Ethiopian Journals Online (EJOL)
Not a member yet
4332 research outputs found
Sort by
Thermal analysis of solar dryers
No full textA general purpose solar crop dryer for drying various agricultural products such as coffee, fruits, vegetables, medicinal plants, etc. is simulated. The simulated solar collector consists of the solar air heater (solar collector) which uses low-emissivity glass cover and an integral d1yer chamber attached to the collector where the products to be dried are placed, Fig. I. A thennal solar-collector model was developed to detennine the available useful energy for heating the ambiellt air from the available meteorological data such as global and diffuse radiation and ambient air temperatllre. Basic heat transfer equations for single-plane glass glazing are derived and techniques for the solution of these equations are presented. A computer programme was written to predict the collector outlet temperature, mass flow rate and other engineering variables from the input of the meteorological data and collector parameters. Results of the system simulation are presented in graphical fonn suitable for system pe1formance detennination. From the incident solar flux, ambient air temperature and solar collector parameters, the average yearly values of useful energy, the collector outlet air-temperature and air volume flow-rate are also predicted and presented graphically. In this paper, the theoretical analysis of the drying chamber was effected through an example, as it very much depends on experi~entally detennined parameters which also depend on the particular type of product to be dried
ASSESSMENT OF POWER RELIABILITY AND IMPROVEMENT POTENTIAL BY USING SMART RECLOSERS
No full textThis paper presents the use of smart reclosers forimproving reliability of a distribution system of one ofthe major cities of Ethiopia. As frequent powerinterruptions are posing a huge problem to the life ofthe people and the economy, finding a solution to theproblem is very essential. Electric reliability hasaffected social well being, public health, watersupply, communication service, and economic growthin the country. This study presents the assessmentresult of the power distribution reliability for the cityand the possibility of using smart reclosers forimproving the urgent and pressing power interruptionproblem. The smart reclosers are key elements forfault detection, isolation and restoration.TheWindMilsoftware has been used to verify theimprovement of the reliability indices for thedistribution system. The simulation result of thedesigned model with three reclosers in each feederand tie-recloser between connected nearby feedersshows that the application of smart reclosersimproves the reliability of the distribution network by75% in comparison with the reliability of currentlyexisting system
CONTACT STRESS ANALYSIS OF INVOLUTE SPUR GEAR BY FINITE ELEMENT METHOD (FEM)
No full textIn this paper the contact stress in rolling-sliding contact of involute spur gear and the effect of coefficient of friction was analyzed. To achieve this, first, three dimensional involute spur gear pairs were developed in Solid works 2012 Premium and the 3D model was exported to ANSYS workbench 14.5. Next, the analysis was done using both FEM and Hertizian stress formula. Thus, it is found that the contact stress both in the FEM and analytical method is the similar with minimum variation. In addition, it is found that the contact stress increases with higher coefficient of friction between the areas of contact. Therefore, the analytical method can be applied for gear design of high load carrying capacity effectivel
MULTI-STAGE AND MULTI-RESPONSE PROCESS OPTIMIZATION IN TAGUCHI METHOD
No full textProduct quality is all about reducing variations of key performance indicators. However, product manufacturing often, requires multiple processes with multiple indicators, which make reducing variation a complex task. There are tools used to optimize a single stage process independently which ensure local optimization only. However, there is no clear approach that leads to system optimization. The aim of this study is, therefore, to develop a tested approach to optimize multi-stage and multi-response industrial process. For this purpose, a conceptual model with its optimization procedure was developed and then validated by taking Harar Brewery as a case. Variations under each stage were evaluated before and after optimization. As a result, the new model with its optimization procedure is proven to ensure the stability of a process with reduced variatio
Enhanced energy detector using adaptive wiener filter in cognitive radio
No full textThe rising demand in wireless services and applications bring channel constraint on the existing frequency spectrum allocation scheme. By addressing the concept of cognitive radio technique (CR), it is possible to mitigate the scarcity of spectrum channel and underutilization of spectrum efficiencies. Various techniques and methods have been used to maximize the probability of detection (Pd) in CR system. Energy detector spectrum sensing is one of the transmitter based techniques, which detects the presence of primary user in cognitive system. However performance indicator metrics show that the energy detector algorithm has poor performance when the received SNR value is too low as compared to the system threshold value. In this paper the performance of conventional and enhanced energy detector (using adaptive Wiener filter) is examined based on Receiver Operating Curve (ROC) and Complementary Receiver Operating (CROC). Simulation results are plotted for different threshold values, false alarm probability, detection probability and the received SNR. Results for AWGN and Rayleigh flat fading channel models are presented. The Simulations results in Rayleigh flat fading channel show that, the insertion of adaptive Wiener filter in conventional energy detector has improved the probability of detection by 8% and reduce the probability of miss detection by same amount
