Swirl Burner Research Articles

Numerical investigation of ammonia/coal co-combustion in a low NOx swirl burner

Co-firing of ammonia (NH3) and coal in boilers is a promising technology to reduce CO2 emissions from power plants. However, NH3/coal co-firing in swirl burners can impact the original flame structures and may lead to serious NOx emission issues due to the high concentration of nitrogen in NH3. In this study, a numerical simulation of a low-NOx swirl burner was carried out to investigate the changes in flame structure, carbon emission, and characteristics of NOx generation. The results show that, when the NH3 co-firing ratio is 10 cal%, the flame retains its original swirl structure, and a significant increase in NO formation is observed in situations with different injection positions. When the co-firing ratio increases to 20 cal% and 30 cal% and NH3 is injected through the central air pipe, the flame changes from a swirl flame to an elongated flame. The injected NH3 is wrapped by the low-temperature primary air, separating the NH3 from the high-temperature zone, and pyrolysis becomes the primary reaction pathway for NH3 consumption rather than oxidation. The NO concentrations measured at the chamber outlet can be reduced to 11.2mg/Nm3, which is lower than that from coal combustion. As the co-firing ratio further increases to 50 cal%, NH3 and air are well mixed and burned, forming a “candle-type” flame. This results in a significant rise in NO emissions.

Küçük Bir Girdaplı Yakıcı ve Fırında CO2 Seyreltmesinin Alev Stabilizasyonu ve NOX Emisyonu Üzerindeki Etkileri

There is a rapid increase in the demand for low NOx hydrogen/methane combustion systems in recent years, and low NOx and low emission burners and furnaces are promising platforms for such applications. Combustion instability is the most important drawback in low NOx burners and uniform thermal field. This paper investigates the influence of CO2 in methane and hydrogen on combustion instability. Numerical simulations were conducted using a small swirl burner and furnace combustion system. The effects of fuel type and CO2 dilution rate, which is a major contributor of hydrogen and methane combustion, on the combustion characteristics and instability are investigated. Combustion instability decreases for higher CO2 dilution rates.

Numerical simulation study on combustion characteristics and NOx emission of COREX gas swirl burner and boiler

To tackle the problem of high nitrogen oxide (NOx) emissions and low combustion efficiency when burning COREX gas, a type of high-calorific gas that is a by-product of the Ouye furnace, a low-NOx burner with broad applicability was designed. In this study, the feasibility of flue gas recirculation technology applied to a swirl burner of COREX gas was investigated through Fluent numerical simulation, and the effect of central flue gas recirculation ratio on burner combustion characteristics and NOx generation rate was analyzed. The results indicated that the implementation of central flue gas recirculation reduced the NOx concentration from 288.4 mg/m3 to 101.48 mg/m3. To further reduce nitrogen oxide emissions, an interlayer structure for flue gas was proposed to be installed between the COREX gas and internal secondary air. Flue gas was extracted from the tail of the boiler and introduced into the interlayer structure, thereby delaying the fuel and internal secondary air mixing while achieving uniform diffusion of low-temperature and low-oxygen flue gas into the combustion zone. Compared with the structure without flue gas interlayer, the NOx concentration can be reduced from 101.48 mg/m3 to 61.54 mg/m3 by passing the flue gas through the central passage and flue gas interlayer with a mass ratio of 3:5.The study demonstrated that the low-NOx burner designed in this paper had sound inhibition effects on thermal and prompt NOx. Based on both actual operation and simulation results, it was observed that the outlet NOx concentration could be regulated within 100 mg/m3 across varying boiler loads. This marks the first application of a low-NOx burner for the combustion of COREX gas in China.

Toward the use of LES for industrial complex geometries. Part II: Reduce the time-to-solution by using a linearised implicit time advancement

