Share PDF

Search documents:
  Report this document  
    Download as PDF   
      Share on Facebook

13ICE-0172

Author, co-author (Do NOT enter this information. It will be pulled from participant tab in MyTechZone)

Affiliation (Do NOT enter this information. It will be pulled from participant tab in MyTechZone)

Copyright © 2012 SAE International

ABSTRACT

The Selective Catalytic Reduction (SCR) based on Urea Water- Solution (UWS) injection is an effective technique to reduce nitrogen oxides (NOx) emitted from diesel engines. A 3D numerical computer model of the injection of UWS and their interaction with the exhaust gas flow and exhaust tubing is developed to evaluate different configurations during the development process of such a DeNOx system. The model accounts for all relevant processes appearing from the injection point to the entrance of the SCR-catalyst especially during evaporation and thermolysis of droplets, hydrolysis of isocyanic acid in gas phase, heat transfer between wall and droplets and spray/wall-interaction. The results of droplet evaporation are compared with experimental values The spray wall deposition found to be varying with temperature and residence time. Simulated results of variation of wall deposition, film deposition area, evaporated mass with respect to residence time done using CFD code AVL FIRE. The rebound and thermal breakup of the droplets significant factors at elevated temperatures which lead to depletion of spray wall thickness.

1. INTRODUCTION

Among various techniques of reducing diesel NOx to levels required by strict worldwide emission regulations, the most realistic solution is ammonia-selective catalytic reduction (SCR), which uses ammonia as a reducing agent. Due to toxicity and handling problems of ammonia, the Urea-Water Solution(UWS) is used and the feasibility of such SCR systems is investigated and underlined by the European commission, reports that Urea-SCR systems are the most promising approach to comply with EURO IV and V emission standards. Urea-SCR has been widely used since the 1980 to reduce NOx emissions from exhaust gas in stationary applications. More

recently, the technology has been applied to mobile diesel engine applications such as heavy duty trucks, ships, etc. The development of Urea–SCR system is still a challenging task for mobile diesel engines.

In modern automotive applications, the urea-water solution(UWS) based selective catalytic reduction (SCR) is a promising method to control NOx emissions. Urea-Water- Solution (UWS) containing 32.5 wt% urea; brand name: AdBlue) is sprayed into the hot exhaust stream and subsequently the reducing agent (ammonia, NH3) is generated by evaporation of water, thermolysis of urea and hydrolysis of isocyanic acid (HNCO) [1]. The resulting spatial distribution of the reducing agent upstream to the catalyst of the converter is a crucial factor for the conversion of NOx.

In actual exhaust configurations impingement of droplets on the catalyst and the walls cannot be avoided due to the slow evaporation and thermolysis and due to the inertia of the droplets [2]. Especially in passenger car applications [3], where dosing systems without air-assisted atomizer have to be applied in combination with small tube diameters, a noticeable amount of spray impacts on the exhaust tube surfaces. Spray impingement causes local cooling of the wall. Deposition of droplets and wall film formation can occur if the surface temperature decreases below a critical temperature. Evaporation from the wall film leads to further cooling and an increasing risk of formation of melamine complexes [4].Analyzing the literature, several studies on the evaporation and thermolysis of UWS from sprayed droplets can be found, e.g. [2], [5], [6], and [7]. However the evaporation of the evaporation of the UWS not yet completely known in the heated environment. To the best of our knowledge there are no studies published on the effects of spray impact on surfaces and wall film formation at the injection of UWS. Therefore UWS spray, evaporation and interaction with hot gas stream and walls is considered in this work. To predict the generation and

1

distribution of the reducing agent a detailed three-dimensional numerical model of the behavior of the UWS spray in the exhaust system is developed and implemented in the commercial CFD code Fire v8.3 from AVL [8].Evaporation of the two-component droplets in the gas phase is described using the Rapid Mixing (RM) model, which considers the influence of urea concentration and variable fluid properties. The vapor pressure of urea is derived from experimental data [9] to calculate the thermal decomposition of the urea particles. Thus, the physical conditions of the droplets are determined as an important boundary condition in case of impingement on the walls and the catalyst. The used spray/wall-interaction model of Kuhnke [10] accounts for dry and wet as well as for cold and hot walls by using dimensionless numbers which are influenced by the thermo-physical properties of the droplets. Heat transfer between spray and wall is described according to Wruck[11]. The film on the wall is modeled as a two- component fluid of urea and water coupled by momentum, species, and energy balances to the gas phase and the walls.

