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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Enhance the heat transfer in a heat treatment furnace through improving the combustion process in the radiation tubes

Elmabrouk, Elmabrouk Mohamed January 2011 (has links)
Radiation tube burner systems are widely used as in-direct heating systems in heat treatment furnaces. Saving energy through improving the combustion and heat transfer process in the radiation tubes has become a pressing issue in the heat treatment industry over the last few years. The material structure during the process of heat treatment is predominantly determined by the temperature. The heat treatment processes usually require an even temperature distribution inside the furnace over a long duration to achieve the desired material properties. Due to the rising energy costs and the environmental concerns regarding the combustion emissions, reducing energy consumption has become a significant area of concern for the industry. This research is aimed at improving the combustion process in the radiation tubes to enhance the heat transfer to the heat treatment chamber, therefore achieving the objective of making energy savings. As a measure of improving the energy efficiency in the heat treatment furnace, Single End Radiant Tube burners (SET) were used to replace Vtube combustion systems. The energy efficiencies of the SET and V-tube combustion systems were experimentally verified in two full-scale working furnaces. This project carried out a quantitative analysis of the combustion and heat transfer processes in the radiation tubes and the heating chamber using Computational Fluid Dynamics (CFD) simulations. One of the main objectives of the project was to optimize the design of the furnace heating system with the aid of validated CFD simulations. Experimental data obtained in the full-scale working furnaces were used for the validation of the CFD simulations and to provide the boundary conditions for the CFD cases. Experimental instruments were installed in two heat treatment furnaces to measure the fuel flow rates in the radiation tubes, their surface temperatures and the temperature distribution inside the heat treatment furnaces. The fuel flow rates were used to determine the energy efficiencies of the heat systems and the fuel inlet conditions for the CFD cases. The measurements of the outer surface temperatures of the radiation tubes were used to determine an average temperature as the boundary conditions for CFD simulations of the combustion process inside the SET burner. The temperature distributions inside the furnace heating chamber were examined by measuring the temperature at specific points using a thermocouple matrix. This temperature measurement provided data to validate the CFD simulations of heat transfer inside the furnace heating chamber. Three series of CFD simulations were carried out in this project. The cases in the first series of CFD simulations were based on the SET burner. The flow mixing, combustion and heat transfer process in the SET burner were analyzed in the baseline CFD case, and the influence of radiation models on the CFD simulations were investigated. The design parameters, such as the effect of the burner diameter, were also verified in the baseline CFD studies. Using the baseline case as a reference, a numerical study was carried out to explore the applications of advanced combustion technologies, such as high temperature air combustion (HiT AC) and two-stage combustion with preheated air in the SET. The results of the CFD simulations were used to determine the heat flux rates through the SET burner wall into the heating chamber, which were used as the boundary conditions for the CFD simulations of the heating chamber in the second series of CFD cases. A good agreement was found between the numerically predicted temperatures at specified points inside the furnace heating chamber and the experimentally measured temperatures at the same points. This demonstrated that the heat flux rates from the SET burners can be applied in CFD-aided design to optimize the operational conditions of similar or super-size furnaces with confidence. Finally, a case study of CFD-aided design was carried out in the third series of CFD simulations of the heat transfer process inside the super-size furnace chamber. CFD simulations were used to verify if six SET burners are sufficient to provide the required temperature distribution inside the chamber and provide optimum locations for the SET burners.
2

Proposed design of a novel expander-turbine for use with compressed air energy storage for generating electricity

