Energy configuration optimization of submerged propeller in oxidation ditch based on CFD

Transcription

1 IOP Conference Seres: Earth and Envronmental Scence Energy confguraton optmzaton of submerged propeller n oxdaton dtch based on CFD To cte ths artcle: S Y Wu et al 01 IOP Conf. Ser.: Earth Envron. Sc Vew the artcle onlne for updates and enhancements. Related content - Effects of computatonal grds and turbulence models on numercal smulaton of centrfugal pump wth CFD H L Lu, M M Lu, L Dong et al. - Cavtaton smulaton and NPSH predcton of a double sucton centrfugal pump P L, Y F Huang and J L - Numercal study of cavtaton flows nsde a tubular pumpng staton X L Tang, W Huang, F J Wang et al. Ths content was downloaded from IP address on 1/07/018 at 10:09

2 Energy confguraton optmzaton of submerged propeller n oxdaton dtch based on CFD S Y Wu, D Q Zhou and Y Zheng College of Energy and Electrcal, Hoha Unv, Nanjng, Jangsu, Chna E-Mal:wusyuan571@163.com Abstract. The submerged propeller s presented as an mportant dynamc source n oxdaton dtch. In order to guarantee the actvated sludge not depost, t s necessary to own adequate drve power. Otherwse, t wll cause many problems such as the awful mxed flow and the great consumng of energy. At present, carryng on the nstallaton optmzaton of submerged propeller n oxdaton dtch mostly depends on experence. So t s necessary to use modern desgn method to optmze the nstallaton poston and number of submerged propeller, and to research submerged propeller flow feld characterstcs. The submerged propeller nternal flow s smulated by usng CFD software FLUENT6.3. Based on Naver-Stokes equatons and standard k turbulence model, the flow was smulated by usng a SIMPLE algorthm. The results ndcate that the submerged propeller nstallaton poston change could avod the condton of back mxng, whch caused by the strong drve. Besdes, the problem of sludge depost and the low velocty n the bend whch caused by the drve power attenuaton could be solved. By adjustng the submerged propeller number, the least power densty that the mxng drve needed could be determned and savng energy purpose could be acheved. The study can provde theoretcal gudance for optmze the submerged propeller nstallaton poston and determne submerged propeller number. 1. Introducton The oxdaton dtch s a knd of Intermttent Cycle Extended Aeraton System (ICEAS), treatment capacty of whch s better than other bologcal treatment system because of unque mxng performance. In order to make certan of unque mxng performance and treatment effect, mscble lquds must crcular flow wth certan current velocty n oxdaton dtch. When the mscble lquds velocty s too slow, t s easy for sludge to depost at the bottom of the pool. Sludge settlng not only decrease oxdaton dtch workng volume, but also led to energy consumpton ncrease. The problem of sludge settlng and energy consumpton have ntmate connect wth flow feld dstrbuton n oxdaton dtch. In ths study, the prmary msson s research submerged propellers nfluence on oxdaton dtch flow feld. At present, carryng on the submerged propeller nstallaton optmzaton n oxdaton dtch mostly depends on experence. When the dstance between the submerged propellers and the gude walls s too short, the condton of back mxng whch caused by the strong drve s appear. When the dstance between the submerged propellers and the gude walls s too far away, the problem of sludge depost and the low velocty n the bend whch caused by the attenuaton of the drve power also wll appear. The submerged propellers nternal flow s smulated by usng CFD (computatonal flud dynamcs). Through changng submerged propeller nstallaton poston and submerged propeller number, the Publshed under lcence by Ltd 1

