Arnaud Lacarelle

Spatio-Temporal Characterization of a Conical Swirler Flow Field Under Strong Forcing

In this study, a spatio-temporal characterization of forced and unforced flows of a conical swirler is done based on Particle Image Velocimetry (PIV) and Laser Doppler Anemometry (LDA). The measurements are performed at a Reynolds number of 33,000 and a swirl number of 0.71. Axisymmetric forcing is applied to approximate the effects of thermoacoustic instabilities on the flow field at the burner inlet and outlet. The actuation frequencies are set at the natural flow frequency (Strouhal number Stf ≈ 0.92) and two higher frequencies (Stf ≈ 1.3 and 1.55) that are not harmonically related. Phase locked and phase averaged measurement are used as a first step to visualize the coherent flow structures. Secondly, Proper Orthogonal Decomposition (POD) is applied to the PIV data to characterize the effect of the actuation on the fluctuating flow. Measurements indicate a typical natural flow instability of helical nature in the unforced case. The associated induced pressure and flow oscillations travel upstream to the swirler inlet where generally fuel is injected. This observation is of critical importance with respect to the stability of the combustion. Harmonic actuation at different frequencies and amplitudes affects the mean-field profile most at the outlet, while the coherent velocity fluctuations are strongly influenced at both inlet and outlet. On one hand, the dominant helical mode is replaced by an axisymmetric vortex ring if the flow is forced at the natural flow frequency. On the other hand, the natural flow frequency prevails at the outlet under forcing at higher frequencies and POD analysis indicates that the helical structure is still present. The presented results give new insight into the flow dynamics of a swirling flow burner under strong forcing.Copyright

Spatiotemporal Characterization of a Conical Swirler Flow Field Under Strong Forcing

In this study, a spatiotemporal characterization of forced and unforced flows of a conical swirler is performed based on particle image velocimetry (PIV) and laser Doppler anemometry (LDA). The measurements are performed at a Reynolds number of 33,000 and a swirl number of 0.71. Axisymmetric forcing is applied to approximate the effects of thermoacoustic instabilities on the flow field at the burner inlet and outlet. The actuation frequencies are set at the natural flow frequency (Strouhal number St f ≈0.92) and two higher frequencies (St f ≈1.3 and 1.55) that are not harmonically related to the natural frequency. Phase-averaged measurement are used as a first step to visualize the coherent flow structures. Second, proper orthogonal decomposition (POD) is applied to the PIV data to characterize the effect of the actuation on the fluctuating flow. Measurements indicate a typical natural flow instability of helical nature in the unforced case. The associated induced pressure and flow oscillations travel upstream to the swirler inlet where generally fuel is injected. This observation is of critical importance with respect to the stability of the combustion. Harmonic actuation at different frequencies and amplitudes does not affect the mean velocity profile at the outlet, while the coherent velocity fluctuations are strongly influenced at both the inlet and outlet. On one hand, the dominant helical mode is replaced by an axisymmetric vortex ring if the flow is forced at the natural flow frequency. On the other hand, the natural flow frequency prevails at the outlet under forcing at higher frequencies and POD analysis indicates that the helical structure is still present. The presented results give new insight into the flow dynamics of a swirling flow burner under strong forcing.

Time domain modelling and stability analysis of complex thermoacoustic systems

Abstract A methodology allowing for a modular setup of complex acoustic systems is developed. The transfer behaviour of the individual subsystems is formulated in time domain. Subsystem descriptions can be obtained by analytical considerations, numerical methods, or experimental data. Once the complex subsystems have been characterized experimentally, changes in system geometry can be implemented easily by exchanging or adding subsystems. To validate the modelling approach, experiments are conducted in an acoustic test rig with a combustor-type geometry. Results are compared to predictions from the model, demonstrating accuracy in frequency and time domain. Application to thermoacoustic instabilities arising in lean-premixed combustion is given. The influence of a modified fuel distribution on an unstable operating point of a lean-premixed combustor is studied and validated with experimental data. Additionally, a study on the parameters governing the flame transfer function is performed to generate a stability map of a model combustor. An advantage of the state-space approach is that stability of a thermoacoustic system can be determined by simply solving a matrix eigenvalue problem. This is in strong contrast to the traditional approach, where the complete model is formulated in frequency domain with infinite-dimensional transfer functions. The time domain approach is based on the methodology presented by Schuermans et al. [1]. In contrast to their work, however, subsystems are not obtained from modal expansions but are characterized by using system identification techniques. Additionally, accuracy of the time domain model is verified by experiments.

