Flame Heat Release Research Articles

Effect of Piston Crevices on the Numerical Simulation of a Heavy-Duty Diesel Engine Retrofitted to Natural-Gas Spark-Ignition Operation

Three-dimensional computational fluid dynamics internal combustion engine simulations that use a simplified combustion model based on the flamelet concept provide acceptable results with minimum computational costs and reasonable running times. Moreover, the simulation can neglect small combustion chamber details such as valve crevices, valve recesses, and piston crevices volume. The missing volumes are usually compensated by changes in the squish volume (i.e., by increasing the clearance height of the model compared to the real engine). This paper documents some of the effects that such an approach would have on the simulated results of the combustion phenomena inside a conventional heavy-duty direct injection compression-ignition engine, which was converted to port fuel injection spark ignition operation. Numerical engine simulations with or without crevice volumes were run using the G-equation combustion model. A proper parameter choice ensured that the numerical results agreed well with the experimental pressure trace and the heat release rate. The results show that including the crevice volume affected the mass of a unburned mixture inside the squish region, which in turn influenced the flame behavior and heat release during late-combustion stages.

Effect of orientation on the burning and flame characteristics of PMMA slabs under different pressure environments

To investigate the combined effects of orientation and environmental pressure on flame characteristics, steady burning experiments of inclined polymethyl methacrylate (PMMA) slabs were conducted at reduced environmental pressures ranging from 0.5 to 1 atm. According to the visual observations of flame appearance and fitting results of burning rate at different inclination angles, diffusion flames can be classified into three regions: pool-fire region, transition region and wall-fire region. Experimental result shows that flame pulsation frequency depends only on sample width in pool-fire region, however, mainly on fuel inclination angle in wall-fire region. A theoretical analysis is performed to predict the flame pulsation frequency as a function of modified gravity and the outlet diameter of gas-phase flame at different fuel inclination angles. In addition, the dimensionless flame pulsation frequency St is found to correlate well with the modified Froude number Fr∗, St=0.461/Fr∗0.53. Flame length shows a tendency to peak at 0.8 atm except 2.5 cm wide samples. Correlations between flame length and heat release rate are found with a power-law exponent of 1.41 for pressures smaller than 0.8 atm and 1.04 for larger pressures. Both experimental result and the developed correlations of flame length suggest that the roles of molecular diffusion and air entrainment played in diffusion burning are closely related to the environmental pressure and sample dimension.

Flame stabilization of supersonic ethylene jet in fuel-rich hot coflow

The stability limit of a supersonic ethylene jet flame in a fuel-rich hot coflow was examined by investigating the influence of the injection pressure, which was varied from 2.0 atm to 4.5 atm, and of the equivalence ratio of the coflow, which was varied from 1.2 to 1.6. The flames were investigated with time-resolved chemiluminescence and schlieren images, as well as a large-eddy simulation of combustion. The results show that, with increasing injection pressure, the flame state changes from stable to unstable and blow-off, and the flame brush thickness, heat release, and height of coflow decrease. The flame stability limits decrease as the equivalence ratio of coflow increases. Lastly, a large-eddy simulation was performed to investigate the mechanism of flame stabilization, and the numerical simulation results are in good agreement with the experimental results. It was found that the stability of a supersonic flame is affected by the chemical time scale and flow time scale.

Numerical prediction of the Flame Describing Function and thermoacoustic limit cycle for a pressurised gas turbine combustor

ABSTRACTThe forced flame responses in a pressurized gas turbine combustor are predicted using numerical reacting flow simulations. Two incompressible1large eddy simulation solvers are used, applying two combustion models and two reaction schemes (4-step and 15-step) at two operating pressures (3 and 6 bar). Although the combustor flow field is little affected by these factors, the flame length and heat release rate are found to depend on combustion model, reaction scheme, and combustor pressure. The flame responses to an upstream velocity perturbation are used to construct the flame describing functions (FDFs). The FDFs exhibit smaller dependence on the combustion model and reaction chemistry than the flame shape and mean heat release rate. The FDFs are validated by predicting combustor thermoacoustic stability at 3 and 6 bar and, for the unstable 6 bar case, also by predicting the frequency and oscillation amplitude of the resulting limit cycle oscillation. All of these numerical predictions are in very good agreement with experimental measurements.

