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  • 8/10/2019 CFD Experimental and numerical investigations of jet active.pdf

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    Experimental and numerical investigations of jet active

    control for combustion applications

    Vincent Faivre

    and Thierry Poinsot

    Institut de Mecanique des Fluides de Toulouse, Allee du Pr. Camille Soula, 31400

    Toulouse, FRANCE

    Abstract. Controlling the mixing of a gas (usually fuel) issuing from a tube into surrounding

    air is a basic problem in multiple combustion systems. The purpose of the present work is

    to develop an actuator device to control the mixing enhancement of an axisymmetric non-

    reactive jet. The actuators consist of four small jets feeding the primary jet flow. These four

    jets are oriented to add an azimutal component to the velocity field. The influence of jets

    deflection and position along the main jet duct is discussed. Schlieren photographs and Planar

    Laser Induced Fluorescence measurements are used to compare the efficiency of the three

    configurations of interest. The effect of control-to-main mass flow rates ratio is quantified

    through hot wire anemometry results. Large Eddy Simulations (LES) of both forced and

    unforced configurations are also performed. The objectives of the numerical part of this work

    are to understand the actuator effect and to validate LES as a tool to study active control.

    Submitted to:Journal of Turbulence

    To whom correspondence should be adressed ([email protected])

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    Active jet control for combustion applications 2

    1. Introduction

    Combustion instabilities may occur in closed combustion chambers, resulting from the

    coupling between acoustics and combustion [1, 2, 3, 4, 5, 6, 7]. They can appear in many

    combustors such as gas turbines or industrial furnaces. Those instabilities are responsible

    for noise, vibrations and sometimes complete device failure [8]. Being able to control such

    phenomena is an important research path in the combustion community. There are two ways

    to control a flow. Passive control consists in modifying the geometry of the burner [9] and/orthe combustion chamber ; on the other hand active control consists in injecting external

    energy through actuators [10, 11]. The quality of the control depends directly on the design

    of the actuators. Some of them are specific to combustion applications but most actuation

    techniques are encountered in both reactive and non reactive applications : loudspeakers

    [10, 12], synthetic jets [13], flaps [14]. The purpose of the present study is to quantify,

    experimentally and numerically, the effects of forcing on the aerodynamic field in a model

    configuration : a non-reactive jet of air.

    The actuators are designed to modify the flow in two ways (figure 1) :

    a radial fluid injection into the main jet enhances its mixing with the ambient air [15, 16], a swirl addition drastically changes the aerodynamic pattern of the flow and can be used

    to stabilize the flame [17].

    To obtain these effects simultaneously the actuators of the present work consist of four

    small jets feeding the primary jet flow. These four jets are oriented to add an azimutal

    component to the velocity field. To visualize the effect of the actuators on the main flow,

    hot-wire anemometry, Schlieren photographs and Planar Laser Induced Fluorescence (PLIF)

    measurements are used.

    Introducing swirl to increase mixing or to stabilize flames is not new [17, 18, 19, 20, 21,

    22, 23]. The effect of swirl jets has been studied in details by many authors and the objectiveof the present work is not to repeat these analysis. Here, the objective is to design a control

    device (the four jets) which can be easily tuned to adjust swirl and turbulence levels without

    any geometry modification. Another objective is to minimize pressure losses which are often

    encountered in swirling devices using vanes for example.

    The interaction of the actuators jets with the main jet can also be simulated using Large

    Eddy Simulations. Such LES provide a detailed insight into the physical mechanisms which

    is difficult to obtain experimentally. At this stage, LES cannot be used for all regimes to

    investigate the most efficient actuation mode : for such a task, experiments are more efficient.

    However LES is more powerful to investigate one regime in details so that a combination

    of experiments and LES is used in this work where the strategy is the following : first,

    experiments are used to establish which actuator configuration is the most efficient in terms of

    mixing enhancement ; LES of this configuration only are then performed to understand how

    the actuator actually affects the main jet.

