| 11. |
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| 12. |
- Amiri, Kaveh
(författare)
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Experimental investigation of a Kaplan runner under steady-state and transient operations
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Hydropower is a key part in electricity production nowadays. Hydropower electricity production rose to 3579.5 TWh in 2013, ranked as the second source of electricity production in the world after fossil fuels. It is the principle source of renewable electricity production, producing 16.2% of the electricity in 2013, accounting for 78% of the renewable electricity production in the world. Specifically in Sweden, hydropower is the main source of electricity production producing 47.5% of the required electricity. Nuclear, biomass, and wind placed in the following positions in the ranking in 2013 with 38.4%, 6.5%, and 4.3%, respectively.Besides meeting electricity demand with an environmental-friendly method, hydropower has a unique and important role which is grid regulation: balancing electricity production and consumption. Gas turbines and hydraulic turbines called “Primary reserves” are the only electricity production systems that can be used for fast regulations due to their short start-up time from 1 to 60 s. The obvious environmental problems, air pollution, and costs associated with gas turbines make hydropower a prime alternative whenever applicable. In Sweden, the share of fossil fuels in electricity production is small; 2.8% in 2012 with an average annual growth of -0.7% in the period 2002-2012. Hence, hydropower is practically the only available source used to regulate the grid fluctuations resulting from deregulated market and fast growth of intermittent power generation systems, i.e., solar and wind energy. Hydraulic turbines are subject to frequent off-design and transient operations because of their grid regulation responsibility. Such operating conditions decrease turbine’s efficiency and affect its lifetime significantly. Off-design and transient operation of hydraulic turbines may induce unpredicted pressure fluctuations on the stationary and rotating parts of the turbine. Special attention should be dedicated to the effects of such phenomena on the runner blades because of their importance on the efficiency of the turbine, and their vulnerability to the pressure fluctuations.This thesis presents an experimental investigation on the effects of off-design and transient operation of an axial hydraulic turbine on velocity fields and pressure fluctuations exerted on the runner and the draft tube of a turbine. The investigation was performed on a 1:3.1 scaled model of a Kaplan turbine known as Porjus U9. The main objective was to investigate the effect of operating point on pressure and velocity fluctuations in the runner and the draft tube. Another objective was to study the effect of transient operation on pressure fluctuations exerted on the runner and the draft tube, to investigate the formation and mitigation process of a rotating vortex rope (RVR) within the draft tube. Finally, the effect of the swirl leaving the runner and the draft tube bend on the performance of the turbine was investigated. The study involves pressure measurements on the runner blades and draft tube walls of the turbine, laser Doppler anemometry (LDA) measurements within and after the runner, and particle image velocimetry (PIV) measurements within the draft tube.The pressure and LDA results acquired during steady state operation of the turbine showed different sources of fluctuations on the runner at different operating points resulting in symmetric and asymmetric fluctuating forces on the runner. The pressure measurements during transient operating conditions exhibited pressure fluctuations exerted on the runner during load variations and elucidated some aspects of formation and mitigation process of RVR within the draft tube. The PIV measurements performed after the draft tube bend of the turbine focuses on the physical phenomena resulting in flow asymmetry after the draft tube bend of hydraulic turbines affecting their efficiency.
