Swirling Spray Flames Research Articles

Advanced multiscale modal and frequency analysis of swirling spray flame near to lean blowout

This study provides an in-depth characterization of Jet-A1 swirled flames' dynamics using advanced decomposition techniques on high-speed OH* chemiluminescence images. Research conducted in a 300-kW combustor with global fuel-to-air ratios Φ = 0.36, 0.24, and 0.18 reveals dominant low-frequency components under ultra-lean conditions through statistical and wavelet analyses. Integrating Proper Orthogonal Decomposition (POD), Dynamic Mode Decomposition (DMD), and Multiscale Proper Orthogonal Decomposition (mPOD) enhances understanding by detailing the energy and frequency of flame modal structures. Observations show that as Φ decreases, flame size and intensity reduce, while fluctuations increase, indicating a transition towards unstable combustion dynamics. Furthermore, the frequency and intensity of burst phenomena increase, particularly in higher modes, indicating the system's approach to lean blowout (LBO). The application of nonlinear time series analysis, including autocorrelation (AFC) and phase space reconstruction, to mPOD eigenvalues, detects early warning signs of LBO. At lower Φ, the ACF reveals significant and frequent bursts, indicating high instability and intermittent behavior. Phase space reconstruction shows distorted orbits with a spiral-like structure, characteristic of substantial damping and irregular behavior near blowout conditions.

Large-eddy simulation investigation on the effects of inlet swirl/Reynolds number and fuel heating value on a turbulent kerosene spray flame

To progress toward the high efficiency and low emission aviation engine technology, it is mandatory to understand the complex phenomena including kerosene spray atomization, evaporation, turbulent mixing and combustion in the engine combustor. Nowadays with the rapid growth of computational capacity over the world, large-eddy simulation (LES) performs a more and more important role in understanding the complex multi-phase combustion process of kerosene fuels in aviation engine configuration. In the present work, LES is employed to investigate a swirling kerosene spray jet flame in a model combustor, which has been studied experimentally. The turbulent combustion is modeled by the flamelet generated manifold (FGM) approach with a 3-component surrogate kerosene skeletal mechanism. The averaged gas velocity and temperature predicted by LES generally agrees well with the experimental measurements. Local extinction is observed at the inner shear layer of the swirling spray flame, due to droplet-turbulence-flame interactions. A parametric study with 11 cases is then performed to investigate the effects of inlet swirl/Reynolds number and fuel heating value on the kerosene flame. As the swirl number increases, the enhanced turbulent mixing between kerosene fuel and air facilitates the combustion process, but combustion instabilities may occur if the swirl number is too high. A higher Reynolds number can promote the turbulent combustion via a stronger recirculation zone, but it also has strong dilution effects on the combustion field. Variations in the fuel heating value directly impact the temperature field, but the flow field, major species concentrations and droplet phase statistics are only slightly affected. These LES results give clues to achieve optimal combustion performance in realistic aviation engine configurations via better arrangements of fuel and air injections.

Spray flame synthesis of the NASICON-structure Na3Zr2Si2PO12 solid electrolyte nanoparticles for solid-state Na+ batteries

NASICON structural Na3Zr2Si2PO12 is considered as a promising solid electrolyte for all-solid-state sodium ionic batteries owing to the high ionic conductivity at room temperature, good moisture, thermal and chemical stability, and wide electrochemical window. However, there are still no reliable and scalable synthesis method for such multi-element complicated structural oxides. In this work, NZSP electrolyte particles are successfully synthesized by swirling spray flame synthesis and the solid electrolyte pellets are made by solving the problems of multiple elements sintering and densification. The SSA of the as-synthesized raw NZSP nanopowders with reduced amount of EHA is the largest with size of about 10 nm for the gas-to-particle route and suitable pathway in flame. The raw nanoparticles are of core-shell structure with ZrO2 cores and amorphous shells of other elements due to the distinctive temperatures for gas-to-solid transformation of different elements. The disk-shape pellets compassed by non-NASICON-structure NZSP raw nanopowders become NASICON structure after reactive sintering and two-step sintering. The NZSP solid electrolyte pellet with stoichiometric P content sintered at 1250/1100 °C for 12 h by two-step sintering method exhibits the highest ionic conductivity of 0.65 mS/cm at room temperature (25 °C) with activation energy of 0.370 eV and highest relative density of 98.6 %. The excellent conductivity and the high-yield production capability of the swirling spray flame synthesis make it a promising synthesizing method for wider application of NASICON structural NZSP solid electrolytes.

