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Item Modelling of conventional and microchannel delugeable tube bundle for a direct air-cooled steam condenser(University of Namibia, 2023) Angula, EsterThis study presents the thermal performance modelling of conventional and microchannel delugeable tube bundle which incorporated into a second stage of the induced Hybrid (Dry/Wet)) Dephlegmator (HDWD). Hybrid dry/wet dephlegmator was proposed to replace a Conventional Dephlegmator (CD) of a direct Air-Cooled Condenser (ACC) system, coupled to 30 MW steam turbine of a generating unit. The Conventional Delugeable Tube Bundle (CDTB) thermal performance was modelled and configured using one- and two - dimensional models by employing heat and mass transfer approach. The geometric parameters for both first and second stages tube bundles of HDWD were derived from that of CD of ACC system of the considered generation unit. Therefore, the thermal performance of CDTB was optimised to ensure highest performance, and appropriate geometric parameters that are corresponding to that of CD and available space for installation. The best CDTB configuration’s performance was compared to similar existing Delugeable Round Tube Bundle (DRTB) in literatures. When the tube pitch varied from 25 mm to 38 mm, the DRTB’s heat transfer rate and air-side pressure drop were found to be in the range of 1.4 to 2 times and 1.3 to 2.2 times that of CDTB, respectively. The Microchannel Delugeable Tube Bundle (MDTB) thermal performance was analysed using semi-empirical model, which comprises of microchannel heat transfer and flow correlations. The size and geometric parameters of MDTB was equivalent to that of CDTB, and the only difference was that, the MDTB has microchannel/ ports on the steam-side. The MDTB thermal performance was found to increase as the hydraulic diameter of the channels decreased. The thermal performance comparison of the CDTB and MDTB at bundle, component and system levels was carried out. At all the levels, the MDTB performance was higher than that of CDTB. However, this higher performance came at expense of higher steam-side pressure drop. As the ambient air temperature increased from 32 ℃ to 44 ℃, the MDTB’s heat transfer rate for 140 𝜇𝑚 hydraulic diameter was found to be 9 to 6 % higher than that of CDTB. The air-side pressure drop variations between the two bundles were insignificant. Furthermore, the thermal performance of HDWD incorporated with either CDTB or MDTB was compared to that ii of CD at component and system levels. At system level, and as the ambient air temperature increased from 32 ℃ to 44 ℃, the heat transfer rate of the CD was found to be 27 % to 31 % less than that of HDWD incorporated with either CDTB or MDTB. The Air-side pressure drop for HDWD was found to be about 41 % higher than that of CD. Additionally, the steam turbine output and net powers were found to be higher when the ACC was incorporated with HDWD than when CD was incorporated into ACCItem Failure of pump systems operating in highly corrosive mine water at Otjihase mine(2018) Hango, Silas ItheteIn the Namibian Otjihase underground mine, water is pumped from natural underground reservoirs to the surface using mild steel pumps that have cast iron valves and shaft sleeves coated with a Ni-Cr-Fe alloy. As these components failed very frequently in the highly corrosive mine water environment, it became necessary to provide recommendations for alternative pump materials. The Ni-Cr-Fe coated, carbon steel shaft sleeves were mechanically worn by abrasion in contact with debris trapped in packing glands. The highly corrosive mine water contained solid soil particles, which contributed to internal erosion-corrosion of the pump components. Once the protective coating was breached, the exposed steel corroded rapidly when reacting with the corrosive mine water, resulting in leakages. The main objective of this work was therefore to characterise the steel and cast iron components used in the pump system, determine methods to improve the tribocorrosion resistance of the pump components and recommend a hardfacing material with improved performance in the tribocorrosive mine environment. To simulate and study the synergistic effect of electrochemical and mechanical interaction between the pump components and highly corrosive mine water, the hardness and electrochemical response in synthetic mine water of the following proposed bulk materials were tested: Hastelloy G30, ULTIMET, Stellite 6B and ToughMet 3. Hastelloy G30 demonstrated good corrosion resistance, but had low hardness and poor abrasion resistance. ToughMet 3 had high hardness, but low corrosion resistance. As ULTIMET and Stellite 6B both had high hardness and good corrosion resistance, they were selected for further investigation to assess sliding abrasive wear and tribocorrosion behaviour in synthetic mine water. The possibility of enhancing the corrosion resistance of ULTIMET and Stellite 6 (not 6B) alloys as protective coatings by adding minor amounts of ruthenium was investigated. ULTIMET and Stellite 6 powders were each mixed with nominal 0.3 wt% Ru and nominal 0.6 wt% Ru additions. The powders with no Ru, nominal 0.3 wt% Ru and nominal 0.6 wt% Ru were then thermally sprayed by the high velocity oxy-fuel flame (HVOF) process onto a carbon steel substrate, and compared to a Cr2O3 coated steel as a benchmark. The powders and the coatings were characterised using optical and scanning electron microscopy with energy dispersive X-ray spectroscopy, and X-ray diffraction. Hardness, sliding abrasive wear, and corrosion and tribocorrosion behaviour of the coatings and the substrate in synthetic mine water were then determined. Comparison of the hardness showed that the nominal 0.3 wt% Ru ULTIMET coating had higher hardness than the same coatings with no Ru and nominal 0.6 wt% Ru, the nominal 0.6 wt% Ru Stellite 6 coating had the highest hardness and overall, the Stellite 6 coatings had higher hardness values than both ULTIMET and Cr2O3 coatings. At pH 6, the ULTIMET and Stellite 6 coatings with and without Ru additions had low corrosion current densities and consequently low corrosion rates in synthetic mine water. For the ULTIMET coatings, the corrosion rates decreased as the Ru content increased. The Stellite 6 coating had slightly lower corrosion current densities and corrosion rates than ULTIMET under all tested conditions. Stellite 6 coatings had lower abrasive wear rates at the tested loads than the other materials. The lowest abrasive wear rates were recorded with additions of nominal 0.3 wt% Ru (5 N), and nominal 0.6 wt% Ru (10 N). As expected, tribocorrosive wear rates increased with increasing load for all alloys. Ruthenium additions to ULTIMET and Stellite 6 coatings decreased the tribocorrosive wear rate. The best tribocorrosion resistance was achieved by the Stellite 6 coatings. Stellite 6B bulk samples and Stellite 6 coatings with Ru had higher hardnesses, lower corrosion rates, lower wear rates and lower tribocorrosion rates than the carbon steel substrate, Cr2O3 coating, and ULTIMET bulk material and coatings. Stellite 6 coating with nominal 0.6 wt% Ru exhibited lower corrosion rates at pH 6 and 3 than the ULTIMET coating. Therefore, the Stellite 6 coating were recommended for use in pump shaft sleeves and the Stellite 6B bulk alloy in valves at Otjihase Underground Mine. The cost saving for using the proposed alternative materials was calculated as US$ 8 546.68 (R 128 200.20) per year. This is a major economic incentive for Otjihase Underground MineItem Engineered kinetics about photocatalytic degradation of textile wastewater(2006) Shigwedha Nditange