Numerical Study of Aerodynamic Interaction of Compressor and Combustor Flows

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    2002 37th lntersociety Energy Conversion Engineering Conference (IECEC)

    IECEC 2002 Paper No.

    20176

    NUMERICAL STUDY OF AERODYNAMIC INTERACTION

    OF

    COMPRESSOR AND COMBUSTOR FLOWS

    K. Su

    and C.

    Q. Zhou

    Purdue University Calumet

    2200 169th Street

    Hammond. Indiana46323

    [email protected]

    ABSTRACT

    Numerical study of the aerodynamic interaction of

    transient compressor and combustor flows has been

    conducted using the KIVA-3V code. The simulation

    was based

    on

    the solution of Navier-Stokesequations

    with models of turbulence, sprays and chemical

    reactions. A typical annular combustor, including the

    diffuser, the secondary channels and the liner, was

    modeled. The transient inflow was assumed by

    specifying the pressure oscillation in a

    form

    of

    sinusoidal function. Aeroacoustic characteristics of

    the diffuser-liner flow were revealed. Three flow

    patterns, namely the quasi-steady, transition and

    steady patterns corresponding to the frequency

    ranges of n

    L

    EO.

    EO 5

    n

    5

    320 and

    n

    2 320 Hz, were

    classified. it is found that for the quasi-steady pattern,

    combustion flow is in quasi-steady state with flow

    properties dependent of time; for the steady pattern,

    the combustion flow can be treated as

    if

    in steady

    state, in which the influence of oscillation can be

    ignored; and the flow in the transition pattern behaves

    in-between. They significantly influence the gas

    turbine combustion.

    NOMENCLATURE

    ap

    amplitude factor

    AM reference area

    CO discharge coefficient

    E

    activation energy

    H combustion heat

    k turbulent kinetic energy; chemical reaction

    constant

    L length

    M

    Mach number

    ma gas mass flow rate

    m,

    fuel mass flow rate

    n oscillation requency

    P pressure

    P3 combustor inlet pressure

    R

    universal gas constant

    S source term

    st

    Strouhal number

    t

    time

    temperature

    U , U velocity

    ud droplet velocity

    Z coordinate

    P pressure difference

    ,$ variable

    P density

    r effective diffusivity

    r droplet relaxation ime

    INTRODUCTION

    Offdesign conditions of compressor lead to

    distortions, and moreover, reversing of gas flow at the

    interface

    of

    compressor and combustor. During these

    procedures, physical processes in individual

    components of the gas turbine engine are strongly

    coupled. Influences between connected components

    such as compressor and combustor are important to

    the engine performance. CFD simulation for gas

    turbine combustor, from the compressor exit to the

    combustor exit, is needed for investigation of the

    interaction between the compressor and combustion

    flows.

    For a long time, diffuser and liner are simulated

    separately due to the limitation of the capacity of

    computers (Correa and Shyy, 1987; Tolpadi. 1995).

    As the accuracy of physical models and the capacity

    of computers increase, efforts

    on

    numerical simulation

    of the whole combustor, including the diffuser,

    secondaly flow channels and the liner. have been

    made. Crocker et al. (1998) conducted the numerical

    simulation of combustor flows from the exit of

    compressor to the turbine inlet. Su and Zhou

    (2000)

    studied the effect of non-uniform nflow of the diffuser

    on

    combustion using the KiVA-3V code. Now it is

    possible to numerically investigate the aerodynamic

    interaction between compressor and combustor.