2.CFD METHODOLOGY

2.1Evaporation Model Including

Urea Thermolysis

The influence of urea on the evaporation of water from a UWS droplet is investigated theoretically by different evaporation models considering droplet motion and variable properties of UWS and the ambient gas phase. To evaluate the influence of solved urea on the evaporation of water, three different evaporation models are used:

Liquid phase

Rapid Mixing Model (RM Model) [12, 13]:

Within the RM model infinite high transport coefficients are assumed for the liquid phase, resulting in spatial uniform temperature, concentration and fluid properties in the droplet, but the quantities will change in time. The variation of urea

concentration of the droplet can be evaluated by

 

dYdtu = − ṁmvap

(1)

Diffusion Limit Model (DL Model) [14]:

Neglecting internal convection, only diffusive transport of energy and mass are assumed. The diffusion equation for

species and energy in the droplet is solved considering variable fluid properties.

∂

 

 

G

 

∂2

+

 

 

 

∂

 

 

∂G

 

+

 

 

 

 

∂

∂Y

 

 

u

∂ Yu

1

 

∂P

G1

∂

 

u

Gr

u

 

dr

∂Yu

 

t

= r2

w2

w +ρ

 

w +

u

 

w

 

 

dt − r

(2)w

∂

 

 

 

 

∂2

 

 

 

 

∂l

 

 

 

 

 

 

 

 

∂

(3)

∂Tt

 

= r2

 

+

w +l1

∂w + r

 

 

 

drdt −

r

∂Tw

 

 

∂wT2

 

 

 

 

with

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

d

=

ρ lcp,

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

r denotes a radial convective velocity which accounts for the variable density field within the droplet.

Effective Diffusion Model(ED model):

The ED model accounts for internal circulation due to forced convection. It considers internal circulation by an empirical correction of the transport co-efficients G and ld

Gas phase:

For the gas phase the quasi-steady model [13] is used. The differential equations for droplet mass and temperature can be derived from energy balance as follows.

 

 

dmdt

= −πDdrg,refGg,refSh* ln(l+BM)

(4)

 

 

dTdt

= mṁvapcp,

cp,vap,

T

T

−h

(5)

where,

BM =

Yvap,s

Yvap,

and

 

 

 

 

 

 

1 Yvap,s

 

 

 

 

 

 

BT = ( +BM)x −

1,

=

Cp,vap,

N

*

1

 

*

 

 

Cp, ,

 

Le

 

dm

 

 

 

l ,

*

ln(

+BT)

 

(6)

dt

= −πDd Cp,vap,

 

 

 

with

 

 

 

 

 

 

 

 

 

 

 

BT = Cp,vap, vapT

Ts

 

 

 

 

 

 

 

The RM and DL models describe the physical limits of infinite high and only diffusive transport. As the temperature and the urea concentration changes strongly during evaporation,

2

dmu a dt

variable fluid properties are used. For the calculations, it is additionally assumed that no crystallization of urea occurs. The droplets are assumed to be spherical throughout the evaporation and decomposition processes [15].

In the modeling of thermolysis rate, there are different formulations for the thermolysis rate in literature The reaction rate is described by an Arrhenius-type equation.. The kinetic model used by Kim [9] reads as

= − re ,A. −Ea

RT (7)

with the kinetic parameters A=382 1/s, Ea= 2.94e7 J/mol. Birkhold et al. [2] use the experimental data from Kim et al. [9] for a parameter fit and get A=0.42 kg/ms, Ea=69 000 J/mol..

For thermolysis following reaction is considered

(NH2)2CO(s orl)

NH3(g)+HNCO(g) H=+185KJ/mol

And rate equation is

Ea

dα

(8)

(−RT )

dt = −A.( − ) .

where( denotes the urea conversion fraction α given by where α= re (t=0)- re (t))/ re (t=0) Buchholz [16] suggests A=1.e6 1/s, Ea = 7.3e4 J/mol and n=0.3.

All of the above approaches are implemented in FIRE and can be selected in the spray evaporation GUI section. According to the most comprehensive validation related to the automotive SCR application the approach of Birkhold (equation (2.41)) has been established as the default setting. The frequency factor A and activation energy Ea are input parameters in the GUI with default settings as given above

2.2 Mass, Heat Transfer and Evaporation

In the present work, a transfer temperature, uniform gas model derived by Dukowicz [17] is applied. The model assumes spherical symmetry, a quasi steady gas film around the droplet, uniform droplet properties, and phase and thermal equilibrium at the droplet surface. The mass balance of the droplet is given by,

 

dmdt

= Ad.

