Numerical Simulation of the Dispersion of a Plume of Exhaust Gases from Diesel and Petrol Engine Vehicles

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1 World Academy of Scence, Engneerng and Technology Numercal Smulaon of he Dsperson of a Plume of Exhaus Gases from Desel and Perol Engne Vehcles H. ZAHLOUL, and M. MERIEM-BENZIANE Absrac The obecve of hs wor s o sudy numercally he dsperson characerscs of desel engne vehcle polluans a an dle condon n an dealzed and smplfed envronmen. The Fluen code wh he - eddy dsspaon urbulen model was used for he numercal smulaon of he parculae concenraons of exhaus plume NOx, CO and HC. The smulaed resuls mach very well wh he expermenal ones close o he source of emsson bu decay o he amben concenraons much slower. Despe he dfference n he nal concenraons, he polluans decayed exponenally n all drecons. The effecs of he nal emsson concenraon and ex velocy have been nvesgaed. Keywords Concenraon, dsperson, exhaus plume, polluan I. INTRODUCTION MISSIONS from moor vehcles are he man source of Ear polluon n mos urban ces. The dsperson of polluans s hghly dependen on local weaher and drvng condons. A he cy level, he ncrease n urbulence and he dmnuon of he sably of he boundary layer over urban areas have resuled n changng he dsperson of polluans from auomobles and road raffc. One mporan feaure of he urbulen flow s s ably o ranspor and mx he polluan much more effcenly han a lamnar flow. Idlng emssons are consdered more harmful because he polluans canno be dspersed by he wae creaed by a movng vehcle. A specfc dle mode s one n whch he engne vehcle s pared along he sree wh he moor swched on. They have been varous numercal and expermenal sudes on he dsperson of polluans emed by moor vehcles. Mos of he expermenal ones were carred ou eher n wnd unnels or on roadsde [1], []. For he numercal nvesgaons, hey were concenraed on he developmen of a numercal model for smulang he dsperson process In hs wor, he FLUENT code wh he - urbulen model was used o sudy he dsperson of polluan emed by a moor vehcle whch s operang a dle under dealzed and smplfed condons. The obaned resuls were analyzed and compared o he expermenal ones [3]. H. Zahloul s wh he Deparmen of Mechancal Engneerng, Unversy Hassba Benboual of Chlef, Algera (phone : ; fax : ; e-mal : zahloulh@yahoo.fr) M. Merem Benzane s wh he Deparmen of Mechancal Engneerng, Unversy Hassba Benboual of Chlef, Algera, (e-mal : mbmadd001@yahoo.fr). II. GOVERNING EQUATIONS In order o descrbe he process of he dsperson of exhaus polluans n amosphere accuraely, he effecs of urbulen mxng, convecon, dffuson, emperaure varaons and speces ranspor mus be consdered. The governng equaons for he problem suded can be wren as [4]. A. Connuy equaon : ρ r + dv( u ) = 0, (1) Where ρ s he flud average densy; U s he mean velocy componen n he drecon of coordnae. Momenum equaon : U U ρu U + pδ + U 3 δ + ρg + () ( ρu U ) = 0 Where p s he flud average pressure, s he flud lamnar vscosy, g s he gravaonal acceleraon n he drecon of coordnae. In hs sudy, he Reynolds sresses n Eq. () can be wren as U U U U U (3) ( ρ ) = + δ x x 3 Where s he flud urbulen vscosy and can be expressed as = c (4) / Where c s a consan of he model. and can be solved by he urbulen energy ( ) equaon and he urbulen dsspaon ( ) equaon : B. Turbulen energy( ) equaon : ( ) U U ρ Γ + U U + ρ = 0 (5) 885

2 World Academy of Scence, Engneerng and Technology C. Turbulen dsspaon equaon : ( ) U U Γ + U ρu C + C ρ 0 1 = (6) Where C 1 and C are consans of he model, Γ and be deermned va Bousssnesq approxmaons. D. Energy conservaons equaon Γ can T Dp u ( ρh) + ( ρuh) = ( + ) Σ( h J, ) + + τ = 0 D u Where h s he sac enhalpy, s he molecular conducvy, s he effecve molecular conducvy and can be expressed as = / Pr, s ' he dffuson flux J, of he h chemcal speces due o he concenraons gradens of he h speces. E. Speces ranspor equaon: J ', ( C ) + ( uc ) = + R, (8) ρ Where C s he mean concenraon of he h speces and R s he source erm due o he chemcal reacons. The ranspor equaons descrbed n hs secon were dscrezed by he fne volume mehod and he compuaons were performed usng he FLUENT code. III. NUMERICAL SIMULATION In hs par, we focus on he feasbly of usng a numercal code for flud dynamcs o smulae he concenraons of gaseous polluans (NOx, CO, HC). Then, we compare he resuls of our calculaons wh measuremens aen from he leraure [3] A. Expermenal daa [3] (7) Fg. 1 Schemac of caresan coordnae sysem and vehcular exhaus plume geomery Expermens were carred ou n a sem-enclosed space ha has suffcen space for he plume o dsperse freely n all drecons. The expermenal se permed repeaable measuremens n he vehcular exhaus plume under realsc and reproducble condons.the expermens were also performed wh he followng parameers: - Sable weaher condons (wea wnd) and n he evenng o avod convecon curren due o he hea of he sun. - Temperaure: 0- C -Relave humdy: 50-60% -No movng vehcle bu wh runnng moor Inal condons: a) Enry of flud: All he properes of he flud a he nle are nown [1]. b) Speed: - The ex velocy of polluan = 4.8 m / s. - The ar velocy n sable weaher condon = 0.3 m / s. c) Temperaure: - The emperaure a he oule of he exhaus ppe s 380 K. - The emperaure of he compuaonal doman s 93 K. d) Concenraon: -producs of he exhaus gases from he combuson of desel and perol engnes are gven by he followng able: The expermens were carred ou o gan nsgh no he evoluon of he vehcular polluans nsgh he feld regon of he exhaus plume of wo moor vehcles (Desel and Perol) under dle condons and oban daa for esng he performance of he numercal mode. In he expermens, he es vehcles were operaed under dle condon on he es se and experenced no prevalng wnd. The caresann coordnae sysem (x, y, z) s fxed on he ground, as shown n Fg. 1. The orgn of he coordnae sysem s on he cenerlne of he alppe ex of he vehcle. The hegh of he alppe above he ground s H. 886