Nonlinear mode transition mechanisms of a self-excited Jet A-1 spray flame

Low-frequency combustion instabilities arise in an aero-engine gas turbine combustor under idle/sub-idle conditions, particularly when pilot-mode non-premixed combustion is sustained. Despite the fundamental importance of such instabilities, little is known about how they are initiated and grow in the system. Here, we present nonlinear mode transition processes using several analysis methods, including Fourier/Hilbert transforms, phase portraits, spectrograms, low-order analytic modeling, and high-speed flame visualization methods. Our results demonstrate that a non-premixed Jet A-1 spray flame yields an intermediate-amplitude, quasi-periodic L1 mode oscillation at 50 Hz for a low pilot equivalence ratio, and the frequency gradually increases with increasing fuel flowrates. Subsequent to a critical point (103 Hz), the system undergoes a discontinuous mode transition, giving rise to the formation of an extremely large pressure oscillation with an L2 mode structure at 244 Hz. Several key triggers were found to induce the mode shift: (i) generation of a high intensity pulse in the flame's heat release rate due to the spontaneous ignition of unburned reactant mixtures, (ii) emergence of a large-scale vortical structure and its interaction with a partially premixed flame front, and (iii) development of self-sustained limit cycle oscillations driven by periodic convection of a hot spot – a mechanism known as entropy wave propagation. Our study identifies the occurrence of a large-amplitude peak followed by a local minimum intensity, analogous to the activation energy concept, as an essential step for entry into a new state with large-amplitude limit cycle oscillations.

An experimental study of the detailed flame transport in a SI engine using simultaneous dual-plane OH-LIF and stereoscopic PIV

Understanding the detailed flame transport in IC engines is important to accurately predict ignition and combustion phasing, rate of heat release and assess engine performance. This is particularly important for RANS and LES engine simulations, which often struggle to accurately predict flame propagation and heat release without first adjusting model parameters. Detailed measurements of flame transport in technical systems are required to guide model development and validation.This work introduces an experimental dataset designed to study the detailed flame transport and flame/flow dynamics for spark-ignition engines. Simultaneous dual-plane OH-LIF and stereoscopic PIV are used to acquire 3D measurements of unburnt gas velocity (U⇀gas), flame displacement speed (SD) and overall flame front velocity (U⇀Flame) during the early flame development. Experiments are performed in an optical engine operating at 800 and 1500 RPM with premixed, stoichiometric isooctane-air mixtures. Analysis reveals several distinctive flame/flow configurations that yield a positive or negative flame displacement for which the flame progresses towards the reactants or products, respectively. For the operating conditions utilized, SD exhibits and inverse relationship with flame curvature and a strong correlation between negative SD and convex flame contours is observed. Trends are consistent with thermo-diffusive flames, but have not been quantified in context of IC engines. Flame wrinkling is more severe at the higher RPM, which results in a broader SD distribution towards higher positive and negative velocities. Spatially-resolved distributions of U⇀gas and SD are presented to describe in-cylinder locations where either convection or thermal diffusion is the dominating mechanism contributing to flame transport. Findings are discussed in relation to common engine flow features, including flame transport near solid surfaces. Findings provide a first insight into the detailed flame transport within a technically relevant environment and are designed to support the development and validation of engine simulations.

