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    International Journal of Dynamics of Fluids

    ISSN 0973-1784 Vol.2, No.1 (2006), pp. 55-71

    Research India Publications

    http://www.ripublication.com/ijdf.htm

    Numerical Simulation of Shock-Free Shear Layer

    Interaction in Reacting Flows

    M. Deepu1, S. S. Gokhale

    2and S. Jayaraj

    3

    1Lecturer in Mechanical Engineering

    N S S College of Engineering, Palakkad, Kerala-678 008, India

    Email: [email protected] in Aerospace Engineering

    Indian Institute of Technology Madras-600 036, India

    Email: [email protected] in Mechanical Engineering

    National Institute of Technology Calicut, Kerala-673 601, India

    Email: [email protected]

    Abstract

    The performance of any supersonic combustor system depends on efficient

    injection and complete burning. Computational analysis of the flow field

    associated with supersonic combustors is presented. Results are obtained by

    numerically solving unsteady, compressible, turbulent Navier-Stokes

    equations, using Unstructured Finite Volume Method (UFVM) incorporating

    RNG based - two equation model and time integration using three stageRunge-Kutta method. The developed numerical procedure is based on the

    implicit treatment of chemical source terms by preconditioning. Reaction is

    modeled using an eight step H2-air chemistry. The code is validated against

    standard experimental data. The analysis could demonstrate the effect of

    interaction of oblique shock wave with hydrogen stream in its mixing withcoaxially flowing air and subsequent reaction.

    Key words: SCRAMJET, Supersonic combustor, Reduced chemistry, Point

    implicit method, FVM.

    IntroductionThe recent interest in single stage to orbit trans-atmospheric vehicle has lead to the

    development of a hypersonic flight [1-5], which incorporates a supersonic combustor.

    Supersonic Combustion Ramjet engine (SCRAMJET) benefits from the better

    performance of air breathing propulsion system. Scramjets need a combustor that

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    56 M. Deepu, S.S. Gokhale and S. Jayaraj

    should have efficient mixing and combustion of fuel with air at supersonic speeds

    without much pressure loss [5, 6]. Many experimental and numerical analyses have

    been reported during the last few decades regarding the characteristics of the complex

    flow field resulting due to fuel air mixing and combustion. Many fuel injection and

    flame holding techniques which can have efficient fuel oxidant mixing with accurate

    burning rate have been developed but there is always a trade-off between mixing

    effectiveness and pressure drop occurring inside the combustor affecting the total

    propulsive thrust available at the nozzle.

    Performance of a supersonic combustor system depends on efficient injection and

    complete burning. Future hypersonic vehicles are expected to require the performance

    and operability benefits from air breathing propulsion systems as it is providing high

    specific impulse [3]. The use of supersonic combustors in such vehicles requires

    efficient supersonic combustion in combustor lengths short enough to be compatiblewith practical engine sizes.

    Combustion modeling simulates the chemical and physical evolution of a complex

    reactive flow system by numerically solving the governing time dependent

    conservation equations of mass, momentum and energy. The reaction zone propagates

    as a consequence of strong coupling between combustion chemistry and the

    appropriate fluid mechanical process. The combustion model should accurately

    predict the strong interaction between the energy released from chemical reactions

    and the dynamics of fluid motion. The chemical reaction leads to the generation of

    gradients in temperature, pressure and density, which in turn influence the transport of

    mass, momentum and energy. Macroscopically these gradients may lead to the

    development of vorticity or affect diffusion of mass and energy. On a microscopicscale, these gradients develop turbulence and thereby influence mixing and flame

    velocities. The interaction of fuel with oxidizer is very difficult to predict in such

    systems due to high non linear dependence of various flow parameters and chemical

    source terms. Therefore predicting strong interplay between chemistry and fluid

    dynamics in reacting flows is the real challenge in combustion modeling. The use of

    supersonic combustors in atmospheric vehicles requires efficient combustion within

    combustor lengths which is short enough for practical engine sizes. Supersonic

    reacting flow field can be simulated by adding finite rate chemistry to standard

    compressible Navier Stokes equations. Both turbulence and chemical kinetics are

    important, since the residence time inside the combustor is much smaller. Explicit

    treatment of all conservation terms with reaction chemistry results in stiff equationsand it will degrade the performance of numerical method as flow field and chemical

    kinetics with differing time scales need to be solved simultaneously. Existence of

    several non-equilibrium states creates challenges in solution procedure. Governing

    equations of turbulent shear layer flows involving finite rate chemistry are often

    difficult to solve due to stiffness (Ratio of largest time scale to smallest). Stiffness

    will degrade the performance of numerical methods. While handling two differing

    time scales, both can be advanced equally in pseudo time. In other words, it can be

    treated as way of rescaling the equation in time such that both phenomena evolve at

    comparable pseudo time scales resulting in higher time steps for integration.