    Aerodynamic interaction of transient compressor

    and combustor flows is one of the most complicated

    problems in gas turbine combustion. Most offdesign

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    0-7803-7296-4/02/$20.00 02002

    IEEE 43

    1

    mailto:[email protected]:[email protected]
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    conditions are induced by unexpected compressor

    behaviors. Among them, surge and stall are the most

    common cases that would influence the durable

    combustion performance and result in heavy

    damages to the engine. Surge is a large-sale

    reversed flow through the entire engine, including

    compressor and combustor. Rotating stall is milder

    situation that may occur just before the surge. In

    rotating stall, pockets of stagnant air rotate around the

    blade rows, causing local regions of flow blockage

    and distortion. Because CFD codes at this stage

    cannot deal with reversed flows at boundaries very

    well, the effort in this study mainly focused

    on

    the

    general and mild case, that is, the combustion with

    transient distorted inflows from the compressor.

    In this study, the aerodynamic mechanism of

    interactions of transient compressor and combustor

    flows was investigated through the numerical

    simulation. Effects of the inflow oscillation

    on

    combustion were analyzed. These results will be

    helpful in the development of the gas turbine

    combustor.

    THEORETICAL APPROACH

    Compressible, threedimensional, transient

    reacting flows in gas turbine combustor were solved

    using a time-marching method with models of

    turbulence, sprays and chemical reactions. Details of

    the theory can be found in references (Amsden et al.,

    1989; Amsden, 1993.1997).

    Gas Phase Eauations

    The governing equations are time-averaged,

    threedimensional, transient Navier-Stokes equations.

    Turbulence

    is

    modeled using the standard

    k-E

    model

    along with the wall function treatment for near-wall

    regions. The transient form of the three-dimensional

    conservation equations may be written in general for

    a conserved variable @as

    where p is the fluid density, r the effective diffusion

    coefficient, U the fluid velocity, and S he source term

    which depends on the equation being considered.

    Continuity, momentum, energy, turbulence and

    species equations were solved, with the dependent

    variable representing 1, velocity, internal energy,

    turbulent kinetic energy and dissipation, and species,

    respectively. The temperature field is obtained from

    the thermochemical look-up table by linear

    interpolation based on the calculated values of the

    mixture fraction and its variance.

    Chemical Reactions

    For combustion, chemical reactions proceed

    kinetically n the KIVA-3V code. The fuel was Jet-A in

    a chemical formula of CIZH~~. A simplified kinetic

    mechanism with 17-species and 23-step was

    employed (Kundu et al., 1999). Chemical rate

    expressions are evaluated by a partially implicit

    procedure. The reaction rate constants are in the

    form of Arrhenius

    where

    E

    is the activation energy, and A is the

    constant. With the reactions rates determined for the

    mechanism, the chemical source terms in the species

    equations were obtained. The mixing-controlled

    turbulent combustion model based on the eddy-

    dissipation was used for the turbulence combustion.

    Liquid Phase Esuations

    The Lagrangian method was used in the liquid

    phase modeling. The fuel was assumed

    to

    inject into

    the combustor as a fully atomized spray which

    consists of spherical droplets. Liquid sprays are

    represented by a discrete-particle technique, in which

    each computational particle represents a number of

    droplets of identical size, velocity, and temperature.

    Droplet properties are determined by using the Monte

    Carlo sampling method. The log-normal rule was

    accepted to describe the droplet size distribution at

    injection. The equation of motion of a spherical

    droplet is given by the following equation

    (3)

    where ud is the droplet velocity and t he relaxation

    time of the droplet. Physical properties of the gas

    phase are calculated on the temperature averaged

    between droplets and the surrounding gas.

    The changes of mass, momentum and energy of

    droplets from conservation equations are added into

    the source terms of the governing equations. The

    momentum exchange is treated by implicit coupling

    procedures to avoid the prohibitively small time steps

    that would otherwise be necessary. The accurate

    calculation of mass and energy exchange is ensured

    by automatic reductions in the time step when the

    exchange rates become arge.

    Turbulence effects on the droplet motion are

    accounted for in the following way. When the time

    step exceeds the turbulence correlation time,

    turbulent changes in droplet position and velocity are

    chosen randomly from probability distributions for

    these changes. The interaction time between the

    droplet and the eddy is taken as the minimum

    of

    the

    eddy lifetime and the transit time required for the

    droplet to cross the eddy.

