High-Speed PIV Investigation of Coherent Structures in a Swirl-Stabilized Combustor Operating at Dry and Steam-Diluted Conditions

Transcription

1 Lisbon, Portugal, 9-1 July, 1 High-Speed PIV Investigation of Coherent Structures in a Swirl-Stabilized Combustor Operating at Dry and Steam-Diluted Conditions Steffen Terhaar 1,*, Christian Oliver Paschereit 1 1: Technische Universität Berlin, Chair of Fluid Dynamics, -Hermann-Föttinger-Institut-, Müller-Breslau-Str. 8, 163 Berlin, Germany * correspondent author: steffen.terhaar@tu-berlin.de Abstract Flow instabilities are known to influence the combustion process in many beneficial and adverse ways. In the current study the occurrence of helical flow instabilities featuring a Precessing Vortex Core in a swirl-stabilized combustor is investigated at dry and steam-diluted conditions. Velocity fields and flame positions are measured using high-speed Particle Image Velocimetry and OH* chemiluminescence. The temperature field is estimated using a Quantitative Light Sheet technique and validated with previous suction pyrometer measurements. The combined results reveal strong changes in the flame position, velocity field, and temperature field with increasing rates of steam dilution. The transition between the flame and flow field shapes were abrupt and no intermediate states could be observed. Coherent flow structures were extracted by the application of Proper Orthogonal Decomposition. At isothermal conditions, a helical instability arising near the combustor inlet was found. This structure was completely suppressed for the dry flame. At very wet conditions, the helical instability reappeared and was similar in appearance and frequency to the isothermal case. At intermediate rates of steam dilution, a third flame and flow field type was encountered that featured a helical instability of lower frequency located further downstream than in the isothermal and very wet case. A possible explanation for the occurrence and suppression of the instabilities is provided taking the estimated temperature field into account. 1. Introduction The injection of high amounts of steam, generated by the exhaust heat of the turbine, into the gas turbine combustion chamber promises high cycle efficiency without the need of a separate steam turbine (Jonsson and Yan ). Hereby, the plant complexity can be reduced, resulting in lower installation and operational costs. Furthermore, the steam addition has extremely beneficial effects on the emission formation and, additionally, allows for clean and safe combustion of hydrogen containing fuels and pure hydrogen (Göke et al. 11). In the scope of the European Advanced Grant Research Project GREENEST, stable combustion with steam contents of up to % of the air mass flow was achieved in a swirl-stabilized model combustor. In the combustor the swirling flow expands at the burner exit due to circumferential forces, leading to radial pressure gradients coupled with an adverse axial pressure gradient along the centerline. If the swirl intensity is sufficient, vortex breakdown occurs and a stagnation point, followed by a recirculation zone, is formed in the center. Recent measurements of the flow field revealed that the steam addition had a significant influence on the flame position, the mean reacting flow field, and the turbulence characteristics, including turbulent length scales (Terhaar et al. 11). It is well known that swirling flows with vortex breakdown often exhibit large-scale helical flow instabilities (Fernandes et al. 6; Gallaire and Chomaz 3; Oberleithner et al. 11). The vortex core is displaced from the center and precesses around the combustor axis. This precession is commonly referred to as a Precessing Vortex Core (PVC) (Syred 6). The role of the instability on the combustion characteristics has been evaluated in a number of studies (Huang and Yang 9; Paschereit et al. ; Syred 6). Two major contributions of coherent helical flow structures to the combustion process have been identified: Increased mixing, and coupling with thermoacoustic instabilities

