Effect of static mixer geometry on flow mixing and pressure drop in marine SCR applications

Int. J. Nav. N Archit. O Ocean Eng. (2 2014) 6:27~388 http p://dx.doi.org/110.2478/IJNAO OE-2013-01611 pISSN: 20092-6782, eISS SN: 2092-67900

ⓒ SNAK, 2 2014

Effectt of statiic mixerr geomettry on flo ow mixinng and pressure drop in marine SCR app plicationns Taew wha Park1, Yoonmo Sung1, Taekyung K Kim1, Inwon Lee2, Gyung gmin Choi3 aand Duckjooll Kim3 1

2

G Graduate Proggram, School of Mechanicaal Engineering g, Pusan Natio onal Universitty, Busan, Korrea Gloobal Core Ressearch Center for Ships andd Offshore Pla ants, Pusan Na ational Univerrsity, Busan, Korea K 3 Pusan Clean n Coal Center, Pusan Nation nal Universityy, Busan, Koreea

ABSTRACT T: Flow mixinng and pressurre drop characcteristics for marine m selectiive catalytic rreduction appllications weree investigated numerically too develop an efficient staticc mixer. Two different d mixerrs, line- and sw wirl-type, werre considered.. The effect off vane angles on the relativve intensity, unniformity indeex, and pressu ure drop was iinvestigated in n a swirl-typee mixer; these parameters are a dramatica ally affected bby the mixer geometry. g Thee presence off a mixer, rega ardless of thee mixer type, leed to an imprrovement of ap pproximately 220% in the mixing perform mance behind tthe mixer in comparison c too not having a mixer. In parrticular, there was a tradeof off relationship p between thee uniformity annd the pressure drop. Con-sidering the m mixing perforrmance and th he pressure dro rop, the swirl-type mixer wa as more suitabble than the line-type mixerr in this study. KEY WORD DS: Selective catalytic c reducction; Static m mixer; Uniform mity index; Pressure drop; M Marine diesel engine.

INTODUCT TION In order tto meet stringeent future emisssion regulatioons from the environmental e protection ageency (EPA), especially e withh respect to redducing nitrogenn oxides (NOx), various techhnologies such h as basic internal engine m modifications, fuel f switching,, direct water iinjection, exhaaust gas recircu ulation (EGR), and selectivee catalytic redu uction (SCR) hhave been reco ommended forr marine diesell engines. Thee internationall maritime orgganization (IM MO) has regulaated NOx emisssions from marine m vessels.. The IMO’s T Tier III standarrd requires thatt marine vesseels must reduce NOx emissio ons by 80% beetween 2010 and a 2016 (Die-selNet, 2008)). SCR is one of the most promising techn hnologies for accomplishing a this aggressivve regulation. As a reducingg water solution agent, urea iss preferred in marine m SCR ap pplications beccause of its saafety and low toxicity. t Urea w n (UWS), con-taining 32.5w wt% urea, is injjected into thee hot gas stream m from exhausst manifolds, and a then, NH3 is generated by b dewatering,, thermolysis, aand hydrolysiss processes. Th he urea-SCR rreactor should d be controlled d to ensure high gh de-NOx perfformance, low w NH3 slip, andd low urea connsumption. To maximize thee de-NOx efficiency and min nimize the NH H3 slip, a contro olled turbulentt mixing proceess for two-phhase flow, such h as UWS witth the exhaustt gas stream, and a a highly uuniform flow in i front of thee SCR reactor m must be obtainned. Flow mixxing is a comm mon device unit operation in a large number of processes, and a it is used inn many differeent applicationss Correspondinng author: Gyunngmin Choi, e-mail: choigm@ @pusan.ac.kr This is an Opeen-Access articcle distributed under u the termss of the Creative Commons Attribution Noon-Commerciall License (http:///creativecomm mons.org/licensees/by-nc/3.0) which permits unrestricted noon-commercial use, distributioon, and reprodu uction in any medium, proviided the originaal work is propeerly cited.

