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A thesis submitted to the Faculty of Engineering, University of the Witwatersrand, Johat.nesburq, in fulfilment of the requirements for the degree of Doctor of Philosophy Johannesburg, 1998 / This thesis contributes to accurate, practicable techniques of ascertaining and assessing site performance of large refrigerating machines chilling water 'for cooling deep South African mines. It applies to all vapourcompression machines cooling fluids in steady, continuous processes. To assess whether a water chilling machine is performing satisfactorily, both its actual performance, and the corresponding normal or optimal performance of which it is capable, must be ascertained. Both requirements r esent difficulties on site. in particular, the traditional "heat balance" method of verifying the apparent performance obtained from measurements in the water circuits does not prove that such performance is accurate. The calibration of typical site instrumentation is not assured, so an "acceptable" heat imbalance may conceal large but similar errors - which thus also balance out - in the apparent constituents of the heat balance. Three methods of independently ascertaining actual performance, so verifying apparent performance, are presented, The first is an enhanced method, applicable to custom-built machines as well as conventional ones, of ascertaining the efficiency of the actual refrigerating process from measurements in the refrigerant circuit. This detects errors concealed in an "acceptable" heat balance. Where some refrigerant-circuit measurements are unavailable, an inexact version of this method still indicates the relative likelihood of the apparent performance being acceptably accurate. The third method, where these two are inadequate, is ascertaining actual performance using available measurements and fundamental machine modelling. Such modelling is also the most versatile method of predicting corresponding normal or optimal performance. A computer program simulating complete mine water chilling installations is used here. Actual performance can then be meaningfully assessed and appropriate remedial action justified, as shown in seven case studies. An outcorr.e for conventional water chillinq machines with a centrifugal compressor is that keeping heat exchangers clean may prejudice efficiency under part-duties lf a machine has been designed for optimum efficiency at full duty. An alternative control philosophy of maximising the machine load may then yield better performance. If these techniques are included ill an automated system of fault diagnosis, they will be of most use to burdened mine staff, who are generally not refrigeration experts.

This report investigates the application of digital computer simulation models to the analysis and optimization of complex mine water reticulation systems- A simulation program is developed and documented, Guidelines in the construction and use of mine water models are applied in a case study of water quality and quantity aspects of Unisel Gold Mine.

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering. / The reliability of using isotropically consolidated specimens in the laboratory determination of liquefaction characteristics of cohesionless sands, as is common practice today, is evaluated. Two series of triaxial tests involved the conventional undrained loading of isotropically and anisotropically consolidated specimens. The third series of tests utilized anisotropically consolidated specimens loaded by following special undrained stress paths. Test results revealed that liquefaction characteristics obtained from isotropically and anisotropically consolidated specimens are different. The effective angles of friction mobilized at peak shear stress and at steady state were greater for anisotropically consolidated specimens. Loading stress path is seen to have no effect on the liquefaction characteristics derived from conventional loading of anisotropically consolidated specimens. The critical stress ratio line is observed to be the line determining the initiation of liquefaction so that any stress path attempting to cross this line, regardless of direction of approach, immediately shows strain-softening behavior leading to liquefaction. / AC 2018

Die wear is always an important issue in hot forming processes, such as in forging and extrusion. Die life affects the economics of process to product and in order to optimise die life, the mechanism of wear should be approached scientifically. The aim of this work is to provide a systematic method for predicting and quantifying wear occurring in the extrusion of INCONEL 718 (IN718), nickel superalloy. To characterise wear, the process prediction which contributes to it must be identified and quantified. First, material characterisation was carried out using the Gleeble physical materials simulator. Then a set of unified viscoplastic constitutive equations was developed suitable for modelling microstructural evolution of IN718, i.e. evolution of average grain size, dislocation density and recrystallisation under hot forming conditions, which enabled resulting flow stress to be calculated and the microstructure of formed parts to be predicted. Second, heat transfer and friction during the forming process were investigated, by upsetting cylinders and performing ring tests on IN718. The heat transfer experimental work centres rounded the development of a reliable method for the measurement of the sub-surface temperatures in the bottom die during upsetting. The experimental values of sub-surface temperatures under various lubrication and forging conditions were analysed. A theoretical approach was proposed for the determination of the values of effective heat transfer coefficient and effective friction factor, and comparisons of experimental results and those from FE simulations were made and satisfactory matchings were obtained. Finally, integration of the material model and derived boundary conditions using subroutines for FEA are presented. Qualitative studies of abrasive die wear carrying out in a FE package, DEFORM, on the effect of various hot forming cases are shown. The numerical results are compared with the observations from mechanical measurements and metallurgical examinations for the studied die. Good correlations are found for most cases, which prove the presented methods can be used effectively in the prediction of die wear. Also, further work is suggested to enhance the modelling capabilities.