    NUMERICAL SCHEME

    The gas phase solution procedure is based

    on

    a

    finite volume method called the ALE (arbitrary

    Lagarangian-Eulerian) method. The equations are

    differenced in integral form with the volume of a

    typical cell used as the control volume and with

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    divergence terms transformed to surface integrals.

    Spatial difference is formed on a finitedifference

    mesh that subdivides the computational region into a

    number of small cells that are hexahedrons. The

    nodes may be arbitrarily specified functions of time,

    thereby allowing a Lagrangian, Eulerian or mixed

    description. Transient solution is marched out in a

    sequence of time steps. The block-structured BFC

    mesh is employed for the complex geometry of gas

    turbine combustor. The discretization and the

    numerical algorithm were described in references

    (Amsden et al., 1989; Amsden, 1993,1997).

    A typical gas turbine combustor was modeld in

    this study. The combustor is annular with 12 domes

    equally spaced along the circumferentialdirection. A

    single dome sector of 30

    span of the combustor,

    which includes a swirler and a fuel nozzle and a set of

    primary and secondary holes, was simulated with

    periodic boundaly conditions on two sides. A grid of

    80,000 cells for the entire combustor was used.

    The computations were performed at the medium

    power condition of the combustor with pressure

    of

    8.4

    MPa and temperature of 600 K. The inflow was

    simply assumed a sinusoidal function

    e = [I + psin(&t)]

    4

    where ap is the amplitude factor of the oscillation and

    ap = 0.01, n the oscillation frequency. Flow

    oscillations with different frequencies in the combustor

    are illustrated n Fig.1. Flows at the exhaust nozzle of

    gas turbine engine usually are chocked during actual

    operation. Therefore, pressure at the outlet of

    combustor can be considered constant when inflow

    distortion appears, which simplifies the simulation.

    PrrannrnJaur

    illha

    -gl+mCtsrtB

    Fig.1 Flow oscillations in the liner.

    RESULTS

    AND

    DISCUSSIONS

    Flow characteristics of combustor with diffuser

    and the secondary flow channels were analyzed.

    Time histories of pressure, mass flow rate, and heat

    release rate were discussed.

    Combustor

    Flow

    AnalvSis

    Fig2 plots velocity distributions of combustor flow

    at the longitudinal section. Airflow from the

    compressor first enters the prediffuser where airflow

    velocity reduces and the static pressure rises. It is

    then split into three branches: the central stream that

    supplies air to the combustor dome, and two streams

    that feed the outer and inner annuli through the

    diffuser dumps where flow velocity is further reduced

    and static pressure rises. All the three flows then go

    into the liner through penetration holes.

    h,

    . . .

    Fig2 Typical combustor velocity field

    Gas pressure becomes smaller when flows enter

    the liner due to pressure loss through the

    penetrations.

    It

    is more uniform than in diffuser

    because of more straight and open flow channel and

    the subsequent slowing down gas flow. Thereby,

    physical and chemical processes in the combustion

    flow can be considered to proceed under constant

    pressure. There is a recirculation downstream of the

    swirler where all the critical processes, such as fuel

    injection, spray evaporation, turbulent mixing and

    chemical reactions occur. For this reason pressure

    and mass flow rate in combustion zone are typically

    used in the analysis. Following is the dilution zone

    where the fresh air enters the liner through

    penetration holes, and mixes with the hot gas to

    generate uniform exit temperature distributions. Mass

    flow rate and pressure distributions are shown in

    Figs.3 and 4. It is seen that the mass flow rate

    increases along the axial direction due to the gas flow

    accumulation from the swirler and penetration holes.

    On the other hand, pressure drop along the axis is

    small because of the low gas velocity in combustion

    and dilution zones. and it becomes

    a

    little larger only

    in the converging section near the exit where gas

    velocity increases.

