Application of the Ultrasound Velocity Measuring Technique to Stirred Vessel Flows with Multi Element Stirrers

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1 15 1. ISUD 1st Internatinal Sympsium n Ultrasnic Dppler Methds fr Fluid Mechanics and Fluid Engineering September 9-1 I, 1996 Paul Scherer Institut, 5232 Villingen PSI, Switzerland Applicatin f the Ultrasund Velcity Measuring Technique t Stirred Vessel Flws with Multi Element Stirrers P. Wachter, W. Steidl, M. Heiken, F. Durst Department f Fluid Mechanics, University f Erlangen-Nuremberg, Cauerstr. 4, Erlangen, Germany ABSTRACT 1. Intrductin In many fields f the chemical industry, stirred vessel flws are f great imprtance. Flws f this kind are als f interest t peple wrking in the field f sewage water treatment. Whereas the nature f stirred vessel flws is knwn and methds have been develped t upscale labratry experiments, details f the flw are unknwn but becme f increased imprtance fr detailed layuts f stirred reactrs. Measuring techniques, such as laser Dppler anemmetry (LDA) usually prvide very accurate infrmatin but require ptical access t the measuring pint. This means the use fthe LOA-technique ill: full scale reactrs is nrmally cmplicated and requires extensive experiencef the scientist. Furthermre, these measurements are usually time cnsuming and yield mre detailed infrmatin than is needed in praxis. Because fthis, an ultrasnic measuring technique (UVP-mnitr) has been applied fr flw field studies by the authrs, prviding velcity prfile infrmatin alng the penetratin length f an ultrasund wave. Apprpriate signal prcessing yields lcal velcity infrmatin f mean flw prperties. This infrmatin prvides an insight int the flw field f stirred reactr flws with multi element stirrers, studied in the present fluid mechanic research wrk. 2. Backgrund In waste water treatment plants bidegradatin fpllutants takes place in stirred and aerated tanks. Flakes fsludge which cnsists ut fdifferent clnies fbacteria are respnsible fr the bilgical decmpsitin. These flws must be distributed hmgeneusly in the basins t use the whle vlume. Furthermre depsits culd disturb the prcess essentially. Therefre a minimum bttm velcity is required fof waste water treatment plants by.

2 16 German authrities. T guarantee the desired flw field fr different tank gemetries labratry scale experiments are made. In the present wrk a hyperblid stirrer which was develped at LSTM-Erlangen is emplyed. This agitatr is distinguished by its special shape, which ensures an attached flw directly abve the stirrer surface. Separatins and related flw lsses are thus minimised. The transprt ribs n the stirrer surface cause the waste water t flw ff in a radial directin and therefre enhance the circulatin f the tank cntents. Owing t its energy input near the bttm and the related high bttm velcities, the hyperblid stirrer pssesses gd suspensin qualities cmbined with lw energy cnsumptin. Increased mvement f the water surface, and additinal entry f xygen during denitrificatin are avided. 3. Test rig Fig. 1: Schematic drawing f the test rig A test sectin was set up, as shwn in figure 1 and 2 fthe present abstract. The test rig was cnstructed t achieve gemetrical similarity with the mst rectangular basins which are used in practice. The vlume f the tank can be reduced by a variable wall and s it is als pssible t reach the shape f a square tame Light sheet techniques were emplyed in rder t characterize the flw field generated by ne, tw and three hyperbli~ stirrers munted in a test sectin. A UVP-mnitr was used t map ut the flw field. Prir t the main investigatins, tests were made t select the mst suitable scattering particles fr measuring thrugh a wall made f plexiglas. The backgrund f these experiments was t determine an installatin specificatin fr this kind f stirrer with an ptimal relatin fperatinal and investment csts. T reach this aim stir up tests were carried ut with artificial sludge, whereby the velcity f the fluid was

3 17 Fig. 2: Pht f the tank measured at a representative pint as a functin f the rtatinal speed f the stirrer, the fining level and the relatin f length t width f the tarue The measuring psitin was fund n the basis f flw mapping. We determined that there is a regin at the edge f the basin, where the axial cmpnent f the velcity is mre than 99% f the resultant velcity. S we can characterize the effectiveness f the hyperblid stirrer (the minimum rtatinal speed t ensure suspensin f the artificial sludge) with just ne measurement f ne velcity cmpnent in the vessel. These results can be transferred t industrial plants. The UVP-mnitr was emplyed in rder t map ut the entire flw field inside f the water cntainer. Lcal velcity infrmatin in hrizntal planes is sh\\tn, fr example, in figures 3 and 4. Infrmatin has als been acquired n turbulence prperties and the lcal energy dissipatin f the stirrer inside f the water cntainer (see figures 5 and 6). y = 7 L~;;;i u:, = 3 ["-:/5) Fig. 3: Hrizntal flw mapping f ne plane

4 , , z= GO (mm) ~ic = 3 (mis) 200 [mmls) O~-~-~"""'''''''''''''''''''''...'''''''''...--';''---';'---' tn.1\) I\) (..) C.).,. 0 en tn 0 c.n Fig. 4: Vertical flw mapping f ne plane The turbulent flwfield is cmpsed f vrtices with a different pwer density. These fluctuatins fthe lcal mean velcity cntribute t an effective mixing in a stirred tank. The vrtices decay step by step in smaller units and ultimately dissipate. T stir a mixture gently a hmgeneus distributin f the dissipated energy is strived fr in the whle tank. 'I = 197 Em..!!] u~=3rmfs.j! y~x z=8{mm] u..,=3 [m/5j Fig. 5: Turbulence intensity f a hrizntal plane Fig. 6: Lcal energy dissipatin in a vertical plane

5 19 z=60 [mmj l\i =3 [mis] 100 ~ l--"--+--"':...r-""---+t---r-ti 200 [mmls] tn tn ~ I\) tn (,.) (,.) tn./>. Fig. 4: Vertical flw mapping f ne plane The turbulent flwfield is cmpsed f vrtices with a different pwer density. These fluctuatins f the lcal mean velcity cntribute t an effective mixing in a stirred tan1e The vrtices decay step by step in smaller units and ultimately dissipate. T stir a mixture gently a hmgeneus distributin f the dissipated energy is strived fr in the whle tanle y=197fmm] Ut;r>= 3 [mlsj lcal energydisslpatin V:'; Ii 2CC{; ~ It.!~.,..,..."..,.."..,..., '!'.,..--~.,..---=-=.,..--._= : "1sa ~ ;~~ ~ 7.#6 turulence intensity z i y~x g :::::::::::::JL;..;..;...:...J...:...:::==~=~O g g g ~ 6.32 ~ 5,0'2 400 ~ :l.~ ~ 3,':1 Ii 2~2 :f;,001 ~ z=6immj u",,=3[mlsj Fig. 5: Turbulence intensity f a hrizntal plane Fig. 6: Lcal energy dissipatin in a vertical plane