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where a definned degree of homogeneity h of o a fluid is deesired (Regner et al., 2006). In I particular, a mixing devicce, e.g., a staticc mixer is usuaally installed too improve the rate of decom mposition for urea u to NH3 an nd to enhance tthe uniformity y of the spatiall distribution off NH3 and isoccyanic acid (HN NCO) (Munnaannur and Liu, 2010). Howev ver, the increasee in the system m pressure dropp because of miixing must be minimized. m There havve been many attempts to deevelop static m mixers for mobile SCR appliccations, and coomputational fluid f dynamicss (CFD) has beeen widely used in the design optimizatioon of spray no ozzles, flow mixing m characte teristics, NOx reduction r pro-cesses, and uurea decompossition (Thakur et al., 2003; Z Zheng et al., 2009; Zheng ett al., 2010; Zhhang et al., 200 06; Zhang andd Romzek, 20007; Birkhold ett al., 2007; Nguyen et al., 20010; Larmi and d Tiainen, 2003 3; Chen and W Williams, 2005 5; Battoei et al.., 2006). Thakuur et al. (2003)) provided an extensive reviiew of static mixers m in the processing p inddustry, presentting guideliness for the selectiion of static mixers. m Zheng et e al. (2009) ddeveloped seveeral types of mixers, m includinng cone, 2-stag ge, and butter-fly mixers. Thhey also invesstigated the efffect of in-pipe mixing devicees on urea dep posits with resppect to mixer configurations c s and various eexhaust gas tem mperatures. Zh hang et al. (20006) introduceed a simple flo ow mixer withh twisted bladees based on ann original deltaa wing mixer foor the purpose of creating booth swirling an nd turbulent flo ows. Turbulennt flow has a do ominant effectt on the flow m mixing index or o uniformity index i in the shhort distance immediately i behind the flow w mixer (Zhan ng et al., 2006;; Zhang and Roomzek, 2007).. In allowinng a sufficient mixing m length by reducing thhe occupied spaace, it is necesssary to developp a proper statiic mixer with a high mixing pperformance as a well as a low w pressure droop. However, there t is insuffiicient research on the relationship betweenn flow mixing ccharacteristics and pressure drops d in the mar arine engine fieelds. In this stuudy, both line- and a swirl-type mixers were cconsidered; eacch mixer was divided d into thrree cases of vaane angles: 30º,, 45º, and 60º. The effects off a mixer’s geeometric structu ture on the flow w mixing charracteristics andd the resulting pressure dropp c finnite volume, three-dimensio t onal (3-D) CFFD code; FLU UENT (versionn were investiggated numerically using a commercial 6.3.26). The ppurpose of thiss study was to evaluate the efffect of mixer geometry on the t relative inteensity, uniform mity index, andd pressure dropp with the objeective of enhan ncing the de-N NOx efficiency.. Additionally, information ppertaining to th he selection off proper static m mixers was proovided based on n the correlatioon between thee uniformity ind dex and the preessure drop.

NUMERICA AL METHOD DS AND CON NDITIONS Analysis mod del The geom metry of the SC CR system, wh hich includes the position of the spray injector, mixer, SSCR reactor, and a measuringg points, is show wn in Fig. 1. Line-type L and swirl-type mixxers, as shown n in Fig. 2, hav ve 36 vanes eaach. A swirl-ty ype mixer wass developed in this study withh several uniqu ue features: a ssimple design for f production; a variable vanne angle to gen nerate differentt he pipe. Both ttypes of mixerrs with variouss swirl flows; aand the flexibillity for installaation and to coontrol the mixeer volume in th vane angles w were simulatedd in 3-D to inveestigate the floow pattern, turb bulence characcteristics, and uuniformity of water, w which iss assumed to bee UWS at the SCR S catalyst en ntrance.

Fig. 1 Computatioonal domain fo or the SCR system.

