180 research outputs found

    Mathematical model of a vapour absorption refrigeration unit

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    By means of carefully devised assumptions, a simple linear model is presented for an absorption refrigeration unit employing either water-lithium bromide or ammonia-water refrigerant-absorbent pairs. Absorption systems are an alternative to vapour compression systems by being thermally activated. Such heat energy may come from the sun or even from hot exhaust gases from a particular engineering process. A thorough investigation of the optimal operating temperatures is necessary to ensure effective operation of the system. By means of this simulation, the system response to varying absorber, generator and condenser temperatures was analyzed.peer-reviewe

    Drug Resistant Streptococcus pneumoniae (DRSP) in the Maltese Islands

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    The DRSP prevalence rate for the Maltese Islands was investigated. Consecutive samples were obtained, both from adults and children, from September 2000 through April 2002. Penicillin-intermediately-resistant isolates amounted to 27%, erythromycin-resistant isolates 31%, and clindamycin-resistant isolates 19%. The oxacillin disk was found to be an effective screening method for the detection of penicillin resistance. An association was found in patients who had DRSP, as well as diabetes and/ or cardiovascular disease. Finally, an investigation of the local antibiotic consumptions over the period 1997-2000, for the National Health Service was conducted. The highest consumption rates were obtained with co-amoxiclav, amoxicillin, erythromycin, cephalexin and ciprofloxacin. The results obtained here call for more judicious use of antibiotics. In addition, the setting up of a local DRSP surveillance unit is mandatory. Moreover, the use of molecular techniques to investigate specific genes, such as ermAM and mefE associated with macrolide-resistance, should be introduced as part of investigational laboratory work.peer-reviewe

    Lightweighting of double-decker buses

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    The bus industry is currently undergoing extensive transformation as cities around the world push for the rapid introduction of electric buses. Lightweighting of bus structures is identified by leading experts as one of the key technologies necessary to enable and assist this revolution in the industry. Alexander Dennis Ltd. (ADL) is the UK’s largest bus manufacturer and a worldwide leader in the construction of double-decker buses. ADL consider lightweighting to be one of the three main technological pillars of the company and have thus supported various ongoing research programmes with this EngD research programme funded in collaboration with WMG, University of Warwick. This thesis summarises the outcomes of the EngD programme, the primary objective revolving around the identification of innovative yet feasible lightweighting opportunities applicable to ADL double-decker buses. A systematic review of the state-of-the-art of bus lightweighting followed by a critical analysis of ADL bus structures led to initial feasibility studies of various lightweighting opportunities which in turn led to a lightweighting proposal. An innovative lightweight upper-deck structure design was conceived, developed and proposed to ADL. The holistic redesign of the system achieved a 42% weight reduction whilst also significantly lowering the bus centre of gravity hence enabling further lightweighting of other primary structures. The redesigned upper-deck structures necessitates the novel introduction into the bus industry of two key technologies necessary for its realisation; braided fibre reinforced polymer beam structures and coated polycarbonate glazing. A study on the feasibility of utilising fibre reinforced composites to manufacture cost-effective curved structural beams was carried out. A state-of-the-art review identified a composite manufacturing process consisting of a bladder-assisted consolidation of braided commingled thermoplastic preforms as ideally suited for the bus industry. Tooling was designed and machined to allow demonstrator beams to be manufactured using the proposed method. A finite-element methodology, that would enable the design of these composite beam structures, was proposed and verified though correlation of simulation performance data with data collected from three point bend tests carried out on test beam structures. Design guidelines including considerations of manufacturing volumes and costs were prepared for use by ADL. Investigations on the feasibility of polycarbonate glazing application within the bus industry identified gaps in the knowledge of lifetime performance of polycarbonate glazing exposed to bus industry specific conditions. A novel testing set-up was designed to assess the performance of commercially available coated polycarbonate glazing exposed to a harsh daily bus washing environments. Following the successful identification of a suitable coating system, a demonstrator manufacture programme was set-up. This led to the successful manufacture and planned installation on in-service buses of polycarbonate glazing panels achieving 57% component weight reduction when compared to the current laminated-glass glazing panel

