Studies have been made of optical waveguides produced in GaAs and InP epitaxial layers. Of the possible waveguiding mechanisms present in these devices the contribution from the photo-elastic effect (strain-induced refractive index changes) dominates. Stresses in evaporated metal films and their control have been investigated. Strain-induced waveguides have been used to produce a novel directional-coupler structure with a short coupling length (~2mm). In GaAs bias has been applied to control the amount of light at the output of each of the two waveguides forming these couplers and it has been possible to isolate the light in either the excited or the coupled waveguide. A new theoretical model, based on finite difference techniques, has been developed and used to analyse strain-induced, slab and rib waveguide structures. Results obtained have been compared with those from other methods. Theoretical predictions of guiding properties in GaAs strain-induced waveguides give good agreement with experimental results in all cases. Optical waveguiding in InP layers using the same photoelastic mechanisms, assessed experimentally, indicates that the refractive index changes are similar to those in GaAs but slightly larger. One of the first measurements of the nonzero electro-optic coefficient, r41, of InP is described. Guiding properties vary little with time in both InP and GaAs. The reflection of light guided in a single-mode photoelastic waveguide into a second perpendicular guide using a vertical etched facet running at 450 to the direction of propagation is proposed for providing bending with negligible loss and some experimental results are reported.

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Approved for public release; distribution is unlimited / The objective of this thesis is to synthesize signals with a dynamically change power spectral density, in a SDR transmitter, utilizing the most appropriate channels, modulation schemes and transmission rates for communication, based on the noise profile (AWGN plus interferences) of the link, in order to achieve performance within some predefined acceptable levels. The objective is obtained by simulation. / Major, Hellenic Army

Approved for public release; distribution is unlimited. / This research presents the results of a combined numerical and experimental study of heat transfer and pressure drop behavior in a compact heat exchanger (CHE) designed with drop-shaped pin fins. A numerical study using ANSYS was first conducted to select the optimum pin shape and configuration for the CHE. This was followed by an experimental study to validate the numerical model. The results indicate that the drop shaped pin fins yield a considerable improvement in heat transfer compared to circular pin fins for the same pressure drop characteristics. This improvement is mainly due to the increased wetted surface area of the drop pins, and the delay in the flow separation as it passes the more streamlined drop shaped pin fins. The data and conclusions of this study can be used in heat exchanger design for large heat flux cooling applications as in gas turbine blades, and high-power electronics. / Lieutenant Junior Grade, Tunisian Navy

A regional assessment of the potential effects of introducing transmission facilities in alternative corridors east of Lake Winnipeg was conducted. The primary focus of the study was on the impact of transmission facilities on outdoor recreation. Several proposed transmission corridors were compared on the basis of the potential negative impact on five factors: existing development providing outdoor recreation facilities, polices and planning objectives east of Lake Winnipeg, natural resources used for outdoor recreation, ecological processes and natural ecosystems and the appearance of the landscape. The West Corridor and W₁ trunk along the east shore of Lake Winnipeg was considered to have the least impact on outdoor recreation, and further study of this alternative was recommended. / 1978