2 Lisbon, Portugal, 9-1 July, 1 It is commonly accepted that the PVC increases flame stabilization due to better mixing, and recirculation of hot exhaust gases, and enlargement of the flame surface (Galley et al. 11; Huang and Yang 9; Stöhr et al. 11). A possible coupling of the PVC with thermoacoustic instabilities has been proposed (Huang and Yang 9; Steinberg et al. 1; Syred 6), but the involved mechanisms need further clarification (Coats 1996; Moeck et al. 1). The burner geometry and the operating conditions play an important role in the manifestation of helical instabilities (Syred 6). Roux et al. (Roux et al. ), for instance, carried out a combined numerical and experimental study and observed that the PVC, which was present at isothermal conditions, was completely damped in the reacting case. Boxx et al. (Boxx et al. 1) observed both a flame featuring a PVC and a flame without a PVC in the same burner at different operating conditions. The damping of the PVC is often attributed to the lower swirl intensities near the centerline, which are caused by dilatation and reduced transference of angular momentum into the central region due to the increased viscosity of the burnt gases (Roux et al. ; Syred 6). However, the density distribution is also expected to influence the occurrence of flow instabilities. Of particular interest for the interpretation of the results of this study are the work from Monkewitz and Sohn (Monkewitz and Sohn 1988) about instabilities in hot jets and the work from Lim and Redekopp (Lim and Redekopp 1998) about instability conditions for variable density swirling jet flows. Both applied linear stability analysis and found that axial velocity and temperature gradients in the same direction destabilized the flow. In contrast, gradients in opposite directions, as they can be found at combustion conditions, stabilized the flow. Steam dilution of the fuel-air mixture can lead to strong changes in the combustor flow field and flame position (Terhaar et al. 11). They identified three different flow field shapes with significantly differing mean velocities and turbulence characteristics. The transition between the shapes was not continuous and no intermediate states were found. In the present paper it will be shown that the changes in the flow field shapes are closely linked to the occurrence of coherent structures including a PVC. The flow field, flame position, temperature field, and the coherent structures with steam dilution are investigated by means of high-speed Particle Image Velocimetry (PIV), OH* chemiluminscence imaging and a Quantitative Light Sheet (QLS) technique. The QLS technique is extended to estimate the spatial temperature distribution, which is shown to play an important role for the development of the coherent structures. The remainder of this study is structured as follows: The generic burner, the operating conditions, and the measurement techniques, including the extension of the QLS technique to estimate temperature distributions, are described. Subsequently, for each of the three flame shapes encountered, the averaged flow fields, flames, and estimated temperature fields are presented. Proper Orthogonal Decomposition (POD) is applied to the data and the extracted coherent structures are presented and interpreted. Finally, a link between the flow field shapes, the flame position, the occurrence of coherent structures, and the near-nozzle temperature distribution is drawn.. Experimental Approach Generic Burner The generic burner used in this study is shown in Figure 1. Air and steam are premixed before entering the burner at the swirl generator, where fuel is injected through 16 holes arranged in a circle in the bottom plate of the burner. The swirling flow mixes with natural gas fuel in the annular passage to the combustion chamber. The swirl generator is based on the movable block principle (Leuckel 1967) with variable radial and tangential passages. By rotating the lower blocks, the area of the passages and the resulting swirl number can be varied. For the presented results it was adjusted to a fixed value of. - -

3 Lisbon, Portugal, 9-1 July, 1 The burner outlet radius mm and the centerbody diameter of 3 mm yield a hydraulic diameter of mm. The hydraulic diameter and the bulk velocity at the burner outlet are used for normalization throughout this work. A 3 mm long silica glass tube with an inner diameter of mm was used as the combustion chamber. The water-cooled exhaust tube had a length of 7 mm. At the outlet an end orifice with a contraction ratio of 8:1 was used in order to suppress self-excited thermoacoustic instabilities. The influence of the orifice on the flow field has been shown to be negligible for the tested combustor operating conditions (Terhaar et al. 1). However, for reference measurements Figure 1 Schematic of the generic at isothermal conditions, it was removed in order to avoid burner strong distortions of the flow field due