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    Active jet control for combustion applications 3

    Figure 1. Generic scheme of the SW90, A90 and A45 configurations.

    Nozzle ( ) h (mm)

    A90 90 30

    A45 45 30SW90 90 8

    Table 1. Control parameters for each configuration of actuator.

    2. Experimental facility

    Figure 1 shows a generic scheme of the nozzle equipped with the actuator. The exit diameter

    of the main jet (D) is 10 mm while the exit diameter of each small secondary jet is 2mm.

    Previous investigations have shown that the exit diameter of the small jet (d) is one of the

    main parameters having an influence on the control efficiency [24]. Here, two other parameters

    are tested : the orientation of the four small jets relative to the main one () and the distance

    between the actuator and the main jet exit (h). To evaluate the importance of these parameters,

    three configurations of actuators have been tested (table 1).

    For all configurations, the control jets are tangential to the main one to add an azimutal

    component to the velocity field.

    The nozzle is connected to a cubic box so that the jet flow is confined. The characteristic

    length of the box is 10 diameters of the main jet. The outlet of the box ends directly into

    free atmosphere. The effects of the box size were investigated numerically : even though the

    uncontrolled flow was slightly affected by the box length, the effects of control on this flowwere independent of it.

    The main air flow is delivered by an hot air generator. This device allowsto reachmass

    flow rates up to 30 g/s at 400 C.Temperature regulation is performed using PID regulation.

    The control flow rate is measured by a DANTEC S2140 Mass Flow Transducer which has

    been specially modified for the present work to measure unsteady flow rates up to 1 g/s at a

    frequency up to 500Hz. The ratio between the mass flow rate of the control flow ( mact) and

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    Active jet control for combustion applications 4

    the mass flow rate of the main jet ( mjet) is :

    q

    mact

    mjet(1)

    It is adjusted with a MOOG servovalve which works in both continuous and pulsated regime.

    The jet spreading is characterized by the mean and rms velocity fields measured with a single

    hot-wire probe. The flow is also visualized through Schlieren photographs to measure its

    spreading angle and through PLIF snapshots to characterize the flow structure.

    2.1. Velocity measurements

    Velocity profiles are obtained using a DANTEC Streamline constant temperature anemometer

    coupled to a CHARLYROBOT displacement table which allows micrometric probe shifting.

    The single hot-wire calibration is performed on DANTEC 54H10 calibrator. Each part of

    the anemometry rig is linked to a computer equipped with a KEITHLEY DAS 1800ST/HR

    acquisition card. Velocity signals are recorded at a frequency of 25000 Hz during 1 second.

    2.2. Optical diagnostics

    Two optical diagnostics have been used for the present study : Schlieren technique and Planar

    Laser Induced Fluorescence (PLIF). For the Schlieren visualization the main jet is preheated

    (70 C). Since the present experiments are carried out using non fluorescent gases, PLIF

    measurements require the flow to be seeded with acetone. Acetone fluorescence is induced

    by a laser sheet (Nd YAG l=266nm in UV, I0=40mJ, sheet thickness=500nm). An intensified

    CCD camera sensitive to the acetone fluorescence wave length is used to collect PLIF images.

    3. Numerical setup

    The principle of LES is to resolve the larger scales of turbulence while modelling the smaller

    ones [7]. LES is therefore a good tool to predict the effect of control on the flow because large

    structures are certainly essential in this mechanism.

    LES of the experimental configuration are performed using the parallel CFD code

    AVBP developed at CERFACS in Toulouse (www.cerfacs.fr) and at IFP in Paris, France.

