Coaxial Flames Research Articles

Shear Coaxial Methane–Oxygen Injector Mixing and Combustion Examined by Laser Absorption Tomography

Mixing length scales for methane–oxygen shear-coaxial single-element rocket injectors were experimentally assessed using laser absorption tomography. The laser spectroscopy technique enables quantitative and spatially resolved measurements of carbon monoxide (CO) and temperature within the near-field region, probing rovibrational absorption lines of CO near 5 μm. Multiple injector designs were examined with differing oxidizer post recess depths to compare mixing characteristics. All tests were performed at an oxidizer-to-fuel ratio of 3 with a total propellant flow rate of 0.350 g/s. Planar measurements were taken at 16 axial positions, which collectively captured the first 57 mm of each flame. A tomographic inversion method was applied to obtain radially resolved distributions of temperature and CO mole fraction for each axial position, from which two-dimensional images of the thermochemical structure were generated. Characteristic mixing parameters are defined and extracted from features within the species and temperature profiles to visualize the spatial evolution of CO production. Increasing oxidizer post recess improved mixing due to enhanced shear-induced turbulence and associated radial diffusion of species and temperature; however, the enhancement was nonlinear. This work establishes the first use of laser absorption tomography to directly measure mixing length scales associated with temperature and species profiles in coaxial flames.

On the spatial characteristics of the flame transfer function for a GCH4-GO2 swirl coaxial flame

In this study, the combustion instability characteristics of gaseous methane–gaseous oxygen atmospheric flame were investigated. A swirl coaxial injector was used under external modulation with a central methane flow and a loudspeaker. The flame transfer function with a photomultiplier tube was examined for possible errors, and an injector transfer function was defined to study the influence of the input perturbation on the cold gas flow without combustion. Both the results of flame transfer function and injector transfer function were affected by acoustic resonance. However, a higher mode of resonance did not appear in the flame transfer function. To analyze this difference, the flame movement was visualized using a high-speed camera by capturing the OH* chemiluminescence signal. Furthermore, a local flame transfer function was defined to examine the flame response zone-by-zone through a light signal. A higher-mode reaction appeared again in the local flame transfer function. It was found that flame separation due to modulation was the cause of the difference among the flame transfer function, injector transfer function, and local flame transfer function. By capturing the entire flame, its periodicity could be lost; hence, the local flame transfer function could prevent misinterpretation of the flame fluctuation.

Assessment of Large Eddy Simulation for the prediction of recessed inner tube coaxial flames

Assessment of Large Eddy Simulation for the prediction of recessed inner tube coaxial flames

Evaluation of Large-Eddy Simulation Coupled with an Homogeneous Equilibrium Model for the Prediction of Coaxial Cryogenic Flames under Subcritical Conditions

Large Eddy Simulations of liquid O2/gaseous H2 coaxial flames at subcritical pressure conditions are reported in this paper. These simulations reproduce the experimental Mascotte cases A1, A10 and A30, operating at 1, 10 and 30 bar, respectively, and for which temperature measurements and experimental visualisations are available. The main objective of this work is to assess the accuracy of the multi-fluid Homogeneous Equilibrium Model (HEM) described in Pelletier et al. (Computers & Fluids, 2020) for rocket engine applications. Of particular interest is the comparison with the experimental temperature measurements from Grisch et al. (Aerospace science and technology, 2003). To that purpose, numerical simulations are conducted with care, in order to ensure a proper statistical convergence and estimate the influence of the grid resolution for each case. Despite the crude assumptions—no surface tension and no atomisation model, for instance—that are made with the HEM used in this work, results are found to be in reasonable agreements with the measurements for case A10, even with the coarser grid. For case A30, a fine mesh resolution is required to capture the low intensity recirculation zone downstream of the inner jet necessary to reproduce the shape of the experimental profile. Finally, case A1 simulations, with the lowest Weber number, show large departures with the experimental measurements. This is expected to be due to a deficiency of the model to properly reproduce the two-phase dispersed flow.