Effects of heat release and imposed bulk strain on alignment of strain rate eigenvectors in turbulent premixed flames

The impact of combustion heat release and bulk compressive strain on local alignment between the flame front normal and strain rate eigenvectors (principal strain rates) are investigated using simultaneous laser-induced fluorescence imaging of OH and tomographic particle image velocimetry in turbulent premixed CH4/O2/N2 counterflow flames with Karlovitz numbers (Ka) of 1.3 and 2.7. The bulk strain rate imposed by the counterflow introduces a preferential alignment of the most compressive principal strain rate, s3, with the flame normal, n. Dilatation induced by heat release acts as a competing mechanism that promotes the alignment of the most extensive principal strain rate, s1, with n. In the counterflow flames, the preferential s3-n alignment prevails and remains dominant across the entire flame. This alignment stands in stark contrast to observations from previous studies in turbulent Bunsen flames or flames in isotropic turbulence, indicating the significance of bulk strain rate in determining local strain-flame alignment. The effects of increasing turbulence intensity on strain rate-flame front alignment are twofold; on the one hand, turbulence diminishes the s3-n preferential alignment that is associated with the bulk strain field by increasing flame surface wrinkling and reducing the tendency of the flame front normal and s3-eigenvectors to align with the axis of the counterflow. On the other hand, turbulence reduces the impact of heat release and enhances preferential s3-n alignment approximately 1 mm ahead of the flame front, reflecting the characteristic alignment of compressive strain and scalar gradients in turbulent non-reacting flows. The effects of bulk strain are also observed in strain rate alignment statistics based on the fluctuating velocity fields, although the impact is less pronounced than for the statistics based on the full velocity fields.As a result of this complex interplay between heat release, turbulence, and bulk strain rate, the flame-tangential strain rate is on average extensive, and the flame-normal strain rate is dominantly compressive except for an approximately 0.8 mm wide region near the flame front where it is extensive due to dilatation. The compressive bulk strain in the counterflow was also shown to compress the length scales over which the strain rate and its alignment are affected by flame heat release. The present finding is important for developing turbulent flame models to accommodate the effect of bulk strain rate that is inherently associated with practical burner geometries, and the length scale dependence of the bulk strain effect could be a consideration for determining the cutoff scale in the context of large-eddy simulations.

The role of precessing vortex core in two combustion regimes: Numerical simulation studies

Large Eddy simulation (LES) with finite rate chemistry was used to investigate the combustion dynamics in a lab-scale PRECCINSTA combustion chamber. Transient three dimensional numerical simulations were carried out at two different thermal powers (10 kW and 35 kW) with a fixed equivalence ratio of 0.7. The predicted results were compared with the experimental data and good agreements were found between them. In the cold flow field under both conditions, a precessing vortex core (PVC) in the inner shear layer (ISL) existing between the swirling jet and the inner recirculation zone (IRZ). However, two different flow and combustion dynamics were observed when combustion occurred. At thermal power of 10 kW, there was a V-shaped flame and the combustion of the flame was stable. The PVC disappeared and the vortices arrangement was symmetrical in the ISL. However, at 35 kW, there was a M-shaped flame with a PVC in the ISL and combustion instability triggered. In depth analysis of the characteristics of flow, temperature and heat release field, we found that the flame surface was wrinkled periodically by the PVC which enhanced the mixing between the cold fresh gas and hot burned products. Then, the mixture was ignited locally and heat release was rapid in the middle of the combustion chamber. These effects were directly related to the periodic vortices motion which was induced by PVC. It was confirmed that the influence of PVC on flame surface and heat release is an important factor for triggering the combustion instability at thermal power of 35 kW. The zone division based on different roles of flow/flame and thermoacoustic coupling was also discussed to illustrate the combustion instabilities caused by PVC.