̇

 

 

 

 

 

(9)

where, the time derivative of the droplet mass

 

 

depends on

the specific

vapor

mass

flux

 

vaporizing from or

 

 

 

 

 

 

 

 

 

 

d

 

condensing on the droplet surface

̇. The energy balance of

the droplet is

ℎ .Ad . ̇ +

Ad

 

 

 

 

d.

,d.dTdt

=

Ë™

 

̇

 

(10)

By introducing a specific surface energyË™

flux

 

 

 

 

 

 

 

 

̇ =

 

 

 

 

 

the balance equations of the massAand energy can

 

 

be re-arranged to

 

 

 

 

 

 

 

 

dm

=

Ë™

̇vap

 

 

 

 

 

 

 

(11)

dt

̇

 

+

â„Ž

. ̇vaṗ

 

 

 

 

 

d. ,d.dTdt =

Ë™

 

 

 

(12)

Equations 14-15 define the transient behavior of the droplet

provided that the expressions

˙and

̇vaṗ are known.

The energy transfer flux ˙is calculated

˙ = Ad.α.(

g – d)

(13)

α is a general heat transfer coefficient, which is typically

derived from Nusselt correlations [18]. The fraction of specific

heat to mass flux at the droplet surface

̇vap is evaluated based

on analogy considerations. It is assumeḋthat the differential

equations and boundary conditions for the heat and mass transfer problems are similar. Based on a rigorous theoretical study, Dukowicz [16] derived the following expression,

ṁvap

=

∞

Le. vap,

vap,∞

.(1

vap, )

, )

(14)

q̇

 

vap,

,

.(

vap,

 

3.SPRAY/WALL-INTERACTION

3.1Spray/Wall-Interaction Model

The physical mechanisms occurring during the interaction of spray with a wall are very complex, because the behavior of the impinging droplet is influenced by a variety of parameters such as droplet properties like velocity, diameter, fluid properties

3

and surface properties like wall temperature, surface roughness or wall film height [19]. Thus semi-empirical models based on dimensionless variables are developed for numerical simulations (e.g. [20]). The used model of Kuhnke[21] considers all relevant impingement phenomena by a classification into the four regimes deposition, splash, rebound and thermal breakup based on the two parameters

3.2 Spray Impingement Heat Transfer

The cooling of the wall due to spray impingement plays an important role, because only if the dimensionless wall temperature T*(ratio of wall temperature to saturation temperature) decreases below the critical value (usually1.4) deposition of droplets will occur and a wall film will develop. The implemented heat transfer model is based on the approach of Wruck[11]. When a droplet impinges on a hot wall, a short period of direct contact between fluid and solid exists until a vapor cushion is formed. For small droplets the heat transferred during this direct contact is determining the total heat transfer, because of the insulation effect of the vapor cushion [11].The heat transfer is calculated assuming the contact by two semi- infinite bodies, droplet and wall, which are in contact for a certain time with an effective contact area. It is based on one- dimensional transient heat conduction in both contact partners

Figure 1. Regime map for UWS of spray/wall-interaction according to Birkhold [8]

3.3Wall film Modelling

Deposition of droplets leads to a wall film which is modeled with a 2D finite volume method in the Fire v8 wall film module [8]. Gas and wall film flow are treated as separate single phases, coupled by semi-empirical boundary conditions. The film is transported due to shear forces, gravity and pressure gradients.

3.4Continuity Equation

The continuity equation transformed into conservation of film thickness assuming a spatially constant wall film thickness in a cell, equation can be solved explicitly, if velocity components and the source term are known.. development of a wall film using physical properties like kinematic viscosity and the surface tension. The viscosity increases with increasing urea concentration [22] and leads to an increase of the film thickness and shear stress and a tendency to a laminar film profile [22]. For high urea concentrations the above approach may only be a rough estimation of the film dynamics. The viscosity increases with increasing urea concentration [21] and leads to an increase of the film thickness and shear stress and a tendency to a laminar film profile [22]. For high urea concentrations the above approach may only be a rough estimation of the film dynamics. Evaporation and thermolysis from wall film is based on Fick’s law of diffusion. Wall film energy equation and the evaporation routines are coupled in this study through a time step adaptation to avoid numerical instabilities and to solve the steep gradients of heating and evaporation of the wall film. The influence of flow conditions, exhaust tube properties and spray parameters on the film formation can be evaluated with the developed model. The derived models implemented in the CFD-code Fire v8.3 help to predict real processes during the layout of exhaust tube configurations and injector mounting positions with respect to the spatial distribution of the reducinagent upstream the catalyst.