Ali, Sadiq January 2013 (has links)
Electricity generation is highly carbon intensive, associated with complex externalities. The preceding thirteen decades have witnessed improvements in efficiencies of rotary machines employed' for electricity generation through enhanced metallurgy, understanding of fluid mechanics and thermodynamics; consequently improving their conventional designs without much heed to environmental degradation, understanding of energy resources, and sustainability, until emergence and realisation of disastrous climate changes. This forced the environmentalists and scientists to explore new frontiers, and to harness renewable energies in any possible way. The situation required an innovative approach and unconventional design to satisfy the need of time. The aim of this research is to recommend a novel design for an expander-turbine that is suitable for generating electricity using compressed air as working fluid; investigating material suitability for cryogenic temperatures. The underlying endeavour is to promote reliable quality renewable energy generation that is sustainable, economical, and suggesting zero-carbon solution in commercial and domestic environment. The results of this venture are very promising and are expected to address UK's commitments to Kyoto protocol for reducing carbon footprint in energy generation. The methodology adopted was to investigate efficiency of individual mechanisms in relevant rotary machines that helped in identifying individual portfolio components that may improve efficacy of turbines if put together.
3

Optimisation of wind turbine blade structural topology

Buckney, Neil January 2013 (has links)
Wind turbines become more cost effective as they grow larger; however the blade mass increases at a greater rate than the power. For a continued size increase, reducing the mass of the blades is necessary. Additionally, lighter blades lower overall turbine costs because the loads on the rest of the structure are decreased. Therefore, the use of lightweight blades can have a significant impact on the cost of wind energy. To achieve blade mass reductions, an alternative structural layout is generated using topology optimisation. The result is a topology which varies along the blade length, transitioning from a structure with trailing edge reinforcement to one with offset spar caps. An alternative beam topology optimisation method is developed that enabled a buckling constraint to be applied. The structural efficiency of the topologically optimised blade is then assessed using shape factors and performance indices, measures which have been expanded to account for asymmetric bending of beams with multiple materials. The utility of shape factors is first demonstrated on six example beam sections before being applied to the blade. To demonstrate application to a more refined design, the performance of a 100m wind turbine blade is assessed , using maps to visualise the structural efficiency. The effect of using carbon fibre and offsetting the spar caps is evaluated, providing a greater understanding of the improved designs. Overall , the results show that wind turbine blades can be improved with structural layouts that take advantage of favourable bend-bend coupling between the out-of-plane and in-plane directions. Because traditional design concepts do not account for bending coupling, a missed opportunity for further mass reduction exists. To this day, the structural topology of the blades has remained fixed despite increasing length and changing loads. Topology optimisation and structural efficiency analysis are shown as methods used to challenge this design convention and reduce blade mass, thereby lowering the cost of wind energy.
4

Extending the lifetime of wind turbine gearboxes

Igba, Joel Ejiroghene January 2017 (has links)
Wind turbines (WTs) are a proven source of clean energy with wind power energy harvesting technologies supplying about 3% of global electricity consumption in 2014. However there is an increasing demand on maintenance and operational improvements since turbines have been plagued with downtime problems ofmajor components e.g. gearboxes, which in particular are known to have a higher downtime per failure than other WT subassemblies. This is as a result of two reasons. First, WT gearboxes have historically suffered from early failures due to the underestimation of operational load conditions. Second, WT gearboxes have very complex repair procedures needing heavy lifting equipment such as external cranes for repair and replacement. This downtime results in revenue loss for the customer. Hence, for a company like Vestas Wind Systems AlS, who designs, manufactures and services WTs dealing with the gearbox downtime issue is of great importance. This thesis focuses on the gearbox challenge specific to Vestas organisational context based on a research journey undertaken by the author whilst embedded in Vestas as an employee. It focuses on the 2MW fleet of gearboxes serviced by Vesias globally. Furthermore, the thesis addresses two dimensions of the industrial problem - (i) investigating the gearbox problem, i.e. the issue with reliability and maintainability, and identifying solutions for improving these, and (ii) improvement of Vestas internal processes which contribute to delivering maintenance and repair solutions for gearboxes e.g. the capturing and reusing of maintenance and repair data for failure and reliability analysis. These two strands of the research equip decision makers within Vestas with tools and techniques for making decisions concerning the maintenance and repair challenges. Hence, enabling the company to improve performance of the gearboxes and extending the life of gearboxes. The main outcomes of this thesis are the development of new and novel in-service decision-making models (and tools) which are currently adding 'value to Vestas. First, a preventive maintenance optimisation model was developed by applying state of the art approaches used in industries like aerospace and marine, to historical gearbox in-service data from Vestas operational WTs. This model estimates the optimal interval for preventive replacements, repair and inspections of gearboxes. The benefit to Vestas is that the model helps WT managers to make timely decisions regarding planning and scheduling maintenance, which can reduce the downtime considerably and avoid consequential failures, hence resulting in cost savings for the company. Second, a novel extreme vibration model was developed using the automated condition monitoring data from operational gearboxes. This model can help in detecting failures on the high speed and intermediate speed stages of the gearbox as early as one month in advance. The model was recently developed and validated and is soon to be implemented in the organisation but it is expected to help avoid consequential failures and reduce downtime due to the ability to plan and schedule maintenance early as soon as a fault is detected. Third, a decision support framework (with an accompanying tool) for repair cost estimation, gearbox damage classification and feedback of repair data to design, was developed using a soft systems approach. In addition, a data repository has been created which contains repair statistics that is used for analysis purposes for guiding repair decisions and of the design of new gearboxes. The developed models, framework and tools are now being used across the organisation by engineers and service personnel, and by Vestas external repair providers, which has led to savings for Vestas in the order of hundreds of thousands of Euros yearly.
5

