CFD simulation with multiphase flows in porous media and open mineral storage pile

Transcription

1 Computatonal Methods n Multphase Flow V 421 CFD smulaton wth multphase flows n porous meda and open mneral storage ple S. Torno, J. Toraño, I. Dego, M. Menéndez, M. Gent & J. Velasco School of Mnes, Unversty of Ovedo, Span Abstract In open storage ples n bulk solds port termnals, power statons and cement factores, not only the sold and porous barrer behavour n front of the ple s mportant, but also the effect that porous and sold barrers produce when they are behnd the ple. Consderng the dust propagaton behnd the ple, the sold barrers are more effectve than the porous ones. But, the effect of the porous barrers on the wnd velocty dstrbuton, manly n zones between the barrers and the ples should be taken nto account, regardng the total dust emsson to the atmosphere and ts propagaton. In these studes, wnd and dust concentraton measurements n laboratory and feld were carred out. A hot-wre anemometer (Veloccalc Plus (TSI)) was used n the wnd study and two dust collectors (E-Sampler, Met One Instruments, Inc., Oregon, USA) were used to measure the dust concentraton (Total Partcle lke PM10); a meteorologcal staton (E-Sampler, Met One Instruments, Inc., Oregon, USA) attached to a PM10 collector was also employed. All these measurements were used to adjust the 3D CFD computatonal model (Ansys CFX 10.0): In the wnd case through a k-epslon turbulence model and the dust case by Lagrangan method. These adjusted models allow us to carry out several smulatons combnng the effect of sold and porous fences n front of and behnd the ple, as well as ple shape modfcatons and behavour analyss accordng to the dust emsson from several specal confguratons and ts relaton to the wnd gust preferental drectons. Keywords: partcle emsson, Computatonal Flud Dynamcs (CFD), open storage ple, ar polluton. do: /mpf090361

2 422 Computatonal Methods n Multphase Flow V 1 Introducton The movement of mnerals n open storage ples produces partcle emssons to the atmosphere, whch were studed to mnmze ther effect n the area more or less near the ple. In prevous research [1], the use of sold barrers n storage ple protecton whch reduces dust emsson to 66% n the worst envronmental and ndustral condtons, was shown. Nevertheless, the use of ths sold barrer produces a hgh velocty vorte between the barrer and the ple, causng a greater dust emsson to the wndward sde of the ple, [2 4]. The authors, based on ther own eperence and other research lke [5] and [6], changed a sold barrer for a 30% porosty barrer, provng that the dust emsson s reduced by 78%. The 30% porosty s obtaned from research carred out by [7], who have determned from the Partcle Trackng Velocmtry that a porosty of 30% s the more effectve. Once the barrer effect (sold and porous ) n front of the open storage ple s studed, the need of studyng the barrer effect behnd the ple arses, thus the ple s protected aganst the wnd wth a barrer n front of t and the emtted dust s collected wth a barrer behnd the ple. In ths paper, the barrer effectveness, sold and porous, n each of the studed poston, s shown. The 3D Computatonal Flud Dynamcs or CFD code Ansys CFX 10.0 s used to carry out the smulatons, through the K-epslon turbulence model [8], whch was adjusted by the measurement campagns carred out n the Mne of Carbonar S.A. stuated n Asturas (North of Span). 2 Epermental descrpton 2.1 Introducton The scale tests were carred out n the nstallatons of Carbonar, whch are shown n fg. 1. The equpment conssted of a metalc cone of 52cm n heght (h) and 69cm n dameter, a metalc ventlaton ppe of 3m n length and 300mm n dameter, whch s connected to a 12kw fan, whch generates the wnd flow. Two barrers are added to these elements, whch are placed n front of and behnd the cone. 2.2 Barrer n front of the cone In ths case, the barrer has 52cm n heght and 1.78m n length such as t was eplaned n [1] for the sold barrer, and t s placed at 25cm from both the ppe and the cone. In fg. 2 dstrbuton of the elements used n the scale tests for the porous barrer, s shown.

3 Computatonal Methods n Multphase Flow V 423 Fgure 1: Zone where the equpment was nstalled. 2.3 Barrers behnd the cone. In ths case the barrer s placed at 3.5m behnd the cone and t has 1m n heght and 2.04m n length. In fg. 3 the dstrbuton of the elements for measurements n feld wth the dust collectors at 2.5m behnd the barrer, s shown. 2.4 Velocty and concentraton measurements. Flow velocty measurements were carred out wth a hot-wre anemometer (Veloccalc Plus (TSI)), whch was also used by other authors lke [9] and [10], n 73 measurement ponts dstrbuted n the cone zone and the barrer. Two dust collector (E-Sampler, Met One Instruments, Inc) measurng total partcles and PM10 (<10 mcron) were used to measure dust concentraton. Meteorologcal condtons, relatve humdty of atmosphere, temperature, wnd velocty and the wnd drecton were measured by a meteorologcal staton (E- Sampler, Met One Instruments, Inc) attached to a PM10 collector. These feld measurements were used to valdate CFD smulaton. 3 Modellng The mathematcal model s based on three equatons: contnuty, momentum and energy. These epressons form a partal dfferental equaton system, coupled n lnear form and they were used to acheve the soluton. The porosty nput n the model s carred out from the research developed by [11] and [12] n whch the absolute porosty value s calculated by two coeffcents, the Lnear Resstance Coeffcent (C R1 ) and the Quadratc Resstance Coeffcent (C R2 ): Determnaton of these coeffcents s ncluded n the dscretsaton of the Naver-Stokes equatons. Ths dscretsaton produces a set of equatons whch

