Thermoacoustically driven flame motion and heat release. variation in a swirl-stabilized gas turbine burner

Transcription

1 Thermoacoustically driven flame motion and heat release variation in a swirl-stabilized gas turbine burner investigated by OH and CH 2 O LIF and OH chemiluminescence W. HUBSCHMID 1, A. INAUEN 1, S. SCHENKER 1, N. TYLLI 1, R. BOMBACH 1, F. GÜTHE 2, W. KREUTNER 1 1 Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 2 ALSTOM (Schweiz) AG, CH-5405 Baden-Dättwil, Switzerland Abstract Laser-induced fluorescence (LIF) of OH and CH 2 O and OH* chemiluminescence (CL), all phase-locked to the dominant acoustic oscillation, are used to investigate phenomena related to thermoacoustic instability in a swirl-stabilized industrial scale gas turbine burner. Sinusoidal phase-averaged flame motions in axial (main flow) direction are obtained from different schemes of defining the flame position. Qualitative agreement between data and theoretical analysis of the observed flame motion, interpreted as originating mainly from variation of the burning velocity, is obtained. The heat release variation during an acoustic cycle is determined from the sinusoidally varying total OH* CL intensity. In addition, the modulation of the fuel concentration by the sound wave is obtained from LIF of acetone, which was used as fuel tracer. 1

2 2

3 NOMENCLATURE A 0, A 1 parameters of gas flow (Eq. A4) BZ c bottom flame zone adiabatic sound velocity c fuel fuel concentration CZ CL CM d central flame zone chemiluminescence center of mass diameter of combustion chamber h m f molar formation enthalpy I LIF CL intensity laser-induced fluorescence p ac acoustic pressure Q & heat release rate into a volume element S T air t TZ u u' cross section element of combustion chamber inlet air temperature time top flame zone inlet gas velocity turbulence intensity V volume element v b burning velocity v gas,0 stationary part of phase-averaged gas velocity tot v gas phase-averaged gas velocity v r axial component of flame velocity relative to gas 3

4 δ amplitude of variation of v ( t) v r v r temporal average of v r v var phase-dependent gas velocity x, y transverse coordinates r z axial coordinate z fl flame position in axial direction z 0 temporal average of z fl z fl,max (min) maximum (minimum) of fl z z fl = z z fl, max fl,min & (t) axial component of flame velocity in laboratory z fl α phase angle of maximal flame velocity relative to gas γ ratio of heat capacities ( c p / cv ) ζ = A 1 /ω ϑ angle between flame front and x-y-plane ρ 0 average gas density φ φ ψ ω fuel-air equivalence ratio phase angle of forward zero passage of flame motion phase angle of maximum LIF intensity angular frequency of sound 4

5 1 INTRODUCTION Modern industrial gas turbines are operated at very lean premixed flame conditions in order to reduce the emission of NO x. In this preferred range of operation the occurrence of thermoacoustic instabilities is favored. The temporal variation of the heat release is known to be the main source of pressure waves in the system. Resonance of the combustor and feedback of the pressure variation on the fuel/air mixing and on the turbulence fluctuation lead to selective amplification of distinct oscillation frequencies. One important engineering goal is to devise a theoretical description of the observed phenomena allowing predictions of growth conditions for pulsation modes and the determination of the asymptotic pressure level of oscillation. Current theoretical description uses simplified models as a basis for numerical calculations. Therefore, experimental information on thermo-acoustically unstable combustion is needed to validate the numerical simulations, as well as to guide the further development of burners and combustors. Optical techniques like LIF (laser-induced fluorescence), CL (chemiluminescence), or PIV (particle imaging velocimetry) are among the measurement tools employed in the gas turbine industry. Lee and Santavicca (2005) give a detailed overview of diagnostics of combustion instabilities in gas turbines. Most of previous experimental works are related to determination of heat release, fuel concentration, flow field and/or acoustic quantities. We note here a selection of articles concerned mostly with optical measurements on model gas turbine burners for the determination of heat release and fuel concentration: Lee et al. (2000) used OH* CL to determine the heat release in a lean premixed model combustor. This quantity was compared to the reaction rate determined from the flame surface density as obtained from OH LIF. Their results showed good agreement between these distinct measurements. Ferguson et al. (2001) used high-speed OH* and CH* CL imaging to determine the global heat release rate and the flame surface area, respectively, of stable and unstable conical flames in a Rijke 5

6 Tube combustor. Krebs et al. (2002) and Khanna et al. (2002) determined the flame response of swirl stabilized flames to externally driven oscillations. Using OH* CL as a measure for the heat release rate they compared flame response and incoming velocity fluctuations. Giezendanner et al. (2003) reported on the combination of quasi-simultaneous phase-resolved LIF and CL measurements of OH, CH, CH 2 O, and of OH* in order to observe phenomena related to combustion instabilities in a swirl-stabilized diffusion burner. Bellows et al. (2006) investigated the saturation of the pressure fluctuations with growing heat release by means of OH* and CH* chemiluminescence. Measurements of fuel concentrations in acoustically unstable engines have been done by Venkataraman et al. (1999), who combined phase-resolved CH* CL with fuel/air mixing data obtained by means of LIF of the fuel tracer acetone. They conclude that both, flame area changes and equivalence ratio fluctuations drive the combustion instability in their lean premixed bluff-body-stabilized combustor. The technique of acetone LIF has been applied also by Schneiders et al. (2001) to investigate the mixing processes in gas turbine burners. In this article we investigate the effect of phase-averaged periodic flame motion, which is observed in an industrial scale swirl-stabilized gas turbine burner that features thermoacoustic instability. We applied phase-locked LIF of OH and CH 2 O to localize the flame zone and to detect its motion at the frequency of the acoustic oscillation. To complete the picture, CL of OH* together with mentioned LIF techniques was applied to determine the variation of the heat release. Furthermore, we employed LIF of acetone to show the modulation of the fuel concentration by the acoustic oscillation. All measurements were phase-resolved with respect to the dominant acoustic oscillation, in order to observe the variation of the measured quantities during an acoustic period, and to determine their relative phase relationship. Various levels of external sound forcing ranging from zero to maximum power of the loudspeaker system were applied by us to stabilize the 6

