Natural Acoustic Modes Research Articles

Numerical investigation of combustion instability in a liquid rocket engine: Interaction effect between hydrodynamics and acoustic mode

In this paper, we present the numerical evidence of multiple bifurcations in thermoacoustic instability as we decrease the equivalence ratio (ϕ) from fuel-rich (ϕ = 1.5) to fuel-lean (ϕ =0.59) within a single-element coaxial liquid rocket engine burning hydrogen peroxide and methane. An axisymmetric compressible Large Eddy Simulation solver in an open-source platform OpenFOAM combined with a global methane chemical kinetical mechanism was used to model the turbulent mixing and combustion process. After the validation of grid independence and simulation results, the synchronization phenomenon between hydrodynamic instability, acoustics and combustion is the main focus of this paper. The results here first revealed that in a direction of decreasing equivalence ratios, the first transition to limit cycle state from combustion noise state was observed via intermittency. A second bifurcation was also observed, leading to quasi-period thermoacoustic instability from the limit cycle state. Intrinsic vorticity modes and natural acoustic modes could be regarded as oscillators. The competition and cooperation between the two modes led to a phenomenon beyond limit cycle states. The vortex dynamics of thermoacoustic systems are dominated by the interaction of intrinsic vorticity modes and natural acoustic modes and exhibit an adjustment of vortex shedding frequencies, resulting in different stability regions. Herein, the flame index was performed in more detail to distinguish between the premixed flame and diffusion flame. Results indicated that the burning mode was an important driver in promoting instability whilst it also served as a medium for competition and cooperation between the natural acoustic modes and intrinsic vorticity modes.

A Combined Delayed Detached Eddy Simulation and Linearized Navier–Stokes Equation Study on the Generation and Reduction of Aerodynamic Noises Inside Steam Turbine Control Valve With Acoustic Liner

Abstract This study was aimed at numerically investigating the source, generation mechanism, and strategy for reducing aerodynamic noises inside a steam turbine control valve. A delayed detached eddy simulation was performed to extract the three-dimensional unsteady turbulent flow structures formed within the serpentine flow passage of the turbine valve. Acoustic analogies, spatial Fourier transform, and spectral proper orthogonal decomposition on the delayed detached eddy simulation-simulated flow data were complementarily combined to clarify the generation mechanism of tonal and broadband aerodynamic noises. The results showed that broadband noises were produced by wall-attached jet flow and turbulent mixing flow between the annular wall jets and central reverse flow. High-intensity tonal noises were generated by the excitation of multi-order natural acoustic modes of the bell-shaped valve spindle. The intensive acoustic pressure pulsations concentrated inside the bell jar and propagated along the diffuser to the downstream turbine chamber. A novel ring acoustic liner was designed using the acoustic impedance model to reduce the valve noises without sacrificing the flow performance. The noise reduction effectiveness was evaluated by solving the linearized Navier–Stokes equations in the frequency domain.

Vortex-Resonant Sound Interaction in a Duct Containing an Orifice

This paper reports the results of a set of experiments involving the passage of flow through a constriction - a single orifice - located in the middle of a duct. Geometric variations during testing include the length of the duct, orifice size, and orifice thickness. Acoustic oscillations in the duct were measured by systematically varying the flow rate for each. combination of geometric variables. Hot wire measurements were performed, in the region downstream of the orifice, under select conditions. The results indicate the excitation of clearly audible, discrete sharp tones of high amplitude that abruptly shift frequencies as the flow rate is increased within a certain range. The excited frequencies are related to fairly high harmonics of the natural acoustic modes of the duct. Simultaneous measurements from the hot wire and the microphone show the same frequency. This suggests the prevalence of a "lock-on" type of phenomena as reported earlier with two orifices. The effect of increase in the orifice diameter is to decrease the range of frequencies when the sharp tones are heard. Some evidence is presented that suggests the critical role played by the thickness of the orifice in exciting these sharp tones.