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Fig. 2 Configuration C of static mixeers: (a) line-ty ype mixer, and d (b) swirl-typpe mixer. The test cconditions are listed l in Table 1. The compuutational grid iss composed off approximatelyy 650,000 hexaahedral and te-trahedral cellss using FLUEN NT/GAMBIT (ANSYS, Incc., USA). The mesh structuree became densser toward the spray injectorr and the mixerr because of thee high gradientts of velocity, ttemperature, an nd species conccentrations. mputational connditions with various mixerr types and po ositions. Table 1 Com Case

Mixxer type

1

W W/O

2

Swiirl, 30º

3

Swiirl, 45º

4

Swiirl, 60º

5

Line (up--down), 45º

Position of mixer and spray

7 D and 3 D from m inllet boundary

mary of initiaal and boundarry conditions. Table 2 Summ Item

Boundaary condition of computtational domain

Initiaal condition off spraay injection

Conditions ns and value

Sectionn Inllet

Exhaust gas g velocity: 220m/s, 573K

Ou utlet

Pressure outlet: atmospphere

Wall

Adiabaticc / No-slip

Caatalyst

Porous media: m 1/α = 1..24  107, C 2 = 10.59

Sp pray material

H2O (liqu uid water), 30 0K

Veelocity

25m/s

An ngle

70º

SM MD: (Rosin-R Rammler)

Mean diaameter: 35µm,, Spread param meter: 3.5

Injjection type

6 hole, so olid cone typee

Boundary coonditions The modeel assumes thatt the exhaust gas g is fully devveloped and it consists of 77% % N2 and 23% % O2 by mass. A time step off 1ms was deteermined to be sufficient s to prroduce results iindependent off the choice off time step. The he injector conssists of 6 holess with 0.45mm diameter. Thee initial and bou undary conditiions used in the calculation are a shown in Ta Table 2. The iniitial conditionss of the exhausst gases were set based on full f load and 9900rpm on a 920kW 9 diesel engine (HYUN UNDAI HiMSE EN). The tem-perature of thee exhaust gas was w set to 573K K, and the inlett velocity was 20m/s. 2

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Numerical procedure The flow region in the SCR system is divided into three regions: the 1st region is the turbulent flow region in the upstream and downstream of the SCR reactor; the 2nd region is the laminar flow region in the SCR reactor; and the 3rd region is the region that contains the dispersed two-phase flow in the surrounding spray injector. The Lagrangian discrete phase model is used which contains sub-models for droplet dispersion, drag, and evaporation. An injection type of solid cone for primary breakup model is used to inject water liquid. To minimize computational time and stable convergence strategy, secondary breakup model for droplet breakup and collision are not considered in this study. The injected drop-size was found to follow a RossinRammler distribution with a mean diameter of 35 microns and spread parameter of 3.5. For an incompressible, unsteady twophase turbulent flow, the 3-D Reynolds-averaged Navier-Stokes (RANS) governing equations for mass, momentum, species concentration, and energy were solved. The standard κ-ε turbulent model was used to calculate the turbulent quantities. The continuity, momentum, and energy equations are expressed as follows in Eqs. (1)-(3), respectively.

ui 0 t ui ui u j p     t x j xi x j

(1)

  u j ui    t     x x  j  i 

    

T uiT      t  T         t xi xi x j   Pr Prt  xi 

(2)

(3)

The turbulent kinetic energy κ and the rate of energy dissipation ε are computed from a standard two-layer κ-ε turbulent model.  ui    t xi x j  ui     t xi x j

  t          xi 

  t          xi 

   G   

(4)

     C 1G  C 2   

(5)

 u i 2 , and  t  C    x j

(6)

G denotes the production rate of κ and is given below:

G   ui u j

 u u j ui t  i   x x j  j xi

In the above equations, the coefficients are as follows: C 1 = 1.44, C 2 = 1.92,    = 1.3. C  = 0.09,  = 1.0, and