A-posteriori error estimation using higher moments in computational fluid dynamics

Russant, Stuart January 2015 (has links)
In industrial situations time is expensive and simulation accuracy is not always investigated because it requires grid refinement studies or other time consuming methods. With this in mind the goal of this research is to develop a method to assess the errors and uncertainties on computational fluid dynamics (CFD) simulations that can be adopted by industry to meet their requirements and time constraints. In a CFD calculation there are a number of sources of errors and uncertainties. An uncertainty is a potential deficiency that is due to a lack of knowledge of an activity of the modelling process, for example turbulence modelling. An error is defined as a recognisable deficiency that is not due to a lack of knowledge, for example numerical discretisation error. The process of determining the level of errors and uncertainties is termed verification and validation. The work aims to define an error estimation method for verification of numerical errors that can be produced during one simulation on a single grid. The second moment solution error estimate for scalar and vector quantities was proposed to meet these requirements. Where the governing equations of CFD, termed the first moments, represent the transport of primary variables such as the velocity, the second moments represents the transport of the primary variables squared such as the total kinetic energy. The second moments are formed by a rearrangement of the first moments. Based on a mathematical justification, an error estimate for vector or scalar quantities was defined from combinations of the solutions to the first and second moments. The error estimate was highly successful when applied to six test cases using laminar flow and scalar transport. These test cases used either central differencing with Gaussian elimination, or the finite volume method with the CFD solver Code_Saturne to conduct the simulations, demonstrating the applicability of the error estimate across solution methods. Comparisons were made to the numerical simulation errors, which were found using either the analytical or refined solutions. The comparisons were aided by the normalised cross correlation coefficient, which compared the similarity of the shape prediction, and the averaged summation coefficients, which compared the scale prediction. When using the first order upwind scheme the method consistently produced good predictions of the locations of error. When using the second order centred or second order linear upwind schemes there was similar success, but limited by influences from solution unboundedness, non-resolution of the boundary layer, the near-wall gradient approximation, and numerical pressure error. At high Reynolds numbers these caused the prediction of the location of error to degrade. This effect was made worse when using the second order schemes in conjunction with the constant value boundary condition. This was the case for the scalar or velocity simulations, and is caused by the unavoidable drop to first order accuracy during the near-wall gradient approximation that is required for the second moment source term approximation. The prediction of the scale demonstrated a dependence on the cell Peclet number. Below cell Peclet number 4 the increase of the estimate scale was linearly related to the increase of the error scale. The estimate scale consistently over-predicts by up to a factor of 3. This allows confidence that the true error level is below that which is predicted by the error estimate. At cell Peclet numbers greater than 4 the relationship between the scales remained linear, however, the estimate begins to under-predict the estimate. The exact relation becomes case dependent, and the highest under-prediction was by a factor of 10. In such circumstances a computationally inexpensive calibration can be done.
6

