Paper # 070DI-0287 Topic: Diagnostics

8th U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute

and hosted by the University of Utah May 19-22, 2013

Time-Resolved Particle Image Velocimetry of Non-Reacting Flow in a Swirl-Stabilized

Combustor without and with Foam Inserts for Acoustic Control

Joseph Meadows* and Ajay K. Agrawal** Department of Mechanical Engineering

University of Alabama, Tuscaloosa, AL 35406

Abstract

Combustion noise and thermo-acoustic instabilities are of great importance in highly critical applications such as rocket propulsion systems, power generation, and jet propulsion engines. In our previous studies, we have utilized foam inserts to suppress combustion noise and thermo-acoustic instabilities in lean premixed (LPM) and lean direct injection (LDI) combustion systems. While these studies demonstrated efficacy of the foam insert concept to mitigate noise and instability in gaseous and liquid fuel combustion systems, the actual mechanisms involved have not been understood. Present study is a step towards overcoming this gap by examining the non-reacting flow field without and with foam inserts using time-resolved Particle Image Velocimetry (PIV). The experimental setup consists of a swirl-stabilized combustor without and with the foam inserts placed at the dump plane. Seeded air flow enters the combustor through the mixing chamber located upstream of the dump plane. Baseline measurements taken without the foam insert are compared to the measurements acquired with foam insert. Although the flow field inside the annulus of the foam insert was optically inaccessible, measurements immediately downstream of the foam insert provide insight into the instantaneous flow field and turbulence characteristics. The study highlights the role of the foam insert on flow and turbulence structure, which ultimately affects the acoustics behavior of the combustor in a favorable manner.

1. Introduction

Combustion dynamics in gas turbine engines has gained significant attention form the combustion community since low-emission engines are increasingly employed in the industry. This paper investigates the effects of foam inserts on the non-reacting turbulent flow field of a swirl stabilized system in an effort to gain initial insight into the flow characteristics for a reacting system. In a reacting swirl stabilized combustor, such as a gas turbine, thermo-acoustic instabilities can develop if the pressure fluctuations are in phase with the heat release fluctuations, as stated by the Rayleigh’s criterion [1]. The mechanisms of combustion instabilities are extremely complex because of the coupling between several physical phenomena such as, unsteady flame propagation, unsteady flow field, acoustic wave propagation, vortex flame interactions, and natural or forced hydrodynamic instabilities [2]. __________________________ *Graduate Research Assistant, Department of Mechanical Engineering, 3050 SERC. **Robert F. Barfield Professor, Department of Mechanical Engineering, 3072 SERC.

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Vortices naturally occur in non-reacting and reacting swirl stabilized systems, however the interactions of the vortices with the flame front and acoustic field allow a coupling of the pressure fluctuations and heat release fluctuations. The acoustic field can excite hydrodynamic flow instabilities, which lead to large, organized vortical structures near the flame front and can cause disturbances in the heat release rate [3]. Another mechanism for instabilities occurs in the corner recirculation zone where the vortices recirculate hot products that serve as ignition source for incoming reactants. A review of flame vortex interaction has been published by Renard et. al. [4].

One of the most important features of swirl stabilized flow is the formation of shear layers and the vortex breakdown region. Shear layers in a swirl-stabilized combustor are caused by the interaction of fluid streams with different velocities and the vortex breakdown is a result of the formation of a stagnation point in the flow followed by regions of reversed flow [5]. Reversed flow region develops into a central recirculation zone commonly known as vortex breakdown bubble. Figure 1 illustrates the shear layers and the vortex breakdown bubble in a swirl-stabilized combustor. In a non-reacting flow, the unstable vortical structures formed in the shear layers are convected out of the flow domain and can be thought of as a convective instability whereas in reacting flows the vortices can couple with other mechanisms to result in instabilities throughout the flow domain i.e. global or absolute instability [5].

Figure 1: Swirl Stabilized Flow with Vortex Breakdown Bubble and Shear Layers

Wicksall [6-9] investigated the effect of flow field on the flame for different fuels using particle image velocimetry (PIV) and OH planar laser induced fluorescents (PLIF) and observed that the flame stabilization and flow disturbances are susceptible to the fuel composition. Experimental investigation of detailed turbulence characteristics has been difficult in the past, because of limited temporal resolution of non-intrusive optical diagnostic capabilities. However, recent advancements in time-resolved PIV make it possible to resolve flow structures of turbulence for a wide range of length and time scales. O’Conner and Lieuwen analyzed the multidimensional disturbance field caused by transverse acoustic excitation of a swirling annular nozzle flow and a premixed-swirl stabilized flame using time-resolved PIV at a framing rate of 10 kHz [10]. It was shown that the flow field near the nozzle is a superposition of acoustic and vortical disturbances, and that different disturbances were observed in different portions of the flow. O’Conner and Lieuwen also investigated the vortex breakdown bubble in a transversely excited swirl flow [3]. Steinberg et al. [11] utilized stereoscopic PIV, OH PLIF, and OH* chemiluminescence to investigate the vortex structure and their interaction with the flame region. The flow fields were found to contain either periodically shed toroidal vortices or helical precessing vortex cores, and the helical vortex cores contribute to exciting thermo-acoustic instabilities. These laser diagnostic techniques are very effective tools to investigate combustion dynamics and thermo-acoustic instabilities.

