Comparative Study on Electrical Discharge and Operational Characteristics of Needle and Wire-Cylinder Corona Chargers

Transcription

1 50 Jounal of Electcal Engneeng & Technology, Vol. 1, No. 4, pp. 50~57, 006 Compaatve Study on Electcal Dschage and Opeatonal Chaactestcs of Needle and We-Cylnde Coona Chages Panch Inta* and Nakon Tppayawong** Abstact - The electcal dschage and opeatonal chaactestcs of needle and we-cylnde coona chage based on cuent measuements fo postve and negatve coonas wee evaluated and compaed. A sem-empcal method was used to detemne the on concentatons n the chagng one and at the outlet of both chages. Results fom expemental nvestgaton evealed that magntudes of the chagng cuent fom the we-cylnde chage wee appoxmately 3.5 and tmes smalle than those fom the needle chage fo the postve and negatve coonas, espectvely. The on numbe concentatons at the outlet fo postve coona of both chages wee hghe than fo negatve coona at the same voltage. Flow and electc felds n the chagng one of both chages wee also analyed va numecal computaton. Stong electc feld stength one was dentfed and led to hgh chagng and patcle deposton. Effect of patcle deposton on the evoluton of dschage cuent was pesented. It was shown that ons loss nsde the we-cylnde chage was hghe than the needle chage. The patcle deposted on the coona electodes and on the gounded cylnde caused a geat educton n chagng effcency of both chages. Keywods: Electcal dschage, Needle chage, We-cylnde chage, Patcle deposton 1. Intoducton Coona dschage poduces hgh concentaton unpola ons that ae used to poduce hghly chaged aeosols va feld- and dffuson-chagng mechansms. It s often necessay to estmate the numbe of chage on a patcle of any gven se, based on the condtons unde whch t acques the chage. The elatonshp between the aveage numbe of elementay chages and the se of the patcles detemnes the electcal moblty. It s a wdely and successfully used technque fo aeosol chages. Both needle and we-cylnde coona chages have been utled and evaluated [1, ]. Howeve, to ou knowledge, no study has been caed out to compae the electostatc chaactestcs. Ths pape pesents an attempt to chaactee and make compason between electcal dschage popetes of the two chages. In ths pape, the electostatc popetes n tems of voltage-cuent elatonshps of both we-cylnde and needle coona chages wee compaed and dscussed fo postve and negatve coonas n the chagng ones. A sem-empcal method, based on on cuent measuement and electostatc chagng theoy, was used to calculate aveage on concentaton n the chagng one fo each chage [3]. Effect of patcle deposton on the evoluton * Dept. of Mechancal Engneeng, Chang Ma Unvesty, Chang Ma, 5000 Thaland. (panch_nta@yahoo.com) ** Dept. of Mechancal Engneeng, Chang Ma Unvesty, Chang Ma, 5000 Thaland. (nakon@dome.eng.cmu.ac.th) Receved Decembe 15, 005 ; Accepted Novembe 14, 006 of dschage cuent was pesented. A numecal model was employed to descbe flow and electc felds n the chagng ones to gve a bette undestandng on the opeaton of the chages.. Expementaton The needle and we-cylnde coona chages tested n the pesent study ae shown n Fg. 1. The needle chage s geometcal confguaton s smla to the chage used by Henande-Sea, et al. [] (Fg. 1a). It conssts of a coaxal coona-needle electode placed along the axs of a cylndcal tube wth tapeed end. The needle electode s made of a stanless steel od, 3 mm n damete and 49 mm n length, endng n a shap tp. The angle of the needle cone s about 9 o and the tp adus s about 50 μm, as estmated unde a mcoscope. The oute cylndcal s made of an alumnum tube, 30 mm n damete and 5 mm n length wth concal shape. The angle of the cone s about 30 o and the ofce damete s about 4 mm. The dstance between the needle electode and the cone apex s mm. The coona-needle electode head s connected to an adjustable DC hgh voltage supply, whle the oute electode s gounded. The we-cylnde coona chage has a geometcal confguaton smla to that used by Kesknen, et al. [4] (Fg. 1b). It conssts of a coaxal coona-we electode placed along the axs of a metallc cylnde tube. The oute cylndcal s made of an