    AVBP solves the full compressible Navier-Stokes equations on 2D or 3D meshes. Meshes

    can be structured, unstructured or hybrid. The numerical approach is based on finite-volume

    schemes using the cell-vertex method. The scheme provides third-order accuracy both in

    space and time. It has been tested on multiple simple (isotropic turbulence, vortices, acoustic

    waves, channel, etc...) and complex cases [25, 26, 27, 28, 29, 30].The boundary conditions treatment is based on the NSCBC approach [31, 32] : acoustic

    waves are identified and treated independently on boundaries to satisfy the boundary

    conditions. At the inlet of the main jet and of the four actuator jets, velocity profiles are

    imposed. At the box outlet, a non reflecting condition is fixed [31, 7]. On all walls, no slip

    conditions are imposed (figure 2).

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    Active jet control for combustion applications 5

    Figure 2. Computational domain. 1 : actuators. 2 : main flow. 3 : jet blowing in the box.

    The subgrid-scale model is the WALE (Wall Adapting Local Eddy viscosity) model

    [25, 33]. This model is based on the square of the velocity gradient tensor and degenerates

    correctly near the walls.

    All simulations are three-dimensional, the mesh is hybrid and contains 1 million cells for

    600.000 points. Figure 2 shows the computational domain. Note that, even for the unforced

    case, the actuators are included in the mesh.

    4. Experimental results : determination of the optimal configuration

    To quantify the actuators effect, a criterion based on the jet spreading angle measured on

    Schlieren views is used (figure 3). The estimated value of the jet spreading angle includesthe large structures of the shear layer. This diagnostic is sufficient to compare solutions and

    identify the most efficient actuators.

    Figure 3. Schlieren photographs of the unforced flow (a) and the forced one (b). Evaluation

    of the jet spreading angle.

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    Active jet control for combustion applications 6

    Different values of control-to-main mass flow rate ratio (q) have been investigated and

    figure 4 shows the jet spreading angle evolution as a function of q. For every actuator the jet

    spreading angle increases with q but not in the same way. Note that the jet angle significantly

    varies with control ranging from 19 (unforced) to 70 (forced cases).

    Figure 4. Jet spreading angle vs. control-to-main mass flow rate ratio (q) for A45, A90 and

    SW90 configurations.

    4.1. Effect of actuators deflection ()

    The efficiencies of A90 (=90) and A45 (=45

    ) actuators are compared to investigate the

    influence of (see figure 1). The A90 configuration is the most efficient : except for very low

    values of q, the jet controlled by the A90 actuator spreads more than the one controlled by

    the A45 actuator at the same control mass flow rate. The orientation of the four small jets hastherefore an effect on the control efficiency.

    One possible explanation is that, for equal control flow rates, the azimutal component

    added to the velocity field is larger for the A90 than for the A45 configuration. The A90

    actuator only adds an azimutal component to the velocity field while the A45 actuator adds

    both azimutal and axial components. Therefore the main flow sweeps the actuator effect along

    the jet axis more easily.

    4.2. Effect of the distance from actuators to main jet exit (h)

    The efficiencies of SW90 (h=8mm) and A90 (h=30mm) actuators are now compared toinvestigate the influence of h (see figure 1) on the jet angle. Figure 4 shows that the distance

    h between the actuator and main jet exit influences the control efficiency. For each value

    of q, the jet spreading is higher when the flow is controlled with the SW90 actuator which

    has the smallest h value. This result shows that the control effect vanishes along the nozzle

    presumably because of wall friction and confinement. As the SW90 configuration has been

    identified as the most efficient it was retained for all further studies.

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    Active jet control for combustion applications 7

    4.3. Effect of control flow rate on the velocity field

    Figure 5 shows the effect of increasing q on the radial profiles of mean axial and rms velocities

    at x/D=6 where D is the main jet exit diameter. Those profiles are obtained by using a single

    hot wire probe. Each profile is normalized by the mean centerline velocity at the jet nozzle

    exit of the free jet configuration (Uc0). Figure 5 shows that higher q values lead to larger

    mixing enhancement : the increase of q leads to a decrease of the mean centerline velocity

    and to an expansion of both mean and rms velocity profiles. Velocity profiles on figure 5 alsoshow that the entrainment of the ambient air is favored when increasing the control-to-main

    mass flow rate ratio.