to the subcritical flow state (Escudier et al. 6). The combustion air was preheated to K and perfectly mixed with overheated steam upstream of the generic burner. The steam content Ω is defined as the ratio of the mass flow rate of steam to the mass flow rate of air. ( ) While the steam content was varied between and.3, the total mass flow was kept constant for all measurements at steam air kg/h. Table 1 gives an overview of the investigated combustor operating conditions. The inlet temperature as well as the adiabatic flame temperature, calculated using CANTERA and the GRI 3. mechanism (Smith et al.), were kept constant at 6 K and 18 K, respectively, in order to assure a constant acceleration over the flame. Consequently, with increasing steam addition the equivalence ratio had to be increased in order to compensate for the higher specific heat capacity and lower amount of air at steam diluted conditions. As a result, the theoretical combustor power P is increased with higher steam contents. Table 1 Combustor operating conditions. Flame Shape T in (K) T ad (K) T fl (K) Ω Re P (kw) Isothermal V Trumpet like Annular Annular Heat losses to the combustor wall result in lower flame temperatures than the adiabatic flame temperature For this setup and the investigated temperatures, a simple correlation was found to be valid (Göke and Paschereit 1; Terhaar et al. 1):. () Velocity Measurements Figure shows a sketch of the experimental setup for the velocity measurements in the combustion chamber. Flow velocities were measured using high-speed Particle Image Velocimetry (PIV) at a horizontal plane containing the combustor axis. The PIV system consisted of a dual cavity diode pumped ND:YLF laser and a high-speed CMOS camera. The camera is able to operate in full frame mode (1px x 1px) up to fps. In double frame mode this resulted in a recording speed of - 3 -

4 Lisbon, Portugal, 9-1 July, 1.7 khz of the PIV system. A light sheet optic formed a laser sheet with a thickness of 1 mm and the beam waist located slightly beyond the measurement area. The time delay between the pulses was set according to the expected flow velocities between 1 and 16µs. The strong out-of-plane velocity component associated with swirling flows required a rather short pulse separation in order to minimize lost particle pairs. Titan dioxide (TiO ) particles were seeded into the flow upstream of the swirl generator using a brush based powder disperser to ensure Figure Sketch of the experimental setup a very homogeneous particle distribution. Reflections of the incoming laser light at the silica glass were minimized by using beam dumps for the sheet and primary reflections. Further reflections could be eliminated by sandblasting parts of the silica glass. The steam led to corrosion of the glass, which required regular substitution. The images were post-processed with a final interrogation area of 16px x 16px. The interrogation window overlap of % resulted in a spatial resolution of 1.8mm. The data was filtered for outliers and interpolated from adjacent interrogation areas. OH* Chemiluminescence Measurements An intensified CCD camera with a band pass filter at 38 nm was used to give the spatial distribution of OH* chemiluminescence, which correlates with the heat release and the intensity of the chemical reaction. Time resolved data of the integral heat release fluctuations were recorded using a band pass filtered (38 nm) photo multiplier tube. Temperature Estimation Distributions of the density and temperature field inside the combustor were estimated using the Quantitative Light Sheet (QLS) technique. The QLS technique is mainly used as an alternative to Laser Induced Fluorescence (LIF) measurements for mixing experiments, where the flow is only partly seeded (Findeisen et al. ; Roehle et al. ; Voigt et al. 1997). However, the principle is transferrable to density measurements in a homogeneously seeded flow. The big advantage of the QLS technique is the simplicity of the setup compared to Rayleigh or Raman scattering based techniques. The experimental setup is almost identical to the PIV setup and makes the QLS technique very suitable to be used simultaneously to PIV measurements. In the QLS technique the amount of scattered light is used to derive the spatial distribution of the seeding density, which can be correlated to either mixture fractions or the fluid density. The recorded intensity signal does not only depend on the seeding particle density but also on several other factors. However, the