7 acoustic oscillation rendering phase-locking more easily. Different techniques for quantifying the phase-averaged motion of the flame are presented and compared with each other. The observed flame motion is supposed to originate from the periodical variation of the burning velocity and, in part, from the periodical variation of the gas velocity. The phenomenon of periodic flame motion was previously reported at various conferences, cp. Hubschmid et al. (2002) and Schenker et al. (2003). Hassa et al. (2002) observed flame motion by means of OH* CL in a liquid fuel combustor operated at pressures up to 20 bar. In a premixed swirlstabilized combustor a periodic motion of the flame was observed by Bellows et al. (2007). OH LIF and CH 2 O LIF were applied at a large number of operation conditions of the investigated burner for both, the determination of the flame motion, and as well the (approximate) determination of the periodic variation of heat release, in order to study the phase relation between these two quantities. Well-known restrictions have to be considered to deduce local heat release rates also from local OH LIF intensities. Model calculations for laminar premixed combustion show (Glassmann 1996) that the OH concentration rises approximately proportional to the temperature at the boundary between fresh and burnt gases, but decreases slowly afterwards whereas the temperature still increases slightly. The local heat release rate can therefore not be obtained from the local OH LIF intensity. However, the assumption holds that the OH LIF intensity integrated over an appropriately chosen volume is an approximate measure for the global heat release rate in this volume; see the further discussion in Sec The governing relations of thermoacoustic instabilities in combustion are reviewed here shortly. In case that only the variation of the heat release rate is considered as source of sound, generation and propagation of sound obeys the following wave equation (Ingard 1968): 2 pac 2 t c ( 2 p ac ) = Q& ( γ 1) (1) t V 7

8 In (1), p ac is the sound pressure, Q & is the heat release rate in the volume element V = x y z. By convention, x and y are the transverse coordinates, and z is the axial coordinate, which coincides with the main flow direction (see Fig. 2). Furthermore, the ratio γ of specific heats and the adiabatic sound velocity c appear in Eq. 1. Other source terms, as the turbulence source term, are neglected in Eq. 1, as they are assumed to be distinctly smaller than the heat release term. The dissipation terms are not written explicitly either. In case of sound pressure amplitudes, which are not small compared to the average pressure, a generalized non-linear sound equation has to be applied instead of Eq. 1 (Poinsot et al. 2001). The heat release rate per cross section element S = x y of the combustor chamber is Q& S = c fuel v ( t, x, y ) b ( t, x, y) cosϑ h m f. (2) In Eq. (2), vb ( t, x, y) is the burning velocity, i.e. volume of reactant gas burnt per unit cross section of the flame brush and per unit time (the flame brush, or shortly flame, is assumed to be infinitely small), ϑ is the angle of the flame brush to the x-y-plane, c fuel ( t, x, y) is the fuel concentration at the coordinates x and y of the flame, and m h f is the molar reaction enthalpy. Hence, variation of heat release originates from either variation of c fuel ( t, x, y) and/or vb ( t, x, y). The variation of vb ( t, x, y) can be detected as motion in axial direction of the flame, and/or from a variation of the angle ϑ. c fuel ( t, x, y) as well as vb ( t, x, y) depend on the presence of acoustic waves. How burning velocity and turbulent flame speed (relative speed of flame front, resp. some iso-line of the reaction variable, relative to gas flow) are related with each other is discussed in detail by Filatyev et al. (2005). According to Schuermans et al. (2004), varying sound pressure modulates the fuel concentration at the injector of the given burner. With some time delay the induced variation of the fuel concentration reaches the flame. On the other hand, the turbulent flame speed 8

9 depends also on the presence of sound waves: The theoretical analysis of Zimont and Lipatnikov (1995) and others (cp. also Polifke et al. 2000, or Filatyev et al. 2005) relates turbulent flame speed to laminar flame speed s L and turbulence intensity u'. Both quantities vary with the presence of sound waves: s L depends on the temperature and on the fuel concentration and therefore on sound pressure; furthermore Poinsot et al. (1987, 2001) showed that strong sound waves generate vortices and thus enlarge the turbulence intensity u'. A comprehensive overview on instabilities in gas turbine engines is given in the recent book edited be Lieuwen and Yang (2005). See here especially the articles by Ducruix et al. (2005), Huang et al. (2005), Menon (2005), Lieuwen (2005), and Dowling and Stow (2005). Earlier Paschereit et al. (2000, 2001, 2002), Krüger et al. (2002), and Krebs et al. (2002b), among others, discussed specific problems of combustion instabilities in gas turbines. A review on combustion dynamics and its control can e.g. be found in Candel (2002). 9

10 2 EXPERIMENTAL SET-UP Most of the experiments were performed on a commercial single swirl-stabilized gas turbine burner (Alstom EV burner), which for the experiments was operated at atmospheric pressure (Power P ~ 700 kw). The burner is shown in Fig. 1; in detail it has been described by Döbbeling et al. (2005). Figure 2 presents a schematic drawing of burner, combustor, together with the optical experimental setup. The fuel/air supply, upstream of the burner, and the exhaust are not shown. Natural gas is injected along the air slots of the burner cone and from a lance situated in the cone axis. The preheated air (673 C < T < 743 C) enters from the plenum through the slots separating the two half-cones of the burner. Fuel and air mix in the burner cone and enter the cylindrical combustion chamber with a high swirl. At the sudden expansion into the combustion chamber, recirculation zones are formed in the central and in the outer region. This is confirmed by numerical simulations (Biagioli 2006, Flohr et al. 2002) as well as by LDA measurements in a water flow simulation (Paschereit et al. 1999). The position of the flame on the burner axis is close to the stagnation point of the central recirculation zone. Part of the combustion chamber, from the burner exit plane to approximately one chamber diameter length downstream, is made of UV transparent air cooled quartz glass. Dependent of the burner operating conditions, acoustic oscillations at distinct frequencies established inside the combustion chamber, one of them being clearly dominant. Forcing the dominant frequency with two sets of four loudspeakers each (total power 1200 W) enhanced the continuity of the combustion instability. One of the sets was mounted upstream of the burner in the air plenum, the other one at the exhaust. The sound pressure in the combustion chamber was measured with a calibrated water-cooled microphone positioned close to the flame zone, i.e. approximately 1.5 chamber diameters downstream of the burner exit plane. 10

