Please read the abstract (Summary) in the 00front part of this document / Dissertation (M Eng (Mechanical Engineering))--University of Pretoria, 2006. / Mechanical and Aeronautical Engineering / unrestricted

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The study of two phase flow, where a finely divided solid is suspended in a fluid medium, has received considerable attention in the last five years due to the increased number of systems in which this phenomena occurs. The two most important areas involving the gas- particle mechanism are nuclear reactor heat exchangers and liquid and solid rockets.

This study aims to rationalize the consumption of thermal energy in residential buildings by recovering heat from wastewater inside the building before entering the central sewage network outside the building, by conducting an analytical study for a residential tower in Syria to find out the coverage percentage of the heat energy recovered from wastewater for the heating and domestic hot water loads needed for the tower, and calculating the percentage of reduction in carbon dioxide (CO2) gases. It is a simple technology as the thermal recovery system consists of three main components, which are in order: a wastewater tank, heat exchangers, and a heat pump. The research begins with an introduction that consists of the importance of wastewater and the waste heat energy it carries. After that, there are some case studies, research problem, its importance, the aim of the research, and finally the research methodology. In the first chapter, we talked about the concept of heat recovery from wastewater in general, methods of heat recovery, and the most important advantages and disadvantages of this process. It also includes an identification of the main parts used in this technology and how it works, especially the exchangers and the heat pump. This chapter also addresses the problem of forming a layer of biofilms on the surface of heat exchangers from the wastewater side and the most important methods used to treat it. We move on to the second chapter, in which we review the most important facilities for heat recovery from wastewater that have been viewed. Then comes the third chapter in which the heat recovery process was conducted for a nine storey residential tower in Syria, each floor has four apartments, where we first calculated the rate of wastewater flow for the entire tower, and we proposed a heat recovery system (physical model) inside the tower. Then the mathematical equations for heat recovery and the solution of these equations were developed based on some necessary assumptions needed in the solution process to know the most important results desired in this field. It also included the calculation of the coverage ratio of the heat energy recovered from the wastewater for the domestic hot water and heating loads, as well as the calculation of the mass and percentage of the reduction of carbon emitted to the atmosphere. Then simple economic feasibility was also conducted in this chapter to know the daily financial savings as a result of using this technology. The research ends with the most important conclusions and future research that have been reached and the conclusion of the research. The most important results show that the average coverage percentage of heat energy recovered from wastewater for heating load in residential buildings ranges between [30-56%]. It was also found that the average coverage percentage of heat energy recovered from wastewater for domestic hot water load ranges between [65-100%].

All-aluminum microchannel heat exchangers are designed to significantly reduce refrigerant charge requirements, weight, reduced brazed joints, and decreased potential for leakage by increasing reliability. Al 3003 alloy is corrosion resistant and can be formed, welded, and brazed but the issue with all-aluminum heat exchangers is localized corrosion (pitting) in corrosive environments. Currently, there is no universally accepted corrosion test that all coil manufacturers use to characterize their products. Electrochemical testing method of cyclic polarization was employed in this investigation and relevant parameters including electrolyte corrosive agent and its concentration, electrolyte pH, and applied potential scan rate was varied to find an optimal set of parameters. Results of cyclic polarization of Al 3003 in electrolytes containing various concentrations of NaCl were compared with those of the tests in Sea Water Acidified Accelerated Test (SWAAT) electrolyte and it is shown the SWAAT electrolyte (4.2% sea salt acidified to pH of 2.9) is by far stronger (in terms of corrosivity) than typical 3.5% NaCl solution used in most corrosion testing. Corrosion rates (g/m2yr) of Al 3003 measured in this investigation were comparable to those provided by ISO 9223 standard corresponding to C1 through CX categories. Duration of cyclic polarization test is much shorter than that of SWAAT and results obtained in this test is more reproducible compared to those of SWAAT. Scanning electron microscopy micrographs show typical pit depths of about 50 μm.

