analysis of oscillating shear...

11th LDA Symposium

Analysis of oscillating shear layer

C.E.C. Cala, E.C. Fernandes and M.V. Heitor

Centro de Estudos em Inovação, Tecnologia e Políticas de Desenvolvimento, IN+ Dept. Engenharia Mecânica Instituto Superior Técnico

Av. Rovisco Pais, 1049-001 Lisboa Codex PORTUGAL

Abstract This work reports the experimental study conducted on a curved oscillatory boundary layer excited by a single frequency artificial signals. Physical analysis of turbulent shear layer morphology was based on instantaneous sequences of shadowgraph images and turbulent velocity characteristics by Laser Doppler Velocimetry measurements. The data is being interpreted on a phase locked base to allow the characterization of the unsteady inner and outer shear layers, that features to strong temporal and spatial deformations imposed by external induced oscillations. The analysis of vortex pairing dynamics allowed to conclude that the pairing process in this flow corresponds to the jet column mode, as suggested for circular jet flow. The consequences of this flow behavior to the combustion efficiency and pollutant reduction are discussed. 1. Introduction Combustion under sufficiently fuel- lean conditions can have desirable attributes of high efficiency and low emissions. However, as lean limit is approached acoustic or non-acoustic instabilities may arise giving rise to a premature blow-out. It is known that this process is driven by fuel-air mixing characteristics, momentum oscillations of the injected flow or shear layer hydrodynamic instabilities, particularly due to high sensitivity of lean flames to these boundary conditions and also to the low flame speed. All of these combine to narrower the stable operating windows. The consequences of combustion instabilities in combustion chambers are often undesirable due to high-pressure fluctuations (e.g. Sivasegaram et al., 1989) and increased heat transfer to the wall (Azevedo et al., 1994), which may cause performance degradation and structural damage (e.g. Bahr, 1996). In fact, the increased intens ity of combustion at any given point, together with short and spatially localised flames, may raise the likelihood of coupling with acoustic waves, in the context of the Rayleigh criterium, thus causing combustor dynamic problems. It is clear that the amount of potential damage is dependent on the amplitude and duration of the oscillation and on the coupling process between the flame/flow and cavity transfers function. This has led too much of the recent work on the control of combustion instabilities towards their attenuation (e.g. Kemal and Bowman, 1996). In such cases an active control strategy is most promising compared to traditional methods of achieving a desired improvement, but is limited by the sensor/actuators technology available today (see for example the report of Schadow et al., 1997) and by the sparse information covering the behavior of bluff body stabilized flames, under resonance conditions, to different amplitude and frequency of oscillations. In general it is known that in bluff body stabilized flames, there are present two type of oscillations; weak oscillations associated with outer vortex shedding process that may carry reaction if flame crosses the peck of maximum velocity

11th LDA Symposium

and strong oscillations associated with large scale deformation of the recirculation zone, as implied by the presence of strong acoustic pressure gradient (see Fernandes 1998). The reminding question is how flame is affected in the inner shear layer and contributes to the unsteadiness support. Due to the complexity associated with the unsteady reactive flows, and in order to better understand the contribution of the shear layer and vortex unsteadiness to the flame stabilization process, the study is carried out on an non-confined and non-reacting turbulent lean premixed flame stabilized by central recirculation zone, induced by a bluff-body, burning a mixture of C3H8 and air and artificially actuated by acoustic signals. In this context, this work addresses the impact of upstream momentum oscillations, with different amplitude and frequencies, on flow field downstream of the buff-body. The experimental analysis conducted here is based on qualitative measurements undertaken by shadowgraph techniques and quantitative measurements of local phase- locked velocity obtained by combining an LDV with a microphone. The data is being interpreted on a phase locked base to allow the characterization of the unsteady reacting shear layer features to strong temporal and spatial deformations imposed by external induced oscillations. 2. Experimental Procedures 2.1. Experimental Setup and Experimental Techniques

The set-up used throughout this work is that shown in Figure 1. The burner consists of a nozzle with 80mm exit diameter. A 56mm diameter bluff-body was centrally placed in order to stabilize the flame.