11 For the OH LIF experiments, the frequency doubled light of a pulsed Nd:YAG/dye laser system (Quantel YG781/TDL50) at 286 nm was used to excite the P 1 (7) line of the A X (1 0) transition of the OH radicals. This line is rather insensitive to variations of temperature in the range considered. The repetition rate of the Nd:YAG laser was fixed to 20 ± 0.2 Hz. A specially designed phase-lock loop (PLL) filter allowed for locking of the laser repetition rate to the dominant thermo-acoustic frequency, which was around 120 Hz for all the experiments. The period of the acoustic oscillation was divided into 24 intervals, i.e. 15 phase resolution. At each phase angle of one measurement series, 100 single shot images as well as 10 images of 100 shots summed up on the CCD chip were recorded. Because of time delays in the electronics, the laser pulses are retarded by about 180 with respect to the pressure phase at the microphone. This means that a phase angle of about 270 for the laser pulse corresponded to the maximum sound pressure. By appropriate combination of cylindrical and spherical lenses the laser beam was shaped into a slightly divergent light sheet with a width of about 0.1 m and a thickness of approximately 0.2 mm, propagating from bottom to top of the combustion chamber tube. The laser light sheet was oriented in the y-z plane, y being the propation direction of the laser light, and z being the symmetry axis of the tube, i.e. the main flow direction (see Fig. 2). Using an intensified CCD camera (LaVision FlameStar II) equipped with a band pass filter centered at 310 nm (FWHM 14 nm), the OH LIF signal was detected normal to the plane of the light sheet. The filter chosen allowed to record several rotational lines of the (1 0) transition of OH. For each of the investigated operation conditions of the burner, in addition to LIF of OH, also CL of OH* was registered using the same camera setup. The OH* CL was recorded with the same phase delay as the OH LIF and the same camera position. Exposure time was 0.1 ms. The observation area in flow direction is larger for the CL technique, as compared to the LIF recordings, as it was not limited by the width of the laser light sheet. For the CL experiments only 2-D line-of-sight data were used. In the spectral range of the band pass filter used (see 11

12 above) one has to expect also CO 2 * CL (Nori et al. 2006). However, this does not disturb the final results, as we rely always on relative intensities. Furthermore, CO 2 * shows a broadband emission resulting in only a minor addition to the OH* CL intensities in the small spectral range of the band pass filter used. The formaldehyde (CH 2 O) LIF experiments were done using a slightly different burner and combustion chamber design, which is closer to the arrangements in the real gas turbine. Especially, the stainless steel combustion chamber was of rectangular shape and water cooled. This design only allowed for mounting one set of four loudspeakers (total power 600 W) in the air plenum upstream of the burner mixing section. Two large quartz windows, embedded into the bottom and one of the side walls of the combustion chamber and situated as close as possible to the sudden expansion, provided optical access. Several water cooled microphones allowed for the measurement of both, the frequency and the amplitude of the acoustic oscillations. The frequency doubled output of the same laser system as described above, except for the dye (pyridine 1 = LDS698), was used to excite the A X (4 0) transition at nm, approximately. By means of a combination of the Schott filters KV389 and BG39/3, several vibronic fluorescence emission bands in the spectral range from nm were detected using the same camera type. The burner used for the main part of the experiments did not provide optical access to the fuel / air mixing section. Experiments conducted to observe the modulation of the fuel-air mixing by the sound wave employed a modified version of the burner with an additional fuelair mixing section, part of which was made of quartz. The laser light sheet entered from below covering the whole transparent section length along the symmetry axis. The emitted light was detected at right angles. Visualization of the fuel/air mixing process relies on LIF of acetone. This tracer was added to the fuel prior to mixing. Typical tracer mole fractions were below 2%. Acetone was excited using the fourth harmonic of the Nd:YAG at 266 nm. The laserinduced fluorescence light was collected using the same camera as described above, this time 12

13 equipped with a long pass filter (Schott WG295) to separate the signal from scattered laser light. For identical operating conditions, sound pressures in the combustor were clearly higher with acoustic forcing than without forcing. Full forcing resulted in sound pressures up to 4 times higher than without acoustic forcing. Partial forcing, i.e. only upstream forcing at 70% of full power of the loudspeakers resulted in about 1.3 to 1.7 times higher sound pressures than without acoustic forcing. The sound pressures for flows with flame were - at the same level of power of the loudspeakers - by far larger than the sound pressures for flows of preheated air only. For a typical experimental condition, we found a peak sound pressure of 800 ± 50 Pa with flame and 40 Pa for the flow of air without flame, i.e. fuel replaced by N 2. The added sound intensity by the loud-speakers is therefore by far smaller than the sound intensity generated by the unstably burning flame. With the experimental setup used for the CH 2 O LIF measurements, the naturally occurring sound pressures levels are distinctly lower than with the conventional burner and chamber design. We typically observed pressure oscillations < 300 Pa. In addition, the influence of forcing was significantly lower compared to the original combustor due to both, the combustor design as well as the lower forcing power applied. This complicated the phaselocking of the optical diagnostics equipment to the dominant acoustic oscillation, from which we suppose is resulting mainly the reduced sinusoidal dependence of the LIF intensity on the acoustic phase. An alternative to the phase-locking technique to circumvent this difficulty was proposed by Güthe and Schuermans: They determine the acoustic phase of each laser pulse in their measurements and collect experiments with similar acoustic phases to groups (cp. their paper Phase-locking in post-processing for pulsating flames, submitted to Meas. Sci. Tech. ). This new technique was not yet applied by us, however. 13

14 3 RESULTS AND DISCUSSION 3.1 General results from OH and CH 2 O LIF and from OH* CL The single-pulse OH LIF (resp. CH 2 O LIF) images at an arbitrarily chosen phase of the dominant acoustic oscillation show that the ignition, i.e. the onset of the OH and CH 2 O signals, varies strongly from pulse to pulse. It is situated at locations ranging over a large interval in axial direction. The shape of the flame varies strongly, but there is no characteristic pattern for this at any of the phase angles considered (see Fig. 3a for OH and Fig. 3b for CH 2 O). Images of the flame zone from averages at fixed phases of the dominant acoustic oscillation reveal a more regular behaviour. In the case of acoustic forcing, as well as in the case without acoustic forcing, a periodic variation of the OH LIF (resp. CH 2 O LIF) intensity and a spatial motion of the OH zone (CH 2 O zone) is observed. Figure 4a) shows the results of an OH LIF experiment with sound forcing, displaying the flame in 45 -intervals of the phase angle. The cross-section of the OH zone with the laser light sheet is a v-shaped domain in the center, with an opening angle of about 40 with respect to the plane normal to the main flow direction. Additional OH zones are seen in the outer recirculation zones. Qualitatively, the OH LIF intensity oscillations are more pronounced in the outer zones, and maxima of intensity are shifted in phase, compared to the flame zone in the center. At the same physical conditions, also OH* CL was recorded. Figures 4b and c show the results of the phaseresolved CL measurements for same circumferential orientation of the burner as in the LIF experiment and with the burner was rotated by 90, respectively. An axial asymmetry, originating from the asymmetric design of the burner is recognized. Distinctly observable is a general asymmetry of the intensity distribution, i.e. higher CL intensity in the upper part of the figure. 14

