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2016, vol. 44, br. 2, str. 115-124
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Numerička analiza uticaja ugaone brzine na proces mešanja nenjutnovskog fluida u horizontalnoj mešalici sa dva lopatična kola
Numerical analysis of rotational speed impact on mixing process in a horizontal twin-shaft paddle batch mixer with non-Newtonian fluid
aVojnotehnički institut - VTI, Beograd, Srbija bInstitut Goša, Beograd, Srbija cUniverzitet u Beogradu, Elektrotehnički fakultet, Srbija
e-adresa: slavce@yahoo.com
Projekat: Istraživanje i optimizacija tehnoloških i funkcionalnih performansi ventilacionog mlina termoelektrane Kostolac B (MPNTR - 34028)
Primena savremenih mernih i proračunskih tehnika za izučavanje strujnih parametara ventilacionih sistema na modelu energetski izuzetno efikasnog (pasivnog) objekta (MPNTR - 35046)
Sažetak
U radu je izvršena numerička analiza uticaja broja obrtaja na efikasnost mešanja paste od sojinog brašna sa vodom i vazduha. Mešanje se vrši u delimično ispunjenoj, horizontalnoj mešalici koja ima dve osovine sa po 7 lopatica. Osovine se obrću istim ugaonim brzinama u suprotnim smerovima. Određen je stepen mešanja paste sa vazduhom, utrošak snage i disperzivni indeks mešanja, koji predstavljaju važne podatke u prehrambenoj industriji. Numeričke simulacije trodimenzionalnog laminarnog strujanja unutar mešalice su izvedene primenom multifaznog modela Volume of Fluid (VOF), za određivanje zapreminskog udela pojedinih faza. Korišćen je komercijalni softver ANSYS FLUENT, a određivano je nestacionarno strujno polje. Definisane su dve faze, od kojih se prva obuhvatala pastu od sojinog brašna i vode, a druga vazduh. Pasta od sojinog brašna sa 67% vode predstavlja nenjutnovski fluid. Reološke karakteristike paste, kao ulazni podaci za numeričke simulacije, su određene eksperimentalno za temperaturu od 250C. Za potrebe nestacionarne simulacije, oko lopatičnih kola unutar mešalice, je generisana dinamička numerička mreža. Numeričke simulacije su izvedene za dve radne ugaone brzine osovina sa lopaticama: 10 o/min i 25 o/min. Numeričke simulacije multifaznog strujanja, sojine paste i vazduha, su pokazale da je mešanje faza utoliko bolje ukoliko je ugaona brzina lopatičnih kola veća, jer veća količina vazduha ulazi u pastu. Najveće trenje se javlja na vrhovima lopatica i u direktnoj je srazmeri sa ugaonom brzinom osovina. Rezultati su pokazali da se uticaj ugaone brzine na indeks disperzivnog mešanja može zanemariti. U aktivnoj zoni, mešanje je efikasno sa visokim indeksom disperzivnog mešanja. Primenjena numerička metoda daje korisne podatke karakterizacije strujanja koji mogu biti od velike koristi tokom dizajniranja mešalica, kao i u optimizaciji parametara mešanja tokom proizvodnje. U budućem radu, pažnja će biti posvećena analizi uticaja oblika i položaja lopatica na zapreminski udeo vazduha koji ulazi u pastu, utrošak energije kao i indeks disperzivnog mešanja, što će svakako dovesti do značajne uštede u procesu proizvodnje. Dobijeni rezultati se mogu primeniti za različite aplikacije u prehrambenoj , hemijskoj, biohemijskoj i farmaceutskoj industiji.
Abstract
Nowadays, the priority of food industry processes is based on mixing of the various flavour pastes with the air. Optimal mixing process in the results means the instantaneous preparation of the homogenous and easily spreadable paste while consuming the lowest power. For achieving the brand precise and repeatable flavour, of major importance are, beside the filling parameters, the mixing setup parameters. This work presents the influence of rotation velocity, as one of the setup data, on: the paste quality and the mixing index, obtained by the CFD. The paste was prepared of soy flour and water, and described by the experimentally determined rheological characteristics. Mixing vessel, of the twin-shaft paddle type, introduced mixing by the laminar shear-thinning. This mixing process was modelled through 3D fully transient numerical simulations employing the volume of fluid (VOF) multiphase model of the Eulerian-Eulerian approach. In two cases, the influence of the angular velocity to the mixing-index value can be neglected. At the mixing area two regions were marked off. First one and dominant, was the region of active mixing preoccupied with high shear flow, described with high mixing index. Opposite, in the passive mixing region, the flow was almost rotational, represented by low mixing indexes. The rotation velocity change made the significant influence to wall shear stress levels achieving the highest values at the tips of the paddles.
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