    Comparison of the numerical result has been done with the standard axisymmetric

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    Numerical Simulation of Shock-Free Shear Layer 57

    reacting free shear flow experimental measurements of Cheng et al. [7] The predicted

    heat release and species production rates are found to have reasonable agreement with

    the experimental results. The analysis has been extended to study the mixing and

    reacting behavior of hydrogen injection issuing into hot supersonic air stream with

    angled inlet.

    Governing Equations of Compressible Reacting FlowsThe conservation form of equations, which govern a two dimensional turbulent

    compressible flow can be expressed in a generic form for axisymmetric flows as

    1 ( )U F rGS

    t x r r

    + + =

    (1)

    Where

    ( )

    2

    , F=

    xx

    r xr

    rxx xr r eff

    i

    i

    u

    u

    uvu

    Tv E u v K xU E

    ux

    uYx

    uY

    +

    + + + =

    ( ) ( )

    2

    0

    0

    1

    G= and0

    r

    r rx

    r xr

    rr r rx eff

    r

    r

    i

    r i

    v

    uv

    v

    T PE v u K rr S

    v Hr

    Hv

    r

    v Y

    +

    + + + +

    =

    Here

    2 2 12

    3 3

    rxx eff

    u u rvP

    x x r r

    = + +

    rxr rx eff

    u v

    r x

    = = +

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    58 M. Deepu, S.S. Gokhale and S. Jayaraj

    2 2 12

    3 3

    r rrr eff

    v u rvP

    r x r r

    = + +

    2 12

    3

    r reff

    v u rv

    r x r r

    = +

    In the above versions of formulations, the effective viscosity of the flow field

    tleff += (2)

    From Sutherlands law laminar viscosity (Ns/m2) for different temperature (K) is

    found as

    l

    =1.458

    10-6

    4.110

    5.1

    +TT

    andt

    is found from turbulence model

    The effective thermal conductivity

    eff l t K K K= + (3)In which

    Pr

    Cpk ll

    =

    t

    t

    t

    Cpk

    Pr

    =

    For the present analysis modified - model called Renormalisation Group (RNG)model was used. Yakhot et al. [8] proposed this model, which systematically removes

    all the small scales of turbulence motion from the governing equation by expressing

    their effects in terms of large scales and a modified viscosity. Following are the RNG

    - model.

    ( ).( ) [ ]

    k eff u div grad H

    t

    + = +

    (4)

    ( )( ) [ ]

    k eff div U div grad H

    t

    + = +

    (5)

    Here the turbulence source terms are defined as2

    *

    1 22 and H 2 .t ij t ij ijH E C E E C

    = =

    Turbulent viscosity is defined as2

    tC

    = (6)

    Closure coefficients are evaluated as

    C=0.0845, ==1.39, C1=1.42 , C2=1.68/

    ij ij(2E .E ) =

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    Numerical Simulation of Shock-Free Shear Layer 59

    and C1*=

    3

    0

    1

    )/1(1

    +

    C

    0=4.377, =0.012Value of constant is adjustable which is found from near wall turbulence data. Allother parameters are explicitly computed as part of RNG process. As noted in the

    source term of the governing equations contains the chemical source terms.

    . Theseterms represent the generation or destruction of species i due to chemical reaction. For

    any chemical reaction with NRnumber of elementary reactions involving NSnumber

    of species

    1 1

    .' ''

    1 1

    ' ''

    ( ) ( '' ' )

    S S

    S S

    ji ji

    j j

    N N

    ji i ji i

    i i

    N N

    i ji ji f bji i

    C C

    C k C k C

    = =

    = =

    =

    (7)

    Where i=1,2,3.NS represents species and j=1,2,3NR represents reactions

    Net change in concentration of any species can be found as. .