Numerical and experimental investigations of Darrieus wind turbine start-up and operation

Du, Longhuan January 2016 (has links)
The performance of small, H-Darrieus vertical axis wind turbines has been investigated numerically and experimentally with particular attention paid to turbine performance at low tip speed ratios (low Reynolds number) and to turbine self-starting. Comprehensive wind tunnel measurements have been performed to provide accurate aerofoil data at low Reynolds numbers and high angles of attack; a unique requirement for vertical axis wind turbine (VAWT) starting studies. Two-dimensional CFD models and blade element momentum (BEM) models were created and assessed to provide new insight into turbine performance for different wind conditions and into different turbine geometries in order to guide the design of the experimental investigation. The experimental and numerical studies have demonstrated that design parameters including turbine solidity, blade profile, blade pitch angle and blade surface roughness have strong influences on turbine performance and turbine self-starting capability. Although other authors have conducted numerical studies of the effect of these parameters, this work represents the first experimental validation for turbine performance at low tip speed ratios. In contrast to some previous studies it is shown that there is no advantage to be gained from the use of cambered blades and that symmetrical blades set at small negative incidence provide the best design solution. It is also shown that increasing the turbine’s solidity can significantly improve self-starting capability and that for a given solidity, increasing the rotor radius with a corresponding increase of blade chord improves performance further. However, these starting performance gains are achieved at the expense of a small loss of peak power output. In addition, bio-inspired blades with tubercle leading edges are demonstrated to be able to significantly improve the turbine self-starting capability by introducing a more gradual stall characteristic. These results are the only reported measurements of the effect of tubercle leading edges on vertical axis wind turbines. Finally, a novel, real-time on-board pressure measurement system was developed and employed to examine the instantaneous blade pressure distribution and its variation when the turbine is rotating. The complex flow physics including dynamic stall, laminar separation and flow curvature were successfully recorded and provide unique, unsteady data to increase our knowledge and understanding of the transient aerodynamics of the H-Darrieus wind turbine. The experimental results were also compared with the available CFD and BEM predictions. It is demonstrated that BEM based approaches are highly sensitive to the quality of the aerofoil data that is provided as input to the model. This thesis provides validation of previous work on the question of whether H-Darrieus wind turbines can start without external assistance and in the light of this research a set of revised design rules are proposed to achieve self-starting turbines.
7

Experimental and theoretical investigation of a three-phase direct contact condenser

Al-Muhammedawi, Hameed B. Mahood January 2016 (has links)
In the present work, for the first time, an experimental and theoretical study of the heat transfer characteristics of a bubble type three-phase direct contact condenser has been carried out. The experiments were conducted using a Perspex column of 70 cm in total height and 4 cm inner diameter, as a direct contact condenser. The active column height throughout the experiments was 48 cm. Pentane vapour at three different initial temperatures (40℃, 43.5℃ and 47.5℃), was used as the dispersed phase while tap water at a constant temperature (19℃) was used as the continuous phase. Seven different dispersed phase mass flow rates and five different continuous phase mass flow rates were tested. The experiments considered the transient temperature distribution along the direct contact condenser, the steady-state temperature distribution, the volumetric heat transfer coefficient, the heat transfer rate per unit volume and the holdup ratio. Also, the efficiency and capital cost of the direct contact condenser were estimated, and the heat transfer of the three-phase direct contact condenser during flooding was studied. Theoretical models describing the direct contact condenser were developed. These models included the transient temperature distribution, the steady-state temperature distribution and the volumetric heat transfer coefficient. These models implicitly involved new derivations for the surface heat transfer coefficient, the two-phase bubble size, the relative velocity of two-phase bubbles, the drag coefficient and the added mass of the two-phase bubble. All expressions were derived analytically except for the transient temperature distribution along the condenser which was found numerically, using MATLAB. The results showed that the mass flow rate ratio has a significant effect on the heat transfer characteristics of the condenser, while the initial temperature of the dispersed phase has only a slight effect. The models developed were fitted the experimental data well.
8