Small-angle scattering technique uses the scattering of radiation (e.g. neutrons or X-rays) at small angles to probe large-scale structures withjn matter, up to thousands of Angstroms. It is proven a valuable tool for investigating precipitation in reactor pressure vessel (RPV) steels and Fe-Ga alloys offering a statistical average over a large volume of samples. RPV steels, being of crucial importance for the longevity of a nuclear reactor, have been a long-standing theme for investigations. The main topics of such investigations are the effects of irradiation upon the steels and the consequent implications on their macroscopic properties. In this thesis, small-angle neutron scattering is employed to investigate irradiation induced precipitates in low- and high-Cu RPV steels. After irradiations with protons to low damage levels, precipitates could be clearly observed only in the high-Cu RPV steels. Stable preirradiation formed features are attributed to precipitation of carbides. Fe-Ga binary alloys have attracted much attention due to the still unexplained high magnetostriction they exhibit. To investigate the composition of nanoheterogeneities in a Fe-Ga sample, anomalous small-angle X-ray scattering is employed exploiting the energy dependence of the Fe and Ga atoms near their respective absorption edges. The nanoprecipitates are found to have a Fe3Ga stoichiometry.

We study the low temperature magnetic ordering of rare-earth pyrochlores. The dominant magnetic interaction: nearest neighbour antiferromagnetic Heisenberg exchange, is frustrated with a macroscopic ground-state degeneracy. This degeneracy is lifted by weaker interactions, stabilising long-range order. First we study the dipolar governed gadolinium stannate with an external magnetic field. Factorising the Hamiltonian in terms of ten quadratics provides exact solutions to the over-constrained model with fields orientated along highly symmetrical directions. Next we study the isostructural gadolinium titanate: the much more complex magnetism is indexed by a different propagation-vector to gadolinium stannate due to further neighbour exchange interactions. This material is controversial: elastic neutron scattering and Mössbauer experiments have been using contradictory interpretations. We propose a new state which appears to resolve this inconsistency. Finally we model erbium titanate, which is approached differently due to the dominant crystal-field. Existing elastic neutron scattering data is reexamined and found inconsistent with the state currently discussed in the literature so we suggest an unusual multiple-q state. The spins are not orientated along the expected crystal-field direction: a consequence of frustration. Energetics are studied phenomenologically. We suggest that experimentally observed gapless spin-waves control transfer of spin density between different q-points of the proposed state.

This thesis presents the work done to characterise two superconducting materials. We study BiPd, a non-centrosymmetric superconductor which is theoretically expected to show signs of spin singlet and triplet mixing due to the strong spin-orbit scattering of its composing elements. We map the field-temperature superconducting phase diagram along two crystal directions using Small Angle Neutron Scattering (SANS), magnetisation and \(µ\)SR measurements and determine the microscopic parameters defining the superconducting state. We also uncover a rare behaviour displayed in low-\(k\) superconductors, the Intermediate Mixed State, which causes domains of vortex lattice with constant spacing to coexist with Meissner domains at low applied fields. Finally we show evidence that, unlike what was expected, the superconductivity in BiPd behaves conventionally. The second material studied is Nb3Sn, widely used to produce large magnetic fields in various devices such as MRI machines. We investigate the superconducting state of several polycrystalline samples with different tin concentrations, as recent evidence point towards a lack of change of the upper critical field with varying Sn doping, in contradiction with older measurements that see a drop in H\({c2}\) associated with the apparition of a structural (martensitic) crystalline transition. Using SANS, we show that these recent results were likely not measuring the bulk state of Nb3Sn and that we find large variation of H\({c2}\) with Sn concentration. We also present indications that the vortex lattice is influenced by non-local effects at large fields by measuring the change in the vortex lattice structure with field. Lastly, our measurements are consistent with a full single gap behaviour in Nb3Sn.

The grain growth behaviour during reheating between 950 ºC and 1300 ºC of as-cast Al-Nb steel (containing 0.019 wt% Nb and 0.057 wt% Al) and rolled Nb-containing steel (containing 0.028 wt% Nb and 0.031 wt% Al) have been investigated. In particular the role of microalloying element segregation during casting and, hence the spatial distribution of microalloying precipitates, on grain boundary pinning during reheating has been considered. The Al-Nb containing steel has been examined in separate initial conditions, including as-cast (segregated structure), homogenised and forged (reduced separation of segregated bands) samples. It was found that microalloy segregation occurred between the dendritic and interdendritic regions, where the secondary dendrite arm spacing (SDAS) was 150 ± 50 μm. Nb showed strong segregation into the interdendritic regions resulting in a higher number density of Nb(C,N) precipitates (2.64 × 104 /mm2) compared to the dendritic region (0.73 × 104 /mm2). However, Al did not show strong segregation resulting in relatively well-distributed AlN precipitates in the matrix (1.29× 104 /mm2 in the interdendritic region and 1.89× 104 /mm2 in the dendritic region). After forging, the separation between the segregated bands was reduced to 65 ± 10 μm from the previous 150 ± 50 μm in the as-cast sample. The increased Nb content in the rolled Nb-containing steel compared to the Al-Nb steel gave a greater extent of segregation in the solute-enriched regions resulting in a larger number density of Nb(C,N) present (5.9× 104 /mm2), whilst the separation between in the segregated bands in the as-rolled Nb-containing steel was 35 ± 10 μm.