4.CFD MODEL

The amount of UWS injected into gas stream is set to produce a stoichiometric amount of ammonia inside the converter. Due to the toxicity and handling problems of ammonia, the non- toxic and commonly available reducent urea is used, and is injected in front of the SCR converter. The present study allows to judge different SCR exhaust system configurations with respect to conversion and local distribution of reducing agent. For our study emphasis is given to on evaporation followed by hydrolysis and thermolysis of UWS which are contributing for formation and distribution of ammonia. The simulation is done considering the gas phase reaction module of Fire v8.3 to characterize the ammonia distribution along entry to the SCR.

4

Inlet

Homogeneous reactions

Outlet (rear)

Location of the injector

Figure. 2 Typical SCR system for CFD study

Table 1: Properties of Urea Water Solution

Exhaust

 

Exhaust temperature

573–723 K

Wall temperature

300 K

Flow rate

0.23575 kg/sec

Spray

 

Injection velocity

5- 27 m/s

Injection temperature

293 K

Injection mass in the SCR domain

0.4gms

5.RESULTS AND DISCUSSION

Spray Validation: To evaluate the influenc e of varying injection velocities, CFD simulations are done for injection UWS of 0.4gms in SCR domain for 0.4 secs from an orifice of 0.1mm outer diameter and at various injection velocities of 5- 27m/s with average particle sizes of 8e-005m. The spray distribution at injection velocity 15m/sec is sh own in the Fig 3. It is observed that axial velocity varies w ith respect to droplet size. Small droplets are accelerated due to momentum transfer between exhaust gases and spray drop lets. At lower velocities the momentum transfer to UWS drop let is lower so the residence time of the droplet increases which results in enhanced evaporation. The variation of mass fr action of water in SCR domain from the simulated results are shown in figure 4 at exhaust gas temperature 673K.The mass fr action of water at different gas velocities are compared and found faster evaporation at slower exhaust gas velocities.

5

Figure. 3 Variation of droplet diameter at 623K after 0.4sec

water

0.0805

 

 

 

 

5m/sec

 

 

 

 

 

 

 

 

0.08

 

 

 

 

 

 

 

 

 

 

10m/sec

 

 

 

 

 

 

 

 

 

0.0795

 

 

 

 

15m/sec

 

 

of

 

 

 

 

 

 

0.079

 

 

 

 

20m/sec

 

 

fraction

 

 

 

 

 

 

 

 

 

 

27m/sec

 

 

0.0785

 

 

 

 

 

 

 

 

 

 

673K

 

 

0.078

 

 

 

 

 

 

Mass

 

 

 

 

 

 

 

0.0775

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0.077

 

 

 

 

 

 

 

 

0.0765

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

0.2

0.4

0.6

0.8

1

Residence time sec

Figure.4 Variation of mass fraction of water with residence time at various injection velocities

water

0.088

 

 

 

 

8.3m/sec

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0.086

 

 

 

 

10.8m/sec

 

 

 

 

 

 

 

 

 

0.084

 

 

 

 

 

 

 

of

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

fraction

0.082

 

 

 

 

 

 

 

0.08

 

 

 

 

 

 

 

Mass

0.078

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0.076

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

0.2

0.4

0.6

0.8

1

Residence time, s

Figure. 5 Variation of mass fra ction of water with residence time for two different exhaust gas velocities at UWS injection velocity 27m/sec at 673K