Acoustic Field in a Duct Resulting from Vortex Shedding from Multiple Restrictors

This paper reports an experimental investigation of the acoustic field in a duct due to vortex shedding from multiple restrictors. The spacing between two restrictors was varied very closely in these experiments. It was observed that high amplitude acoustic field was established in the duct when the restrictors were closely spaced and that the highest amplitude of acoustic oscillations occurred for a certain specific spacing of the restrictors depending upon other conditions. It was also observed that the dominant frequencies locked on to the different possible natural acoustic modes that could be established in the duct.

Effects of Intrinsic Flame Instabilities on Thermoacoustic Oscillations in Lean Premixed Gas Turbines

Abstract Thermoacoustic instabilities are commonly encountered in the development of aeroengines and rocket motors. Research on the fundamental mechanism of thermoacoustic instabilities is beneficial for the optimal design of these engine systems. In the present study, a thermoacoustic instability model based on the lean premixed gas turbines (LPGT) combustion system was established. The longitudinal distribution of heat release caused by the intrinsic instability of flame front was considered in the model. Effects of different heat release distributions and characteristic parameters (Lewis number Le, Zeldovich Number β, and Prandtl number Pr) of the premixed gas on thermoacoustic instability behaviors of the LPGT system were investigated based on the established model. Results show that the LPGT system features have two kinds of unstable thermoacoustic modes. The first one corresponds to the natural acoustic mode of the plenum and the second one corresponds to that of the combustion chamber. The characteristic parameters of the premixed gases have a large impact on the stability of the system and can even change the system from stable to unstable state.

A Reduced-order Model for Lock-on via Vortex-combustion-acoustic Closed-loop Coupling in A Step Combustor

ABSTRACT The lock-on of the frequency of acoustic oscillations to the frequency of the vortex shedding in a reacting flow in a backward-facing step combustor is demonstrated using a reduced-order model. Individual models for the flow, flame and acoustic models are developed. A two-dimensional flow field with growth and shedding of vortices is modeled using a potential flow model derived via a conformal mapping of the step geometry. Combustion and acoustic models are derived using thermo-diffusive equations and the one-dimensional Galerkin method, respectively. A modulation of heat release rate fluctuations is thus enabled by vortices advecting past the flame, overcoming the time and space-localized assumptions in vortex kicked oscillator models. Comparisons with results from kicked oscillator models show a better prediction of the dominant frequencies during stable and unstable operations. A Reynolds number sweep is performed to reveal the transition from stable to unstable operation and thus the onset of instability. A shift in the dominant frequency of pressure fluctuations from the natural duct acoustic mode to the natural vortex shedding mode is observed, indicative of a lock-on. The results compare well with past experimental and computational data for similar geometry and flow conditions.

Low-Order Modeling to Investigate Clusters of Intrinsic Thermoacoustic Modes in Annular Combustors

Abstract The intrinsic thermoacoustic (ITA) feedbackloop constitutes a coupling between flow, flame and acoustics that does not involve the natural acoustic modes of the system. One recent study showed that ITA modes in annular combustors come in significant number and with the peculiar behavior of clusters, i.e. several modes with close frequencies. In the present work, an analytical model of a typical annular combustor is derived via Riemann invariants and Bloch theory. The resulting formulation describes the full annular system as a longitudinal combustor with an outlet reflection coefficient that depends on frequency and the azimuthal mode order. The model explains the underlying mechanism of the clustering phenomena and the structure of the clusters associated with ITA modes of different azimuthal orders. In addition, a phasor analysis is proposed, which encloses the conditions for which the 1D model remains valid when describing the thermoacoustic behavior of an annular combustor.