Large Eddy Simulations of n-heptane and n-dodecane binary blends in swirling multi-component spray flames

Well-understanding and mastering Sustainable Aviation Fuels (SAF) mixture composition as well as the potential of their initial component concentrations’ impact on flames is clearly of critical importance in today’s effort and energy transition. In this study, the focus lies on conducting Large Eddy Simulations (LES) to comprehend the impact of species concentration changes in well-controlled multi-component fuel blends on flame structures. The SICCA-spray rig from the EM2C laboratory operated with three blends of n-dodecane and n-heptane in varying proportions, is specifically addressed and investigated in light of the available data. To conduct these simulations, the dynamically thickened flame model and an evaporation multi-component sub-model are coupled with a reduced chemistry mechanism for n-heptane and n-dodecane binary blends. Across all investigated blends, the simulated swirling spray flame predictions align well with the experimental measurements confirming the suitability of the proposed modeling. For this configuration, the alterations in species concentration do not appear to significantly impact the overall flame structures and characteristics when observed from an average perspective. However, localized differences are identified, revealing notable composition effects. The simulation outcomes indicate that the early consumption of n-heptane contributes to stabilizing the flame, whereas the vaporization of n-dodecane is the primary factor responsible for combustion occurring further downstream. These effects are closely tied to the evaporation properties of each fuel compound and their concentration proportions within the blend, as expected. This insight highlights the intricate relationship between fuel properties, their concentrations within blends, and the resulting combustion behavior, shedding light on the complexities of multi-component fuel combustion characteristics.

Effect of low fuel temperature on combustion deterioration of kerosene swirling spray flames using OH-PLIF

The low fuel inlet temperature poses challenges to the flame stability of aircraft engine combustors. In order to investigate the combustion deterioration of swirl-stabilized kerosene spray flames under conditions of low fuel inlet temperature (T < −16 °C), a comprehensive system was established, comprising a burner commonly used in aircraft engines and a cooling setup. Upon analyzing OH-PLIF images, it becomes evident that sub-zero Celsius temperatures significantly lead to a decrease in OH radical concentration, resulting in a diminished heat release rate. The strongest signals decrease by a minimum of 5 times. Additionally, through the analysis of LIF spectra and detuned images, the differences between OH-LIF and kerosene-LIF are beneficial for recognizing the kerosene particles from the OH-LIF signals at the same time. For atomization assessment, employing the LOG operator blob detection method, it is found that at sub-zero Celsius temperatures, the overall count of fuel LIF particles experiences a slight reduction, while the count of larger particles increases across various air flow rates. This observation highlights an inferior droplet distribution at lower fuel temperatures. The image processing of kerosene LIF particles opens up the possibility for a rapid evaluation of atomization, especially when assessing combustion deterioration in kerosene spray flames through the use of a simple OH-PLIF technique.

Stability and structure of lean swirling spray flames with various degrees of prevaporization

Achieving full premixing and complete evaporation in lean prevaporized premixed combustors is challenging as it depends on the spray injector characteristics, prevaporisation strategy, and flow conditions. This article experimentally explores the stability and structure of a turbulent swirling n-heptane spray flame under various degrees of prevaporization. The results show that preheating the air to 343 K and 393 K has little effect on the lean blow-off velocity, while recessing the fuel injection significantly decreases the lean stability limit. To correlate these limits, various attempts to define a Damköhler number were made, but unlike previous studies with no prevaporisation, the difficulty in defining laminar flame speed in the present case does not allow a single correlation to work for all degrees of prevaporization. Four stable cases that differ in equivalence ratio, air preheat temperature, and fuel injection recess are investigated using one-dimensional PDA, OH* chemiluminescence and CH[Formula: see text]O-planar laser-induced fluorescence (PLIF). Cases without fuel injection recess or air preheat exhibit a conical-shaped heat release zone near the shear layers. Preheating the air to 393 K reduced the Sauter mean diameter, increased prevaporization, and enabled a second heat release zone downstream of the fuel injection. Recessing the fuel injection by 25 mm reduced droplet velocities and led to a semi-spherical instead of a conical heat release zone. The CH[Formula: see text]O-PLIF signal without injection recess was high along the central axis and its distribution resembled that observed for spray jet flames. In contrast, with recessed spray injection, CH[Formula: see text]O was mainly found outside the central recirculation zone and only appeared inside during lean blow-off; similar to previous work with premixed flames. These findings show that different methods of prevaporization, which only differ by subtle changes in droplet characteristics, strongly impact flame stability. The present data can be used for turbulent flame modelling focusing on sprays and finite-rate kinetics.