most important parameters can be described by a simplified model (Findeisen et al. ; Freund et al. 11):. (3) In this model the measured intensity of the scattered light ( ) consists of three parts. The first term represents the light intensity scattered by the seeding particles. It depends on the distribution of the particle density, on the local light sheet intensity ( ), and on a factor ( ) that accounts for different viewing angles in the measurement plane. For details see (Findeisen et al. ; Freund et al. 11). The second term stems from reflections of the light at the silica glass combustion chamber ( ), and the last term ( ) represents the camera dark current. Other physical effects such as light extinction, multiple scattering, and illumination of the background due to seeding are neglected in the model and in the scope of this study. - -

5 Lisbon, Portugal, 9-1 July, 1 In order to correct for the background light and dark current ( and ), reference pictures ( ) without seeding ( ) were recorded and subtracted from the raw images:. () For the correction of the local light sheet intensity ( ) and the viewing angle ( ) images at isothermal conditions and, hence, an almost homogeneous particle density distribution ( ) were recorded:. () With the known homogeneous particle density the particle density distribution for a measured light intensity distribution can be obtained: In order to convert particle density distributions into fluid density distributions it must be assumed that the scattering cross section of the particles does not change inside the measurement plane and that the velocity field does not cause changes in the particle density. The melting temperature for the TiO seeding is higher than any temperatures reached in the combustor. Hence, it can be assured that the size distribution of the particles does not change. Changes in the seeding density inside smaller eddys could be observed to a limited extent, even in the homogeneously seeded isothermal case. This is assumed to stem from the higher density of TiO compared to air. Particles are transported out of the core of the eddy due to centrifugal forces. However, these inhomogeneities disappear in the averaged raw image. Thus, only averaged density and temperature fields can be extracted with the proposed technique. Due to the fact that several parameters determine the total amount of scattered light, the QLS technique is commonly used as a relative technique only. This means that without a reference density no absolute values can be deduced from the results. From Equation 6, it follows that the density distribution can be obtained if the density distribution of the homogeneous image is known and it is assumed that the particle density is proportional to the fluid density: (6). (7) From the density distribution, the temperature distribution can be readily derived if ideal gas behavior and negligible pressure drop over the flame are assumed. However, a comparison with temperature data obtained using a suction pyrometer (Göke and Paschereit 1) showed that the absolute values differed up to %, while the shape of the temperature field was reproduced very well,. The main reasons are assumed to be long term variation in the laser energy and the seeding intensity, which directly influence the measured light intensity and are not corrected for. A more reliable possibility for scaling was found to be the temperature in the region of burnt gases, which can be estimated from Equation. A comparison of the reconstructed temperature data to radial profiles measured using a suction pyrometer (Göke and Paschereit 1) in a similar setup (steel combustion chamber) at identical operating conditions is provided in Figure 3. The good agreement between the suction pyrometer data and the scaled temperature profiles underlies the reliability of the QLS technique for qualitative temperature estimations. Additionally, if scaling is possible, as in the present work, quantitative distributions can be T (K) = QLS = Suction Pyr. =.3 QLS =.3 Suction Pyr Figure 3 Comparison of estimated temperature profiles to data from (Göke and Paschereit 1) at x D h - -