Figure 1. Experimental set-up The burner was placed on a milling machine table in such a way that the profiles were obtained by burner displacement. The arrangement allows three orthogonal movements, (x, y and z direction) with an error less than 0.5mm. A blower-motor assembly through a plenum with 860x740x500mm, in order to reduce the pressure fluctuation, feed the airflow. A plate orifice connected to a “U” manometer made the control. The propane was supplied through two rotameters allowing a maximum flow rate of about 4.2g/s at PTN, with an error of 0.05g/s. The gases are

11th LDA Symposium

passed through screens and honeycomb to prevent swirling and breakdown the large vortical structures in order to improve the mixing process. The velocity field was measured with a multiline Laser Doppler Velocimetry (LDV) system operating in the forward scatter mode. The coherent light from an Argon-Ion laser with 2W maximum power at λ=514.5nm (green laser beam) was focused through an optical system. The beam is splited in two with an approximately equal power; one of them could be shifted continuously up to 16MHz by a rotating grate. The optical system (splitter and rotating grate) is that used by Ferrão, (1993). This system was configured for a receiving angle of 8.272º. The dimension of the measuring volume, resulting of beams interception, is 44 x 44 x 606µm with 3.57µm fringes spacing. A lens of 310mm focal length collects this control volume and focuses it in the photomultiplier, which transforms it into a sinusoidal electric signal. Three loudspeakers, Uni-Power model s-50, with 16, introduced the controlled excitationΩ and 50W, attached near the exit of the jet, shifted geometrically 120º. The driven input was sinusoidal signals from an audio-amplifier Rotel KB 890 feeded by a Philips PM5131 generator with a working range of 0.1Hz up to 2MHz. The flow motion was recorded with a high-speed camera, (Kodak Motion Corder Analyzer, Model SR-Ultra) at 5000 frames per second with a freezing time of 1/10000s. 2.2. Data Processing

The understanding of the turbulent shear layer behavior is very important for the development of many technological processes and practical devices. An accurate description of the shear layer at a given location is for that very important. This problem can be approached, in an experimental manner, by introducing controlled disturbance within the fluid at some point, and then to study the behavior of this disturbance downstream (Hussain and Reynolds, 1970). In the presence of such kind of flow, with coherent structures, the instantaneous values, according Hussain and Reynolds (1970) are defined to be composed by three terms:

)()(~

)( tutUUtU iiii ′++= (3.1)

Were )(tU i , is the instantaneous value, iU is the long-time average mean, )(

~tU i is the

periodic component (the contribution of the organized waves, the coherent structures) and )(tu i′ is the turbulent fluctuations.

11th LDA Symposium

Figure 2 Time and phase averages of a random signal Figure 2. Illustrates a typical acquisition scheme implemented for conditional sampling, in order to extract the organized wave from the random turbulence (upper signal). The lower is the reference signal from external source, used as clock to start the acquisition process. Conditional samplings were performed on the Data Translation-Fulcrum board; model DT3009, witch combines a TMS320C40 DSP (Digital Signal Processing) from Texas Instruments (for details see Fernandes and Heitor, 1998). Signals, velocity fluctuations and pressure (reference signal) were recovered at 600Hz, the believed fundamental response of the flow. Each record for phase averaging was composed by a set of 20 time histories. With the same board an average of 60 Fast Fourier Transform for velocity was performed. 3. Results and Discution 3.1. Mean Field

These flows were excited by single frequency artificial acoustic signals. Physical analysis of turbulent oscillating shear layer, as shown in the shadowgraph images presented in Figure 3, was obtained by providing the characterization of local morphology of the flame front and shear layer characteristics for different amplitude of oscillations in order to assess the non- linear response of the reacting shear layer.

UU~

u′U(t)

UU~

u′U(t)

11th LDA Symposium

Figure 3. Experimental Conditions: StL=0.52, L=7mm, φ=0.6, BR=64%, Reacting Flow Under excitation the flame front is wrinkled, due its interaction with the induced vortices, as can de seen in Figure 3. This reduces the local flow velocity and increases the heat exchange between the hot products and cold mixture, which prevents a premature extinction, widerrig the stable operating window. This interaction between flame front (inner shear layer) and unburned mixture (outer shear layer) increases the mixing between the hot products and the cold reactants. The vortex rolling up of the outer shear layer engulfs the cold reactants witch are heated and burned during the shear layers interaction. It is expected that this interaction can improve the combustion efficiency and reduce the pollutant emissions, reducing the amount of unburned gases. As suggested by Haile et al (1998), the visible length of the flame is shortened witch can reduce the life time of burned gases in regions of high temperature, reducing then the NO thermal formation. This shortness of the flow, under excitation, should be seen in the Figure 4, where it is shown the mean axial velocity evolution along the central line. The applied excitation corresponds to a Strouhal number of 0.52, based on the flow thickness (L=7mm).