Phase-locking particle image velocimetry measurements of acoustic-driven flow interactions between tandem deep cavities

Acoustic-driven flow interactions between tandem deep cavities, which manifest as resonances between the natural acoustic standing-wave mode and the intrinsic shear-layer vortex structures, were experimentally investigated by using a pressure transducer array, the planar particle image velocimetry (PIV) technique, and phase-locking PIV measurements. Specifically, in the phase-locking PIV measurements, a field-programmable gate array-based phase-determination strategy was used to improve the phase-locking accuracy. The pressure measurement results demonstrated that under certain Reynolds numbers, significantly intensified acoustic pressure pulsations were excited once the magnitude of the acoustic resonance occurring inside the tandem deep cavities reached almost three times the magnitude of the dynamic pressure head at the channel inlet. Beyond that, the planar-PIV results illustrated the elevated turbulent flow quantities, such as the expanded velocity gradients, amplified shear-layer momentum thickness, intensified velocity fluctuations, and statistical Reynolds shear stresses. Subsequently, a proper orthogonal decomposition (POD) analysis was conducted to successfully extract the dominant flow modes underlying the acoustic-driven flow interactions, namely, the cavity-to-cavity flow mode and the counterrotating shedding vortex mode. The first POD mode gave rise to essential flow streaks that shuttled synchronously between the tandem deep cavities, while the second POD mode contributed to the streamwise vortex-shedding motions. Finally, the phase-locking PIV results comprehensively revealed the spatiotemporal evolutions of the coherent flow structures (the upper shedding vortices and the recirculation zones beneath) and their centroid trajectories. The findings of this study could be useful for revealing the flow–acoustic coupling mechanisms in related industrial facilities.

Combustion instability analysis on a partially premixed swirl combustor by thermoacoustic experiments and modeling

The combustion instability of a partially premixed swirl burner is investigated by experimental tests and numerical modeling. The experimental results show that the combustion dynamics of the partially premixed swirl combustor under different thermal power rates and equivalence ratios can be classified into three regimes: combustion noise, intermittent oscillation and limit cycle oscillation. The characteristics of each combustion regime are clarified: combustion noises occur with small pressure oscillation amplitudes and unapparent frequencies when the equivalence ratio approaches 1; the limit cycle oscillations present in strong single frequencies within the relative lower (6223–8563) and higher (10,796–13536) ranges of Re number; the intermittent oscillations observed within a middle range (8634–10796) show multiple peaks in their spectrums. To obtain the relationship between flame transfer function and thermoacoustic modes of the combustor, the CFD/system identification and low order thermoacoustic network model are developed. The thermoacoustic modes are found to be influenced by the interactions between natural acoustic mode and flame response factors. The modeling results reveal that the intermittent oscillations show two modes with positive growth rates, while the limit cycle oscillations are dominated by one mode. Moreover, the combustion instability is found more sensitive to FTF but not acoustic boundary in the modeling.

Combustion modes and driving mechanisms of pressure fluctuation in a novel vortex-tube combustor with quasi-steady and stratified properties

The operating limit and pressure fluctuations of a localized stratified vortex-tube combustor (LSVC) are investigated experimentally by taking methane as fuel at the inlet temperature of 300 K and the combustor pressure of 1 atm over a range of equivalence ratios (0.1–1.0). The combustion modes are distinguished according to the properties of pressure fluctuations and the driving mechanisms of each combustion mode are explored in combination with numerical simulation. The results show that the LSVC can realize stable combustion with pressure fluctuation amplitudes always less than 4 kPa in a large operating range, and the flame front is always continuous. The operating flammability limit experimental range can be divided into five regimes with different combustion modes. The first and fifth combustion modes are steady combustion with pressure fluctuation amplitudes less than 1 kPa, and the second to fourth ones are quasi-steady combustion with pressure fluctuation amplitudes between 1–4 kPa. The spectrum analyses of pressure fluctuations show that there are one low-frequency peak around 300 Hz and one high-frequency peak around 1500 Hz, which are dominated by the first and third axial natural acoustic modes of resonant oscillation combustion, respectively. In the first and fifth combustion modes, the resonances are both weak due to the influences of low flow rate and laminarization respectively. In the second mode, the thermo-acoustic coupling oscillation and the resonance are excited simultaneously, yielding the highest pressure fluctuation amplitude in the entire operating range. The high-frequency resonance causes the high-frequency pressure fluctuation of the third mode. Both the unsteady heat release and flow field affect the pressure fluctuation in the fourth mode. The former produces the low-frequency fluctuation, which can resonate with the natural frequency and excite a weak thermal-acoustic coupling.