15 A large number of further OH LIF measurements with changed experimental parameters (inlet temperature, inlet flow velocity, global fuel-air ratio, mode of fuel injection) resulted in changes of mean flame position, and of amplitudes and phase angles for the observed varying quantities (OH LIF intensity, flame motion). They otherwise confirmed the general behaviour noted above. The laser light sheet in its usual position close to the burner exit plane, covered the main heat release zone and, thus, the main area of OH formation. In smaller concentrations, OH is found also more downstream, especially in the outer flame zones; in the central zone, however, the OH concentration is very small. This was shown by measurements shifted downstream by about the sheet width. For the analysis of the phase-averaged OH LIF measurements (i.e. multiple measurements at fixed phase angles and subsequently averaged), the 2-D distribution of the OH LIF intensity was split into small sub-zones of about 1/30 th of the chamber diameter. It appeared that in the center, as well as in the outer zones, the sub-zones had rather similar temporal behaviour (with respect to intensity variation and to flame motion). We collected therefore these subzones to larger zones: in a central flame zone (CZ) and two peripheral zones close to the burner wall, named bottom (BZ) and top flame zones (TZ; "bottom" and "top" refer to the setup geometry; see Fig. 3a, rightmost picture). The temporal behaviour in the CZ is rather different to the one in BZ and TZ. The temporal behaviour in the transient zones in between mark the transition from central to peripheral zones; it varies strongly from sub-zone to subzone. A small part of the combustion chamber at the very bottom is masked by mechanical elements of the test rig. A sine function can well be fitted to the phase averaged OH LIF intensity data, separately integrated spatially for CZ, BZ, and TZ. Figures 5a and 5c show the phase-averaged OH LIF intensities, integrated over the central flame zone, as a function of the oscillation phase of an 15

16 experiment without and with sound forcing, respectively. The experiments were operated at T in = 723 K, u = 40 ms -1 and φ = We observe a variation in OH LIF intensity in the CZ of about ±7% and ±9%, respectively, during one oscillation period, with a slight change of the phase for the maximum of intensity. (The observed pressure amplitude was higher by a factor 1.9 in the case of acoustic forcing: 500 Pa vs. 930 Pa.) In the transition zones the intensity variation is reduced and the phase is shifted somewhat, compared to the CZ, whereas in the BZ and TZ averaged LIF intensities vary stronger again. The relative statistical fluctuations of the single-pulse LIF intensities for the central flame zone at a given phase gave relative intensity fluctuations (standard deviation divided by mean intensity) of 26 32% for the various phase angles. Therefore, standard deviations of the mean values are about 3% for N = 100 pulses. This agrees approximately with the deviations of the mean values of the various phases from the fit curve, see Fig. 5c. An image of a CH 2 O measurement averaged at a given phase is shown in Fig.3b. Variation of the CH 2 O intensity in BZ during the acoustic period is shown in Fig. 6a. Identifying maximum of CH 2 O intensity with maximum of heat release, we conclude that oscillation of sound pressure and of heat release rate are shifted by about 90 (Maximum of sound pressure is at about 270 ). This means that observed heat release in the BZ adds only little to sound intensity (Rayleigh criterion). This behaviour is expected for the whole flame in the quasisteady state where sound intensity added has just to compensate for radiation and dissipation of sound. (Behaviour in the other flame zones was similar to the one in BZ; however, as fitting of the intensity variation with a sine function was rather weak, we do not consider it as proven.) 16

17 3.2 Flame motion from phase-averaged OH and CH 2 O LIF measurements As the flame in the gas turbine burner is rather spread out, various quantities can be defined to represent its position and its motion. For the observation of a flame motion, the absolute value of the flame position is of minor importance; rather a relative value with an arbitrary offset is required. In a first definition, the flame position is identified with the phase-averaged center of the OH LIF intensity in axial direction, here called center of mass (CM). This quantity is averaged along the radial coordinate y, i.e. along the light sheet propagation, separately for the CZ, as well as for the BZ and TZ of the combustion chamber. A sine function can well be fitted to the phase-averaged OH LIF intensity data, separately spatially integrated for the CZ, BZ and the TZ. Figures 5b and 5d show the evolution of CM in the CZ as a function of the oscillation phase for the experiments without and with sound forcing, respectively, with the experimental conditions as defined in the previous sub-section. The total variations of the CM flame position scaled by the chamber diameter, z fl / d, are and , respectively. Results for the flame motion in the sub-zones of CZ are presented in Fig. 7. They show that the phase-averaged flame motion has the character of a piston motion. We conclude from this result that the angle ϑ between flame front and x-y-plane does not vary strongly with the acoustic oscillation. We observe for the whole sample of measured operation conditions that the phases at the maximum of OH LIF intensity in CZ are larger than the phases at backward zero passage by about 60 to 120, i.e. they turned out to be at about the phases of minimum flame position. Repeated series of phase-averaged measurements under identical conditions gave absolute deviations 10 from the mean values of phase angles of minimum flame position and of maximum OH LIF intensity. From the experiment with CH 2 O LIF, we found, in the BZ, a shift of 70 between phase at backward zero passage and phase at the maximum of the CH 2 O LIF intensity (Fig. 6b). The corresponding results in the other flame zones gave a similar 17

18 result. However, here the fits with a sine function of the respective data were rather weak. That phase angle of maximum OH LIF (resp. CH 2 O LIF) intensity and of minimum flame position in a given flame zone are related to each other reflects the relation between heat release rate, burning velocity v b, and fuel concentration c fuel, see Eq. 2. LIF intensities are thereby used as an approximate measure for the heat release rate. For a quantitative analysis, one has to relate the flame velocity as observed in the laboratory frame, to the flame velocity relative to the gas flow. In the Appendix, we describe a procedure for such a reduction. Assumptions and approximations in this simplified analysis are: neglect of phase-dependent gas velocity; homogeneity in the transverse axes (in x-y plane) of the flow field in the combustor; time-averaged flow direction parallel to axis; stationary external conditions; infinitely small thickness of flame brush in propagation direction, and therefore equality between turbulent flame speed and burning velocity. The calculation shows that in case of a spatially inhomogeneous flow with a large negative gradient of the mean flow velocity, the phase of the maximum flame velocity relative to the gas is at about the minimum flame position, whereas in case of a homogeneous flow field, maximum flame velocity relative to independent the gas is at the zero backward passage of the flame. That the gas velocity does not depend on the acoustic phase is clearly an approximation. From the sound particle velocity a phase-dependent modulation of the gas flow velocity is expected. We refer here to Duan et al. (2005), who determined by phaseresolved laser Doppler anemometry a periodic oscillation of the gas velocity in a gas turbine model combustor exhibiting thermoacoustic instability. Under the additional assumption that the phase difference between oscillations of c fuel and v b is small, we conclude that the phase ψ of maximum of LIF intensity has to be in the phase-interval between backward zero passage and minimum flame position. This assumption is justified, if main change of variation of v b is caused by variation of c fuel. 18