    Previous researches only focused on either the

    diffuser or the liner flow, but the influence of

    compressor flow on combustion cannot be found this

    way. Our effort is on the simulation of combustor flow

    from the diffuser inlet to the exit of the combustor to

    reveal the coupling of diffuser and combustor flows

    under the inflow oscillation.

    Aerodvnamic Characteristics

    Figs.3 and 4 display the histories of pressures

    and mass flow rates -at the diffuser inlet and In the

    combustion zone of the liner with inflow oscillations of

    n

    = 80, 240 and

    320

    Hz. They clearly illustrate

    pressure and mass flow rate oscillations under inflow

    pressure oscillations. It is seen that amplitudes of

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    pressure oscillations in the liner become smaller than

    at the inlet for oscillations of

    n

    = 80 and 320 Hz. it

    means that the pressure oscillation is dampened in

    the flow and through the penetration holes. However,

    for the inflow oscillation of n

    =

    240 Hz, the oscillation

    is amplified. It indicates that some kind of resonance

    happens

    to

    the combustion flow, which makes the

    pressure oscillation stronger.

    :~~

    :

    .2

    1

    *

    -inlet

    --cormustion

    zone

    7.8

    I

    0.000

    O.OM 0.008 0.012 0.016

    Tima

    IS)

    a) = 80 Hz.

    8.4

    e

    6 0

    0 000 04 0 008 0

    012

    r im 1

    (b) = 240 Hz.

    0

    O W 4 0 8 0.012

    rima

    IS)

    (c) = 320 Hz.

    Fig.3 Pressure histones

    I

    I

    -

    P

    .4

    2

    .

    .2

    O O W 0004 OW8 0012 0016

    Time

    I*

    (a) n = 80 Hz.

    0.000 0.004 0 008

    0.012

    rims 5 )

    (b) = 240 Hz.

    0 61

    I

    z

    -inlet

    --cormustion

    zone

    0

    0.OW 0.003 0.006

    O.OD9 0.012

    r i m e IS

    (c) = 320 Hz.

    Fig.4 Mass flow rate histories.

    Meanwhile, amplitude of mass flow rate

    oscillation decreases as the frequency increases, as

    indicated in Flg.4. It can be explained that the mass

    flow rate oscillat ion is weakened by the forced mixing

    resulted f he oscillation itself, and this effect is

    intensified as the frequency increases. It is clear that

    the mass flow rate oscillation in combustion zone is

    much milder than at the liner exit. Usually the mass

    flow rate

    in

    combustion zone is about

    30

    -

    40

    % of

    the total mass flow rate. As mass flows accumulate in

    the liner, fluctuations of mass flow rates gradually

    become augmented. Please note that combustion

    performance mainly dependson flow properties in the

    combustion zone. Therefore, it mitigates the variation

    of combustion performance. At the frequency of

    n

    =

    240 Hz, pulsation n mass flow rate is stronger than at

    the frequency of

    n

    = 80 Hz due to the resonance of

    the flow.

    When the oscillation frequency is lower than =

    80 Hz, combustion flows can be considered in quasi-

    steady state. Flow properties change through

    convection relatively fast compared

    to

    the oscillation

    itself, and at any instant the combustion can be

    described as if it were in steady state. Due to

    insulation of l iner walls and outflow boundary

    condition, the combustion flow can only experience

    less than 1/4 of the oscillation cycle while going

    through the whole liner. Therefore, when the inflow

    oscillation frequency 5 80 Hz, the combustion flow

    can be dealt with using theoretical methods for the

    steady combustion under the operating condition

    changing with time.

    On the other hand, when the oscillation

    frequency is higher than n = 320 Hz, the combustion

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    flow is like in steady state. From Fig.1, it is found that

    the gas flow experiences more than one oscillation

    cycle during residing in liner. Effective oscillation

    along the axial direction is well developed. Because

    the oscillation propagates at sonic speed in the gas

    flow, flow properties can respond to the oscillation

    quickly and be mixed up well in the liner. Therefore,

    pulsations with flow properties could be moderated

    comparatively for oscillation flows with frequencies of

    2 320.