3 actuatng length and power consumpton of dfferent submerged propellers are compared, the submerged propellers nstallaton poston whch could satsfy the oxdaton dtch need can be dentfed, and submerged propeller number s the least at the same tme. Ths research could provde references for the further study on optmzaton of submerged propellers.. Materals and methods.1. Expermental set up The computatonal doman s the basc oxdaton dtch model. The sngle straght segment s 90 meters n length, and the bg sem-crcle radus s 9 meters and the small semcrcle radus s 4.5 meters. Each channel s 9 meters wde, wth a water depth of 5 meters. Central wall wdth s 0.5 meters. The oxdaton dtch dagrams are shown n fgure 1 and fgure. Fgure 1. Dagram of the oxdaton dtch (unt: m). Fgure. A-A cross secton (unt: m). From fgure 3 and fgure 4, there have 11 cross sectons n oxdaton dtch. The cross secton sze s 5 meters 9 meters, and each of cross secton has 9 measurement ponts. Fgure 3. Samplng cross sectons locatons. Fgure 4. Schematc dagram of samplng locatons... Mathematcal modellng..1. Flow feld modellng. In order to predct flow feld n oxdaton dtch, the study use sngle-phase flow of three dmensonal CFD model. The Reynolds-averaged, Naver-Stokes equatons that govern the 3D, steady-state, ncompressble flow n oxdaton dtch as follows: Contnuty equaton: Momentum equatons: u 0 j t j p j j 1 P u u j ( uu ) [ ( )] The t n above equatons s determned wth the k turbulence model: (1) ()

4 t The dstrbuton of k and are calculated from the followng sem-emprcal modeled transport equatons: k Equaton: Equaton: C k k t k u ( ) G k u [ t ] C1 G C k k u u j u G t[ ] j... Smulaton of submerged mpellers. The submerged propellers n oxdaton dtch s the only energy source. Apart from producng axal flow velocty, the rotatng mpeller would also create eddy currents around tself. The eddy current would affect the dstrbuton of flow feld near the mpeller. However, ts effect was small when compared to the whole flow feld n the dtch, so t was gnored n the smulaton. The mpeller man functon s to pass energy to the flud to obtan an axal flow velocty. Each submerged propellers has two blades. The radus of wheel hub and mpeller s 0.16 meters, 0.9 meters. The blade thckness from root to empennage smooth blendng, the blade root thckness s 40 mllmeter and the empennage thckness s 18 mllmeter. For the submerged propellers, when the blade mountng angle s 5 degree and rotatonal speed s 6.18rad/s, the submerged propellers power consume s the least. There are two knds of work condtons. As shown n fgure 1, the submerged propellers one s just workng on the work condton 1, the nstallaton poston of submerged propellers.5 meters blow water surface. All submerged propellers are workng on work condton. The nstallaton poston of submerged propellers one and submerged propellers three can be found from fgure, and the nstallaton poston of submerged propellers two s the same as submerged propellers three n the vertcal secton. j (3) (4) (5) Fgure 5. The fan model for a submerged mpeller. As shown n fgure 5, the movng regon and the fxed area compose the computatonal doman. A cylndrcal movng zone wth 0.4 meters n wdth and 0.95m n radus s created for each mpeller. The flud n the movng zone obtaned momentum and energy by axal thrust and the momentum and energy passed to the rest of the flud n the dtch followng the momentum and energy equatons. In ths study, three knds of mesh whch have dfferent densty standard are desgned. The mesh parameter and computed result wll be shown n table 1. Wth mesh element ncrease, the power s 3