    Thermal management in electrical machines

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    Apart from protecting electrical machines from temperature related failures, thermal management of electrical machines is employed in the design phase of electrical machines to achieve higher efficiencies and higher power densities. There are many techniques that can be used to predict and monitor the thermal performance of electrical machines. Some of the most common techniques for predicting the thermal performance is the use of lumped capacitance thermal networks and, more recently, Computational Fluid Dynamics (CFD) tools. On the other hand, temperature sensors located in critical areas of the machine windings or winding resistance monitoring are used for machine monitoring during operation. For this Special Issue, which addresses the thermal management of electrical machines, topics of interest included numerical models for heat transfer in electrical machines, CFD analysis of fluid and heat flow in electrical machines, thermal testing of electrical machines, thermal design methodologies used in electrical machines, temperature distribution in electrical machine windings, temperature measurement techniques in rotor and stator windings of electrical machines and common temperature related faults in electrical machines.peer-reviewe

    End winding cooling in electric machines

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    The fluid flow field and temperature distribution within the end region of a Totally Enclosed Fan Cooled (TEFC) induction motor have been investigated both experimentally and using Computational Fluid Dynamics (CFD) techniques, in order to improve the thermal performance. The flow field and the distribution of heat transfer coefficients over the end windings and internal surfaces (mainly frame and end shield) are characterised for a typical end region configuration. This is then used as a base case in order to investigate the impact configuration changes have on the fluid flow field and heat transfer characteristics in the end region of TEFC induction motors. Common parameters governing the flow field inside the end region are varied, allowing recommendations for improved design and further research recommendations to be made. CFD techniques are successfully applied to model the end region, including the copper where the heat generated is a function of temperature. Through these numerical techniques a good understanding of the flow field was made possible which enabled the author to propose and test configuration changes which improve the heat transfer characteristics in the region. These changes were validated experimentally

    End winding cooling in electric machines

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    The fluid flow field and temperature distribution within the end region of a Totally Enclosed Fan Cooled (TEFC) induction motor have been investigated both experimentally and using Computational Fluid Dynamics (CFD) techniques, in order to improve the thermal performance. The flow field and the distribution of heat transfer coefficients over the end windings and internal surfaces (mainly frame and end shield) are characterised for a typical end region configuration. This is then used as a base case in order to investigate the impact configuration changes have on the fluid flow field and heat transfer characteristics in the end region of TEFC induction motors. Common parameters governing the flow field inside the end region are varied, allowing recommendations for improved design and further research recommendations to be made. CFD techniques are successfully applied to model the end region, including the copper where the heat generated is a function of temperature. Through these numerical techniques a good understanding of the flow field was made possible which enabled the author to propose and test configuration changes which improve the heat transfer characteristics in the region. These changes were validated experimentally

    Optimisation and Management of Energy Generated by a Multifunctional MFC-Integrated Composite Chassis for Rail Vehicles

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    With the advancing trend towards lighter and faster rail transport, there is an increasing interest in integrating composite and advanced multifunctional materials in order to infuse smart sensing and monitoring, energy harvesting and wireless capabilities within the otherwise purely mechanical rail structures and the infrastructure. This paper presents a holistic multiphysics numerical study, across both mechanical and electrical domains, that describes an innovative technique of harvesting energy from a piezoelectric micro fiber composites (MFC) built-in composite rail chassis structure. Representative environmental vibration data measured from a rail cabin have been critically leveraged here to help predict the actual vibratory and power output behaviour under service. Time domain mean stress distribution data from the Finite Element simulation were used to predict the raw AC voltage output of the MFCs. Conditioned power output was then calculated using circuit simulation of several state-of-the-art power conditioning circuits. A peak instantaneous rectified power of 181.9 mW was obtained when eight-stage Synchronised Switch Harvesting Capacitors (SSHC) from eight embedded MFCs were located. The results showed that the harvested energy could be sufficient to sustain a self-powered structural health monitoring system with wireless communication capabilities. This study serves as a theoretical foundation of scavenging for vibrational power from the ambient state in a rail environment as well as to pointing to design principles to develop regenerative and power neutral smart vehicles

