UDOM IR :: Browsing by Author "Mdee, Ombeni J."

Browsing by Author "Mdee, Ombeni J."

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Analysis of ore hand-crushed size distribution for grinding process in the artisanal and small-scale gold mines production: a case study of Nholi mine, Bahi district
(College of Engineering and Technology, University of Dar es Salaam, 2022) Tembo, Malugu M.; Mdee, Ombeni J.
In many parts of the world, artisanal and small-scale gold mines (ASGM) employed numbers of people compared to large-scale mines. The ASGM has been played a crucial role in poverty alleviation and rural development, particularly in developing countries. The mined ore is crushed and ground for beneficiation of gold. The grinding process involved the size reduction of blasted materials to acquire minerals of interest. This paper presents the size distribution of hand-crushed for blasted materials using sieve analysis and digital image processing. The sieve test results for hand-crushed blasted material of three muck-piles were ranged between 17 mm and 36 mm that is equivalent to 20% and 80% passings respectively, with an average of 26 mm. The digital image processing results were about 19.8 mm for 20% passing and 58 mm for 80% passing, with an average of 36.06 mm. The hand-crushed provided a wide range of material distribution than the required size in the grinding unit and increased oversize by 18%. The hand-crushed of blasted materials provides different sizes distribution leading to increase the residence time and power consumption of the grinding units.
Analysis of performance characteristics predicted from several experimental data and conversion methods for pumps as turbine application using statistical techniques
(University of Dar es Salaam, 2020) Mdee, Ombeni J.; Kimambo, Cuthbert Z. M.; Nielsen, Torbjorn K.; Kihedu, Joseph
Different performance characteristics have been indicated when running centrifugal pumps in the reverse direction. The water flows from the discharge side of the pump to the suction side to run in the reverse direction and generate the mechanical rotational energy for the micro-hydropower plant. The current study evaluates the extent of variation of performance characteristics predicted by several experimental data from different pump-specific speeds and conversion methods. The performance characteristics discussed include the head, flow rate, efficiency and specific speed. The flow rate and head of a pump operating in pump mode divided with the characteristic of the pump operating in the reverse mode, at the best efficiency point, the resulting coefficient of determination (R 2 ) values were of 0.890 and 0.708, respectively. Also, the graph of head versus flow rate coefficients, which is a second- order polynomial function, has shown the value of R 2 of 0.954 for pump-specific speed ranging between 9 and 94 rpm. However, the pump in the reverse mode has smaller performance characteristics for efficiency and specific speed compared to the pump mode operation with R 2 of 0.966 and 0.999, respectively. Furthermore, schematic empirical statistical models were developed to predict the performance characteristics of several conversion methods using pump data obtained from the manufacturers.
Analytical Evaluation of Head and Flow Rate Off-Design Characteristics for Pump as Turbine Application
(The American Society of Mechanical Engineers, 2019) Mdee, Ombeni J.; Kimambo, Cuthbert Z.; Nielsen, Torbjorn K.; Kihedu, Joseph
Head and flow rate are the important parameters for proper selection of centrifugal pump. However, the reversed operation of centrifugal pump leads to the off-design characteristic of head and flow rate. This paper presents an analytical model developed by using the system curves and velocity relations derived from pump application. Also, the differential technique is applied to the analytical model to develop the off-design characteristics of head ratio and flow rate ratio relations. The off-design characteristic relations were compared with literature and available conversion methods. Then, the analytical model coefficient (AMC) with the range between −4 and +4 was developed from the off-design characteristics of head ratio and flow rate ratio relations. The AMC value was equal to 1 when the pump operates in turbine mode and pump mode at the pump best efficiency point (BEP) and extended to either side up to ±4 when tested with literature data. Therefore, the analytical model consists of the off-design head and flow rate characteristics, when simplified leading to the AMC that could be applied to select the possible boundary limits of head and flow rate for different pumps.
Assessment of hydropower resources in Tanzania. A review article
(EDP Sciences, 2018) Mdee, Ombeni J.; Nielsen, Torbjorn K.; Kimambo, Cuthbert Z.; Kihedu, Joseph
The hydropower resources have become an attractive means of generating electricity to the off-grid network, especially in rural areas. This article assesses the small, mini and large hydropower resources and identified to reach 5.3GW for electricity generation in Tanzania. The technology development discussed comprises of hydro turbine manufacturers, classification and turbine selection. The barriers presented include the presence of sediment in the hydropower reservoirs, financial viability, policy and regulatory issues related to hydropower development. This reviewed article serves the investors and policymakers to understand the status of hydropower resources in Tanzania.