The strong increase in computational power observed during the last few years has allowed to use Large Eddy Simulation (LES) for industrial configurations. Nevertheless, the time-to-solution is still too large for a daily use in the design phases. The objective of this work is to develop a new time integration method to reduce the time-to-solution of LES of incompressible flows by allowing the use of larger time step. The projection method, probably the most commonly used method in the context of LES of incompressible flow, is generally applied using explicit time advancement which constrains the time-step value for stability reasons (CFL and Fourier constraints). The time step can then be small with respect to the physical characteristic times of the studied flow. In this case, an implicit time advancement method, which is unconditionally stable, can be used. However, this leads to non-linear resolution of momentum equation which can strongly increase time-to-solution because of non-linear iterations inside a physical iteration. To relax the stability constraints while minimising the computational cost of an iteration, a linearised implicit time advancement based on Backward Differentiation Formula (BDF) scheme is proposed in this work. The linearisation is performed using an extrapolated velocity field based on the previous fields. This time integration is first evaluated on a turbulent pipe test case. It is observed a time-to-solution up to five times lower than the explicit time integration while keeping the same accuracy in terms of mean and fluctuating velocity fields. To incorporate this new time advancement method in the automatic mesh convergence developed in Part I, a time-step control method based on the local truncation error is used. The resulting automatic time-step and mesh procedure is evaluated on a turbulent round jet case and on PRECCINSTA configuration, a swirl burner which is a representative case of an industrial aeronautical injection system. This new procedure leads to a time-to-solution up to three times lower than the previous procedure, presented in Part I.

Influence of blade angle in the outer secondary air for swirl burner on the flow, combustion, and slagging characteristics in the pre-combustion chamber under the air-staged condition

Influence of blade angle in the outer secondary air for swirl burner on the flow, combustion, and slagging characteristics in the pre-combustion chamber under the air-staged condition

Effect of acoustic excitation on the combustion and emission characteristics of methane-ammonia-air swirling flame

The blending characteristics of ammonia with conventional fuels have recently received much attention from researchers. This paper investigated the effect of acoustic excitation on the combustion and emission characteristics of methane-ammonia-air swirling flame on a laboratory-scale swirl burner. The high-speed schlieren imaging system was used to visualize the flame flow under acoustic excitation. The emission characteristics of CO and NOx at different excitation frequencies (70–270 Hz) and sound pressure levels (SPL, 117–128 dB) were determined. Results show that adding ammonia causes a significant increase in NOx emission and an increase in CO emission under fuel-rich conditions. The flame shows a layered V-shape with periodic development under weak low-frequency (70 Hz) acoustic excitation and an inverted cone with step distribution under high-frequency (270 Hz) acoustic excitation. Low-frequency acoustic excitation can reduce CO and NOx emissions under fuel-lean conditions. Especially for the excitation condition of 120 Hz (Prel = 0.3), NOx and CO emissions are reduced by 203 ppmvd and 170 ppmvd, respectively. However, when the excitation is stronger, the CO emission will rise sharply due to the destruction of the flame structure. Besides, the more intense acoustic excitation will cause a decrease in CO emission and an increase in NOx emission under fuel-rich conditions.

Large-Eddy Simulation of swirled flame stabilisation using NRP discharges at atmospheric pressure

In this work, the use of Nanosecond Repetitively Pulsed (NRP) discharges is investigated numerically to improve the stabilisation of the flame in a swirl burner. The studied configuration is the PACCI burner test rig set up at KAUST, in which it was experimentally demonstrated that NRP discharges can effectively enhance flame stability. Simulations are performed with the reactive compressible Navier–Stokes solver AVBP, using a recently developed phenomenological model for plasma-assisted combustion of methane-air premixed flame. The ability of the numerical model to reproduce the main flow behaviours is first assessed by comparison with PIV measurements in cold flow conditions. Then, a lean atmospheric pressure case (ϕ=0.67) without plasma actuation is simulated and compared with OH* chemiluminescence image from experiment. Both numerical and experimental results reveal an unstable turbulent flame, with intermittent attachment to the burner exit. Finally, two operating conditions with NRP discharges are simulated. The NRP discharges correspond to 10 kV pulses applied at a frequency of 20 and 30 kHz, respectively corresponding to a discharge power 0.72 and 1.17% of the thermal flame power. First, a significant stabilisation effect is observed when applying the discharges, which efficiently mitigate the flame lifting. This effect is quantitatively demonstrated by tracking the flame centre of gravity, which shows that the flame moves upstream under the influence of the plasma, in good agreement with experimental observations. Results also show that a net power gain is obtained by applying NRP discharges to combustion by comparing the thermal flame power to the plasma power. In particular, an increase of plasma actuation efficiency from 4 to 6 as been observed in the cases studied by increasing the plasma power.