6 Lisbon, Portugal, 9-1 July, 1 obtained. Concerning the measurement uncertainty, it must be assumed that due to the challenging measurement environment of confined reacting flows, in terms of seeding quality and reflections, the measurement error may be significantly increased compared to the QLS measurements for isothermal mixing, where accuracies of % are possible (Voigt et al. 1997). 3. Results and Discussion Time Averaged Flow Fields Figure shows velocity vectors of the time-averaged isothermal flow field superimposed on the normalized turbulent kinetic energy. Three regions can be identified: an annular jet emanating from the mixing tube, an internal recirculation zone (IRZ) generated by vortex breakdown, and an outer recirculation zone (ORZ) induced by the area change and the confinement. In between the regions, turbulent shear layers are produced, with very high levels of turbulence. The observed flames, flow fields, and temperature distributions at different levels of steam dilution are shown in Figure. Figure a shows the typical V-shaped flame, encountered at dry conditions ( ), with the main reaction zone located close to the combustor inlet in the inner shear layer zone. The region of burnt gases and hot temperatures reaches upstream until the centerbody. Compared to the isothermal flow field, the opening angle of the jet is considerably wider and the negative velocities inside the IRZ are uniform. The flame shown in Figure b was encountered at steam levels of - 1 Figure Isothermal flow field. Velocity vectors are superimposed on the normalized turbulent kinetic energy. Lines indicate the zones of zero axial velocity.. It shows a trumpet-like shape with a very long axial distribution of the heat release and the highest rates near the centerline in the IRZ. As in the case of the V-flame, the region of hot temperatures reaches up to the centerbody. The corresponding flow field features a jet region with a remarkably small opening angle. The third flame shape (Figure c-d), encountered for high levels of steam dilution, shows a detached annular flame located further downstream in the combustion chamber. The hot gases do not penetrate into the region near the centerbody. While the flame at extremely wet conditions of is longer and located even further downstream compared to, the temperature distributions and flow fields remain very similar. Furthermore, a very good similarity to the isothermal flow field is found. Coherent Structures The extraction of coherent structures from the flow field measurements is achieved by the application of Proper Orthogonal Decomposition. POD is a well-established technique in fluid mechanics (Berkooz et al. 1993). It projects the turbulent flow field on an orthonormal vector base that maximizes the turbulent kinetic energy content for any subset of the base. It therefore allows for an accurate description of the turbulence data using only a limited number of modes. Figure 6 shows the results of the POD analysis for the V-flame ( ), the trumpet like flame ( ), and an annular flame ( ). For comparison purpose the results of an isothermal measurement are also shown. For every operating point a time series of 3 snapshots was decomposed in a nozzle-near region ( and ). The normalized through plane vorticity of the first pairs of POD modes containing coherent structures and the spectra of the corresponding mode coefficients are presented. Note, for the trumpet like flame (Figure 6c) the third mode is additionally shown, as the second and third mode represent one coherent structure Norm. turb. intensity (-) - 6 -

7 16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 9-1 July, 1 - Norm. OH*-chem. (-) 1 Temperature (K) Temperature (K) Norm. OH*-chem. (-) 6 16 d) Annular flame at Ω c) Annular flame at Ω Norm. OH*-chem. (-) b) Trumpet like flame at Ω 1 Temperature (K) a) V-flame at Ω Temperature (K) 16 Norm. OH*-chem. (-) Figure Flame pictures, velocity vectors superimposed on the Abel-deconvoluted OH*chemiluminescence images, and estimated temperature field with increasing steam dilution. -7-

8 Lisbon, Portugal, 9-1 July, 1 PSD (a.u.) m=1 m= Strouhal number (-) a) Isothermal flow field 6 6 PSD (a.u.) m=1 m= Strouhal number (-) b) V-flame at 6 6 PSD (a.u.) m=1 m= m= Strouhal number (-) c) Trumpet like flame at PSD (a.u.) m=1 m= Strouhal number (-) d) Annular flame at 6 Figure 6 Normalized vorticity of the first POD Modes and spectra of the corresponding coefficients. The first two POD modes of the isothermal flow field (Figure 6a) resemble asymmetric vortices traveling along the shear layers. Their appearance is typical for a helical instability featuring a PVC (Oberleithner et al. 11; Stöhr et al. 11). The structure at isothermal conditions possesses about 1% of the turbulent energy and oscillates at a normalized frequency of. The peak in the spectra of the POD coefficients of the first two POD modes is spread and indicates some jitter in the precession frequency of the PVC. For the V-flame (Figure 6b) no similar structures could be found, as those in the isothermal case. The two most dominant modes describe stochastic or very slow movements of the recirculation zone. The spectra of both mode coefficients do not show any significant peaks. Also, for the coefficients of modes containing less energy no clear peak in the spectra can be found. This indicates that in the flow field of the V-flame no helical periodic coherent structure is present. 