Convective Time Delay Research Articles

Investigation on self-excited thermoacoustic instability and emission characteristics of premixed CH4/NH3/air flame

Ammonia is one of the promising carbon-free alternative fuels for low-carbon power generation. This study investigated the self-excited thermoacoustic instability of premixed CH4/NH3/air flame in a swirl burner. The work aims to determine the effects of ammonia mole fraction (χNH3), equivalence ratio (Ф) and combustion chamber length (CL) on flame morphology and self-excited flame dynamics. In addition, the emission characteristics of CH4/NH3/air flame were also investigated. Results show that the thermoacoustic instability of the 1/4 wave mode is excited when 0.8≤Ф≤1.2 and χNH3 ≤ 0.3. There are apparent pressure fluctuations and heat release fluctuations in the combustion chamber. The fluctuating frequency decreases with increasing CL. The maximum fluctuating frequency and oscillation intensity appear near Ф=1.0. When χNH3 increases, the amplitude of fluctuating pressure and heat release rate decrease obviously, and the instability range narrows to Ф=1.0–1.1 simultaneously. The low-order acoustic network model successfully predicted the instability transition and frequency decreasing trend caused by increasing ammonia. The increase in the convective time delay is responsible for this transition. Emission analysis indicates that NOx emission can be controlled within 100 ppm under fuel-rich conditions, but the CO emission increases sharply. Thermoacoustic instability and nitrogen oxides can be simultaneously controlled under fuel-rich conditions when χNH3>0.3. However, this will be limited by unburned reactants.

Experimental study on effects of ammonia enrichment on the thermoacoustic instability of lean premixed swirling methane flames

Ammonia (NH3) has recently emerged as a promising carbon-free energy carrier. Further development and application of NH3 as fuel in the gas turbine industry can significantly reduce the emissions of carbon dioxide (CO2) and contribute to the achievement of a carbon–neutral society. This study experimentally examined the thermoacoustic instability characteristics of a laboratory-scaled lean premixed gas turbine model combustor operated with different NH3 blending ratios with methane (CH4). Experiments conducted under a wide range of inlet velocities and equivalence ratios suggest that NH3 concentration is critical in determining the characteristics of the instability. Specifically, when the NH3 proportion is less than 50 %, the addition of NH3 causes a mode transition of the instability. However, when the content of NH3 is greater than 50 %, it is shown that the instabilities are suppressed, indicating that the addition of a certain amount of NH3 can enhance the stability of CH4 flames. Additional analysis of flame dynamics reveals that the introduction of NH3 causes the lengthening of the flame front and weakens heat release rate fluctuations in the flame root regions. Further Proper Orthogonal Decomposition (POD) analysis of the flow field shows that the instability modes are strongly coupled with periodic vortex motions of the flow dynamics along the shear layers. Finally, the mode shifting phenomena is successfully predicted by low-order thermoacoustic network modeling. It is suggested that the change in convective time delay caused by NH3 addition is responsible for such transitions.

Reactive control of second Mack mode in a supersonic boundary layer with free-stream velocity/density variations

We consider closed-loop control of a two-dimensional supersonic boundary layer at $M=4.5$ that aims at reducing the linear growth of second Mack mode instabilities. These instabilities are first characterized with local spatial and global resolvent analyses, which allow us to refine the control strategy and to select appropriate actuators and sensors. After linear input–output reduced-order models have been identified, multi-criteria structured mixed $H_{2}$ / $H_{\infty }$ synthesis allows us to fix beforehand the controller structure and to minimize appropriate norms of various transfer functions: the $H_{2}$ norm to guarantee performance (reduction of perturbation amplification in nominal condition), and the $H_{\infty }$ norm to maintain performance robustness (with respect to sensor noise) and stability robustness (with respect to uncertain free-stream velocity/density variations). Both feedforward and feedback set-ups, i.e. with estimation sensor placed respectively upstream/downstream of the actuator, allow us to maintain the local perturbation energy below a given threshold over a significant distance downstream of the actuator, even in the case of noisy estimation sensors or free-stream density variations. However, the feedforward set-up becomes completely ineffective when convective time delays are altered by free-stream velocity variations of $\pm$ 5 %, which highlights the strong relevance of the feedback set-up for performance robustness in convectively unstable flows.