Visualization and analysis of the impinging spray and combustion characteristics in a lateral swirl combustion system (LSCS)

A lateral swirl combustion system (LSCS) was proposed to improve the fuel/air mixing and combustion processes, and previous results have validated its excellent combustion performance in direct injection diesel engines. To further explore the fuel/air mixture mechanism of the LSCS, the effects of convex edge height ha = 2–8 mm (corresponding to interferential angle δ = 133°-14°) on the impinging spray and flame were investigated visually in a constant volume combustion system employing high-speed shadowgraph, direct photography and two-color methods. The experimental results suggested that the combustion chamber with ha = 6 mm (δ = 48°) exhibited superior results in terms of total spray area and combustion. Regardless of whether ha increased or decreased from 6 mm, all of which impaired the synthesis result of the wall-flow-guided and interferential effects. Specifically, when the ha was 2 mm, the wall-flow-guided effect was decreased, and the excessive δ corresponding to 133° caused the neighboring wall jets to approximate a head-on collision, which inevitably resulted in the part of fuel accumulation. Modified ha to 8 mm (δ = 14°), the neighboring wall jets developed nearly in the same direction, diminishing the interferential capacity. Similar higher-temperature and higher-soot areas were observed during the burn-out process for ha = 2 and 8 mm compared to what was obtained at ha = 6 mm, except that the bright area of ha = 2 mm was located near the convex edge at the bottom, while ha = 8 mm appeared in a higher position. Therefore, the LSCS, through exerting the comprehensive effects of wall-flow-guided and interferential interactions, obtains the optimal fuel/air mixing and combustion effects.

Thermal combustion characteristics of stabilized swirl burner for Prevaporized Partially Premixed Flames.

Thermal combustion characteristics of stabilized swirl burner for Prevaporized Partially Premixed Flames.

Toward the use of LES for industrial complex geometries. Part I: automatic mesh definition

With the constant increase of computational power for the past years, Computational Fluid Dynamics (CFD) has become an essential part of the design in complex industrial processes. In this context, among the scale resolving numerical methods, Large-Eddy Simulation (LES) has become a valuable tool for the simulation of complex unsteady flows. To generalise the industrial use of LES, two main limitations are identified. First, the generation of a proper mesh can be a difficult task, which often relies on user-experience. Secondly, the ‘time-to-solution’ associated with the LES approach can be prohibitive in an industrial context. In this work, these two challenges are addressed in two parts. In this Part I, an automatic procedure for mesh definition is proposed, whereas the Part II is devoted to numerical technique to reduce the LES ‘time-to-solution’. The main goal of these works is then to develop an accurate LES strategy at an optimised computational cost. Concerning the mesh definition, because LES is based on separation between resolved and modelled subgrid-scales, the quality of the computed solution is then directly linked to the quality of the mesh. However, the definition of an adequate mesh is still an issue when LES is used to predict the flow in an industrial complex geometry without a priori knowledge of the flow dynamics. This first part presents a user-independent approach for both the generation of an initial mesh and the convergence of the mesh in the LES framework. An automatic mesh convergence strategy is proposed to ensure LES accuracy. This strategy is built to guarantee a mesh-independent mean field kinetic energy budget. The mean field kinetic energy is indeed expected to be mesh independent since only turbulent scales should be unresolved in LES. The approach is validated on canonical cases, a turbulent round jet and a turbulent pipe flow. Finally, the PRECCINSTA swirl burner is considered as a representative case of complex geometry. First, an algorithm for the generation of an unstructured mesh from a STL file is proposed to generate a coarse initial mesh, before applying the mesh convergence procedure. The overall strategy including automatic first mesh generation and its automatic adaptation paves the way to use LES approach as a decision support tool for various applications, provided that the ‘time-to-solution’ is compatible with the applications constraint. A second paper, referred as Part II, is devoted to the reduction of this time.

Effects of biomass particles size and shape on combustion process in the swirl-stabilized burner reactor: CFD and machine learning approach

When planning the development of the energy sector, significant attention is given to the energy from the renewable sources, amongst which the biomass has an important role. Computational fluid mechanics and machine learning models are the powerful and efficient tools which allow the analysis of various heat and mass transfer phenomena in energy facilities. In this study, the in-house developed CFD code and machine learning models (Random Forest, Gradient Boosting and Artificial Neural Network) for predicting the biomass trajectories, particle mass burnout and residence time in a swirl burner reactor are presented. Pulverized biomass combustion cases (fine straw, pinewood and switch grass) with various mean diameters (ranging between 60 and 650 μm) and different shape factors (within the range 0–1) are considered. The results of numerical simulations revealed a noticeably nonlinear dependence between the input values (particle types, sizes and shapes) and the output values (particle trajectories, mass burnout and residence time), mostly due to the complex swirling flow in the reactor. For particles with the mean diameters within the ranges considered, the mass burnout of particles generally decreases as the biomass particle shape factor increases. The residence time of pulverized biomass in the reactor shows in most cases a decreasing trend as the particle shape factor increases. Artificial Neural Network showed the best predictions for both particle mass burnout (RMSE = 0.083 and R2 = 0.937) and particle residence time (RMSE = 1.145 s and R2 = 0.900), providing the reliable assessment of these important indicators in the combustion process.

