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- KurzbeschreibungThe aim was to find out if, and if yes subject to what restrictions, the vane could be<br>used as a viscometer, even for viscoelastic fluids. The rationale of such an attempt is<br>based on the fact that in all viscometers, which require fluid inertia to be neglected,<br>a viscometric flow does not exist, no matter how small Re is. Prominent examples<br>are cone&8211;plate flow and torsional (i.e. plate&8211;plate) flow. Secondary motions always<br>affect the ideal local flow kinematics. Yet, only at sufficiently large Re&8211;numbers<br>do these changes in the local velocity field lead to measurable global relationships,<br>which are used in determining ?. For purely rotational devices this is theM&8211;O relationship.<br>The fluids used were taken from the class of aqueous polymer&8211;solutions. Three<br>different types of polymers were used, namely an industrial one (polyacrylamide),<br>a biopolymer (hydroxypropyl guar) produced industrially by adding polypropylene<br>to guar gum and the biopolymer xanthan gum with its helical backbone. For<br>either type of polymer solution three or four different concentrations were used.<br>Using a Couette viscometer (concentric cylinder, CC) the flow curves were obtained.<br>It turned out that, depending upon concentration, the flow curves differed quantitatively<br>but showed qualitatively similar behavior. While a Cross&8211;like model sufficed<br>for the two biopolymers, a Carreau&8211;Yasuda&8211;like model was required for the polyacrylamide<br>solutions. Irrespective of these details a master curve allows the flow<br>curve to be determined for any concentration without actually measuring ?(?? ).<br>To use the vane as a viscometer requires its characteristic curve (ChC) Ne = Ne(Re)<br>to be established. To this end various Newtonian fluids (NFs) were used, in our case<br>various silicon oils of lowmolecular weight. The influence of ? on the ChC is largest<br>in the creepingmotion regime (CMR), when Ne = c/Re is bound to hold. In our case<br>up to Re &732; 10 one is in the CMR, where c = 13.51 was established experimentally.<br>Since this constant differs rather drastically from the one used in our commercial<br>CC&8211;viscometer (of similar dimensions as the vane device) it is clear that substantially<br>different flow fields have to prevail in these two devices.<br>For non&8211;Newtonian fluids (purely viscous or viscoelastic ones) ? is not a constant<br>but rather depends upon ?? . Thus, ?ref was utilized (in our case the solvent water<br>was used as the reference fluid) to define a reference Reynolds number Reref.<br>If Ne Re = c characterizes the CMR then a change of Ne Reref vs Reref to Ne Re vs<br>Re requires, in a log&8211;log plot a shift along a 45? line. The magnitude of this shift furnishes<br>?. For each O the function ? = ?(O) can thus be determined. Equating then<br>?(O) with the viscometrically established flow curve ? = ?(?? ) furnishes a relation<br>between O and ?? . For O small enough (small Re) the relation is linear, i.e. ?? = c?O<br>prevails. Although c? differs slightly from fluid to fluid , the fluid&8211;independent approximation<br>c? &732; 4 (in our case) produces sati
- AutorAlejandro Peugnet
- VerlagCuvillier Verlag
- Seiten180 Seiten
- Gewicht246 g
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