VENTILATION AND DEDUSTING OF MELTING SHOPS

Transcription

1 Second Inernaonal Conference on CFD n he Mnerals and rocess Indusres CSIRO, Melbourne, Ausrala 6-8 December 1999 VENTILATION AND DEDUSTING OF MELTING SHOS Rober KICKINGER 1, hlpp GITTLER 1, Mrko JAVUREK 1 and Johann LEHNER 1 Johannes Kepler Unversy, Fludmechancs Deparmen, Alenbergersr. 69, 4040 Lnz, AUSTRIA VOEST-ALINE Indusreanlagenbau GmbH, Turmsr. 44, 4031 Lnz, AUSTRIA ABSTRACT CFD smulaons of venlaon and dedusng sysems n melng shops wh an elecrc arc furnace are presened. The models nclude free and forced convecon, mulphase flow and complex geomery. Specal emphasze s gven o he modellng of he dus phase: An Euler-Lagrangan dscree random walk approach, he sochasc ranspor of parcles (ST) model and an algebrac drf flux model are brefly revewed and appled n smulaon. Whle applcaon of he ST-model was unsuccessful, boh he Euler-Lagrange approach and he drf flux model perform well, wh he drf flux model beng more compuaonally effcen. Sudes on he nfluence of blower (ID fan) power and he effec of urbulence modellng (sandard and RNG k-ε model) on dus dsperson are ncluded. The numercal sudes resuled n he opmzaon of blower power ogeher wh he crane/rolley arrangemen and he canopy hood desgn. NOMENCLATURE c p specfc hea capacy D p parcle dameer Gr Grashof number k urbulen knec energy m D oal mass of dus parcles n plan m mass of dus parcle p pressure Re Reynolds number T emperaure me u velocy u parcle velocy x j j h coordnae drecon n space α volume fracon of dus β hermal expanson coeffcen ε urbulen dsspaon rae λ hermal conducvy µ dynamc vscosy densy of ar densy of dus parcles INTRODUCTION In elecro seel makng processes a large amoun of dus s creaed: Durng chargng off-gases are generaed drecly above he elecrc arc furnace (EAF) due o he burnng of ol conanng scrap. Durng melng he off-gases are emed no he plan by he EAF hrough openngs for he elecrc power supply and he oxygen lance. Usually a dedusng sysem consss of wo pars: he prmary off-gas sucon sysem aached o he EAF va he elbow duc and he secondary offgas sucon fed on he roof of he melng shop (see Fg. 1 and Fg. ) above he EAF. These sucon sysems mus be desgned properly n order o acheve a mnmum dus concenraon a workng areas a a gven sucon mass flow or vce versa. As ypcal power consumpons for he sucon sysem are n he range of 1 - MW here s a grea poenal for savngs. Ths paper deals wh he smulaon and desgn of secondary dedusng sysems by means of CFD. Basc deas have been consdered by Bra (1997) and a smlar sysem was suded by Ish e al. (1997). Fgure 1: Indusral plan wh varous venlaon nles and oules. GEOMETRY AND GRID A very dealed modellng of a melng-shop s no praccable due o he complex geomery. The queson whch componens of an ndusral plan have a large mpac on he flow paern canno be answered easly. For hs sudy we ncluded he followng componens no our CFD model: boundares of he plan (floor, sdewalls, roof) canopy hood and paron sde walls elecrc arc furnace off-gas sucon venlaon nles and oules plaforms and large ransformers crane, rolley and scrap baske (for he smulaon of he chargng process) Some of hese are dsplayed n Fg. 1 and Fg.. The benefs of unsrucured meshng echnques and adapve grd refnemen have been consequenly used o creae a compuaonal grd whch s dense n relevan regons and coarse elsewhere. 19

2 Crane Scrap Baske Floor Canopy Hood Trolley EAF Fgure : Confguraon (Trolley mouned beween craneways). MATHEMATICAL MODELLING The Raylegh number gβ TH c µλ 3 14 Ra (1) ~ where H 30 m s he ypcal hegh of he buldng, ndcaes a hghly urbulen plume, hence a urbulen formulaon was followed: Governng Equaons The Reynolds-averaged mass, momenum and energy equaons are consdered. Ths se of conservaon equaons s well known and no gven here. Turbulence Modellng To close he above equaons he sandard k-ε model (Launder and Spaldng, 197) and he RNG k-ε model (Yakho and Orszag, 199) have been used. The RNG model exends he sandard k-ε