In many continuous combustion processes, such as those found in aeroengines or gas turbines, the flame is stabilized by a swirling flow formed by aerodynamic swirlers. The dynamics of such swirling flames is of technical and fundamental interest. This article reviews progress in this field and begins with a discussion of the swirl number, a parameter that plays a central role in the definition of the flow structure and its response to incoming disturbances. Interaction between the swirler response and incoming acoustic perturbations generates a vorticity wave convected by the flow, which is accompanied by azimuthal velocity fluctuations. Axial and azimuthal velocities in turn define the flame response in terms of heat--release rate fluctuations. The nonlinear response of swirling flames to incoming disturbances is conveniently represented with a flame describing function (FDF), in other words, with a family of transfer functions depending on frequency and incident axial velocity amplitudes. The FDF, howev...

Dynamics of Swirling Flames

Transverse combustion instabilities: Acoustic, fluid mechanic, and flame processes

A review of cavity-based trapped vortex, ultra-compact, high-g, inter-turbine combustors

Nonlinear interaction between a precessing vortex core and acoustic oscillations in a turbulent swirling flame

A review of active control approaches in stabilizing combustion systems in aerospace industry

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 © 2008 by ASME

http://berndnoack.com/publications/Lacarelle2009asmej.PDF

Spatiotemporal Characterization of a Conical Swirler Flow Field Under Strong Forcing

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 © 2010 by ASME and Alstom Technology, Ltd.

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

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 stabi...

/pdf/time-domain-modelling-and-stability-analysis-of-complex-8igxatkzix.pdf

Time domain modelling and stability analysis of complex thermoacoustic systems

Jets in crossflow are widely used in the industry for homogenization or cooling tasks. Recently, pulsating jets have been investigated as a mean to increase the scalar mixing efficiency of such configurations, whether for a single jet or for an array of jets. To avoid the disadvantages of mechanically actuated flows (costs, maintenance), a new injector based on a fluidics oscillator has been designed. Four injectors have been implemented in a generical jet in crossflow configuration and the mixing efficiency of the setup was compared with the one of the same setup equiped with standard non oscillating jets. With help of high-speed concentration measurement technique, the scalar mixing quality of both setups was measured at three positions downstream of the injection plane. In all the cases tested, the fluidics injectors present a better temporal homogenization, characterized with the Danckwerts unmixedness criterion, than the standard jets. For a defined mixing quality, a decrease of the mixing length by approximately 50% can be achieved with the fluidics injectors. Furthermore, the new injectors exhibit a mixing quality which is less sensitive to variations of the jet to crossflow momentum. The flapping motion of the fluidics injectors induces a wider azimuthal spreading of the fluidics jets immediately downstream of the injection location. This increases the macro- and micromixing phenomea which lead then to the high gains in mixing quality. It is thus demonstrated that fluidics oscillators present a strong potential to improve the passive scalar homogenization of jet in crossflow configurations.Copyright © 2011 by ASME

Increasing the Passive Scalar Mixing Quality of Jets in Crossflow With Fluidics Actuators

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 © 2007 by ASME.

Arnaud Lacarelle

Papers

Spatiotemporal Characterization of a Conical Swirler Flow Field Under Strong Forcing

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

Time domain modelling and stability analysis of complex thermoacoustic systems

Increasing the Passive Scalar Mixing Quality of Jets in Crossflow With Fluidics Actuators

Study of the Vortex Breakdown in a Conical Swirler Using LDV, LES and POD