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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.
Fulltext: content.csg -  PDF
Plný tet: SKMBT_C22013102415150 - 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.
Fulltext: content.csg - PDF
Plný tet: SKMBT_C22012112614030 - 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 Viscosimetry of Microdispersions - Detection of Apparent Wall Slip
Pěnkavová, Věra ; Večeř, M. ; Wein, Ondřej
The rheometric data measured using three various sensor geometries (PP60, ZZ40DIN and KK-sensor) are compared in this contribution at the case of water solution of polymer welan. The KK-sensor, constructed at our department, combines advantages of geometry both plate-plate and concentric cylinder geometry. It is shown that KK-sensor is comparable to commercial sensors. It is presented for water kaolin dispersions that apparent wall slip does not disappeared neither for low consistent dispersions. The values of slip coefficients are even higher in the case of less consistent samples.
Fulltext: PDF
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Experiments on Large Bubbles Rising in an Inclined Flat Channel
Tihon, Jaroslav ; Pěnkavová, Věra ; Vejražka, Jiří
The motion of single air bubbles in an inclined flat water channel is investigated experimentally. The electrodiffusion technique of near-wall flow diagnostics is applied to measure the wall shear stress distribution under large bubbles rising along the upper channel wall. The directionally sensitive, two-strip electrodiffusion probes are used to detect the location of flow reversal in a thin liquid film separating the moving bubble from the wall. The electrodiffusion measurements are synchronized with the visual observation of bubble movement done by a high-speed camera. The experiments are carried out for three different channel heights (1.5, 4, and 8 mm) and cover a wide range of channel inclination angles (from horizontal to vertical position). The experimental results on bubble motion characteristics obtained in stagnant and co-flowing liquid arrangements are confronted with the recent predictions and scaling provided by theoretical models.
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