    Figure 5. Mean radial profiles of axial velocity (up) and rms axial velocity (down) at x/D=5

    for different control flow rates.

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    Active jet control for combustion applications 8

    Figure 6. Mean radial profiles of axial velocity. Comparison between LES and experiments.

    Unforced case (q=0).

    5. Numerical results

    In addition to experimental characterization, the actuators effect was also studied using

    simulations to :

    understand the phenomena induced by control and identify the mechanisms responsiblefor mixing enhancement,

    validate LES as a tool to study active control by comparing LES and experimental results.

    5.1. LES validation for the unforced case

    LES is validated first on the unforced case. Figure 6 shows radial profiles of mean and rms

    axial velocities at different distances from the jet nozzle exit (x/D=3 and 7) for both numerical

    and experimental tests. LES seems to be able to reproduce the experimental results : the

    mean profiles extracted from the simulations are in good agreement with the experimentalones. The amplitude of the velocity fluctuation is slightly overestimated at x/D=3, especially

    in the mixing zone. Nevertheless, figure 7 shows that LES predicts the entrainment rate,

    in agreement with experiments and literature on free round jets [9]. Both experimental and

    numerical streamwise mass fluxes are calculated by integrating the mean velocity profile,

    assuming that the flow is axisymmetric.

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    Active jet control for combustion applications 9

    Figure 7. Normalized streamwise mass flux. Comparison between LES, experiments and

    Gutmark and Grinstein data [9].

    5.2. LES validation for the forced case

    Figure 8 shows radial profiles of mean and rms axial velocities at different distances from thejet nozzle exit (x/D=3 and 7) for both numerical and experimental tests. The configuration

    corresponds to a control mass flow rate of 15% of the main mass flow rate (q=0.15). As for

    the unforced case, figure 8 demonstrates that LES reproduces both mean and rms velocity

    when control is on. The agreement between numerical and experimental results is better for

    the forced than for the unforced case. It is probably due to the fact that the large structures

    involved by the control are correctly captured by LES.

    5.3. Effects of control

    Figure 9 presents the mean axial velocity field in a transverse plane at a distance of 0.5 mmupstream from the nozzle exit. Two-dimensional vectors have been superimposed to the field :

    each of those is the projection of the local three-dimensional velocity vector on the plane. As

    expected, figure 9 shows a global rotation movement of the flow inside the nozzle. LES

    reveals that each of the four actuation jets induces a longitudinal vortex. This mechanism is

    also found for classical jets in cross flow (JICF) where the interaction between jet and cross

    flow leads to a contra-rotating vortex pair (CVP) [34]. In the present case, one vortex only is

    observed because the other one is constrained by the wall and has no place to develop.

    The jet spreading after the jet nozzle exit can also be visualized by computing the velocity

    in a cylindrical coordinate system. Figure 10 shows the orthoradial velocity (u) field at a

    distance of x/D=1 from the jet nozzle exit for both forced and unforced configurations. The

    level of ureached in the forced configuration is very high compared the unforced case (scales

    are different in both cases). It shows that the control device has the expected effect : it adds

    swirl to the flow. This swirl is responsible for the mixing enhancement with the ambient air.

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    Active jet control for combustion applications 10

    Figure 8. Mean radial profiles of axial velocity. Comparison between LES and experiments.

    Forced case (q=0.15).

    Figure 9. Actuator effect on the flow inside the nozzle : mean axial velocity field and 2D

    velocity vectors.

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    Active jet control for combustion applications 11

    Figure 10.Flow field of orthoradial velocity componentat x/D=1 for both forced and unforced

    cases.

    Figure 11. Effect of control on the flow. Vortex detection using Q criterion. Control is

    activated at t=T0. Animation (1.57Mo).