3 World Academy of Scence, Engneerng and Technology TABLE II Gaseous polluans concenraons for each vehcle [3] Engne NO X (ppm) CO (ppm) HC (ppm) VEHICLE A DIESEL VEHICLE B PETROL In he process of smulaon, he dmensons of he negraed doman were aen o be 15m x 6m x 3m. The dameer of he alppe was 0.03m and he hegh of he alppe above he ground was 0.3m. The exernal boundary of he negraed doman was defned as free sream boundary. Fg. 3-a Comparson of NOx concenraon beween he expermenal daa and smulaed resuls IV. RESULTS AND DISCUSSION A. Concenraons of CO, NOx and HC of vehcle A (Desel) Fg 3-b Dsrbuon of NOx concenraon [ppm] Fg. -a Comparson of CO concenraon beween he expermenal daa and smulaed resuls Fg. 4 Comparson of HC concenraon beween he daa and smulaed resuls expermenal Fg -b Dsrbuon of CO concenraon [ppm] We noce ha he smulaed resuls show a good agreemen wh he expermenal daa Fg. a and fg. 3a show ha he concenraons of CO and NOx decreased rapdly along he downsream dsance of he vehcular exhaus plume. The concenraons of CO and NOx a he alppe ex were 55 and 140 by volume respecvely and dropped exponenally o less han 1 ppm whn a dsance of 3.0m. Smlarly, n fg. 4, he HC concenraon decreased rapdly along he downsream dsance. Ths concenraon was 4.5 and dropped exponenally o less han 1 ppm from a dsance of m. 887

4 World Academy of Scence, Engneerng and Technology B. Concenraons of CO and NOx of vehcle B (perol) Fg. 5 Comparson of CO concenraon beween he expermenal daa and smulaed resuls Fg. 8 Comparson of HC concenraon beween vehcle A and B Fg. 7 shows ha he desel vehcle (vehcle A)has he hghes level of NOx concenraon, whle he perol engne vehcle B) has he lowes of hese polluans. Furhermore, Fg. 8 shows ha he desel engne has he lowes concenraon of HC emsson whle he perol engne has he hghes of hese polluans. I can be concluded ha here are sgncan dfferences of polluan concenraons n he exhaus plume for dfferen vehcles under dle condon and he dfference depends on he ype of he vehcle, he vehcle echnologe level and he condons of he vehcle. Fg. 6 Comparson of NOx concenraon beween he expermenal daa and smulaed resul D. Smulaon resul of flow velocy of he exhaus plume We noce ha he smulaed resuls show a good agreemen wh he expermenal daa Fg. 5 and fg. 6 show ha he concenraons of CO and NOx decreased rapdly along he downsream dsance of he vehcular exhaus plume. The concenraons of of CO and NOx a he alppe ex were 57 and 5.5 by volume respecvely and dropped exponenally o less han 1 ppm whn a dsance of 3.5m C. Comparson of NOx and HC concenraons beween vehcle A and B (numercal and expermenal) Fg. 9 Flow velocy profle along X axs Fg 10 Dsrbuon of flow velocy of he exhaus plume Fg. 7 Comparson of NOx concenraon beween Vehcle A and B Fg. 9 and fg. 10 show ha he evoluon of polluan plume velocy n he longudnal drecon X. One noces ha here s a sudden decrease of he flow speed of exhaus gases. I decreases exponenally wh a value of 4.8m/s a he ex of he alppe o reach he ar amben speed for a dsance of m. 888

5 World Academy of Scence, Engneerng and Technology V. CONCLUSION Idlng emssons of a vehcle s consdered more harmful on human healh because he polluans canno be dspersed by he wae creaed by a movng vehcle. Ths paper nvesgaes numercally he dsperson characerscs of he exhaus plume when a vehcle s operang under dle condon by consderng he dsperson of wo vehcles (desel and perol) from s pon of emsson o some dsance downsream. Also, he CFD code FLUENT was used o analyze he dsperson of gaseous polluan usng he - eddy dsspaon model. The model was valdaed wh NOx, CO and HC concenraons measured expermenally. The model was able o smulae closely he expermenal resuls close o he source bu he smulaed resuls oo a longer dsance o decay o amben values. REFERENCES [1] K. T., Knapp, S. B., Teada, Cenral Calorna vehcle parculae emsson sudy, Fnal repor US, Envronnemenal Proecon Agency, 000, MD-46 [] D. B. Kelson, W. F. Was, Fne parcle emsson on Mnnoesa hghways, Fnal repor, Mnnesoa, 001, MN/RC [3] Z. Nng, C. S., Cheung, Y. Lu, Expermenal and numercal sudy of he dperson of moor vehcle polluans under dle condon, Amospherc envronmen, vol.39, 005, pp [4] Km e all, On he predcon of concenraon varaons n a dspersng heavy-duy ruc exhaus plume usng - urbulen closure, Amospherc envronmen, vol.35, 001, pp