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| 13. |
- Andersson, Jim
(författare)
-
Systems Analysis of Chemicals Production via Integrated Entrained Flow Biomass Gasification : Quantification and improvement of techno-economic performance
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Lignocellulosic biomass gasification is a promising production pathway for green chemicals, which can support the development towards a more sustainable society where fossil fuels are replaced. To be able to compete with fossil fuels, a highly efficient production of biomassbased products is required in order to maximize overall process economics and to minimizenegative environmental impact. Large production plants will likely be required to obtain favourable economy-of-scale effects and reasonable production cost. Entrained flow gasification (EFG) is a favourable technology due to its suitability for large-scale implementation and ability to produce a high quality syngas from various biomass feedstocks. In order to estimate overall energy efficiency and production costs for gasification-based biorefineries, it is important to be able to characterise the gasifiers’ performance. This in turnrequires reliable estimations of the gasification process. Integration of EFG-based biorefineries with existing pulp mills or other large-scale forestindustries can be achieved by integration of material and/or energy flows, as well as by coutilisation of process equipment. This could potentially induce both technical and economic added-values. At chemical pulp mills, an important feedstock for green chemical production may be the black liquor from the pulp production, since it provides an attractive combinationof advantages. The black liquor availability is, however, directly correlated to the pulp production (i.e. the mill size) and the potential green chemical production volume via pure black liquor gasification (BLG) is therefore limited.In this thesis, two systems are considered that expand on the BLG concept with the intent to increase the chemical production volume, since this could generate positive economy-of-scale effects and is a rather unexamined area. In addition to this, an EFG configuration entailing a lower availability related risk for the considered host pulp mill is also considered. The threeconsidered integrated systems are: (i) co-gasification of biomass-based pyrolysis oil blended with black liquor for methanol production, (ii) parallel operation of BLG and solid biomass EFG for methanol or ammonia production, and (iii) replacing the bark boiler with solid biomass EFG for methanol or ammonia production. These system solutions establish a combination of material, energy and equipment integration. The main aim of this thesis is to increase the knowledge of the characteristics of entrained flow biomass gasification systems and their opportunities for integration in existing industries for production of green chemicals (methanol and ammonia). An appropriate modelling framework that combines chemical modelling on a high level of detail with holistic industrial site modelling is designed and used to identify and quantify energetic and economic addedvalues for the integrated biorefineries. Mathematical process integration models based on Mixed Integer Linear Programming (MILP) of pulp mills are used to study integration of the biomass gasification systems with the mills. An iterative modelling approach is applied between the process integration model and the detailed biomass gasification models based on Aspen Plus or a Matlab-based thermodynamic equilibrium model. As a complement to themodelling framework, a multi-scale equivalent reactor network (ERN) solid biomass-based EFG model is developed to be able to identify and study influential parameters on the gasifiers’ performance in the Aspen Plus platform. This is approached by considering the effect of mass and heat transfer as well as chemical kinetics. The results show that replacing a recovery or a bark boiler with EFG for green chemicals production improves the overall energy system efficiency and the economic performance,compared to the original operation mode of the mill as well as compared to a stand-alone gasification plant. Significant economy-of-scale effects can be obtained from co-gasification of black liquor and pyrolysis oil. Co-gasification will add extra revenue per produced unit of methanol and reduces the production cost significantly compared to gasification of pure pyrolysis oil. In general, integrated EFG systems producing methanol sold to replace fossilgasoline are shown to constitute attractive investments if the product is exempted from taxes. Ammonia produced via EFG is per unit of produced chemical significantly more capital intense than the corresponding system producing methanol. The economic viability in the considered ammonia configurations is therefore found to be lower compared to methanol.The ERN model is shown to be able to estimate key performance indicators such as carbon conversion, cold gas efficiency, syngas composition, etc. for a real gasification process, showing good agreement with experimental results obtained from a pilot scale gasifier. This simulation tool can in future work be implemented in more global models to study and use to improve the techno-economic performance of EFG-based biorefineries, by quantifying theinfluence of important operational parameters. The main conclusion from this work is that production of green chemicals from biomass EFG integrated with a pulp mill is techno-economically advantageous compared to stand-alonealternatives. It is also concluded that the pulp mill size will be decisive for what integration route is the most favourable. Integration of an individual BLG plant with a pulp mill of maximum size would be the most economically beneficial alternative. However, the possibility to increase the green chemical production from a given black liquor volume improves the viability for integration in smaller mills. Increasing the production volume would therefore result in the highest efficiency and economic benefits given mill sizes up to300 kADt/y. From a resource perspective, this would however lead to an increased demand for biomass import to the mill, and this expansion could be limited by the overall availability of biomass resources.Keywords: Pulp mills, integration, biomass, gasification, green chemicals, methanol, ammonia.
