Phase Of Flame Transfer Function Research Articles

Experimental Study of the Dynamics of Lean Premixed Hydrogen Flames in a Multi Jet Combustor

Abstract In this study, the flame dynamics of lean premixed hydrogen jet flames are experimentally investigated. Acoustic and optical measurements are used to capture the response of a bundle of jet flames to acoustic forcing. Using helium as a fuel surrogate, we simulate the change in acoustic properties in the burner during the determination of cold burner transfer matrix measurements. We investigate the influence of the equivalence ratio and the addition of methane as well as the interaction of the individual flames to evaluate the scalability of the results to systems with more flames. It is shown that the changes in the dynamic flame response can primarily be explained by the flame length, which changes both with the methane share and with the equivalence ratio. It can be observed that with small changes in the equivalence ratio, the flame length and the flame transfer function (FTF) change in the same way as with a small change in gas composition. To assess the scalability of these results, we deactivate some of the jet flames and analyze how the overall response to acoustic forcing changes. We find that the FTF phase is not affected by the number of active flames. Analyzing the respective gain values, significantly stronger responses are measured for a few flames, but only small difference can be measured above a certain number of neighboring flames so that the lab scale results can also be expected to be valid for industrial configurations with a high number of flames.

The effect of flame curvature and flame base movement on the frequency response of a conical Bunsen flame

We create a general model for the tracking of a premixed flame sheet. This model is based on a Lagrangian formulation of the interface motion, which is more computationally efficient than the level-set formulation solved on a grid. We apply this model to a conical Bunsen flame in which flame-base oscillations and flame stretch effects are also considered. We demonstrate that the flame-front dynamics, the heat release rate perturbations, and the flame transfer function (FTF) are affected by three features: (i) the velocity field locally perturbing the flame-front, (ii) the flame-base oscillations normal to the steady flame-front propagating along the flame and (iii) the flame-base oscillations tangential to the flame-front reducing and increasing the flame-length. We provide a physical explanation for the saturation of the FTF phase shift with increasing forcing frequency, which is found experimentally. We show why a n−τ model with fixed τ cannot model this simple flame and that, when building a quantitatively-accurate model of a thermoacoustic system containing this flame, the flame dynamics need to be simulated directly or modelled more accurately than with the n−τ model. The flame model used in this paper has been designed to be differentiable so that it can be used for rapid data assimilation using Laplace’s method. Code is provided so that the user can generate the FTF of a conical premixed flame with any set of parameters and any flame-base trajectory.

On the flame transfer function models for laminar premixed conical and V- flames considering the stretch effect

This paper investigates a predictive model that considers the impact of stretch on the dynamic response of laminar premixed conical and V- flames; the flame stretch consists of two components: the flame curvature and flow strain. The steady and perturbed flame fronts are determined via the linearized G-equation associated with the flame stretch model. Parameter analyses of the effects of Markstein length L, flame radius R and unstretched flame aspect ratio β are also conducted. Results show that the flame stretch reduces the steady flame height, with this effect being more significant for larger Markstein lengths and smaller flame sizes. The effects of flame stretch on perturbed flames are evaluated by comparing the flame transfer function (FTF) considering the flame stretch and not. For flames of different sizes, the impact of flame stretch on FTF gain can be divided into three regions. When both β and R are relatively small, due to the decrease in steady flame height and the impact of flow strain, the FTF gain increases. As β and R gradually increase, the FTF gain of the conical flame oscillates periodically while the FTF gain of the V-flame decreases, primarily due to the flame curvature enhancing the flame front disturbance and the wrinkle counteracting effect. When β and R are large, a disruption in wrinkle counteracting effect ensues, leading to a significant increase in FTF gain. Furthermore, as the actual flame height is reduced, the flame stretch also reduces the FTF phase lag which is related to the disturbance propagation time from the flame root to the tip.

Influence of pilot H[formula omitted] injection on methane-air swirled flame stabilization and acoustic response

Large Eddy Simulation is used to investigate the effect of localized pilot H2 injection on the Flame Transfer Function (FTF) of a premixed CH4-air swirled flame. The response of a perfectly premixed methane-air flame is compared to the one with an additional pilot hydrogen injection that supplies 10% of the original power. Numerical simulations are validated against experiments in terms of global FTF values at selected forcing frequencies, acoustic pressure and velocity signals, CH* flame images and flame root position dynamics. The unforced cases are first considered showing that, when H2 is injected in the center, the flame becomes slightly lifted with a localized diffusion front above the pilot injection. As in the experiments, LES retrieves that hydrogen pilot injection leads to a global redistribution of the heat release rate towards the flame root due to higher burning rates which translates to an overall FTF gain reduction over the entire frequency range explored. Then, the flame acoustic response for the two injection strategies is scrutinized at two distinct forcing frequencies: 240 Hz where the FTF gain difference is maximum, and 590 Hz where the FTF phase shift is maximum. LES reveals that, despite H2 pilot injection does not modify the dynamics of the large vortical structures shed in the external shear layer of the swiling jet which are synchronized by the acoustic forcing, the redistribution of the heat release rate towards the flame base weakens their interaction with the flame tip, explaining partly the FTF gain reduction at the two selected frequencies. In addition to that, a marked axial movement of the internal recirculation zone is observed at 240 Hz during the forcing cycle. For the pilot injection, it leads to an oscillation of the lifted flame root while, for the CH4-air case, the flame anchoring point is not affected. This additional oscillation leads for the pilot case to heat release rate fluctuations acting in phase opposition with respect to those observed at the flame tip, generating a further drop of the FTF gain at this specific frequency. The increased burning rate at the flame root and the flame length reduction of the pilot hydrogen flame also affect the characteristic time lag of the flame response. For both frequencies f = 240 Hz and 590 Hz, the phase shift between the two injection strategies is proportional to the flame length reduction caused by hydrogen injection. These simulations confirm that pilot hydrogen injection is an efficient way to reduce the acoustic response of swirled flames over a large frequency bandwidth.