    1

    RN

    i i

    j j

    C C=

    =

    (8)

    And the net production of species is given by

    iii WC..

    = (9)

    iW represents the molecular weight of individual chemical species.

    Reaction rates in the above equation can be found from Arrhenius law

    expj

    j

    j

    N jf j j

    f jb

    eq

    Ek A T

    RT

    kk

    k

    =

    =(10)

    Herejeqk is the equilibrium reaction constant.

    An eight-step reaction mechanism proposed by Evans and Schexnayder [9] has been

    used here for modeling hydrogen-air chemistry for which the reaction steps and

    reaction rates are summarised in Table 1.

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    60 M. Deepu, S.S. Gokhale and S. Jayaraj

    Table 1: Reaction steps and reaction rates of eight step mechanism.

    Forward Rates Backward Rates

    Reaction A N E A N E

    H2+M=H+H+M 5.51018 -1.0 51987.0 1.81018 -1.0 0.0O2+M=O+O+M 7.21018 -1.0 59340.0 4.01017 -1.0 0.0H2O+M=H+OH+M 5.21021 -1.5 59386.0 4.41020 -1.5 0.0OH+M=O+H+M 8.51018 -1.0 50830.0 7.11018 -1.0 0.0H2O+O=OH+OH 5.81013 0.0 9059.0 5.31012 0.0 503.0H2O+H=OH+H2 8.41013 0.0 10116.0 2.01013 0.0 2600.0O2+H=OH+O 2.21014 0.0 8455.0 1.51013 0.0 0.0H2+O=OH+H 7.51013 0.0 5586.0 3.01013 0.0 4429.0Here M is a third body. The reaction rates are expressed in Arrhenius law form.

    Units of A are multiple of cm3.

    mole-1.

    s-1.

    E is in Joules

    Evaluation of thermodynamic properties specific heat at constant pressure, enthalpy

    and entropy are respectively found from standard thermodynamic data from Mc Bride

    et al. [10] as.

    2 3 4ip

    i i i i i

    cA B T C T DT E T

    R= + + + + (11)

    2 3 4

    2 3 4 5i i i i i i

    iH B C D E F A T T T T RT T

    = + + + + + (12)

    2 3 4ln

    2 3 4

    i i i ii i i

    S C D E A T BT T T T G

    R= + + + + + (13)

    For each species two sets of coefficients (Ai-Gi) are used for the temperature intervals,

    one applicable from 300K up to 1000K and the other applicable from1000K up to

    3000K. Total energy of flow field is given by

    ( )221

    2

    1vu

    phYE

    iN

    i

    ii ++= =

    (14)

    Temperature is worked out from the above equation using the Newton-Raphsonmethod from the modified energy equation. The pressure is calculated from the

    resulting temperature as follows

    1

    iN

    i

    i i

    Yp R T

    W

    =

    = (15)

    Numerical ModelingTo analyse the SCRAMJET combustor flow field unstructured finite volume method

    is employed here. The unstructured grid techniques are able to idealise and analyse

    geometrically complex problems with more ease compared to conventional structured

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    Numerical Simulation of Shock-Free Shear Layer 61

    grid method. The finite volume Runge-Kutta scheme suggested by Jameson et al. [11]

    is used in the present analysis.

    The conservation equation for a control volume can be written as

    .. 0i

    i i i

    faces

    dUV F ds S V

    dt+ = (16)

    Vi is the cell volume and ds is the area of elemental sides. This is a system of ordinary

    differential equation. For obtaining the solution this has to be integrated with respect

    to time.

    For a control cell ABCD as shown in Fig. 4.1 above FVM formulation can be writtenas

    ( )ij ABCD ij

    Ud fdy gdx Sd t

    + =

    (17)

    Here f and g are the Cartesian components of flux vector. The area is evaluated from

    the vector product of the diagonals as

    1

    2ABCD AC BD

    x x = (18)

    [ ]1

    ( )( ) ( )( )

    2

    ABCD C A D B D B C Ax x y y x x y y = (19)

    The cell averages of the derivative of different flow variables for the surface

    described by a quadrilateral ABCD can be evaluated as follows.

