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Visualization of cryogenic helium flow using of laser-induced-fluorescence
Drahotský, Jakub ; Šiler, Martin (referee) ; Urban, Pavel (advisor)
This work evaluates the laser induced fluorescence method (LIF) for the vizualization of the cryogenic helium flows for experimental study of the Rayleigh–Bénard convection at the Institute of Scientific Instruments (ISI) in Brno in the labs of Cryogenics and Superconductivity group. As the tracer particles, we considered the metastable molecules He2*, that are created by the ionization of helium with themfemtosecond laser pulse and then they are excited by the lasers with appropriate wavelengths for the invocation of the fluorescence. On the basis of the research study, I have evaluated the current laser equipment at the ISI as insufficient for the experiment realization. At the end of this work, there is a list of a laser technique and other devices which are necessary for the adaptation of the LIF method in the experiments at ISI.
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Design of a nitrogen Dewar vessel for electron microscopes
Brauner, Tomáš ; Voňka,, Jakub (referee) ; Urban, Pavel (advisor)
Observation of biological samples in electron microscopes in a state closest to the native state requires reaching cryogenic temperatures, most often with the use of liquid nitrogen (LN2). For this purpose a Dewar vessel with cooled copper thumb for cooling the sample holder and anticontamination shield inside the vacuum chamber of the scanning electron microscope (SEM) and focused ion beam (FIB) source was designed. Characteristics of the designed LN2 Dewar vessel were validated by measuring executed on the real functional sample. The reached temperature at the tip of the thumb under estimated heat load 2 W was 90,8 K. Corresponding refilling interval of LN2 is about 11 h. Experimentally measured data correlate with theoretically calculated characteristics. The designed Dewar vessel with a cooled thumb fulfills the required properties specified in the thesis assignment with a margin.
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Cryogenic cell for study of water ice in ESEM microscope
Krutil, Vojtěch ; Kubíček, Jaroslav (referee) ; Urban, Pavel (advisor)
The presented thesis focuses on designing a cryogenic cell for the study of water ice in an environmental scanning electron microscope (ESEM). This cell allows the study ice in the temperature range 80 K – 300 K in a nitrogen gas environment with a pressure of up to 400 Pa. The cell is cooled by a flow cooling system, where liquid or gaseous nitrogen is used as a refrigerant. The cell consists of a double-walled vessel with vacuum insulation, a flow-through heat exchanger, a sample well, and a cooled cell lid. The heat exchanger was designed to be able to dissipate the heat load at the level of 1 W. The exchanger is equipped with an electric resistance heater with an output of approximately 60 W, enabling heating of the sample at speeds of up to 100 K·min-1. The design also includes an LN2 gateway located on the door of the vacuum chamber of the microscope, to which the capillaries of the heat exchanger for the intake and outlet of refrigerant are connected. During the experimental verification of the cryogenic cell in the test vacuum chamber with a pressure of GN2 ~ 400 Pa, the limit temperature of 77.5 K on the sample well was reached.
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Role of thermal near field in cryogenic Rayleigh-Bénard convection
Věžník, Tomáš ; Schmoranzer,, David (referee) ; Urban, Pavel (advisor)
This bachelor thesis is focused on the eect of the thermal near feld (NF) radiation on (i) the onset of Rayleigh-Bénard convection (RBC) and (ii) the thermal inuence on the temperature sensors used in RBC studies. In the frst part, the theoretic base for both studied areas, RBC and NF, is presented. Further, the ways of fnding the onset of RBC, the temperature profles, the basic equations for heat transfer in far feld and near feld and the description of the apparatus used for the measurements of radiative heat transfer are given too. In the practical part, the process of measurement is presented and the model that we obtained for describing the dependence of the coefcient of radiative heat transfer between two Cu surfaces is demonstrated. Afterwards, the data are used to analyse the eect of NF on the onset of convection, and to calculate the inuence on the temperature profle measurements with cryogenic temperature sensors and, at last, the comparison of total heat transferred over the experimental cell with NF heat transfer over the thermal boundary layers arising in RBC is done. A goal of this thesis was to fnd out whether the eect of NF can explain the contradictory results of determining the onset of convection in cryogenic RBC (expressed by critical Rac) based on the measurements of the heat transfer in RBC and of the eectivity of heat transfer at high values of Ra numbers in cryogenic experiments. The current evaluations clearly demonstrate that the eect of NF on studied phenomena is by several orders of magnitude smaller than other relevant eects and can thus justly be neglected.