model by an analycally derved dfferenal formula for he effecve vscosy accounng for low-reynolds-number effecs and oher feaures such as an addonal erm n he ranspor equaon for ε. Modellng Naural Convecon The rao of buoyancy and neral forces aron Sde Walls Transformer Gr gh ~ 5 () Re u 1 6 where u 6 m/s s a ypcal velocy n he plume, ndcaes srong buoyancy conrbuons o he flow. To accoun for naural convecon he Boussnesq approxmaon and he deal gas law have been used n smulaons. Boundary Condons Sandard wall funcons (Launder and Spaldng, 1974) have been used and all walls have been consdered smooh. Venlaon nles and oules have been modelled as pressure boundary condons. The off-gas sucon oule was modelled as velocy boundary condon. Typcal values n he secondary off-gas duc range from 8 o 5 m/s. Elecrc Arc Furnace The furnace s emng energy and ho gases as well as dus no he plan. In smulaons he mass flow and emperaure of emsson gases have been prescrbed. These parameers have been compued by an EAF smulaon ool developed by VAI and descrbed n Hofer e al. (1997). In he CFD smulaons radaon was negleced n general. Dffuson/Convecon Transpor Model We consder a Speces Transpor Model whou chemcal reacons and lamnar mass dffuson, gven by µ m' ( m' ) + ( u jm' ) ( ). (3) j j Sc j Here m' s he mass fracon of speces, µ s he urbulen vscosy and he urbulen Schmd number s se o Sc 0.7 (4) Ths model s used n he smulaon of he chargng process o judge how effcen a confguraon sucks off dus loaden ar. Of course hs model s no capable of represenng sedmenaon and smulaons apply o he dus parcles, ha move wh he flow of he connous phase. Euler-Lagrangan Dscree Random Walk Model In hs model he rajecory s of a dus parcle s compued by negrang he force balance du m FD + Fg + Fm + Fp d (5) n a Lagrangan reference frame. Here 18µ CD Re FD m ( u u ) D 4 (6) denoes he drag force and D u u Re µ (7) s he relave Reynolds number. C D s he drag coeffcen for smooh sphercal parcles followng Mors and Alexander (197). The erm Fg mgs (1 ), (8) where g s s he projecon of he gravaon vecor on he coordnae along parcle rajecory, regards o gravaon and accouns for sedmenaon effecs. F m 1 m d d ( u u s he force necessary o accelerae he surroundng flud (added mass concep). Because of he large dfference n densy ~ 3000 (10) he nfluence of he added mass force s small, however s ncluded n he smulaons. dp Fp m u (11) ds s he force acng due o pressure gradens n he flud. Turbulen dsperson s modelled by a dscree random walk mehod (sochasc rackng echnque), see Fluen ) (9) 130

3 Inc. (1998). The effec of he dspersed phase on he connous phase s realsed by source erms n he connous phase equaons. The connous and dspersed phase equaons are solved alernaely unl he soluons n boh phases have converged. The modelled parcle sze dsrbuon s gven n Table 1, where γ ndcaes he mass fracon for each parcle dameer. Ths dscree specrum s represenave for a measured connous specrum. D p [µm] γ [%] u D [m/s] Table 1: arcle sze dsrbuon and drf veloces. Sochasc Transpor of arcles Model Whle he Dscree Random Walk mehod s based on he averagng of a large number of random parcle rajecores he sochasc ranspor of parcles (ST) model follows a rgorous mahemacal approach. Turbulen dsperson of parcles n gas flows abou a mean rajecory s calculaed usng sascal mehods: The concenraon of parcles abou a mean rajecory s represened by a mulvarane Gaussan probably funcon (DF) where ( x, ) e s / 3 3/ (8π ) 1 σ, (1) 3 x µ s. (13) 1 σ The geomercal poson µ () represens he mos lkely poson of a parcle (he cener of he parcle cloud). The varances σ of he DF are based on he degree of parcle dsperson due o urbulen flucuaons and hey can be expressed as a funcon of he mean square velocy flucuaons and he parcle velocy correlaon funcon (Baxer and Smh, 1993). Therefore he modellng of he parcle velocy correlaon funcon deermnes he parcle dsperson (Wang, 1990). The mean rajecory s obaned by solvng he ensemble-averaged equaons for all