D DAVID PUBLISHING. Q Energy flux (W) Nu Nusselt number (hd/k) Nomenclature. Greek Symbols

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1 Journal of Energy an Power Engineering 9 (2015) oi: / / D DAVID PUBLISHING Effect of Injection Parameters on Evaporation an Thermolysis Characteristics of UWS (Urea-Water-Solution) in SCR (Selective Catalytic Reuction) Systems Saashiva Prabhu S 1, Nagaraj Shivappa Nayak 2 an Kapilan Natesan 3 1 Canara Engineering College, Visvesvaraya Technological University, Karnataka , Inia 2 Caleonian College of Engineering, Glasgow Caleonian University, CPO Seeb 111, Sultanate of Oman 3 Nagarjuna College of Engineering an Technology, Visvesvaraya Technological University, Karnataka , Inia Receive: September 18, 2014 / Accepte: November 03, 2014 / Publishe: February 28, 2015 Abstract: UWS (optimize Urea-Water Solution) injection system is require to increase the NH 3 conversion efficiency of urea-base SCR (Selective Catalytic Reuction) system of moern automobiles The focus of the current stuy is to o parametric stuies by simulation in a three-imensional moel using CFD (Computational Flui Dynamics) coe AVL FIRE Simulations were carrie out to stuy the characteristics of evaporation an thermolysis UWS consiering the effect of injection velocity, uration of injection, injection angle an for ifferent types of injection In the case of the injection velocities up to m/sec, the ammonia concentration continues to increase It is foun that as the uration injection ecreases, the concentration of ammonia increases In case of continuous injection, the flow rate is less which results in lower velocity of injection, lesser atomization an slower evaporation resulting lesser conversion of UWS into NH 3 Shorter uration of injection leas better atomization with increase velocity of injection which results in faster evaporation an thermolysis Key wors: Evaporation, thermolysis, SCR, UWS, injection parameters Nomenclature A Surface area of roplet (m 2 ) A Pre-exponential factor kg/(m s) B M, T Spaling numbers C Concentration C p Heat capacity J/(kg K) D Diameter (m) E a Activation energy (J mol -1 ) h Heat of reaction (J mol -1 ) Le Lewis number m Mass (kg) m Mass flux (kg/s) Q Energy flux (W) Nu Nusselt number (hd/k) Corresponing author: Saashiva Prabhu S, tech associate professor, research fiels: IC engines an emission control ssprabhu97@gmailcom q Specific energy flux (W/m 2 ) S Source term (kg/s) Sh Sherwoo number t Time (s) T Temperature (K) u Velocity (m/s) Σv Diffusion volume Y Mass fraction Greek Symbols Surface tension (N/m) h Heat transfer coefficient (W/m 2 /K) Density (kg/m 3 ) Diffusion coefficient (m 2 /s) Absolute viscosity (Ns/m 2 ) Shear stress (N/m 2 ) Kinematic viscosity (m 2 /s) Ey viscosity (m 2 /s)

2 Effect of Injection Parameters on Evaporation an Thermolysis Characteristics of UWS (Urea-Water-Solution) in SCR (Selective Catalytic Reuction) Systems 223 Subscipts 1, 2 Directions, phase,g Droplet, gas I Interface l,vap Liqui, vapor ref Reference g Gas phase m Mass s Soli phase sat Saturation v Vapour w Wall Superscipts Characteristic 1 Introuction There are several methos to eliminate NO x emitte from exhaust of iesel engines namely Humi Air Metho, Exhaust Gas Recirculation, Water Injection, High Scavenging etc Compare to these methos of removal NO x from the exhaust gases, the SCR (Selective Catalytic Reuction) base on UWS (Urea Water Solution) is an effective technique to reuce NO x (Nitrogen Oxies) emitte from the iesel engines without altering the performance of the engines UWS (containing 325 wt% urea, bran name: A Blue) is spraye into the hot exhaust gas stream which gives reucing agent NH 3 after evaporation an thermolysis [1] The major part the SCR system is the injection system where the injection parameters play a major role in evaporation an thermolysis characteristics The impingement of roplets on the catalyst an on walls cannot be avoie ue to slow evaporation an thermolysis, as well as the inertia of the roplets in actual exhaust configurations [2] Birkhol et al [3] evelope a moel that accounts for the injection of UWS, spray, thermolysis, spray-wall interaction etc an implemente into CFD (Computational Flui Dynamics) coe an valiate the results with