19 For OH LIF an additional shift for the phase ψ of the intensity oscillation has to be taken into account, as OH is still present after the heat release reactions: If one assumes in a model calculation that after the formation the concentration of OH by further reactions decays exponentially, one finds that this additional phase shift, between variation of heat release rate and of OH concentration, lies also between 0 and 90 ; it depends on the ratio of the decay constant to the angular frequency ω; it is reduced if the OH is transported out of the observation volume, as is the case for OH in the central zone. This phase shift was observed by Giezendanner (2003) in a swirl-stabilized diffusion fame burner, comparing the phases of OH LIF with CH 2 O LIF, CH LIF, or OH CL (as presumed indicators of instantaneous heat release). In consideration of all the mentioned effects, we find qualitative agreement between theoretical analysis and experimental data on the phase relation between oscillations of LIF intensity and of flame motion. For the OH LIF experiment, depicted in Fig. 5d, assuming a homogeneous velocity field and neglecting a phase dependent gas velocity, one obtains from the calculation done in the Appendix that the variation δ vr of the flame velocity in axial direction is δ vr = 0.045u (u is inlet gas velocity), inserting the total amplitude of the flame motion as obtained from the CM definition of flame position. Instead of the CM, the rising slope of the OH LIF intensity profile could also be used as a measure for the flame position. For an experiment with acoustic forcing, Fig. 8 shows for the central zone the normalized profiles of the phase-averaged OH LIF intensity along the axial burner direction at the phase angles of 135 and 315, which represent about minimum and maximum position, respectively. Plotting the axial positions of the rising edge versus the oscillation phase for the values 0.3 to 0.8 (in intervals of 0.1) of the normalized OH LIF intensities, one finds a mean value for z fl / d of , whereas the corresponding value of z fl / d for CM motion was This shows the ambiguity in the definition of the flame 19

20 position by means of OH LIF. However, the phase of minimum flame position, i.e. the position closest to the burner edge, within the accuracy of the measurement, was the same for both definitions of the flame position. 3.3 Flame motion from single-pulse OH LIF images Instead of obtaining the flame motion from phase-averaged pictures one may proceed by obtaining a flame front trace for each single-pulse image. It is based on the fact that the OH concentration rises steeply in the main heat release zone. In the present work, a median filter was applied iteratively to the raw images of the experiment with sound forcing, in order to smooth out smaller holes in the intensity images. The smoothed image was thereafter binarized with a threshold set at the empirically found level of 15% of the maximum image intensity value. To obtain the outline of the flame in the binarized image, a morphological operation is applied, which returns 0 when all 4 connected pixels have the same intensity value (either 0 or 1). Figure 9a shows a representative of the single-pulse OH LIF images; diameter at the exit of the swirl burner is about 55% of the image height. Figure 9b shows the flame front trace obtained by applying the binary image processing scheme described above. Performing the operations described above for all 100 single-pulse images for a given phase of the dominant thermoacoustic oscillation, and adding the resulting flame front images, an image of the distribution of flame fronts is obtained. In Fig. 10, such images corresponding to every third measured phase angle are shown. In the images, brighter areas indicate a preferential position for the flame front. It should be noted however, that the rise as well as drop in image intensity gives a contribution to the flame front image. However, as these regions are typically well separated, this is not a serious limitation. To determine the flame front motion, the central part of the image was subdivided into 10 pixel high horizontal slices. For each of these slices, the intensity contributions in the vertical direction were added to 20

21 form an intensity histogram along the image width. The first hill after the burner exit was subsequently fitted with a Gaussian, and for each slice the horizontal position corresponding to the maximal value of the Gaussian was chosen as the flame position. To reduce noise, the flame front positions of four such slices were added. In this way, the variation of the phaseaveraged flame front position over a period of the dominant thermoacoustic oscillation is obtained. The phase angle of the minimal flame position in the central flame zone found with this method is very close to the one obtained from the center of mass motion of OH; the obtained amplitude of the motion is z fl / d of and thus somewhat larger than the value obtained from the rising slope in the phase-averaged images (cp. previous subsection). From the amplitudes z fl / d determined by any of the three techniques described above, one obtains variations of the flame velocity that are of the order of a few percent of the burner inlet flow velocity. 3.4 Global heat release variation from OH* CL The phase-averaged 2-D OH* CL signals obtained from integration over the whole flame zone are a measure (see below) for the total heat release rate at a specific phase angle. In consistency with the LIF measurements, the total CL intensity also varied sinusoidally during the period of a sound oscillation. The observed CL intensity variations over one period of the oscillation were ±14% and ±13%, respectively, for two orientations of the burner (differing in azimuth angle by 90, cp. subsection 3.1), whereas the observed phase angle of the maxima of the CL intensity differed by 21. The values for the total CL intensities for the two burner orientations under otherwise identical conditions represent repeated measurements of the same quantity. 21

22 In order to determine the variations in heat release from CL intensity variations, we performed a series of not phase-resolved experiments with a variation of the global fuel-air equivalence ratio φ for combustion of natural gas. As we assume that a relevant if not overwhelming part of heat release variations in thermoacoustic instabilities results from φ-variations, we obtain in this way a scale for mentioned heat release variations. To cover the domain of interest for the thermoacoustic instabilities in the investigated burner, four measurements with different values in the interval 0.42 φ of the global φ were performed and the corresponding intensities I CL(φ ) were determined (Fig. 11). An approximately linear relationship between φ and I ) was observed. These measurements extend the φ-domain covered by Higgins et al. CL(φ (2001), Lee and Santavicca (2003), and Nori and Seitzmann (2007) to smaller values of φ. The variation of the heat release rate (or of the fuel-air equivalence ratio, resp.) on the normalized OH* CL intensity for the phase-resolved experiments on thermoacoustic instability can now approximately be determined from the derivative β of this function at the mean φ-value of the variation (i.e. φ 0 ), approximated by φ φ0 φ 0 = ICL ( φ) ICL ( φ0) β. (3) I ( φ ) CL 0 Here I φ ) and I ) are the intensities of OH* CL for φ 0 and for a value φ close to φ 0, CL( 0 CL(φ respectively. We obtain for the measurements depicted in Fig. 11, dependent on the mean value φ 0, that 0.25 β Taking into account this value of β, the variation of ±13.5% for the OH* CL intensity corresponds to about ±3.5% of variation of heat release rate during an acoustic period. Measurements at other operation conditions result in variations of up to ±35% of variation of OH* CL intensity, i.e. of variations of heat release rate of up to about ±9%. According to the plane wave solution of Eq. 1, sound waves with pressure amplitudes of up to Pa are 22