    Determination of concentration of the aqueous lithium–bromide solution in a vapour absorption refrigeration system by measurement of electrical conductivity and temperature

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    Lithium–bromide/water (LiBr/water) pairs are widely used as working medium in vapour absorption refrigeration systems where the maximum expected temperature and LiBr mass concentration in solution are usually 95 °C and 65%, respectively. Unfortunately, published data on the electrical conductivity of aqueous lithium–bromide solution are few and contradictory. The objective of this paper is to develop an empirical equation for the determination of the concentration of the aqueous lithium–bromide solution during the operation of the vapour absorption refrigeration system when the electrical conductivity and temperature of solution are known. The present study experimentally investigated the electrical conductivity of aqueous lithium–bromide solution at temperatures in the range from 25 °C to 95 °C and concentrations in the range from 45% to 65% by mass using a submersion toroidal conductivity sensor connected to a conductivity meter. The results of the tests have shown this method to be an accurate and efficient way to determine the concentration of aqueous lithium–bromide solution in the vapour absorption refrigeration system.peer-reviewe

    Potential Sea Level Rise Inundation in the Mediterranean: From Susceptibility Assessment to Risk Scenarios for Policy Action

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    Coastal ecosystems and anthropic activities are prone to be affected by the negative impact of marine-related processes induced by climate change, such as erosion, flooding and permanent inundation. Studies aiming at defining potential risk scenarios represent a valuable tool for the identification of the most suitable coastal adaptation measures. After outlining sea level rise implications at the Mediterranean scale, this paper deals with inundation risk scenarios for the years 2050 and 2100 for the north-eastern sector of the Island of Gozo (Malta), central Mediterranean Sea. The analysis, carried out by applying an index-based procedure, firstly required the evaluation of the susceptibility to inundation of the investigated coastal stretch under different sea level projections. Then, the spatial combination of inundation susceptibility with the exposure and vulnerability of the area allowed identification of the most critical sectors in terms of coastal risk. The results of the analysis showed that, under the worst-case climate scenarios, 5.5% and 8.1% of the investigated coastal sector are prone to very high inundation risk (Class R4) in 2050 and 2100, respectively. In particular, the bays of Ramla and Marsalforn, which are characterized by significant economic and touristic activities, were found to be the sites where the expected impacts of future sea level rise will be higher if no management strategy and adaptation action are taken in the near future

    The effect of shank-space on the thermal performance of shallow vertical u-tube ground heat exchangers

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    One parameter that may affect the performance of a ground source heat pump is the shank-space, the center-to-center distance between the two branches of a vertical U-tube used in a ground heat exchanger. A 3D steady-state computational fluid dynamics (CFD) model of a U-tube ground heat exchanger was used to investigate the influence of varying shank-space on the thermal performance of two isolated vertical shallow U-tubes, one 20 m deep and the other 40 m deep, given that most existing research focuses on systems making use of deeper boreholes. The models adopt an innovative approach, whereby the U-junction at the bottom of the U-tube is eliminated, thus facilitating the computational process. The results obtained show that, although the temperature drop across the U-tube varies for different shank-spaces and is lowest and highest for the closest and the widest shank-spaces, respectively, this temperature drop is not linear with increases in shank-space, and the thermal performance improvement drastically diminishes with increasing shank-space. This indicates that, for shallow U-tubes, the temperature drop is more dependent on the length of the pipework.peer-reviewe
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