Experimental investigation on a rotary compressor-expander heat pump performance

Kouhestani, Mohammad Salehi January 2016 (has links)
This research investigates the performance and impacts of integration of an internal expander within a typical heat pump system for the UK climate. In this novel system, the expansion process takes place within an expander rather than a traditional expansion valve, and it is expected that recovered work by the expander could provide a portion of the required power to drive the compressor. This project comprises a literature review, test rig design and fabrication, control system design instrument selection and calibration, data-logging system setup and installation, implementation of the BS standard for heat pumps performance testing, implementation of tests, and data collection according to relevant standards and data analysis. A test rig is developed and improved throughout this research. More than 20 sets of tests have been carried out between 2011 and 2014, and the most appropriate of them are presented and discussed. To determine the impact of expander integration, the rotary compressor-expander unit was tested with and without an expander, and the system COP, capacity, and isentropic efficiency are discussed. This thesis commences by introducing key drivers of this research, the energy crisis, global warming issues, and the contribution of the domestic sector to energy consumption in the UK. Then it continues by classifying heat pumps according to their energy sources. A detailed review and evaluation of major advanced heat pumps and refrigeration technologies then follows. Test rig design and development (according to the BS EN14511-2:2011) are discussed and outlined comprehensively. The test scenarios are presented, obtained data are discussed, test outcomes are evaluated, and the issues and their causes are addressed in detail. To address the issues and interpret the achieved results, theoretical analysis is carried out as well, then actual outcomes are compared with theoretical anticipated results and the differences are addressed. Results of compressor tests and system integration are presented separately and discussed comprehensively. The last chapter outlines conclusions and recommends further possible work on this research topic and offers practical suggestions to improve the performance of this developed system. However, this research reveals that such integration of an expander and compressor is not suitable in practice for typical UK winter conditions, though it may offer some benefits for extreme winter conditions where the ambient temperature drops to -10°C or below. In addition, it reveals that the designed compressor is not able to overcome a significant temperature increase (which is the case for medium-temperature heating applications and retrofit applications) due to internal refrigerant leakage.
9

Carbon assessment of wind power

Chapman, Samuel S. B. January 2015 (has links)
The Earth is facing huge implications from Anthropogenic Global Warming and peaks in the production of finite fossil fuels. Decision-makers have to choose strategies for combating these dual problems whilst ensuring minimal costs to society and the environment. Unfortunately, renewable technologies in particular have doubt associated with their ability to reduce total life cycle greenhouse gas (GHG) emissions of electricity due to uncertainty in estimates. This thesis analyses historic associated GHG estimates of wind farms, the largest renewables contributor to electricity generation in the UK, to reduce the uncertainty inherent in estimates and better understand critical factors that influence estimation. Through harmonisation of published life cycle GHG emissions estimates, they are reduced by 56% to between 2.9 and 37.3gCO2e/kWh. Average values for onshore and offshore wind power are calculated as 16 and 18.2gCO2e/kWh respectively and exhibit similar characteristics in their life cycle GHG emissions. Ormonde Offshore Wind Farm is analysed using a novel hybrid approach and gives total baseline GHG emissions of 17.5gCO2e/kWh and is the largest wind power installation to be analysed to date. Finally, an estimate of the effect of load variability of wind on thermal plant in the UK system is calculated. It is shown that this effect may reduce the net emissions saving from wind power relative to the whole UK system’s savings when wind power is included.
10

Development of an ultrasonic sensing technique to measure lubricant viscosity in engine journal bearing in-situ