Corner Recirculation Zone

Reactants

Combustor Liner

Vortex Breakdown Bubble Inner and Outer Shear

Layers

Swirler

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Thermo-acoustic instabilities are of great concern in gas turbine engines and methods of mitigating them can be categorized into passive and active techniques. [12-16]. The active techniques usually involve the actuation of the fuel and or air delivery system, while the passive techniques consist of modifications to the combustor geometry, for example, by baffles, resonators, and acoustic liners [12]. Instabilities tend to occur at frequencies associated with the combustor’s natural longitudinal, radial, azimuthal, or bulk modes [17], so passive techniques such as Helmholtz resonators, matching combustor upstream and downstream lengths, gradual diameter changes, and locating an orifice at the antinodes of the quarter wave are all proven to be moderately effective methods at reducing pressure amplitudes, but their implementation in practical combustors is often difficult [13].

Agrawal and Vijaykant [18] developed a passive technique to mitigate combustion instabilities in a swirl-stabilized, lean premixed (LPM) combustor using open-cell foam insert placed at the dump plane. The foam insert is a ceramic matrix alloyed with hafnium carbide/silicon carbide (HfC/SiC) layered coating to resist temperatures up to 1800 ˚C [18]. Experimental studies with the foam inserts demonstrated their effectiveness in mitigating instabilities over a wide range of operating conditions with reduction in acoustic pressure levels of up to 30 dB [19-22]. In an effort to gain preliminary understanding of the flow field with and without the foam insert, Agrawal and Sequera [23] employed a simplified computational fluid dynamics (CFD) analysis of swirling flows with axisymmetric geometry using Reynolds averaged turbulence model and turbulent premixed combustion model based on the work of Zimont [24].

Foam inserts have been shown to be effective in mitigating thermo-acoustic instabilities; however the underlying physical mechanisms are still unknown. The present study is the first step in the direction intended to investigate the flow field by time-resolved PIV technique. Turbulent characteristics, time averaged velocity and vorticity fields, and instantaneous velocity and vorticity fields will be discussed for a non-reacting swirl-stabilized flow system without and with a foam insert located at the dump plane. 2. Experimental Approach A. Test Setup

Figure 2 shows the swirl-stabilized combustor oriented vertically and operated at atmospheric pressure. The air flow consists of a primary air source and a seeded air flow source. The seeded air flow source is introduced into the primary air source approximately 2 meters upstream of the test setup to allow for mixing. Seeded air and primary air flows are introduced into a plenum containing marbles to breakdown large vortical structures. Then, the air flow passes through an annular pipe and proceeds towards an axial swirler flush mounted at the dump plane of the combustor. The swirler consists of six vanes at 28˚ to the horizontal plane to produce a swirl number of 1.5. Once the air passes through the swirler, the air enters a rectangular combustor with 84.7 x 84.7 x 95.3 mm dimensions.

The primary air flow rate is controlled with a manual valve and measured using a laminar flow element (LFE) with a reported calibration error of +/- 5 liters per minute (lpm). The pressure drop in the LFE was measured with a differential pressure transducer and the pressure in the LFE was measured with an absolute pressure transducer. The measured flow rate was corrected for temperature as specified by the LFE manufacturer. The seeded air flow rate is measured using a sonic nozzle and flow rate is controlled by the upstream supply pressure. The upstream and downstream pressures were measured using absolute pressure transducers. The temperature was also measured using a type-K thermocouple with built in cold junction compensation. All data were acquired using a compact DAQ device supplied by national instruments. National Instruments LabView programming language was used for real time data analysis.

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Figure 2: Schematic of Swirl-Stabilized Combustor

The conical diffuser shaped foam insert used in this study has an outer diameter of 7.3 cm. The inner diameter of the foam insert at the inlet (dump plane) and outlet is 2.7 cm and 4.2 cm, respectively. The length of the foam insert is 5.1 cm, see figure 3. The foam insert geometry was selected based on previous work conducted by Sequera and Agrawal [25]. The porosity and pore density of the material is 0.85 and 24 ppcm, respectively. The foam insert is wrapped in graphite foil to prevent flow from exiting the outer wall of the annulus. As shown in Figure 2, a Plexiglas plate with a circular hole in it is placed at the exit plane of the foam insert to prevent undesirable flow conditions.

