The performance of a vapor compression system is known to be affected by the ingestion of liquid droplets in the compressor. In these multiphase flows, the liquid and the vapor phase are tightly coupled. Therefore the interphase heat, mass and momentum transfer as well as droplet dynamics including droplet breakup and droplet-wall interactions play a vital role in governing these flows. Only thermodynamic analyses or two-dimensional mean-line calculations are not sufficient to gain an in-depth understanding of the complex multiphase flow field within the compressor. The objective of this research was to extend the current understanding of the operation of a multistage centrifugal compressor under two-phase flow conditions, by performing three-dimensional computational analysis. In this work, two-phase flow of a single constituent (refrigerant R134a) through a two-stage, in-line centrifugal compressor was analyzed using CFD. The CFD model accounted for real gas behavior of the vapor phase. Novel user defined routines were implemented to ensure accurate calculations of interphase heat, mass and momentum transfer terms and to model droplet impact on the compressor surfaces. An erosion model was developed and implemented to locate the erosion "hot spots" and to estimate the amount of material eroded. To understand the effects of increasing liquid carryover, the mass flow rate of the liquid phase was increased from 1% to 5% of the vapor mass flow rate. The influence of droplet size on the compressor performance was assessed by varying the droplet diameter at the inlet from 100 microns to 400 microns. The results of the two-phase flow simulations were compared with the simulation involving only the vapor phase. Liquid carryover altered the flow field within the compressor, and as a result, both impellers were observed to operate at off-design conditions. This effect was more pronounced for the second impeller. The overall effects of liquid carryover were detrimental to the compressor performance. The erosion calculations showed maximum erosion potential on the blade and shroud of the first impeller. The results from this investigation provided new and useful information that can be used to support improved design solutions. / Ph. D.

Yes / The desalination of saltwater is a viable option to produce freshwater. All the desalination processes are energy-intensive and can be carried out on a large scale. Therefore, producing freshwater using renewable energy sources is the most desirable option considering the current energy crisis and the effect that fossil-fuel-based energy has on our carbon footprint. In this respect, the tray-type still, one of several solar power desalination still varieties, is popular owing to its straightforward design, economic materials of construction, and minimal maintenance requirements, especially in isolated island regions with restricted energy and natural water supplies. The traditional tray-type solar power has a few drawbacks, such as the inability to recover latent heat from condensation, reduced thermal convection, a large heat capacity, and comparatively minimal driving power through evaporation. Therefore, the improvement of heat and mass transfer capabilities in tray-type stills has been the subject of many studies. However, there is a lack of a comprehensive review in the open literature that covers the design and operational details of multistage solar stills. The purpose of this paper is to present a thorough overview of the past research on multistage solar stills, in terms of configurations, capabilities, and cutting-edge options. In comparison to a unit without a salt-blocking formation, the review indicates that a multistage distillation unit may run continuously at high radiation and generate pure water that is around 1.7 times higher than a unit without a salt-blocking formation. The most effective deign is found to be “V”-shaped solar still trays that attach to four-stage stills, since they are less expensive and more economical than the “floor” (Λ-shape) design, which requires two collectors. Additionally, it can be stated that the unit thermal efficiency, solar percentage, and collected solar energy (over the course of a year) increase by 23%, 18%, and 24%, respectively, when the solar collectors are increased by 26% (at the constant inflow velocity of the water).

Yes / Phenol is one of the most toxic and harmful pollutants in industrial wastewater streams, the removal of which is therefore of critical importance. The use of reverse osmosis (RO) systems as a means of treating wastewater is continuously growing. This research investigates the effect of operating parameters on the performance of five different multistage RO configurations coupled with a trickle bed reactor (TBR) using model-based simulation. The results were compared, and an analysis was then performed to identify which hybrid TBR and multistage RO arrangement rejected the most phenol content. The basis for comparison was four performance metrics of permeate concentration, rejection, recovery, and specific energy. The study found that the flow rate and concentration have little effect on the operation unless there is a concurrent increase of both. It was also found that the four-performance metrics used were interlinked and affect the quality and quantity of the final freshwater product.

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Yes / Despite the fact of being intensive energy consumption, MSF is a mature technology that characterised by a high production capacity of high-quality water. The multistage flash (MSF) desalination process is one of the prominent thermal desalination used in the industry of seawater desalination to produce high quantity and high quality of freshwater. However, this process consumes large amount of energy and faces thermal limitations due to its high degree of exergy destruction at several units of the process. Therefore, the research of MSF is still existed to elevate the performance indicators and to resolve the concern of high energy consumption. To rectify these limitations, it is important to determine the units responsible in dissipating energy. This study aims to model an industrial MSF process validated against real data and then investigate the exergy destruction and thermodynamic limitations of the process. As a case study, Azzour MSF seawater desalination plant, located in Al Khiran in Kuwait is under the focus. A comprehensive model is developed by analysing several published models. Specifically, the calculation of exergy destruction has embedded both physical and chemical exergies that identified as a strong point of the model developed. As expected, the highest exergy destruction (55.5%) occurs within the heat recovery section followed by the brine heater with exergy destruction of 28.26% of the total exergy destruction. This study identifies the sections of the industrial process that cause the highest energy losses.