6 St (-).1.1. Isothermal Trumpet like flame Annular flame Re (-) x 1 Figure 7 Strouhal number at different operating conditions

9 Lisbon, Portugal, 9-1 July, 1 The first POD mode of the trumpet-like flame (Figure 6c) shows a similar structure as the first mode of the V-flame. This mode also did not show any significant periodicity. The second and third POD modes form a skew symmetric structure similar to the first two POD modes at isothermal conditions but shifted significantly downstream. The coherent structure described by the POD modes oscillated at a significantly lower normalized frequency than in the isothermal case and the energy content is similar at about 1%. The POD modes of the flow field of the annular flame (Figure 6d) show clear evidence of a similar helical structure as in the isothermal case. In contrast to previous observations (Roux et al. ), the structure is not suppressed by the combustion but considerably amplified (1% of turbulent energy). The streamwise decay of the vortices is slower and also the peak in the spectra is higher and sharper. The normalized frequency of the instability remains very similar to the isothermal case at. Additional measurements at various operating points (Figure 7) showed that the normalized frequency for the annular flames and the isothermal flow field is constant at. For the trumpet like flame a slight dependence on the operating conditions (e.g. equivalence ratio) was found. This can be explained by the fact that the structure of the trumpet like flame is located mainly in the region of burnt gases, which, of course, is considerably more influenced by the operating conditions than the region upstream of the flame. Simultaneous OH*-chemiluminescence measurements with a photo multiplier tube showed that no periodic fluctuation in the integral heat release were caused by the helical structures. This has been observed previously (Moeck et al. 1) and was attributed to the skew symmetric type of the helical flow structures and the resulting skew symmetric heat release fluctuations. Instability Mechanisms The skew symmetric coherent structures found at isothermal conditions that are associated with the PVC can be attributed to a self-excited hydrodynamic instability (Gallaire and Chomaz 3; Oberleithner et al. 11). Key elements for a sufficiently strong growth of the instabilities are, in addition to the distribution of tangential velocities, the shear layers of the axial velocity. The growth rate of instabilities, as shown by classical linear stability analysis (Michalke 196), directly scales with the inverse shear layer thickness of the axial velocity component. In the case of non-isothermal flow temperature gradients and the corresponding density gradients were also shown to influence the growth of instabilities (Lim and Redekopp 1998; Monkewitz and Sohn 1988). Temperature and velocity gradients in the same direction promote instabilities and gradients in opposite directions were shown to suppress instabilities (see Figure 8). Figure 9 shows radial profiles of the axial velocities normalized with the bulk velocity at the burner outlet and normalized estimated temperatures for a V-Flame, a trumpet like flame, and an annular flame. For the V-flame (Figure 9a) at positions with steep gradients of the axial velocity, the temperature gradient is in the opposite direction. This is assumed to stabilize the shear layers. Additionally, the higher viscosity of the hot gases, which reach upstream until the centerbody, causes lower transfer of angular momentum to the central region and, thereby, lower tangential velocities near the centerline (Syred 6). Both effects are assumed to suppress the helical instability. This is similar in the nozzle-near region of the trumpet-like flame (Figure 9b). However, the axial velocity gradient also remains steeper further downstream (x ), where the temperature gradients are less pronounced and sufficient angular momentum is assumed to be transferred to the central region (Syred 6). This u (arb. unit) u T * y destabilizing T * (arb. unit) Figure 8 Destabilizing or stabilizing effects of a temperature gradient on an axial velocity shear layer