23 generated in the flame zone by these variations of the heat release rate. These sound waves are superposed to the sound waves formed previously, and in a quasi-steady state they compensate for the losses of sound and radiation of sound out of the combustion chamber. 3.5 Modulation of fuel concentration Furthermore, by means of a single tracer LIF experiment with acetone, using a slightly different burner with optical access to the fuel / air mixing section (cp. Sec. 2), we proved that the fuel concentration c fuel is modulated for the given type of burner. We found that the (electronically) phase-averaged LIF intensity varied by about ±6 % during the period of the sound oscillation, see Fig. 12. The temporal variation of c fuel in the mixing region reaches with some time delay the flame zone and causes there a periodic variation of the heat release rate. Thus, we confirmed that this coupling of the sound wave back to the flame is one of the important feedback mechanisms to build up thermoacoustic instability in the given type of burner. 23

24 4 SUMMARY AND CONCLUSIONS We showed that phase-locked 2-D LIF and CL techniques are useful tools for the investigation of thermo-acoustic phenomena in commercial gas turbine burners, and thus for the further development of gas turbine burners. Main effects of acoustic instability studied in this paper are the phase-averaged flame motion and the phase-averaged variation of the heat release. Phase-locking was applied at the dominant frequency of acoustic oscillation. Phaseresolved data of the flame motion were obtained from the phase-averaged images as also directly from the single-pulse images. To distinguish the specific behaviour of various zones of the flame, quantities were in part determined separately for the peripheral and the central flame zones. Flame motion, together with data on the time-dependent gas velocity, gives information on the variation of the burning velocity in unstable combustion. We observed for a large number of measurements at various operation conditions that phaseaveraged OH LIF intensity and flame position of the various flame zones vary sinusoidally with the acoustic frequency. Oscillations are observed in the case of external sound forcing as well as without sound forcing. Phase angles of flame motion and OH LIF intensity variation varied for repeated measurements with identical conditions by 10. Measurements with phase-averaged CH 2 O LIF also showed a sinusoidal behaviour of intensity variation and flame motion. The amplitudes of flame motion in axial (main flow) direction strongly depend on the definition of the flame position, i.e. center of mass or rising slope from phase-averaged images, or flame front trace from single-pulse images. Phase angles of the flame motion, however, do within the measurement accuracy not depend on the choice of definition. We obtain qualitative agreement between experiment and theoretical analysis of the observed flame motion, interpreted as originating mainly from variation of the burning velocity. The 24

25 relative weight of the different contributions to the phase-averaged flame motion has still to be clarified by further investigations. Phase-averaged CL intensities integrated over the whole flame region vary sinusoidally by about ±13%...±35% around their mean values, the amount of variation depends on the operation condition. We showed by variations of the global fuel-air equivalence ratio φ that in the relevant domain of very lean combustion with natural gas there is an approximately linear dependence between variations of OH* CL intensities and variations of heat release. From these calibration measurements we conclude that spatially integrated phase-averaged heat release rates in unstable combustion varied in the range ±3.5%...±9%. One important feedback mechanism in the formation of the thermoacoustic instability in the given type of burner is the modulation of the fuel concentration c fuel by the acoustic oscillation. This was proved by phase-locked LIF measurements of acetone in the mixing part of a slightly modified burner. The acetone had been added to the fuel as a tracer. We are grateful for financial support by the Federal Office of Energy (BFE) and by KTI (Kommission für Technologie und Innovation), which made this investigation possible. The participation of O. Paschereit in planning and supervising the experiments, and of M. Zajadatz, R. Lachner, C. Motz, all of Alstom (Schweiz), for help during the experiments, especially for running the Alstom test rig, is highly appreciated. We thank P. Flohr, B. Schuermans, F. Biagioli, V. Bellucci, all of Alstom (Schweiz), and M. Füri of ETH Zürich for helpful discussions. 25

26 APPENDIX: QUANTITATIVE APPROACH TO THE PHASE- AVERAGED FLAME MOTION IN A SIMPLIFIED MODEL We consider the heat release zone, also called flame, or a part of it, to be a volume characterized in transversal extent by the coordinates x and y. In case of a piston-like motion of the flame, as we assume it, and which is suggested by experiment, the flame moves back and forth along the combustion chamber axis. To simplify the analysis, this axis is supposed to be parallel to the (time-averaged) flow direction. Actually, this is not the case for the flow field of the investigated burner, as there is non-negligible radial in-flow of reactants towards the center of the combustor, as was confirmed by LES calculations (Biagioli 2006). However, considering the whole CZ, as it was defined above, we may assume that this in-flow of reactants into CZ in transverse direction is small compared to the in-flow in axial direction. The experimentally determined flame position of some section of the flame volume is defined as the average z-coordinate z fl of the section considered, i.e. the CZ in our case. All timedependent quantities are assumed to be phase-averaged with respect to the dominant acoustic oscillation with the angular frequency ω. The laboratory flame propagation velocity & (t) is tot the difference between the gas velocity v gas ( t) at z fl (t), and the propagation velocity v ( t) in opposite direction, of the flame relative to the gas flow: z fl r, tot gas z& ( t) = v ( z, t) v ( t) (A1) fl fl r We assume furthermore that v r ( t) oscillates sinusoidally with the frequency ω, the amplitude of the oscillation being δ v r ; i.e. we assume a stationary process, which is under constant external conditions (constant φ, T in, u) approximately true for forced and as well for unforced acoustic instability: [ 1 + δ cos( ω α) ] v r ( t) = v r t. (A2) 26

