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Cryogenic He experiment on Natural Turbulent Convection
Králík, Tomáš ; Urban, Pavel ; Musilová, Věra ; Hanzelka, Pavel ; Srnka, Aleš ; Repisky, Andrej
Cryogenic helium gas is a suitable fluid for study of natural turbulent convection at very high Rayleigh numbers. We present experimental method and new result on Reynolds and Péclet numbers characterizing large scale circulation (LSC) of fluid at Rayleigh numbers within the range from 1011 to 1015. Observation of LSC is based on measurement of local temperature fluctuations of convecting helium. For experiments we used a specially designed cylindrical cell of height L = 0.3 m and diameter D = 0.3 m with minimized parasitic effects on studied convection, previously published on CryoPrague 2006 [1].
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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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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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Fuzzy pressure controller for helium bath cryostats
Hanzelka, Pavel ; Jura, P.
Pressure stability of about 10 Pa/h is necessary for precise measurementsof the helium evaporation rate of the low loss cryostats. A pressure controller fulfilling this requirement was designed and tested. It consists ofan absolute pressure sensor, an electronic fuzzy controller and a proportional electromagnetic valve. Computer simulation of the system behaviour was used for the controller tuning. The minimization of the final deviationwas preferred to the reaction speed. The operational properties of the controller were verified at the tests of a helium dewar. The controller was utilised at the development of a small helium cryopump.
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A new construction of rotor of helium expansion turbine
Dupák, Jan ; Hanzelka, Pavel ; Michalička, Petr ; Tuček, L. ; Ustohal, V.
The aim of our work was to develop a rotor of the expansion turbine designed for a helium liquefier that would enable operation in different modes and achieving of the desired effectiveness at same time. For meeting theserequirements the rotor should enable precise operation of the turbine in the range of 140 000 to 350 000 rqm at minimum heat flow to the low-temperature part of the turbine. Considering the low thermal conductivity and a high strength, the low-temperature part was made of VT3-1 titanium alloy. This contribution is focused mainly on problems of creation of mechanically strong and thermally resistant joint of this part of the rotor with the part working at room temperature.
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