Large Eddy Simulations of complex multicomponent swirling spray flames in a realistic gas turbine combustor

Large Eddy Simulations of the realistic liquid fueled gas turbine combustor LOTAR operated at ONERA are performed for two fuels; a conventional JetA-1 and an alternative alcohol to jet fuel At-J, each modeled by a 3-component formulation. JetA-1 is composed of n-dodecane, methyl-cyclohexane and xylene each corresponding to the major hydrocarbon families found in real fuel. At-J is a synthetic drop in fuel composed of only branched chain alkanes, iso-octane, iso-dodecane and iso-hexadecane. Analytically Reduced Chemistry and multicomponent spray evaporation model coupled to the dynamic thickened flame turbulent combustion model are employed to understand the processes involved in turbulent spray flames in the LOTAR configuration. The objectives are to predict and understand the potential effects of staged vaporisation and consumption of the fuel components, and their impact on the spray flame structures. Simulations confirm the role of preferential evaporation in establishing and stabilising the reaction zone. JetA-1 evaporation zones extend deep into the rich burnt gasses resulting in a combustion regime with the possibility of droplet clusters burning individually. At-J which is more volatile, leads to complete combustion with the majority occurring due to the premixed lean reactions of the smaller pyrolysed components. The need to further include models capable of identifying and handling combustion regimes encountered in such spray flames is hence highlighted. This work is intended as a starting point for improving multicomponent spray modelling and requires additional experimental data for validation.

Swirling spray flames dynamical blow out induced by transverse acoustic oscillations

Recent experiments on a laboratory scale annular system comprising multiple injectors (namely, MICCA-Spray), indicate that combustion instabilities coupled with azimuthal modes may induce large amplitude oscillations, which under certain conditions, lead to blow out of some of the flames established in the system, a phenomenon designated as dynamical blow out (DBO). An attempt is made in the present investigation to reproduce this phenomenon in a linear array of injectors (namely, TACC-Spray), where the acoustic field is externally applied to flames established by injector units that are identical to those used in the annular combustor. The acoustic field is generated by driver units placed on the lateral sides of a rectangular cavity. The pressure level induced in TACC-Spray can reach a peak value of 1700 Pa in a frequency range extending from 680 to 780 Hz, which corresponds to the typical frequency of azimuthal instabilities observed in the annular system. A theoretical model based on dimensional analysis serves to guide the choice of operating conditions that may lead to the DBO phenomenon. Experiments carried out in TACC-Spray and MICCA-Spray are then used to determine the DBO boundary, define the conditions that need to be fulfilled to observe this phenomenon, and gather high-speed visualizations providing some insights on the mechanisms that induce blow out.

Rapid Gas-Phase Synthesis of the Perovskite-Type BaCe0.7Zr0.1Y0.1Yb0.1O3-δ Proton-Conducting Nanocrystalline Electrolyte for Intermediate-Temperature Solid Oxide Fuel Cells.

Perovskite-type proton-conducting materials, such as BaCe0.7Zr0.1Y0.1Yb0.1O3-δ (BCZYYb), are very attractive for the next-generation equipment of electrochemical energy conversion and storage owing to their excellent conductivity in the intermediate-temperature range (300-750 °C), as well as good thermo-chemical stability, coking resistance, and sulfur tolerance. However, the lack of a reliable and cost-effective synthesis method for such multi-component co-doping oxides limits their large-scale application. In this study, for the first time, we successfully synthesize BCZYYb electrolyte nanopowders by using a rapid, scalable flame-based gas-phase synthesis method with two different barium precursors Ba(NO3)2 and Ba(CH3COO)2, named as BCZYYb (N) and BCZYYb (CA). The as-synthesized nanoparticles exhibit good crystallinity of the pure orthorhombic perovskite BCZYYb phase. BCZYYb (CA) shows more uniform doping with the element ratio of 1:0.74:0.12:0.08:0.1, which is very close to the theoretical value. The shrinkage and surface SEM (scanning electron microscope) results indicate that the flame-made powders have superior sinterability compared to the sol-gel-made powders because of the smaller primary particle size (∼20 nm). Electrochemical impedance spectroscopy tests show that BCZYYb (CA) sintered at 1450 °C has the highest protonic conductivity of 1.31 × 10-2 S cm-1 in wet H2 when operating at 600 °C and still maintains a high-level conductivity of 1.19 × 10-2 S cm-1 even when the sintering temperature is reduced to 1350 °C, which is mainly attributed to uniform doping and good sinterability. The activation energy for the conductivity of BCZYYb (CA) is also significantly lower than that of conventional electrolytes, which suggests much better conductivity in the intermediate (∼600 °C) and even lower operating temperature. The excellent conductivity performance combined with the high-throughput production capability makes the swirling spray flame a promising synthesis method for promoting the BCZYYb electrolytes from lab to industrial-scale solid oxide fuel cells.