Estimation of storage tank capacities for different roofing areas for rainwater harvesting in Dodoma urban, Tanzania
(University of Dar es Salaam, 2022) Mdee, Ombeni J.; Tembo, Malugu M.; Mwegoha, William J. S.
This study uses a mass balance model and economic analysis technique to present an estimation of roofing areas and storage tank capacity for Rainwater Harvesting (RWH) system. The water-saved benefits were estimated using the monthly rainfall of over 39 years from 1981 to 2020 and five roofing areas. The proposed roof-storage-ratio method presents the minimum requirement of roof area and storage tank size when the ratio value is closer to 1. The benefit-cost ratio and percentage of reliability indicated the optimal roofing areas ranging between 200 and 300 m2 for storage tank capacity between 20 and 25 m3, with a minimum discount rate of 5%. The increased capacity of storage tank and roofing area would also be a potential factor to increase the investment cost for installing the RWH system.
Evaluation of rainfall extreme characteristics in Dodoma urban, a central part of Tanzania
(2022) Mdee, Ombeni J.
The occurrence of low rainfall in semi-arid areas including Dodoma urban leads to a shortage of water. This paper addresses the evaluation of rainfall extreme characteristics by analyzing the variability indices, probability distribution, and return periods. The daily rainfall index shown the magnitude of rainfall varied unpredictably within annual wetted days. The study area experienced a period of one to two months per year with extreme seasonality as evaluated using the rainfall seasonality index. The standardized anomaly index indicated the equivalent of 60% of 20 years experienced the dried years with unpredicted occurrence. The Weibull distribution was selected out of Fifteen probability functions when ranked using Kolmogorov–Smirnov and Anderson-Darling tests. The return periods of rainfall had an average rainfall of 576 mm and were predicted using seventeen plotting position methods and Weibull distribution. Therefore, the magnitude of rainfall in the semi-arid areas would not easily be estimated but using more than one technique would improve the evaluation of rainfall characteristics
Identification of potential sites for surface runoff harvesting in the semi-arid area for developing cities. A case study of Dodoma urban, Tanzania
(Springer, 2021) Mdee, Ombeni J.; Tembo, Malugu M.
The rapid growth of socioeconomic activities brings the overpressure to the water sources in the semi-arid areas. This paper identifies potential sites for surface runoff harvesting from seasonal rainfall. The potential sites were determined using the analytic hierarchy process (AHP) and weighted linear combination (WLC) techniques with four parameters including runoff depth, drainage density, slope, and lineament density under the GIS environment. The runoff depth was estimated using the Soil Conservation Service method with the three factors such as land use, rainfall, and soil parameters. The normalized weights obtained from the AHP technique were multiplied with the individual weights for each parameter using the WLC technique to acquire the weighted classes (WCs) for defining suitable for surface runoff harvesting. The covered area of about 5.5% for a very high suitable site was obtained followed by 76.4% of the highly suitable site with the WC ranged from 433 to 500% and 372 to 433%, respectively. The covered area of about 18.1% for the medium to a very low suitable site for harvesting surface runoff with the WC between 271 and 372%. The developed potential site map would provide access for harvesting the surface runoff to improve the water shortage during long dry periods, minimize flooding, or reduce the land use across the drainage network in the semi-arid areas.
Measurement methods for hydropower resources: a review
(E.W. Publications, 2018) Mdee, Ombeni J.; Kimambo, Cuthbert Z.; Nielsen, Torbjorn K.; Kihedu, Joseph
Each hydropower plant is usually designed for specific amount of discharge and gross head. This paper presents the methods used to measure the discharge and gross head variables with additional of assessment tools. Discharge measurement methods covered include bucket, float, velocity-area, dilution, hydraulic structure and advanced discharge measurement while gross head measurement methods covered include topographic and DEM maps, hose level with pressure gauge, sight level, spirit level and advanced survey instruments. The assessment tools presented include SMART MINI-IDRO, Integrated Method for Power Analysis, VIPIDRO ASTE 4.0, Hydromatch, Hydrohelp and Renewable Energy Technology Screen tool. The assessment tools used to estimate the potential of hydropower resources in terms of energy, hydropower plant components and financial analysis. Therefore, this review paper indicated the possibility of conducting a pre-feasibility study of hydropower estimation by using the open access tools and accessible equipment in the carpentry and mason works, especially in the developing countries.