The catalyst is the core of SCR reactor. In this study, a honeycomb type SCR catalyst filter was adopted. If the catalyst filter is constructed physically from a numerical simulation without any simplifications, the grid of the model will reach a level that is beyond the calculation capabilities of most computing systems. Therefore, an approach of a porous media model was adopted

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to simulate thhe flow in the catalyst c filter. The T mass and m momentum tran nsfer in the rad dial direction w was ignored because the axiall velocity is doominant in the SCR filter (Jeeong et al., 20005). In this simple model, the t pressure chhange (drop) iss defined by a combination oof Darcy’s Law w and an additiional inertial looss term along the SCR filter (Fluent, 2007)):

1   p    v  C2  v 2  m 2  

(7))

p  2.73v 2  199.72v

(8))

where ∆p is tthe pressure drrop, µ is the laaminar fluid vi viscosity, α is the t permeabilitty of the mediu ium, C2 is the pressure-jumpp coefficient, v is the velocityy normal to th he porous face,, and ∆m is th he thickness off the medium. To obtain the two unknownn c of th he simple part cell analysis as a shown in Figg. values, α and C 2 , a quadraatic equation off Eq. (8) is deriived from the calculation 3. Appropriatte values for α and a C 2 can be b calculated uusing Eqs. (7) and a (8) as show wn in Table 2.

h respect to inllet bulk velocity. Fiig. 3 Pressure drop distributtion of the parrt cell in the SCR filter with

RESULTS A AND DISCUS SSION Distributionss of velocity an nd water masss fraction in SC CR system Fig. 4 illuustrates the coontours of the velocity and w ding the sprayy water concenttration of partss of the SCR system, includ injector, mixeer, and reactor. Case 1 represeents the calculaation condition ns of no mixer.. Cases 2, 3, annd 4 represent the calculationn conditions off swirl-type mixxers with vane angles of 30º,, 45º, and 60º, respectively. Case C 5 represennts the calculattion conditionss of a line-typee mixer with a vane angle off 45º. Consider erable disturban nces occur surrround the spraay injector for all cases. Thee flow patterns behind the mixxers differ with h the mixer typpe. In the swirll-type mixers of o cases 2, 3, annd 4, the centraal recirculationn zone (CRZ) iss generated neaar the mixer fieelds, and the C CRZ increases with w increasing g vane angle. T The line-type mixer, m howeverr, does not exhhibit a CRZ, because b the maain flow near the mixer mo oves up and down. d The vellocity distributtion and waterr concentrationn of case 1 in front of the SCR S reactor arre concentrated d in the center region. Thiss phenomenon may promotee serious probleems, such as catalyst c filter damage d and NH H3 slip (NMR RI, 2011). For the t remainder of the cases (ccases 2-5), thee velocity and cconcentration of o water are relatively well ddistributed in co omparison to case c 1. In particcular, Fig. 4(b)) demonstratess that the mass fraction of watter for case 3 appears a well mi mixed with inpu ut gases in frontt of the SCR reeactor. Based on o these resultss, NH3 slip, low w mixing of UW WS with hot ex xhaust gases, annd catalyst filer damage in fro ont of the SCR R reactor have the t potential too be mitigated w without using any a guide vanees, such as bafffles.

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Fig. 4 Contours of (a) velocity and (b) water concentration for different cases with a calculation time 1.5s. Effect of mixer geometry on turbulent flow characteristics Turbulent flow occurs when instabilities in a flow are not sufficiently damped by viscous action and the fluid velocity at each point in the flow exhibits random fluctuations (Turns, 2000). Turbulence can be depicted as fluctuations in a fluid flow. When working with chemicals as in an SCR reactor, typically a high level of flow fluctuations is preferable for the mixing of UWS with exhaust gases. It is common to define the relative turbulent intensity for the velocity as follows:

TI 

u U

(9)

where u is the root-mean-square (RMS) of the turbulent velocity fluctuations ( U  U ) at a particular location over a specified period of time, and U is the average of the velocity for the instantaneous value (U) at the same location over the time period. For the same theory, the standard deviation of the temporal variation of Reynolds averaged velocity (  ) and its relative intensity ( RI   / U ) are adopted. In this RANS simulation, U  U is the temporal variation of Reynolds averaged velocity, not the turbulent velocity fluctuations because U represents the Reynolds averaged velocity. Fig. 5 shows  for 0.3s for different measuring positions and mixer geometries. For case 1 (no mixer), there were few changes in velocity at positions 1, 2, and 3. However,  significantly increased near the SCR filter because a large recirculation zone formed in the diffuser region. When the flow enters the porous zone, it aligns with the channel direction and the higher pressure is located around the catalyst entrance and the center line (Chen et al., 2004). The results of all cases at positions 4 and 5 exhibit a similar tendency. At position 5,  for all cases is relatively lower than that at positions 2, 3, and 4. Case 4 has the highest  after the mixer owing to a large CRZ. This behavior can be explained as a result of the vorticity magnitude generated by the induced flow direction of the mixers. Vorticity is a measure of the rotation of a fluid element as it moves in a field, and is defined as the curl of the velocity vector:

  V

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(10)

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Fig. 5 History of the t standard deviation d (σ) uup to 0.3s for different d meassuring points aand different cases. c

Fig. 6 Contouurs of differen nt cases of (a) vorticity mag gnitude and th he iso-surface of vorticity att 500 1/s and (b)) pressure, botth obtained with a calculation time of 1.55s. Fig. 6 shoows the contouurs of the vorticcity magnitudee and the iso-su urface of the vorticity magnititude for all casses. Except forr case 1, the otther cases exhiibit a high vortticity magnitudde regardless of o the mixer ty ype. This is beecause the intro oduced flow iss concentrated at the center region r for the case 1, while the line-type and a swirl-type models disperrse the flow to o the wall. Forr d the mixer. A case 5 (line-tyype mixer), thhe vorticity maagnitude was rrelatively high by the turbuleent flow immeediately behind small-scale vvortex may be generated direectly behind thhe mixer, and vanishes rapid dly after passinng through it. For the swirl-m is longer with streaam-wise axis than t that for th he line-type miixer. In additio on, the volumee type mixers, tthe vorticity magnitude of the vorticitty as shown inn Fig. 6 increasses because thee CRZ graduallly increases with w the vane anngle. The resu ults above indi-cate that the size and lengthh of the CRZ are controlledd by the mixerr shape. The sw wirl-type mixeer creates both h turbulent andd

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swirling flow ws. Therefore, it i is important to select the apppropriate mix xer in order to maximize flow w mixing and minimize m NH3 slip. The reacction for NOx reduction in SCR S catalysts rrequires a suff fficient reaction n time within a certain temp perature range.. The NO and the NH3 do not n react at low wer temperaturres and insuffiicient flow mix xing. NH3 slipp is a means of o NH3 passingg SCR reactor unn-reacted. through the S

Figg. 7 Distributio ons of relativee intensity (RII) for different measuring pooints and a cases usinng a calculatio on time of 1.5s. ometries. In alll Fig. 7 preesents relative intensity (RI) distributions aat 1.5s for diffferent measuriing positions aand mixer geo mixer cases, there are relattively small ch hanges in the R RI at position 1. RI substan ntially increasees at position 2 after passingg mixers. After thhis point, RI grradually decreaases, and exhib bits uniform diistributions as it approaches the t SCR filter.. through the m The maximum m RI values weere as follows: 1.54% at posiition 1 for casee 1, 1.96% at po osition 5 for caase 2, 2.75% att position 2 forr case 3, 4.67% at position 2 for case 4, and d 2.99% at possition 2 for casse 5. Incorporaating the mixerr is most effecttive for impro-ving mixing pperformance im mmediately beh hind the mixerr when the van ne angle is greaater than 45º. A Although case 4 possesses thee highest RI forr all measuringg positions, the levels of the m mean RI for casse 3 are constaantly maintaineed along the strream-wise axiss from measuriing positions 2 to 3 as shown n in Fig. 8. Frrom an operatiional point of view v with resppect to the durrability and thee likelihood of system failure,, case 3, a swirl-type mixer w with a 45º vane angle, is more favorable thann other cases.