Intrinsic thermoacoustic modes in an annular combustion chamber

Thermoacoustic instabilities originate from the interaction of unsteady heat release rate associated with flames and the acoustic modes of a combustor. A feedback loop not involving the natural acoustic modes has been observed in single-flame configurations with anechoic terminations: an acoustic wave emitted by the flame travels upstream, and the associated velocity fluctuation again excites the flame. This feedback cycle gives rise to thermoacoustic modes intrinsic to the flame. An analytical model for an annular thermoacoustic system is formulated, and the existence of intrinsic modes of various azimuthal orders is demonstrated. The spectrum of an annular combustor is computed with a three-dimensional thermoacoustic Helmholtz solver. The configuration resembles those commonly found in gas turbines. In addition to the observations in previous studies, numerous intrinsic modes are found, with frequencies close to the lowest acoustic modes. All of the intrinsic modes can be grouped into clusters, at frequencies corresponding to multiples of the inverse flame response time delay. It is demonstrated that the newly observed intrinsic modes belong to the same mechanism that has recently been studied in single-sector/flame configurations. An analysis of the evanescent character of cut-off azimuthal modes explains the pattern in the spectrum. The underlying physical mechanism is generically present in any annular combustion chamber and a possible source of instability.

Thermoacoustic combustion instability of propane‐oxy‐combustion with CO2dilution: Experimental analysis

In this study, the effect of CO2 dilution on the thermoacoustic stability of propane-oxyfuel flames is studied in a non-premixed, swirl-stabilized combustor. The results, obtained at a fixed combustor power density (4 MW/m3 bar) and global stoichiometric equivalence ratio (Φ = 1.0), show that the oxy-flame is stable at 0% and low CO2 concentrations in the oxidizer. A self-amplifying coupling between heat release and pressure fluctuations was observed to occur at the CO2 concentration of 45%, which matches the point of flame transition from a jet-like to a V-shaped flame resulting from the formation of inner recirculation zone. The observed frequency for both the pressure and heat release oscillations is 465 Hz and the ensuing thermoacoustic instability is believed to have been resulted from vortexes and flame interactions. Subsequent to the coupling of the oscillations at the CO2 concentration of 45%, their amplitudes grew at 50% to 60% CO2 dilution levels. The maximum amplitude was observed at 60% CO2 concentration after which, as CO2 dilution level increases, the acoustic amplitude and that of its counterpart in the heat release spectrum decreased due to damping (energy dissipation) arising from heat loss and viscous dissipation. An increase in hydrogen concentration in the fuel and a decrease in the combustor power density were observed to lower the acoustic amplitude. Furthermore, a frequency shift is observed with a change in the combustor firing rate, which shows that the mode scales with the flow velocity, and therefore, unlikely to be a natural acoustic mode of the combustor. This study, therefore, reveals thermoacoustic instability in non-premixed oxy-combustion driven by changes in flame dynamics and macrostructures as the CO2 concentration in the oxidizer mixture varies.