84.7 mm

84.7 mm

152 mm

610 mm

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95.3mm

Seeded and Primary Air Flow

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84.7 mm

Foam Insert OD = 73 mm Inlet ID = 27 mm Outlet ID = 42 mm

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Top View Without Foam Insert

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Figure 3: Photograph of Foam Insert (Measurement Scale is in Inches)

B. Time-Resolved PIV System

Velocity measurements in the flow field were obtained using time-resolved PIV technique. Quantronix Hawk-Duo 532-120-M Nd:YAG laser with a wavelength of 532 nm and a 18 mJ/pulse at the 4 kHz repetition rate is used for the experiments. The time between the two laser pulses was 50 µs. A Photron SA5 Fastcam camera with a Sigma 105 mm focal length lens and a 1024 x 1024 pixel resolution was used at a frame rate of 4 kHz. A TSI divergent sheet optic, with f = -25 mm cylindrical lens, combined with a 500 mm spherical lens, was used to create a laser sheet 85x85x1 mm. The laser sheet entered the square combustor through the center plane and will be referred as the x-y plane. The spatial resolution for the experiment is 84.25 µm per pixel, which corresponds to velocity resolution of 1.68 m/s. A TSI six jet atomizer was used to introduce olive oil seed particles of approximately 1 µm diameter in the air flow.

Velocity field calculations were performed using Insight 4G data acquisition, analysis, and display software from TSI. The velocity was computed using a three pass Recursive Nyquist Grid. The initial interrogation window size of 64 x 64 pixels with 50% overlap grid spacing was used. The computed velocity vectors then undergo local median vector validation with a reference vector used as the median value of all vectors in the neighborhood. The maximum allowed difference between the computed vector and the reference vector is two times the reference vector. The results from the previous pass are used to optimize the spot offset for the next pass and the interrogation window size is reduced by one half the previous pass. This process is repeated until an interrogation window of 16 x 16 pixels is obtained. A Fast Fourier Transform (FFT) is used to compute the correlation function, and the location of the correlation peak is determined by fitting a Gaussian curve to the highest pixel and its four nearest neighbors. The measured data were rejected based on three criterion: passing the median test as mentioned above, a peak to noise ratio of 1.5, and an absolute velocity magnitude greater than 15 m/s because it would not be a physical representation of the actual flow. A recursive filling using the local mean was used to fill the holes in the vector field. The filling procedure sorts the holes by the number of valid neighbors found initially. It first fills the holes with the most valid neighbors since they have the best chance to be filled; it then fills the holes with the second most valid neighbors, in which the holes filled in the previous pass are also treated as valid neighbors. C. Data Analysis

The average velocity field was calculated from an ensemble average of the velocity field. Spectral analysis for both the radial (x) and axial (y) components of the velocity was performed at specific locations in the flow field with high vorticity. A normalized autocorrelation function (equation 1) was used and then the frequency spectrum was determined using equation 2. For the present sampling rate of 4 kHz, a spectral resolution of 1 Hz and maximum resolvable frequency of 1 kHz was achieved. The vorticity was calculated using equation 3 and only the out of plane component

Top View Side View

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was computed. The turbulent kinetic energy (TKE) was calculated by taking the standard deviation of the velocity with time at every point in the flow field and is defined in equation 4, N is the number of vector plots.

, , , ,

, (1)

(2)

(3)

  ∑ (4)

3. Results and Discussion

In this section, averaged data are presented followed by instantaneous and spectral data. The air flow rate in this experiment is approximately 0.002 kg/s with 30 % of the airflow entering through the seeder. The camera framing rate of 4 kHz will allow 2000 image pairs per a second for velocity calculations. The time between the instantaneous flow field data is 0.5 ms, and a total of 2000 instantaneous flow field data are used for data analysis. A. Time-Averaged Flow Field

Figure 4 shows the time-averaged velocity field without and with the foam insert. The slight asymmetry in the flow field is attributed to minor misalignment of the combustor and/or foam with the inlet of the flow. Without foam insert, we can observe the corner recirculation zone due to the sudden change in flow area at the dump plane. With foam insert, the corner recirculation zone is absent which also agrees with the CFD study by Sequera and Agrawal [23]. Foam insert also affects the vortex breakdown by reducing the size and extent of the central recirculation zone. Figure 5 compares axial velocity profile with foam insert at y = 48 mm to those at y = 8 mm and 48 mm without foam insert. The maximum axial velocity for the two cases is similar at y = 48 mm. The shear layers are located on both sides of the velocity peaks, and the central recirculation zone is located between the velocity peaks. The foam insert affects the location of peak velocities, and demonstrates a larger spreading angle of the shear layer and a larger central recirculation zone without foam insert. The changes in the central and corner recirculation zone and the inner and outer shear layer dynamics change the flame vortex interaction in a reacting flow field and consequently will change the heat release characteristics of the flame.