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| 14. |
- Asplund, Matthias
(författare)
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Wayside Condition Monitoring System for Railway Wheel Profiles : Applications and Performance Assessment
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The railway is an important mode of transport, due to its environmental friendliness, high safety level, and low energy consumption combined with a high transport capacity, among other factors. The Swedish railway network is old, there has been almost no expansion of the network during the past few decades, and more traffic is expected. Therefore, there is currently a demand for more track capacity and, in the short term, the existing network is expected to deliver the increased capacity. The railway operators in the network have a large impact on train delays, and wheel failures are one large contributor of delays. Delays destroy capacity and, therefore, capacityconsuming failures, such as abnormal wheels, need to be minimised. This can be achieved by using appropriate condition monitoring for the wheels on the track to find potential capacity consumers before failures happen. Therefore, the condition of the wheel-rail interface is important, since the state of the wheel influences that of the rail and vice versa. The monitoring of rail profiles is already being performed, but the monitoring of wheel profiles is still in the development phase. This thesis treats the applications and performance assessment of a wheel profile measurement system (WPMS), and presents case studies focusing on its system and measurement performance. The proposed applications concern how the information from the WPMS can be integrated with information from other data sources and with physical models to obtain a true current picture of the wheel behaviour. The thesis investigates the measurement performance of the WPMS by using a paired T-test and a number of quality measures, e.g. the reproducibility and repeatability, the precision-to-tolerance ratio and the signal-to-noise ratio. In conclusion, this thesis shows that the WPMS works well with an expected level of reliability in a harsh climate with respect to its measurement and system performance. By combining other data with the data from the WPMS, potentially abnormal wheels can be found in an early stage if the proposed new maintenance limit for the wheel parameter of the flange height is implemented. Furthermore, through adding a physical model to the process, the real contact condition of the actual wheel-rail interface can be evaluated and measurement deviations can be found. However, the wheel parameters, as well as the entire profile, need a high measurement quality with little variation, which seems to be an issue with respect to the measurement performance when advanced calculations are to be done. Therefore, a new approach for evaluating measurement performance has been developed using established statistical tools and quality measures with predefined acceptance limits; with the help of this approach, one can differentiate between the variation in the measurements originating in the different measurement units and the variation originating in the wheels. This new approach can be applied to judge the measurement performance of wheel profile condition-monitoring systems, and can also be implemented for other condition-monitoring systems to evaluate their measurement performance. Finally, this approach promotes the development of a condition-based maintenance policy by providing more reliable information for maintenance decision makers.
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| 15. |
- Atashipour, Seyed Rasoul
(författare)
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Structural Analysis of Deep Composite Box-Type Components with application to a proprietary stabilising timber wall element
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The stabilising wall element of the Trä8-system is a deep box-type beam/column element which is made of different types of timber composites including framing members of gluedlaminated timber (glulam) and sheathings of laminated veneer lumber (LVL). This element is used as a ―shear wall‖ in multi-storey buildings up to four storeys. To be able to optimise thecomposite stabilising element and use its maximum potential and efficiency, it is necessary to have sufficient fundamental knowledge and information about the structural behaviour and influences of different mechanical properties and geometrical parameters and dimensions of the sub-elements.In this thesis, structural analysis and design principles of the Trä8 stabilising wall element are discussed, including the early stage of erection during assembly to the final usage and residence. A list of required fundamental basic analyses are presented, including accurate deflection analysis for the serviceability limit state, local pre- and post-buckling of the LVL sheathings, global buckling as well as the lateral-torsional buckling of the stabilising element in different design situations. This thesis is then focused on deformations and local instability. For each type of structural