An Active Flow Control Approach for Spatially Growing Mixing Layer

Abstract A control approach is proposed in this study that controls the growth of a spatially growing mixing layer in an open loop. Due to the similarity of the mixing layer dynamics in the temporal and spatial domains, the control is exercised in the temporal domain that leads to optimization of the layer in the spatial domain. The proportional-integral-derivative (PID) control in the temporal domain uses Navier–Stokes equations as the plant model. Apart from being prohibitively expensive, control in the spatial domain presents convective time delay problems. These barriers are circumvented by solving the control problem in the temporal domain very rapidly, based on a given set point in the linear regime. Once the set point in the temporal domain is reached, the corresponding initial conditions in the temporal domain are mapped to inflow boundary conditions in the spatial domain. With these mapped inflow boundary conditions in the spatial domain, the spatial development of the mixing layer is followed, which confirms the efficacy of control in the spatial domain. The present control methodology offers a new paradigm for control of the spatially growing mixing layer.

Effect of Spray-Wall Interaction on Thermoacoustic Instability Prediction by Flame Transfer Function and the Convective Time Delay Method

Abstract This study numerically investigates the effect of spray-wall interactions on thermoacoustic instability prediction. The large-eddy simulations (LES)-based flame transfer function (FTF) and the convective time delay methods are used by combining the Helmholtz acoustic solver to predict a single spray flame under the so-called slip and film spray-wall conditions. It is found that considering more realistic film liquid and a wall surface interaction model achieves a more accurate phase lag in both of the time lag evaluations compared to the experimental results. Additionally, the results show that a new time delay exists between the liquid film fluctuation and the unsteady heat release, which explains the larger phase value in the film spray-wall condition than in the slip condition. Moreover, the prediction capability of the FTF framework and the convective time delay methodology in the linear regime is also presented. In general, the instability frequency differences predicted using the FTF framework under the film condition are less than 10 Hz compared with the experimental data. However, an underestimation of the numerical gain value leads to requiring a change in the forcing position and an improvement in the numerical models. Due to the ambiguous definition of the gain value in the convective time delay method, this approach leads to arbitrary and uncertain thermoacoustic instability predictions.

Excitation of the precessing vortex core by active flow control to suppress thermoacoustic instabilities in swirl flames

In this study, we apply periodic flow excitation of the precessing vortex core at the centerbody of a swirl-stabilized combustor to investigate the impact of the precessing vortex core on flame shape, flame dynamics, and especially thermoacoustic instabilities. The current control scheme is based on results from linear stability theory that determine the precessing vortex core as a global hydrodynamic instability with its maximum receptivity to open-loop actuation located near the center of the combustor inlet. The control concept is first validated at isothermal conditions. This is of utmost importance for the proceeding studies that focus on the exclusive impact of the precessing vortex core on the combustion dynamics. Subsequently, the control is applied to reacting conditions considering lean premixed turbulent swirl flames. Considering thermoacoustically stable flames first, it is shown that the actuation locks onto the precessing vortex core when it is naturally present in the flame, which allows the precessing vortex core frequency to be controlled. Moreover, the control allows the precessing vortex core to be excited in conditions where it is naturally suppressed by the flame, which yields a very effective possibility to control the precessing vortex core amplitude. The control is then applied to thermoacoustically unstable conditions. Considering perfectly premixed flames first, it is shown that the precessing vortex core actuation has only a minor effect on the thermoacoustic oscillation amplitude. However, we observe a continuous increase of the thermoacoustic frequency with increasing precessing vortex core amplitude due to an upstream displacement of the mean flame and resulting reduction of the convective time delay. Considering partially premixed flames, the precessing vortex core actuation shows a dramatic reduction of the thermoacoustic oscillation amplitude. In consideration of the perfectly premixed cases, we suspect that this is caused by the precessing vortex core-enhanced mixing of equivalence ratio fluctuations at the flame root and due to a reduction of time delays due to mean flame displacement.