Effect of obliquity on impingement heating characteristics of co-axial swirling flame jet - An experimental and numerical investigation

Flame jet impingement has been studied as an indispensable technique for direct heating applications suitable for metal processing industries. The current experimental and numerical investigation pursues to unfold the fluid dynamics and heat transfer characteristics for an obliquely impinging co-axial swirling flame jet. Co-axial flame used in the present work contains outer swirling flame together with an inner non-swirling flame. Steady state heat balance technique using one dimensional energy balance across the impingement plate has been employed to determine the local heat flux distribution. Due to obliquely impinging swirling jet, development of an asymmetric thermal boundary layer takes place on the target surface. The geometry being studied here develops a complex flow field which has also been investigated numerically using RANS based ‘k-e’ RNG turbulence model. The effect of various operating parameters such as inclination (θ = 0°, 15°, 30° and 45°), impingement distance (Z/Dh = 3, 5, 7 and 9), Reynolds number (Re(s) = 2500, 4000, 5500, 7000 and 8500) and equivalence ratio (ϕ = 0.9, 1.0, 1.1 and 1.2) have been studied on the flow field development and resulting heat transfer distribution.

Waste tires pyrolysis oil and its blend with diesel fuel spectral flame analysis, temperature contours, and emissions

The purpose of this research paper is to investigate the energy valorization from the pyrolysis of waste tires using coaxial continuous flame burners. For such an investigation, both the Tire Pyrolysis Oil (TPO) along with its blend (B1) with light diesel oil (LDO) were prepared, physically and chemically characterized, and then combusted in coaxial burners. To distinguish the spectral emission peaks, for the vaporization and combustion zone and the hot recirculation zone of the produced flame, flame spectroscopy techniques were utilized. The mass-specific emission indices and the axial inflame temperatures were used as indicators of the flame radiation intensity in the flame zones. The coaxial flame was inspected for excess air factors of 1.33, 1.04, and 0.9. In the study, a higher radiation intensity of TPO/LDO blend flame was recognized in the B1 flame length which was taller than that of LDO by 48% at λ=1.33. Furthermore, a decrease in CO emissions levels was also noticed in the combustion of waste tire pyrolysis oil and B1 by an average of 4%–12% compared to that from HDO, and the NOx emissions were also reduced by 8.5%–26%. The highest axial inflame temperature of 1,205 °C was recorded for TPO at condition λ=0.9, where the presence of oxygen molecules in fuel helps flame improvements.

Ensemble Combustion of Liquid Oxygen / Gaseous Hydrogen Coaxial Flames with Transverse Acoustics

ABSTRACT Experiments were performed to investigate the ensemble combustion of a linear array of three coaxial liquid oxygen/gaseous hydrogen turbulent non-premixed jet flames, under the influence of transverse acoustic forcing. The results are systematically compared to the results from a predecessor study of the same ensemble of flames under the same conditions, but without acoustic forcing. Both pressure nodes (PN) and pressure antinodes (PAN) were investigated as a function of the outer-to-inner momentum flux ratio. Shadowgraphs and OH* emission imaging showed that PNs distort the flames more than PANs, similar to previous observations in single flames. A wave amplification mechanism (WAM) identified previously to be present both in single flames and in the unforced ensemble of flames was found to also be present under acoustic forcing. In the predecessor study without acoustics, the WAM tended to self-organize within the ensemble into an out-of-phase varicose nesting that minimized large-scale collisions. In the present study, PN forcing tended to organize the WAM structures into a sinuous nesting, which also minimized collisions. PAN forcing, on the other hand, promoted destructive in-phase varicose nesting that promoted large-scale collisions, causing subsequent breakdown into smaller scales. Dynamic Mode Decomposition revealed natural frequencies and modes in addition to the acoustic quantities. Discrete combustion waves related to the WAM propagated at the same normalized average velocities regardless of whether acoustics were present in the ensemble or not. However, the trajectories are grouped into different curves than for single-element flames.

Ensemble Combustion of Liquid Oxygen/ Gaseous Hydrogen Coaxial Flames

ABSTRACT Experiments were performed to investigate the ensemble combustion of a linear array of three coaxial liquid oxygen/gaseous hydrogen turbulent non-premixed jet flames. The experimental conditions were carefully controlled to duplicate those of a previous single element experiment as closely as possible, so that the single element flame could be closely compared with the present three element flame. All ensemble and inter-element effects were due to the turbulent combustion only; acoustic effects were intentionally suppressed. Abundant interactions were found both in the time averaged sense as well as in the dynamics of the larger scale motions. The multielement flames were found to be axially longer, and dominant frequencies were found to emerge for the multielement flames that were not present for the single element flames. A dynamic wave amplification mechanism previously identified for single element flames was found to remain present for multielement flames. However, the mechanism did not operate independently for each element in the ensemble, but instead entered into phase relationships with the neighboring elements. The phase relationship possibly encouraged the dynamics between jet flames that allowed the dominant frequencies to emerge. Several indications of such a phase relationship were found, but not all indications were observed to occur all of the time. It is possible that the coordination of the wave amplification mechanism between the flames was weak or intermittent. Other simultaneous processes probably also contributed to the overall flame behavior.