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Thermoelectric generator (TEG) is a solid state technology based on the Seebeck effect that can generate electrical power from waste heat. For continuous electrical power generation heat exchangers are integrated into the “cold side” and the “hot side” of the TEG such that a temperature difference across the TEG can be established and maintained. This thesis will focus on characterizing two different flat-plate cold side heat exchanger prototypes specifically designed for dissipating the thermal loads from TEG modules. The majority of TEGs modules available have a flat geometry design and a square shape with typical dimension of 40 mm × 40 mm or 56 mm × 56 mm. To maximize the net electrical power generated by the TEGs the cold side heat exchanger is required to have uniform surface temperature distribution, and excellent heat transfer performance with minimum pressure drop. To achieve the previously mentioned requirements, two flat-plate heat exchanger prototypes having two distinct heat transfer techniques were investigated. Each heat exchanger is designed to accommodate an array of 14 TEG modules arranged in two parallel rows with 7 TEGs per row a typical arrangement for large waste energy harvesting applications. The first heat exchanger prototype utilized single-phase forced convection through 140 minichannels (1 mm × 1 mm × 90 mm long) as a heat transfer technique. The second prototype utilized 14 liquid jets, 3 mm in diameter and 40.3 mm apart, impinging on a flat surface located 5 mm above. Each impinging jets was positioned at the centre of the TEG cooling area. An experimental facility was constructed in order to test the minichannels heat exchanger and the impinging jets thermally and hydrodynamically. The heat transfer, pressure drop and temperature distribution results were compared to determine the most appropriate cold side heat exchanger prototype for the TEG POWER system. The TEG POWER system is a waste heat recovery system designed to recoup waste heat from the exhaust gases of commercial pizza ovens. The TEG POWER system is capable of harvesting waste thermal energy produced by an establishment and utilize it for electrical power generation and thermal storage purposes. Heat transfer results indicated that for a given mass flow rate the minichannels heat exchanger has better heat transfer performance compared to the impinging jets heat exchanger. The minichannels heat exchanger design had a thermal conductance of 238 W/C at 0.19 kg/s coolant flow rate compared to 111 W/C for the impinging jets heat exchanger. The total pressure drop and the minor losses for each heat exchanger prototype were measured experimentally. For the minichannels heat exchanger, the total pressure drop is 23.3 kPa at flow rate of 0.235 kg/s. Comparatively, the total pressure drop for the impinging jets heat exchanger was 27.4 kPa at the same flow rate. Fittings losses for the minichannels heat and impinging jets heat exchanger were found to be 50% and 80% respectively. The maximum total measured drop corresponded to pumping power requirements of 5.7 W and 6.8 W for the minichannels and impinging jets heat exchanger respectively. Local and average temperature distributions and their influence on the electrical power generated were studied for both heat exchanger prototypes. It was found that the minichannels heat exchanger offers more uniform surface temperature distribution per row of TEGs compared to the impinging jets heat exchanger. Therefore the minichannels heat exchanger is well suited for cooling two rows of TEGs simultaneously. Based on the thermal and hydrodynamics comparison results the minichannels heat exchanger prototype is recommended for implementation in the TEG POWER system. / Thesis / Master of Applied Science (MASc)

The next generation of nuclear power sources, Gen. IV, will include an emphasis on small, modular reactor (SMR) designs, which will allow for standardized, factory-based manufacturing and flexibility in the design of power plants by utilizing one or several modular reactor units in parallel. One of the reactor concepts being investigated is the Molten Salt Reactor concept (MSR), which utilizes a molten salt flow loop to cool the reactor and transfer heat to the power conversion cycle (PCS).Here, the use of a supercritical carbon dioxide (S-CO2) Brayton cycle is assumed for that PCS. The purpose of this thesis is to investigate the heat exchange between these two systems and to determine the suitability of a common heat exchanger concept, the shell-and-tube heat exchanger (STHE). This was accomplished using a code written in Python programming language that optimized the geometry ofa baffled STHE for a range of conditions the reflect MSR power plants currently in the design or concept stages. Star-CCM+ computational fluid dynamics (CFD)software was used to visualize the flow patterns of molten salt and CO2 in these STHE designs, and it was also used to determine heat transfer coefficients and pressure drops. These values were compared to those calculated by the optimizer code in order to validate its results. Finally, modularity analysis was performed for these STHE designs. Trends were generalized from these results that will contribute to judgments about the suitability of STHE’s for use with MSR’s and S-CO2.

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