The research in this dissertation addresses reduced rank adaptive signal processing, with specific emphasis on the multistage Wiener filter (MWF). The MWF is a generalization of the classical Wiener filter that performs a stage-by-stage decomposition based on orthogonal projections. Truncation of this decomposition produces a reduced rank filter with many benefits, for example, improved performance. This dissertation extends knowledge of the MWF in four areas. The first area is rank and sample support compression. This dissertation examines, under a wide variety of conditions, the size of the adaptive subspace required by the MWF (i.e., the rank) as well as the required number of training samples. Comparisons are made with other algorithms such as the eigenvector-based principal components algorithm. The second area investigated in this dissertation concerns "soft stops", i.e., the insertion of diagonal loading into the MWF. Several methods for inserting loading into the MWF are described, as well as methods for choosing the amount of loading. The next area investigated is MWF rank selection. The MWF will outperform the classical Wiener filter when the rank is properly chosen. This dissertation presents six approaches for selecting MWF rank. The algorithms are compared to one another and an overall design space taxonomy is presented. Finally, as digital modelling capabilities become more sophisticated there is emerging interest in augmenting adaptive processing algorithms to incorporate prior knowledge. This dissertation presents two methods for augmenting the MWF, one based on linear constraints and a second based on non-zero weight vector initialization. Both approaches are evaluated under ideal and perturbed conditions. Together the research described in this dissertation increases the utility and robustness of the multistage Wiener filter. The analysis is presented in the context of adaptive array processing, both spatial array processing and space-time adaptive processing for airborne radar. The results, however, are applicable across the entire spectrum of adaptive signal processing applications. / Ph. D.

In this dissertation, we focus on developing sampling-based algorithms for solving stochastic linear programs. The work covers both two stage and multistage versions of stochastic linear programs. In particular, we first study the two stage stochastic decomposition (SD) algorithm and present some extensions associated with SD. Specifically, we study two issues: a) are there conditions under which the regularized version of SD generates a unique solution? and b) in cases where a user is willing to sacrifice optimality, is there a way to modify the SD algorithm so that a user can trade-off solution times with solution quality? Moreover, we present our preliminary approach to address these questions. Secondly, we investigate the multistage stochastic linear programs and propose a new approach to solving multistage stochastic decision models in the presence of constraints. The motivation for proposing the multistage stochastic decomposition algorithm is to handle large scale multistage stochastic linear programs. In our setting, the deterministic equivalent problems of the multistage stochastic linear program are too large to be solved exactly. Therefore, we seek an asymptotically optimum solution by simulating the SD algorithmic process, which was originally designed for two-stage stochastic linear programs (SLPs). More importantly, when SD is implemented in a time-staged manner, the algorithm begins to take the flavor of a simulation leading to what we refer to as optimization simulation. As for multistage stochastic decomposition, there are a couple of advantages that deserve mention. One of the benefits is that it can work directly with sample paths, and this feature makes the new algorithm much easier to be integrated within a simulation. Moreover, compared with other sampling-based algorithms for multistage stochastic programming, we also overcome certain limitations, such as a stage-wise independence assumption.

Typical deep level mines use up to 27 ML water per day for mining operations. Multistage centrifugal pumps up to 2500 MW are used in an upward cascading manor to dewater the shaft. The dewatering systems at some mines are automated to enable surface control. Automation of the pumps is typically based on the best practice procedure known when implemented. Best practice procedures are used to ensure safe pumping operations. It was found that pump failures could still occur even with the best practice implemented. Unexpected failures of pumps are of major concern because they can result in the flooding of a mine. Flooding increases the risk of environmental damage and injury to the mining personnel. An additional concern is the maintenance cost of multistage centrifugal pumps. Overhaul cost of a seized multistage centrifugal pump is almost R1-million. The aim of this study was to improve established best practice procedures for pump automation. This could be achieved by investigating the general root cause of failures of automated pumps. Additional instrumentation and protection devices to prevent similar incidents were examined. Revised system control parameters were developed to ensure that the pumps operated within the design specifications. The improved best practices proved to prevent failures as a result of overheating and cavitation. Increasing the pump reliability and availability enabled surface control. The control of the automated dewatering system realised an electricity cost saving of R6-million. The automated system also made it possible to calculate the real-time pump efficiency within 5%. Previous best practice procedure was found to be inadequate to prevent all possibilities of failure. Additional precaution measurements were added to prevent pump failure. / MIng (Mechanical Engineering), North-West University, Potchefstroom Campus, 2015

Typical deep level mines use up to 27 ML water per day for mining operations. Multistage centrifugal pumps up to 2500 MW are used in an upward cascading manor to dewater the shaft. The dewatering systems at some mines are automated to enable surface control. Automation of the pumps is typically based on the best practice procedure known when implemented. Best practice procedures are used to ensure safe pumping operations. It was found that pump failures could still occur even with the best practice implemented. Unexpected failures of pumps are of major concern because they can result in the flooding of a mine. Flooding increases the risk of environmental damage and injury to the mining personnel. An additional concern is the maintenance cost of multistage centrifugal pumps. Overhaul cost of a seized multistage centrifugal pump is almost R1-million. The aim of this study was to improve established best practice procedures for pump automation. This could be achieved by investigating the general root cause of failures of automated pumps. Additional instrumentation and protection devices to prevent similar incidents were examined. Revised system control parameters were developed to ensure that the pumps operated within the design specifications. The improved best practices proved to prevent failures as a result of overheating and cavitation. Increasing the pump reliability and availability enabled surface control. The control of the automated dewatering system realised an electricity cost saving of R6-million. The automated system also made it possible to calculate the real-time pump efficiency within 5%. Previous best practice procedure was found to be inadequate to prevent all possibilities of failure. Additional precaution measurements were added to prevent pump failure. / MIng (Mechanical Engineering), North-West University, Potchefstroom Campus, 2015