problem, accurate, but simple and general methodology is employed to incorporate a large number of effective geometric and property parameters. In this way, a fast evaluation of the influence of different parameters is possible for a wide range of values without needing several time-consuming 3-D finite element (FE) simulations. In several cases, the accuracy and validity of the obtained solutions and formulae are examined and confirmed by comparing their results to those based on the 3-D FE simulations. First, different composite beam theories including shear effects are employed for deflection analysis of the stabilising timber element. Next, an accurate energy-based methodology isdeveloped rendering an explicit formula that gives accurate predictions for the deflection of the stabilising element within a medium range of sheathing or web thicknesses and/or stiffnesses. For weak shear webs, however, this model is not sufficient. Therefore, another new model based on the partial composite interaction theory is developed for accurate deflection predictions of the element having weak shear webs. This model contains simple but accurate formulae for deflection analysis of any similar composite box-type of element with an arbitrary range of geometry and property parameters.Next, shear buckling of the rectangular LVL-panel between the framing members are analysed using differential quadrature (DQ) numerical technique as well as an explicit analytical solution and a formula is established for accurate and fast prediction of the shear pre-buckling of the LVL. This study is extended by considering the effect of each individual lamina of the LVL as well as general orthotropic material properties using a laminated theoryand the DQ solution approach. Further, the post-buckling of the LVL panel is studied based on a simple analytical method with rotating stress fields together with the accurate 3-D FE simulations. A simple formula is then proposed for accurate prediction of the shear post-buckling resistance of the LVL-panel in the stabilising element. As a result of the thesis concerning the two main focus areas, deformations and local instability, explicit analytical formulae are presented for the design of the box-type stabilisingelement with respect to deflections including shear effects in the serviceability limit state and with respect to local shear buckling of the sheathings of the stabilising element including the post-critical area in the ultimate limit state.
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| 16. |
- Backman, Ylva
(författare)
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Students’ Reasoning about Learning and Well-being in School : On the epistemic privilege of Swedish early adolescent students
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The recent emphasis on performance in standardized testing of Swedish students is visible in reports from both national and international agencies. According to the reports, there has been a steep decline in the internationally measured performance of Swedish students in school since the 1990s. In fact, the performance of Swedish students in the Programme for International Student Assessment (PISA) has declined more dramatically than any other OECD country in the last decade. This has influenced a national debate about factors promoting knowledge acquisition. Alongside the drop in performance, young Swedish people’s mental problems have increased since the 1990s. The impact of one’s well-being on performance has been widely acknowledged in previous research, and two explicit targets in both current andprevious Swedish national curriculum for the compulsory school are learning and well-being. Meanwhile, the importance of encouraging early adolescents’ (10–14 years) participation in improving their own standards is acknowledged in several recent reports from influential non-governmental organizations such as the WHO and UNICEF. However, advancing students’ development requires a keen understanding of their current situation, something impeded by the fact that data on early adolescents is relatively scarce. Furthermore, a large amount of previous research has been focused on risk factors related to illness in children, rather than factors promoting well-being. The aim of this thesis is to explore early adolescent students’ reasoning about learning and well-being in school. A qualitative design with open-ended writing tasksand interview questions was constructed in order to facilitate the incorporation of students’ reasoning about learning and well-being into the research, in accordance with the study’s salutogenic point of departure and the specific standpoint epistemology that I propose and use in the thesis.The empirical data includes (1) written reflections by 200 students in grades 5–9 from 11 classes in four different schools (rural and urban), which were part of the Swedish compulsory school system and which were located in two municipalities in the northern part of Sweden, and (2) interviews with 24 students, from 12 to 15 years old, from two municipally run schools, which were also part of the compulsory school program in northern Sweden. Four