D.Ing. / The heating of water with hot-water heat pumps is extremely energy-efficient. With the refrigerant R-22 hot water temperatures of 60° C to 65° C are possible. However, these temperatures are low in comparison with the temperatures obtained from other methods of water heating, for instance electrical geysers. Should higher water temperatures be obtained, the applications of hot-water heat pumps will increase. This is possible by using a zeotropic refrigerant mixture as working fluid. A R-22 and R-142b zeotropic refrigerant mixture shows exceptional potential in achieving hot water temperatures. The condensing coefficients need to be predicted correctly to optimize the condenser design. Unfortunately, there is a lack of detailed literature available on condensing coefficients for the recommended mass fractions of R-22 with R-142b at condensing temperatures of 60° C or more. Micro-fin tubes perform outstanding in enhancing heat transfer and are widely used to save energy. Unfortunately, there is also a lack of detailed literature on condensing coefficient at the recommended mass fractions of R-22/R-142b refrigerant mixtures condensing in micro-fins, twisted tapes and high fins at temperatures of 60° C or more. In this study condensing coefficients of R-22 and the zeotropic refrigerant mixture R-22 with R-142b were obtained in smooth tubes at mass fractions of 90%/10%, 80%/20%, 70%/30%, 60%/40%, 50%/50%. The experimental data were used to evaluate some of the methods that are commonly used to predict condensing coefficients. Experiments were also conducted at the same zeotropic mass fractions, to compare three different methods of heat transfer enhancement to that of the smooth tubes namely: micro-fins, twisted tapes and high fins. All measurements were conducted at an isobaric inlet pressure of 2.43 MPa. The test sections consisted of a series of eight tubes with lengths of 1 603 mm. The smooth tubes had an inner diameter of 8.11 mm. With the R-22/R-142b zeotropic refrigerant mixture condensing in smooth tubes, it was observed in the sight glasses that a predominantly stratified wavy flow regime exists at low mass fluxes, from 40 kg/m2s to 350 kg/m2s. The refrigerant mass fraction decreased the condensing coefficient by up to a third on average from 100% R-22 to a 50%/50% mixture of R-22 with R142b. A predominantly annular flow regime was observed at mass fluxes of 350 kg/m2s and more. At this flow regime the condensing coefficients were not strongly influenced by the refrigerant mass fraction, decreasing only by 7% as the refrigerant mass fraction changed from 100% R-22 to a 50%/50% mixture of R-22 with R142b. When the experimental data were compared with three methods that are commonly used to predict condensing coefficients it was found that the flow pattern correlation of Dobson and Chato (1998) gave the best predictions for R-22. The Silver (1964) and Bell and Ghaly (1964) method gave the best predictions for the R-22/R-142b mixtures. When the three heat transfer enhancement methods were compared with smooth tubes it was found that micro-fins were more suitable as an enhancement method than twisted tubes or high fins. It was also found that the condensing coefficients and pressure drops decrease as the mass fractions of R-142b increases.

M.Ing. / Deposition of scale on heating surfaces is a major problem in industry as well as households. The scale that forms on the heating surfaces acts as an insulator and results in decreased heat transfer effectiveness . These are two main approaches to prevent or reduce scaling. Although these approaches are claimed to be efficient, there is a need to evaluate or verify their efficacy. This calls for a method which should preferably enable quantitative and rapid evaluation of these techniques in the laboratory. A reliable, rapid and quantitative measurement method which was comprised of stripping the scale from heat exchanger pipes with a 10% acetic acid solution and measuring the Ca concentration in the acid after stripping, was developed. A total of 11 tests, 7 to test the reproducibility, 2 to test a physical water treatment device and 2 to test the effect of zinc, were conducted. The reproducibility amongst different pipes, and amongst different experiments could not be achieved. Attempts to explain the inconsistency through statistical analysis of the data showed that, the inconsistency in the results could partly attributed to chemical differences, particularly changes in calcium concentration and TDS. Metal contamination, particularly zinc could also be responsible for a part of the inconsistency.

M.Ing. / An experimental investigation was conducted into the heat transfer characteristics of horizontal smooth, micro-fin and herringbone tubes during in-tube condensation. The study focused on the heat transfer coefficients of refrigerants R-22, R-134a and R-407C inside the three tubes. The herringbone tube results were compared to the smooth and micro-fin tube results. The average increase in the heat transfer coefficient when compared to the smooth tube was found to be as high as 322% with maximum values reaching 336%. When compared to the micro-fin tube, the average increase in heat transfer coefficient was found to be as high as 196% with maximum values reaching 215%. A new unified correlation was also developed to predict the heat transfer coefficients in a herringbone and micro-fin tube. The correlation predicted the semi-local heat transfer coefficients accurately with 96% and 89% of the data points falling in the ± 20% region for the herringbone and micro-fin tube respectively. The average heat transfer coefficients were also accurately predicted with all the data points for the herringbone tube and 83% of the data points for the micro-fin tube falling in the ± 20% region. The trend of the new correlation also fitted the data accurately and the conclusion was made that the correlation is accurate and could be used successfully in practice.

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2014. / In this thesis we will establish effective numerical schemes appropriate for the solution of a non-linear partial differential equation modelling heat transfer in one dimensional longitudinal fins. We will consider the problem as it stands without any physical simplification. The main methodology is based on balancing the non-linear source term as well as the application of numerical relaxation techniques. In either approach we will incorporate the no-flux condition for singular fins. By doing so, we obtain appropriate numerical schemes which improve results found in literature. To generalize, we will provide a relaxed numerical scheme that is applicable for both integer and fractional order non-linear heat transfer equations for one dimensional longitudinal fins.

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