parcles represened by he DF. Algebrac Drf Flux Model Ths mulphase flow model for nerpenerang meda n general solves he connuy and momenum equaon for he mxure and a volume fracon equaon for each addonal phase. I allows he phases o move a dfferen veloces and he drf velocy of phase j u u u, (14) D, j j m where u m s he mxure velocy, s gven by an algebrac expresson. Snce he maxmum mass fracon of dus s very low n hs applcaon, α < 0.003, (15) m he conrbuon of he dus phase o he mxure densy and mxure velocy can be negleced: m (16) u m u Therefore he drf velocy of he prmary phase vanshes u D, ar 0 (17) and he drf velocy of he dus phase s compued by an analyss of Eq. 5: We consder an unacceleraed moon (local equlbrum beween he phases) and neglec he nfluence of pressure gradens (due o Eq. 10) and oban 4D ud g. (18) 18µ CD Re for he drf velocy of he dus parcles. Wh he drag coeffcen C D as gven by Mors and Alexander (197) Eq. 18 s a quadrac equaon for he drf velocy and numercal values for dfferen parcle dameers are gven n Table 1. The assumpon of local equlbrum can be jusfed by he fac, ha parcle acceleraon occurs only n shor me scales (and herefore n shor spaal lengh scales) before gravaon and drag forces equal each oher. Wh he nfluence of urbulen dffuson he volume fracon equaon for he dus phase reads ( α) + µ ( Sc ( αu ) α ( ) αu D ) (19) As a consequence of Eq. 16 he volume fracon equaon s decoupled from he oher feld equaons (for he mxure), whch are (agan due o Eq. 16) equal o hose of he gas phase. Boundary condon for he volume fracon equaon are as follows: rescrbed volume fracon a nles and zero graden a vercal walls. Sedmenaon on non-vercal walls was mplemened by source erms n he neghbourng cells. RESULTS AND DISCUSSION All smulaon resuls have been obaned usng he commercal CFD code Fluen 5 (and he former verson Fluen/UNS) exended by user defned rounes. Domnaed by naural convecon, he flow n he venlaed plan wh a hea source s que sensve and convergence n numercal smulaon can only be reached by usng an unseady solver. Ths ndcaes ha no seady soluon exss for hs ype of flow, see also Reynolds (1998). If he plan has only a few venlaon nles he whole flow paern (n smulaon) shows grea dependance on he drecon vecor of he nflowng ar. Ths parameer was deermned by separae CFD smulaons akng no accoun he complex geomery (nose reducon devces and wndow shades) of he venlaon nles (Mer, 1998). Velocy vecors durng he melng process are dsplayed n Fg

4 walk approach s exremely CU nensve as s necessary o compue a large number of parcle rajecores o ge some sascal safey. 7.e-6 6.e-6 1µm EAF 5.1e-6 4.1e-6 3.1e-6.1e-6 1.0e-6 100µm EAF 0.0e-0 Fgure 4: Conours of dus concenraon [kg/m 3 ] n headhegh over plaforms. Fgure 3: lume velocy vecors durng melng. ST-Model Whle hs model was succesfully appled o smple flows (Baxer and Smh, 1993), s applcaon for hs parcular flow proved as very dffcul and suffers from a prncpal drawback: As mos of he dus s sucked off by he venlaon and only a small percenage of he oal amoun of dus sedmens n he plan (as a consequence of urbulen dsperson), he mean cloud rajecores leave he compuaonal doman hrough he venlaon oule and very large maxmum cloud dameers are necessary o accoun for he effecs of sedmenaon. In combnaon wh long parcle resdence mes before sedmenaon occurs hs leads o excessve smulaon mes prohbng he successful applcaon of hs model n he presen form and wh he compuer hardware avalable. Run 1 Run Run 3 Run 4 m D [kg] c D [10-6 kg/m 3 ] c O [10-3 kg/m 3 ] dm O [10-3 kg/s] Table : Resuls from dscree random walk smulaons. Euler-Lagrangan Dsree Random Walk Model Table gves resuls from four runs showng he sascal naure of he dscree random walk model. In hs sudy up o rajecores wh up o seps n space per DM-eraon have been compued. The dus concenraon n he off gas c O (and herefore he mass flow of dus n he offgas dm O) scaer n a very narrow bandwdh whle he oal amoun of dus n he plan m D and he average dus concenraon n