Instability Control by Premixed Pilot Flames

Premixed flames of swirl-stabilized combustors (displaced half-cone) are susceptible to thermo-acoustic instabilities, which should be avoided under all operating conditions in order to guarantee a long service life for both stationary and aircraft gas turbines. The source of this unstable flame behavior can be found in a transition of the premix flame structure between two stationary conditions that can be easily excited by fuel fluctuations, coherent structures within the flow, and other mechanisms. Pilot flames can alleviate this issue either by improving the dynamic stability directly or by sustaining the main combustion process at operating points where instabilities are unlikely. In the present study, the impact of two different premixed pilot injections on the combustion stability is investigated. One of the pilot injector (pilot flame injector) was located upstream of the recirculation zone at the apex of the burner. The second one was a pilot ring placed at the burner outlet on the dump plane. A noticeable feature of the pilot injector was that an ignition device allowed for creating pilot premixed flames. The present investigation showed that these premixed pilot flames were able to suppress instabilities over a wider fuellair ratio range than the conventional premixed pilot injection alone. Furthermore, it was possible to prevent instabilities and maintain the flame burning near the lean blowout when a percentage of the fuel was premixed with air and injected through the pilot ring. NO x emissions were significantly reduced.

CH*/OH* Chemiluminescence Response of an Atmospheric Premixed Flame Under Varying Operating Conditions

In this work the relationship between the ratio of the global CH* and OH* flame chemiluminescece and the global equivalence ratio of a technically premixed swirl-stabilized flame is investigated. The burner allows for a modification of the premix fuel injection pattern. The global flame chemiluminescence is monitored by a high-sensitivity light spectrometer and multiple photo-multipliers. The photo-multipliers were equipped with narrow optical band-pass filters and recorded the flame’s OH*, CH* and CO2* chemiluminescence intensity. To ensure an approximately uniform equivalence ratio distribution in the combustion zone, the spatial OH* and CH* flame chemiluminescence was recorded simultaneously with one ICCD camera using a special optical setup, which incorporated among other things one fully reflective and one semi-reflective mirror and appropriate optical filters. The flame chemiluminescence intensity was mapped for a range of equivalence ratios and air mass flows. The mapping shows that (as stated for perfectly premixed flames in the literature) the OH*, CH* and CO2* intensity of the investigated flame depends linearly on the air mass flow and exponentially on the equivalence ratio (i.e., I = km * φβ ). Hence for the investigated operating conditions (i.e., quasi premix conditions) the global CH*/OH* intensity can be employed as a measure of the global equivalence ratio for the operating conditions investigated in this work. However, the contribution of broadband CO2* chemiluminescence in the wave length range of CH* chemiluminescence has to be accounted for.Copyright

Combination of Image Postprocessing Tools to Identify Coherent Structures of Premixed Flames

A combination of postprocessing tools of OH*-chemiluminescence snapshots is used to characterize the coherent structures of two types of premixed burners: a bluff body and an industrial swirl burner. Two methods are combined to extract the structures: a phase-averaging algorithm and the proper orthogonal decomposition. The first method is based on the estimation of the instantaneous phase of the snapshots relative to a (local) time-resolved signal. A phase-sorting-phase-averaging algorithm then reconstructs the evolution of the flame at a chosen frequency over one cycle. The proper orthogonal decomposition method is used as a filter to smoothen the snapshots. Both methods provide insight into the physical mechanisms of coherent structures in the two premixed flames under consideration. The snapshots of the bluff-body combustion exhibit a symmetric structure. This indicates that the von Karman vortex street in the cold flow is suppressed by the addition of heat in the shear layer. Three coexisting flame structures of the swirl burner in the combustion chamber could be identified: a natural helical structure of the burner and two axisymmetric modes. Increasing the amplitude of acoustic forcing at the natural flow frequency changes the helical structure to an axisymmetric one.