    It

    is seen that amplitude of mass flow rate

    oscillation is about 5 % of time averaged value at

    inflow oscillation of n = 320 Hz. Therefore, it is

    reasonable to consider that the combustion flows with

    inflow oscillation of

    Z

    320 Hz is in steady flow

    pattern. Consequently,

    all

    the theory and analyses for

    steady combustion can be applied. Unlike quasi-

    steady combustion flow with

    5 80

    Hz, operating

    conditions for this case can be considered constant.

    Combustion flow with the inflow oscillation of 80 s

    B

    320 Hz can be classified as in transition flow

    pattern. Combustion with the transition flow pattern

    behaves in between that for quasi-steady and steady

    flow patterns. In this frequency spectrum, effective

    oscillation along the axial direction is gradually

    established.

    As

    mentioned before, oscillations of flow

    properties are gradually mitigated except at the

    frequency around

    n =

    240 Hz when resonance

    happens

    to

    the flow. Thus, the variation of

    combustion flow in transition pattern is simply the

    combination of the two aerodynamic processes, and

    would be more complicated.

    Forced combustion oscillation flows in the three

    flow patterns, i.e., quasi-steady, transition and steady

    flow patterns, have different effects on aerodynamic

    characteristics and combustion performance.

    Corresponding oscillation frequency, = 80 and 320

    Hz, which are naturally called the quasi-steady

    frequency and the steady frequency, are critical

    to

    the

    combustion behaviors.

    Fig.5 shows the ratio of amplitudes of pressure

    oscillations at the inlet and in the liner. it is seen that

    the pressure oscillation is dampened in transportation

    in gas flow because of the viscosity of the flow and

    the resistance of the flow channels between cold and

    combustion regions. However, for the case with

    n

    =

    240 Hz the amplitude of the pressure oscillation

    becomes larger than at the diffuser inlet. Apparently,

    aeroacoustic resonance occurs at the frequency of

    240 Hz for the diffuser-liner system. Generally, the

    damp of flow oscillation can protect combustor from

    any undesirable impact of off-design conditions from

    the compressor and make combustion more stable,

    but it would also limit acceleration capability of the

    engine and increase pressure

    loss

    in combustor.

    However, aeroacoustic resonance intensities the flow

    oscillation, and might result in deterioration of

    combustion performance and lead to damage

    to

    combustor structure.

    Fig.6 plots the amplitude of mass flow rate

    oscillation in combustion zone. It is found that for the

    frequency range of

    5 80

    Hz (quasi-steady flow

    pattern), the amplitude of mass flow rate oscillation

    remains about unchanged, but for 80 i i

    320

    Hz

    (transition flow pattern) the amplitude decreases as

    oscillation frequency n increases, and finally for 5

    320

    Hz

    (steady flow pattern) the oscillation amplitude

    becomes so weak that it can be ignored. Also, it is

    obvious that the mass flow rate oscillation become

    stronger when the resonance happens at the

    frequency = 240 Hz. It will change the flow

    properties in the combustion zone, and subsequently

    affect the combustion performance.

    Quasi-Steadv Flow Pattern n 5 80

    As

    discussed above, theoretical and empirical

    analyses for steady combustion can be applied to

    combustion flow with inflow oscillation in quasi-steady

    flow pattern. Please note that flow properties oscillate

    with time. Correspondingly, combustion performance

    and aerodynamic characteristics can be achieved

    through theoretical and empirical methods for the

    steady combustion. Considering the combustor as a

    simple flow channel, the mass flow rate of the

    combustor can be obtained by

    m,

    =

    C A , G

    5)

    where CO s the discharge coefkient, A,r the

    reference cross section area,

    p

    the gas density, and

    LIP the pressure difference throughout the flow

    channel. In the simulation, temperature of the

    transient inflow was simply assumed constant. Thus,

    gas temperature mainly depends on the mass flow

    rate in the combustor. Because the temperature rise

    due to the combustion is more than 2000

    K,

    much

    larger than the inlet temperature of the diffuser, then

    the combustion temperature is obtained as

    T

    H,ml/m.,

    where I is the fuel combustion heat.