5.1 Droplet Evaporation Characteristics

Some studies have been done on droplet evaporation at different temperature for the droplet sizes higher than the droplet which are really seen in SCR applications [23].Main objectives of our study are to understand the evaporation characteristics of UWS droplet in various ambient temperatures numerically for stationary droplet. So far, to our knowledge, there have been few useful experimental data capable of quantifying the evaporation of UWS droplet over a wide range of temperatures for moving droplets. As typically appeared in the evaporation process of multicomponent fuel droplet or emulsified mixture droplet, the phenomenon of violent droplet fragmentation which is often referred to as micro explosion is also observed during the evaporation of UWS droplets at elevated temperatures[23]. More volatile component trapped in droplet interior due to diffusional resistance can be heated beyond local boiling point and hence undergoes internal superheating. This may eventually lead to the onset of homogeneous nucleation, whose extremely rapid rate of gasification causes intense internal pressure build-up and thereby disruptive droplet fragmentation. In this study, it was explored how micro explosion affects both the evaporation of UWS droplet and the formation of urea derived deposits under a variety of ambient temperatures [23]. More volatile component trapped in droplet interior due to diffusional resistance can be heated beyond local boiling point and hence undergoes internal superheating. In this study, it was explored how micro explosion affects both the evaporation of UWS droplet and the formation of urea derived deposits under a variety of ambient temperatures. In this experiment, observations were conducted for the UWS droplets evaporating at stagnant state. However, in actual spray systems, droplet internal circulation is induced by the inertia acquired during its injection. A relative motion between droplet and ambient gas results in shear at interface, which leads the liquid near droplet surface to recirculate internally temperatures. The numerical study of evaporation taken for lower temperatures ranging 373-473K and compared with experimental data of wang et.al[23] done for stationary droplet. However at higher temperatures the microexplosion of droplets may require additional boundary conditions which are not considered here. The droplet evaporation behavior at 373,423 and 473K are shown in Figs. a-c

Figure 6. A sample photograph of evaporating UWS Droplet [23]

 

1.1

 

 

 

Simulated

 

 

 

 

 

 

1

 

 

 

Experiment

 

 

 

 

al

 

 

 

 

 

2(mm2/mm2)

0.9

 

 

 

373K

 

 

 

 

0.8

 

 

 

 

 

 

 

 

 

i

 

 

 

 

 

2/D

0.7

 

 

 

 

o

 

 

 

 

 

D

 

 

 

 

 

 

0.6

a

 

 

 

 

 

 

 

 

 

0.5

 

 

 

 

 

0

20

40

60

80

 

 

 

t/D 2(sec/mm2)

 

 

 

 

 

i

 

 

 

1.2

 

 

 

 

Simulated

 

1

 

 

 

 

 

Experimental

 

 

 

 

 

 

 

 

 

)

0.8

 

 

 

 

 

 

423K

2(mm2/mm2

 

 

 

 

 

 

 

 

0.6

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

i

 

 

 

 

 

 

 

 

 

2/D

0.4

 

 

 

 

 

 

 

 

Do

 

 

 

 

 

 

 

 

 

 

0.2

 

 

 

 

 

 

 

 

 

 

b

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

20

40

60

80

100

 

 

 

 

 

t/D 2(sec/mm2)

 

 

 

 

 

 

 

 

 

o

 

 

 

6

 

1.2

 

 

 

 

 

 

 

 

 

 

 

 

Simulated

 

 

1

 

 

 

 

Experimental

 

 

 

 

 

 

 

0.8

 

 

 

 

473K

 

)

 

 

 

 

 

 

2(mm2/mm2

 

 

 

 

 

 

0.6

 

 

 

 

 

 

0.4

 

 

 

 

 

 

i

 

 

 

 

 

 

2/D

 

 

 

 

 

 

 

 

 

 

 

 

 

o

 

 

 

 

 

 

 

D

 

 

 

 

 

 

 

 

0.2

 

 

 

 

 

 

 

 

c

 

 

 

 

 

 

0

 

 

 

 

 

 

 

0

50

100

150

200

250

300

 

 

 

 

t/D 2(sec/mm2)

 

 

 

 

 

 

 

i

 

 

 

Figure 7(a-c). Comparison of simulated results of droplet evaporation with experimental results at 373K, 423K and 573K [23]

As shown in Fig 7a-b, UWS droplets evaporating at both 373 and 423 K exhibit almost linear history. The numerical results show almost same trend but slight variation in slopes and droplet history is divided into 2 stages. At 373K only water gets evaporated but at 423K the urea melting also may also take place since melting temperature of urea is 406K.Thermolysis reaction can also take place at higher tempearatures the simultaneous evaporation and thermolysis of urea droplet may lead to more uniformity in droplet even after depletion. Experimental observations show that at 373K the even after full lifetime the reduction of droplet size is only nearly 60% This is because liquid component constituting UWS droplets is completely evaporated at these points and hereafter almost no more reduction in droplet overall size is observed which is seen increasing with temperature.