2 Panch Inta and Nakon Tppayawong 51 Aeosol nlet To hgh voltage supply Coona needle electode To hgh voltage supply Aeosol outlet Aeosol nlet Coona w e electode Aeosol outlet Oute electode Oute electode (b) we-cylnde Fg. 1 Schematc dagams of the needle and we-cylnde chages D C hgh voltage powe supply (L eybold Ddactc 5171) A Slca gel dye Flte Test cha ge K ethley Electom ete 6517A + Scanne cad 65 Chagng cuent Extenal Compute, Data Loggng Flte and Faaday cage Ionc cuent Vacuum pum p M ass fl ow cont ol l e Fg. Expemental setup fo electcal dschage chaacteaton alumnum tube, 8 mm n damete and 10 mm n length. The we electode s made of a stanless steel we, 300 μm n damete and 10 mm n length. An adjustable DC hgh voltage supply s used to poduce the coona dschage on the we electode whle the oute metallc electode s gounded. The electcal dschage chaacteaton expements wee caed out usng the setup sketched n Fg.. Clean dy a flow was egulated and contolled by means of a mass flow mete and contolle, typcally n the ange between 0 30 lte/mn. The a flow was foced by a vacuum pump. An adjustable DC hgh voltage powe supply (Leybold Ddactc model 5171, 500 mv peak-topeak pple voltage, and 0.5 ma maxmum cuent) was used to mantan the coona voltage dffeence, geneally n the ange between 1-10 kv. The chagng cuent fom the coona electode was measued dectly wth the senstve electomete (Kethley 6517A electomete ncopoatng a Kethley 65 scanne cad ntefaced to a pesonal compute va an RS-3 nteface) va the oute electode. It s desable to know the on numbe concentaton wthn the chagng one, N, because the Nt poduct s the man paamete contollng the effcency of patcle chagng. In ths study, the onc cuent at the chage outlet was measued by fltaton method. An a sample was dawn nto a shelded Faaday cage wth a flte though whch all the a passed. The flte was equpped wth a fne collecton metal gd, and was electcally solated fom the Faaday cage and gound. Hgh effcency patculate-fee a (HEPA) flte was used fo these expements because the collecton effcency fo small a ons was vey hgh. In the Faaday cage, the chages wee emoved fom the a steam by the flte and the esultng onc cuent flow was montoed wth the senstve electomete. Both the chagng cuent and onc cuent wee ecoded by a pesonal compute and used fo calculaton of coespondng total on numbe concentaton.

3 5 Compaatve Study on Electcal Dschage and Opeatonal Chaactestcs of Needle and We-Cylnde Coona Chages 3. Theoy On the bass of Maxwell s theoy, the electc feld between the two electodes of the chage s gven by ρ dv E = ε whee E s the electc feld, ρ s the space chage densty, and ε 0 s the vacuum pemttvty. The on cuent densty, j = ρu wth the mean onc velocty, u = ZE, whee Z s the electcal moblty of the ons. The on cuent densty can be expessed on the bass of the on cuent, I, though the nne suface aea, A, of the oute electode of the chage as 0 I A (1) ρ ZE = () The space chage defnes n tems of the on concentaton N, s ρ = Ne whee e s the elementay chage. Substtutng nto equaton (), the mean on numbe concentaton n the chagng one of the chage n the absence of aeosol patcles s gven by N I = (3) ezea 3.1 Ion Concentaton n Chagng Zone of the Needle Chage The nne suface aea of the metallc cone (chage outlet) whee the on cuent s collected, and s gven by ( ) ( ) A = π + + L (4) 1 1 whee 1 and ae the nne and oute ad of a concal fustum, and L s the length of the chagng one. If the space chage effect s neglected, the electc feld stength was estmated to be gven by the smplfed equaton V E = (5) d whee V s the appled voltage, and d s the dstance between the electode tp and the cone apex. Substtutng equaton (4) and equaton (5) nto equaton (3) gves N Id = π L ezv ( ) ( ) The N t poduct (mean on numbe concentaton mean aeosol esdence tme) s a fundamental paamete to descbe the opeaton of an aeosol chage, because t s ntmately elated to the extent to whch the patcles can become chaged. The mean aeosol esdence tme of the patcles n the chagng one of the chage s gven by ( 1 1 ) 3Q a (6) π L + + t = (7) whee Q a s the aeosol flow ate. Fo the standad aeosol flow of 5.0 lte/mn, the mean aeosol esdence tme n the chagng one s 814 s at atmosphec pessue. 3. Ion Concentaton n Chagng Zone of the We-Cylnde Chage The nne suface aea of the oute electode of the chage s gven by A= π L (8) If the space chage effect s neglected, the electc feld, E, n the chagng one s gven by Gauss s law, E V = (9) ln ( ) 1 whee 1 and ae ad of the nne and oute electode, espectvely. In the same way, substtutng equaton (8) and equaton (9) nto equaton (3), the mean on concentaton s gven by N ( ) I ln 1 = (10) π LZeV The obstucton of the flow caused by the we electode was neglected due to the vey thn ( 1 << ) we electode. The mean esdence tme of patcles n the chagng one of the coona-we chage s equal to π L t = (11) Q a Fo the standad aeosol flow of 5.0 lte/mn, the mean