Vortex dynamics during acoustic-mode transition in channel branches

The vortex dynamics during acoustic mode transition in channel branches were experimentally investigated with phase-locking particle image velocimetry (PIV) measurements. Particularly, a real-time waveform recognition approach, based on an offline pressure analysis by dynamic mode decomposition (DMD) and a real-time computation by field programmable gate array, was established. In the offline DMD analysis, energetic pressure DMD modes during acoustic mode transition were extracted from pressure data measured by a pressure transducer array and found to agree well with the natural acoustic standing-wave modes numerically determined from an acoustic modal analysis. The acoustic mode transition process was classified into three successive phases: Phase-I: hybrid acoustic modulations, Phase-II: no acoustic modulation, and Phase-III: third-order acoustic modulation. Subsequently, the vortex dynamics corresponding to Phase-I and Phase-III were determined by phase-locking PIV measurements with the real-time waveform recognition approach. The results are summarized as follows. (1) The vortex dynamics coupled with the first acoustic standing-wave mode in Phase-I were related to the first shear layer hydrodynamic mode in channel branches. (2) The vortex dynamics coupled with the second acoustic standing-wave mode in Phase-I were recognized as the signatures of the second shear layer hydrodynamic mode. (3) However, in Phase-III of the acoustic mode transition, modulated by the third acoustic standing-wave mode, the corresponding vortex dynamics fully developed into a second shear layer hydrodynamic mode. This work provides a better understanding of the complex vortex dynamics of channel flows with broad implications for industrial piping systems.

Excitation of high frequency thermoacoustic oscillations by syngas in a non-premixed bluff body combustor

A laboratory-scale bluff body combustor is mapped for its stability and flame dynamics of non-premixed flames with three fuels, namely, pure H2, H2CH4 mixture, and H2CO mixture, the last one representing syngas. Unsteady pressure measurements and high-speed OH* and CH*/CO2* chemiluminescence imaging are simultaneously performed. The combustor behaviour with syngas is markedly different than the other two in exciting high frequency oscillations, typically at the third harmonic longitudinal acoustic mode of the duct at high air flow Reynolds numbers (Re). In contrast, the H2CH4 excites only the fundamental longitudinal mode, and pure H2 excites up to the first harmonic. The latter two are observed to lock on to the shear layer mode of the bluff body wake, whereas the H2CO case locks on to thrice the Strouhal number associated with the shear layer mode, commensurate with the excitation of the third harmonic natural acoustic mode. Time-averaged OH* and CO2* chemiluminescence images show large-scale structure for the H2CH4 case compared to heat release rate zones aligned with the shear layer in the pure H2 and H2CO cases. Cross-sectionally averaged chemiluminescence profiles exhibit a streamwise stagger in the peaks of OH* and CO2*, suggesting two heat release rate zones, that could excite the acoustic oscillations. The instantaneous profiles indicate a convective delay between the two heat release rate zones that is close to the third harmonic acoustic time scale. The sequential of H2 oxidation to OH followed by CO oxidation by OH to form CO2 are considered to be responsible for the high frequency excitation in the case with the H2CO mixture when compared to the other two cases.

Effect of syngas composition on high frequency combustion instability in a non-premixed turbulent combustor

An experimental investigation on the effect of syngas composition (H2/CO) on thermodynamic instability of a bluff-body combustor is studied. Three syngas compositions, namely, 25%H2–75%CO, 50%H2–50%CO and 75%H2–25%CO, and pure hydrogen and 75%H2–25%CH4 are acoustically characterized by varying the inlet Reynolds number, Re, in the range of 2200–8100. The combustor is observed to undergo two modes of thermo-acoustic instability across the above Re range for all syngas compositions and pure hydrogen, whereas for the H2CH4 composition, the instability manifests at a single frequency ∼130 Hz. For all the syngas compositions, the pressure oscillations exhibit low frequency (130 Hz) at low Re, and bifurcate to higher frequency (∼500 Hz) at mid-range Re values, while pure hydrogen exhibits bifurcation from the low frequency to the first harmonic (∼260 Hz). The high frequency observed with the syngas compositions is at the third harmonic natural acoustic mode of the combustor and matches thrice the Strouhal frequency of the hydrodynamic oscillations of the shear layer separating from the bluff body. Further investigation into the nature of the dynamic behaviour across the listed fuels is performed using simultaneous time resolved imaging of OH* and CO2* chemiluminescence and two-component particle image velocimetry. The peak in the CO2* chemiluminescent intensity is found to be axially staggered downstream of that of OH*. This is due to sequential oxidation of H2 and CO, the former producing OH as an intermediate consumed by the latter. Spatio-temporal analysis of the time-resolved data reveals that the presence of the flame in the shear layer of the bluff-body plays a significant role in modulating the high frequency acoustic excitation. Further, a spatial Rayleigh map is derived based on both the OH* and CO2* chemiluminescence, which reveals the bluff-body shear layer dynamics to yield the key driving and damping regions in the present study.