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Flow instabilities due to torsional oscillators in superfluid helium
Skokánková, Tamara ; Schmoranzer, David (advisor) ; Urban, Pavel (referee)
This Thesis is devoted to the investigation of superfluid helium flows due to torsional oscillators. In its first part, flow due to a torsionally oscillating disc suspended on a tungsten filament is studied (building upon the work of A. C. Hollis Hallett from 1952). Measurements of the motion of the torsionally oscillating disc were performed in superfluid helium at temperatures between 1.265 K and 2.157 K at saturated vapour pressure. Time traces of the disc angular deflection were obtained, and critical parameters related to the turbulent flow stability were determined. In laminar flow, scaling of drag forces with the dimensionless Donnelly number was verified. Based on these results, and comparison with the original work, a scenario of the decay of turbulent flow was suggested. The second part of this work is focused on the development and construction of a similar experiment for mK temperatures. A new type of oscillator was designed, a so-called "pillbox", and a series of testing measurements was performed both at room and mK temperatures.
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Design of a sample holder with electrical contacts for UHV SEM/SPM
Krutil, Vojtěch ; Vlček, Ivan (referee) ; Urban, Pavel (advisor)
The presented thesis focuses on designing a sample holder for an UHV SEM/SPM microscope suitable for using in low temperature (20 K - 300 K) applications. This newly designed sample holder is equipped with ten spring-loaded contacts for electrical connection of a transport pallet to the sample holder, which will be equipped with a temperature sensor and a heating element. Two quadruples of contacts are reserved for the sample and the temperature sensor and the remaining pair for the heating element. A thorough research study of commercially available sample holders indicates that the holders for the intended use are not available on the market. In the low-temperature tests of the newly designed sample holder, the limit temperature of the 24 K was reached in a test vacuum chamber with a flow cooling system. The ambient temperature was 300 K. The contact function was successfully verified by measuring the transient electrical resistance at the fixed and the spring contact sections. Additionally, a modification of the sample holder for high temperature (300 K - 700 K) was suggested.
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Temperature profiles and temperature fluctuations in turbulent Rayleigh-Bénard convection
Drahotský, Jakub ; Mantia,, Marco La (referee) ; Urban, Pavel (advisor)
Tato práce popisuje výzkum zaměřený na studium vertikálních teplotních profilů a fluktuací v turbulentní Rayleighově-Bénardově konvekci. Experiment byl proveden v "Barrel of Ilmenau" obsahující válcovou experimentální celu s průměrem 7,15 m a výškou 4,7 m ("the aspect ratio" = 1,5) naplněnou suchým vzduchem. Teplotní profily a fluktuace byly studovány podél vertikální osy cely mezi horní a spodní deskou spodní deskou v rozmezí Rayleighova čísla (Ra) 1E11 4E12. Teplotní profily byly změřeny novou metodou využívající systém s optickým vláknem Luna ODiSI-B, který byl pořízen týmem z Ilmenau. Systém umožňuje měřit teplotní profil ve všech bodech podél celého vlákna současně s prostorovým rozlišením 5 mm.
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The Project of Educational cycle trail in the protected landscape area in Broumovsko
Urban, Pavel ; Šafránek, Jiří (advisor) ; Kuna, Petr (referee)
Title The Project of Educational cycle trail in the protected landscape area in Broumovsko. Objectives The aim of this work is to create project of Educational cycle trail in rarely visited areas in the protected landscape area in Broumovsko. The main outcome of this work is a brochure which should be easily accesible in Toutist Information Centres in the area as well as on the Internet, where the tourist are allowed to download this file for free. Method The first part of this work analyzes accessible information found in printed and internet sources and gained thanks to dialogues with specialists working in PLA Broumovsko. Afterwards these gained information and facts were organized and used for selecting material for two educational cycle trail and also the path was defined. In the second part I worked on the evaluation of these two paths in the form of questionnaire, which was distributed to those who rode their bikes along the educational cycle trails. On the basis of dialogues with experts dealing with the protected landscape area, such as people working in the administration of the PLA and people engaged in travel tourism organizations in this PLA, I made an analysis of the current situation in the protected area and afterwards, analysis of a possible contribution of this work. Then two...
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