4 Panch Inta and Nakon Tppayawong 53 aeosol esdence tme n the chagng one s 74 s at atmosphec pessue. 3.3 Ion Concentaton at Chage Outlet Fom the onc cuent at the chage outlet, the on numbe concentaton, N, can be calculated fom the expesson I N = (1) eq whee I s the onc cuent, e s the elementay chage and Q s the volumetc a flow though the flte. The onc cuent measuements wee tanslated nto on numbe concentatons gven the total a flow ate though the chage. The onc concentaton was then used as an nput fo the chagng models. 3.4 Electc Feld nsde the Chage A numecal model was developed to nvestgate flow and electc felds n the chagng one of the chage to gve a bette undestandng on the opeatng of the chages. The model s conssts of two pats; flow feld and electc feld modelng. Fo electostatc feld modelng, the Posson s equaton fo the electc potental can be used. Howeve, the space-chage effect on the electc feld can be neglected ( ρ = 0) fo low aeosol concentaton, typcally less than 10 1 patcles/m 3, and wth low patcle chage level [5, 6]. Fo the pesent chage confguatons, the esultng Laplace s equaton n the -D, axsymmetc, cylndcal coodnates s gven as follows: Fo the bounday condtons used, constant potentals ae appled to the coona-electode (V = coona voltage), the oute electode ( ), and the eo gadent condtons s appled to the boundaes wth out walls. 3.5 Flow Feld nsde the Chage Flow condtons nsde the chage wee assumed to be steady, ncompessble and lamna. Based on the pncple of momentum consevaton, the ncompessble Nave- Stokes equatons can be appled n ths case. In these axsymmetc geometes, the contnuty and Nave-Stokes equatons used n ths model can be wtten n the -D cylndcal coodnates as follows. Contnuty equaton: 1 + = 0 ( u ) ( u ) Nave-Stokes equatons: Fo the adal component (n -decton), u u uθ 1 p u + u = ρ u + μ ( u) μ Fo the axal component (n -decton), u u 1 p 1 u u u + u = + μ + ρ u (17) (18) (19) 1 V V + 0 = (13) Once the electc potental s obtaned, the electc feld stength n the - and - dectons can be calculated by the followng equatons V V E =, E = (14) In the smplfed geomety, the electc feld stength ae expessed by V E(,) = E() =, (15) ln ( ) 1 E (,) = 0 (16) Fo the ccumfeental component (n θ -decton) u u 1 u u u u θ θ θ θ μ θ + u = μ + (0) whee u s the velocty component n the -decton, u s the velocty component n the -decton, u θ s the velocty component n the θ -decton, p s the pessue and μ s the knematc vscosty of a. Fo the bounday condtons used, no slp bounday was appled to all the sold walls ncluded n the computaton doman, and fxed velocty bounday condton was appled to the aeosol nlet. The velocty at nlet was calculated fom the flow ate though the chage. Unfom velocty pofle was assumed at the sheath and aeosol nlet acoss the coss

5 54 Compaatve Study on Electcal Dschage and Opeatonal Chaactestcs of Needle and We-Cylnde Coona Chages Q = 5 lpm No-slp condtons stuctued gd was used. A total of about 6,000 meshes wee dstbuted n computatonal doman of the coonaneedle chage (Fg. 3a.), whle the coona-we chage, a mesh wth 4,000 meshes was used (Fg. 3b.). No-slp condtons V = 3.5 kv Outlet bounday Axs of symmety No-slp condtons Q = 5 lpm No-slp condtons Outlet bounday V = 7.5 kv Axs of symmety 4. Results and Dscusson Fg. 4 shows cuent-voltage chaactestc of the postve and negatve coonas poduced fom the needle and we-cylnde coona chages. In the needle chage, the spak-ove phenomena occued fo the postve coona voltages lage than about 5.0 kv and negatve coona voltages lage than about 4.5 kv. Above these values, the cuent was found to exhbt a fluctuaton n an uncontollable manne and no measuement could be made. Meanwhle, the we chage was able to opeate stably at 10 kv appled voltage. Howeve, at the same coona chagng cuent, A 1.0x10-4 postve, coona-needle negatve, coona-needle postve, coona-we 8.0x10-5 negatve, coona-we 6.0x x10-5.0x coona voltage, kv Fg. 4 Cuent-voltage chaactestcs n the chagng ones of the needle and we-cylnde chages 1.0x10 16 postve, coona-needle (b) we-cylnde Fg. 3 Numecal domans of the needle and we-cylnde chages secton of the nlet tubes. Numecal computaton was pefomed usng a commecal computatonal flud dynamc softwae package, CFDRC TM. The soluton doman was dvded nto a numbe of cells known as contol volumes. In the fnte volume appoach of CFDRC TM, the govenng equatons wee numecally ntegated ove each of these computatonal cells o contol volumes. Fg. 3 shows physcal and computatonal domans as well as coss sectons of the computatonal mesh dstbuton n the chagng one fo each chage. A on numbe concentaton, ons/m 3 8.0x x x x10 15 negatve, coona-needle postve, coona-we negatve, coona-we coona voltage, kv Fg. 5 Vaaton n on numbe concentaton wth appled voltage nsde chages