sub-studies were conducted and presented in four journal articles. The data in sub-studies I, II, and IV consisted of the written reflections described above, while the data in sub-study III consisted of the interviews described above. The first sub-study concerned early adolescent students’ previous positive experiences and indicated that the students found aspects both within and beyond the classroom relevant for having a good time in school. In more detail, the students’ positive experiences concerned (i) interaction with teachers, (ii) freedom of choice regarding work and workmates, (iii) the atmosphere for discussions, (iv) school subjects and success, (v) learning processes in outings, (vi) friends, and (vii) primary (basic) needs. The results show a perceived complex relation between learning and well-being in school, which may well be studied further in future research. The second sub-study investigated students’ preferred states of affairs. The students emphasized a variety of views of what kind of structures, content, actions and attitudesmay have a positive impact on the learning environment in school. The variousreflections provided by the students were understood as falling under four themes: (i) influencing educational settings; (ii) striving for reciprocity; (iii) managing time struggles; and (iv) satisfying well-being needs. Besides providing perceived opportunities for change in school concurring with previous research, the results also point towards great variety and certain inconsistencies in the students’ perceptions about factors promoting learning.In the third sub-study, the interview data was analyzed using a distinction between decision methods and criteria of rightness, which has rarely been used in empirical research. Six forms of variety in the students’ moral reasoning were found, denoted as follows: (i) interpersonal variety in decision method dimension, (ii) intrapersonal variety in decision method dimension, (iii) interpersonal variety in criterion of rightness dimension, (iv) intrapersonal variety in criterion of rightness dimension, (v) interpersonal variety between the two dimensions, and (vi) intrapersonal variety between the two dimensions. The use of the distinction between decision methods and criteria of rightness enabled a nuanced understanding of varieties in students’moral reasoning, and may be useful in large-scale studies, for example, to explore the rate of occurrence of each respective form of variety. Future research could also examine the potential context dependence of the different forms of varieties. In the fourth sub-study, five perceived bidirectional crossovers of subjective wellbeing (i.e., perceived two-way transmissions between happiness and other aspects within the school domain) were noted in the students’ written reflections. These were: (i) happiness and learning, (ii) happiness and school engagement, (iii) happiness and appreciation of subjects or lesson content, (iv) happiness and others’ happiness, and (v) happiness and prosocial behavior. Besides providing novel hypotheses aboutbidirectional crossovers of subjective well-being, this sub-study supplied a systematic framework for interpreting qualitative outputs and provided conceptual as well as analytical direction for future research about happiness in education. I provide a definition of the formal expression “bidirectional crossovers” of subjective well-being, which may provide analytic categories for future research, while the more informal expression “circles of happiness” could be used in education, allowing teachers and students to conceptualize everyday events in the classroom. In the final parts of this thesis, students’ reasoning about the conditions for and effects of learning and well-being in school in all the four sub-studies are compared with previous empirical research through the use of established and novel definitions and theory. The comparison shows considerable correspondence between the students’ reasoning and previous research, providing evidence to the effect thatSwedish early adolescent students’ reasoning is trustworthy regarding the conditions for and effects of learning and well-being in school. There are also novel researchgenerating hypotheses formulated directly based on the students’ reasoning. The trustworthiness and novelty of the students’ reasoning about the conditions and effects of learning and well-being in school provides evidence for an epistemic privilege for the students participating in this study. This constitutes a starting point for futureresearch in which the claims about epistemic privilege in the standpoint epistemology proposed here can be further tested for Swedish early adolescent students in general.KEYWORDS: education, epistemic privilege, happiness, learning, moral reasoning, standpoint epistemology, well-being
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| 17. |
- Borris, Matthias
(författare)
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Future trends in urban stormwater quality : effects of changes in climate, catchment characteristics and processes and socio-economic factors
