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    DOI 10.1007/s12591-015-0268-4

    O R I G I NA L R E S E A R C H

    Numerical Study on Binary Nanofluid Convection

    in a Rotating Porous Layer

    Jyoti Sharma1 Urvashi Gupta2 R. K. Wanchoo2

    Foundation for Scientific Research and Technological Innovation 2016

    Abstract The present paper investigates the convection in a binary nanofluid layer in porous

    medium under the influence of rotation using DarcyBrinkman model. A set of partial differ-

    ential equations based on conservation laws for binary nanofluid convection are solved using

    Normal mode technique and one term weighted residual method. The problem is analyzed

    for both stationary as well as oscillatory convection for free-free boundaries of the layer.

    The oscillatory motions come into existence for bottom heavy configuration of nanoparticles

    in the fluid layer. As far as thermal Rayleigh number is concerned, it does not show muchvariation with respect to different nanoparticles (alumina, copper, titanium oxide, silver) for

    bottom heavy configuration. Rotation parameter is found to stabilize the system significantly.

    Keywords Binary convectionNanofluidDarcyBrinkman modelRotationBrownianmotion Thermophoresis

    Introduction

    Nanofluid is a highly influential term which is being discussed within the heat transfer

    community over a wide spectrum. Nanofluid represents the suspension of nanometer-sized

    particles (oxides, nitrides, ceramics, metals and semiconductors) in base fluids (water, ethyl-

    ene glycol, oil). The idea of introducing nanofluids first came into the mind of Choi [1] who

    claimed that the heat transfer can be enhanced with the addition of nanoparticles in the fluid.

    Buongiorno [2] developed a system of conservation equations for nanofluids incorporating

    the effect of Brownian motion and thermophoresis. Using this model, Tzou [3] studied the

    thermal convection in nanofluids analytically using eigenfunction expansion and found that

    B Urvashi Gupta

    [email protected]; [email protected]

    1 Energy Research Centre, Panjab University, Chandigarh 160014, India

    2 Dr. S.S. Bhatnagar University Institute of Chemical Engineering and Technology, Panjab University,

    Chandigarh 160014, India

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    critical Rayleigh number is reduced with the addition of nanoparticles. By using single term

    Galerkin approximation Kuznetsov and Nield [4] found the expression for thermal Rayleigh

    number and the condition of overstability in porous medium using DarcyBrinkman model.

    Further, Bhadauria et al.[5] and Yadav et al. [6] made an extension of the thermal instability

    problem by introducing Coriolis force term due to rotation in momentum equation for porousand non-porous medium respectively and concluded that rotation increases the stability of

    the system. Gupta et al. [7] studied the nanofluid convection under vertical magnetic field

    and found that stability rises with rise in magnetic field parameter.

    When the nanoparticles are added in a binary fluid such as salty water, it is known as a

    binary nanofluid. Double diffusive convection in nanofluid/convection in binary nanofluid is

    like a triple diffusion process in which variations are caused by three different components

    heat, nanoparticles and solute which have different rates of diffusion. Very few investigations

    have been done until now on the convection in binary nanofluids. Double diffusive convection

    in a nanofluid layer for flow in a porous medium was first studied by Kuznetsov and Nield

    [8]. The complex expressions for Rayleigh number have been approximated to get simplified

    expressions. Further, Yadav et al. [9] and Gupta et al. [10] investigated the convection in

    a binary nanofluid layer independent of the restrictions on parameters. The influence of

    magnetic field on binary nanofluid convection was considered by Gupta et al. [ 11]. They

    have made the valid approximations in the complex expressions for analytical study and

    alumina-water nanofluid is used for numerical investigation of the problem.