experimental ata of Kim et al [4] Evaporation from the wall film leas to further cooling an an increasing risk of formation of melamine complexes [1] Birkhol et al [2] theoretically investigate the evaporation of water from a single roplet of UWS by a rapi mixing moel an a iffusion limit moel The results show that the ecrease in vapor pressure ue to increasing concentration of urea in the roplet results in a continuous increase of the roplet temperature an a slower evaporation compare to pure water Helen et al [5] use water instea of UWS in a CFD stuy an estimate the concentration of the reucing agent from the water vapor concentration To the best of our knowlege, there are no much of parametric stuies publishe on the effects varying the UWS injection parameters like injection velocity, injection uration on spray evaporation an thermolysis with consieration of wall interaction To preict the generation an istribution of the reucing agent a etaile three-imensional numerical moel is evelope an implemente in the commercial CFD coe AVL FIRE [6] The evelope moel for SCR esign optimization is capable of accurately preicting the effect of some of injection parameters on: (1) the atomization process of the injecte liqui jet; (2) the vaporization an thermolysis processes of UWS roplets in the heate an turbulent exhaust gas environment; an (3) the spray/wall interaction 2 CFD Methoology 21 Evaporation Moel Incluing Urea Thermolysis The influence of urea on the evaporation of water from a UWS roplet is investigate theoretically by ifferent evaporation moels consiering roplet motion an variable properties of UWS an the ambient gas phase The separate moels for evaporation at gas an liqui phases are iscusse below: Liqui phase: RM moel (Rapi Mixing moel) [7, 8]: Within the RM moel, infinite high transport coefficients are assume for the liqui phase, resulting

3 224 Effect of Injection Parameters on Evaporation an Thermolysis Characteristics of UWS (Urea-Water-Solution) in SCR (Selective Catalytic Reuction) Systems in spatial uniform temperature, concentration an flui properties in the roplet, but the quantities will change in time The variation of urea concentration of the roplet can be evaluate by: Y u t m = - Y u m (1) where the mass flow from liqui to gaseous phase is efine to be negative Gas phase: For the gas phase, the quasi-steay moel [7, 8] is use Evaporation rate preiction is sensitive to choice of property values Analytical expressions for the iffusive transport fluxes are obtaine by integration of transport equations for mass an enthalpy outsie the roplet The ifferential equations for roplet mass an temperature can be erive from energy balance as follows: where, B M m =-pd r G Sh ln(1 + B ) (2) * g, ref g, ref M T m æc ( T T ) ö vap - p, vap, ref g = h ) - vap m c ç B p, çè T ø Y -Y = 1-Y vap, s vap, g B T vap, s an: x = (1 + B ) -1 an: c x = c M pvapref,, The Eq (2) is moifie as: with B T m Sh * * Nu Le pgref,, gref, * D Nu B T cpvapref,, (3) l =-p ln(1 + ) (4) c ( T T ) pvapref,, g- s = hvap As the temperature an the urea concentration change uring evaporation, variable flui properties are use The roplets are assume to be spherical throughout the evaporation an ecomposition processes [3] In the moeling of thermolysis, there are ifferent formulations mentione in the literature The reaction rate is escribe by an Arrhenius-type equation use by Kim et al [4] as follows: m urea æ Ea ö =-m A exp - urea (5) ç è RT ø with the kinetic parameters A =382 s -1, E a = 294E + 7 Jmol -1, R-universal gas constant, Birkhol et al[2] use the experimental ata from Kim et al [4] for a parameter fit an the values for A =042 kg/ms, E a = 69,000 J/mol For thermolysis, following reaction is consiere: (NH 2 ) 2 CO (s or l) NH 3 (g) +HNCO (g) H = +185 kj/mol an the rate equation is: a æ E n a ö =-A (1 -a) exp r - (6) ç è RT ø where the urea conversion fraction α is given by: m ( t = 0) -m ( t) urea urea a= m ( t = 0) urea an Buchholz [9] suggests A = 1E6 s -1, E = 73E4 Jmol -1 an n = 03 r a All of the above approaches are implemente in CFD coe AVL FIRE uner spray evaporation an gas phase reaction moules The approach evelope by Birkhol et al [2] has been establishe in numerical coe an efault values of input parameters, frequency factor A r an activation energy E a are given as mentione above 22 Mass, Heat Transfer an Evaporation In the present work, a uniform gas moel erive by Dukowicz [9] is applie The moel assumes spherical symmetry, a quasi steay gas film aroun the roplet, uniform roplet properties, phase an thermal equilibrium at the roplet surface The mass balance of

