The impinging solar receiver, bearing the merits of high heat transfer coefficient and compact structure, has a great potential in the field of solar dish Brayton system. Despite the wide application of cylindrical structure in the impinging solar receiver, the research on orifice array optimization against curvature surfaces is rare.In this paper, the main objective is to study the heat transfer and pressure drop characteristics of an orifice impinging array under a constant mass flow rate and a constant surface temperature boundary condition for the future impinging receiver design. Various orifice shapes were studied via numerical tools (Ansys Fluent 14.0) and their performances in both pressure drop and heat transfer coefficient were compared. The upstream fillet orifice was found to have the lowest pressure drop with moderate compromise in heat transfer coefficient. Moreover, a mathematical optimization model, based on empirical correlations, was developed for the orifice impinging array on the convex cylindrical surfaces. This model can provide an appropriate range of orifice number and orifice diameter, from which the key factors of the array including the ratio of height and orifice diameter H/D, orifice interval, number of orifices in each tier circumferential and tier numbers can be calculated. Several validation cases were also conducted by Ansys Fluent.

The high amount of scientific and communications data produced by low earth orbiting satellites necessitates economical methods of communication with these satellites. A volumetric phased array for demonstrating horizon-to-horizon electronic tracking of the NASA satellite EO-1 was developed and demonstrated. As a part of this research, methods of optimizing the elemental antenna as well as the antenna on-board the satellite were investigated. Using these optimized antennas removes the variations in received signal strength that are due to the angularly dependent propagation loss exhibited by the communications link. An exhaustive study using genetic algorithms characterized two antenna architectures, and included optimizations for radiation pattern, bandwidth, impedance, and polarization. Eleven antennas were constructed and their measured characteristics were compared to those of the simulated antennas. Additional studies were conducted regarding the optimization of aperiodic arrays. A pattern-space representation of volumetric arrays was developed and used with a novel tracking algorithm for these arrays. This algorithm allows high-resolution direction finding using a small number of antennas while mitigating aliasing ambiguities. Finally, a method of efficiently applying multiple beam synthesis using the Fast Fourier Transform to aperiodic arrays was developed. This algorithm enables the operation of phased arrays combining the benefits of aperiodic element position with the efficiency of FFT multiple beam synthesis. Results of this research are presented along with the characteristics of the volumetric array used to track EO-1. Experimental data and the interpretations of that data are presented, and possible areas of future research are discussed.