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Climate change and progressing urbanization cause numerous environmental concerns, including the impacts on urban drainage. Such impacts were addressed during the last two decades with focus on hydraulic overloading of drainage systems and the means of overload remediation by stormwater management. However, modern urban drainage also serves to provide and protect broad environmental services chiefly by controlling stormwater quality. During the past 40 years, a sizeable investment has been made in urban drainage systems to improve stormwater quality and protect receiving water ecosystems. Such investments are at risks, because of impaired performance of stormwater quality controls now and in the future for the following reasons: (i) Hydraulic and Pollution overloading (ii) the aging of stormwater management systems and (iii) insufficient attention paid to socio-economic issues. The primary objectives of the thesis that follows is to address the above issues by examining future trends in stormwater quality and the influential factors affecting these trends. Trends in urban stormwater quality, in response to projected changes in the climate, urban catchments and their drainage systems, and environmental practices and policies, were studied by systematically describing these changes by a set of scenarios, which were then applied to several test catchments in simulations with two well-established computer models of urban drainage (US EPA SWMM and WinSLAMM). In runoff simulations, stormwater quality was described Total suspended solids (TSS) and three heavy metals, namely Cu, Pb and Zn. The assessment of uncertainties in the simulation process and potential changes in sewer pipe materials further inspired two additional studies: Potential improvements in modelling trace metal transport and control by clarifying the role of coarse sediments on road surfaces, and water quality implications of using sewer pipes made from three different materials. Simulations with up-scaled rainfall data produced changes in stormwater quality, depending on the type of storm events. Generally pollutant loads increased due to climate changes characterized by higher depths and intensities of rainfall in future scenarios. Storms with low to intermediate depths and intensities showed the highest sensitivities to climatic changes, because runoff producing areas increased with higher storm intensities (i.e., leading to contributions of pervious areas), and sufficient pollutant supplies on catchment surfaces; for high intensity events, such supplies were quickly exhausted. TSS loads exported from catchments with low imperviousness were most sensitive to climatic changes, but the magnitudes of TSS loads were low compared to those from catchments with high imperviousness. Furthermore, potential changes in catchment characteristics and drainage systems were identified to be of importance. Future scenarios combining changes in climate and socio-economic factors showed that the impacts on stormwater quality caused by climatic changes were smaller than those caused by changes in socio-economic factors. However, future urbanization impacts on stormwater quality could be controlled by incorporating modern stormwater management measures in future catchments. Simulations of such controls indicated that they were highly effective in protecting the stormwater quality. Finally it was noted that the two applied computer models produced somewhat different results and high uncertainties when assessing the future stormwater quality. This was due to their different descriptions of the underlying processes. Hence, it was desirable to examine the feasibility of improving stormwater quality modelling, particularly with respect to heavy metals. During laboratory experiments coarse particles were identified to potentially release significant amounts of heavy metals (mostly in the particulate bound phase) during runoff events. Site/runoff event specific factors (e.g., traffic intensity and street sweeping routines) and characteristics of the particles (i.e. organic content) were identified as influential factors affecting the release of heavy metals. This finding may help improve the description of pollutant transport processes in stormwater quality models. Laboratory experiments showed that various pipe materials (PVC, concrete and corrugated steel) affected the stormwater quality differently, depending on the characteristics of the stormwater used in experiments. The concrete pipe contributed to increase pH of the transported stormwater. Metal concentrations were mostly unaffected in the PVC pipe, decreased in the concrete pipe (due to particle deposition and metal adsorption to the pipe surface), and while Zn concentrations increased in the corrugated steel pipe due to elution, Cu and Pb concentrations were reduced (due to particle deposition in the corrugations). Since the impact of climatic changes on stormwater quality was relatively small compared to changes in socio-economic factors, future efforts to maintain or improve stormwater quality should focus on implementing pollutant abatement strategies, including implementation of well-designed and maintained stormwater treatment measures.