Effects of hydrogen addition on the forced response of H2/CH4 flames in a dual-nozzle swirl-stabilized combustor

The effects of hydrogen addition on the forced response of H2/CH4 flames are analyzed in a dual-nozzle swirl-stabilized combustor. The hydrogen volumetric content in the fuel is varied from 0% to 40%. Flame transfer function (FTF) is used to compare the forced response of the flames. The FTF gain featuring the local maximum and minimum values, which occurred commonly in the FTFs under all hydrogen contents, is determined by two different mechanisms: the change in the flame angle and the flame roll-up phenomenon. Among two mechanisms, the flame roll-up phenomenon has a more important role in determining the FTF characteristics. In addition, hydrogen addition attenuates the local maximum gains and decreases the FTF phase slope. The change in the flame roll-up behavior, which is induced by a short and compact flame distribution at high hydrogen contents, is the primary reason of these differences in the FTF.

Effect of fuel line acoustics on the flame dynamics of H2/CH4 syngas in a partially premixed gas turbine combustor

This paper investigates the dynamic response of flames, considering H2-enriched CH4 fuels that are submitted to upstream mechanical perturbations in a model gas turbine combustor, resulting in fuel velocity oscillations. Flame transfer function (FTF) and cold-flow transfer function (CTF), which represent, respectively, the dynamic responses in the flow of the flame and fuel induced by a velocity modulation in the fuel feed line (FFL), are derived. The gain of FTF shows a low-pass behavior and gradually increases until a specific modulation frequency is reached, which is identical to the frequencies of the corresponding peak gains of the CTF. The periodic variation of OH* intensity is observed from the images collected with a high-speed OH-PLIF. The high oscillation near the injector indicates that the fuel velocity perturbation leads to the amplification of the heat release fluctuation at specific modulation frequencies. The singularity frequencies corresponding to the peaks of the gain of both transfer functions coincide with the resonance frequencies in the FFL. The phase of FTF and CTF decreases as the H2 ratio increases and the phase of FTF is well fitted into a single line by nondimensionalization through Strouhal number. Most of these findings on the characteristics of combustion instability and acoustical characteristics of FFL can be used as a crucial methodology of H2 combustor design, such as an anti-resonance fuel/air feed line, which can ensure the stable operation of combustion system by avoiding H2 induced combustion instability failure.

Relationship of gain and phase in the transfer function of swirling flames

Gain and phase are two key characteristics of flame transfer function (FTF) in evaluating unsteady flame response. Few studies have investigated the correlation between gain and phase of FTF compared to the extensive literature on the gain characteristics. In this study, we measured the gain and phase of acoustic-excited swirling flames with different flow rates, fuels, equivalence ratios, and burner structures on a single nozzle premixed swirl burner. We identified, for the first time, the consistent variation of FTF gain and derivative of phase over a wide range of acoustic frequencies, and revealed an essential linear relationship between the extremal frequencies of gain and phase derivative, demonstrating that the gain and phase of FTF are not independent. We further proposed an analytical decomposition of the periodically oscillated swirling flame in which FTF equals the combined complex vector of all the perturbing mechanisms. The synchronized variation characteristics of the gain and phase derivative for different flames and acoustic excitations can be explained using a two vector model based on vector decomposition. The distinct time lags and angular velocities of the perturbing vectors were determined by the different spatial positions of the flame tip and base. The local extrema of the FTF modulus and phase derivative are caused by the phase interference of the sub-vectors. Multiple complex vectors affecting heat release rate should be considered under high frequency.

Two-step approach for pressure oscillations prediction in gas turbine combustion chambers

Currently, gas turbine manufacturers frequently face the problem of strong acoustic combustion-driven oscillations inside combustion chambers. These combustion instabilities can cause extensive wear and sometimes even catastrophic damage of combustion hardware. This requires prevention of combustion instabilities, which, in turn, requires reliable and fast predictive tools. We have developed a two-step method to find a set of operating parameters under which gas turbines can be operated without going into self-excited pressure oscillations. As the first step, an unsteady Reynolds-averaged Navier–Stokes simulation with the flame speed closure model implemented in the OpenFOAM® environment is performed to obtain the flame transfer function of the combustion set-up. As the second step time-domain simulations employing low-order network model implemented in Simulink® are executed. In this work, we apply the proposed method to the Beschaufelter RingSpalt test rig developed at the Technische Universität München. The sensitivity of thermoacoustic stability to the length of a combustion chamber, flame position, gain and phase of flame transfer function and outlet reflection coefficient are studied.