Schirru, Michele January 2016 (has links)
This work presents a novel technique to measure viscosity in-situ and in real time in engine component interfaces by means of an ultrasonic technique. Viscosity is a key parameter in the characterization of lubrication regime in engine parts because it can be related to friction in the contact, and to the lubricant film thickness. Ultrasound is a non-destructive and non-invasive technique that is based on the reflection of sound from interfaces. The reflection from a solid-air boundary can identify, for instance, the presence of a crack in a material, while reflection from a solid-liquid interface can help detecting the properties of the liquid sample. Reflection of longitudinal waves measures fluid film thickness and chemical composition, while the reflection of ultrasonic shear waves measures the fluid viscosity. The viscosity measurements based on ultrasonic reflection from solid-fluid boundaries are referred to as reflectance viscometry techniques. Common ultrasonic reflectance viscometry methods can only measure the viscosity of Newtonian fluids. This work introduces a novel model to correlate the ultrasonic shear reflection coefficient with the viscosity of non-Newtonian oils by means of the Maxwell model analogy. This algorithm overcomes the limitation of previous models because it is suitable for the analysis of common engine oils, and because it relies only on measurable parameters. However, viscosity measurements are prohibitive at the metal-oil interfaces in auto engines because when the materials in contact have very different acoustic impedances the sound energy is almost totally reflected, and there is very little interaction between the ultrasonic wave and the lubricant. This phenomenon is called acoustic mismatch. When acoustic mismatch occurs, any valuable information about the liquid properties is buried in measurement noise. To prove this, the common reflectance set-up was tested to measure the viscosity of different lubricants (varying from light base oils to greases) using aluminium as solid boundary. More than 99.5% of the ultrasonic energy was reflected for the different oils, and accurate viscosity measurement was not possible because the sensitivity of the ultrasonic measurement at the current state of the art is of ±0.5%. Consequently, the discrimination by viscosity of the oil tested was not possible. In this study a new approach is developed. The sensitivity of the ultrasonic reflectance method is enhanced with a quarter wavelength matching layer material. This material is interleaved between metal and lubricant to increment the ultrasonic measurement sensitivity. This layer is chosen to have thickness and mechanical properties that induce the ultrasonic wave to resonate at the solid-liquid interface, at specific frequencies. In this work, resonance is associated with the destructive interaction between the wave that is incident to the matching layer and the wave that is reflected at the matching layer-oil interface. This solution brings a massive increment in the ultrasonic measurement sensitivity. The matching layer technique was first tested by enhancing the sensitivity of the aluminium-oil set-up that was affected by acoustical mismatch. A thin polyimide layer was used as a matching layer between aluminium and the engine oil. This probe was used as ultrasonic viscometer to validate the sensing technique by comparison with a conventional viscometer and by applying a temperature and pressure variation to the samples analysed. The results showed that the ultrasonic viscometer is as precise as a conventional viscometer when Newtonian oils are tested, while for Non-Newtonian oils the measurement is frequency dependent. In particular, it was noticed that at high ultrasonic frequency only the viscosity of the base of the oil was measured. The ultrasonic viscometer was used to validate the mathematical model based on the Maxwell analogy for the correlation of the ultrasonic response with the liquid viscosity. At a second stage, this technique was implemented in a journal bearing. The ultrasonic viscometer was mounted in the shaft to obtain the first viscosity measurement along the circumference of a journal bearing at different rotational speeds and loads. The ultrasonic viscometer identified the different viscosity regions that are present in the journal bearing: the inlet, the regions characterized by the rise in temperature at the contact and the maximum loaded region were the minimum film thickness occurs. The results were compared with the analytical isoviscous solution of the Reynolds equation to confirm that the shape of the angular position-viscosity curves was correct. Finally, the method was preliminarily tested on a coated shell bearing to show that the coating presents in bearing, like iron-oxide or babbit, is a good matching layer for the newly developed ultrasonic viscometer technique. This means that ultrasonic transducers, with sizes as small as a pencil tip, have the potential to be mounted as viscometers in real steel bearings where the coating layer in contact with the fluid acts as a matching layer. Overall, the results obtained showed that this technique provides robust and precise viscosity measurements for in-situ applications in engine bearings.

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