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Models of AWS Material Functions
Wein, Ondřej ; Pěnkavová, Věra
New classes of 4-parameter models of fluidity for the shear-thinning non-Newtonian liquids are introduced, including their local pseudosimilarity representation. The models cover both purely-viscous (PV) and visco-plastic (VP) flow behavior. New 4-parameter model of slipidity (Navier slip coefficient) is introduced, as well as its modification for retarded AWS (slipidity decreasing with rising shear stress) and wall dilatancy (negative slipidity). Their asymptotic behavior, as well as their asymptotic sub-models are identified and prepared for implementation in the software AWSWork.
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The Deflocculation of Kaolin Suspensions – the Effect of Various Electrolytes
Pěnkavová, Věra ; Guerreiro, M. ; Tihon, Jaroslav
The deflocculation effect of conventional additives to kaolin suspensions is evaluated from the results standard rheological measurements. Several widely used electrolytes (NaOH, Na2CO3, Na2SiO3, SHMP = sodium hexametaphosphate, and CMC = sodium salts of carboxymethylcellulose) have been tested. The optimal concentrations of these deffloculants, in respect to reaching the maximum reduction of initial suspension viscosity, are found. The stability of deflocculated kaolin suspensions against sedimentation is evaluated and different aspects of the observed flow enhancement discussed. Inorganic electrolytes are found to be more effective in viscosity reduction, but on the other hand, low-molecular organic CMC additives produce more stable final suspensions.
Plný tet: SKMBT_C22013102415150 -  PDF
Plný text: content.csg - PDF
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Rheology of Colloidal Dispersions: Deflocculation of Kaolin Dispersions
Guerreiro, M.I.A. ; Růžička, Marek ; Pěnkavová, Věra
All inorganic additives in their optimal concentrations proved to be effective at reducing the initial viscosity to 1%. In the case of NaOH two different trends were observed: firstly this additive behaved as a deflocculant agent where, at its optimal range of concentrations, the viscosity of kaolin dispersion reduced to its minimal value, secondly when used at concentrations above its optimal range, sodium hydroxide behaved like a flocculant agent increasing strongly the viscosity of kaolin dispersions. The possible cause was attributed to an excess of Na+ and OH- species in bulk media that promoted the reversible effect. Similar effect was also reported for NaCMC at high and medium molar weight, were the viscosity firstly decreased to a certain concentration of additive and then above that concentration started to increase. Thus low molar weight NaCMC was the most effective among the organic additives since it decreased the viscosity to 10% of initial viscosity. Qualitative evaluation showed that organic additives promoted more stable dispersions, in contrast to poor stability offered by the inorganic additives, with exception of NaOH that provided dispersions with good stability inside of its optimal range.
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Wall Shear Stress Induced by Taylor Bubbles in Inclined Flow Channels
Tihon, Jaroslav ; Pěnkavová, Věra ; Vejražka, Jiří
The motion of single air bubbles in flat channels is experimentally investigated. The electrodiffusion technique of near-wall flow diagnostics is applied to measure the wall shear stress distribution under large rising bubbles. The measurements are synchronized with the visual observation of bubble movement by a high-speed camera. The analysis of video records provides information on the bubble shape and terminal velocity. The experiments are carried out for three different channel configuration (with heights of 1.5, 4, and 8 mm), cover a wide range of channel inclination angles (from horizontal to vertical position), and dealing with both the bubbles in stagnant and in co-flowing water.
Plný tet: SKMBT_C22012112614030 - PDF
Plný text: content.csg - PDF
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The Transitional Regime of Backward-Facing Step Flows
Tihon, Jaroslav ; Pěnkavová, Věra ; Havlica, Jaromír ; Šimčík, Miroslav
The backward-facing step flow is investigated experimentally (electrodiffusion technique) and numerically (CFD software Fluent) at moderate Reynolds numbers achieved in a water channel. The direction sensitive sensors are used to measure the wall shear rate behind the step and detect the near-wall extent of different flow-recirculation regions. 2D numerical simulations provide additional information on the global flow rearrangement caused by the change of operational parameters (channel expansion ratios, inlet flow conditions). At the transitional flow regime, the near-wall flow exhibits an unsteady character with a high sensitivity to external low-frequency perturbations (intrinsic flow instability). The inlet pulsatile forcing is found to affect strongly the overall flow structure behind the step, resulting in the reattachment length reduction and the reverse flow intensification.
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AWS viskozimetrie mikrodisperzí - detekce zdánlivého skluzu na stěně
Pěnkavová, Věra ; Večeř, M. ; Wein, Ondřej
V tomto příspěvku je ukázáno porovnání dat ze tří různých viskozimetrických senzorů s různou geometrií (PP60, ZZ40 DIN a KK-senzor). KK-senzor, vyvinutý na našem pracovišti, spojuje výhody senzorů typu válec-válec a deska-deska. Na příkladu vodného roztoku welanu je ukázáno, že KKsenzor je srovnatelný s běžnými komerčními senzory PP60 a ZZ40 DIN. Na příkladech vodných kaolinových disperzí (čistě vodných i s přídavkem ztekucovadla hexametafosforečnanu sodného) je ukázáno, že ani v případě disperzí o relativně nízké konzistenci zdánlivý skluz na stěně nemizí. Hodnoty skluzových koeficientů jsou naopak u nížekonzistentních vzorků vyšší.
Plný text: PDF
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Experimentální studium pohybu bublin v nakloněném plochém kanálu
Tihon, Jaroslav ; Pěnkavová, Věra ; Vejražka, Jiří
Elektrodifúzní diagnostika přístěnného toku je použita pro měření rozložení smykové rychlosti pod velkou bublinou stoupající podél stěny nakloněného kanálu. Směrově citlivá čidla umožňují též detekci reverze toku v tenkém kapalném filmu oddělujícím pohybující se bublinu od stěny kanálu. Elektrodifúzní měření jsou synchronizována s vizuálním pozorováním pohybu bubliny pomocí rychloběžné kamery. Analýza těchto videozáznamů poskytuje informace o tvaru a rychlosti bublin. Experimenty jsou provedeny pro tři různé výšky tokového kanálu a pokrývají široký rozsah úhlů jeho naklonění. Získané výsledky charakterizující pohyb bublin jak v nehybné kapalině, tak i v kapalině proudící vzhůru kanálem, jsou konfrontovány s předpověďmi teoretických modelů.
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