6 Panch Inta and Nakon Tppayawong 55 on cuent, A 1.5x x x coona voltage, kv postve, coona-needle negatve, coona-needle postve, coona-we negatve, coona-we Fg. 6 Vaaton n on cuent and concentaton wth appled voltage at the chage outlet 1.5x x x10 1 voltage, magntude of the chagng cuent was makedly smalle fo the we chage, compaed to the needle one. Even at the maxmum possble appled voltage unde pesent setup (10 kv), the chagng cuents fom the we chage wee appoxmately 3.5 and tmes smalle than those fom the needle chage just befoe the spak-ove fo the postve and negatve coonas, espectvely. Geneally, the cuents fo negatve ons wee slghtly hghe than those fo postve ons. Ths was expected because negatve ons have hghe electcal moblty than postve ons. A hgh on concentaton n the chagng egon of a chage s desable fo hgh aeosol chagng effcency. The on concentaton was appoxmately popotonal to the on cuent. Ths on cuent nceased wth the electc feld, hence appled voltage. Ths was the case fo both chages, as clealy depcted n Fg 5. on numbe concentaton, ons/m 3 adal dstance fom we electode, m we electode oute electode axal dstance, m (b) we-cylnde Fg. 7 Dstbutons of electc feld stength nsde the needle and we-cylnde chages The numbe concentaton of the postve and negatve ons leavng the coona-needle and coona-we chages wee shown as a functon of the coona voltage n Fg. 6. In the needle chage, negatve coona (.e. negatve on geneaton) appeaed at about. kv, whle a lage voltage, about.5 kv, was needed fo the onset of postve coona. In the case of postve ons, on concentaton appeaed to depend on coona voltage only wthn a naow nteval of voltages. Fo lage voltages, on numbe concentaton adal dstance fom needle electode, m needle electode axal dstance, m oute electode adal dstance fom needle electode, m needle electode oute electode adal dstance fom we electode, m oute electode we electode axal dstance, m (b) we-cylnde axal dstance, m Fg. 8 Patcle tajectoes nsde the needle and wecylnde chages

7 56 Compaatve Study on Electcal Dschage and Opeatonal Chaactestcs of Needle and We-Cylnde Coona Chages x10 9.0x10 8.0x10 chagng cuent, A 7.0x10 6.0x10 5.0x10 4.0x10 3.0x10.0x10 coona-needle chage, 3.5 kv, 5 lpm coona-we chage, 7.5 kv, 5 lpm 1.0x tme, mn Fg. 9 Evoluton of chagng cuent fo each chage wth opeatng tme pactcally became a constant, ndependent of the appled voltage. Meanwhle, the on concentaton of negatve ons nceases wth nceasng coona voltage. Fo the coonawe chage, the onset of negatve coona was about.8 kv, and postve coona was about 3.0 kv. Fo both cases, on concentatons wee also found to ncease wth nceasng coona voltage only wthn a naow voltage nteval. Fo lage voltages, on numbe concentatons deceased slghtly wth nceasng coona voltage. As t can be seen fom Fg. 6, at the same coona voltage, the magntude of the on numbe concentaton was makedly smalle fo the we-cylnde coona chage, compaed to the needle coona chage. Ths was attbuted to hgh ons loss nsde the we-cylnde chage. Nonetheless, the needle chage was found to become unstable at lowe appled voltage. It was also evdent that the on numbe concentatons fo postve coona of both chages wee slghtly hghe than those fo negatve coona. Ths was because negatve ons have hghe electcal moblty than postve ons, hence they wee moe lkely to mpact and depost on the chage s wall. When the appled voltage nceased, on numbe concentaton and electc feld stength wee found to ncease, nducng a bette patcle chagng ate and moe patcle loss due to deposton on the electodes and on the wall. Examnaton of flow and electc felds n and aound the chagng egons fom numecal smulaton esults fo both chages evealed that (Fgs. 7, 8) thee exsted egons of stong electc feld n close poxmty to the tp of the needle and the we. Flow speed n these egons was faste n case of the needle chage than that of the wecylnde chage. Fg. 9 llustates the evoluton of chagng cuents measued fo both chages opeatng at the coespondng stable coona voltages wth tme. A decease n chagng effcency as shown by a declne of chagng cuent wth tme n Fg. 7 was a dect esult of evoluton fom a stable dschage to spak-ove o acng phenomenon. Both the needle and we chages smlaly exhbted a contnuous educton n chagng cuent. Howeve, cuent fom the needle chage was found to declne n a hgh ate than that fom the we-cylnde chage. The buldup of patcles deposted on the wall appeaed to affect the chage pefomance advesely. (b) we-cylnde Fg. 10 Patcle deposton nsde the chagng egons fo both chages