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| 18. |
- Broström, Robert
(författare)
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Design implications for visual-manual interfaces in cars
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- During the past decades there has been an extensive evolution in computer and communication technologies. In the last ten years these technologies have become an integrated part of our cars via in-vehicle information systems. Considerable efforts are put into research and development of the interface between the in-vehicle information systems and the driver. Due to the fact that vision is imperative in driving, the focus is in particular on the visual-manual interface and on apt usability evaluation methods.The purpose of this thesis is to study design implications for the development of visual-manual interfaces in cars. A point of departure is that the practice of the usability concept, usability evaluation methods, and compliance tests has implications for the design process.Four papers are appended to the thesis. Paper I focuses on differences between rotary control and touch screen interfaces in terms of driving simulator and glance metrics. Paper II investigates correlations between data from customer evaluations and expert evaluations for visual-manual interfaces. Paper III searches for the presence of and possible explanations for individual glance strategies in a driving simulator study. Paper IV investigates effects of individual glance strategies on the NHTSA visual-manual distraction test in a driving simulator set-up.In the thesis it is concluded that the optimal input device is dependent on the task being performed, that is, the touch screen is more efficient for alphanumeric input while rotary control performs list scrolling more efficiently. In addition, drivers seem to adapt their glance behaviour based on the display position. It is evident that efficiency metrics correlate between several usability evaluation methods. Hence, it is possible to generalise results from expert evaluations at an early design phase, to customer survey samples based on car ownership. Concerning glance behaviour, it is clear that individual glance strategies exist. Especially interesting are two glance strategies, long glancers and frequent glancers, that affect the results of the NHTSA compliance testing. These glance strategies have to be controlled for in the test situation. Finally, a framework is proposed that combines a classification of visual-manual actions with different visual fixations. The task analysis framework has the potential to analyse actions in terms of required visual resources, thus already providing an evaluation of visual-manual interfaces in the concept phase.
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| 19. |
- Doroudgarian, Newsha
(författare)
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High Performance Bio-based Composites : Mechanical and Environmental Durability
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The presented work is a part of the ongoing effort on the development of high performance bio-based composites with enhanced durability, under static and dynamic mechanical loading including the exposure to elevated humidity. The impact of relative humidity on the performance of cellulosic fibers (natural and regenerated), bio-based resins and their composites was studied. The material performance was rated against the data for glass fiber epoxy, as the reference. The comparison of water absorption results for unreinforced resins and for composites showed that the cellulosic reinforcement is primarily responsible for the transport and uptake of moisture in the composites. The effect of chemical treatment on the cellulosic fibers, as a protection against moisture, was evaluated. However, the treatment did not improve the moisture resistance in composites significantly. Quasi-static tensile tests revealed that some of the bio-based resins and their composites performed very well and comparable to the composites of synthetic epoxy, even at high humidity. However, any structural material is supposed to hold mechanical loads over a long service time and most often in harsh environmental conditions. Hence, tension-tension fatigue tests were performed on the fiber bundles as well as on the composites. The fibers of choice as the reinforcement for further mechanical testing were regenerated cellulose fibers (RCF), mainly owing to the stable geometry and properties. Due to the high nonlinearity of RCF, the fatigue tests were limited in number and the focus was on analyzing the mechanisms underlying the fatigue behavior rather than on constructing S-N curves. Strain evolution of the bio-based composites during the dynamic fatigue was very similar to that observed in the static fatigue (creep). It confirmed the strong influence of viscoelastic and viscoplastic phenomena on the overall performance of the material under the rapid loading conditions in fatigue. Since the durability of composites greatly depends on the material’s ability to stand the internal damages (e.g. debonding, microcracking, delaminations), the interfacial properties in the bio-based composites were addressed. To investigate the fracture toughness of bio-based composites, the double cantilever beam (DCB) tests were carried out, under static and dynamic loading. Moreover, the DCB results were utilized as a measure of the fiber chemical treatment’s efficiency to improve the adhesion between RCF and the resin. The nonlinearity of RCF strongly influenced the results obtained from DCB tests, which complicated the analysis regarding the effectiveness of the fiber surface treatment. Nevertheless, this study brings forward the issues that have to be dealt with, in order to characterize and predict the performance of these composite materials with highly nonlinear reinforcing fibers. Overall, the results presented in this thesis give an insight into the behavior of bio-based composites, at various environments and under different types of mechanical loading. Based on these findings, the potential use of these materials in structural applications can be assessed.