4 Effect of Injection Parameters on Evaporation an Thermolysis Characteristics of UWS (Urea-Water-Solution) in SCR (Selective Catalytic Reuction) Systems 225 the roplet is given by: m = Am (7) vap where, the time erivative of the roplet mass m epens on the specific vapor mass flux m vap vaporizing from or conensing on the roplet surface A The energy balance of the roplet is given by: T (8) mc =D h Amvap + Q p, vap By introucing a specific surface energy flux Q q = the balance equations of the mass an energy A can be re-arrange as: m m = Q vap æ ö T m vap = ç1 +D p, vap mc Q h ç q çè ø q (9) (10) Eqs (9) an (10) efine the transient behavior of the roplet provie that the expressions Q an m are q known The energy transfer flux Q is calculate as: Q = Ah ( T -T ) g (11) h is a general heat transfer coefficient The mass an energy fluxes transferre between the iniviual roplets an the gas phase are applie as aitional source terms in the balance equations of the flui flow in their Eulerian formulation, it is possible to formulate any urea ecomposition reaction as homogeneous gas phase reaction 23 Wall Film Moeling Deposition of roplets leas to a wall film which is moele with a 2D finite volume metho in numerical coe Gas an wall film flow are treate as separate single phases, couple by semi-empirical bounary conitions The film is transporte ue to shear forces, gravity an pressure graients Continuity equation: ( u ) ( u ) 1 = t x y ra cell S m (12) The usual step woul be to employ the momentum equations for solving the velocity components u 1 an u 2 Let us assume that the velocity components are known as well as the source term ( u ) 1 ( u ) 2 Now, the convective terms, is x y evaluate an Eq (12) can be solve explicitly The viscosity increases with increasing urea concentration an leas to an increase of the film thickness an shear stress an a tenency to a laminar film profile [6] 24 Film Velocity Profiles One of the key features of the FIRE wall film moel is the use of analytical film velocity profiles instea of a momentum equation As inertial effects are ignore, the profiles introuce here are steay state As long as the film is thin an no transient forces are taken into account, steay state conitions are reache within fractions of a secon The istribution of shear forces across the film irectly relates to the velocity profile Using Boussinesq hypothesis for turbulent ey viscosity ε, the velocity profile of film ue to applie shear force τ can be written as: t = ( n+e) u r y (13) Finally, the mean film velocity, which is necessary for equation is obtaine by integrating over film thickness: u f æ pö = 2 rg - + 3t 6m çè x ø where I is interfacial shear force 25 Evaporation an Thermolysis from Wall Film I (14) The multi-component moel evelope for evaporation an thermolysis of UWS roplets which

5 226 Effect of Injection Parameters on Evaporation an Thermolysis Characteristics of UWS (Urea-Water-Solution) in SCR (Selective Catalytic Reuction) Systems are impinging at the walls The UWS roplets are heate up an ue to the low vapor pressure of urea compare to the vapor pressure of water, the water evaporates first from the roplet which leas to a spatial urea concentration graient with a maximum at the roplet surface Thus solve urea at the roplet surface causes a ecrease of the vapor pressure of the water For this purpose, a two stage process is assume, where evaporation of water takes place initially from the multi species film an if the mass fraction reuces by 5%, then thermolysis of urea takes place base on an Arrhenius approach Evaporation wall film is base on Fick s law of iffusion which is given by: m é ê r ( D + D ) ù ú c ë - û y v 12 t = 1 c ê I ú (15) This is the evaporation mass flux (kg/(s m 2 )), or evaporation rate, which epens primarily on the following parameters: (1) Concentration graient has probably the greatest influence on evaporation rate If the gas above