    The influence of rotation on binary nanofluid convection in porous medium using Darcy

    Brinkman model for free-free boundaries is investigated in this work. Due to the presence of

    rotation, Coriolis force term is added in the momentum equation and DarcyBrinkman model

    is used to write conservation equations for the system as given by Kuznetsov and Nield [8].These equations are made non-dimensional by making the variables dimensionless and small

    perturbations are imposed on initial solution to get perturbed equations. Further normal mode

    technique and one term weighted residual methods are used to find the solution of perturbed

    equations. We have three diffusing components; heat, solute and nanoparticles and hence

    the problem becomes much more complex for oscillatory convection and cannot be studied

    analytically. Thus to study the problem we take numerical values of various parameters

    involved for alumina, copper, titanium oxide and silver in water based nanofluids and use

    the software Mathematica to solve equations for thermal Rayleigh number for each value

    of wave number for the two types of convection . To plot the stability curves for oscillatory

    convection, interpolation is used. Tabulated values are also provided wherever needed toanalyze the problem completely.

    Formulation of Problem and Relevant Partial Differential Equations

    A rotating binary nanofluid layer with angular velocity is heated and soluted from below

    as shown in Fig.1.The temperatures T1 and T0, the volume fractions of nanoparticles 1and0 and the solute concentrationsC1and C0 are taken at the bottom and top of the layer,

    respectively. The basic equations which express the convection in a rotating binary nanofluidlayer in the light of DarcyBrinkman model (Buongiorno[2], Yadav et al. [6] and Kuznetsov

    and Nield[4,8]) are

    uD=0, (1)f

    uD

    t= p+ 2uD

    uD

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    Fig. 1 Sktech of the physical

    system

    + p+(1) {(1T(TT0)C(CC0))}g+ 2

    (uD), (2)

    (c)m T

    t+(c)f uD T= km2T+ (c)p

    DB T+ DT T T

    T0

    +c DT C2C, (3)

    t

    + uD

    =

    DB+DT TT0

    , (4)

    C

    t+ uD

    C= DS2C+DC T2 T. (5)

    Here, Eqs. (1)-(5) are the conservation equations for mass, momentum, thermal energy,

    nanoparticles and solute, respectively.The physical parameters are: fluid velocity uD =(u,v,w)(m/s), volume fraction of nanoparticles , timet(s), coefficient of diffusion due to

    Brownian motionDB (m2/s), coefficient of diffusion due to thermophoresisDT(m

    2/s), fluid

    temperatureT(K), density of the fluid at upper boundary 0

    (kg/m3), fluid pressure p(Pa),

    fluid viscosity(N s/m2), thermal volumetric coefficientT(K1), solutal volumetric coef-

    ficientC, acceleration due to gravity g(m/s2), medium effective viscosity(N s/m2), fluid

    density (kg/m3), fluid specific heat c(J/kg K), medium conductivitykm (W/m K), medium

    porosity, medium permeability (m2), nanoparticles densityp (kg/m3), nanoparticle spe-

    cific heatcp(J/kg K), solute concentrationC, diffusivity of solute DS, diffusivity of Dufour

    typeDT Cand diffusivity of SorettypeDC T. We introducedimensionless variables as follows:

    (x,y,z)= (x, y, z)/d, t= tm /d2, u= uDd/m ,p= p/m ,

    =

    1

    10 , C= C

    C0

    C1C0 , T= T

    T0

    T1T0 ,with m= km

    (c)f, = (c)m

    (c)f, (6)

    where the superscript denotes the dimensionless variable. Thus using Eqs. (6),Eqs.(1)(5)(after removing the superscript ) are

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    u=0, (7)Dn

    PR

    u

    t= p+Dn2uuRm k Rnk+ RD Tk+ Rs

    L sCk+ T a(uk),

    (8)

    T

    t+u T= 2T+ NB

    LN T+ NDNB

    LNT T+ST C2C, (9)

    1

    t+ 1

    u= 1

    LN2+ ND

    LN2T, (10)

    1

    C

    t+ 1

    u C= 1

    Ls2C+SC T2 T, (11)

    where the non-dimensional parameters are: Prandtl number PR = fm ; Darcy num-ber Dn

    =

    d

    2 ; Nanofluid Lewis number LN

    = mDB

    ; Solute Lewis number L S

    = m

    DS;

    Thermal Darcy-Rayleigh number RD = gTd(T1T0)m ; Solute Rayleigh number Rs =gCd(C1C0)