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| 20. |
- Filippov, Andrei
(författare)
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Self-Diffusion and Microstructure of Some Ionic Liquids in Bulk and in Confinement
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An ionic liquid (IL) is a salt, which usually is in the liquid state at normal temperature and pressure. The properties of ILs can be adjusted for various processes and applications by choosing different combinations of ions. Similar to other salts, ILs contain only ions with positive (cations) and negative (anions) charges in equal proportions. However, to prevent solidification, ions in ionic liquids usually contain bulky organic chemical groups, which, apart from electrostatic interactions, promote other types of interactions between ions, such as: (i) van-der-Waals interactions; (ii) hydrogen bonding; (iii) - stacking, etc., depending on the particular chemical structure of the ions. All these interactions, in combination, may lead to formation of specific microstructures in ILs, which may vary with temperature caused by changing thermal rotational and translational energies of the ions. Ions in these microstructures may have preferential orientations relative to each other, maintain anisotropic properties similar to those in liquid crystals or, in some specific cases, may even separate into microscopically organised liquid phases. Therefore, the dynamics of ILs may also be dependent on their microstructure. In many practical applications ionic liquids are placed on surfaces or in confinements. Solid surfaces introduce extra forces, which may be specific to the charge of the ions or/and to functional groups in the ILs. The geometry and interactions of ions in confinements or/and pores of materials may also disrupt specific bulk microstructures of ILs. Both confinement effects and interactions of ions with surfaces are manifested in the translational dynamics of the ions. One of the most direct and informative methods to study translational dynamics of ILs is pulse-field-gradient nuclear magnetic resonance (PFG-NMR).In this thesis the results of PFG-NMR studies on a few classes of ILs are reported: (i) the historically “standard” (since Walden’s discovery in 1914) ionic liquid, the ethylammonium nitrate (EAN) and (ii) halogen-free orthoborate-based phosphonium, imidazolium and pyrrolidinium ILs with varied structure and lengths of alkyl chains in cations, and varied structures of orthoborate anions. These ILs were studied in bulk at different temperatures, and also in confinements, such as between parallel glass and Teflon plates and in mesoporous Vycor glass. It was found that diffusion coefficients of cations and anions in EAN, phosphonium and pyrrolidinium orthoborate ILs in bulk are different, but according to the standard Stocks-Einstein model, they correspond to diffusion of ions in homogeneous liquids. A change in the chemical structure of one of the ions results in a change in both the diffusion coefficient of the oppositely charged ion and the activation energy of diffusion for both ions in an IL. Similar effects were observed from the chemical shifts and diffusion coefficients measured by NMR for imidazolium orthoborate ILs dissolved in polyethylene glycol solutions, in which imidazolium cations strongly interact with PEG molecules, further affecting the diffusion of orthoborate anions via electrostatic interactions. A liquid-liquid phase separation was suggested for a few phosphonium and pyrrolidinium bis(mandelato)borate ILs, in which a divergence of diffusion coefficients and activation energies of diffusion for cations and anions was detected at temperatures below ca 50 °C. In addition, a free-volume theory was invoked to explain the dependences of density of ILs on the alkyl chain length in cations.It was also found that for a phosphonium bis(salicylato)borate IL confined in 4 nm mesoporous Vycor glass the diffusion coefficients of ions increase by a factor of 35! This phenomenon was explained by the dynamic heterogeneity of this IL in micropores and empty voids of the Vycor glass. For EAN IL in confinements between glass and Teflon plates, the diffusion of ethylammonium cations and nitrate anions is significantly anisotropic, i.e. slower in the direction of the normal to the plates and faster along the plates compared to diffusion of the ions in bulk. A plausible explanation of this PFG NMR data is that EAN forms layers near polar and non-polar solid surfaces. A similar phenomenon, to a lesser extent, was also observed for phosphonium cations of bis(mandelato)borate, bis(salicylato)borate and bis(oxalato)borate confined between glass plates. The results of these studies may have implications in modeling tribological performance, i.e., friction and wear reduction for contact pairs of different materials lubricated by various classes of ionic liquids.
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