Fig. 8 Distributions of meaan RI with resp pect to measuring points for or diffferent cases uusing a calculaation time of 1.5s.

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Relationship between presssure drop and d uniform flow w m flow and UWS U distributiion across the front of the catalyst c filter are necessary to obtain a maximum m per-A uniform formance for de-NOx and a minimum NH H3 slip in the S SCR system. The T flow uniformity index iss a commonly used indicatorr of the flow diistribution deggree for after-trreatment applic ications (Welteens et al., 1993 3; Bressler et aal., 1996; Girarrd et al., 2006)). In this study,, a similar defi finition is adop pted to evaluatte the distribu ution degree off water concenntration at a ceertain locationn along the pipee:

1 n UI c  1   2 n i 1

C

i

C

2

C

(11))

where, UIc iis the uniform mity index for the t water conccentration, Ci is the local concentration c of water, C is the averagee concentrationn of water, and n is the numbeer of cells. Gennerally, higher UIc indicates better b fluid mixxing. Fig. 9 shoows the UIc with w respect to o the calculatioon time (from 0.3 to 1.5s) fo or the stream-w wise positions 1 to 5 and forr cases 1 to 5. T The effects of the t mixer on th he UIc are quitte strong, regarrdless of the ty ype of mixer. C Compared to caase 1, the casess with mixers im mprove the U UIc by approxiimately 20% inn positions 2 and a 3, approxim mately 30% inn the position 4, 4 and approxi-mately 5% inn position 5. Foor all cases, th he UIc increassed along the stream-wise diirection. The m maximum valu ues of the UIc were 98.5% ffor case 4, 96% for case 3, 94 9 % for case 22, 93.6% for caase 5, and 90% for case 1. Th The sequences for f the UIc att different posittion are as folllows: case 3 > case 5 > case 1 > case 4 > caase 2 before th he mixer at the position 1; casse 4 > case 5 > case 3 > case 2 > case 1 behhind the mixerr at the positionn 2; case 4 > caase 3 > case 5 > case 2 > casse 1 at position ns 3 and 4; andd case 4 > case 3 > case 2 > case c 5 > case 1 in the right frront of the cataalyst filter at th he position 5. T These results in ndicate that thee UIc increasess with increasee in the vane angle a of the sw wirl-type mixeer, and the casees for the swirrl-type mixer generally g show w better mixing performance than t the line-ty ype mixer, evenn though the liine-type mixer outperforms itt behind the mixer m and in thee ormance is due ue to a small-sccale vortex gen nerated by thee up and down n induced flow w region of the diffuser. This superior perfo behind the linne-type mixer that t vanishes rapidly r in a shoort region. Con nversely, for th he swirl-type m mixer, a large-sscale vortex inn terms of CRZ Z is generated in i a long region n. Therefore, oobtaining a certtain distance to o mix of the UW WS with the ex xhaust gases iss important, annd the swirl-typpe mixer is bettter than the linne-type mixer with respect to o uniform flow w and the high her mixing per-formance in ppractical SCR applications. a

Fig. 9 Historry of uniformiity index ( UIc ) for H2O from 0.3 to 1.5s for different m measuring poiints and cases.