Reactive control of isolated unsteady streaks in a laminar boundary layer

This study is motivated by controlling transient growth and subsequent bypass transition of the laminar boundary layer to turbulence. In experiments employing a model problem, an active roughness element is used to introduce steady/unsteady streak disturbances in a Blasius boundary layer. This tractable arrangement enables a systematic investigation of the evolution of the disturbances and of potential methods to control them in real time. The control strategy utilizes wall-shear-stress sensors, upstream and downstream of a plasma actuator, as inputs to a model-based controller. The controller is designed using empirical input/output data to determine the parameters of simple models, approximating the boundary layer dynamics. The models are used to tune feedforward and feedback controllers. The control effect is examined over a range of roughness-element heights, free stream velocities, feedback sensor positions, unsteady disturbance frequencies and control strategies; and is found to nearly completely cancel the steady-state disturbance at the downstream sensor location. The control of unsteady disturbances exhibits a limited bandwidth of less than 1.3 Hz. However, concurrent modelling demonstrates that substantially higher bandwidth is achievable by improving the feedforward controller and/or optimizing the feedback sensor location. Moreover, the model analysis shows that the difference in the convective time delay of the roughness- and actuator-induced disturbances over the control domain must be known with high accuracy for effective feedforward control. This poses a limitation for control effectiveness in a stochastic environment, such as in bypass transition beneath a turbulent free stream; nonetheless, feedback can remedy some of this limitation.

Coupling of flame geometry and combustion instabilities based on kilohertz formaldehyde PLIF measurements

In the past, flame geometry was found to play an important role in determining when strong pressure fluctuations associated with a combustion instability occur in a gas turbine model combustor. The goals of this study were to use flame surface area as a more accurate way to quantify the flame shape and the heat release rate, in lieu of chemiluminescence; to accurately resolve both the spatial structure and the time history of the heat release fluctuations, and to show that kilohertz formaldehyde planar laser-induced fluorescence provides a new way to achieve these goals. The dual-swirl burner, developed at DLR Stuttgart by Meier et al. was operated using dimethyl ether (DME) to study fluctuations in the flame surface area, flame brush, flame length, and heat release rate. To understand the instability, accurate measurements are needed of the correlation between heat release rate fluctuations and pressure fluctuations. Thus heat release rate must be recorded as a function of time and space. However conventional chemiluminescence offers only a line-of-sight measurement. High-speed CH2O PLIF was applied to study the motion of flame surfaces in response to the pressure oscillations. Results show that the flame surface density fluctuated at the acoustic frequency (typically 320Hz). The frequency of the combustion instability was found to increase as either the gas velocity or the flame speed was increased. These trends are consistent with previous n-τ theories that model the convective time delay. Strong pressure oscillations occur for conditions when the flame is nearly flat; the instability disappears when the flame becomes V-shaped. The explanation is that all points on a flatter flame have the same convective time delay but this is not true for the V-flame geometry.

Assessment of thermoacoustic instabilities in a partially premixed model combustor using URANS approach

The paper presents a numerical study of the mechanisms driving thermoacoustic instabilities in a lean partially premixed combustor in conditions representative of gas turbine combustion systems. Various combustion models and modeling approaches able to predict the onset of thermoacoustic instabilities are examined and applied to the experimental test rig in order to assess their validity. The influence of the imposed acoustic and thermal boundary conditions on characterization of the coupling between heat release rate fluctuations and the acoustic field is investigated. Predicted data is used to improve the understanding of mutual interactions between pressure fluctuations and unsteady heat release in the unstable combustors which play an essential role in characterizing limit-cycle behavior. The mean convective time delay between heat release and the perturbation in the flow is used to determine the stability condition of the combustor. The study shows that heat transfer is important parameter regulating pressure oscillations.