workng areas c D scaer by a facor ~. Ths ndcaes ha he number of parcles racked and he number of seps s suffcen bu accuracy could be ncreased by calculang more and longer parcle rajecores. However he dscree random Fgure 5: Conours of dus concenraon [kg/m 3 ] on a vercal plane. Algebrac Drf Flux Model When applyng hs model o secondary phases wh nonunform parcle sze dsrbuon one approach s o compue a mass weghed average drf velocy and solve one ranspor equaon for he volume fracon. The alernave (followed n hs arcle) s o solve an addonal equaon for he volume fracon of each parcle sze (as gven n Table 1). Ths approach provdes nformaon abou he nfluence of parcle sze on dsperson. The concenraon of 1µm- and 100µmparcles s dsplayed n Fg. 4 clearly ndcang, ha 100µm-parcles prmarly sedmen n he vcny of he EAF, whle 1µm-parcles are dsrbued over he whole plan. The overall dus concenraon (sum of all ndvdual parcle sze concenraons) on a vercal plane s gven n Fg. 5. The nfluence of clean ar enerng hrough a venlaon nle s clearly vsble. The oal mass of dus n he plan m D and he average dus concenraon n workng areas c D are gven n Table 3. In general he drf flux approach s robus and economc n erms of compuaonal resources compared o he dscree random walk approach. Furhermore conrary o he Euler- 13

5 Lagrange model effcen parallelsaon n he framework of a commercal CFD package s no problem. 0.1µm 1µm 10µm 100µm sum m D [kg] c D [10-3 g/m 3 ] Table 3: Dus phase smulaon resuls (Drf flux model). Dscusson of Dus hase Smulaon Resuls As can be seen from Tables and 3 smulaon resuls for he oal mass of dus parcles n he plan m D and he average dus concenraon n workng area c D obaned by he drf flux model are ~ 4 mes hgher han for he Euler-Lagragan dscree random walk approach. Ths large dfference mples some quesons: How can hs large dfference be explaned? Is he drf flux model overshoong or he Euler-Lagragan approach undershoong dus concenraon? ossble explanaons (nclude among ohers) a oo small choce of he urbulen Schmd number Sc n he drf flux model resulng n oo hgh parcle dsperson. The measured value of average dus concenraon n workng areas c D kg/m 3 s very close o he Euler-Lagrangan smulaon resul. However hs s no very useful n answerng he queson, whch approach s closer o realy, because he dus concenraon n boh models bascally depends on he volume fracon of dus a he EAF (he only source of dus n he model). Ths parameer was compued by DynEAF (Hofer e al., 1997) and s very dffcul o measure and herefore has no been valdaed by measuremens so far. However dspe he quanave dscrepancy he qualave correlaon of smulaon resuls of boh models s good and boh models can be appled successfully for he comparson of confguraons. whle he gas n he complemenary par of he plan s nalzed o conss of speces 1 (clean ar) m ' 0 and m' 1 1elsewhere. (1) By solvng he dffuson/convecon ranspor equaon Eq. 3 we oban graphs ϕ() as shown n Fg. 6 by negrang he mass fracon m' () over he compuaonal doman and dvdng hrough he nal mass of speces a me m' ( ) dv ϕ ( ). () m' ( 0) dv Fgure 7: Conours of mass fracon of spece m' [1] durng chargng (Confguraon 1) Fgure 6: Mass fracon for wo dfferen confguraons. Geomercal Opmzaons When opmzng confguraons, boh he melng process and he chargng process mus be consdered. Evaluaon of confguraons wh regard o he melng process s done as dealed above wh resuls from he drf flux model or he Euler-Lagrangan approach. To judge how effcen a confguraon sucks off dus loaden ar durng chargng he dffuson/convecon ranspor model s used n smulaons: From observaons he area where dus and hea are generaed durng he chargng process s known. The gas n hs area s nalzed a me 0 o conss of speces (dus-loaden ar) exclusvely, m ' 1 and m' 1 0 a dus generaon area, (0) Fgure 8: Conours of mass fracon of spece m' [1] durng chargng (Confguraon ). As one major desgn mprovemen we descrbe he opmzaon of he crane/rolley arrangemen: In Confguraon 1 he hermal plume s spl by he crane/rolley arrangemen (see Fg. 7) and one par of he plume s dreced owards he roof of he plan resulng n a large recrculaon regon above he rolley and n poor sucon performance (Fg. 6). Ths effec s due o he unapproprae desgn of he crane/rolley arrangemen (see Fg. 9) whch forces a quas wo-dmensonal flow paern beween he craneways. Ths effec s compleely suppressed by he opmzed Confguraon (Fg. 8). In hs opmzed confguraon he rolley s no mouned above bu beween he craneways o preven he quas wo-dmensonal splng of he plume and o ensure, ha off-gases can easly flow around he whole arrangemen on 133