    Ds; Nanoparticle Rayleigh number Rn = (p)(10)gd

    m; Basic-density

    Rayleigh number Rm = [p 0+(10)]gdm

    ; Taylor number T a=

    2d2

    2; Diffusivity

    ratioND= DT(T1T0)DB T0(10) ; Particle density increment NB= (c)P

    (c) f(10);

    Dufour parameter ST C= DT C(C1C0)m (T1T0) ;

    Soret parameter SC T= DC T(T1T0)m (C1C0) . (12)

    Initial Flow and Disturbance Equations

    Initially, the fluid layer is assumed to be in a state of rest, so the physical quantities: tempera-

    ture, concentration of solute and nanoparticles volume fraction vary in the vertical direction

    only i.e.

    u=0, T= TI(z) , =I(z) , C=CI(z) , p= pI(z) , (13)where the subscript I denotes the initial flow. Let us apply Eqs. (13) to Eqs.(7)(11) and

    use the fact that for most nanofluids Lewis number is large and diffusivity ratio is small

    (Buongiorno[2]), we get

    TI= I= CI= 1z . (14)Let us apply small perturbations on the initial flow and write

    u

    =0

    +u, p

    = pI

    +p, T

    =TI

    +T, C

    =CI

    +C,

    =I

    +, (15)

    where the superscript - denotes the perturbed variable. Using Eqs. (15) on the set of Eqs.

    (7)(11), linearizing and using Eqs. (14) (after dropping the superscript -), we get

    u=0, (16)Dn

    PR

    u

    t= p+ Dn2uu Rnk+ RD Tk+ Rs

    L SCk+

    T a(uk), (17)

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    T

    tw= 2T ND

    LN

    T

    z+

    z

    2NDNB

    LN

    T

    z+ST C2C, (18)

    1

    t 1

    w= 1

    LN2+ ND

    LN2 T, (19)

    1

    C

    t 1

    w= 1

    L S2C+SC T2T. (20)

    Using the identity F

    = ( F) 2Ffor any vector fieldF, on Eq.(17) and

    using Eq. (16), we getDn

    PR

    tDn2 +1

    Dn

    PR

    t2 Dn4 + 2

    w+ Rn2H RD2HT

    Rs

    Ls 2

    HC+T a

    2w

    z2

    =0, (21)

    where2 = 2x2

    + 2y2

    + 2z2

    and2H= 2

    x2+ 2

    y2.

    Results and Discussions

    The differential equations (21) and (18)(20) form an eigenvalue problem which will be

    solved using the normal mode technique. Let

    (w, T, C, )=(W(z), T(z),(z),(z)) exp(i kxx+i kyy+st). (22)Using (22), in the above mentioned equations, we get

    s Dn

    PR Dn D2 a2+11+ s Dn

    PR

    D2 a2 D n D2 a22+T a D2W

    s Dn

    PR Dn D2 a2+1RDa2T Rs

    L Sa2Rna2

    =0, (23)

    1

    W+

    1

    L S D2 a2

    s

    +SC T

    D2 a2

    T=0, (24)

    WsT+ D2 a2T+ NBLN

    (DT D )2NDNBLN

    DT+ST CD2 a2=0, (25)

    1

    W

    1

    LN

    D2 a2 s

    ND

    LN

    D2 a2T=0, (26)

    with D ddz

    ,a=(k2x+k2y )1/2.We write s= i , where is real and is a dimensionless frequency. The conditions for

    free- free boundaries are

    W= D2W= T== =0 at z= 0 and z=1. (27)Let the trial functions satisfying the conditions (27) as

    (W, T, , )=(A,B, C,D) sin z. (28)By making use of orthogonality of the trial functions we obtain four equations in four

    unknowns A, B, C and D. Elimination of these unknowns from the obtained set of equa-

    tions produces the eigenvalue equation as

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    Rn

    1+ D n J+ Dns

    PR

    J ND

    JLs

    J ST C

    + s

    LN

    +J2 SC TST C+

    (J+s)

    J+s Ls

    Ls

    a2

    J

    LN+ s

    . J SC TRs 1+ D n J+

    Dns

    PR a2

    Ls

    RD J+ s Ls

    1+ D n J+ Dns

    PR a2

    L s

    + J ST C JLN + s

    J SC T

    J

    1+ D n J+ Dns

    PR

    2+ 2 T a

    RD

    1+ D n J+ Dns

    PR

    a2

    (J+s) J

    LN+ s

    J+ s Ls

    J

    1+ D n J+ DnsPR

    2 + 2 T aL s

    Rs

    1+ D n J+ Dns

    PR

    a2

    L s

    =0, (29)

    where J= 2

    +a2

    .