8 Panch Inta and Nakon Tppayawong 57 Patcle collecton on the electodes and walls fomed dendtes buld-up (Fg. 10), causng a change n the needle and we suface geomety and modfyng the dschage egmes. Ths gave se to a geat change n the electc feld aound the electodes whch lessened the chagng effcency. 5. Concluson In ths study, needle and we-cylnde coona aeosol chages wee constucted and tested. Compason of both chages pefomance based on the electcal dschage chaactestcs and opeaton was caed out and dscussed. A sem-empcal method, based on cuent measuements, was used to calculate aveage on concentaton n the chagng one and at the outlet fo each chage. Addtonally, a numecal model was developed to nvestgate flow and electc feld pattens n the chagng one fo each chage to gve a bette undestandng on the opeaton. Non-unfom dstbutons of electc feld stength and hgh speed flow wee dentfed. Chagng cuent and on numbe concentaton as a functon of coona voltage wee evaluated. Effect of patcle deposton on the coona electodes and on the gounded cylnde was pesented. The chagng cuent and on concentaton n the chagng one wee found to ncease wth coona voltage fo each chage. Ion as well as patcle losses nsde both chages nceased wth appled voltage. Results fom on cuent measuement at the outlet of both chages evealed that on loss wthn the we-cylnde chage was hghe than the needle chage. Patcle deposted on the coona electodes and the gounded cylnde fo each chage also nceased wth opeatng tme. Acknowledgements Fnancal suppot fom the Natonal Electonc and Compute Technology Cente (NECTEC), Natonal Scence and Technology Development Agency (poject no. NT-B--E ) s gatefully acknowledged. coona one, Jounal of Aeosol Scence, vol. 34, pp , 003. [3] P. Inta and N. Tppayawong, Appoach to chaacteaton of a dode type coona chage fo aeosol se measuement, KIEE Intenatonal Tansactons on Electophyscs & Applcatons, vol. 5-C, pp , 005. [4] J. Kesknen, K. Petanen, and M. Lehtmak, Electcal low pessue mpacto, Jounal of Aeosol Scence, vol. 3, pp , 199. [5] D. R. Chen and D. Y. H. Pu, Numecal modelng of the pefomance of dffeental moblty analyes fo nanomete aeosol measuement, Jounal of Aeosol Scence, vol. 8, pp , [6] R. P. Camata, H. A. Atwate, and R. C. Flagan, Space-chage effects n nanopatcle pocessng usng the dffeental moblty analye, Jounal of Aeosol Scence, vol. 3, pp , 001. Panch Inta He eceved BS.Tech.Ed. degee n electcal engneeng fom Rajamangala Lanna Unvesty of Technology n 001 and M.Eng. degee n enegy engneeng fom Chang Ma Unvesty n 003. He ecently gaduated wth Ph.D. n mechancal engneeng at Chang Ma Unvesty. Hs nteests nclude electoaeosol measuement technology, automaton contol and nstumentaton. Nakon Tppayawong He eceved B.Eng. and Ph.D. degees n mechancal engneeng fom Impeal College, UK n 1996 and n 000, espectvely. He s cuently an assstant pofesso at Chang Ma Unvesty. Hs eseach nteests ae HV applcatons n aeosol system, enewable enegy. Refeences [1] L. Unge, D. Boulaud, and J. P. Boa, Unpola feld chagng of patcles by electc dschage: effect of patcle shape, Jounal of Aeosol Scence, vol. 35, pp , 004. [] A. Henande-Sea, F. J. Alguacl, and M. Alonso, Unpola chagng of nanomete aeosol patcle n a