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Černé povrchy pro použití v kryotechnice
Králík, Tomáš ; Hanzelka, Pavel ; Musilová, Věra ; Srnka, Aleš
Surfaces with high absorption or emission of thermal radiation are often needed in cryogenic and space applications. It is more difficult to realize a sufficiently black surface applicable in cryogenic systems than to make highly reflective surfaces. Experimental results on black surfaces like epoxy coatings, paints and thin films concerning the thermal radiative properties (emissivity and absorptivity) in the temperature range from 20 K to 300 K of the source of the thermal radiation are presented.
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Návrh a testování kryostatu s experimentální celou pro turbulentní tepelnou konvekci
Urban, Pavel ; Hanzelka, Pavel ; Musilová, Věra ; Srnka, Aleš ; Skrbek, Ladislav
We have designed an experimental cell of He cryostat for the study of the turbulent natural convection at very high Rayleigh numbers (1e6 < Ra < 2e15) with cryogenic 4He gas as a working fluid. The main goal of our study is to resolve the question about the transition to an ultimate Kraichnan regime within a range of available Ra, as similar experiments with cryogenic 4He gas performed in Chicago, Grenoble, Oregon and Trieste gave controversial results. The Grenoble group claimed to observe the Kraichnan regime above Ra = 1e11 in their experiments. This phenomenon has not been observed in other laboratories. Using our newly developed cylindrical convection cell of variable aspect ratio G with very thin stainless steel wall is well suited to resolve this issue as well as remaining controversy about the Nu(Ra,Pr,G) dependence.
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Přenos tepla napříč superizolacemi z pokojových do nízkých teplot
Hirschl, Ch. ; Králík, Tomáš ; Laa, Ch. ; Musilová, Věra ; Schmid, T. ; Stipsitz, J.
For more than 30 years thermal measurement results and analytical formulas are available to calculate the heat transfer through Super Insulation (SI). According to the temperature dependence of the emissivity for aluminium, the most commonly used material for the reflecting surfaces, derivations based on the Stefan-Boltzmann-Law for radiative heat transfer have been established. Together with conductive terms this leads to formulas for the calculation of heat transfer through SI. Using new measurement results of temperature-dependent emissivity and conductivity we have performed heat flux calculations applying common software tools. The heat transfer through the SI and the temperature distribution inside the SI calculated with this new approach has been compared with the established formulas and measurement results.
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Návrh héliového kryostatu pro studium turbulentní tepelné konvekce při kryogenních teplotách
Srnka, Aleš ; Hanzelka, Pavel ; Musilová, Věra ; Urban, Pavel ; Skrbek, Ladislav
We present the design of a cryostat with an experimental cell for turbulent thermal convection using cryogenic helium gas. It is based on our long-term experience in cryostat design, especially low-loss cryostats for NMR magnets. The top and bottom of the convection cell are made of thick Cu plates. The cylindrical stainless steel wall consists of three parts; the middle one is exchangeable and allows for varying the cell aspect ratio. Another goal of our cell design is reduction of the parasitic heat flux from the copper plates via the cylindrical wall to its interior. As the cell operates up to 3.5 bar, we have paid significant attention to the safety of the cryostat and simulated critical situations such as a sudden vacuum loss.
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Helium cryopump for electron optical devices design and preliminary cryogenic testing
Dupák, Jan ; Hanzelka, Pavel ; Musilová, Věra
A helium cryopump for electron optical devices was designed and preliminary cryogenic parameters of the first pump sample were tested. Low LHe consumption should enable economical operation of the pump without recycling the evaporated He. The pumping cryopanel is cooled by normal LHe down to 4.2 K. A LN2 bath is utilized for the LHe bath thermal shielding. Two additional heat radiation shields cooled by the vapours of the consumed cryoliquids are installed to decrease the LHe and LN2 losses. Three chevron screens placed between the input office and the cryopanel are used to protect the internal parts against direct heat radiation from the parts at ambient temperature. The chevron systems were replaced by low emissivity screens in the preliniary tests. Predicted low evaporation rates of LHe and LN2 were achieved.
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