Closed-Loop Flow Control of a Forebody at a High Incidence Angle

The flowfield around an axisymmetric forebody at a high angle of attack () produces a significant side force. This side force results from an asymmetric pressure distribution around the body due to an asymmetric vortex configuration. Numerical studies of open-loop control using mass blowing slots near the tip of the model have shown a proportional response of the side force over a range of momentum coefficient amplitudes. From the open-loop simulations, a prediction-error minimization method was employed to formulate a linear time-invariant model, which captured the dynamics of the side force response to different mass flow rates applied to either the port or starboard actuator. Based on the linear time-invariant model, a proportional–integral control law was developed for set-point tracking a prescribed side force. The development of the linear time-invariant model, and corresponding linear time-invariant feedback solution are presented to illustrate the model’s capabilities and limitations. The ability to track a set-point signal based on the linear time-invariant model and corresponding proportional–integral control law are shown. The results indicate that the bandwidth of the controller is limited to frequencies below the convective frequency due to the convective time delay. Finally, the linear time-invariant feedback solutions are compared to Navier–Stokes feedback simulations, which show very good agreement.

Amplitude-Dependent Flow Field and Flame Response to Axial and Tangential Velocity Fluctuations

The current paper investigates the nonlinear interaction of the flow field and the unsteady heat release rate and the role of swirl fluctuations. The test rig consists of a generic swirl-stabilized combustor fed with natural gas and equipped with a high-amplitude forcing device. The influence of the phase between axial and azimuthal velocity oscillations is assessed on the basis of the amplitude and phase relations between the velocity fluctuations at the inlet and the outlet of the burner. These relations are determined in the experiment with the multimicrophone-method and a two component laser Doppler velocimeter (LDV). Particle image velocimetry (PIV) and OH*-chemiluminescence measurements are conducted to study the interaction between the flow field and the flame. For several frequency regimes, characteristic properties of the forced flow field and flame are identified, and a strong amplitude dependence is observed. It is found that the convective time delay between the swirl generator and the flame has an important influence on swirl-number oscillations and the flame dynamics in the low-frequency regime. For mid and high frequencies, significant changes in the mean flow field and the mean flame position are identified for high forcing amplitudes. These affect the interaction between coherent structures and the flame and are suggested to be responsible for the saturation in the flame response at high forcing amplitudes.

The combined dynamics of swirler and turbulent premixed swirling flames

The dynamics of premixed confined swirling flames is investigated by examining their response to incident velocity perturbations. A generalized transfer function designated as the flame describing function (FDF) is determined by sweeping a frequency range extending from 0 to 400 Hz and by changing the root mean square fluctuation level between 0% and 72% of the bulk velocity. The unsteady heat release rate is deduced from the emission intensity of OH * radicals. This global information is complemented by phase conditioned Abel transformed emission images. This processing yields the distribution of light emission. By assuming that the light intensity is proportional to the heat release rate, it is possible to deduce the distribution of unsteady heat release rate in W m −3 and see how it evolves with time during the modulation cycle and for different forcing frequencies. These data can be useful for the determination of regimes of instability but also give clues on the mechanisms which control the swirling flame dynamics. It is found from experiments and demonstrated analytically that a swirler submitted to axial acoustic waves originating from the upstream manifold generates a vorticity wave on its downstream side. The flame is then submitted to a transmitted axial acoustic perturbation which propagates at the speed of sound and to an azimuthal velocity perturbation which is convected at the flow velocity. The net result is that the dynamical response and unsteady heat release rate are determined by the combined effects of these axial and induced azimuthal velocity perturbations. The former disturbance induces a shedding of vortices from the injector lip which roll-up the flame extremity while the latter effectively perturbs the swirl number which results in an angular oscillation of the flame root. This motion is equivalent to that which would be induced by perturbations of the burning velocity. The phase between incident perturbations is controlled by the convective time delay between the swirler and the injector. The constructive or destructive interference between the different perturbations is shown to yield the low and high gains observed for certain frequencies.