5 decrease. After consderng factors such as the computer capacty and actual result, the scale of grd s selected for further research. The mesh sze of movng regon s 0.0m and the mesh sze of fxed area s 0.4m. Table 1. Mesh parameter and result...3. Boundary condtons. The rotaton dsc n operaton s set as the movng wall, and then the rotatonal speed, rotaton-axs orgn and drecton of the movng wall are set, respectvely. The roughness constant and the roughness heght of the movng wall are set as 0.5 and m, respectvely. The free surface s regarded as symmetry. The non-slp boundary condton s suppled for the others, whch ncluded the bed, the sde and central walls, and the gude wall. The boundary condton between the movng and fxed meshes s nteror. Snce all of wastewater flow rate compare wth the average desgn velocty n the dtch (0.15m/s) s relatvely small, the nflow and outflow are not beng counted n the computaton...4. Software package. The FLUENT package was chosen for mplementng ths case. Pressure based solver and the control-volume based SIMPLE dscretzaton scheme were used. All of the parameter settngs held ther default values n FLUENT6.3. The total CPU tme for a converged soluton was about 39h, or 7000 teraton steps. 3. Results and dscusson 3.1. Determne submerged propellers nstallaton poston On workng condton 1, when the dstance from gude walls to submerged propellers s 5 meters, 10 meters, 15 meters, 0 meters, 5 meters, Choosng the central measurement ponts of cross sectons, flow velocty n dfferent cross sectons s measured. The result s shown n fgure 6. Because the dstance from gude walls to submerged propellers s greater than 0 meters, the lqud homogeneous dstrbuton s completed. As we all know, the velocty dstrbuton depends on the vscous couplng n vertcal heght at ths tme. As shown n fgure 6, flud wll keep the velocty gradent that the top and the mddle layer of velocty s greater than the bottom layer velocty. For the dfferent submerged propellers nstallaton poston, when the lqud flow reach to the cross secton of 4, the velocty n the oxdaton dtch s less than 0.15m/s, t s easy for sludge to depost at the gude walls. Wth the software FLUENT, the torque could be read n dfferent workng condtons. Usng the formula of horse-power, the submerged propellers power could be determned. Horse-power formula: P M (6) The submerged propellers power for the dstance from gude walls to submerged propellers wll be shown n table. When the rotatonal speed s 6.18 rad/s and the dstance from gude walls s 10 meters, the power consume s the last. The results ndcate that the change of the nstallaton poston of submerged propeller could avod the condton of back mxng, whch caused by the strong drve. Besdes, the problem of sludge depost and the low velocty n the bend whch caused by the attenuaton of the drve power could be solved. 4

6 Fgure 6. Flow velocty (m/s) versus n dfferent cross sectons. Table. The submerged propellers power for the dstance from gude walls to submerged propellers. 3.. Flow feld analyss Accordng to the analyze of fgure 6, the velocty s less than 0.15m/s at the gude walls, t s easy for sludge to depost. In order to meet the need of oxdaton dtch, under the workng condton of two, three submerged propellers are openng at one tme. Measurements are carred out n the computed dtch. The velocty magntude s probed at several samplng pont, located at three dfferent layers, the top layer 0.5 meters blow water surface, the mddle layer.5 meters blow water surface, and the bottom layer 0.5 meters from the oxdaton dtch bed. As shown n fgure 7. When the top lqud flow just reach to the frst straghtaway, the velocty of straghtaway nner sde s less than the velocty of straghtaway outer flank. Ths s because the functon of the second curved condut effluent. The lqud flow dstrbuton s homogeneous n the drecton of flow and soon after. The range of velocty s from 0.0 meters to 0.3 meters n frontal of the frst straghtaway. Under the acton of submerged propellers, the velocty of top lqud flow sn t ncrease mmedately, on the role of frcton drag the velocty s decrease on the contrary. The range of velocty s declne from 0.0 5

7 meters to 0.15 meters. Untl reach to the cross sectons of 3, the hgh-speed of lqud flows to central and hgher layer. The velocty of top lqud flow rebounded to 0.0 meters. The velocty of the frst curved condut outer flank s greater than the velocty of the frst curved condut nner sde. The velocty of the frst curved condut nner sde s also great than 0.0 meters, the sludge settlng phenomenon wll not take place. The nner sde velocty s low and the outer flank s hgh at the curved condut export. The velocty fluctuaton condton of the second straghtaway s smlar to the frst straghtaway. The trend of velocty change s decrease frst and then. The range of velocty s from 0.35 meters to 0.15 meters after equspaced. (a) (b) (c) Fgure 7. Velocty (m/s) profles n the horzontal secton of the dtch ((a) Top layer, (b) Mddle layer, (c) Bottom layer). Fgure 8. Flow velocty (m/s) versus n dfferent cross sectons ((a) velocty at the cross secton of 1 and 8, (b) velocty at the cross secton of 3 and 7, (c) velocty at the cross secton of 4 and 6, (d) velocty at the cross secton of 9 and 11). 6