Figure 6 shows contour plots of the time-averaged vorticity without and with foam insert. An important physical phenomenon observed is the difference of vortices in the corner recirculation zone. Although the flow in the annulus and inside the foam insert is optically inaccessible, some of the flow passes through the porous region of the insert and effectively eliminates the corner recirculation zone observed without foam insert. Contour plots of the turbulent kinetic energy can be seen in figure 7. Turbulence is also eliminated in the corner recirculation zone with foam insert. Since the shear layer spreading angle has decreased with foam insert, the velocity and the vorticity increases in the central recirculation zone. This also increases the turbulence in the central recirculation zone, which plays a vital role in the stabilization of a flame.

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Figure 4: Time Averaged Velocity Magnitude Contour Plots without (left) and with (right) Foam Inserts with Velocity Vectors

Figure 5: Axial Velocity Profiles at Different Axial Locations

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Figure 6: Time-Averaged Vorticity Contour Plots without (Left) and with (Right) Foam Insert with Velocity Vectors

Figure 7: Turbulent Kinetic Energy Contour Plots without (Left) and with (Right) Foam Insert with Velocity Vectors

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B. Instantaneous Flow Field and Spectral Variations

This section focuses on the instantaneous flow field, in particular the formation of vortices. Figure 8 shows instantaneous vorticity contours at time intervals of 1 ms. The regions of high vorticity without foam insert are in the corners and near the combustor walls. The vorticity in the corner corresponds to the corner recirculation zone, and the vorticity near the combustor wall is attributed to the shedding of vortices in the inner and outer shear layers. The regions of high vorticity with foam insert are located in the central region and are attributed to the inner and outer shear layers and the vortex breakdown region. The shear layer spreading angle is decreased with foam insert which changes the shape of the central recirculation zone. The flow structures formed with foam insert displace in the axial direction with each time interval more than the structures formed without foam insert. Vortices are convected out of the flow domain at a faster rate with foam insert.

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Figure 8: Instantaneous Vorticity Contours with a time difference of 1 ms (Top to Bottom) without (Left) and with (Right) foam Insert

with Velocity Vectors

As an initial investigation into the spectral behavior of the velocity components, points were selected in regions of

high vorticity for cases without and with foam insert. The spectral plots for a single point in both cases are presented. Without foam insert, the point A is located near the interaction between the shear layer and corner recirculation zone as shown in figure 4 (-32.5 mm, 10 mm). With foam insert, the point B in figure 4 (-16 mm, 56 mm) is located near the inner shear layer and the central recirculation zone. These locations were selected since recirculation zones and vortices form in the shear layers of the flow field. The spectral analysis resolved frequencies up to 1 kHz but a range of 10 to 100 Hz is shown in figure 9 because dominant frequencies were not observed outside this range. Figure 9 shows that the frequencies of oscillation for the axial and radial velocities are coupled. The foam insert affects the peak frequency and intensity. The spectral analysis presented in this study illustrates a powerful analysis technique that will be employed in future work with reacting flow systems without and with thermo-acoustic instabilities.

Figure 9: Spectral Analysis of Velocity Components at a Point of High Vorticity without (Left) and with (Right) Foam Inserts

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(A.U

.)

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4. Conclusions The swirl stabilized flow field with foam insert at the dump plane has been measured for the first time using time-

resolved PIV technique. The results show that the corner recirculation zones are eliminated and the shear layer spreading angle is decreased with the addition of the foam insert. The decrease in shear layer spreading angle causes the central recirculation zone to narrow and an increase in velocity, vorticity, and turbulent kinetic energy in the central recirculation region. The asymmetry and randomness of the instantaneous velocity and vorticity field was eliminated with the calculation of the time-averaged flow field. This study offers a preliminary insight into the flow behavior of a reacting flow system. In combustion systems where instabilities are observed the quantitative analysis of the spectral behavior in the velocity flow field is needed and the technique used to compute this behavior is demonstrated in this study. Future work will focus on three dimensional reacting flow fields of lean premixed combustion and lean direct injection combustion in an effort to quantify the mechanisms for mitigating thermo-acoustic instabilities with foam insert. Acknowledgements

This research was supported in part by Ultramet Corporation through funding from the US Navy. Joseph Meadows was supported by the Department of Education Graduate Assistance in Areas of National Needs (GAANN) Fellowship program.

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