blockers are present. There has also been research showing the optimal modulation scheme for effective jamming. However, we are not aware of work analyzing the effect of modulated adjacent channel blockers on communication system performance. The approach taken in this paper is a theoretical derivation followed by numerical analysis aimed to quantify the effect of receiver nonlinearity on communication system performance as a function of (1) receiver characteristics, (2) blocking signal powers, (3) signal and blocker modulation format, and (4) phase-synchronized/non-synchronized blocker reception. The work focuses on M-PSK modulation schemes. For high blocker powers and non-linearity, the Es/No (Eb/No) performance loss can be as high as 4.7 dB for BPSK modulated signal and BPSK modulated blockers when received in sync with the desired signal. When blockers have a random phase offset with respect to the desired signal, the performance degradation is about 2 dB for BPSK modulated desired and blocker signals. It was found that for an BPSK transmitted signal with phase-synchronous blockers, the SER (BER) deteriorates the most when the blocking signals are of the same modulation. The effect is reduced, but still significant, as the modulation order of the signal of interest or the blockers, or both increases. / Master of Science / This thesis analyzes the effect of non-linear components in wireless receivers on communication system performance. We consider that two strong radio frequency signals adjacent in frequency to the desired signal enter the receiver and cause signal distortion known as 3rd order intermodulation distortion. We analyze the effect on the symbol error rate (SER) in the presence of two modulated blockers. SER defines the ratio of erroneously detected symbols to the total number of transmitted symbols and is a function of the modulation scheme and radio channel conditions. The SER analysis is done for Phase Shift Keying (PSK) modulated signals and blockers for different receiver types and blocker power levels. This thesis derives the theoretical SER expressions followed by numerical analysis aimed to quantify the effect of receiver non-linearity on communication system performance as a function of (1) receiver characteristics, (2) blocking signal powers, (3) signal and blocker modulation formats, and (4) phase-synchronized/non-synchronized reception of blockers. We justify the need for these new SER expressions and verify them via simulations. The thesis shows that modulated blockers can significantly impact communication system performance if the blockers are strong with respect to the signal of interest and if the device is highly non-linear. The work also shows that the performance degradation is a function of the blocker signal characteristics, but there are ways to overcome this loss by design or management. This has important implications on the management of spectrum in the new shared spectrum bands, where heterogeneous systems and devices will coexist with strong signals coming from nearby transmitters, radars or TV stations, among others.

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.

no / In recent years there have been major advances in our understanding of the mechanisms of phase separation in polymer and copolymer blends, to the extent that good control of phase-separated morphology is a real possibility. Many groups are studying the computational simulation of polymer phase separation. In the light of this, we are exploring methods which will give insight into the mechanical response of multiphase polymers. We present preliminary results from a process which allows the production of a two-dimensional finite-element mesh from the contouring of simulated composition data. We examine the stretching of two-phase structures obtained from a simulation of linear Cahn-Hilliard spinodal phase separation. In the simulations, we assume one phase to be hard, and the other soft, such that the shear modulus ratio ... is large (... ). We indicate the effect of varying composition on the material modulus and on the distribution of strains through the stretched material. We also examine in some detail the symmetric structures obtained at 50% composition, in which both phases are at a percolation threshold. Inspired by simulation results for the deformation of these structures, we construct a "scaling" theory, which reproduces the main features of the deformation. Of particular interest is the emergence of a lengthscale, below which the deformation is non-affine. This length is proportional to ... , and hence is still quite small for all reasonable values of this ratio. The same theory predicts that the effective composite modulus scales also as ..., which is supported by the simulations.

Yes / In the design of phase shifters, the modeling equations are too complicated and require some approximations to be derived correctly by hand. In response to this problem, this paper presents a generalized concept, algorithm, and MATLAB code that provide the exact modeling equations of the transmission parameters and the scattering parameters of any 90^o wideband stub-loaded phase shifter. The proposed code gives the modeling equations in term of variables for any number of stubs and characteristic impedance value by utilizing the symbol-based analysis of the MATLAB. It also illustrates the results as a function of normalized frequency relative to the center frequency f_o, and can be and can be tailored to any user-defined frequency range. As a matter of comparison, a three-stub wideband 90^o stub-loaded phase shifter is simulated using CST Microwave Studio and experimentally fabricated on Rogers RT5880 dielectric substrate with dimensions of 30×40×0.8 〖mm〗^3. The comparison reveals the accuracy of the proposed computerized modeling with -10 dB impedance bandwidth equal to 90% (0.55 fo-1.45 fo), (90∓5 degrees ) phase difference bandwidth equal to 100% (0.5 fo-1.5 fo), and negligible insertion loss. The novelty of this work is that the proposed code provides the exact modeling equations of the stub-loaded phase shifter for any number of stubs regardless the complexity of the mathematical derivations.

The entrainment process in two-phase buoyant jets differs significantly from their singlephase counterparts, and is not well understood. Entrainment models developed for singlephase flow are often used in two-phase jetting simulations, albeit with limited success. In this work, Particle Image Velocimetry (PIV) and shadowgraph flow visualization experiments have been conducted on submerged round gas jets of varying speeds and nozzle diameters with the goal of improving our understanding of the entrainment process in a two-phase (gas-liquid) jet. The total entrainment estimated using the PIV measurements is higher than the respective values suggested by a common empirical model developed for singlephase buoyant jets. A two-phase theoretical entrainment model used for comparison shows an overestimation of entrainment, but predicts the increase in the rate of entrainment with axial distance from the jet nozzle seen in the PIV results. This thesis also presents advances in PIV processing methodology that were developed concurrently with the entrainment research. The novel Spectral Phase Correlation (SPC) allows for particle displacement to be determined directly from phase information in the Fourier domain. Some of the potential benefits of the SPC explored here include (1) avoidance of errors introduced by spatial peak-finding routines; (2) use of a modal analysis that can be used to provide information such as correlation quality; and (3) introduction of a means of incorporating information from multiple image windows. At low image noise levels, the method performs as well as an advanced CC-based method. However, difficulties unwrapping the aliased phase information cause the SPC's performance to degrade at high noise levels. / Master of Science