    ( )( ) ( )( )1

    2A C B D B D A C

    ABCDUdy U U y y U U y y= (20)

    Relation for the x derivative is

    1U Ud

    x x

    =

    (21)

    ( )( ) ( )( )

    ( )( ) ( )( )A C B D B D A C

    A C B D B D A C ABCD

    U U U y y U U y y

    x x x y y x x y y

    =

    (22)

    A

    B

    C

    D

    Fig. 4.1. An arbitrary shaped plane quadrilateral

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    62 M. Deepu, S.S. Gokhale and S. Jayaraj

    Relation for the r derivative is

    1U Udr r

    = (23)

    ( )( ) ( )( )

    ( )( ) ( )( )A C B D B D A C

    A C B D B D A C ABCD

    U x x U U x x U U

    r x x y y x x y y

    =

    (24)

    In order to avoid oscillations near shocks and other uncoupled errors, the numerical

    solution procedure needs to incorporate artificial dissipation. For the present analysis

    the numerical dissipation terms are formed with Laplacian and Biharmonic operators.

    The dissipation operation can be written as

    (2) (4)

    ( ) ( ) ( )i i iD U d U d U = + (25)where d

    (2)(Ui) represents the contribution of the undivided Laplacian operator and

    d(4)

    (Ui) is the contribution of the Biharmonic operator. Biharmonic operator does the

    background dissipation to damp high frequency uncoupled error modes.

    Laplacian operator for the ith

    cell is given by

    =

    =6

    1

    2)(

    k

    iki UUU (26)

    where, the summation in k is taken over all control volumes which have the common

    interface with ith

    cell.

    The Biharmonic operator is given by

    )(2

    1)( 22

    6

    1

    )4()4(

    ik

    k k

    k

    i

    i

    iki UUt

    V

    t

    VUd

    +

    = =

    (27)

    The Biharmonic operator is not operated near shock as this will produce pre and

    post shock oscillations.

    Here

    )(,0max)2()4()4(

    ikik k =

    With the addition of artificial dissipation terms the space discretisation will become

    ( )( ) ( ) 0i

    i i i

    d UV R U D U

    dt

    + = (28)

    Explicit treatment of all conservation terms with reaction chemistry results in stiff

    equations and it will degrade the performance of numerical method. Flow field and

    chemical kinetics with differing time scales need to be solved simultaneously.

    Bussing and Murman [12] introduced the method of preconditioning the

    conservation equations in conjunction with chemical source terms alone being treated

    implicitly. Such a method has the advantage of both explicit and implicit methods.

    While handling two differing time scales, both can be advanced equally in pseudo

    time. In other words, it can be treated as way of rescaling the equation in time such

    that both phenomena evolve at comparable pseudo time scales resulting in higher time

    steps for integration.

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    Numerical Simulation of Shock-Free Shear Layer 63

    Thus the modified equation is

    1 ( )

    U F rGSJ St x r r

    + + = (29)

    The point implicit formulation of the time stepping can be written as

    ( )

    ( )

    ( )

    (0) ( )

    0 (1) (0) (0) (0)

    1

    1 (2) (0) (1) (0)

    2

    3 (3) (0) (3) (0)

    3

    1 (3)

    ( )

    ( )

    ( )

    n

    i i

    i i i i

    i i i i

    i i i i

    n n

    i i i

    U U

    tiSJ U U R D

    Vi

    tiSJ U U R D

    Vi

    ti

    SJ U U R DVi

    U U U

    +

    =

    =

    =

    =

    = +

    (30)

    The point implicit scheme in which all the six chemical species terms are treated

    implicitly and all other terms explicitly. The preconditioning Matrix SJ used for this

    purpose is given by

    =

    2 2 2

    2 2 2

    2 2 2

    2 2 2

    2 2 2

    2 2 2

    . . .

    . . .

    . . .

    1 . . . . .

    1 . . . . .

    . . . . . . . . . . . . . . . . . . . .

    . . . . . 1

    H H H

    i i i

    H O H O

    O O O

    i i i

    H O H O

    H O H O H O

    i i i

    H O H

    t t tY Y Y

    t t tSJ

    Y Y Y

    t t tY Y Y

    O

    (31)

    Code Validation and Test ResultsCoaxial Hydrogen-Air Supersonic Mixing and Reaction

    Numerical simulations were performed on an axisymmetric configuration consisting

    of coaxial jets (Hydrogen and vitiated air) exiting at supersonic velocities intoambient air. Cheng et al. [7] made measurements of temperature and composition

    using ultra-violet Raman scattering and laser induced predissociative fluorescence

    techniques. These experimental results are used for validation. The flow conditions

    have been summarized in Table 2.