Thermolysis of the droplets: Once urea-water-solution (UWS) spray is injected into hot exhaust gas stream before SCR catalyst, water content first evaporates from UWS. Then, ammonia is generated in-situ through both thermal decomposition of urea The influence of urea concentration on the evaporation of UWS has been studied. The decrease in vapor pressure due to increase in concentration of urea in the droplet results in continuous increase in droplet temperature and slower evaporation as compared to water. Fig.8 shows the variation of mass fraction of NH3 and HCNO with residence time. The concentration of both the species starts increasing up to residence times of 0.4secs at all temperatures 573, 623 and 673K and decreasing thereafter. The NH3 formed till 0.4 sec starts getting consumed during oxidation and HCNO undergoes hydrolysis reaction and gives ammonia. So overall concentration of both NH3 and HCNO starts decreasing after 0.4 sec

 

0.082

 

 

 

 

 

0.081

 

 

 

300DEG573K

 

 

 

 

 

of water

0.08

 

 

 

350DEG623K

 

 

 

 

0.079

 

 

 

400DEG673K

fraction

 

 

 

 

0.078

 

 

 

 

 

 

 

 

 

Mass

0.077

 

 

 

 

 

 

 

 

 

 

0.076

 

 

 

 

 

0.075

 

 

 

 

 

0

0.2

0.4

0.6

0.8

Residence time, s

Figure 4. Variation of mass fraction of water with residence time

 

6.00E-04

 

 

 

 

573K

 

 

 

 

 

 

Concentration

5.00E-04

 

 

 

 

623K

 

 

 

 

 

673K

4.00E-04

 

 

 

 

 

3.00E-04

 

 

 

 

 

2.00E-04

 

 

 

 

 

3

 

 

 

 

 

NH

1.00E-04

 

 

 

 

 

 

 

 

 

 

 

0.00E+00

 

 

 

 

a

 

 

 

 

 

 

0

0.2

0.4

0.6

0.8

1

Residence time, s

 

1.20E-03

 

 

 

 

 

 

573K

 

 

 

 

 

 

 

 

 

 

 

 

 

 

of HCNO

1.00E-03

 

 

 

 

 

 

623 K

 

 

 

 

 

 

 

673K

8.00E-04

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

fraction

6.00E-04

 

 

 

 

 

 

 

 

 

 

4.00E-04

 

 

 

 

 

 

 

 

 

 

Mass

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2.00E-04

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

b

 

 

 

 

 

 

 

 

 

 

 

 

 

0.00E+00

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

0.2

0.4

0.6

0.8

1

Residence time, s

Figure8(a-b).Comparision of simulated results thermolysis a) Variation of NH3 concentration) Variation of HCNO mass fraction with reference to residence time

7

5.2 Spray Wall Interaction

Birkhold et al[8] developed spray wall interaction model for UWS and which Spray impingement on hot surfaces, can lead to a better spatial distribution of the reducing agent due to thermal breakup. This also yields better conversion to ammonia, because small droplets with an enha nced heat and mass transfer are generated. Additionally heat is transferred from the hot surface to the droplets durin g contact and accelerates evaporation. As the film is transport ed downstream in regions with less spray impingement, the evaporation of water from the film leads to higher urea concenntrations. After evaporation of water is completed, thermolysis of urea begins in these areas. Spray impact also results in cooling of surfaces and can lead to wall film formation if the surface temperature decreases below a critical value. The critical transition temperature for UWS is determined within this study. Spray impact also results in cooling of surfaces and can lead to wall film formation if the surface temperature decreases below a critical value. The influence of flow conditions , exhaust tube properties and spray parameters on the film for mation can be evaluated with the developed model.. The spray wall deposition behavior is monitored using 3 major parameters of wall deposition namely wall film deposition, film deposition area and evaporated mass from the wall.

Wall deposition: The simulated results obtaine d for total film mass with respect to residence times are compared at 573K and 623K .The wall film deposition occurs at higher rates at lower temperature, due slow evaporation of UW S droplets and thermal decomposition. However the film for mation at later stages found to proceed at same rates for bot h temperatures 573K and 623K.Film deposited at lower te mperature will undergo further evaporation and thermolysis thereby showing similar trend as that of higher temperature at later stages

6E-10

 

5E-10

 

 

 

 

 

 

mass,kg

4E-10

 

 

 

 

 

 

3E-10

 

 

 

 

 

 

film

573K

 

 

623K

 

 

 

 

 

 

 

 

Total

2E-10

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1E-10

 

 

 

 

 

 

 

0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

0.1

0.2

0.3

0.4

0.5

 

 

 

 

Residence time,s

 

 

 

Figure. 5 Variation of total film mass with residence time

Film deposition area: It is evide nt from the Regime map for spray/wall-interaction according to Kuhnke[10] the spray transforms to splash at higher te mperatures due to the area of encroachment and lower mass o f the droplets which caused due faster evaporation. At higher temperatures the rebound of the droplets will increase spray wall area. At thermal breakup is also more predominant at higher temperature and the deposition area will be more and film thickness reduces.