Predicting the ignition sequences in a separated stratified swirling spray flame with stochastic flame particle tracking

Stochastic flame particle tracking in conjunction with non-reacting combustor simulations can offer insights into the ignition processes and facilitate the combustor optimization. In this study, this approach is employed to simulate the ignition sequences in a separated dual-swirl spray flame, in which the newly proposed pairwise mixing-reaction model is used to account for the mass and energy transfer between the flame particle and the surrounding shell layer. Based on the flame particle temperature, the particle state can be classified in to burnt, hot gas, and extinguished. The additional state of hot gas is introduced to allow the flame particles with high temperature to survive from nonflammable region and then potentially to ignite the nearby favourable regions. The simulations of the separated stratified swirl spray flame reveal two different ignition pathways for flame stabilization. The first showed that some flame particles from the spark would directly enter the main recirculation zone resulting from the velocity randomness and then ignite both sides of the combustor simultaneously. The second showed that flame particles from the spark would ignite the traversed regions following the swirl motion inside the combustor. The predicted ignition sequences were compared with the evolution of flame morphology recorded by high-speed imaging from experiments, showing qualitative agreement.

Studies on a swirling heptane spray flame by large-eddy simulation

Swirling spray flames are widely encountered in gas-turbine combustors, rocket and ram-jet engines and internal combustion engines. Large-eddy simulation (LES) with different sub-grid scale (SGS) stress and combustion models is frequently used for studying the combustion mechanism of liquid fuel and design of combustors. In this paper, LES with a statistical-moment combustion model was used for studying a swirling spray flame. The statistical results for velocity, temperature and species concentration were validated by both experimental results and DNS database, showing the reliability of the simulation results. The instantaneous results give the spiral structures of flames formed by swirl, which can intensify combustion.

Effects of liquid fuel/wall interaction on thermoacoustic instabilities in swirling spray flames

Effects of liquid fuel/wall interaction on thermoacoustic instabilities in swirling spray flames

On the impact of fuel injection angle in Euler-Lagrange large-eddy simulations of swirling spray flames exhibiting thermoacoustic instabilities

On the impact of fuel injection angle in Euler-Lagrange large-eddy simulations of swirling spray flames exhibiting thermoacoustic instabilities

On the impact of fuel injection angle in Euler–Lagrange large eddy simulations of swirling spray flames exhibiting thermoacoustic instabilities

This study deals with the fundamental problem of combustion dynamics in gas turbine combustors operating with liquid fuel. In this framework the present work proposes the study of an academic liquid fueled combustor sensitive to thermoacoustic instabilities, simulated via high-fidelity Large Eddy Simulations. The experimental setup addressed is SICCA-spray from EM2C laboratory featuring both stable and unstable flames depending on the combustion chamber length. The proposed analysis, based on the Euler-Lagrange modeling approach, studies the impact of the spray injection angle θ on both the stable flame and the triggering of the longitudinal combustor acoustic mode when using a longer quartz tube. For the liquid injection modeling, the FIM-UR semi-empirical model is adopted with three different θ values: θ=35∘,45° and 60°. In stable conditions, the spray angle is proven to have a negligible impact on the flame anchoring point, however, the mean flame length and fuel distribution are found to be slightly modified by the velocity at which droplets enter the combustion chamber. For the thermoacoustically unstable conditions, two well-established stable limit cycles with the same frequency and similar amplitudes are obtained when fuel is injected at θ=45∘ and 60°. Contrarily, the system stabilizes when θ=35∘ pointing to the importance of the dynamics of the liquid film layer formed inside the injector for this setup. Likewise, this liquid film layer dynamics and its modeling appear critical as already suggested by previous studies on the same configuration. The detailed analysis of the thermoacoustically unstable different predictions is then performed through the investigation of the spatial fields of the local Rayleigh index obtained following the novel extension in the frequency domain of the Rayleigh criterion complemented by the application of Dynamic Mode Decomposition. It confirms that the injection angle of the liquid spray has a significant effect on the thermoacoustic response of the system. Indeed the influence of θ on the dynamics of the liquid fuel when entering the combustion chamber is proven to have an impact on the synchronization mechanism governing the liquid phase with respect to acoustics sustaining the observed limit cycles. More specifically, couplings at the liquid phase level are evidenced by introducing two novel indices correlating the fluctuations of liquid fuel volume fraction and evaporation rate with pressure.