Figure 4. Mean axial velocity along the central line with and without oscillation; U=8m/s, BR=64%

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11th LDA Symposium

As suggested by Fernandes (1998) the combustion process can trigger flow instabilities forcing the shear layer to roll-up. In order to understand the behavior of the shear layer under external excitation and to ga in some control over the dynamics of the shear layer development, isothermal flow is analyzed. Only the reactants were present in the same amount needed to perform the reacting process, this way the instabilities from combustion are isolated. Of course, the reactive flow (combustion flow) is much more complicated than the non-reactive flow (isothermal flow) witch is presented in this work, however, an understanding of simpler flows is a prerequisite to dealing with complex flows. Figure 5 a) and b) show high-speed motion pictures of non-forced and forced flow respectively. From Figure 5 b) can be seen the enhancement of the vortex rolling-up and the pairing process. Zaman and Hussain (1980) suggested that the occurrence of this large-scale structures in turbulent shear flow, characterized by coherent vortical fluid, with length scales of the order of the shear flow width play a dominant role in the entrainment and mixing. An important feature of the dynamics of these structures is the pairing. A better understanding of the behavior of these structures should be given by quantitative measurements. The region marked with a circle, in Figure 3, will be analyzed in detail.

Figure 5. Flow sequence, outer shear layer, acquired at 5000Fps with an exposition time of 1/10000s. Isothermal flow φ=0.6, U=8m/s a)Unforced flow b)Forced flow at 600Hz In Figure 6 it is shown the mean velocity, mean vorticity and spectra for inner and outer shear layers. A gradual process of energy transfer from the forcing frequency (fundamental) to the first subharmonic is shown. This frequency–halving according to Zaman and Hussain (1980) revels that the pairing process is going on. The analysis of the unsteady field (next) will show that this process starts approximately at z/R=0.25. The spectra analysis doesn’t show any future pairing down stream, as suggested by Zaman and Hussain (1980) this should be due the limited length before the end of potential core. This suggests that the activity of pairing in inner shear layer corresponds to the jet column mode and, in this particular issue; this flow behaves as a jet. Up stream of the plane z/R=0.5, another energy distribution at higher frequencies is identified at 3f/2. This distribution should indicate the motion of small scales structures or the non-linear response of the flow to the forcing signal. According to Husain and Hussain (1982), Zaman and Hussain (1980) also found peaks of energy at 3f/2 for axisymmetric jet flow under controlled excitation, in previous work. The energy spectra reveal a hump

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at 30Hz, which should revel the bubble oscillation, as suggested by spectra of non-forced flow.

Figure 6. Mean velocity, vorticity and energy spectra for inner and outer shear layer; U=8m/s, BR=64% Since there are spatial and temporal variations in formation, shape, size, orientation, convection velocity, interaction and breakdown of large scales structures, as suggested by Zaman and Hussain (1980), the mean flow alone doesn’t reveal clearly their occurrence and dynamical role. There, for a deeper understanding, conditional measurements of velocity were taken without combustion. 3.2. Unsteady Velocity field

In Figure 7 time history velocity and vorticiy field, for the reactants alone, with no flame present, is shown. It is seen that the vectors near the bottom and along the shear layers are unsteady which can improve the mixing process. There is a reasonable velocity gradient in radial direction, which is favorable for vortices growing.

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11th LDA Symposium

Figure 7. Two-dimensional representation of phase averaged velocity vectors and phase averaged vorticity, over half- cycle of oscillation; U=8m/s, BR=64%. The contour map reveals two regions of high vorticity. The inner region (internal shear layer) with positive vorticity witch indicates counterclockwise rotating vortices and the outer region (external shear layer) with negative vorticity, the clockwise rotating vortices. The high vorticity at the bottom (Time 1) indicates the vortices shedding points, from where they are convected down stream. For the inner shear layer; with the center of the vortex at about z/R=0.25 (Time 4) the vorticity starts to decrease witch indicates the occurrence of pairing. From this picture should be expeculated that the pairing process at Time 6 is completed and the merged vortexes are convected down stream as one new identity and, as suggested by Figure 6, no more pairing occurs. At sometime a new vortex is starting to grow up in this shear layer. On other hand in the outer shear layer the vortex is stronger and has longer lifetime. Taking again Time 1 as starting point it is seen that the vortex grow from this time until more less Time 5 where the pairing process starts and go on until Time 8. In fact, from the spectra, Figure 6, it is evident that the merging process in the outer shear layer occurs while the vortices are convected downstream but in the inner shear layer the merged vortex are only convected after the end of the pairing process. Video images of vorticity field confirm this statement. But, in reality, the inner vortex has other dynamical behavior. What is seen from Figure 6 should be due the recovering process selected. In fact all signal were recovered at 600Hz, but since there is a pairing process the frequency is halved at the merging region and, the occurrences starts to happen two times slowly. So the recovering process doesn’t have enough ability to seen what is going on with a half-frequency. To overtake this malfunction of the implemented process of recovering the signal it is suggested a two folded recovering, at the fundamental frequency and at its subharmonic. With combustion, as suggested by Driscoll et al (1994), it is expected that