Influence of vortex-excited acoustic resonance on flow dynamics in channel with coaxial side-branches

The influence of vortex-excited acoustic resonance on flow dynamics in a straight channel with coaxial side branches was experimentally determined. The energetic vortex structures superimposed in the shear layer, formed at the intersection between the straight channel and the side-branches, are closely coupled with the natural acoustic modes of the side-branches, resulting in significantly intensified acoustic standing waves in the side-branches. Two configurations with symmetric (length-to-diameter ratio: L1/D = L2/D = 10) and asymmetric (L1/D = 10, L2/D = 5) arrangements of the side-branches were used to investigate the clearly different flow dynamics with and without acoustic resonance. In the experiments, wall pressure measurements and acoustic modal analyses were first conducted to determine the frequency lock-on range of the vortex-excited acoustic resonance. Subsequently, the flow fields in two configurations were measured with planar particle image velocimetry. The results indicated that the vortex-excited acoustic resonance resulted in significant flow fluctuations inside the branches together with expanded shear layers and the enlarged vortex structures. Vorticity fluctuations and Reynolds shear stresses were also increased significantly when subjected to acoustic modulation. In terms of the proper orthogonal decomposition analysis, two different motions of the energetic flow structures were identified as interacting with the acoustic resonance: the vertical synchronous oscillation mode and the streamwise vortex-shedding mode. Correspondingly, the phase-averaged low-order flow fields, determined by the mode coefficients of these two correlative modes, clearly indicated the flapping motion of the mainstream and the periodic oscillation of the shear layer.

Analysis of the natural acoustic modes of a gas turbine combustor using isothermal CFD simulations

Thermoacoustic instabilities usually result from the coupling between the oscillatory heat release and one or more natural acoustic modes of the combustion system. When the shifting of system frequencies caused by the unsteady heat release is limited, the calculation of natural modes allows to identify which of them are excited by the flame once changes in flow temperature and composition due to combustion are considered.In this paper, isothermal computational fluid dynamics simulations are performed to predict the natural modes of a heavy-duty gas turbine combustor in reactive conditions. Combustion and heat transfer are neglected in the numerical analysis to simplify the model and limit the computational effort. The natural frequencies resulting from isothermal simulations are then corrected using a rather basic post-processing approach to account for temperature and gas composition changes due to combustion process. Frequency and amplitude of the calculated modes are finally compared to experimental measurements to evaluate the ability of the acoustic analysis to capture frequency and spatial shape of the combustor natural modes excited by the flame.

Shape effects of single axisymmetric cavity in a circular duct on flow induced acoustic oscillations

