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Sökning: L4X0:1402 1544 > (2015-2019) > (2016)

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71.
  • Zhang, Yingying (författare)
  • Thermodynamic Analysis and Screening ILs/DESs-based Absorbents for CO2 Separation
  • 2016
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • CO2 separation plays an important role in both biofuel production, and CO2 capture and storage (CCS) implementation to deal with global warming. The available CO2 separation technologies are either energy-intensive or require large-scale operations, and it is crucial to develop novel CO2 separation technology in order to optimize the energy uses and the amounts of CO2-absorbents/adsorbents.Recently, ionic liquids (ILs) have been proposed as potential liquid absorbents for CO2 separation with remarkable properties. A lot of ILs have been synthesized for this purpose. The CO2 absorption capacity/selectivity and the energy use have been considered in screening ILs, while the amounts of ILs needed have seldom been considered in the screening process. Meanwhile, the high-cost, toxicity and poor biodegradability of the conventional ILs limit their applications in large-scale. Deep eutectic solvents (DESs) have emerged as a new type of ILs, and in particular, those based on choline salts (i.e. choline-based DESs) show additional advantages in cost, environmental impact and synthesis. Choline-based DESs have been synthesized and the research work related to CO2 separation with this series of DESs and their aqueous solutions has been carried out. However, it is still unclear which absorbent can achieve a better performance for CO2 separation.The choice of absorbents for CO2 separation depends on gas streams, and the performances of absorbents for CO2 separation relate to the energy uses and the amounts of absorbents needed. In this thesis work, four gas streams (i.e. flue gas and lime kiln gas from the combustion of fossil-fuels, biogas from the anaerobic digestion of biomass as well as bio-syngas from the gasification of biomass) with different temperature, pressure, CO2 concentration and gaseous components were considered, and CO2 separation from four gas streams was analyzed thermodynamically based on Gibbs free energy change. The analysis shows that biogas is the CO2 stream with the lowest theoretical energy penalty. Therefore, biogas was chosen as a specific CO2 stream for further evaluating the performances of CO2 absorbents.In evaluation, the conventional ILs were first analyzed and screened for CO2 separation from biogas with three options (i.e. option 1: the CO2 dissolution enthalpy and CO2 working capacity, option 2: the energy use, and option 3: the energy use and the amount of IL needed). The investigation shows that the screen of ILs is strongly related to the operational condition and the screening criteria. In the option of “the energy use and the amount of IL needed”, the operational condition was optimized based on the minimum Gibbs free energy change, and the energy use and the amount of IL needed were considered in screening. While in other screening options, the operational conditions were presumed and the amounts of ILs needed were not considered. Therefore, the option of “the energy use and the amount of IL needed” is more reasonable compared to the other two options. The performances of these screened conventional ILs were further compared with those of the commercial CO2 absorbents. It shows that the conventional ILs are promising CO2 absorbents due to lower energy uses or lower amounts of ILs needed combined with the advantage of non-volatility.The research work on choline-based DESs and their aqueous solutions for CO2 separation was surveyed and reviewed. Generally, the properties of choline-based DESs are similar to those of conventional ILs. Considering the additional advantages of low-cost, non-toxicity and biodegradability, choline-based DESs are more promising for CO2 separation. However, due to the limited available research work, further studies need to be carried out from experimental measurements to model developments. The performances of choline-based-DESs for CO2 separation from biogas were analyzed. Based on the option of “the energy use and the amount of absorbent needed”, the choline-based-DESs were screened and then compared with the conventional ILs and the commercial CO2 absorbents. The comparison results show that the choline-based-DESs are more promising for CO2 separation from biogas due to the non-volatility, lower energy uses or lower amounts of absorbents needed. In addition, CO2 separation from other CO2 streams was further investigated. It shows that the physical absorbents are more suitable for the CO2 streams with high CO2 concentration (i.e. biogas, lime kiln gas and bio-syngas), while the chemical CO2 absorbents are more suitable for that with low CO2 concentration and high temperature (i.e. flue gas). Considering the high amounts of physical absorbents, further study needs to be carried out with techno-economic analysis.
72.