 

10

 

 

 

 

9

 

 

8

 

area,cm²

7

 

6

 

5

623K

 

Film

4

 

3

 

 

 

 

2

 

 

1

573K

 

0

 

 

 

0

0.1

0.2

0.3

0.4

0.5

 

 

Residence time,s

 

 

Figure 6. Variation of Film area with

residence time

Evaporated mass: The deposited UWS over the surface evaporates; the evaporation rate aries with film thickness and the temperature. The simulated results of evaporated mass are compared at temperatures 573 K and 623K. (Fig.7). More quantity of film mass getting eva porated at lower temperature compared to that at higher te mperatures since more mass accumulates at lower temperatures in the form of film.

 

1.35E-07

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

mass,kg

1.125E-0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

9E-08

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Evaporated

6.75E-08

 

 

 

 

 

 

 

 

623K

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4.5E-08

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

573K

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2.25E-08

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

0.1

0. 2

 

 

 

 

 

0.3

0.4

 

 

0.5

 

 

 

 

 

 

Residen ce time,s

 

 

 

 

 

 

 

 

Figure.7 Variation of evaporated mass with residence time

8

ACKNOWLEDGMENT

Authors would like to thank Department of Mechanical Engineering of Canara Engineering College, Manipal Institute of Technology and NITK Surathkal for the support to write this paper.

NOMENCLATURE

a thermal conductivity

Aarea

,Spalding numbers heat capacity,

Cdrag co-efficient

Ddiameter,

g

gravity

h

heat of reaction

h

specific enthalpy,

 

Lewis number

m

mass,

ṗ

mass flow

pressure

Q

 

˙energy flux

 

Nusselt number

 

Re

Reynolds number

 

qË™

specific energy flux

 

r

radius,

 

Sh

Sherwood number

 

t

time,

 

T

temperature,

 

u

velocity,

)

w

dimentionless radius, (r/

vapour mass fraction

Y

mass fraction

 

Greek symbols

αheat transfer coefficient

heat conductivity,

ρdensity

diffusion coefficient,

Subscipts

∞ ambient

d,g droplet,gas

l,vap

liquid, vapor

ref reference

D drag

[ ⁄ ]

[m 2]

[ ⁄ K]

[m]

[m/s] [J/mole][ ⁄ ]

[k[⁄kg]

[Pa]

[W]

[W/m[ 2] m]

[s] [K] [m/s]

[W/m2/K] [W/m K] [Kg/[ ⁄3]

g gas phase

Superscipts

characteristic

6.CONCLUSION

In the present work, simulation is done for evaporation characteristics in UWS injection and spray wall interaction. The simulated results of single droplet evaporation at various ambient temperatures are compared with the experimental results of Wang et al[23]. Droplet evaporation at various injection velocities are compared and found increased evaporation for lower injection velocities. As microexplosion occurs at higher temperatures the numerical results are able to predict the droplet fate unless some change in boundary conditions or using user defined functions.The three parameters of the film deposition wall film deposition, film deposition area, evaporated mass from the wall are taken to account to study the behavior of wall film deposition. Thus the formation of a wall film can be calculated precisely as well as the secondary breakup of droplets at the wall. The spray wall deposition found to be varying with temperature and residence time, the variation seems to follow the kunkhe spray wall interaction map. The rebound and thermal breakup of the droplets significant factors at elevated temperatures which lead to depletion of spray wall thickness. The spray film formation at later stages found to proceed at same rates for both temperatures.

REFERENCES

[1]M. Koebel, M. Elsener, and T Marti. NOx-reduction in diesel exhaust gas with urea and Selective Catalitic Reduction. Comb. Sci. and Techn., 121:85–102,1996.

[2]F. Birkhold, U. Meingast, P. Wassermann, and O. Deutschmann. Modeling and simulation of the injection of urea-water-solution for automotive SCR DeNOx- systems. Applied Catalysis B:, 2005.