Study of burner geometry effects on non-premixed flame response under acoustic excitation

The study of the flame under the acoustic excitation contributes to the study of flame self-excited thermoacoustic instability. Although non-premixed combustion is widely used in industry, research on its combustion instability characteristics is relatively few. This paper experimentally studies the response of a non-premixed swirl flame under excitation. The geometry modifications are the length of the inlet section which is set to be 0.245 m, 0.345 m and 0.445 m and the installation of separation plates. Through the analysis of flame response behavior at different excitation frequencies, combined with the calculation of acoustic mode, we find that for the 0.245 m and 0.345 m inlet length cases, the maximum flame heat release fluctuation responds near 144 Hz. This mode is due to the resonance of the fuel pipe. For the 0.445 m inlet length cases, the maximum flame heat release responds near 134 Hz. This mode is a mixed mode of fuel pipe and combustion chamber. The maximum flame response occurs at the point where the pressure fluctuation reaches the maximum in the inlet section. The image analysis shows the same mode distinction trend between short and long inlet length. Besides, the removing of separation plates leads to a more stable response of the flame in low inlet airflow rate. However, at high airflow rate, the flame tends to be more unstable. Moreover, the higher acoustic forcing frequency than quarter-wave mode will cause the quarter-wave resonant frequency to appear. The results of our work can benefit the implications of thermoacoustic instability in non-premixed flame.

Experimental study on the maximum temperature and flame extension length driven by strong plume in a longitudinal ventilated tunnel

In order to study the maximum temperature and flame extension length when flame impinges on the tunnel ceiling, experiments were carried out with different longitudinal velocities, heat release rates and ceiling-source heights considered. The results showed that: (1) The maximum temperature without ventilation was not sensitive to heat release rate and ceiling-source height, and the longitudinal velocity was a key factor for the reduction of the maximum temperature. A new hypothesis based on the decease of real effectual heat release rate in a ventilated tunnel was given and a new model to predict the maximum temperature at the flame impingement region was proposed. (2) The correlation of the flame extension length was built with the modified flame height and heat release rate. And, the cylindrical and ellipse flame shape hypotheses were used to verify the experimental data in this study. It was found that the theoretical value of the flame extension length agreed well with the corresponding experimental results. In addition, the cylindrical flame hypothesis was more consistent with experimental data, compared to ellipse flame hypothesis.

A novel application of silicone-based flame-retardant adhesive in plywood

A silicone-based elastomer filled with vinyl-silane treated aluminum hydroxide was used to replace conventional polyurethane-based adhesive to provide a flame-retardant adhesive for plywood. The shear strength and fire performance of such a silicone-based (SI) adhesive glued plywood (SI/plywood) were investigated and compared to those of the polyurethane-based (PU) adhesive glued plywood (PU/plywood). The shear strength of the SI/plywood [(0.92 ± 0.09) MPa] was about 63% lower than that of the PU/plywood at room temperature, but it was less sensitive to water (62% reduction for the PU/plywood and 30% reduction for the SI/plywood after hot-water immersion at 63 °C for 3 h). The fire performance of plywood was assessed by a simulated match-flame ignition test (Mydrin test), lateral ignition and flame spread test, cone calorimetry, and thermocouple measurements. With a higher burn-though resistance and thermal barrier efficiency, and lower flame spread and heat release rate, the SI/plywood exhibited a superior fire-resistance and reaction-to-fire performance and improved fire-resistance as compared to the PU/plywood. The SI adhesive generated an inorganic protective layer on the sample surface that visibly suppressed glowing and smoldering of the plywood during combustion. The SI adhesive was also combined and reinforced with cellulosic fabric (CF) or glass fabric (GF) to prepare composite plywood (SI/CF/plywood and SI/GF/plywood) with improved fire performance. The cone calorimetry and thermocouple measurements indicated that the use of CF or GF in SI/CF/plywood and SI/GF/plywood, respectively, suppressed the delamination and cracking of the composite plywood and promoted the formation of an effective thermal barrier during smoldering and flaming combustion. Particularly, the SI/GF/plywood exhibited the most effective fire barrier with no crack formation, and the lowest heat release rate among the plywood types investigated in this study.