the film is alreay saturate with vapor, the graient goes to zero an evaporation eclines; (2) The influence of temperature, reflecte by the temperature epenent properties c I an D 12 an time epenant properties D t Wall film energy equation an the evaporation routines are couple in this stuy through a time step aaptation to avoi numerical instabilities an to solve the steep graients of heating an evaporation of the wall film The erive moels implemente in the numerical coe help to preict the real processes uring the layout of exhaust tube configurations an injector mounting positions with respect to the spatial istribution of the reucing agent at the upstream of the catalyst Birkhol et al [2] suggeste the moel for urea ecomposition rate which is in the form: m urea æ Ea ö =-AreaD A exp - filmpatch (16) ç è RT ø Injection Angle Fig 1 The imensions of CFD moel They utilize the experimental ata from Kim et al [4] for a parameter fit an get A = 042 kg/ms, E a = 69E + 3 J/mol, n = 1 D filmthick is the local film thickness 3 CFD Moel The moel evelope by Kim et al [4] is use for our stuies Fig 1 show the imensions CFD moel 4 Results an Discussion 41 Effect of Injection Velocity Droplet evaporation: the evaporation of single roplet is stuie earlier by Wang et al [10] is for larger roplet compare those which are really seen in SCR applications Grout et al [11] use the epenency of 2 -law an obtaine the expression for evaporation time During evaporation an subsequent thermolysis process the parameters of UWS injection play major role Birkhol et al [2] i numerical stuies on UWS evaporation an thermolysis behavior using CFD coe AVL FIRE an valiate with experimental results of Kim et al [4] at ifferent exhaust gas flow rates an temperatures In our work, for the experimental set up evelope by Kim et al [4], the simulations are one to optimize the NH 3 formation in the SCR converter The initial roplet size is 50 µm an UWS is injecte at start time 01 s an en time 04 s There are no correlations to preict the roplet sizes of UWS with injection velocity But Kim et al [4] have given some experimental values at the ownstream of the injection point but they are not exact values to be use in our simulation Obviously at the nozzle tip, the velocity will be higher So a range of velocity m/s is chosen to stuy the behavior of UWS at ifferent temperatures

6 Effect of Injection Parameters on Evaporation an Thermolysis Characteristics of UWS (Urea-Water-Solution) in SCR (Selective Catalytic Reuction) Systems 227 Figs 2a-2c inicate the faster evaporation UWS at higher injection velocity an higher temperature The higher velocities, ie, > 50 m/s roplets o not fin sufficient time for evaporation an thermolysis an simulation iverges in such cases The roplet evaporation an their size iminishing along the length of SCR converter are shown in the Figs 2a-2c Evaporation from wall: Nagaraj et al [12] numerically preicte that more film eposition occurs Sauter mean iameter,m Sauter mean iameter m Sauter mean iameter,m 475E E E E E E K K 106m/sec K 106m/sec 106m/sec Fig 2 Comparison of roplet evaporation characteristics with various injection velocities at (a) 573 K (b) 623 K an (c) 673 K Evaporate Film mass,kg Fig Evaporate film mass at various injection velocities at lower temperatures At temperature 300ºC, the evaporation behavior is stuie for ifferent injection velocities As the injection velocity increases, the roplet size reuces which enhances the evaporation (Fig 3) Thermolysis behavior: NH 3 istribution for various injection velocities at 573 K, 623 K an 673 K are compare in Figs 4a-4c The NH 3 conversion efficiency is foun to be high at elevate temperature At higher injection velocities, the spray penetration increases an roplet size ecreases which leas to incomplete evaporation an thermolysis ue to ecrease resience time an reuce inertial effect However, as the roplet size ecreases with increase in injection velocity, resulting in better atomization of the roplets which may lea to faster evaporation an thermolysis At 673 K, maximum NH 3 concentration is for injection velocity in the range of m/s Figs 4a-4c show NH 3 concentration at transient injection conitions for temperatures 573 K, 623 K an 673 K, respectively 42 Effect of Injection Duration Evaporation behavior: to stuy the behavior of UWS evaporation an thermolysis for various injection urations, the injection uration ranging from 01 s to 05 s is set in the simulation The quantity 033 gms of UWS is injecte for ifferent injection urations resulting ifferent velocities of injection It is observe that from Fig 5a that the evaporation is faster for shorter injection uration The slope of the curve

