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28 Potable Water

CFD modeling improves venturi ozone injection performance Computational fluid dynamics (CFD) modeling is a valuable tool in the design of sidestream venturi injection piping systems in municipal ozone treatment James R Jackson and Srikanth S Pathapati of the Mazzei Injector Company llC explain how CFD has been used to resolve perfonnance issues

The usc of nluhi-ph1SC C()rnplHlshytional fluid dynamics CfD j modeling provides new insigh t 11110 the complc-ltiry of mixed flogtS lnd senCS as lhmlI pflgtQicrivc and design tool in designing sid(~ stream Icnmrj injection (SV1) for municipll ozone cont~l1ing designs According to Mazui Injector Company LLC SVI system manufacturer the corr~C1 use of mixed fluw hytlrwlk-s in SVJ piping design can significantly lI1prOI tlu perfomlancc of OlOrll SVI systems reg1rdless of the degrlC of mixing provided by wcondary gtS mixing dCliccs

hUlicipal wallT rrcallllcm facili ti~ muSt he judiciolls in their sdarion of cquipmcru given ~hrinking nvrnue and mmpower in the past decldt Smaller more COStshycfkocrivc water pbm designs have replaced those with largt footprintS extensive finance and hih mairurnanlX In North AmcriGl this trend has led many utiliti~S TO

replace their fine bubblc diffusion b~sins with ~ldcst(cam venruri injection (SVI) for municip~l ozone contacting designs

However the use of SVI presents a chlllerlge 10 the Jaigll engineer trlined in the use of dimensionless numbers ~uch as Ihe Reynolds number This ~nd other traditional Inllytic11 md semi-lnpiriCllmiddotba~middotk of the cnvtIQp metholt[s can be used for hasicdcsign Cltlicubriolls (for example fricliomillosses ) of singlemiddotphasc flows but thcy arc challenged under nonmiddotideal now conditions CTtmelt1 by m SVI sysnm and can Illisreprcscll tht a~~ui11 bchlVior of tlw SVI multi-phase dfluent Om5Cqucntl )1new method musr be used to design SVl piping s)stems

In recent )ears CFD hIS lmergcd as a powerful rool for modeling and uuderstmding IIwhiphase flows in complex systems A ell shyconstnlct(il CFD approlch c~n provide l1Nlepih micro and m](m~ scale insight into flow and spties IranSJXgtM phenomena CFO is founded on closing the Navier-

Stokes equoliolls ~nd uSlSlIdV3nccd numencal methods to ~ccuratcl) characterize multi-phose flows

In till design of a Wlfer plam thl en)H1t-er is Iypically conccnled with he trII1SJXgtrtJlion of a single-phase fluid throughl logicol network of pipes vllves lOd fitTings When presellttlwith an ozone SVI sysrem design however heshe bces very di fferlllt chlllenges 111e nlllltl-ph~se hOl1logcIU)llSdfluent disch3rged b) a Venruri inj(Ctor lgtcgillS 1lt undergo phase separnion after a few pipe dilmelePgt uf rr~vcl (FigutC 2) The lelllJXgtral and spatial extent of phlSt scpartrion de~nds upon the Itlocily uf the mixed flow bubble dianwter and rail ofcoa lescence Evemul11y Ihe effluenT ~rrmifics into a gas phase 1I thl crown of the pipeline a wmer phase along the bottom )f the pipeline and homogenous mixture at Ihe pipelines central core The longer the horiWnal pipe run the more pronounced the stratified phase separltl tion The engintCrs chlllcnge then ISto desigl1 the ozone SVI pi~linc to transport the ozoneshywler mixrurl in a manner thai minimizes phase scplrarion md mainTains mixrurl h(lllogl~leiIY and to cnsure Ihe gas IS evenl) disrribUlt-ci Whlll tlw mixture is dispersed into the bulk water

A partial solution 10 phas wp1rltion uf the SVI cfflue11l is to Ustt stCondlry bulk ner as mixing Jevice Onc such onmlemal delitl the l ipline Flash Reactor (JgtFR) is frequcml) ugted ) homogenize the SVlshysi rnrified pipelinc flow b) plssing it through a set of high velocity n07zlesll1e nonlcs al1gul~r fin) and lIptred orifice add angullr r110lltllWl1to the mixed fluid rhereby re-shtlring and discharging a homogenousl) mixed jer into the bulk pipeline flow

It is noted huwcller that SVI piJXline designs that rn~ou rle fpid phase septnllioll 1Ild ~middotertmiddot l1uid strltificalion ma) deliver gas slugs inw the IFR noz7le manifulds When gl$ slug flow

-FIgure 1 CFD analysis of SVI pipIng and pMgtpOSed modifICations

As-Is ( Modi fication 1 I IModification 2~

S~ sllbcgtw r r_l~- - shy~

~

bullbullbull -shy shy~ ~ shy~ ~_~ _bull

i ~- shy shy- ~

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Figure 2 Muttiphase CFD of pipeline flow colored by vltgtlume frnctioo of gas _ _shyU~

_bull _ bullbull 0

uo

r al _

bull ~ H~

~

amp0-01 ~)~

-shyshy~

occurs tht ll(JnitS arc in~Ip1hJc of homogenizil1g Ihe slrllifilCi 11011 Gas pockets thar slug ffed into the n0771e manifold (lUst

large gas bubhle discharge aT the nonkS reducing thtmiddot mixing and nl~SS r111sf(r of ozone 10 solution C()nscquently slug gas low h~sl signifi~3nt eff~Cf 011 the pcrformancl of an SVI Illvne system

One water plant Ihar cXlxrrcmcd poor porformlIKt due 10 slug gas f((-ci WI S a 15 mgd sylcm thu llSCd ~ PFR to app1) an ozone dosac uf to 3 mgl Primary lt1S mixing WiS provided by tWO dut) Vetlluri

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injlaors operating1 347 gplll ltnch A mlllli-n07z1tmiddot IgtFR provided SICondary g15 mixing into th~ bulk 1lncr flow (Fiure J)

Evaluation of the svrs)Stem during commissioning showed tht mure thm 50 pcrctnt of Ihe injlCt((1 ozone was c()Jlrained in the COnt1l1

basins head space indiGlling eltremcly kw urone rransfer into the bulk Wller flow ExaminaTion of the installation piping showed Ihat Ihe pip layom dirlCted pipeline crolVn gas tol single SCt of nonles whieh en~()lIrtged slug g~s flow inm the nozzle manifold leading It)