The acoustic oscillations excited by flow past an axisymmetric cavity in a circular duct are experimentally investigated. The focus of the work is on the effects of different dimensions and shapes of the cavity, namely, rectangular, V, and U shapes. A large number of transducers are used to simultaneously record the variation of acoustic pressure axially and azimuthally in the duct. The trapped acoustic modes excited in the duct are confirmed by the observation of axial decay of the acoustic pressure amplitude away from the cavity. Shifts are observed between tangential natural acoustic modes of the duct, corresponding to which the amplitudes rise and fall. The subdominant mode amplitude increases simultaneously as the dominant mode decreases during a mode shift. The experimentally observed azimuthal variation of amplitude and phase indicate the prevalence of mixed standing and spinning wave modes, which are separated so as to evaluate the spin ratio, defined as the ratio of the amplitude of the spinning part to the total amplitude comprising both the spinning and standing modes. It is found that the spin ratio reaches low values corresponding to when the mode shifts occur. Accordingly, the ratio of the dominant to subdominant amplitudes registers a spike. The results suggest that, when a dominant mode prevails over a velocity range, the acoustic energy is substantially contributed towards the spinning mode, whereas during conditions of mode shifts, the amplitudes of the competing modes decrease, leaving only their standing wave patterns to prevail mostly. The depth of the cavity affects the natural frequency excited, whereas the length influences the amplitude. The U- and V-shaped cavities offer lesser effective geometric depths to the acoustic modes. The shape of the cavity influences the amplitude excited, with the U-shaped cavity registering the lowest levels. It is expected that the dynamics in the shear layers between the central large-scale vortex and the corner vortices accommodated by the cavity shape contributes to the acoustic oscillations excited.

Investigation of intermittent oscillations in a premixed dump combustor using time-resolved particle image velocimetry

The present work deals with time-resolved investigation of the flow field during acoustic self-excitation by a lean premixed flame in a dump combustor. Simultaneous measurements of unsteady pressure, velocity fields using time-resolved particle image velocimetry (TR-PIV) and CH* chemiluminescence are performed. As the equivalence ratio is varied, conditions of maximum pressure amplitude correspond to the prevalence of intermittent bursts in all the measured quantities. The intermittency is quantified by means of the permutation entropy of the pressure signals. The most significant frequencies are identified with the fundamental natural duct acoustic mode and the large-scale vortex roll-up (“wake mode”) frequency. The simultaneous occurrence of these multiple frequencies and shifts in their relative dominance of frequencies over time, shown by wavelet transform, result in the observed intermittent burst oscillations. Regimes of these ordered oscillations alternate with those of relative silence as well. The dynamic mode decomposition (DMD) of the TR-PIV data during windows of decay and growth of intermittent oscillations in the pressure signal quantifies the contribution of the above dominant modes in terms of their temporal growth rates. Besides the above two modes, the flapping mode and the shear layer mode of the flow field are also indicated. Consecutive PIV realisations and chemiluminescence images in a time-resolved sequence during excitation of high pressure amplitude show the large-scale vortex roll-up to substantially convolute the flame, leading to high heat release fluctuations. A further sequence of consecutive flow/flame fields is also examined as to how oscillations emerge out of a relatively silent time interval. The Kelvin–Helmholtz structures of the shear layer mode prevalent during the silent regime are seen to undergo a collective interaction to form a large-scale structure that excites the vortex and acoustic modes in turn.

Numerical Investigation of Flow Acoustic Coupling in a Half-Dump Combustor

This paper reports on the investigation of combustion instabilities in a methane–air non-premixed half-dump combustor for different flow Reynolds number () using computational aero-acoustic (CAA) simulation. To simulate the flow physics under turbulent flow conditions, detached-eddy simulation in conjunction with generalized eddy-dissipation model is adopted, while a CAA formulation based on Lighthill’s acoustic analogy is used for computing the acoustic field. It is observed that the unsteady pressure signals predominantly arise from either the natural acoustic modes of the duct or the local flow fluctuations in the vortex shedding process downstream of the dump plane, giving rise to different dominant frequencies at different spatial locations at lower and a single dominant locked-on frequency at higher . The nondimensional numbers, that is, Helmholtz and Strouhal numbers, are used to characterize the duct acoustic modes from the vortex shedding modes. Under reacting flow conditions, unsteady heat release and pressure oscillations are monitored to compute Rayleigh index in order to verify if the instability is driving (positive net value) or damping (negative net value) at different frequency levels. Moreover, the predicted Helmholtz and Strouhal numbers are found to be in excellent agreement with the experimental data available in the open literature for a wide range of .