On the Stability and Characteristics of Biogas/Methane/Air Flames Fired by a Double Swirl Burner

CO2-diluted methane fuel is relevant to biogas combustion applications. Despite its poor heating value and low reactivity, which limit its practical applicability, biogas gains popularity as a renewable fuel. However, implementing it in combustion systems requires either modifying or replacing the existing burners. This study investigates the stability, temperature field, and pollutant emissions of CH4/CO2/air-premixed flames fired by a double-swirl burner. A CH4/air mixture of equivalence ratio, Φout, was used in the outer stream, while a CH4/CO2/air mixture was supplied to the inner stream. The CO2 mole fraction, $${x}_{{\mathrm{CO}}_{2}}$$ , in the inner fuel blend varied from 0 to 0.4 for various inner stream equivalence ratios, Φin. The stability diagram of these flames was mapped in terms of Φin verses $${x}_{{\mathrm{CO}}_{2}}$$ for a fixed Φout. Based on the stability map, the inflame temperature field was investigated for six flames. Increasing the %CO2 in the biogas modifies the stability map by increasing the inner stream lean blow-off limits. However, increasing Φout sustains the flame stability, while reducing the CO2 decreases the overall flame blow-off equivalence ratio. Flame size growth with increasing $${x}_{{\mathrm{CO}}_{2}}$$ requires a longer residence time for efficient combustion. The addition of CO2 physically and chemically affects the thermal flame structure, and hence the pollutant emissions. In this burner, ultra-low NOX emission was reported, while an increase in the CO and unburned hydrocarbons (UHC), with increasing $${x}_{{\mathrm{CO}}_{2}}$$ was observed. However, the results show that, for a given $${x}_{{\mathrm{CO}}_{2}}$$ , controlling Φin and Φout could reduce CO and UHC emissions.

An Investigation into Burner Configuration Effects on Premixed Flame Characteristics for LPG Diluted with CO<sub>2</sub>

Meeting stringent emission regulations, the demand for environmentally friendly fuels is increasing by the day. Alternative fuel must be burned alongside conventional fuel to increase the availability of such clean energy sources. The current experimental study investigates the characteristics of the premixed LPG flames with CO2 dilution in tube swirling and non-swirling burners. The study including testing the effects of equivalence ratios, φ, (0.8, 1, 1.2, & 1.4), CO2 dilution ratios (0%, 5%, 7.5%, & 10%), and aspect ratio of the non-swirling burner (2, 4, 6, 8, & 10). Two swirling burners with swirl number was tested, namely 0.78 & 0.48. The dilution of CO2 has been observed lengthens the flame, particularly at higher equivalence ratios and/or flow rates since there is more than one influence, they all agree on a similar influence on flame height. The flame shortens clearly when using a swirling burner. Besides, when increasing the swirl number, the flame height increases slightly. Also, the swirling burner divided the flame's inner core into segments equal to the number of swirl vanes, and a flower-shaped flame was generated at low flow rates. The burner’s aspect ratio affects flame height insignificantly. Flame stability limits increase for a higher equivalence ratio and it enhances due to CO2 addition. The LPG-CO2/air mixture has an improved reply to beat flame flashback. The addition of CO2 expands the flow rate of stable flame by about 40% and 25% for φ = 1 and 1.2 respectively. Utilizing a swirling burner improves flame stability greatly. The limit between flashback and blowout increased by about three times as a result of using a swirling burner.

A method of matching the combustion chamber diameter based on spray characteristics for lateral swirl combustion system

To summarize a method of matching the combustion chamber diameter for lateral swirl combustion system (LSCS), at engine speeds of 1800 r/min and 2500 r/min, under 25%–100% loads and excess air coefficients (φa) of 1.2–2.0, the combustion and emission performance of LSCSs with different combustion chamber diameters was experimentally studied. Experimental results indicate that, with combustion chamber diameter of 88 mm, LSCS obtains the most favorable combustion performance, increasing the indicated thermal efficiency by 0.76%–4.03%. In addition, combustion chamber diameter affects the LSCS combustion performance more evidently under low load (25% load) and high excess air coefficient (φa of 2.0) conditions. Simulation research was performed under typical working conditions (at engine speed of 1800 r/min and 2500 r/min, under 50%–100% loads and φa of 1.2–2.0) to match the optimal combustion chamber diameter, and to study the corresponding spray characteristics. Simulation results show that, the optimal LSCS combustion chamber diameter and fuel spray penetration distance shows approximate liner relation. Therefore, the spray excess penetration coefficient τ is proposed, representing the interaction intensity between fuel spray and combustion chamber wall. For LSCS, when τ reaches its characteristic value of 1.5, the combustion chamber diameter matches well with the fuel spray penetration distance.