Coaxial Multiphase Flame for Continuous‐Flow Assembly of Ternary Nanocomposite Photocatalysts

AbstractFor the low‐cost production of graphene (G)‐included composites, the growth of graphene on a transition metal in hydrocarbon flames is introduced, where the hydrocarbons are adsorbed on the metal surface, followed by catalytic decomposition, leading to continuous G formation. On the other hand, the flame synthesis of G is still a batch process, limiting the practical applications of such materials. Moreover, the hybridization of G for enhanced catalytic activities requires additional reaction and separation steps. Here, a coaxial multiphase flame is generated and combined with a rotating Cu–Ni foil and ultrasonic bath for the continuous‐flow assembly of ternary G composites with low‐cost and rapid implementation. The continuous flows to generate a multicomponent flame consist of MoS2 particle‐laden N2 gas (inner), titanium isopropoxide vapor‐laden CH4‐air (middle), and ethanol liquid (outer), while Cu–Ni foil plies between the flame and ultrasonic bath for the assembly and dispersion of MoS2–TiO2@G composites. The configuration of the composite can be modulated by replacing the MoS2 flow with a CdS flow to construct CdS–TiO2@G. From photocatalytic H2 production and CO2 reduction tests, the developed coaxial flame provides comparable performance with analogous architectures from the usual multistep methods despite the high‐throughput production.

Effect of interactions on impingement heat transfer in odd and even element linear arrays of co-axial flames

Experimental and numerical study of linear arrays of co-axial flames impinging on a flat quartz plate have been performed and interactions and heat transfer characteristics are analyzed. High resolution inverse heat conduction procedure (IHCP) has been employed for determining local heat flux distribution. Strong interactions developed amidst adjacent flames led to asymmetric interactions. Combustion simulations revealed differences in the flow structures of four and five element arrays. Asymmetric interactions resulted in bending of all of the inner flames in four element array whereas central inner flame remains undeflected in five element array. Same has also been observed for six and seven element arrays too. Tilted columnar flows have been identified in even as well as odd numbered linear array of co-axial flames. These tilted flow structures resulted in distortion of recirculation zones (RCZ) of all the flames. Impingement heat flux contours are studied for various impingement spacings (H/Dh = 3, 5 and 7). At smallest impingement spacing, these contour maps revealed footprints corresponding to central inner flame due to symmetrical interactions. Thermal efficiency and heat transfer uniformity of four element array has been observed to be higher than five element array at all impingement spacings.

Transverse Acoustic Forcing of Gaseous Hydrogen/Liquid Oxygen Turbulent Shear Coaxial Flames

Gaseous hydrogen/liquid oxygen turbulent shear coaxial flames were subjected experimentally to both pressure antinode (PAN) and pressure node (PN) transverse acoustic forcing as a function of the gas-to-liquid momentum flux ratio. The response of the flow was recorded using simultaneous high-speed imaging of backlit shadowgraphs and chemiluminescence emission. It was observed that a wave amplification mechanism, defined in Sec. III, previously observed for unforced flows, remained present in the forced flows but was modified by the forcing. The response tended to be axisymmetric for PAN forcing, where the pressure fluctuations are symmetric, and asymmetric for PN forcing, where the velocity fluctuations are asymmetric. Additional analyses performed included dynamic mode decomposition (DMD) and an analysis of the wave trajectories as a function of time. Beyond the main observation that the wave amplification mechanism remained operative not only for unforced flows but also for both kinds of transverse forcing, secondary observations included a greater relative effect on the flow for PN forcing than for PAN forcing and a reduced general response of the flow to the acoustics at higher momentum flux ratios. The velocity of waves observed in the emission were found to depend mainly on the shear layer velocity and not directly on the acoustics.