    The Details of computational domain is given in Fig. 1. The left face of the

    computational domain is given with a supersonic inflow condition in the region of

    supersonic air stream. The burner lips were assumed to non-catalytic. Hydrogen jet

    with above mentioned conditions is introduced at respective position as separate

    boundary condition. Remaining portion in left face is considered as no slip wall. Top

    face is a no slip wall. A supersonic out flow condition is maintained at outlet. Since

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    64 M. Deepu, S.S. Gokhale and S. Jayaraj

    the entire flow field is symmetrical about axis of the coaxial jet, a symmetry condition

    is utilized there. The computational domain was discretised in to 104000 control

    volumes. The initial condition for entire domain is ambient conditions. The grid was

    added to allow specification of the entrainment boundary condition well upstream of

    the coaxial jets. The grid is clustered to give more resolution near the lip regions in

    the transverse direction, and near the burner exit in the streamwise direction. The

    criteria chosen for steady state solution is the density error in successive steps falls

    less than 1.010-06.

    Table 2: Coaxial burner exit conditions.

    H2 Jet Air Jet Ambient

    Air

    Diameter(mm)

    2.36 17.8 120

    Mach

    Number

    1.0 2.0 0

    Pressure

    (MPa)

    0.112 0.107 0.101

    Temperat

    ure (K)

    545 1250 300

    Mass

    Fraction

    21.0

    HY =

    2

    2

    0.245,

    0.175

    O

    H O

    Y

    Y

    =

    =

    2

    2

    0.233,

    0.01

    O

    H O

    Y

    Y

    =

    =

    Figure 2 shows comparison between experimental data and numerical prediction of

    temperature at exit which is 101 mm from exit. Reasonable match has been obtained

    inside core region of the jet and a slight difference in the mixing layers.

    Figure 1: Details of computational domain

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    Numerical Simulation of Shock-Free Shear Layer 65

    The difference is due to initial turbulence level specification given at the inlet and also

    due to the numerical dissipation effects. Figure 3 gives the quantitative comparison of

    water mole fraction profiles at exit. The non-interference of fuel core to the outer

    region can be noticed. The species production rates in the mixing region agree well

    with experimental values. The water mole fraction values at core region could not

    established. The prediction capability of the code developed is proven by this

    comparison.

    Figure 2: Temperature profile at exit.

    Figure 3: Water mole fraction profile at exit.

    The temperature plot for the entire domain is given in Fig. 4. The mixing becomes

    more predominant in the region far from the jet outlets and heat release gradually

    increases in the mixing layer between hydrogen and air. Non reacting fuel core

    extends up to 30 mm from jet outlets. Outer layer between supersonic air stream and

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    66 M. Deepu, S.S. Gokhale and S. Jayaraj

    ambient has almost same temperature through out the length, beyond this non reacting

    core region.

    Figure 4: Field view of temperature for the entire domain.

    Though the typical eight step chemical kinetic mechanism employed here is

    performing well when coupled with two dimensional turbulent compressible Navier-

    Stokes equation; the introduction of seven species has created difficulty in

    computational procedure. The total number of conservation variables increased to

    thirteen including the chemical species which will pose a real challenge while

    applying for scramjet combustors of larger geometry.

    Shock-free shear layer interaction in reacting flows

    In real scramjet combustor, the flow filed is extremely complex. Various mixing

    enhancement studies [13,14] used strut based fuel injectors. The parallel injection hasthe advantage of momentum addition with minimum pressure loss. But the mixing

    efficiency of such a system is very low when compared to transverse injections. The

    mixing behavior of the planar jet can be enhanced by the artificial production of

    recirculation zone due to shock wave-expansion wave pattern. For the present analysis

    an oblique shock is created in the inlet of the flow passage by a 80

    wedge. The oblique

    shock emanated from the inlet will interact with coaxially injected hydrogen stream.

    The interaction of shock with shear layer formed between air and hydrogen streams

    will enhance mixing and thereby promotes reaction. The details of computational

    domain are given in Fig. 5. The flow conditions have been summarized in Table 3.

    Figure 5: Details of computational domain.