4 descrbe the flud propertes, that s, momentum equaton (1) and contnuty equaton (2): ( ) ( ) 0 z w u t = ρ + ρ + ρ, (1) M j j 2 j j S u P u u t u + µ + = + ρ. (2) I II III IV V Fgure 2: Dstrbuton of the elements used for the porous barrers. Fgure 3: Dstrbuton of the elements wth the dust collectors. 424 Computatonal Methods n Multphase Flow V

5 Computatonal Methods n Multphase Flow V 425 where, I s the Acceleraton term, II s the Advecton term, III s the Pressure term, IV s the Dffuson term and V s the Source term (3): S = C u C U u (3) M R1 where, u s the velocty dmensonal component along the 3 drectons (,y,z), U s the velocty vector, U s the speed, P s pressure, ρ s the densty of ncompressble ar, µ s the dynamc vscosty and S M are the source terms n the three drectons; t ndcates the porous barrers n the equaton. C R1 s set to 0 (Kg /m 3 s 1 ) assumng neglgble vscous losses n the ambent ar flow (µ= N s/m 2 ). C R2 s set to (kg/m 4 ), n a 30% porosty, [11]. These coeffcents are related to absolute porosty va K loss (emprcal loss coeffcent) [C R2 ε 2 /ρ], whch s ncluded n Darcy s Law (4): P µ = u + k lossρ U u (4) k In fg. 4, the geometry carred out by usng SoldWorks for the barrer placed n front of the cone, s shown. The doman of the two models has the same dmensons (8525). R 2 Fgure 4: Geometry of the model wth the barrer n front of the cone. In fg. 5, the meshng carred out by usng IcemCFD wth the barrer placed behnd the cone, s shown. Ths model was meshed wth a total of 1,011,030 elements and the other model (barrer placed n front of the cone) was meshed wth a total of 1,107,611 elements. In both cases t can be seen a fner meshng zone, where the dust collectors are stuated, n whch dust concentraton measurements are taken for the smulatons (fg. 5). The flud doman s a 25º ar affected by the gravty laws, not takng nto account the heat transference and consderng k-epslon as the turbulence model.

6 426 Computatonal Methods n Multphase Flow V The boundary condtons have been an Inlet that corresponds to a 12 m/s wnd velocty comng out of a metallc ppe; the ground surface, cone, ppe and barrer as Wall and consderng the rest as Openng. Fgure 5: 3D meshng of the model wth the barrer behnd the cone. 4 Wnd results 4.1 Barrers n front of the cone Takng nto account the results obtaned n the model wth a sold barrer (when porosty s 0%), [1], t was decded to change the barrer porosty to 30%. Ths porous barrer makes the hgh velocty vorte that s produced behnd the sold barrer, dsappear, avodng the dust set n suspenson to wndward zone of the cone. In fg. 6 two planes of mean wnd velocty for both cases, sold and porous barrer, are shown. Besdes, how the porous barrer elmnates the wnd recrculaton between the barrer and the cone, whch s responsble for the dust emsson on wndward surface of the ple, s shown too. These velocty values obtaned from our smulatons have been valdated through velocty measurements n feld for the same ponts. In the sold barrer case, the equaton that relates feld measurements to those of the model s y= wth a correlaton coeffcent R 2 = In the 30% porosty barrer case, the equaton that relates feld measurements to those of the model s y= wth a correlaton coeffcent R 2 = The hgh values of the correlaton coeffcents ndcate the sutablty of the models.

7 Computatonal Methods n Multphase Flow V 427 (a) (b) Fgure 6: Planes of mean wnd velocty for sold barrer (a) and porous barrer (b). 4.2 Barrers behnd the cone When the barrer s behnd the cone, the sold barrer s the most effectve, snce t traps more quantty of dust emtted by the ple than the porous barrer. In fg. 7 two planes of mean wnd velocty perpendcular to the cone for sold and porous barrer, are shown. It can be seen n the porous barrer case, how the ar reaches the dust collectors wth a hgher velocty than n the sold barrer case. Ths s due to the recrculaton zone of low wnd velocty that s produced behnd the sold barrer,

8 428 Computatonal Methods n Multphase Flow V As n the prevous secton, the velocty values obtaned from our smulatons have been valdated through velocty measurements n feld for the same ponts. In the sold barrer case, the equaton that relates feld measurements to those of the model s y= wth a correlaton coeffcent R 2 = In the porous barrer case, the equaton that relates feld measurements to those of the model s y= wth a correlaton coeffcent R 2 = (a) (b) Fgure 7: Planes of mean wnd velocty for sold barrer (a) and porous barrer (b).