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Mixer development generally focuses on the high efficiency of NOx reduction because of its direct impact on the design target; however, it is difficult to determine the total performance of mixing devices based on only the UIc . A more thorough understanding of the flow pattern characteristics and pressure drop is helpful in designing an SCR system with optimal performance for both NOx reduction and system durability. Fig. 10 depicts the relationship between the UIc and pressure drop for different cases. As expected, the case 1 has the lowest pressure loss and UIc , which are 647 Pa and 90%, respectively. The presence of any type of mixer resulted in an improved flow mixing, but a penalty of additional pressure loss. Case 4 had the best mixing performance although the pressure drop was the worst. There is a tradeoff relationship between the UIc and the pressure drop with increasing vane angle for swirl-type mixers. Case 5 has a higher UIc and a pressure drop than case 1. However, the UIc is lower and pressure drop is higher than those for the swirl-type mixers (cases 2-4) at the position 5, as shown in Fig. 9. Furthermore, in a comparison of case 3 and 5, which contain the same angle of 45, case 3 shows higher UIc and a lower pressure drop than case 5, as shown in Fig. 10. Therefore, it is concluded that the swirl-type mixer is more effective than the line-type mixer with respect to the enhancement of UIc and RI. When selecting a static mixer in SCR applications, it is necessary to consider the mixing and the uniform distribution of the UWS in front of the SCR reactor as well as the pressure drop, which is not desirable for the optimization of engine power throughout the system. Finally, it can be concluded that the swirl-type mixer with a vane angle of 45 is more suitable model in this study based on the results of mean RI, UIc and pressure drop. Further research focused on the effect of the vane size becoming larger or smaller may be needed under line- and swirltype mixers. The effectiveness of swirl-type mixers is carefully guessed to be initiated from the larger-scale swirl than those of line-type mixers. It is because the line-type mixer is able to generate the vane-scale swirls whereas the duct-scale swirl is produced by the swirl mixer. Nevertheless, this study may provide useful information for selecting a static mixer in SCR applications.

Fig. 10 Relationship between uniformity index and pressure drop for different cases with a calculation time of 1.5s.

CONCLUSIONS 3-D numerical simulations were performed to investigate the effects of mixer geometry on the mixing performance and the pressure drop. Flow mixing and uniformity can be greatly improved by using a static mixer in the SCR system. Turbulent and swirling flows can also achieve a great improvement for flow mixing with respect to flow recirculation phenomena through a longer distance. In this study, information regarding the selection of proper static mixers was provided based on the correlation between the uniformity index and the pressure drop. The results show that the mixer for SCR applications can be effectively optimized by using a well-designed mixing device. The main results are summarized as follows:

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1) In comparison to the case without a mixer, the cases with a mixer improve the uniformity index by approximately 20% directly behind the mixer, approximately 30% in the diffuser, and approximately 5% immediately in front of the mixer. 2) In the swirl-type mixer, the CRZ is generated directly behind the mixer, and increases with the vane angles of the mixer. Therefore, it is expected that the mixing region is longer with stream-wise axis because the swirl-type mixer creates turbulent and swirling flows. 3) The swirl-type mixer is more effective than the line-type mixer with respect to the enhancement of mixing performance, even though there is a tradeoff relationship between the uniformity index and the pressure drop. Therefore, the swirl-type mixer with a vane angle of 45º is the most suitable model in this study based on the results of both parameters.

ACKNOWLEDGEMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) through GCRC-SOP and this work was supported by the Human Resources Development program (No. 20124010203230-11-1-000) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.