    Stationary Convection

    At the state of marginal stability, when the amplitudes of small disturbances grow or damped

    aperiodically then the transition from stability to instability takes place via a stationary pattern

    of motions which is described bys=i =0. Then the eigenvalue equation(29)reduces to

    RD=

    11 Ls ST C

    Dn J3

    a2 + J2

    a2+ J 2 T a

    a2(1+ Dn J) + LN

    Rn (1SC TST CLs )

    Rs

    1

    SC T

    Rn ND.

    (30)

    For nanofluid convection without rotation, Eq.(30)reduces to

    RD= Dn J3

    a2 + J

    2

    a2+ LN

    Rn Rn ND. (31)

    which is in confirmation with the results of Brinkman model of Kuznetsov and Nield[4].Note that the expression(30)for thermal Rayleigh number is independent of Prandtl number

    and heat capacity ratio.

    Oscillatory Convection

    For oscillatory convections= i = 0,we separate real and imaginary parts of eigenvalueEq.(29)by puttings=i .

    J4

    L sLN+ 2Dn J5

    LsLN+ Dn2J6

    L sLN J4 SC TST C

    LN 2Dn J5 SC TST C

    LN Dn2J6 SC TST C

    LN

    +J3 2 T a

    LsLN J

    3 SC TST C2T a

    LN J

    22

    2

    J22

    L s 2Dn J

    32

    2 2Dn J

    32

    L s Dn

    2J42

    2 Dn

    2J42

    L s J

    22

    LN

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    2Dn J32

    LN Dn

    2J42

    LN Dn

    2J42

    LsLNP2R

    +Dn2J4 SC TST C

    2

    LN

    P2

    R

    2Dn J32

    Ls PR 2Dn

    2J42

    Ls PR 2Dn J

    32

    L sLNPR

    2Dn J32

    LNPR 2Dn

    2J42

    LsLNPR 2Dn

    2J42

    LNPR

    +2Dn J3 SC TST C

    2

    PR+ 2Dn

    2J4 SC TST C2

    PR J

    2T a2

    2

    J2 T a2

    L s J

    2T a2

    LN+ Dn

    2J24

    2P2R+ Dn

    2J24

    Ls P2R

    +

    Dn 2J24

    LNP2

    R +

    2Dn J4

    2

    PR +

    2Dn2J24

    2

    PR

    J2RDa2

    LsLN

    Dn J3RDa2

    L sLN

    +J2 ST CRDa

    2

    LN+ Dn J

    3 ST CRDa2

    LN J

    2NDRna2

    LsLN

    Dn J3NDRna

    2

    L sLN+ J

    2Rna2

    L s + Dn J

    3Rna2

    L s + J

    2ND ST CRna2

    LN

    +Dn J3ND ST CRna

    2

    LN J

    2 SC TST CRna2

    Dn J3 SC TST CRna

    2

    + J2 SC TRsa

    2

    LsLN + Dn J3 SC TRsa

    2

    L sLN

    J2Rs a2

    LsLN Dn J

    3Rs a2

    LsLN+ RD

    2a2

    2 + DnJ RD

    2a2

    2

    +DnJ RD 2a2

    L s PR+ DnJ RD

    2a2

    PRLN Dn J S T CRD

    2a2

    PR

    Rn22

    DnJ Rn

    2a2

    + Dn J N DRn

    2a2

    LNPR

    Dn J Rn2a2

    L s PR

    DnJ Rn2a2

    PR +

    Rs 2a2

    Ls +

    DnJ Rs2a2

    L s

    Dn J S C TRs2a2

    L s PR+ DnJ Rs

    2a2

    Ls PR+ DnJ Rs

    2a2

    L sLN=0, (32)