In this thesis, optimization of the geometry of non-uniform arrays for direction finding yielding unambiguous results is studied. A measure of similarity between the array response vectors is defined. In this measure, the effects of antenna array geometry, source placements and antenna gains are included as variable parameters. Then, assuming that the antenna gains are known and constant, constraints on the similarity function are developed and described to result in unambiguous configurations and maximum resolution. The problem stated is solved with two different methods, the MATLAB optimization toolbox, and genetic algorithm in which different genetic codings are also studied. The performance of the MUSIC algorithm with the optimized array geometries are investigated through computer simulations. The direction of arrival estimates are obtained using the optimized array geometry on the MUSIC algorithm along with the effects of different parameters. Statistics of the true and probable erroneous arrival angles and the probability of gross error are obtained as a measure of performance. It is observed that the proposed optimization process for the array geometry gave rise to unambiguous results for direction finding.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / In the last two decades, multiple input multiple output (MIMO) wireless systems have been subject of intense research due to the theoretical promise of the proportional increase of the communications channel capacity as the number of antennas increases. This outstanding property supposes an efficient use of spatial diversity at both the transmitter and receiver. An important and not well explored path towards improving MIMO system performance using spatial diversity takes into account the interactions among the antennas and the (physical) propagation medium. By understanding these interactions, the transmit and receive antenna arrays can be designed to best âmatchâ the propagation medium so that the link quality and capacity can be further improved in a MIMO system. In this work, we consider the use of spherical harmonics and tensor decompositions in the problem of MIMO channel modeling and estimation. The use of spherical harmonics allows to represent the radiation patterns of antennas in terms of coefficients of an expansion, thus decoupling the transmit and receive antenna array responses from the physical propagation medium. By translating simple propagation-motivated channel models with polarization information into the spherical harmonics domain, we study how propagation parameters themselves and antenna configurations affect MIMO performance in terms of capacity and correlation. A second part of this work addresses the problem of estimating directional MIMO channels in the spherical harmonics domain using tensor decompositions. Considering both single-scattering and double-scattering propagation scenarios, we make use of the parallel factor (PARAFAC) and PARATUCK-2 decompositions, respectively, to estimate the propagating spherical modes, from which the directions of arrival (DoA) and directions of departure (DoD) can be extracted. Finally, we propose and compare two methods for optimizing the coefficients of the spherical harmonics expansion of an antenna array for a prespecified MIMO channel response. / Nas Ãltimas dÃcadas, sistemas de comunicaÃÃo sem fio de mÃltiplas antenas (MIMO - Multiple Input Multiple Output) tÃm sido objetos de intensas pesquisas devido à promessa teÃrica do aumento proporcional da capacidade com o aumento do nÃmero de antenas. Esta propriedade excepcional supÃe um uso eficiente da diversidade espacial no transmissor e receptor. Um caminho importante e nÃo bem explorado no sentido de melhorar o desempenho de sistemas MIMO usando diversidade espacial leva em conta a interaÃÃo entre as antenas e meio de propagaÃÃo (fÃsico). AtravÃs da compreensÃo dessas interaÃÃes, arranjos de antenas de recepÃÃo e transmissÃo podem ser projetados para melhor "casar" com o meio de propagaÃÃo, tal que a qualidade do link de comunicaÃÃo e capacidade possam ser melhoradas em um sistema MIMO. Neste trabalho, consideramos o uso de harmÃnicos esfÃricos e decomposiÃÃes tensoriais no problema de modelagem de canal MIMO e estimaÃÃo. O uso de harmÃnicos esfÃricos permite representar os padrÃes de radiaÃÃo de antenas em termos de coeficientes de uma expansÃo, assim desacoplando as respostas dos arranjos de antenas (transmissoras e receptoras) do meio de propagaÃÃo fÃsica. Traduzindo modelos simples de canais baseados em propagaÃÃo, com informaÃÃes de polarizaÃÃo, para o domÃnio dos harmÃnicos esfÃricos, estudamos como os parÃmetros de propagaÃÃo si e configuraÃÃes especÃficas de antenas afetam o desempenho do sistema MIMO em termos de capacidade e de correlaÃÃo. A segunda parte deste trabalho aborda o problema de estimar canais direcionais MIMO no domÃnio dos harmÃnicos esfÃricos usando decomposiÃÃes por tensores. Considerando tanto cenos de espalhamento simples e de duplo espalhamento, fazemos uso das decomposiÃÃes PARAFAC e PARATUCK2, respectivamente, para estimar os modos esfÃricos propagantes, a partir das quais as direÃÃes de chegada (DoA) e as direÃÃes de saÃda (DoD) podem ser extraÃdas. Finalmente, propomos e comparamos dois mÃtodos de otimizaÃÃo dos coeficientes da expansÃo em harmÃnicos esfÃricos de arranjos de antenas para respostas de canais MIMO prÃ-especificados .

<p> </p> <p>Gas sensors have many applications. Volatile organic compound (VOC) sensors are used for monitoring air quality in homes and office spaces, as well as monitoring manufacturing environments where a wide variety of VOCs can be produced. These gases can include formaldehyde, which can be toxic to humans at concentrations as low as 1 ppm. Other applications for gas sensors include flammable refrigerant detection. With the move towards developing more environmentally friendly appliances, many companies have started to use refrigerants such as R600a (isobutane) and R32 (difluoromethane), which have a much lower global warming potential (GWP) than their predecessors, such as R134a and R410a. While this move is beneficial for the environment, steps to ensure their safe usage have not been widely implemented to date. Therefore, sensors to detect VOCs at or below exposure limits, as well as flammable refrigerants at or below lower flammability limits (LFL), should be developed to ensure undue hazards are identified and mitigated. </p>