8 On the bass of desgn procedure made by the company of LIM n Denmark, when the lqud medum and the oxdaton dtch structure are confrmed, the mnmum power that the oxdaton dtch need can be ensured. UL ( ) 0.15 H f ( ) C / [ ] 0.33 J /kg F 45 W H Q =.3kW r f W W /.3/ 9% 7.69kW R r Table 3. The submerged propellers power on work condton. As shown n table 3, the total power of submerged propellers s 7.74kW, and the mnmum power that the oxdaton dtch need s 7.69kW. As we all know, the power s an mportant parameters for the ratng of mert of submerged propellers. When the total power s less than the mnmum power, the propelled process purpose cannot be realzed. To the contrary, when the total power s greater than the mnmum power too much, ths causes the energy waste. Therefore, when the propelled process meets the requrement, the lower of power s, the better of submerged propellers s. When the submerged propellers are workng on work condton, the submerged propellers could satsfy the need of oxdaton dtch, at the same tme, the power consume s the least. 4. Conclusons An analyss of the numercal smulaton results has been made, and the result s found to be accepted by comparng t wth theores. The study has led to the followng fndngs and conclusons: (1) For the submerged propellers, when the rotatonal speed s 6.18 rad/s and the dstance from gude walls s 10 meters, the power consume s the last. The results ndcate that the change of the nstallaton poston of submerged propeller could avod the condton of back mxng, whch caused by the strong drve. Besdes, the problem of sludge depost and the low velocty n the bend whch caused by the attenuaton of the drve power could be solved. () The dstance from submerged propellers to cross secton s less than 0m; the velocty of lower layer s more than hgher layer, and wth axal dstances ncrease, the hgh-speed of lqud flow to central and hgher layer. It s for sludge to depost at the nner flank and the down cuttng phenomenon s appeared at the outer flank. (3)When the submerged propellers are workng on work condton, the submerged propellers could satsfy the need of oxdaton dtch, at the same tme, the power consume s the least. Nomenclature Q The flow of unt tme, m 3 / s P Power, kw Local resstance factor, 1.0 M Torque, N m L Wre length of oxdaton dtch, m F Wetted area, m The sum of local resstance factor ` P Pressure, Pascal Frctonal resstant factor, 0.03 Effcency, =0% 7

9 W r Power needs by mpulse all lqud, kw U Wetted permeter, m u Velocty component n drecton, m/ s p Water densty, kg / m Coordnate component n j drecton, m/ s C Impulse velocty, m/ s Turbulence Schmdt number for, 1.3 j Subscrpt, j =1,, 3, 1 C Emprcal constant of the k model, C Subscrpt, =1,, 3, C Emprcal constant of the k model, C 0.09 t Eddy vscosty, m/ s C Emprcal constant of the k model, C 1.9 W R Mnmum power, kw H Power needs by each klogram of lqud, J / kg k G f Average turbulence knetc energy per unt mass, m / s 3 Dsspaton rate of the turbulence knetc energy, m / s Producton term of turbulent energy by the mean velocty gradents, kg / m s Acknowledgments Ths research project s sponsored by Natural Scence Foundaton of Chna (No ). References [1] Tan F, Sh W D, Chen B and Cao W D 011 Mxng performance nvestgaton of sewage treatment mxer under mult-rotatonal speed 4th hydraulc machnery & system conf. of Chna (Denver, USA, November 011) pp [] Zhang Y, Huang W D, Gou Q Z, Wang G and Xe R H 009 Industral water & wasterwatert 40(1) [3] Yang H Z, Wang T and Deng R S 007 Journal of chongqng janzhu unversty 9(3) [4] Cao R Y and Fu J Z 001 Chna water & wasterwater 17() 16-8 [5] Stamon A I 1994 Water scence and technology 30() [6] Stamou A I 1997 Water scence and technology 36(5) [7] Luo L, L W M, Deng R S and Wang T 005 Journal of envronmental scences 17(5) [8] Zhao X P, Yang J K and Wu Y Y 009 Heat transfer engneerng. 30(8) [9] Lesage N, Sperando M, Lafforgue C and Cockx A 003 Chemcal engneerng research and desgn 81(A9) [10] Yang Y, Yang J K, Zuo J L, L Y, He S, Yang X and Zhang K 011 Water research [11] Zhang Z C, Zhang X S and Zhang M R 004 Chna leather 33(11) -5 [1] Yang Y, Yang J K, Zuo J L, L Y, He S, Yang X and Zhang K 011 Water research