Modeling and experimental validation of unsteady impinging flames

This study reports on a joint experimental and analytical study of premixed laminar flames impinging onto a plate at controlled temperature, with special emphasis on the study of periodically oscillating flames. Six types of flame structures were found, based on parametric variations of nozzle-to-plate distance ( H), jet velocity ( U), and equivalence ratio ( ϕ). They were classified as conical, envelope, disc, cool central core, ring, and side-lifted flames. Of these, the disc, cool central core, and envelope flames were found to oscillate periodically, with frequency and sound pressure levels increasing with Re and decreasing with nozzle-to-plate distance. The unsteady behavior of these flames was modeled using the formulation derived by Durox et al. [D. Durox, T. Schuller, S. Candel, Proc. Combust. Inst. 29 (2002) 69–75] for the cool central core flames where the convergent burner acts as a Helmholtz resonator, driven by an external pressure fluctuation dependent on a velocity fluctuation at the burner mouth after a convective time delay τ. Based on this model, the present work shows that τ = Re [ 2 j tanh −1 ( 2 δ ω + ( 1 + N ) j ω 2 − j ω 0 2 2 δ ω + ( 1 − N ) j ω 2 − j ω 0 2 ) ] + 2 π K ω , i.e., there is a relation between oscillation frequency ( ω), burner acoustic characteristics ( ω 0 , δ ), and time delay τ, not explicitly dependent on N, the flame-flow normalized interaction coefficient [D. Durox, T. Schuller, S. Candel, Proc. Combust. Inst. 29 (2002) 69–75], because ∂ τ / ∂ N = 0 . Based on flame motion and noise analysis, K was found to physically represent the integer number of perturbations on flame surface or number of coherent structures on impinging jet. Additionally, assuming that τ = β H U , where H is the nozzle-to-plate distance and U is the mean jet velocity, it is shown that β Disc = 1.8 , β CCC = 1.03 , and β Env = 1.0 . A physical analysis of the proportionality constant β showed that for the disc flames, τ corresponds to the ratio between H and the velocity of the coherent structures. In the case of envelope and cool central core flames, τ corresponds to the ratio between H and the mean jet velocity. The predicted frequency fits the experimental data, supporting the validity of the mathematical modeling, empirical formulation, and assumptions made.

Experimental Study on the Role of Entropy Waves in Low-Frequency Oscillations in a RQL Combustor

“Rumble” is a self-excited combustion instability, usually occurring at the start-up of aero-engines with fuel-spray atomizers at sub-idle and idle conditions, and exhibiting low limit frequencies in the range of 50Hzto150Hz. Entropy waves at the (nearly) choked combustor outlet are supposed to be the key feedback mechanism for the observed self-excited pressure oscillations. The experimental study presented here aims at clarifying the role of entropy waves in the occurrence of rumble. A generic air-blast atomizer with a design prone to self-excitation has been incorporated into a thermoacoustic combustor test rig with variable outlet conditions. The thermoacoustic response of the flame was characterized by recording the OH* chemiluminescence, the dynamic pressures, the dynamic temperatures, and by applying PIV. The measurements have shown the occurrence of periodic hot spots traveling with the mean flow with considerable dispersion. Measurements have been conducted with an open-ended resonance tube in order to eliminate the impact of entropy waves on the mechanism of self-excitation. The oscillation obtained, comparable in amplitude and frequency, proved that self-excitation primarily depends on convective time delays of the droplets in the primary zone and thus on the atomization characteristics of the nozzle.

Swirl control of combustion instabilities in a gas turbine combustor

The impact of premixer swirl number, S , and overall fuel equivalence ratio, Φ , on the stability of a model swirl-stabilized, lean-premixed gas turbine combustor has been numerically simulated using large-eddy simulations methodology. Through the use of a premixed flamelet model ( G equation), unsteady vortexflame and acoustic-flame interactions are captured. It is shown that for large values of S , that is, those sufficiently highly for vortex breakdown to occur, the fluctuating pressure amplitudes, p ′, are attenuated significantly (over 6.6 dB reduction). The reduced p ′ amplitudes are accompanied by reduced longitudinal flame-front oscillations and reduced coherence in the shed vortices. Similar p ′ reduction levels are achieved through changes in Φ . Compared to the leanest equivalence ratio simulated ( Φ =0.52), p ′ at stoichiometric is reduced by 6.0 dB. The response of the combustion process to explicit swirl modulation is also investigated. Open-loop control through swirl variation is demonstrated for a lean mixture with significant reductions in fluctuating mass flow rate and p ′ after a convective time delay.