    Figure 11 illustrates the important topological differences between unforced and forced

    cases by displaying isosurfaces of Q criterion used for vortex detection [35]. Without control

    (atT0), the Q isosurfaces are organized in rings which destabilize at a distance of three to four

    nozzle diameters downstream. When control is activated these rings destabilize much faster

    and the jet becomes turbulent right after the nozzle (T0 5 10 4s toT0 1 5 10

    3s).

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    Active jet control for combustion applications 12

    Figure 12. Jet spreading enhancement. PLIF visualizations. Influence of the control flow

    density.

    6. Scaling parameters controlling actuation efficiency

    The previous results have shown that the control-to-main mass flow rate ratio q controls the

    efficiency of the actuators. All those results were obtained by injecting the same gas (air) in

    the main jet and in the four actuators jets. Since it is well known for JICF that density ratio

    between jet and crossflow plays an important role, both experimental and numerical tests are

    repeated injecting a lighter (air-helium mix) or a heavier gas (CO2) in the actuators.

    Figure 12 presents different instantaneous concentration fields (obtained using PLIF) of

    the flow for different values of q and different species injected through the actuators. At a

    constant value of q, lighter gases produce a higher jet spreading angle : for each of the three

    values of q the jet spreading is higher when using the air-helium mix and lower when using

    CO2 than when using air. In those configurations, the value of q is not representative of thecontrol efficiency. The same trend is observed for each value of q between 0 and 0.5, as shown

    on figure 13. On the other hand, when plotting the jet spreading angle vs. the impulsions ratio,

    defined by :

    J

    actU2act

    jetU2jet

    (2)

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    Active jet control for combustion applications 13

    Figure 13. Jet spreading vs. q (left) and J (right). Effect of the control flow density.

    the three curves corresponding to each species collapse and the jet spreading angle grows

    linearly with J.

    Simulations highlight the phenomena responsible for efficiency differences that occurs

    when the control gas density changes. Figure 14 shows concentration isocontours of thespecies injected through the actuators at various distances from the control flow injection.

    The three configurations use the same flow rate ratio (q=0.15) but different gases in the control

    device.

    Figure 14 confirms the experimental results : q is not representative of the control effect.

    LES reveal that the concentration fields exhibit two main modifications, concerning the global

    rotation movement and the interaction between the secondary jets and the main one. The

    global rotation movement can be visualized through the rotation of the vortical axial structures

    around the main jet : the injection of light gas enhances the swirl intensity ; the vortical

    structures turn faster around the main jet axis and so does the main flow. The flow topology

    is also affected by the control gas density through the interaction between the control jets andthe main one. For heavy gas (J 1) the control jets are entrained by the main flow and do not

    penetrate it. On the other hand, for light gas (J 1) the penetration is higher.

    7. Conclusion

    Experimental and numerical investigations of the control of a jet by four radial actuation

    jets have shown that swirl injection (obtained by shifting the axis of actuators jets) is an

    efficient way to control mixing and jet spreading over a very wide range. The effect of the

    deflection and the distance between the control jets and the main jet axis has been studied. The

    most efficient configuration in terms of mixing and jet spreading enhancement is the SW90

    actuator device where actuation jets are located close to the nozzle and oriented to provide

    maximum swirl. Large Eddy Simulations have been validated on both unforced and forced

    cases. The simulations show which mechanisms are responsible for the control efficiency : the

    control jets add swirl (which was expected from the design of the actuators) that enhances the

    jet spreading. However, the LES reveals that the actuator choice (4 small jets) also induces

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    Active jet control for combustion applications 14

    Figure 14. Mean concentration isocontours of the species injected through the actuators.

    Effect of the control gas density. Controled flow with q=0.15

    secondary vortices which may play a role for the reacting cases. Both experimental and

    numerical results performed with lighter or heavier actuator gases confirm that the control

    efficiency is governed by the impulsion ratio J (Eq. 2).

    8. Acknowledgments

    We gratefully acknowledge the support provided by Air Liquide and ADEME.

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    Active jet control for combustion applications 15

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