Effect of microsecond repetitively pulsed discharges on lean blow-off limit and emission of rapidly-mixed ammonia/air swirling flames

This work aims to implement an actuator integrating a swirl burner with electrodes to explore stable and low-emission combustion of ammonia/air. The cavity structure holds a plasma torch driven by low-power microsecond repetitively pulsed discharges (µRPDs) in the nozzle, which serves as a pretreatment for ammonia/air gas before it enters the combustion region. The plasma generates strong excited N2, OH, and atomic H* and O* signals, and fitting N2 spectra indicates a rotational temperature of ∼3000 K and vibrational temperature of ∼4500 K. The flame without plasma is easier to detach from the wall and operate as a cone-shaped structure in the confined quartz tube, while plasma can help attach the flame and extend the lean blow-off limit from ∼0.6 to 0.4∼0.5 in a Reynolds number range of ∼3000 to 7500. Then, optical diagnostics for OH radical and NH2* chemiluminescence are performed to enable analysis of intermediate chemistry. The NH2* signals are distributed at the edge of the OH profile in both attachment and detachment cases. Finally, flue gas analyzers are used to find an optimal lean equivalence ratio where plasma anchors a stable ammonia/air flame with a relatively low emission of approximately 200 ppm NO and zero NH3 and H2.

Effects of pulsed gliding arc plasma on non-premixed CH4/CO2– air flame stability

Effects of carbon dioxide (CO2) addition and pulsed gliding arc (PGA) plasma on methane (CH4) flame stability and pollutant emissions are investigated in this paper. Two main parts are reported: the effects of CO2 addition to the fuel and the impact of PGA plasma on the characteristics of non-premixed CH4/CO2-air flames in a coaxial swirl burner. The burner consists of 2 concentric tubes, the central tube supplies the CH4 flow and the periphery one supplies the airflow. In the central tube, a metal rod is used as a cathode. To produce the PGA plasma, two other electrodes are symmetrically placed around the central one to obtain two PGA plasma zones in the stabilization area of flame. The combustor is a parallelepiped chamber, with a 0.48 m square cross section and 1 m height equipped with six windows on each side. This work primarily focuses on pollutant emissions (NOx and CO), flame structures, and stability. OH* chemiluminescence measurements are used to describe the structure and stability of the flame providing information on the flame lengths and lift-off heights. The lift-off heights and the flame lengths are determined as a function of the CO2 concentrations, ranging from 0% to 50% in the fuel, at a fixed swirl number and equivalence ratio. The results show that CO2 addition to the fuel changes significantly the flame shape and its behavior. The flame becomes more unstable at high CO2 percentages. CO2 addition dilutes reactants, reduces NOx production, and increases CO concentrations in the flue gases. To overcome the instabilities problems due to the addition of CO2, the PGA is used and investigated with different parameters of plasma and flame. PGA plasma is generated by a DC pulsed power generator with HV pulse duration of few microseconds. It is observed that the presence of PGA plasma decreases flame instabilities and reduces CO concentration in the flue gases.

Experimental studies on the particulate matter emission characteristics of a lateral swirl combustion system for direct injection diesel engines

To analyze the particulate emission characteristics of a lateral swirl combustion system (LSCS), experimental research on a single-cylinder diesel engine was done and compared against the Turbocharger-Charge Air Cooling-Diesel Particle Filter Series combustion system (TCDCS) at different conditions. Compared to the TCDCS, the LSCS presents better combustion performance and lower total particle emission characteristics: As for the LSCS, the vast majority of the particle number size distributions shifted downward, indicating a decrease in the particle number concentration. The total particle number and mass concentrations of the LSCS decreased by 8.7–62.4% and 15.2–55.6% at various loads. The number concentration of particles smaller than approximately 8 nm increased for the LSCS, which can be attributed to the higher temperature and more thorough fuel/air mixture, facilitating the oxidation of large particles into small particles. Combined with the simulation, the LSCS perfectly exerts the wall-flow-guided effect, remarkably improving the fuel/air mixing quality and reducing the local over-concentration regions, which can inhibit the formation of particles. Hence, the LSCS effectively reduces the particle number and mass concentrations, exhibiting excellent particulate emission characteristics.