  • Zrida, Hana (författare)
  • Composites with bundle mesostructure: Elastic properties and Damage
  • 2016
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • Many types of composite materials are today used in various types of load carrying structures, due to their excellent strength and stiffness to weight ratio. Simplicity, reliability and low cost of the material processing are important factors affecting the final selection.With the textile reinforced composites, the cost-efficiency is reached by using dry preforms which are impregnated by resin infusion, resin transfer molding etc.; this have made a break-through and have been widely used. Textile composites with bundle meso-structure have been studied in this thesis for elastic properties and damage investigations. The first part of this thesis deals with elastic properties modeling for Non-crimp fabric (NCF) based composites for investigating the effect of meso-structure defects on mechanical properties degradation. The objective of the work is to formulate a model for the NCF composite mesostructure in an attempt to investigate the effect of the waviness on stiffness reduction. Moreover, the stiffness calculation methods for the complex geometry are explained and justified and finally, the different geometrical parameters changes are taken into consideration and included in the calculation.The damage initiation and development is presented is the second part, where woven fabric composites designated for high temperature application were investigated under severe thermal conditions to study their thermal stability and their resistance to thermal damage. The mechanical performance of the same composites was studied. The effect of aging was also investigated. 3D models were realized with Finite elements in order to explain the edge effect on the evolution of the cracks observed during the tensile tests. In addition, the differences and similarities in cracking in different layers were analysed using probabilistic approaches (a simple one as well as Monte Carlo simulations with Hashin’s and also shear lag model) and fracture mechanics arguments.
73.
  • Åkerfeldt, Pia (författare)
  • Additive Manufacturing of Ti-6Al-4V: Relationship between Microstructure, Defects and Mechanical Properties
  • 2016
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • Additive manufacturing (AM) is a relatively new technology that is labelled to be innovative, disruptive, near-net shaping, enabling manufacturing of complex and customised products, for limitless number of applications, directly from the CAD model into real physical parts. For titanium alloys in aerospace applications, AM moreover stands for a reduced material cost, but also for large challenges when considering consistency and qualification of material properties and components in serial production. In the AM process the feedstock material is melted by a heat source that moves according to a building sequence defined by the CAD model. Layer-by-layer the material solidifies into the wanted shape and accordingly the microstructure forms,which determines the average mechanical properties of the manufactured component. However, even if the AM process seems to be very straight forward, the prediction of mechanical and metallurgical properties is complex, partly because of its building in layer nature which generates a complex thermal history dictating the mechanical properties, and partly because of the number of parameters involved during the AM process itself. The objective of the present work was to increase the fundamental understanding of the relationship between microstructure, defects and mechanicalproperties of AM:ed Ti-6Al-4V. Three AM techniques were investigated, namely laser metal-wire deposition (LMwD), electron beam melting (EBM), and gas tungsten arc welding (GTAW) wire feed AM, with the main focus on LMwD. The different techniques were evaluated with regard to microstructure and tensile and fatigue properties. In addition, the EBM Ti-6Al-4V was tested in a hydrogen atmosphere to simulate the working environment for a certain engine application. One of the core findings in the present work was that AM:ed Ti-6Al-4V exhibited a columnar microstructure with elongated prior beta grains growing through several layers following the temperature gradient direction in the built material. To cover the different characteristics of the columnar microstructure, the mechanical properties were evaluated in two orientations of the built Ti-6Al-4V. The mechanical properties, both static and dynamic, were found to be anisotropic, which was further evaluated indetail with respect to the microstructure evolution and defects generated by the AM process. Among the results, when different process conditions were tested, it was concluded that the thickness of the grain boundary alpha along the prior beta grain boundary did not influence the level of anisotropy. However, the prior beta grain boundary was observed to be the weakest microconstituent when the load was applied perpendicular to its prevalence in both tensile and LCF testing. In order to get a better understanding of how the columnar microstructure influences the fatigue properties, the fatigue crack propagation characteristics were investigated with respect to the columnar prior beta grains and crystal orientation. An extensive fractographic study was carried out on all tested specimens. Lack of fusion (LoF) defects were concluded to be the individually most detrimental type of defect to the material properties. The influence of the LoF defects was further concluded to be very dependent on its prevalence in relation to the loading direction; the largest impact on the fatigue life was observed when the LoF defect wasperpendicular to the loading direction. Finally, a part of the aim of the present work was to support the development of a microstructure model that will be implemented in a thermo-mechanical model when simulating AM of Ti-6Al-4V. In order to validate the material model developed, the alpha lath thickness and the fraction of grain boundary alpha were quantified atspecific locations in single and multiple bead walls of GTAW wire feed AM:ed Ti-6Al-4V and compared with the results of the simulated AM process of Ti-6Al-4V.
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