REFERENCES Battoei, M., Doroudian, M. and Lacin, F., 2006. Optimization of exhaust gas distribution on the catalytic converter inletcone diffuser using advanced shape deformation technology, SAE Technical Paper, 2006-01-1439, Pennsylvania: 4 SAE International. Birkhold, F., Meingast, U., Wassermann, P. and Deutschmann, O., 2007. Modeling and simulation of the injection of ureawater-solution for automotive SCR DeNOx-systems. Applied Catalysis B : Environmental, 70, pp.119-127. Bressler, H., Rammoser, D., Neumaier, H. and Terres, F., 1996. Experimental and predictive investigation of a close coupled catalytic converter with pulsating flow, SAE Technical Paper, 960564, Pennsylvania: 4 SAE International. Chen, M. and Williams, S., 2005. Modelling and optimization of SCR-exhaust aftertreatment systems. SAE Technical Paper, 2005-01-0969, Pennsylvania: 4 SAE International. Chen, M., Aleixo, J., Williams, S., Leprince, T. and Yong, Y., 2004. CFD modeling of 3-way catalytic converters with detailed catalytic surface reaction mechanism. SAE Technical Paper, 2004-01-0148, Pennsylvania: 4 SAE International. DieselNet, 2008. Emission Standards, IMO Marine Engine Regulations. [online] Available at: [Accessed March 2013]. Fluent, 2007. Fluent 6.3 User’s Guide. Lebanon: ANSYS Inc. Girard, J. W., Lacin, F., Hass, C.J. and Hodonsky, J., 2006. Flow uniformity optimization for diesel aftertreatment systems, SAE Technical Paper, 2006-01-1092, Pennsylvania: 4 SAE International. Jeong, S.J., Lee, S.J., Kim, W.S. and Lee, C.B., 2005. Simulation on the optimization shape and location of urea injecttion for urea-SCR system of heavy-duty diesel engine to prevent NH3 slip, SAE Technical Paper, 2005-01-3886, Pennsylvania: 4 SAE International. Larmi, M. and Tiainen, J., 2003. Diesel spray simulation and KH-RT wave model, SAE Technical Paper, 2003-01-3231, Pennsylvania: 4 SAE International. Munnannur, A. and Liu, Z. G., 2010. Development and validation of a predictive model for DEF injection and urea decomposition in model SCR deNOx systems, SAE Technical Paper, 2010-01-0889, Pennsylvania: 4 SAE International. Nguyen, T.D.B., Lim, Y.I., Eom, W.H., Kim, S.J. and Yoo, K.S., 2010. Experimental and CFD simulation of hybrid SNCR-SCR using urea solution in a pilot-scale reactor. Computers and Chemical Engineering, 34(10), pp.1580-1589. NMRI, 2011. National Maritime Research Institute 2011. [online] Available at: [Accessed November 2013]. Regner, M., Östergren, K. and Trägårdh, C., 2006. Effect of geometry and flow rate on secondary flow and the mixing process in static mixers-A numerical study. Chemical Engineering Science, 61(18), pp.6133-6141.

Unauthenticated Download Date | 3/22/16 6:45 PM

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Thakur, R.K., Vial, Ch., Nigam, K.D.P., Nauman, E.B. and Djelveh, G., 2003. Static mixers in the process industries-A review. Chemical Engineering Research and Design, 81(7), pp.787-826. Turns, S.R., 2000. An Introduction to combustion: concept and applications. 2nd ed. Boston: McGraw-Hill. Weltens, H., Bressler, H., Terres, F., Neumaier, H. and Rammoser, D., 1993. Optimization of catalytic converter gas flow distributions by CFD prediction, SAE Technical Paper, 930780, Pennsylvania: 4 SAE International. Zhang, X. and Romzek, M., 2007. 3-D numerical study of flow mixing in front of SCR for different injection systems, SAE Technical Paper, 2007-01-1578, Pennsylvania: 4 SAE International. Zhang X., Romzek, M. and Morgan, C., 2006. 3-D numerical study of mixing characteristics of NH3 in front of SCR, SAE Technical Paper, 2006-01-3444, Pennsylvania: 4 SAE International. Zheng, G., Fila, A., Kotrba A. and Floyd, R., 2010. Investigation of urea deposits in urea SCR systems for medium and heavy duty trucks, SAE Technical Paper, 2010-01-1941, Pennsylvania: 4 SAE International. Zheng, G., Palmer, G., Salanta G. and Kotrba, A., 2009. Mixer development for urea SCR application, SAE Technical Paper, 2009-01-2879, Pennsylvania: 4 SAE International.

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