7 228 Effect of Injection Parameters on Evaporation an Thermolysis Characteristics of UWS (Urea-Water-Solution) in SCR (Selective Catalytic Reuction) Systems Mass fraction NH 3 Mass fraction of NH 3 Mass fraction of NH 3 800E E E E E E E E E E E E E E E E E E E E E E E m/sec m/sec 573K m/sec 50m/sec Resiene time (s) 623K K Fig 4 Variation of NH 3 concentration with ifferent injection velocities for pulse injection (a) 573 K (b) 623 K (c) 673 K referring for injection uration s shows the highest evaporation rate while the same is foun ecreasing as injection time increases It is also evient from Fig 5b that roplet iameter ecreases from original size 50 µm at the onset of injection an it Evaporate Liqui mass,kg Sauter Mean Diameter,m Mass fraction of NH E E E E E E E E (c) Fig 5 Variation of NH 3 concentration with ifferent injection urations at (a) Liqui mass evaporate (b) Variation of roplet iameter (c) Mean NH 3 mass fraction at 673 K vanishes at 09 s for injection uration 01 s Thermolysis characteristics show that NH 3 concentration increases with injection urations (Figs 5a-5c) (a) (b) 01-02sec 01-03sec 01-04sec 01-05sec 01-06sec 01-02sec 01-03sec 01-04sec 01-05sec 01-06sec sec 01-03sec 01-04sec 01-05sec 01-06sec

8 Effect of Injection Parameters on Evaporation an Thermolysis Characteristics of UWS (Urea-Water-Solution) in SCR (Selective Catalytic Reuction) Systems Effect of Injection Angle Injection angle is the key factor in spatial istribution on UWS in exhaust gas environment, mixing, heat transfer between UWS an exhaust gas As the injection angle increases, the raial istribution increases This results in better atomization an istribution of UWS with increase ammonia concentration (Fig 6a) The raial istribution causes better mixing an increase resience time Hyun et al [13] have stuie the evaporation an thermolysis behavior for angles -3, 15 an 45 The average film area an film thickness were plotte an results reveal that no film formation occurs at injection angle below 80 an it increases till 140 The two major parameters like film area an film thickness are plotte with respect to resience time (Figs 6c-6) The film forme exists only for few fraction of secon an isappears as evaporation an thermolysis progresses from wall 44 Effect of Type of Injection Injection system in SCR system can be of two types namely continuous an pulse injection with esire frequency In continuous type injection, the esire quantity is injecte continuously with low injection pressure an lesser spray penetration Uniformity of mixing across the cross section cannot be attaine for such kin of injection The continuous injection is moele an simulation is one using require bounary conitions The simulation results obtaine for the same shows the localize urea ecomposition proucts along the length of SCR (Figs 8a an 8b) In pulse type of injection, the require quantity of UWS is injecte for a particular uration an for such cases, raial istribution will be uniform an axial uniformity is attaine by the turbulence of the exhaust gas The results of the same are shown in Fig 7 Mean concentration of NH 3 with resience time in SCR omain is plotte an shown in Figs 7a an 7b for both continuous an pulse injection system Mean mass Mass Fraction NH 3 Film area,cm 2 Film thickness,mm 350E E E E E E E E º 80º 120º 20º 40º 60º 80º 100º 120º 140º 150º (a) (c) º () Fig 6 Variation in (a) NH 3 concentration at 08 m from injection point; (b) NH 3 mass fraction for various injection angle; (c) film area; () average film thicknesses at ifferent injection angles at 673 K (b) 90º 130º 140º 80º 120º 120º 140º