Synthesis of Nanomaterials in a Coaxial Flame

The paper presents the results of experimental studies of the synthesis of fullerenes C60 in a coaxial flame of benzene and acetylene at low pressures; of the synthesis of graphene in a coaxial flame of ethanol and propane, benzene, and acetylene; of the soot formation process in the coaxial flame of propane and ethanol. It has been established that the optimum temperature of a coaxial flame for the formation of fullerenes C60 is 970‒1000 °C with the carbon to oxygen ratio in the internal benzene-oxygen flame C/O ≈ 0.9 ÷ 1. The C/O ratio in an external acetylene-oxygen flame was maintained at a stoichiometric ratio. It was found that the preliminary (before feeding into the burner) treatment of the benzene-oxygen mixture using ultraviolet (UV) radiation with a wavelength of 254 nm promotes an increase in the yield of fullerenes. The synthesis conditions were optimized for: 5‒10 layers graphene in a coaxial flame of acetylene and ethanol; graphene containing more than 10 layers in a coaxial flame of propane and ethanol; one and two-layer graphene in a coaxial flame of ethanol and benzene. The possibility of a significant reduction of the formation of soot particles in the diffusion flame of propane by organizing its coaxial combustion with ethanol is shown.

Scaling of oxygen-methane reacting coaxial jets using x-ray fluorescence to measure mixture fraction

Stoichiometric mixing length Ls of reacting coaxial jet flames is a critical scaling parameter for liquid rocket engine combustors. Previous studies have shown that Ls for shear coaxial flames can be scaled like their nonreacting counterparts using a nondimensional momentum flux ratio J. In addition, stoichiometric mixing lengths of reacting and nonreacting coaxial jets collapse upon a single line by altering J using an effective outer flow gas density. This effective density is calculated from a modified version of the equivalence principle, originally developed by Tacina and Dahm [1, 2] and accounts for the effects of heat release on mixing. However, previous studies also required a second nonphysical scaling constant Sc for the reacting jets, which is not predicted by the equivalence principle [3]. It was originally hypothesized that Sc is attributed to the limitation of hydroxyl (OH) planar laser-induced fluorescence, which only infers Ls. Direct quantitative measurement of conserved scalar fields using conventional optical diagnostics is difficult due to the lack of a tracer that easily fluoresces, survives high temperature oxygen flames, and is not dominated by quenching effects. To measure a conserved scalar field, this work implements x-ray fluorescence of Kr and Ar tracers to obtain quantitative mixture fraction fields. From these mixture fraction fields, stoichiometric mixing lengths for two CH4/O2 flames are calculated and scaled against nonreacting coaxial mixing lengths using the equivalence principle. By directly measuring the stoichiometric mixing length, it is established that the additional constant is a byproduct of the OH measurement technique and the equivalence principle fully captures the scaling. Comparison with high-fidelity simulation of the flame further supports this conclusion. In addition to further strengthening this scaling method, this work represents the first use of x-ray fluorescence to make quantitative conserved scalar measurements in turbulent flames.

Effect of Flow Parameters on the Acoustic Susceptibility of Shear-Coaxial Flames

The effect of density ratio, velocity ratio, momentum ratio, and fuel composition on the susceptibility of shear coaxial flames to periodic pressure waves is studied experimentally. A turbulent diffusion flame, established in an atmospheric-pressure, two-dimensional, shear coaxial combustor, is acoustically forced by a transversely mounted compression driver, and the level of flame acoustic interaction is quantified using chemiluminescence imaging. The parameter of interest is varied by diluting the propellants with helium, argon, or methane by appropriate amounts while holding other parameters relatively constant. Flame response is found to vary exponentially with density ratio, whereas it is found to vary linearly with velocity ratio and fuel composition, at least within the limited ranges tested. Flame response is found to vary exponentially when momentum ratio is varied by varying density ratio, whereas it is found to vary linearly when momentum ratio is varied by varying velocity ratio. Overall, our results support the notions that propellant density ratio is a critical parameter affecting shear coaxial flame–acoustic interactions and that a reduction in propellant density ratio could improve the stability of combustors using such injectors.

Dynamics of a transcritical coaxial flame under a high-frequency transverse acoustic forcing: Influence of the modulation frequency on the flame response

The present investigation is motivated by the problem of high-frequency instabilities in liquid rocket engines. The objective is to gain detailed information on the dynamical response of a transcritical coaxial jet flame submitted to transverse acoustic modulations. Large-eddy simulations (LES) are carried out to examine the effects of the modulation frequency on the flame dynamics when the flame lies in the vicinity of a transverse velocity anti-node, corresponding to a pressure node. It is found that the flame configuration and dynamics notably change with the modulation frequency and that the Strouhal number based on the dense core characteristics essentially determines the response of the system. In the case of a frequency close to the natural frequency of the oxygen jet (St=0.8), the flame motion may be assimilated to that of a flag flapping in the wind. When the flame is modulated at a higher frequency (St=3.2), it features a corrugated surface extending in the axial direction and executing a bulk motion in the transverse direction with no large scale deformation. For both frequencies, the flame is shortened, flattened in the spanwise direction and periodically displaced in the transverse direction following the acoustic field. These simulations are used in a second step to determine the unsteady rate of heat release and the Rayleigh term arising in the acoustic energy balance, giving access to the corresponding source of acoustic energy and to a possible driving mechanism of acoustic instabilities.