    a2J Rn

    L S+ a

    2Dn J2Rn

    L S J

    3

    2 2Dn J

    23

    2 Dn

    2J33

    2

    2 T a3

    2 + J

    3

    L S+ 2Dn J

    4

    L S+ Dn

    2J5

    L S+ J

    2 2 T a

    L S

    +a2J Rn

    + a2Dn J2Rn

    a2J RsL S

    a2Dn J2Rs L S

    + J3LNL S

    +2Dn J4

    LNL S+ Dn

    2J5

    LNL S+ J

    2 2T a

    LNL S a

    2J Rs

    LNL S

    a2Dn J2Rs

    LNL S+ J

    3

    LN+ 2Dn J

    4

    LN+ Dn

    2J5

    LN+ J

    2 2 T a

    LN

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    a2J RnND

    LN a

    2Dn J2RnND

    LN+ Dn

    2J5

    2 P2R Dn

    2J33

    L SP2

    R

    Dn2J33

    LN

    LS

    P 2

    R

    Dn2J33

    LN

    P 2

    R

    + a2Dn J2Rn

    L SPR 2Dn J

    23

    2 PR

    2Dn2J33

    2 PR 2Dn J

    23

    L SPR 2Dn

    2J33

    L SPR a

    2Dn Rn3

    PR

    +a2Dn Rs 3

    L SPR+ 2Dn J

    4

    LNL SPR+ 2Dn

    2J5

    LNL SPR a

    2Dn J2Rs

    LNL SPR

    2Dn J23

    LNPR 2Dn

    2J33

    LNPR a

    2Dn J2RnND

    LNL SPR a

    2JRD

    L S

    a2Dn J2RD

    LS

    a2JRD

    LN

    a2Dn J2RD

    LN

    + a2Dn3RD

    2 PR

    a2Dn J2RD

    LNL SPR+ a

    2J RsSC T

    L S+ a

    2Dn J2Rs SC T

    L S

    +a2Dn J2Rs SC T

    LNL SPR+ a

    2Dn J2RnND ST C

    LNPR+ a

    2JRD ST C

    +a2Dn J2RD ST C

    + a

    2Dn J2RD ST C

    LNPR J

    3SC TST C

    2Dn J4SC TST C

    Dn

    2J5SC TST C

    J

    2 2 T aSC TST C

    +Dn2J33 SC TST C

    P 2R a

    2Dn J2Rn SC TST C

    PR

    2Dn J4SC TST C

    LNPR 2Dn

    2J5SC TST C

    LNPR=0. (33)

    For convection through oscillations, we solve Eqs. (32) and (33) to determine critical

    Rayleigh number for which is real. Thermal Darcy Rayleigh number given by Eq. (30)

    contains four parameters depending on nanofluid properties namely nanofluid Lewis num-

    ber, diffusivity ratio, concentration Rayleigh number and Prandtl number which strongly

    influences the stability of the system. For most of the nanofluids, Lewis number is largeand diffusivity ratio is small and hence coefficient of Rn is large and positive in Eq. (30).

    Thus bottom heavy distribution of nanoparticles must stabilize the system appreciably. Also

    coefficient ofT a is positive in Eq. (30) meaning thereby that rotation parameter has a sta-

    bilizing effect for stationary convection. It is necessary to note that Eqs. ( 32) and(33) for

    oscillatory motions are complex and to find results, some approximations may be made such

    as Lewis number and Prandtl number approach to infinity as used by Kuznetsov and Nield

    [8]. In order to study the present problem completely without using any approximation on

    the variables, let us consider the same numerically for alumina, titanium oxide, copper and

    silver nanoparticles in water based nanofluid.