Building height regulation preparation practices in Addis Ababa: Gaps and implications
No full textContrary to its long urban history, Ethiopia has a very short history of urban planning by local professionals and largescale implementation of plans. As a result, gaps are expected not only in implementation of plans but also in preparation of planning components such as building height regulations. Studies aimed at identifying these gaps and their implications, not only for the study city but also for regional urban centers have, however, been quite scarce. By studying four cases of building height regulation preparation processes for Addis Ababa during the past 22 years, this study is aimed at contributing to filling these gaps. Its findings indicate that there were significant gaps in both the analytic and participatory components of building height regulation preparation processes in the city. The former gap, in particular, had implications for all the major elements of the building height control: its thematic scope, spatial scope, control status, the roles of related building regulations and urban design, and for the development and protection of the green elements of the city which may play vital infrastructure roles. The study recommends basing building height regulation studies on strong analytical or conceptual framework and using not only technical but also political or participatory processes, generally, and revisiting the current building height regulation of Addis Ababa in the light of the general recommendations specifically
Effects of etching process inaccuracy in the malfunctioning level of PCB circuits - a simulaton based analysis
No full textThe PCB is a mechanical base used for interconnection of electronic components. In PCB manufacturing industry, the quality and reliability of circuits is highly dependent upon the accuracy of the manufacturing processes. Among the manufacturing processes, etching is the most sensitive process and needs more accuracy. The etching inaccuracies cause severe problems that may result in degradation of circuit performance and malfunctioning of circuits. In this research the levels of effects of etching manufacturing process inaccuracies on the malfunctioning of PCB circuits is investigated and analysed. The effects of the process inaccuracies on circuits as well as how the process is related with the affecting parameters are addressed. A four-layer multilayer board is considered for the study and the software selected to study the effects of manufacturing inaccuracies is OrCAD PCB designer. Schematic development, PSpice simulation and layout design are made for selected circuits to study the effectsof the etching inaccuracies.According to the results obtained; inaccurate etching results in either larger or smaller width of traces, leaves unwanted conductive materials, and creates unnecessary holes or open circuits and short circuits. In the over etching scenario, when the over etching effect is increased and results in 14.36-ohm resistance, output voltage drops from 48mV to 22mV. For the open circuit scenario, it results in zero output voltage. Similarly, for the short circuit case which is the case of under etching, the output result shows 1.8nV which is very small and it can be considered as zero output voltage. For capacitive effects, the output becomes very small (140uV), unstable and oscillating. For the combined resistive and capacitive effects, the output is nearest to zero (65uV) and oscillating
MODELING, SIMULATION AND PERFORMANCE EVALUATION OF PARABOLIC TROUGH SOLAR COLLECTOR POWER GENERATION SYSTEM
No full textModel of a parabolic trough power plant, taking into consideration the different losses associated with collection of the solar irradiance and thermal losses is presented. MATLAB software is employed to model the power plant at reference state points. The code is then used to find the different reference values which are used as inputs for the TRNSYS programme implemented in the modeling and simulation of the power plant. TRNSYS software is used to simulate the performance of the model at off design weather conditions as well.Keywords: Parabolic Trough Collector (PTC); Heat Transfer Fluid (HTF); TRNSYS power plant model; STEC library; Solar Advisor Model (SAM); TRNSYS solar field model; Solar Electric Generation System (SEGS)
LEAKAGE AWARE HARDWARE AND STOCHASTIC POWER SCHEDULING FOR SMART MOBILE DEVICE
No full textLEAKAGE AWARE HARDWARE AND STOCHASTIC POWER SCHEDULING FOR SMART MOBILE DEVIC
SEISMIC SOIL-STRUCTURE INTERACTION AS A POTENTIAL TOOL FOR ECONOMICAL SEISMIC DESIGN OF BUILDING STRUCTURES
No full textNoting that contemporary seismic design codes have become more and more demanding in terms of requirements related to design forces anddeformations for buildings, this paper attempts to demonstrate that it could be prudent to consider the introduction of soil-structure interaction (SSI) provisions into local codes in order to potentiallyoffset the costs incurred by the high demand for base shear attributed to site amplifications by soft soil sites as per current code requirements. This mostly beneficial effect of site soils is as a result of lengthening of the fundamental period and of the mostly increased effective damping of the overall system due to SSI. After introducing the basic concepts of dynamic SSI, the paper demonstrates that if SSI provisions in some international codes are properly adapted, a substantial reduction in thebase shear force could potentially be achieved so that the sizes of structural elements would also be proportionally less. With this, the paper attempts to address the legitimate concerns of many designengineers regarding the likely escalation of construction costs associated with the stringent requirements of contemporary seismic design spectra for soil sites that are expected to be introduced in the Ethiopian seismic code currently under revision