10 Lisbon, Portugal, 9-1 July, 1 possibly leads to the occurrence of the helical instability in the flow field of the trumpet-like flame at the downstream position. In the case of the annular flame (Figure 9c), the temperature is almost constant across the inner shear layer. Hence, the helical instability is not influenced by the temperature field, and therefore, the resulting instability remains similar in shape and frequency to the isothermal case. The abrupt transitions between the V-flame and the annular flame may be related to a possible feedback loop: The occurrence of the instability featuring the PVC that is found in the annular flame depends on the density gradient across the shear layers of the axial velocity component, and the viscosity in the nozzle-near region. The PVC itself has been shown to enhance mixing (Galley et al. 11) and, hereby, promote a more homogeneous temperature and density distribution, these being more favorable conditions for the occurrence of the PVC. =1 =3 = =1 =3 = =1 =3 = Figure 9 Radial profiles of normalized axial velocities (solid lines) and normalized temperatures (dash-dotted lines) for a V-flame, trumpet-like flame, and annular flame.. Conclusions The flow field and flame characteristics of a swirl-stabilized combustor were investigated under dry and humidified conditions using PIV and OH* chemiluminescence. To estimate the density field and temperature field, a QLS technique has been used and validated. Three types of flame shapes and adjacent flow and temperature fields were found depending on the operating conditions. Both the mean flow field and turbulence characteristics depend significantly on the flame shapes. At isothermal conditions, a helical instability containing a precessing vortex core was found. The dry flame ( ) showed a V-flame anchoring mainly in the inner shear layer. This flow field did not show any periodic helical structures. At a moderate steam level ( ) the flame showed a trumpet-like shape. A skew symmetric coherent structure was found that was located further downstream compared to the isothermal case and precessing at a lower frequency. At high levels of steam dilution ( ), annular flames were encountered. The flow field showed similarity to the isothermal flow field. Consequently, the encountered coherent structures were very similar in appearance and frequency to the isothermal case. A possible explanation for the occurrence and suppression of the helical instabilities is provided by analyzing the temperature fields. Suppression of the instability in the near nozzle region is assumed to stem from stabilizing density gradients along the shear layer for both the V-flame and the trumpet like flame. However, for the trumpet like flame a helical instability was present that was located in the region of burnt gases further downstream. For the annular flame, the temperature and density along the inner shear layer is almost constant. Hence, no influence on the instability is caused and u/u. 1 u/u. 1 u/u. 1 u/u. 1 u/u. 1 u/u. 1 u/u. 1 u/u a) V-flame b) Trumpet-like flame d) Annular flame. 1 u/u

11 Lisbon, Portugal, 9-1 July, 1 the appearance and frequency remains similar to the isothermal case. Regarding the increased mixing of fresh and burnt gasses that is often attributed to the helical instability, a possible feedback mechanism has been identified causing the abrupt transitions in between the flow states.. Acknowledgements The research leading to these results has received funding from the European Research Council under the ERC grant agreement no. 73, GREENEST. The authors would like to thank Andy Göhrs, Eduard Höschele, and the CONFET for assistance in the lab and helpful discussions. 6. References Berkooz G, Holmes P, Lumley JL (1993) The Proper Orthogonal Decomposition in the Analysis of Turbulent Flows. Annual Review of Fluid Mechanics :39-7. doi: 1.116/annurev.fl Boxx I, Arndt CM, Carter CD, Meier W (1) High-speed laser diagnostics for the study of flame dynamics in a lean premixed gas turbine model combustor. Experiments in Fluids :-67. doi: 1.17/s x Coats CM (1996) Coherent Structures in Combustion. Progress in Energy and Combustion Science :7-9. Escudier MP, Nickson a. K, Poole RJ (6) Influence of outlet geometry on strongly swirling turbulent flow through a circular tube. Physics of Fluids 18:113. doi: 1.163/1.7 Fernandes EC, Heitor MV, Shtork SI (6) An analysis of unsteady highly turbulent swirling flow in a model vortex combustor. Experiments in Fluids : doi: 1.17/s Findeisen J, Gnirß M, Damaschke N, et al. () D Concentration Measurements Based on Mie Scattering using a Commercial PIV system. 