9 230 Effect of Injection Parameters on Evaporation an Thermolysis Characteristics of UWS (Urea-Water-Solution) in SCR (Selective Catalytic Reuction) Systems Mass fraction of NH 3 200E E E E-05 Continuous Injection with increase velocity of injection which results in faster evaporation an thermolysis In case of steay state injection, lower velocity of injection leas to lesser atomization whichh may cause slower evaporation of UWS an more oxiation of NH 3 5 Conclusion ns Mass fraction of NH 3 000E E E E E E E ,000 1,200 Number of iterations(1 iteration = 0001 sec) Pulse injection 100 Fig 7 NH 3 formation for continuous an pulse injection system at transient 673 K Fig 8 Effect of temperature in the formation at NH 3 at 673 K for (a) pulse injection; (b) continuous injection fraction of NH 3 for continuous injection foun to be lower In case of pulse injection system, the highest concentration foun to be 280 PPM (by mass) for resience time 09 s at 400 C an it is 180 PPM for continuous injection with 30 injection angle The increase concentration of NH 3 is foun for pulse injection is ue to shorter uration of injection which ens within 04 s Shorter uration of injection leas better atomization In this work, a three-imensional numerical simulation of UWS is one to evaluate the effects of various parameters along with exhaust gas temperaturee on evaporation an thermolysis behavior of UWS The following conclusions are rawn from this work: ( 1) Higher injection velocity causes better atomization an increase evaporation an thermolysis For UWS injection velocities up to m/s the NH 3 concentration continue to increase (2) At higher exhaust gas temperature, the higher injection velocity of UWS resulting higher NH 3 conversion for lesser resience time At lower temperatures, the UWS requires more resience time an the injectionn velocity shoul be less; ( 3) Evaporation from wall increases as injectionn velocity increases As the angle of injection increases, the evaporation an thermolysis processes will increasee ue to better ispersion of UWS an NH 3 concentration n continues to increase (4) Shorter uration of injection in pulse flow gave higher NH 3 concentration compare to that of continuous injection References [1] Koebel, M, Elsener, M, an Kleemann, M 2000 Urea-SCR: A Promising Technique to Reuce NO x Emissions from Automotive Diesel Engines Catalysiss Toay 59: [2] Birkhol, F, Meingast, U, Wassermann, P, an Deutschmann, O 2005 Moeling an Simulation of the Injection of Urea-Water-Solution for Automotive SCR De-NO x -Systems Applie Catalysis B: Environmental 70 (2007): [3] Birkhol, F, Meingast, U, Wassermann, P, an Deutchmann, O Analysis of the Injection of Urea Water Solution for Automotive SCR De-NO x -Systems; Moeling of Two Phase Flow & Spray/Wall Interaction SAE

10 Effect of Injection Parameters on Evaporation an Thermolysis Characteristics of UWS (Urea-Water-Solution) in SCR (Selective Catalytic Reuction) Systems [4] Kim, J Y, Ryu, S H, an Ha, J S 2004 Numerical Preiction on the Characteristics of Spray-Inuse Mixing an Thermal Decomposition of Urea Solution in SCR System In Proceeings of 2004 Fall Technical Conference of the ASME Internal Combustion Engine Division, Long Beach, California USA, [5] Helen, V, Verbeek, R, an Willems, F 2004 Optimization of Urea SCR De-NO x Systems for HD Diesel Engines SAE, [6] FIRE 83 AVL LIST GmbH, A-8020 Graz, Austria, [7] Faeth, GM 1983 Evaporation an Combustion of Sprays Prog Energy Combustion Sci 9(1983): 1-76 [8] Sirignano, W 1983 Fuel Droplet Vapourization an Spray Combustion Theory Prog Energy Combustion System 9 (1983): [9] Dukowicz, J K 1979 Quasi-Steay Droplet Phase Change in the Presence of Convection Informal Report LA MS, Los Alamos Scientific Laboratory, Los Alamos, New Mexico [10] Tae, J W, Seung, W B, an Seung, Y L 2009 Experimental Investigation on Evaporation of Urea-Water-Solution Droplet for SCR Applications AIChE Journal December 2009 Vol55, Issue 12, [11] Grout, S, Bernar, J, Karine, B, an Osbat, P G 2013 Experimental Investigation on the Injection of an Urea-Water Solution in Hot Air Stream for the SCR Application: Evaporation an Spray/Wall Interaction Fuel Volume 106, April 2013, [12] Nayak N The Evaporation an Spray Wall Interaction Behavior of UWS (Urea Water Solution) in SCR (Selective Catalytic Reuction) Systems of Moern Automobiles SAE [13] Shi, X, Deng, J, Wu, Z, an Li, L 2013 Effect of Injection Parameters on Spray Characteristics of Urea-SCR System SAE Int J Engines 6 (2):