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    Numerical Simulation of Shock-Free Shear Layer 67

    Table 3: Flow conditions.

    H2 Jet Air Jet

    Diameter (mm) 6 60

    Mach Number 1.0 3.0

    Pressure (MPa) 0.2 0.107

    Temperature (K) 400 900

    Mass Fraction 21.0HY = 2 0.245OY =

    The computational domain is discretised in to 54,000 control volumes. Figure 6

    gives Mach contour plot. Low velocity regions can be identified along the path of

    progress of the hydrogen jet. Alternate compression and expansion takes place for the

    jet and is not enough to perturb the flow field much in the region near to the jet

    outlets. But the shock wave reflections interfere with the upcoming jet and localised

    low velocity regions are produced. Though these regions are responsible for pressure

    loss of the jet, certainly enhances mixing and reaction. The computed pressure

    contours are depicted in Fig. 7. Due to chemical reaction enormous amount of heat is

    released at mixing layer. The variation of temperature along radial direction for

    various axial locations is plotted in Fig. 8. The mixing and reaction is promoted by

    shock induced vortex generation. Figure 9 shows the variation of the water massfraction along radial direction for various axial locations.

    Figure 6: Mach contour plot.

    Figure 7: Computed pressure contours.

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    68 M. Deepu, S.S. Gokhale and S. Jayaraj

    Figure 8: Variation of temperature for various axial locations.

    Figure 9:Variation of water mass fraction for various axial locations.

    The effect of mixing and subsequent reaction is more down stream areas. The shock

    reflections are responsible for blocking the development of jets in the shear layer and

    thereby creating the low velocity regions. Since the residence time associated with the

    flow field in such areas is more, the reaction is more predominant in such areas

    leading to enhanced heat release. The numerical scheme could demonstrate such a

    complex flow field.

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    Numerical Simulation of Shock-Free Shear Layer 69

    ConclusionA point implicit finite volume solver has been developed and tested for the supersonic

    mixing and reaction of parallel streams of hydrogen and air. The implicit treatment of

    the chemical species in the source term reduces the stiffness considerably and higher

    CFL almost equal to that of non-reacting situations has been obtained. Reasonable

    match has been obtained with experiment for the temperature inside core region of the

    jet and a slight difference in the mixing layers. Though the species production in the

    core of the jet is not predicted well, but it could almost accurately follow that in the

    mixing layer.

    Future works aims at the influence of initial level of turbulence in controlling

    reaction. Though the typical eight step chemical kinetic mechanism employed here is

    performing well when used along with two dimensional turbulent compressible

    Navier-Stokes equation; the introduction of seven species has created difficulty incomputational procedure. The total number of conservation variables has risen to

    thirteen including the chemical species. It will set a real challenge while applying for

    scramjet combustors of larger three dimensional geometry with more number of grids.

    AcknowledgmentsThe first author thanks T. Jayachandran, Head of Fluid Mechanics and Thermal

    Analysis Division of Propulsion group, Vikram Sarabhai Space Centre,

    Thiruvananthapuram, India, and his team members for their valuable suggestions for

    this work.

    Nomenclature

    c Sound velocity

    Ci Concentration of a species

    Cp Specific heat at constant pressure

    Cv Specific heat at constant volume

    D Artificial dissipation

    E Total energy

    Eij Deformation tensor

    Ej Threshold energy of a reaction F&G Flux vectors

    G Gibbs free energy (kJ/kg)

    H Enthalpy

    K Thermal conductivity

    k Reaction rate

    p Pressure

    Q Heat flux

    r Radial direction

    S Source term

    Si Entropy

    t Time

    T Temperature

    U Vector of conservative variables

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    70 M. Deepu, S.S. Gokhale and S. Jayaraj

    V Volume of the control volume

    u Favre average velocity in axial direction

    vr Favre average velocity in radial direction

    Yi Mass fraction of species

    Adjustable closure coefficient

    Kinetic energy dissipation rate

    Molecularity of a reaction

    Kinetic energy of turbulence

    Viscosity

    Kinematic viscosity

    Density

    Shear stress Surface area

    Cell volume

    Rate of production of species

    Subscripts

    f Forward reaction

    b Backward reaction

    i Chemical species

    j Reaction step

    lLaminar

    r Radial direction

    t Turbulent

    Superscripts

    Product

    Reactant

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