[3]C. Enderle, H. Breitbach, M. Paule, and B.

Keppeler.Selective catalytic reduction with

urea -

26th

International Vienna Motor symposium,

April

28-29,

Vienna, Austria, 2005

 

 

[4]H.L. Fang and H.F.M. DaCosta. Urea thermolysis and NOx reduction with and without SCR catalysts.Applied Catalysis B: Environmental, 46:17–34, 2003) reaction importance to Cyanuric Acid Production” American laboratory (1999).

[5]R. van Helden, R. Verbeek, and F. Willems. Optimization of urea SCR deNOx Systems for HD Diesel Engines. SAE, 2004-01-0154, 2004.

9

[6]J.C. Wurzenberger and R. Wanker. Multi-Scale SCR Modeling, 1D Kinetic Analysis and 3D System Simulation.SAE, 2005-01-0948, 2005.

[7]M. Chen and S. Williams. Modeling and Optimization of SCR-Exhaust Aftertreatment Systems. SAE, 2005-01- 0969, 2005.

[8]FIRE 8.3. AVL LIST GmbH, A-8020 Graz, Austria, www.avl.com, 2004.

[9]J.Y. Kim, S.H. Ryu, and J.S. Ha. Numerical prediction on the characteristics of spray-induced mixing and thermal decomposition of urea solution in SCR system. In Proc. 2004 Fall Technical Conference of the ASME Internal Combustion Engine Division,Long Beach, California USA, 2004.

[10]D. Kuhnke. Spray/Wall-Interaction Modelling by Dimensionless Data Analysis. Shaker Verlag,2004.ISBN 3-8322-3539

[11]N. W. Wruck. Transientes Sieden von Tropfen beim Wandaufprall. PhD thesis, RWTH Aachen, 1998

[12]WA Sirignano, “Fuel Droplet Vapourization and Spray Combustion Theory”, Prog. Energy Combustion System 9(1983) 291-322.

[13]G.M Faeth, “Evaporation and Combustion of Sprays”, Prog Energy Combustion Sci. 9(1983)1-76.

[14]R. Kneer, M Schneider , B.Noll S.Wittig “Diffusion Controlled Evaporation of Multi Component Droplet: Theoritical Studies on Importance of Variable Liquid Properties”, Int. Journal Heat Mass Transfer 36(1993)2403-2415

[15]Felix Birkhold, Ulrich Meingast, Peter Wassermann,Olaf Deutschmann “Modeling and Simulation of the Injection

of Urea-Water Solution for Automotive SCR De-NOx- Systems” Applied Catalysis B: Environmental 70 (2007) 119–127

[16]Buchholz, “Einsatz von festem Harnstoff als Reduktionsmittel für die NOx-Minderung nach dem SCR- Verfahren“, Dissertation, Universität Karlsruhe, 2000.

[17]J. K. Dukowicz. Quasi-Steady Droplet Phase Change in the Presence of Convection. Informal Report LA-7997- MS, Los Alamos Scientific Laboratory, Los Alamos, New Mexico, 1979.

[18]Verein Deutscher Ingenieure, editor. VDI-W¨armeatlas,

Berechnungsblätter für den Wärmeübergang.VDI- Verlag,Dässeldorf, 6th edition, 1991.

[19]Chr. Mundo, M. Sommerfeld, and C. Tropea. Dropletwall collisions: Experimental studies of the deformation and

breakup process. Int. J. of Multiphase Flow,21(2):151– 173, 1995.

[20]C. Bai and A.D. Gosman. Development of methodology for spray impingement simulation. SAE, 950283, 1995.

[21]D. Kuhnke. Spray/Wall-Interaction Modelling by Dimensionless Data Analysis. PhD thesis, Universit ¨ at Darmstadt, 2004

[22]L. Jaeger, J. N´yvlt, S. Hor´aˆcek, and J. Gottfried.Viskosit ¨aten von Harnstoffwasserl ¨osungen. Collection Czech. Chem. Commun., 30:2117–2121, 1965

[23]Tae Joong Wang, Seung Wook Baek, and Seung Yeol Lee, “Experimental Investigation on Evaporation of Urea-Water-Solution Droplet for SCR Applications” AIChE Journal December 2009 Vol. 55, No. 123267

ACKNOWLEDGMENTS

Authors would like to thank Department of Mechanical Engineering of Canara Engineering College, Manipal Institute of Technology and NITK Surathkal for the support to write this paper.

10