6th International Symposium on Particle Image Velocimetry, Pasadena, California, USA, September 1-3, Freund O, Schaefer P, Rehder H-J, Roehle I (11) Experimental Investigations on Cooling Air Ejection at a Straight Turbine Cascade Using PIV and QLS. Proceedings of ASME Turbo Expo 11 GT11 June 6-1, 11, Vancouver, British Columbia, Canada Gallaire F, Chomaz J-M (3) Mode selection in swirling jet experiments: a linear stability analysis. Journal of Fluid Mechanics 9:3-3. doi: 1.117/S11361 Galley D, Ducruix S, Lacas F, Veynante D (11) Mixing and stabilization study of a partially premixed swirling flame using laser induced fluorescence. Combustion and Flame 18: doi: 1.116/j.combustflame.1.8. Göke S, Paschereit CO (1) Influence of Steam Dilution on NOx Formation in Premixed Natural Gas and Hydrogen Flames. th AIAA Aerospace Science Meeting, AIAA-1-17, Nashville, Tennessee, USA, January 9-1, 1 Göke S, Terhaar S, Schimek S, et al. (11) Combustion of Natural Gas, Hydrogen and Bio-Fuels at Ultra-Wet Conditions. Proceedings of ASME Turbo Expo 11: Power for Land, Sea and Air June 6-1, 11, Vancouver, Canada Huang Y, Yang V (9) Dynamics and stability of lean-premixed swirl-stabilzed combustion. Progress in Energy and Combustion Science 3:

12 Lisbon, Portugal, 9-1 July, 1 Jonsson M, Yan J () Humidified gas turbines a review of proposed and implemented cycles. Energy 3: doi: 1.116/j.energy..8. Leuckel W (1967) Swirl intensities, swirl types and energy losses of different swirl generating devices. IFRF Doc. Nr. G : Lim DW, Redekopp LG (1998) Absolute instability conditions for variable density, swirling jet flows. European Journal of Mechanics-B/Fluids 17: Michalke A (196) On spatially growing disturbances in an inviscid shear layer. Journal of Fluid Mechanics 3:1-. doi: 1.117/S1161 Moeck JP, Bourgouin J-F, Durox D, et al. (1) Nonlinear interaction between a precessing vortex core and acoustic oscillations in a turbulent swirling flame. Combustion and Flame. doi: 1.116/j.combustflame.1.. Monkewitz PA, Sohn K (1988) Absolute instability in hot jets. AIAA Journal 6: doi: 1.1/3.999 Oberleithner K, Sieber M, Nayeri CN, et al. (11) Three-dimensional coherent structures in a swirling jet undergoing vortex breakdown: stability analysis and empirical mode construction. Journal of Fluid Mechanics 679: doi: 1.117/jfm Paschereit CO, Gutmark E, Weisenstein W () Excitation of thermoacoustic instabilities by interaction of acoustics and unstable swirling flow. AIAA journal 38:1 13. Roehle I, Schodl R, Voigt P, Willert C () Recent developments and applications of quantitative laser light sheet measuring techniques in turbomachinery components. Measurement Science and Technology 11: doi: 1.188/97-33/11/7/317 Roux S, Lartigue G, Poinsot T, et al. () Studies of mean and unsteady flow in a swirled combustor using experiments, acoustic analysis, and large eddy simulations. Combustion and Flame 11:-. doi: 1.116/j.combustflame..1.7 Smith GP, Golden DM, Frenklach M, et al. GRI-Mech Steinberg AM, Boxx I, Stöhr M, et al. (1) Flow flame interactions causing acoustically coupled heat release fluctuations in a thermo-acoustically unstable gas turbine model combustor. Combustion and Flame 17:-66. doi: 1.116/j.combustflame Stöhr M, Sadanandan R, Meier W (11) Phase-resolved characterization of vortex flame interaction in a turbulent swirl flame. Experiments in Fluids 1: doi: 1.17/s y Syred N (6) A review of oscillation mechanisms and the role of the precessing vortex core (PVC) in swirl combustion systems. Progress in Energy and Combustion Science 3: doi: 1.116/j.pecs..1. Terhaar S, Bobusch BC, Paschereit CO (1) Effects of Outlet Boundary Conditions on the Reacting Flow Field in a Swirl-Stabilized Burner at Dry and Humid Conditions. Proceedings of the ASME Turbo Expo 1 Terhaar S, Göckeler K, Schimek S, et al. (11) Non-Reacting and Reacting Flow in a Swirl- Stabilized Burner for Ultra-Wet Combustion. Proceedings of 1st AIAA Fluid Dynamics Conference and Exhibit 7-3 June 11, Honolulu, Hawaii Voigt P, Schodl R, Griebel P (1997) Using the Laser Light Sheet Technique in Combustion Research. Proc. 9th Symp. of AGARD-PEP on Advanced Non-intrusive Instrumentation for Propulsion Engines