Potential of artisanal and small-scale gold mines for economic development in Tanzania: A review
(Academic Journal, 2015) Mdee, Ombeni J.
The availability of mineral deposits for many decades provides an avenue for improving the economic development of the local artisanal and small scale miners in Tanzania. This study documents different possible locations from various available data and show the characteristics of the existing gold deposits extracted and processed by the artisanal and miners using ArcGIS software. It identifies the challenges of utilizing the obsolete equipments, lack of skills and knowledge of mining and mineral processing operations, and improper negotiation between primary mining license owners (PML) and pit holders based on mine plan and design, safety, and environmental management in different artisanal and small-scale Mines (ASMs). However the conceptual model of ASM based on the secondary data gathered was well developed, as well as discussions on the digging, loading and transportation, crushing and mineral processing operations of the existing ASMs. Therefore, the author recommends the documented information in this study as significant in promoting and facilitating further research in Tanzania’s ASM field.
Spatial distribution of runoff in ungauged catchments in Tanzania
(E.W. Publications, 2015) Mdee, Ombeni J.
This paper presents a preliminary map of average annual runoff of Tanzania using a conceptual rainfall runoff model to predict the amount of water directed to base flow of the rivers with the application of Geographic Information System (GIS). Average annual runoff is a fundamental hydrological variable which serves many purposes in hydrological research particularly in stream flow estimation for hydropower plant allocation and development. Still the runoff data can serve during irrigation and flood control situation. The Pacific Southwest Inter-Agency Committee (PSIAC) model was utilized to rate the surface geology, soil-infiltration rate, topography, ground cover, land use, upland and channel erosions parameters which have been applied to estimate the Curve Number (CN). Subsequently, CN and rainfall data have been employed in the Soil Conservation Service (SCS) runoff equation to estimate the direct runoff data. The results of the study show the combination of PSIAC and SCS models were useful in studying of runoff response of the Tanzania’s catchments. Once the preliminary runoff map was evaluated depends on the three climate zones of Tanzania indicated to have high annual runoff in wet zone compared with moderate and dry zones. The runoff map can be applied during preliminary study, however the author acknowledged the primary source of data for the average annual runoff estimation is preferable to be taken at the gauged river flow stations.
A technical evaluation of performance characteristics for pump as turbine application
(Springer, 2019) Mdee, Ombeni J.; Kimambo, Cuthbert Z. M.; Nielsen, Torbjorn K.; Kihedu, Joseph
It is very important to gain a better understanding of pump series of different specific speeds when running in reverse mode. The pump is running in reverse mode to serve as turbine for mechanical rotational energy generation in the hydropower and processing plants. This review chapter employs a technical analysis as a tool to evaluate pump when running in reverse mode by assessing the technological development gathered from the literature. The operational parameters discussed include head, flow rate, power, efficiency, and specific speed. The number of modified pump components includes the blade angle, impeller size and tips, edges of shroud and hub plates, volute casing attached with guide vanes, and modified inlet casing rings and eye enlargement. The internal characteristics also occurred in the form of swirl flow between blades and discharge part, time dependent on flow circulation, radial and axial thrust, blade angle variation, pressure pulsation, cavitation effects, and energy losses between blades, volute casing, and discharge part. Furthermore, the conversion methods discussed were related to pump efficiency, specific speed and constant values to predict the head and flow rate. The conversion methods are mostly used to select the off-the-shelf pumps to run as turbine. Therefore, this paper provides the technological development of running the pump in reverse mode that would give insights for further studies in the four knowledge areas that include operational parameter characteristic, pump component modification, internal flow characteristics at the full- and part-load conditions, and also the development of conversion methods related to pump geometry, fluid properties, performance and system curves to improve the off-the-shelf pump selection.

Browsing by Author "Mdee, Ombeni J."

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