Measurements and Modeling of the Dynamic Response of a Pilot Stabilized Premixed Flame Under Dual-Input Perturbation

Pilot flames have been widely used for flame stabilization in low-emission gas turbine combustors. Effects of pilot flame on dynamic instabilities, however, are not well understood. In this work, the dynamic interactions between main and pilot flames are studied by perturbing both flames simultaneously, i.e., with a dual-input forcing. A burner is used to generate a premixed axisymmetric V-shaped methane flame stabilized by a central pilot flame. Servo valve and sirens are used to produce forcing in the pilot and main flames, respectively. A diagnostic system is applied to measure the flame structure and heat release rate. The effects of forcing frequency, forcing amplitude, phase difference between the two forcing signals as well as the Reynolds number are studied. Both the flame transfer function (FTF) and the flame dynamic position are measured and analyzed. It is found that the total flame response can be modified by the perturbation in the pilot flame. The mechanism can be attributed to the effect of pilot flame on the velocity field of the burnt side. Vortex is found to be able to amplify the pilot–main dynamic interactions under certain conditions. An analytical model is developed based on the linearized G-equation, to further understand the flame interactions through the velocity perturbations in the burnt side. Good agreements were found between the prediction and the experiment results.

Virtual chemistry for temperature and CO prediction in LES of non-adiabatic turbulent flames

Flames stabilization and pollutant formation in confined combustion chambers are affected by heat losses. Reliable numerical simulations must account for such complex phenomena at a reduced CPU cost. The aim of the present work is the development of a comprehensive reduced chemistry model able to account for the impact of heat losses on chemical flame structure. The recently developed virtual chemistry concept has shown promising capabilities to capture detailed chemistry effects with a reduced set of virtual species and kinetic reactions in adiabatic flame conditions. The approach consists in : (i) using a virtual main chemical mechanism coupled to the flow solver equations to predict temperature and heat release, (ii) designing satellite sub-mechanisms dedicated to the description of pollutant formation phenomena. The virtual mechanism is trained to recover the properties of an ensemble of target flames. Here burner-stabilized flamelets are introduced in the reference database to capture the influence of heat-losses on flame heat release and pollutant formation. The present approach is validated in 1D laminar burner stabilized flames and radiative freely propagating flame configurations. Finally the non adiabatic virtual chemistry is used to perform LES of a premixed turbulent combustion chamber submitted to wall heat losses. Comparison against experiment shows that non adiabatic simulation captures well the flame shape, the temperature and the CO prediction.

An Experimental Study on Flame Puffing of a Swirl Partially Premixed Combustion under Varying Mass Flow Rate of Primary Air

It is of practical significance to understand the flame puffing behavior under varying mass flow rate of primary air ṁpri. An experiment was conducted to study the impact of ṁpri on flame puffing in a swirl partially premixed combustor, the puffing behavior of six significant flame properties was examined. The results showed that almost every spectrum had two fundamental frequencies, which is different from the single-peak spectrum of non-swirl flame. The flame heat-release rate, flame area, and flame equivalent width had identical dominant frequency and sub-dominant frequency, both decreased with the increasing of ṁpri. It was attributed to the decreased overall flame temperature caused by the improved mixing of fuel and primary air. All measured frequencies were in the range of 3–14 Hz, but the predicted frequencies from the theoretical models based on non-swirl flame were larger than the measured. This indicates the puffing frequency of swirl flame was much more sensitive to the variation in ṁpri than the frequency of non-swirl flame. Moreover, the amplitude of flame length was the smallest in all properties, with the most weakened oscillating intensity. While the amplitude of the flame area and flame equivalent width were the largest, with the strongest oscillation level. Consequently, the flame puffing is mainly attributed to the oscillation in width direction.

Burst-mode OH/CH2O planar laser-induced fluorescence imaging of the heat release zone in an unsteady flame.