Study of the vortex breakdown in a conical swirler using LDV, LES and POD

Modeling and understanding the vortex breakdown is a key issue of modern Lean Premixed Combustors. The main difficulty of the problem is the unsteady behavior of this type of flow: Large structures resulting from vortex breakdown and the swirling shear-layers, affect directly the flame stabilization leading to heat-release fluctuations and combustion instabilities. Consequently, one needs to capture and understand turbulent coherent structures dynamics for designing efficient burners. This task is particularly challenging since it deals with capturing coherent motions within a chaotic system and should be done using state-of-the art numerical and experimental techniques. The present work focuses on the experimental and numerical study of iso-thermal vortex breakdown in a conical swirler. Experimental investigations were performed with 2D Laser Doppler Velocimetry (LDV) and Hotwire Anemometry at the outlet of the combustor model. Averaged velocity fields and RMS values are showing a strong central recirculation zone. In addition, characteristic frequencies of the flow have been exhibited showing the strong influence of large scale turbulent fluctuation on the flow pattern. These measurements showed also the impact of different outlet geometries on the strength and position of the coherent structures of the flow. Further, Large Eddy Simulation (LES) has been used to obtain a 4D description of the flow. Comparison with LDV profiles showed a good agreement, indicating that the LES tool captures accurately the flow. The LES results were then processed for capturing and identifying the coherent structures. Firstly, characteristic frequencies were analyzed. Here also a good agreement with the experimental data was achieved. Secondly the cores of the vortices were visualized providing a good insight into the unsteady flow pattern. Finally, Proper Orthogonal Decomposition (POD) was applied to the 4D field in order to identify the contribution of different large scale fluctuation modes. The presence of the Processing Vortex Core (PVC) corresponding to a pair of helical structures was captured. Copyright

Planar Investigation of Outlet Boundary Conditions Effect on Isothermal Flow Fields of a Swirl-Stabilized Burner

The swirling flow velocity profiles can be strongly influenced by the outlet conditions of the combustion chamber especially at subcritical flow conditions. The effect of such changes on the mean flow or coherent structures is still unclear. It is investigated in the present work in an industrial swirl inducing burner in cold flow conditions with help of PIV. Proper orthogonal decomposition (POD) as well as acoustic measurements were used to characterize the coherent structures shed from the burner mouth. The combustor length (670, and 2020mm) and the outlet area contraction ratio (1, 0.56, 0.27, and 0.09) are varied. Major changes in the flow field are achieved when using a short combustor and the smallest contraction ratio. For this case, a central jet with streamwise velocity is added to the typical central recirculation zone. The POD analysis of the contraction ratios 1 and 0.09 for the long combustor shows that the first helical mode as well as Kelvin Helmholtz vortices are present with minor changes for both cases. At a contraction ratio of 0.09, some new structures at the jet location and near the combustor wall appear.Copyright

Effect of Fuel/Air Mixing on NOx Emissions and Stability in a Gas Premixed Combustion System

Understanding the mixing properties in lean premixed combustors is of critical importance to realize low NO x emissions and stable combustion over a wide range of operating conditions. This goal can be partially achieved if spatial and temporal homogeneity of the fuel/air distribution is ensured and if the mixing profile remains less sensible to perturbations of the flow field. Fuel/air oscillations are one of the mechanisms leading to thermoacoustic instabilities and should be minimized. In this paper, experimental techniques (Laser Induced Fluorescence, Particle Image Velocimetry, flame transfer function measurements) are used to investigate the response of different mixing profiles of a lean premixed swirl burner to simulated acoustic forcing of different amplitudes. Results for reacting and cold flow investigations are presented. The flame transfer function appeared to be the most reliable tool to predict the impact of the mixing profile on system stability, but the cold flow investigations gave additional information on the mixing mechanisms at the burner outlet.

A Quantitative Link Between Cold-Flow Scalar Unmixedness and NOx Emissions in a Conical Premixed Burner

The feasibility of using cold flow measurements in a conical swirl-inducing burner to predict the fuel/air mixture probability density function (PDF) at the flame location in a staged premixed swirl-inducing burner is discussed in the present work. Particle Image Velocity (PIV) measurements are used to investigate the impact of the flame on the mean and coherent velocity field upstream of the premixed flame of a conical swirl-inducing burner. When the flame does not anchor inside the burner, a good agreement between the reacting and the cold-flow fields is ensured. The scalar mixing field, dominated by coherent concentration fluctuations, is thus marginally affected by the flame. In this case, a correction of the mixture PDF, recorded in a water test rig at the burner outlet with planar laser-induced fluorescence (PLIF), can be used to estimate the mixture PDF at the flame location. This correction is based on the exponential decay of the mixture variance associated with the flame-position information of the reacting flow. An experimental curve fitting and a chemical-reactor network model confirm that the resulting PDF approximates the real unmixedness at the flame better than the measured mixture PDF at the burner outlet. When the flame is anchored inside the burner, the correction approach does not apply anymore, due to the strong flow field changes. The methodology presented allows to quantitatively predict the mixture PDF at the flame location for different total powers, preheating temperatures, and equivalence ratios. The simple mixing model and reactor network model are able to satisfyingly capture the NOx emissions when the flame stabilizes completely downstream of the burner.Copyright