Conventional and distributed combustion regime on a model thermoelectric generator (TEG) performance using a swirl burner/furnace

This study carries out computational fluid dynamics (CFD) technique and MATLAB Simulink to examine the flame characteristics in the combustion chamber and thermoelectrical generator (TEG) performance. TEG converts heat energy into electrical energy by utilizing the Seebeck effect. It is stimulated through the temperature difference between its two surfaces. Temperatures and flame characteristics in the combustion chamber were obtained using the CFD analysis program. In this study, the flame characteristics and temperature profiles through the CFD code are predicted to provide TEG input values so that the model TEG performance can be predicted. For this purpose, colorless distributed combustion technique (CDC) was used to obtain a more uniform thermal field that will be needed to more efficient operate the TEG. The developed swirl burner was modeled to obtain temperature and emission profiles under all combustion conditions studied. The temperature profiles predicted on the wall have become a input to operate the thermoelectric generator. TEG use effects on flame characteristics were numerically investigated. TEG performance results at various temperatures were also examined and explained. The power achieved for the TEG modules placed on the wall has been calculated as 170 W for two cases (21% O2 and 18% O2), and 107.5 W at 15% O2. It is concluded that changes in temperature levels are not much more from the center to the combustor wall when the thermoelectric generators are placed, which suggests that combustion performance is not affected considerably when using it. It is revealed that hybrid low emission combustion-TEG integrated systems can be used under CDC conditions.

Experimental study of particle-laden turbulent reacting flow in a swirl combustor

A test facility for the swirl combustor with staged air injection was set up. The particle-laden turbulent reacting flow was experimentally studied in this facility. The gas and particle instantaneous velocities in the swirl combustor were measured by using the Three-Dimension Particle Dynamic Analyzer (PDA) under two different conditions. The distributions of time-averaged axial and tangential velocities, root mean square of axial and tangential fluctuating velocities, and correlation moments of axial-tangential fluctuating velocities are obtained for both gas and particle phases. The results contain also the distribution of particle mean diameter. The gas and particle fluctuating velocity characteristics exhibit obvious differences compared to their time-averaged velocities.

Impact of spray dynamics on combustion instabilities investigated by changing the atomizer recess in a swirl combustor

The present article is aimed at identifying the impact of spray-wall interaction on combustion instabilities. This is achieved by systematically modifying the liquid fuel atomizer location with respect to the injector outlet in experiments carried out on a single-injector swirling combustor. The injector is characterized by a high swirl number and a relatively high pressure drop, and features a pressure atomizer that delivers liquid heptane fuel in the form of a fine spray. In the first set of experiments, longitudinal self-sustained oscillations (SSOs) are examined by varying the atomizer recess distance and using three combustion chamber lengths. Three distinct instability regions characterized by jumps in amplitude and frequency are observed as a function of the atomizer recess. At lower SSO frequencies (obtained for longer chambers), the pressure fluctuation amplitude in the chamber decreases as the atomizer recess distance is reduced, tending towards a stable operation. An opposite behavior is observed at comparatively higher SSO frequencies (obtained with a shorter chamber), where the system becomes less unstable at higher atomizer recess distances. The oscillation frequency is also found to change and increases as the recess distance is reduced. Three recess distances corresponding to each zone of operation are selected to further analyze the flow and flame structures and dynamics. It is found that the mean velocity profiles only reveal moderate differences in the air flow field, but there are significant changes in the fuel spray distribution in the neighborhood of the injector axis. Laser sheet images of the spray reveal two patterns—one where the spray predominantly interacts with the injector end piece when the atomizer is recessed, and the other where this interaction is minimal. In the latter case, corresponding to a small recess, the fuel spray is directly conveyed into the chamber. It is next found that the spray distribution affects the flame pattern defining two distinct configurations. Finally, the flame response to external disturbances is characterized by flame describing functions (FDFs) that allow carrying out a stability analysis. FDF results obtained with two independent methods indicate that the gain and phase at the three recess positions vary substantially and that this can be used in combination with a low-order model to interpret the instability behavior of the system. This study might serve to guide the modeling of combustion dynamics and help design injectors that are less sensitive to instabilities.