The paper presents simultaneous high-speed (7.5 kHz) planar laser-induced fluorescence (PLIF) of formaldehyde (CH2O) and the hydroxyl-radical (OH) for visualization of the flame structure and heat release zone in a non-premixed unsteady CH4/O2/N2 flame. For this purpose, a dye laser designed for high-speed operation is pumped by the second-harmonic 532 nm output of a Nd:YAG burst-mode laser to produce a tunable, 566 nm beam. After frequency doubling a high-energy kHz-rate narrowband pulse train of approximately 2.2 mJ/pulse at 283 nm is used for excitation of the OH radical. Simultaneously, CH2O is excited by the frequency-tripled output of the same Nd:YAG laser, providing a high-frequency pulse train over 10 ms in duration at high pulse energies (>100 mJ/pulse). The excitation energies enable signal-to-noise ratios (SNRs) of ~10 and ~60 for CH2O and OH PLIF, respectively, using a single high-speed intensified CMOS camera equipped with an image doubler. This allows sufficient SNR for investigation of the temporal evolution of the primary heat release zone and the local flame structure at kHz rates from the spatial overlap of the OH- and CH2O-PLIF signals.

Orientation effect on upward flame propagation over rigid polyurethane foam

Inclined upward (concurrent) flame spread over rigid polyurethane (RPU) foam which can exhibit great fire hazard in real high-rise building fire disasters has attracted more and more attentions in the previous studies. In this work, a series of bench-scale experiments and theoretical analyses were conducted to explore the orientation effect on upward flame spread of RPU foam with sample inclination ranging from 10° to 85°. Results show that the fitted correlations between flame length and pyrolysis length are different due to the increasing inclinations, and can be categorized by a modified Rayleigh number. Meanwhile, the power exponents of the fitted expressions between flame length and heat release rate per unit width can also be classified: the power exponent n = 1, n<1 and n>1, when 10° ≤ θ ≤ 40°, 50° ≤ θ ≤ 70°, and θ ≥ 80°, respectively. Furthermore, the dimensionless mass loss rate is approximately proportional to the quarter power of the modified Rayleigh number. The relationship between upward flame spread rate and sine of inclined angle is approximately exponential, and a critical modified Rayleigh number at which no flame spread can be sustained is proposed. The results of this study can provide a reference to high-rise building fire risk assessment and energy conservation system design.

Numerical simulation of finite disturbances interacting with laminar premixed flames

Compression waves can be generated during combustion processes and subsequently interact with flames to augment their behaviour. The study of these interactions thus far has been limited to shock and expansion waves only. In this study, the interaction of finite compression waves with a perturbed laminar flame is investigated using numerical simulations of the compressible Navier–Stokes equations with single-step chemical kinetics. The interaction is characterised using three independent parameters: the compression wavelength, the pressure ratio of the disturbance, and the perturbation amplitude of the flame interface. The results reveal a wide range of behaviours in terms of flame length and heat release rate that could occur during such an interaction. The results are compared to the classical reactive Richtmyer–Meshkov instability and the role of baroclinic torque and vorticity generation are shown to be primary drivers of the flow instability.

Development of a comprehensive flame transfer function and its application to predict combustion instabilities in a dump combustor

ABSTRACTIn combustion instability analysis, a flame transfer function (FTF) is used to describe the response of a premixed flame to incident acoustic oscillations. A comprehensive FTF is developed that relates the fluctuations in flame heat-release rate to those in density, heat of reaction, turbulent flame speed, and the flame surface area. Each of these four contributions is eventually expressed in terms of the fluctuations in acoustic pressure and velocity through the respective response functions. The flame surface-area response to acoustic oscillations is incorporated into the FTF through the -equation level-set method. In this method, we directly solve for the level-set fluctuations in terms of the velocity fluctuations and then relate the flame surface-area oscillations to . In the absence of turbulent flame-speed fluctuations, the response functions from the present -equation approach are in good agreement with those from the conventional -equation approach (where one writes ). However, when the turbulent flame-speed fluctuations are included, the two approaches differ, principally in the flame response to axial velocity fluctuations. The current FTF, generalized for any mean flame shape, is applied to analyze the response of a V-shaped mean flame located at the cross-sectional interface of a 2D dump combustor. For this flame, the effects of variation in acoustic frequency, mean Mach number, mean temperature, and mean equivalence ratio on the FTF magnitude and phase are investigated. Both axial and mixed mode acoustic perturbations are considered. For the purely axial modes, the response-function amplitudes of density and heat of reaction are seen to be independent of frequency, while those of flame speed and axial velocity exhibit harmonic-like oscillations as a function of frequency. For the mixed-mode fluctuations, the response-function amplitude of density decays monotonically with frequency, whereas the response-function amplitudes of the heat of reaction, turbulent flame speed, and velocity components show decaying oscillatory behavior in frequency. Further, it is observed that the flame-speed response function contributes the most to the overall FTF. Phase analysis of the FTF shows that the transverse velocity response is nearly in phase with the heat-release fluctuations. In addition, density and transverse velocity fluctuations always show constructive interference with pressure fluctuations. The current FTF is also incorporated into a linear modal analysis framework to predict the combustion instabilities in a 2D dump combustor, and the model predictions validated against experiments. Three mean velocity profiles—uniform, parabolic, and turbulent power law—are considered. For the three flames, the first three longitudinal and fundamental transverse unstable modes are predicted.