    Substituting it into Eq.(5) with the gas state equation,

    then

    Because amplitudes of pressure oscillations are

    only

    2 %

    of the time-averaged or mean pressures,

    gas flow throughout combustor can be considered

    approximately at constant pressure. It is found that

    the mass flow rate varies approximately with time in

    an oscillatory trend of a sinusoidal function. With

    Eq.(6). mass flow rate at any instant in an oscillation

    cycle can be estimated. Please note that this

    conclusion

    is

    only suitable for the inflow with the

    oscillation frequency

    S

    80

    Hz. For a constant

    pressure flow process in gas turbine combustor, any

    change in mass flow rate greatly influences the

    combustion performance. Therefore combustion

    performance will exhibit apparent oscillation trends in

    case of oscillation frequency 5

    80

    Hz. Mass flow

    rate oscillations are then approximated by

    interpolating between that for quasi-steady and

    steady patterns.

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    iw *w YYI

    w

    5m

    ~ ~ i ~ t i m h e q u n c y n

    mi

    Fig.5 Ratios of oscillation amplitudes of

    pressures in the liner and at the inlet.

    Transition

    Flow

    Pattern (80 5 n 5 3201

    Oscillation flows in transition flow pattem

    (corresponding

    to

    inflow oscillation frequencies of 80

    s i 320 Hz) exhibit two aerodynamic features: mass

    flow rate oscillation dampens as inflow oscillation

    frequency increases, while the pressure oscillation

    keeps about the same: and oscillation resonance

    happens to diffuser flows at the frequency around =

    240

    Hz. They have a combined influence on

    combustion.

    Effect of Pressure Oscillationon

    Mass Flow Rate

    It is interesting to notice one important feature of

    inflow oscillation in Fig.6, i.e.. the amplitude of mass

    flow rate oscillation decreases as the oscillation

    frequency increases. One exception

    is

    at frequency

    = 240 Hz where resonance occurs. Consider

    simplified one-dimensional flow in a pipe of uniform

    cross section with the oscillation inflow pressure.

    Neglecting viscosity, mass diffusion and convection

    terms, the onedimensional momentum equation

    is

    given as

    7)

    For one-dimensional flow with uniform cross section,

    it can be found after integration,

    where At is the amplitude of the mass flow rate

    oscillation. As the frequency of inflow oscillation

    increases, the amplitude of mass flow rate oscillation

    decreases. It can be extended to explain combustor

    flow oscillation in Fig.6. The amplitude of mass flow

    rate oscillation in transition flow pattem can be

    determined by interpolating between that for quasi-

    steady and steady patterns corresponding to the

    frequencies of = 80 and 320 Hz. Please note that,

    at the frequency of

    =

    EO Hz the amplitude of mass

    flow rate oscillation is obtained by Eq. 6); and when

    = 320 Hz. the mass flow rate oscillation can be

    ignored. It is seen that the abnormality happens for

    inflow oscillations at the frequency of

    =

    240 Hz

    because of the resonance. Since the oscillation

    augment resulted by resonance is hard to predict at

    this stage, the effect of resonance on mass flow rate

    oscillation

    cannot

    be obtained quantitatively.

    Fig.6 Oscillation amplitudes of mass flow

    rates in combustion zone.

    0

    Resonance of Diffuser Flows

    There are some kinds of self-sustained

    oscillations with diffuser flows. Kwong and Dowling

    (1994). and van Lier et al. (2001) studied flow

    oscillations and resonance of the diffuser with gradual

    divergence. According to aeroacoustics, the

    instability of flow is incited when pasting a cavity such

    as diffuser dumps shown in Fig.1 (Rossiter, 1964).