6 all sdes. In combnaon smulaon resuls of boh he melng process and he chargng process provde good valdaon crera o judge he qualy of confguraons and are he bass for opmzaons. Fgure 9: Confguraon 1. Comparson of Turbulence Models Trolley mouned above craneways As ndcaed n Table 4 he RNG model yelds sgnfcanly smaller average values n urbulen knec energy k avg and urbulen dsspaon rae ε avg, whle he average velocy u avg s very smlar. In combnaon wh he dscree random walk model hs resuls n smaller urbulen parcle dsperson and herefore smaller values n dus concenraon. m D denoes he oal mass of dus n he plan and c D s he average dus concenraon n workng areas. k-ε model RNG model u avg [m/s] k avg [m /s ] ε avg [m /s 3 ] m D [kg] c D [10-6 kg/m 3 ] Table 4: Dscree random walk model resuls usng dfferen urbulence models. Fgure 10: Dus/off-gas-sucon-massflow relaon for an napproprae desgned confguraon. Varaon of Blower ower I sounds rval ha ncreasng blower (ID fan) power (and herefore offgas sucon mass flow) resuls n smaller values of dus concenraon. Bu relaons n flud mechancs are mosly nonlnear and for an napproprae confguraon of venlaon nles an ncrease n blower power can acually lead o hgher overall dus concenraons n he plan as shown n Fg. 10. The numercal values n Fg. 10 have been obaned usng he Euler-Lagrangan dscree random walk approach and ndcae he percenage of dus parcles whch are no sucked off and sedmen n he plan. Ths effec can be explaned by he nfluence of a free je from a nearby venlaon nle pushng he plume parly ousde of he canopy hood. However hs wll no happen n a properly desgned melng shop. CONCLUSION CFD s a powerful ool for he desgn of venlaon and dedusng sysems: The smulaon of exsng or projeced sysems reveals he weaknesses of a confguraon. Wh he dealed knowledge obaned by he smulaons opmzaons can be proposed and shown o be effecve. ACKNOWLEDGEMENTS The Drf Flux Model was mplemened by Mrko Javurek. Hs work s fnanced by VOEST-ALINE Sahl Lnz GmbH, Ausra. REFERENCES BAXTER L. L. and SMITH. J.: "Turbulen Dsperson of arcles: The ST Model'', Energy & Fuels, 7, , BIRAT J..: ''Modelsaon e condue des processus sdururques'', La Revue de Meallurge-CIT, Novembre FLUENT INC.: User's Gude for FLUENT 5, Lebanon, NH, USA, HOFER M., STEGER. L., LEHNER J., GEBERT W.: ''Improved EAF Desgn and Operaon usng he DynEAF Smulaon Tool'', Iron & Seel Revew, Vol. 40, No. 9, 35-4, ISHII K., KAWAKAMI H., MURAHASHI Y., TANAKA K.: ''Developmen of New Engneerng Technology Based on Compuer Analyss of Dus Dffuson'', Nppon Seel Techncal Repor No. 74, LAUNDER B. E. and SALDING D. B. : Lecures n Mahemacal Models of Turbulence, Academc ress, London, England, 197. LAUNDER B. E. and SALDING D. B.: ''The Numercal Compuaon of Turbulen Flows'', Compuer Mehods n Appled Mechancs and Engneerng, 3:69-89, MITTER R.: ''rojek Fenserdurchsroemung'', CFD- rakkumsberch, Fludmechancs Deparmen, Johannes Kepler Unversy Lnz, Ausra, MORSI S. A. and ALEXANDER A. J.: ''An Invesgaon of arcle Trajecores n Two-hase Flow Sysems'', J. Flud Mechancs, 55 (): , 197. REYNOLDS A. M.: ''Modellng parcle dsperson whn a venlaed arspace'', Flud Dynamcs Research, :139-15, YAKHOT V., ORSZAG S. A.: ''Renormalzaon Group Analyss of Turbulence. I. Basc Theory'', J. of Scenfc Compung, Vol 1, No. 1, 1-151, 199. WANG L..: On he Dsperson of Heavy arcles by Turbulen Moon, hd Thess, Washngon Sae Unversy,