Leading edge dynamics of lean premixed flames stabilized on a bluff body

This paper examines the dynamics of the flame leading edge in a laminar premixed CH4/air flame stabilized on a bluff body in a channel. Harmonic fluctuations and step velocity change are used to simulate the flame response to acoustic oscillations, which are of primary importance in the study of thermo-acoustic instabilities. We use a fully resolved unsteady two-dimensional code with detailed chemistry and species transport, with coupled heat transfer to the bluff body. Calculations were conducted with different equivalence ratios, body materials, and steady state inlet velocity with step or harmonic perturbations. Results reveal that the flame leading edge dynamics displays a peak response around St = 0.5 suggesting that the leading edge motion is mainly due to the advection of appropriate ignition conditions as a result of the excitement of the wake recirculating flow. There is considerable augmentation of the flame wrinkles generated by the flame leading edge motion as result of the flow–flame interaction. Additionally, we show that a flame that anchors on average further upstream leads to stronger damping of the shear layer vortices and thus weaker vortex-flame interaction and heat release fluctuations. Hence, we identify two different mechanisms by which the flame leading edge location and oscillation amplitude impact heat release fluctuations. The study suggests a stronger dependence of the overall flame wrinkling and heat release fluctuations on the flame leading edge dynamics than recognized previously and the potential role it plays in combustion dynamics.

Impact of Precessing Vortex Core Dynamics on Shear Layer Response in a Swirling Jet

Combustion instability, the coupling between flame heat release rate oscillations and combustor acoustics, is a significant issue in the operation of gas turbine combustors. This coupling is often driven by oscillations in the flow field. Shear layer roll-up, in particular, has been shown to drive longitudinal combustion instability in a number of systems, including both laboratory and industrial combustors. One method for suppressing combustion instability would be to suppress the receptivity of the shear layer to acoustic oscillations, severing the coupling mechanism between the acoustics and the flame. Previous work suggested that the existence of a precessing vortex core (PVC) may suppress the receptivity of the shear layer, and the goal of this study is to first, confirm that this suppression is occurring, and second, understand the mechanism by which the PVC suppresses the shear layer receptivity. In this paper, we couple experiment with linear stability analysis to determine whether a PVC can suppress shear layer receptivity to longitudinal acoustic modes in a nonreacting swirling flow at a range of swirl numbers. The shear layer response to the longitudinal acoustic forcing manifests as an m = 0 mode since the acoustic field is axisymmetric. The PVC has been shown both in experiment and linear stability analysis to have m = 1 and m = −1 modal content. By comparing the relative magnitude of the m = 0 and m = −1,1 modes, we quantify the impact that the PVC has on the shear layer response. The mechanism for shear layer response is determined using companion forced response analysis, where the shear layer disturbance growth rates mirror the experimental results. Differences in shear layer thickness and azimuthal velocity profiles drive the suppression of the shear layer receptivity to acoustic forcing.