On the impact of fuel injection angle in Euler-Lagrange Large Eddy Simulations of swirling spray flames exhibiting thermoacoustic instabilities

On the impact of fuel injection angle in Euler-Lagrange Large Eddy Simulations of swirling spray flames exhibiting thermoacoustic instabilities

On the impact of fuel injection angle in Euler-Lagrange Large Eddy Simulations of swirling spray flames exhibiting thermoacoustic instabilities

On the impact of fuel injection angle in Euler-Lagrange Large Eddy Simulations of swirling spray flames exhibiting thermoacoustic instabilities

On the impact of fuel injection angle in Euler-Lagrange Large Eddy Simulations of swirling spray flames exhibiting thermoacoustic instabilities

On the impact of fuel injection angle in Euler-Lagrange Large Eddy Simulations of swirling spray flames exhibiting thermoacoustic instabilities

Saturation phenomenon of swirling spray flames at pressure antinodes of a transverse acoustic field

This paper describes an experimental investigation of a linearly-arranged multiple-injection system of three swirling spray flames, simulating an unfolded sector of an annular combustor. Here we focus on the central flame placed in the basin of a pressure-antinode (PAN) of a standing transverse wave. Its response, quantified by means of the rms CH* amplitude filtered at the forcing frequency fr, is studied as a function of the acoustic pressure rms amplitude reduced by the bulk aerodynamic pressure, Π, and parameterized by the flame power P. As Π increases, three zones are identified: a linear growth, a transition zone and a saturation zone. Data in the linear growth and transition zone reduced by an adequate parameter merge into a unique self-similar curve whatever P. In the saturation zone, the time-frequency analysis applied to photomultiplier signals shows that the frequency signal at fr degrades in intensity level and uniformity, showing a loss of robustness in the flame response to the acoustic forcing over time. The gas phase displays strongly oscillating aerodynamics that can disrupt the central recirculation zone and destructure the mixing process. The liquid-phase shows a severe space-time modulation in droplet distribution and properties at high Π. Droplets are mainly found in an annular domain, characterized by a large time-dependence and inhomogeneity: a few quite big and slow fuel droplets are detected when the acoustic pressure P′(t) is positive, while a large population of small and rapid droplets is found when P′(t) is negative. In contrast, a little populated central core widens at positive P′(t) and fills up with big and slow droplets at negative P′(t). Such a periodic clustering might thus slow down the evaporation process.

Analysis of low-temperature chemistry in a turbulent swirling spray flame near lean blow-out

An n-dodecane swirling spray flame from the Cambridge flame series is simulated using large-eddy simulation (LES) at conditions near the lean blow-out (LBO) limit. The focus of this study is to examine effects of low-temperature chemistry (LTC) and spray evaporation in turbulent spray combustion. To this end, a first simulation is performed using a finite rate chemistry model with a 55-species skeletal mechanism including LTC in a fully compressible Eulerian-Lagrangian formulation. A second simulation is performed using the same formulation, but with the LTC chemical sub-mechanism deactivated. The interactions of spray and gas-phase mixing and combustion are investigated through the consideration of mean and instantaneous LES results. Chemical explosive mode analysis (CEMA) is extended to account for droplet evaporation in the context of spray combustion, the results of which reveal that the flame is dominated by non-premixed combustion without significant auto-ignitive behavior. CEMA results further show that the effect of spray evaporation on the reaction is two-fold, namely to inhibit reaction near the injector through heat absorption, and to facilitate reaction further downstream by supplying fuel to the gas phase. Mixture fraction-conditioned analysis is then performed to evaluate the importance of LTC in the turbulent spray flame, showing its effect on heat release despite the flame not exhibiting auto-ignitive behavior. A polar mapping is proposed for analyzing the complex interplay of low and high temperature chemistry heat release. The results have consequences for numerical modeling of spray combustion systems where LTC effects are commonly neglected.

Effects of liquid fuel/wall interaction on thermoacoustic instabilities in swirling spray flames

Effects of liquid fuel/wall interaction on thermoacoustic instabilities in swirling spray flames