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11th LDA Symposium

the flame doesn’t create additional velocity gradient inducing vorticity to the reactant side, the outer shear layer. So this pattern should be maintained. In Figure 8 it is shown the effect of forcing on the turbulence intensity down stream. It is seen that forcing, in the inner shear layer, decreases turbulence intensity. As suggested by Petersen and Long (1992) it should be due the ability of the large scales to organize the smallest scales. The same authors suggested that this scale organizing process has direct implication for active control of mixing process.

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Figure 8. Forced flow versus unforced flow Working Conditions: φ=0.6, U=8m/s, BR64% Now combining the information contained in Figures 6, 7 and 8 it is possible to define three distinct regions; region I where the vortex is formed, region II the pairing region and the region III a full turbulent region with the merging process completed. It is expected that in region II, during the merging process the mixing will be increased. 4. Conclusions In this work velocity measurements in the wake of a bluff-body, simulating a flame with a mean velocity of 8m/s and an equivalence ratio of 0.6, are reported for non-reactive flow forced at 600Hz, corresponding to a Strouhal number based on flow thickness of 0.52. This excitation lies in the range where, according to Cho et al. (1998), the vortex pairing can be controlled by means of the fundamental and its subharmonic forcing. The pairing corresponds to the jet column mode, observed by Zamam and Hussain (1980) in a circular jet flow. The data was interpreted on a phase locked base to allow the characterization of the unsteady inner and outer shear layers features to strong temporal and spatial deformations imposed by this external induced oscillations. A Fast Fourier Transform procedure was implemented to characterize the pairing process along the inner and the outer shear layers. With forcing was observed; that the height of the recirculation zone decreases, reduction in the turbulence intensity along the axial direction, reorganizing the small scales. References

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Schadow, K., Yang, V., Culick, F., Rosfjord, T., Sturgess, G. and Zinn, B. (1997). “Active

Combustion Control for Propulsion Systems”, AGARD-R-820, pp.1- 38.

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Sivasegaram, S.; Thompson, B.E. and Whitelaw, J.H. (1989). "Acoustic Characterization Relevant to Gas Turbine Augmentors". J. Propulsion, 5(1), Jan.-Fev., pp. 109-115.

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Enhancement by Active Control ". Springer, in fluid control.

J.F. Driscoll, D.J. Sutkus, W.L. Roberts, M.E. Post and L.P. Goss; (1994). "The Strain Exerced by a Vortex on a Flame – Determined from Velocity Field Images ". Combust. Sci and Thech, vol 96, pp. 213-229.

R.A. Petersen and T. A. Long; (1992). "Controled interations in a forced axisymmetric jet. Part 2. The modulation of broadband turbulence ". J. Fluid Mech. (1992) vol 235, pp. 57-72.

Zaman, K.B.M. and Hussain, A.K.M.F. (1980). "Vortex pairing in a circular jet under controlled excitation. Part 1. General jet response". J. Fluid Mechanics, vol. 101, part 3., pp. 449-491.

Hussain, A.K.M.F. and Zaman, K.B.M. (1980). "Vortex pairing in a circular jet under controlled excitation. Part 2. Coherent strutures". J. Fluid Mechanics, vol. 101, part 3., pp. 493-544. Pan, J.C., Vangsness, M.D., Heneghan, S,P. and Ballal, D.R. (1991). "Scalar Measurements in bluff-bodies Stabilized Flames using Cars Diagnostics". The American Society of Mechanical Engineers (1991), 91-GT-302.

Cho, S.K., Yoo, J.Y. and Choi, H. (1998). "Vortex pairing in an axisymmetric jet using two-frequency acoustic forcing at low to moderate strouhal numbers". Experiments in Fluids 25, pp. 305-315.

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