    Numerical Results and Discussion

    Equation(30) for stationary convection and Eqs. (32) and(33)for oscillatory convection

    are analyzed numerically. Figures25show the stability curves for bottom heavy binary

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    Table 1 Physical properties of water and nanoparticles under consideration

    Physical properties Water Alumina Copper Silver Titanium oxide

    (kg/m3) 997.1 3970 8933 10500 4250

    k(W/m K) 0.613 40 401 429 8.9

    Fig. 2 Effect of different

    nanoparticles on Thermal

    DarcyRayleigh number

    Fig. 3 Effect of porosity on

    Thermal DarcyRayleigh number

    nanofluid convection. The values of various parameters for water based binary nanofluids

    (using results of Buongiorno [2] and Table1in Eqs.(12)) are:For alumina nanoparticles LN= 5000,ND=5,Rn=0.1,PR=5;For titanium oxide nanoparticles LN= 5000,ND=19,Rn=0.107, PR=5;For copper nanoparticles LN= 5000,ND=0.5,Rn=0.26, PR=5;For silver nanoparticles:LN= 5000,ND= 0.5,Rn= 0.31, PR= 5. We fix the other

    parameters as: Ls= 2; T a= 200;Rs= 200; ST C= 0.001;SC T= 1;= 2.5; =0.4;Dn= 0.5. The parameter values for alumina-water nanofluid are used to study theeffects of porosity, Taylor number and solute Rayleigh number on the system.

    Here it is worthwhile to mention that to draw stability curves for oscillatory convection,

    various parameter values in Eqs. (32) and(33) for different wave numbers are used to get acubic equation in2 using the software Mathematica. Thereafter the positive value of2 is

    used (that makes the system unstable) to find the values RD for different wave numbers. For

    graphical representation, interpolation is used with a polynomial of degree seven.

    Figure 2 illustrates the effect of differentnanoparticles on thestability of waterbased binary

    nanofluids. Silver-water is found to be more stable than copperwater which is more stable

    than titanium oxide-water which in turn is more stable than alumina- water for stationary

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    Table 2 Tabulated values of

    RD (oscillatory) for water based

    binary nanofluid with alumina

    nanoparticles for different values

    of=0.4, 0.6, 0.8

    a =0.4 =0.6 =0.8RD (oscillatory)) RD (oscillatory) RD (oscillatory)

    1 3807.737 3968.428 4048.571

    2 947.853 1112.892 1195.439

    3 510.973 677.326 760.266

    4 513.422 679.783 762.727

    5 748.382 914.309 997.049

    6 1208.109 1373.236 1455.584

    7 1937.308 2101.198 2182.932

    Fig. 4 Effect of Taylor number

    on Thermal DarcyRayleighnumber

    Fig. 5 Effect of solute Rayleigh

    number on Thermal

    DarcyRayleigh number

    convection while the effect for oscillatory motions is so small that it is not reflected in graph.

    The destabilizing effect of porosity on the system for stationary mode of convection and

    stabilizing effect for oscillatory motions is shown in Fig. 3. To give the clear picture of

    variation for oscillatory motions, tabulated values of Thermal DarcyRayleigh number for

    different values of porosity are also given in Table 2.The rotation parameter (Taylor number) has a strong stabilizing influence on the layer as

    illustrated in Fig.4while the destabilizing effect of solute Rayleigh number on the system is

    shown in Fig.5for both stationary as well as oscillatory convection. It is noteworthy that the

    mode of convection remains oscillatory as depicted in Figs. 25 and the critical wave number

    rises with the rise in Taylor number [Refer Fig. 4] while it doesnt get much influenced by

    other parameters [Refer Figs.2,3,5].

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    Conclusions

    The binary convection problem is studied numerically to consider the impact of rotation/

    Coriolis force and porosity using DarcyBrinkman model. The relevant partial differential

    equations are solved using the methodologies of superposition of basic possible modes andsingle term Galerkin approximation. The impact of alumina, titanium oxide, silver and copper

    nanoparticles is studied on water based binary nanofluids using the software Mathematica.

    The mode of convection is found to be oscillatory for bottom heavy configuration of nanopar-

    ticles and oscillatory motions are notmuch influenced by nanoparticle properties. Therotation

    parameter is found to stabilize the layer significantly while solute Rayleigh number has a

    destabilizing influence. Porosity destabilizes the system for stationary convection while it has

    a stabilizing impact for oscillatory convection. The critical wave number (where convection

    starts) rises with rise in rotation parameter and remains almost unaffected with the presence

    of nanoparticles, solute and porosity of the system.

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