The wake of an exhaust stack in a crossflow

Relatively few studies have been carried out on the turbulent wake structure of a finite circular cylinder and a stack partially immersed in a flat-plate turbulent boundary layer. There is a need to develop a better understanding of the wakes of these structures, since they have many important engineering applications. This thesis investigates the influence of the aspect ratio on the wake of a finite circular cylinder and the effects of the ratio of jet flow velocity to crossflow velocity (velocity ratio, R) on the wake of a stack in a cross-flow. The wake characteristics of flows over a finite circular cylinder at four different aspect ratios (AR = 3, 5, 7 and 9) were investigated experimentally at a Reynolds number of ReD = 6&#61620;104 using two-component thermal anemometry. Each cylinder was mounted normal to a ground plane and was either completely or partially immersed in a flat-plate turbulent boundary layer. The ratio of boundary layer thickness to the cylinder diameter was 3. A similar turbulent wake structure (time-averaged velocity, turbulence intensity, and Reynolds shear stress distributions) was found for the cylinders with AR = 5, 7, and 9, while a distinctly different turbulent wake structure was found for the cylinder with AR = 3. This was consistent with the results of a previous study that focused on the time-averaged streamwise vortex structures in the wake. In addition, irrespective of the value of AR, high values were observed for the skewness and flatness factors around the free end of the cylinders, which may be attributed to the interaction of the tip vortex structures and downwash flow that dominates this region of the cylinder.The wake characteristics of a stack of aspect ratio AR = 9 were investigated using both the seven-hole pressure probe and thermal anemometry. The seven-hole probe was used to measure the three components of the time-averaged velocity field, while the thermal anemometry was used to measure two components of the turbulent velocity field at various downstream locations from the stack. The stack was mounted normal to the ground plane and was partially immersed in a flat-plate turbulent boundary layer, for which the ratio of boundary layer thickness to the stack diameter was 4.5. In addition, measurements of the vortex shedding frequency were made with a single-component hot-wire probe. The cross-flow Reynolds number was ReD = 2.3 x 104, the jet Reynolds number ranged from Red = 7.6 x 103 to 4.7 x 104, and R was varied from 0 to 3. In the stack study, three flow regimes were identified depending on the value of R: the downwash (R < 0.7), cross-wind-dominated (0.7 < R < 1.5), and jet-dominated (R &#8805; 1.5) flow regimes. Each flow regime had a distinct structure for the time-averaged velocity and streamwise vorticity fields, and turbulence characteristics, as well as the variation of the Strouhal number and the power spectrum of the streamwise velocity fluctuations along the stack height. The turbulence structure is complex and changes in the streamwise and wall-normal directions within the near and intermediate stack and jet wakes. In the downwash and crosswind-dominated flow regimes, two pairs of counter-rotating streamwise vortex structures were identified within the stack wake. The tip-vortex pair and base-vortex pair were similar to those found in the wake of a finite circular cylinder, located close to the free end and the base of the stack (ground plane), respectively. In the jet-dominated flow regime, a third pair of streamwise vortex structures was observed, referred to as the jet-wake vortex pair, which occurred within the jet-wake region above the free end of the stack. The jet-wake vortex pair has the same orientation as the base vortex pair and is associated with the jet rise

Assessment of electricity generation and energy cost of wind energy conversion systems in north-central Nigeria

In this study, the wind energy potential and economic analysis in selected six locations in north central part of Nigeria were investigated using wind speed data that span between 19 and 37 years measured at 10 m height. The performance of small to medium size commercial wind turbine models were examined and economic evaluation of the wind energy in the selected sites was made by using the levelised cost method. The results showed that the cost of energy production per kWh for the selected sites vary between ¢4.02 and ¢166.79. It was shown that Minna is most viable site while Bida is found to be least among the sites considered. Using three selected wind turbine models (in Minna) as case study, an increase in the escalation rate of operating and maintenance cost from 0% to 10%, lead to an increase in the unit energy cost by about 7%. It was further shown that by increasing the escalation rate of inflation from 0% to 5%, the cost of energy decreases by about 29% while the discount rate (return on investment) decreases from 11.54% to 6.23%

Performance evaluation of wind turbines for energy generation in Niger Delta, Nigeria

This paper evaluates wind energy potentials of seven selected locations spreading across Niger-Delta region of Nigeria using wind speed data that extend over 9 to 37 years and subjected to 2-parameter Weibull distribution functions. The performance of four wind turbine models ranging from 35 to 500 kW was simulated in all the locations considered. The results show that the performance of all the wind energy conversion systems gave the least energy output values at Ikom. In addition, annual energy output ranged from 4.07 MWh at Ikom to 145.57 MWh at Ogoja with Polaris America (100 kW) and Zeus Energy (500 kW) wind turbines respectively. It was also observed that, irrespective of the site, G-3120 (35 kW) wind turbine has the highest capacity factor among the models considered. Therefore, for wind energy development, G-3120 model or wind turbine with similar rated wind speed would be most suitable in all the locations. The number of inhabitants that can be served by the energy produced using G-3120 turbine in each location was estimated

