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Biomass and waste gasification and cogeneration by the help of microturbine
Kašpar, Matěj ; Najser,, Jan (referee) ; Baláš, Marek (advisor)
This diploma thesis investigate the question of combined production of heat and power by the gasification of biomass. The introduction describes basic dilemma energy conversion from biomass. Further, special attention at the gasification technology and it´s usage at combined heat and power production is paid. The main section includes schematic proposal cycle with use of microturbine 80 [kWe] as well as basic calculation, which serves as a base for folowing project optimization. For discussed optimization is made concept of configuration, thermal and aerodynamics calculation for both heat exchangers B1 and B2. Last but not least these heat exangers were compared by different aspects such as influence of deposits on power transfer and pressure loss. At the end of the work calculated numeric values are discussed and the project of heat exchanger with it´s advantages for practise are summarised.
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Effect of thermal treatment on mechanical behaviour of Černý vrch bentonite
Slezáková, Pavlína ; Najser, Jan (advisor) ; Hausmannová, Lucie (referee)
The storage of highly radioactive waste from nuclear power plants is currently planned to be solved in the Czech Republic by the construction of a deep repository with a multibarrier concept. The key component of one of the barriers is bentonite, whose sealing properties will help to ensure optimal conditions for the disposal of radioactive waste. The focus of the thesis is on a review of the knowledge on the mechanical properties of bentonite and their change after exposure to high temperatures. The review is complemented by the current knowledge of the czech bentonite BCV from the Černý Vrch locality, which is also the material for the laboratory measurements carried out in this thesis. The key part of the work consists of laboratory tests, necessary for experimental verification of the discussed bentonite properties. Internally developed multipurpose cells (MPC cells) were used for the majority of the laboratory tests, as the standard equipment of soil mechanics laboratories has relatively little application for bentonite barrier research. The aim of the experimental part was to investigate how the swelling capacity and hydraulic conductivity of the material change due to previous thermal loading. The main objective of the work is to obtain an overview of the behaviour of the Czech Ca-Mg...
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Advantages and disadvatages of a hypoplastic model when used in modelling of deep excavations
Jelenová, Věra ; Mašín, David (advisor) ; Najser, Jan (referee)
In building construction, it is important to know in advance how the ground will behave in an area around an excavation. To do this we use numerical models, which predict the behavior of soils. There are many numerical models that more or less precisely predict the behavior of soils. In this thesis I will focus mainly on the hypoplastic model. It predicts the behaviour of soils well. The model has also several disadvantages, especially while predicting stress paths at small strain. I will then also describe an improved hypoplastic model, which clarifies the predictions of the soils behaviour. This thesis is created mainly as a core for a diploma thesis, which should focus on modeling of the soils behaviour, the improved hypoplastic model and excavation near the Bodam lake. This excavation was already a subjekt of a diploma thesis (Becker, 2009), but there only the original hypoplastic model was used. This thesis focuses on the description of some aspects of the mechanical soils behavior. The describtion of the original and the improved hypoplastic model and their parameters follows. In the other part of this thesis we describe the selected buiding projects of excavations in fine-grained soils.
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Strength for short-term stability of unsaturated fine-grained soil with low plasticity
Vilimová, Anna ; Najser, Jan (advisor) ; Kostkanová, Vladislava (referee)
The simplified concept of the strength for short-term stability of unsaturated fine-grained soil, presented in Boháč et al. (2018), proposes the usage of a simple linear strength envelope for unsaturated soils. The assumptions for the applicability of this concept are the applicability of the critical state soil mechanics (CSSM), the applicability of the effective stress concept in unsaturated soils, and the applicability of ϕcʹ as their strength envelope characteristics. The article also discusses the possibility of estimating soil suction with a simplified expression χ = Sr using the effective stress concept in unsaturated soils. This thesis verifies the given parameters based on a set of laboratory experiments, and thus also the applicability of the stated concept. The applicability of the given simplification is also verified using laboratory experiments. The results of laboratory tests didn't confirm the general applicability of the assumptions and showed that the simplification χ = Sr is not applicable.