9 Computatonal Methods n Multphase Flow V Concentraton results Models are valdated when feld measurements concde wth smulatons carred out by CFX. The concentraton value n software Ansys CFX s acheved through `Partcle Trackng`, whch s Lagrangan type solver that smulates partcle trajectores from calculated velocty felds. Ths method starts from the contnuous phase calculaton (ar) and they are used as nput date for the dspersed phase (partcles). The mllons of partcles whch are set n suspenson n realty are smulated from the thousands of representatve partcles wth a certan mass quantty; ths s known as Partcle Number Rate [8]. The concentraton calculaton s obtaned from de calculaton of the area under the Concentraton-Tme curve obtaned by the PM10 dust collector. Ths calculaton s necessary as a permanent phenomenon (concentraton by CFX) and a transtory phenomenon (concentraton n feld) are compared. In table 1 the results of dust concentraton obtaned by modellng and those obtaned n feld for the two types of barrers, n front and behnd the cone, are shown. The porous barrer placed n front of the cone presents less partcle emsson to the atmosphere. The lower concentraton value behnd the cone corresponds to a sold barrer, as ths barrer traps the most partcles avodng they spread farther. As t can be seen, n table 1, the effect of porous barrers s to trap some partcles and decrease the velocty of those crossng the barrer. Table 1: BARRIER TYPE Comparson between concentraton measurements by CFX and n feld. CONCENTRATION (mg/m 3 ) In front of the cone Behnd the cone Epermental Smulaton Epermental Smulaton Sold Porous Concluson Ansys CFX software s a good tool to smulate the problems of dust set n suspenson and ts spreadng n the surroundng area of an open storage ple. When emssons of partculated materal to the atmosphere are mportant t s necessary to use protecton systems n front of the ple and behnd t. These protecton systems are sold and porous barrers and the effect they produce on dust emsson from the ple depend on the poston of ths ple.

10 430 Computatonal Methods n Multphase Flow V The use of a porous barrer n front of the ple decreases the effect of the hgh velocty vorte whch s produced behnd the sold barrer, avodng n ths way the partcle emsson on wndward surface of the ple. Behnd the ple, a sold barrer traps more partcles than a porous one. References [1] Dego, I., Toraño, J., Torno S. & Garca B., Epermental tests and Computatonal Flud Dynamcs (CFD) smulatons of barrers nstalled around open storage ples of raw materals. Advances n Flud Mechancs, 7, pp [2] Lee, S.J. & Lm, H.CH., A numercal study on ow around a trangular prsm located behnd a porous fence. Flud Dynamcs Research, 28, pp [3] Toraño, J., Rodríguez, R., Dego, I., Rvas, J.M. & Pelegry, A., Influence of the ple shape on wnd eroson CFD emsson smulaton. Appled Mathematcal Modellng, 31, pp [4] Dego, I., Toraño, J., Torno, S. & García, B., Epermental tests and Computatonal Flud Dynamcs (CFD) smulatons of barrers nstalled around open storage ples of raw materals. Seventh Internatonal Conference on Advances n Flud Mechancs (AFM 2008), The New Forest, UK. [5] Lee, S.J. & Km, H.B., Laboratory measurements of velocty and turbulence feld behnd porous fences, Journal of Wnd Engneerng and Industral Aerodynamcs, pp [6] Km, H.B. & Lee, S.J., Hole dameter effect on flow characterstcs of wake behnd porous fences havng the same porosty. Flud Dynamcs Research, 28, pp [7] Lee, S.J., Park, K.C. & Park, C.W., Wnd tunnel observatons about the shelter effect of porous fences on the sand partcle movements. Atmospherc Envronment, 36, pp [8] Dego, I., Pelegry, A., Torno, S., Toraño, J. & Menendez M., Smultaneous CFD Evaluaton of Wnd Flow and Dust Emsson n Open Storage Ples. Appled Mathematcal Modellng. Artcle n press. [9] Coleman, H.W., Steele, W.G, Epermentaton and Uncertanty Analyss for Engneers. John Wley & Sons., Inc., New York. [10] Km, H.G., Lee, Ch.M., Lm, H.C. & Kyong, N.H., An epermental and numercal study on the flow over two-dmensonal hlls. Journal of Wnd Engneerng and Industral Aerodynamcs, 66, pp [11] Twary, A., Morvan, H.P. & Colls, J.J., Modellng the sze-dependent collecton effcency of hedgerows for ambent aerosols. Journal of Aerosol Scence, 37, pp [12] Wang, H. & Takle, E.S., A numercal smulaton of boundary-layer flows near shelterbelts. Boundary-Layer Meteorology, 75, pp [13] ANSYS CFX-Solver, Release 10.0: Theory, Flow n porous meda, pp