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Microfluidics in search for catalysts for biomass conversion into liquid fuels
Tesař, Václav
Paper surveys recent developments of combinatorial chemistry characterised by testing many substances and their mixtures simultaneously. Attention focuses on the progress made possible by applying small-scale no-moving-parts fluidics. Encountered problems like small Re and their solutions are demonstrated on author's development of a microfluidic unit for parallel testing of catalysts improving effectiveness of Fischer-Tropsch synthesis of liquid fuels from biomass.
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Holographic interferometry as a tool for visualization of temperature fields in fluids
Dančová, Petra ; Vít, Tomáš ; Lédl, V. ; Trávníček, Zdeněk ; Doleček, R.
Measurement of temperature field in moving fluids is connected with many difficulties. Usage of point temperature measurement methods, such as Constant Current Anemometry (CCA), is limited to frequencies up to 3kHz. This frequency should be the limiting factor for using the CCA in fluid when fast change of temperature occurs. This shortcoming of CCA measurements should be overcome by using of optical methods such as digital holographic interferometry. It is necessary to employ a special holographic setup with double sensitivity instead of the commonly used Mach-Zehnder type of holographic interferometer in order to attain the parameters sufficient for the studied case. This setup is not light efficient like the Mach-Zehnder type but has double sensitivity. The special technique of acquiring and phase averaging of results from holographic interferometry is presented.
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Large-amplitude pulsatile nozzle flow
Tesař, Václav ; Kordík, Jozef
Flowfield inside a simply-shaped nozzle with periodic unsteadiness of so large amplitude that a flow reversal occurs during a part of the cycle was investigated. The range of frequencies was from 20 Hz to 320 Hz, and ratios of the pulsation amplitude to the steady flow were from 1.0 to 4.0. Anemometric velocity measurements were combined with numerical flowfield computations.
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Visualization of the flow around rotating cylinder
Broučková, Z. ; Trávníček, Zdeněk ; Šafařík, P.
The visualization of the flow around a rotating and standing cylinder has been performed in air. This visualization has been performed for two velocities of a freestream for Reynolds numbers Re = 920 a 990 and for four frequencies of the rotation (0 s-1, 5 s-1, 12.5 s-1 and 20 s-1). The results have been compared with the theoretical model – potential flow. The velocity of the freestream has been evaluated by measurement of the frequency of the shedding vortices behind the standing cylinder in the regime of the Kármán vortex street. In the regime of the Kármán vortex street the Reynolds numbers were Re = 110 a 120. The known relations between Stouhal and Reynolds numbers were used.
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Microbubble Generation by Fluidics. Part II: Bubble Formation Mechanism
Tesař, Václav
Author develops a facility for generation of sub-millimetre sized air bubbles in water. A promising and economically viable method, based on author's earlier experience, is fragmentation of bubbles by oscillation while they are still at the aerator exit locations. Understanding the mechanism by which the oscillation affects bubble formation is a pre-requisite to new aerator design. Results obtained so far reveal a strange bubble size independence on the applied acoustic power, frequency of oscillation and the character of aeratoir body.
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Microbubble generation by fluidics. Part I: Development of the oscillator
Tesař, Václav
Author investigates generation of sub-millimetre air bubbles in water. An economical alternative is pulsating the supplied air flow, using a no-moving-part fluidic oscillator. The paper summarises the existing oscillator principles and discusses the author's development of the chosen oscillator – which resulted in a device with behaviour different from what has been so far described in literature.
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Trapped Vortex Ring
Tesař, Václav
Paper discusses an almost unknown and yet interesting operating principle of fluidic nomoving-part devices for flow control. The principle is based on the properties of vortex rings. A standing vortex ring is kept in a semi-toroidal recession positioned opposite to an annular nozzle from whichissues an annular fluid jet. The ring can exist in the recession with two alternative senses of rotation so that the annular jet is led to either the central exit through the centre of the vortex, or to the outer spacepast the outer vortex circumference.
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Enigma of submerged fence skin friction sensor
Tesař, Václav
The problem with the known “sublayer fence” skin friction sensor is the disturbance caused by the fence protruding above the surface. Author comes with the solution preventing the premature transition into turbulence or even flow separation. In his original version of thesensor the fence is submerged slightly below the surface. Detailed investigations revealed the pressure difference output to be dependent on creation of two recirculation regions. An attempt tofacilitate the recirculation, quite surprisingly, has led to the merging of the vortices and total disappearance of a useful output signal.
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