27 For thick flames as in our case v r ( t) differs in general from the burning velocity v b, which is defined as volume of reactant gas burnt per unit cross section and unit time. The temporal averages of v ( t) r ( r v ) and v b, however, are equal. The gas velocity v tot can be decomposed in a stationary part, v gas gas,0, and the phasedependent gas velocity v var ( t ) : v tot gas ( t ) = v + v ( ). (A3) gas,o var t The stationary gas velocity v gas,o depends on the axial coordinate z. In our model case of homogeneous flow field, parallel to the axis, the flame is situated close, in upstream direction, to the stagnation point of the gas flow. (This is not true in our real case of inhomogeneous flow field. Biagioli (2006) showed by numerical simulations that in the center of the combustion chamber the flame zone crosses the recirculation zone having negative axial flow velocity.) We approximate v ( z) linearly: gas,o v z = z + z z z A z z A. (A4) gas, 0( ) v gas,0( 0) ( 0) v' gas,0 ( 0) 0 ( 0) 1 The parameters A 0 and A 1 are positive and defined by the second equation above, and z 0 is chosen so that the relation following equation: A 0 = v holds. From the equations above, one obtains the r [ z ( t) z ] = v ( t) δ v cos( ω ) z& fl ( t) + A1 fl 0 var r t α. (A5) Equation (A5) is a differential equation for z, if the parameters δ v r and α of the relative fl flame propagation velocity and v var ( t ) are known. On the other hand, if known experimentally, (A5) can be used to determine the parameters δ v r and α. z and v var ( t ) are fl The solution of Eq. (A5) with respect to z fl is straightforward in the case that the phase- 27

28 dependent gas velocity can be neglected. One obtains: z δ v r fl = sin( ω t ϕ ) + 2 1/ 2 ω (1 + ζ ) z 0, (A6) with A1 ζ =. (A7) ω The angle ϕ is determined by 2 1/ 2 ζ sin( ω t ϕ ) + cos( ω t ϕ ) = (1 + ζ ) cos( ω t α). (A8) Equation (A6) says that in an inhomogeneous flow field the amplitude of the flame motion is shrunk by the factor 2 2 ) 1/ ( 1+ ζ, compared to the case of homogeneous flow. We may distinguish two limiting cases: a) ζ << 1. We obtain α ϕ + π. This means that the maximum of the flame velocity relative to the gas is at the phase of the backward zero passage of the flame. b) ζ >> 1. We obtain α ϕ + ( 3π / 2 ). This means that the maximum of the flame velocity relative to the gas is at the phase of the minimum flame position. An approximation to (A6), valid for large ζ, is obtained with the following simple argument: In general, at the turning points of the flame motion, z and z min, the flame fl, max fl, propagation velocity relative to the gas is equal to the gas velocity. For large ζ, i.e. for large A 1 compared to ω, we may assume that approximately v gas, 0( z fl,min ) v gas,0( z fl, max ) = 2δ v. (A9) r From the definition of the derivative A 1 in (A6) follows A v, (A10) 1 z fl = 2δ r 28

29 where by definition z = z z. (A11) fl fl, max fl,min Thus, for ζ >> 1 results, in agreement with (A6) 2δ v r z fl =. (A12) ω ζ 29

30 REFERENCES Bellows BD, Neumeier Y, Lieuwen T (2006) Forced response of a swirling, prmixed flame to flow disturbance. J. Prop. Power 22: Bellows BD, Bobba MK, Forte A, Seitzman JM, Lieuwen T (2007) Flame transfer function saturation mechanisms in a swirl-stabilized combustor. Proc. Comb. Inst. 31: Biagioli F. (2006) Stabilization mechanism of turbulent premixed flames in strongly swirled flows. Comb. Theory and Modelling 10: Candel S (2002) Combustion dynamics and control: Progress and challenge. Proc. Comb. Inst. 29: 1-28 Döbbeling K, Hellat J, Koch H (2005) 25 years of BBC/ABB/Alstom lean premix combustion technologies. Proc. ASME Turbo Expo (Paper GT ) Dowling AP, Stow S (2005) Acoustic analysis of gas-turbine combustors, in: Lieuwen and Yang (2005), ch.13. Duan XR, Meier W, Weigand, P, Lehmann B (2005) Phase-resolved laser Raman scattering and laser Doppler velocimetry applied to periodic instabilities in a gas turbine model combustor. Appl. Phys B80: Ducruix S, Schuller T, Durox d, Candel S (2005) Combustion instability mechanisms in premixed combustors, in Lieuwen and Yang (2005) ch. 9. Ferguson D, Richards GA, Woodruff SD, Bernal S, Gautam M (2001) Effect of surface area variation on heat release rates in premixed flames. Paper presented at 2nd joint Meeting of the U.S. Sections of the Combustion Institute, March 2001 Filatyev S.A., Driscoll J.F., Carter C.C., Donbar J.M. (2005) Measured properties of turbulent premixed flames for model assessment, including burning velocities, stretch rates, and surface densities. Combust. Flame 141: 1-21 Flohr P, Schmitt P, Paschereit CO (2002) Mixing field analysis of a gas turbine burner. Proc. IMECE'02 (Paper IMECE ) Giezendanner R, Keck O, Weigand P, Meier W, Meier U, Stricker W, Aigner M (2003) Periodic combustion instabilities in a swirl burner studied by phase-locked planar laserinduced fluorescence. Combust. Sci. and Tech. 175: Glassmann I (1996) Combustion, 3rd edition, ch. 4. Academic Press 30

31 Hassa C, Heinze J, Stursberg K (2002) Investigation of the response of an airblast atomizer combustion chamber configuration on forced modulation of air feed at realistic operating conditions. Proceedings of ASME Turbo Expo 2002, paper GT Higgins B, McQuay MQ, Lacas F., Rolon JC, Darabiha N, Candel S (2001) Systematic measurements of OH chemiluminescence for fuel-lean, high-pressure, premixed, laminar flames. Fuel 80, Huang Y, Wang S, Yang V (2005) Flow and flame dynamics of lean premixed swirl injectors, in: Lieuwen and Yang (2005), ch.10 Hubschmid W, Bombach R, Inauen I, Kreutner W, Schenker S, Zajadatz M, Motz C, Haffner K, Paschereit CO (2002) Sound generating flames of a gas turbine burner observed by laser-induced fluorescence, 30th International Symposium on combustion in Sapporo, Abstracts of Work-in-progress posters, The Combustion Institute Ingard U (1968) Acoustics, in Handbook of physics, 2 nd edition, Condon E.U., Odishaw H., eds., McGraw-Hill Khanna VK, Vandsburger U, Saunders WR, Baumann WT (2002) Dynamic analysis of swirl stabilized turbulent gaseous flames. Proceedings of ASME Turbo Expo 2002, paper GT Krebs W, Hoffmann S, Prade B, Lohrmann M, Büchner H (2002a) Thermoacoustic flame response of swirl flames. Proceedings of ASME Turbo Expo 2002, paper GT Krebs W, Flohr P, Prade B, Hoffmann S (2002b) Thermoacoustic stability chart for highintensity gas turbine combustion systems. Combust. Sci. and Tech. 174: Krüger U, Hüren J, Hoffmann S, Krebs W, Flohr P, Bohn D (2001) Prediction and measurement of thermoacoustic improvements in gas turbines with annular combustion system. J. Eng. Gas Turb. Power 123: 557 Lee JG, Santavicca DA (2003) Experimental diagnostics for the study of combustion instabilities in lean premixed combustors. J. Prop. Power 19, Lee JG, Santavicca DA (2005) Experimental diagnostics of combustion instabilities, in Lieuwen and Yang (2005), ch.16. Lee SY, Seo S, Broda JC, Pal S, Santoro RJ (2000) An experimental estimation of mean reaction rate and flame structure during combustion instability in a lean premixed gas turbine combustor. Proc. Comb. Inst. 28:

32 Lieuwen T (2005) Physics of premixed combustion-acoustic wave interactions, in: Lieuwen and Yang (2005), ch.12. Lieuwen T, Yang V, eds. (2005) Combustion instabilities in gas turbine engines: Operational experience, fundamental mechanisms, and modelling. Progress in astronautics and aeronautics vol American Institute of Aeronautics and Astronautics Menon S (2005 Acoustic-vortex-flame intrractions in gas turbines, in: Lieuwen and Yang (2005), ch.11. Nori VN, Seitzman JM (2007) Chemiluminescence measurements and modeling in syngas, methane and jet-a fueled combustors. Paper AIAA , presented at the 45 th AIAA Aerospace Sciences Meeting, Reno, Nevada Paschereit CO, Gutmark E, Weisenstein W (1999) Coherent structures in swirling flows and their role in acoustic combustion control. Physics of Fluids 11: Paschereit CO, Gutmark E, Weisenstein W (2000) Excitation of thermoacoustic instabilities by interaction of acoustics and unstable swirling flow. AIAA Journal 38: Paschereit CO, Flohr P, Schuermans B (2001) Prediction of combustion oscillations in gas turbine combustors. Paper AIAA , presented at the 39th AIAA Aerospace Sciences Meeting, Reno, Nevada Paschereit CO, Schuermans B, Polifke W, Mattson O (2002) Measurements of transfer matrices and source terms of premixed flames. J. Eng. Gas Turb. Power 124: Poinsot TJ, Trouve AC, Veynante DP, Candel S, Esposito EJ (1987) Vortex driven acoustically coupled combustion instabilities. J. Flui, d Mech. 177: 265 Polifke W, Flohr P, Brandt M modelling of inhomogenously premixed combustion with an extended TFC model. Proceedings of ASME Turbo Expo 2000, paper 2000-GT-0135 Poinsot T, Veynante D (2001) Theoreical and numerical combustion. Edwards Schenker S, Bombach R, Inauen A, Kreutner W, Hubschmid W, Haffner K, Motz C, Schuermans B, Zajadatz M, Paschereit CO (2003) 2-D LIF measurements of thermo-acoustic phenomena in lean premixed flames of a gas turbine combustor. Proceedings of the First International Conference on Optical and Laser Diagnostics (ICOLAD), London, Institute of Physics Publishing 32

33 Schneiders T, Hoeren A, Michalski B, Pfost H, Scherer V, Koestlin B (2001) Investigation of unsteady gas mixing processes in gas turbine burners applying a tracer LIF method. Proc. ASME Turbo Expo 2001, paper 2001-GT-0049 Schuermans B, Bellucci V, Güthe F, Meili F, Flohr P, Paschereit CO (2006) A detailed analysis of thermoacoustic interaction mechanisms in a turbulent premixed flame. Proc. ASME Turbo Expo 2004, paper GT Venkataraman KK, Preston LH, Simons DW, Lee BJ, Santavicca DA (1999) Mechanism of combustion instability in a lean premixed dump combustor. J. Prop. Power 15: 909 Zimont VL, Lipatnikov AN (1995) A model of premixed turbulent combustion and its validation. Chem. Phys. Reports 14:

34 Figure 1: Alstom EV burner. 34

35 Figure 2: Schematic drawing of burner, combustor, together with the optical experimental setup. 35

36 a) b) Figure 3: a) Selection of single-pulse OH LIF intensity images at given phase of the dominant acoustic oscillation. Rightmost picture: Averaged OH LIF intensity distribution; the flame zones TZ, CZ, BZ are specified on the right. Flow is from left to right. b) Selection of single-pulse CH 2 O LIF intensity images at given acoustic phase, and rightmost, averaged CH 2 O LIF intensity distribution. 36

37 Figure 4: a) Phase-averaged OH LIF intensity measurements of the dominant acoustic oscillation at the phases of 0, 45, 90, 135, 180, 225, 270 and 315. b) Phase-averaged OH CL intensity measurements at same conditions as a). c) Phaseaveraged OH CL intensity measurements with rotation of burner by 90 compared to b). The camera position was shifted downstream for b) and c) by about d/8 compared to a). Therefore figures in b,c) have been shifted to the right with respect to a), so that corresponding points in the combustion chamber lie on the same vertical lines. 37

38 Figure 5: a) Variation of the OH LIF intensity in the central zone during an oscillation period and corresponding sinusoidal fit ( ) for experiments without sound forcing. b) Axial coordinate of the center of mass for the OH LIF intensity in the central zone during an oscillation period and corresponding sinusoidal fit ( ) at same conditions as a). c, d) Respective quantities for experiments with sound forcing.. 38

39 Figure 6: a) Variation of the CH 2 O LIF intensity in the central zone during an oscillation period and corresponding sinusoidal fit ( ) b) Axial coordinate of the center of mass for the OH LIF intensity in the central zone during an oscillation period and corresponding sinusoidal fit ( ) at same conditions as a). 39

40 a) b) radial position [1/d] axial position [1/d] z fl /d Figure 7: Center of mass from OH LIF intensity for sub-zones in radial direction of the central zone for phases of 0, 45, 90, 135, 180, 225, 270 and 315 of the dominant acoustic oscillation. 40

41 1.0 normalized intensity phase angle: axial coordinate [1/d] Figure 8: Profiles of OH LIF intensity along the axial direction for the central zone at the phases 135 and 315 of the dominant acoustic oscillation. 41

42 Figure 9 : a) (left): Representative single pulse OH LIF image. b) (right): Flame front trace resulting from a). 42

43 Figure 10 : Superposition of phase-resolved flame front traces for phases of 0, 45, 90, 135, 180, 225, 270 and

44 Figure 11: Chemiluminescence intensity I CL as function of fuel-air equivalence ratio φ. 44

45 Figure 12: Phase-averaged fuel concentration measurement with acetone as tracer. 45

46 top flame zone central flame zone bottom flame zone top flame zone central flame zone bottom flame zone 46