    As it is forming, flow is directed into the cavity. Vortex

    is shed from the leading edge of diffuser and ejected

    in recirculations. The vortex shedding causes

    boundaty layer to periodically separate mainstream.

    When vortex shedding is strong enough. the feedback

    will be able to influence mainstream, and the

    mainstream will undergo pulsation, i.e., the self-

    sustained acoustic oscillation. Variations of the

    recirculation in the diffuser dumps at different times at

    the resonance are shown in Fig.7.

    a) Phase

    0

    b)

    Phase

    90'

    (c) Phase

    180

    Fig 7

    Instantaneous

    recirculations

    at different phases

    in an oscillation cycle.

    The first description of this feedback process was

    credited to Rossiter (1964), who gave a semi-

    empirical formula for the frequencies of oscillation

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    where St = nUu s the Strouhal number corresponding

    to frequency n, the cavity length, U and

    M

    the

    mainstream velocity and Mach number, and K and

    y

    empirical constants with

    =

    0.25 and I/K

    =

    1.75.

    Eq.(9) was originally developed for flows over a

    rectangular cavity. It was applied to the diffuser flow.

    Tab.1 lists oscillation the frequencies obtained from

    Eq.(9) fo rm = 1 and 2, and the numerical simulation.

    It

    is

    seen that there are many possible frequencies

    corresponding to each m from Eq.(9). Block (1976)

    indicated that the cavity would oscillate with a

    characteristic frequency corresponding to

    a

    Strouhal

    number

    on

    the order of St

    < 1.

    Oscillations or

    St

    z 1

    can be ignored due to low power at high frequencies

    in spectra. In this study, only the oscillation frequency

    for m = 1 is considered effective. It is found the

    frequencies from empirical formula for m

    =

    1 and the

    simulation agree

    to

    each other reasonably.

    Steady Flow Pattem

    n

    2

    320

    As discussed before, when the oscillation

    frequency reaches about 320 Hz combustion flow will

    be able to experience one complete oscillation cycle.

    When the oscillation frequency is n 2 320 Hz, fast

    oscillation makes flow properties well averaged, and

    the mass flow rate oscillation well mitigated. It is

    found that pulsation in mass flow rate is about 5 % of

    the averaged value. Thereby, the influence of inflow

    oscillation at this frequency can be ignored, and the

    combustion flow treated as in steady state although it

    is not real steady. Therefore, flow properties and the

    combustion performance can be obtained based on

    the constant operating condition using theoretical and

    empirical approaches for steady combustion. It

    makes the problem much simplified.

    C NCLUSIONS

    Aerodynamic interactions of transient compressor

    and combustor flows were investigated through

    numerical simulations. Following conclusions were

    achieved:

    (1) Three flow patterns, namely quasi-steady,

    transition and steady patterns corresponding to

    the critical frequency arranges of S 80, 80 2 n

    2

    320, and n 2 320 Hz, for gas turbine combustion

    with inflow oscillations were summarized. The

    three flow patterns have different effects on

    combustion.

    (2) Amplitude of mass flow rate oscillation decreases

    as the inflow oscillation frequency increases due

    to the mixing resulted from the flow oscillation.

    Therefore, pulsations in flow properties are greatly

    mitigated as the oscillation frequency increases.

    (3) Resonance originating from the diffuser dumps

    happens to combustion flows when the forced

    inflow oscillation of n

    =

    240 Hz

    is

    applied to

    combustor flows. Resonance leads to more

    fluctuations in flow properties and combustion

    oerformance.

    ACKNOWLEDGEMENTS

    The authors wish to thank the Department of

    Energy for the support of this work under contract

    #540-6288-1121 and A.A. Amsden of the Los Alamos

    National Laboratory for all the help with the KIVA3V

    code.

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