Effects of convection time on the high harmonic combustion instability in a partially premixed combustor

The fuel composition effects of H2/CH4 syngas in a partially premixed model combustor (PP-MC) were examined for the unique phenomenon of combustion instability (CI) frequency/mode shifting (FMS), which is a transition of mode as well as frequency. The increase in the H2 composition of the fuel altered FMS from a longitudinal fundamental mode (≈250Hz) to a 7th harmonic mode (≈1750Hz). The cause and characteristics of this FMS were investigated using OH planar laser-induced fluorescence (OH-PLIF) measured at 10Hz, particle-image velocimetry (PIV), and the flame transfer function (FTF).The convection time (τconv) was assumed to be the key parameter of the FMS. Thus, tests were conducted to determine the air flow rate (V˙air) and equivalence ratio (φ) variation, which are vital parameters of the τconv in terms of the flame length and mixing time. The φ variation caused obvious changes in the flame length and instability frequency/mode, while the V˙air variation did not. The τconv was analyzed by calculating the global convection time (τconv_global) and the real convection time (τconv_real) from the length of the OH-PLIF-based unburned mixture length divided by the averaged mixture nozzle exit velocity. The τconv_real was calculated from the integral of the real velocity determined from PIV. Both calculations showed an inverse correlation between τconv and CI frequency, which particularly signifies that the FMS is controllable and a specific mode of CI can be generated by adjusting the τconv. The FTF was measured to determine the intrinsic characteristics of the flame. The FTF phase was normalized by the Strouhal number (St) and identified a direct relationship between FTF gain and τconv variation.In conclusion, the τconv is the main reason for the FMS. The importance of τconv in understanding the CI characteristics was confirmed in a PP-MC using high H2 fuels.

화염전달함수의 위상차를 이용한 시간지연 분석

Main purpose of the current paper is to show results of time lag analysis using phase information of flame transfer function in order to predict combustion instabilities in a gas turbine combustor. The flame transfer function (FTF) is modeled using a commercial Computational Fluid Dynamics (CFD) code (Fluent). Comparisons of the modeled flame shapes with the measured ones were made using the optimized heat transfer conditions and combustion models. The FTF modeling results show a quite good agreement with the measurement data in predicting the phase delay (i.e. time lag). Time lag analysis results using the phase of FTF shows better combustion instability prediction accuracy than using time lag calculated from the steady state flame length.

Impact of flame base dynamics on the non-linear frequency response of conical flames

The response of laminar premixed conical flames to velocity disturbances is considered theoretically and experimentally with a focus on the impact of the flame base dynamics on the non-linear behavior of the Flame Transfer Function (FTF). Unsteady heat transfer between the flame base and the burner lip is considered to model the flame base response. Predictions for the flame base response Ξ(ω) and flame transfer function F(ω) are compared to measurements over a large range of frequencies. The non-linear behavior of the FTF phase is shown to result from a competition between velocity disturbances contributing to a regular increase of the phase lag with frequency and flame base oscillations leading to a saturation of the phase lag at high frequencies. Increasing the forcing level leads to an early saturation of the phase lag of the FTF at lower frequencies. This analysis demonstrates the important role of flame foot oscillations controlling the saturation of the FTF phase lag.

Scaling the flame transfer function of confined premixed conical flames

The influence of confinement side walls on the response of premixed conical flames submitted to velocity disturbances is investigated experimentally and theoretically. When the flame is sufficiently confined, the burnt gases cannot fully expand at the nozzle outlet. In that case, the confinement ratio Cr=R0/R1 between the burner and flame tube radii and the burnt to unburnt gas volume expansion ratio E=ρu/ρb need to be taken into account in the description of the flame transfer function (FTF). The main effect of confinement is an acceleration of the fresh stream of reactants induced by the over-pressure of the confined burnt gases. Experiments on steady flames reveal that the flame height increases for increasing confinement ratios Cr, leading to a gain shift and a phase drop for the FTF. A theoretical analysis is conducted to model this acceleration and examine its impact on the steady flame shape and flame response to flow disturbances. The change in the FTF phase is shown to be related to a reduction of the mean time lag between heat release rate perturbations and velocity modulations induced by both the velocity acceleration in the fresh reactants and a change in the location of the steady flame front. An expression to scale the FTF of confined flames is derived based on a modification of the classical reduced frequency ω∗ used to scale the response of unconfined flames. This expression includes explicitly the confinement ratio Cr and the volume expansion ratio E. This new dimensionless number enables to plot FTF measured for different confinements with reasonable collapse. It may also be used to transpose FTF gathered on single burners to multiple injection configurations when the burnt gases cannot fully expand due to the presence of neighboring flames.