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Change of clay fill structure induced by applied stress
Bočková, Kateřina ; Najser, Jan (advisor) ; Hájek, Václav (referee)
The open-cast brown coal mining in Northwestern Bohemia produces vast amounts of a lumpy clayey material dredged from the overlying sedimentary layers and deposited into large landfills (clay fills). The clay fills are typical for their double porosity (inside and between lumps), which, when related to the absence of artificial compaction, results in their open and metastable structure. Their structural transition from a coarse-graded (lumpy) material into a fine-graded material (reconstituted clay) is influenced by time, degree of saturation, and stress. The structural transition of Bílina Clay Fill, induced by applied stress, is studied by two scaled-down physical models. Their isotropic compressibility and hydraulic conductivity are tested in a large triaxial cell under increasing stress. A series of standard laboratory tests in a small triaxial cell and oedometer on reconstituted and undisturbed samples provides the limiting characteristic values to the possible behavior of the models. The results showed that the hydraulic conductivity of a saturated clay fill non-linearly decreases as the macrovoids close, approaching the hydraulic conductivity of a reconstituted soil at 540 kPa of the vertical effective stress. Then the macrovoids are filled with the reconstituted soil only, and the trend...
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Modelling of lumpy clay fills
Najser, Jan
MODELLING OF LUMPY CLAY FILLS Jan Najser Abstract An extensive open cast mining of brown coal has been taking place in NW Bohemia since 1940's. During the mining process, overburden clay is placed in the form of irregularly shaped lumps of typical dimensions up to 500 mm into large spoil heaps. The total thickness of the lumpy clay landfills is usually 20-50 m. Their material has a double porosity structure: the porosity of intact clay (intragranular porosity) and the voids between clay lumps (intergranular porosity). The total porosity of the fresh fill can be up to 70%. This soil presents special challenges for geotechnical design, primarily because of its high and non-uniform settlement. Further, the progressive tranformation from the "granular" to "fine-grained" material makes the lumpy clay difficult to characterise and model. The aim of the presented thesis is to describe the behaviour of the landfill and the change of its structure by means of field measurements, the centrifuge and numerical modelling. During site investigation for a motorway over a 20-30 years old landfill, two trial embankments were built and monitored over the period of 3 and 6 years respectively. The subsoil of the embankments was instrumented by hydrostatic levelling profiles, pore pressure transducers and depth reference points...
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Stiffness anisotropy of Brno Tegel determined by bender elements
Krupička, Martin ; Najser, Jan (advisor) ; Malát, Richard (referee)
This Thesis deals with the determination of shear moduli of Brno Neogene clay ("tegel") in various directions and the determination of its stiffness anisotropy. The basic measure of stiffness anisotropy in this thesis is considered to be the degree of anisotropy αG given by the ratio of shear moduli in the horizontal (Ghh) and vertical direction (Gvh). Measurements of shear wave velocity were performed on undisturbed vertically and horizontally oriented samples of the Brno tegel in triaxial cell using piezoceramic sensors, so-called bender elements. As an advanced method for determining the degree of anisotropy of the material, another pair of bender elements was used in the measurements, mounted horizontally on the side walls of the sample in the triaxial cell. Based on these measurements, the values of shear moduli Ghh and Gvh for different stress levels were determined. The ratio of shear moduli then determined the degree of anisotropy αG ≈ 1,43 for the given stresses, as the most reliable value based on performed experiments. Literature review part of the thesis briefly summarizes the existing findings from research of physical and mechanical properties of the Brno tegel.
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Influence of clay fraction on residual strength of soils
Bočková, Kateřina ; Boháč, Jan (advisor) ; Najser, Jan (referee)
This bachelor thesis studies the influence of clay fraction on the residual strength of soils. The introductory part of the thesis outlines the topic of residual strength in soil mechanics. The main part contains a review of research papers that studied the influence of clay fraction on the residual strength of soils. The information from the review is summarized and used in the design of laboratory experiments. A series of ring shear tests were conducted on (i) clay fraction from the "Dobkovičky" area, (ii) sand from the Střeleč quarry and (iii) their mixtures. The experimental results correspond with the reviewed papers: the correlation between the residual strength and the content of the clay fraction is, without the knowledge of mineralogical compostition, insufficient. Therefore, the ring shear test is the most reliable method of determining the residual strength.
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Critical state strength of Brno Clay
Fencl, Michal ; Boháč, Jan (advisor) ; Najser, Jan (referee)
The Thesis deals with the critical state strength of Brno Clay. The critical state strength is an important input parameter in many geotechnical analyses. Further it is used in determining the at- rest coefficient K0 of normally consolidated clays using the Jáky formula. The Thesis first briefly describes the concept of strength for soils and the K0. The critical state strength of the particular soil is then determined by using triaxial conventional and frictionless specimens, and two types laboratory direct shear measurements, namely in the translational and ring shear boxes. The obtained results are discussed and compared with previous published data.
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