Solar energy applications and development in Nigeria: Drivers and barriers

In this study,current perspectives of solar energy utilization as a renewable energy option in Nigeria are examined and discussed from the stand point of sustainable development.The country being a world crude oil and natural gas producer,is over-dependent on the seenergy sourcesforelectricitygeneration and otherenergyapplications.Thishascurrentlyputthecountryatariskofimpendingenergycrisesin view ofthefastdiminishingfossilreserves,inadequaterefining capacitytomeetdomesticconsumption and seriouscasesofenergyinsecurityinrestiveregionswhereexploitationsexist.Inspiteofthevast fossil basedenergyreserves,ameagerelectricityproductioncapacitythatisputat4517.6MWasat December 2012isgeneratedtosupporttheeconomyofateemingpopulationofapproximately170 million people.Nigeriaisnaturallyendowedwithabundantdepositofrenewableenergyresourcesof which solarenergyfromtheSun(beingtheworld0s mostabundantandpermanentenergysource)has for decadesbeenenjoyingveryhighlevelutilizationbyruraldwellersforagriculturalprocessingsinthe country.Itisvastlydepositedwithanestimated17,459,215.2millionMJ/day of solar energy falling on the country0s 923,768km2 land area(approximaterangeof12.6MJ/m2/day in the coastal region to about 25.2 MJ/m2/day in the far north). The different applications to which solar resources have been put and the extent of utilization(including details of existing projects)in the country were thoroughly investigated and discussed.The possible motivations for extensive development of solar energy conversion systems in Nigeria area l so discussed and some of the barriers and challenges are presented. Step sand policy measures to over come the barriers and facilitates the utilization of this resource are suggeste

Implementation and Status of Biogas Technology in Ethiopia- Case of Tigray Region

In Ethiopia, like most developing countries, the majority of households depend on wood and charcoal as their cooking energy resources. This energy has a direct impact on the forest coverage and its emissions have significant health issues. Biogas technology has introduced to address these problems and reduce energy poverty in developing countries. However, the success of this technology depends on many factors such as technology appropriateness, available skill, proper introduction, and government support to create favorable financial incentives. The objective of this paper is to make an overview assessment of biogas development programs in Ethiopia, identify development barriers and provide suggestions to mitigate these barriers. The national and regional biogas program of Ethiopia has implemented to give energy for cooking and to some extent for lighting. However, it has shown limited success over the years. The National Biogas Programme of Ethiopia has shown an overall 50% achievement while the Tigray region registered 55% achievement. In addition, the owner of the biogas facility in Tigray reported a 57% functionality rate but the lowest satisfaction level of 15%.&nbsp; Generally, the region has shown relatively better performance in terms of dissemination and functionality but a low satisfaction rate due to technical limitation, unsteady input, and financial factors. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp

Generating temperature cycle profile from in-situ climatic condition for accurate prediction of thermo-mechanical degradation of c-Si photovoltaic module

The use of climate specific temperature-cycling profile is critical to precisely quantifying the degradation rate and accurately determining the service fatigue life of crystalline silicon photovoltaic (c-Si PV) module operating in various climates. A reliable in-situ outdoor weathering database is pivotal to generating the required climate specific temperature cycle profile. This research utilizes high-resolution data obtained at five minutes interval from installed c-Si PV modules to generate a temperature cycle profile that is representative of a test site in sub-Saharan Africa climate. The study collected a three-year data from 2012 to 2014 on weathering of c-Si PV module located at College of Engineering, KNUST Ghana. The data site is on latitude 6° 40″ N and longitude 1° 37″ W at an elevation of 250 m above sea level. Analysis of the data on temperature variation and thermally induced stresses demonstrates that the region has a profile with a ramp rate of 8.996 °C/h, a hot dwell time of 228 min, cold dwell time of 369 min. Maximum and minimum module temperatures of 58.9 °C and 23.7 °C, respectively; and a cycle time of 86400 s. Comparison with the IEC 61215 standards for terrestrial PV modules qualification reveals percentage changes of – 91%, 2180%, 3590%, 747% for the ramp rate, dwell (hot and cold) and cycle times, respectively. The generated in-situ temperature cycle profile predicts to qualify accurately, c-Si PV modules operating in the sub-Saharan African test site. The systematic technique employed in this study to generate the in-situ temperature cycle profile would be useful to the thermo-mechanical reliability research community. In addition, photovoltaic design and manufacturing engineers may harness the information to create climate specific robust c-Si PV module. Keywords: Temperature cycling, Ramp rates, Temperature gradient, Accelerated Thermal Cycling (ATC), Co-efficient of Thermal Expansion (CTE), Rainflow counting, In-situ data generatio

Analysis of wind speed data and wind energy potential in three selected locations in south-east Nigeria

In this study, the wind speed characteristics and energy potential in three selected locations in the southeastern part of Nigeria were investigated using wind speed data that span between 24 and 37 years and measured at a height of 10 m. It was shown that the annual mean wind speed at a height of 10 m for Enugu, Owerri and Onitsha are 5.42, 3.36 and 3.59 m/s, respectively, while the annual mean power densities are 96.98, 23.23 and 28.34 W/m2, respectively. It was further shown that the mean annual value of the most probable wind speed are 5.47, 3.72 and 3.50 m/s for Enugu, Owerri and Onitsha, respectively, while the respective annual value of the wind speed carrying maximum energy are 6.48, 4.33 and 3.90 m/s. The performance of selected commercial wind turbine models (with rated power between 50 and 1,000 kW) designed for electricity generation and a windmill (rated power, 0.36 kW) for water pumping located in these sites was examined. The annual energy output and capacity factor for these turbines, as well as the water produced by the windmill, were determined. The minimum required design parameters for a wind turbine to be a viable option for electricity generation in each location are also suggested