Large eddy simulations of multiple transcritical coaxial flames submitted to a high-frequency transverse acoustic modulation

This article describes a numerical investigation that explores the dynamics of multiple cryogenic jet flames interacting with high frequency transverse acoustic modes. This research is motivated by the numerous issues associated with high-frequency instabilities in liquid rocket engines. Large eddy simulations are carried out in a complex flow configuration which is turbulent, reactive, transcritical and in the absence or presence of a large-amplitude acoustic motion. The geometry is that of a model scale experimental configuration. Results obtained by exploiting high end computational resources demonstrate the feasibility of such calculations, provide insight in the coupling process, exhibit features which are found in experiments and complement the experimental data. Depending on the acoustic environment (dominated by velocity or pressure oscillations), selective responses of the cryogenic jets and flames can be observed and analyzed. It is found that the flames are more compact when they are made to interact with the transverse acoustic motion and that the dense core length is notably reduced. In the span-wise direction, the flames and dense cores are flattened, a feature which is also found in experiments. The unsteady motion observed experimentally is well retrieved numerically. The simulations highlight the mechanisms that can feed energy in the transverse mode and suggest possible descriptions of the instability driving process. The light methane jet is shaken by the acoustic motion and impacts the dense oxygen stream alternatively on its top and bottom sides. The unsteady heat release rate and the corresponding acoustic Rayleigh term produced by the flames prove to be different according to the flame position regarding the acoustic environment.

NOx emission characteristics in turbulent hydrogen jet flames with coaxial air

The characteristics of NOx emissions in pure hydrogen nonpremixed jet flames with coaxial air are analyzed numerically for a wide range of coaxial air conditions. Among the models tested in simple nonpremixed jet flame, the one-half power scaling law could be reproduced only by the Model C using the HO2/H2O2 reaction, implying the importance of chemical nonequilibrium effect. The flame length is reduced significantly by augmenting coaxial air, and could be represented as a function of the ratio of coaxial air to fuel velocity. Predicted EINOx scaling showed a good concordance with experimental data, and the overall one-half power scaling was observed in coaxial flames with Model C when flame residence time was defined with flame volume instead of a cubic of the flame length. Different level of oxygen mass fraction at the stoichiometric surface was observed as coaxial air was increased. These different levels imply that the coaxial air strengthens the nonequilibrium effect.

Appearance, temperature, and NO x emission of two inverse diffusion flames with different port design

Experiments were carried out to investigate the appearance, temperature distribution, and NO x emission index of two inverse diffusion flames, one with circumferentially arranged ports (CAPs) and the other with co-axial (CoA) jets, both burning LPG with 70% butane and 30% propane. Flame appearances were investigated first with a fixed fueling rate at different airflow rates equivalent to air jet Reynolds numbers (Re) of 1000 to 4500; and then at a fixed airflow rate with different fueling rates equivalent to overall equivalence ratios ( Φ) of 1.0 to 2.0. The CAP flame is found to consist of two zones: a lower entrainment zone and an upper mixing and combustion zone. The CoA flame in most cases is similar to a diffusion flame. The two-zone structure can be observed only at Re larger than 2500. The temperature distributions of the flames are similar at overall equivalence ratios of 1.0 and 1.2 for Re = 2500 , except that the corresponding CoA flame is longer. The flame temperature is higher in the CAP flame than the CoA flame at higher overall equivalence ratios. A measurement of centerline oxygen concentrations shows that the oxygen concentration reaches a minimum value at a flame height of 50 mm in the CAP flame but decreases more gradually in the CoA flame. It can be concluded that there is more intense air–fuel mixing in a CAP flame than the CoA flame. Investigation of the emission index of NO x (EINO x ) for both flames at Re = 2500 and overall equivalence ratios of 1.0 to 6.0 reveals that the EINO x curve of each flame is bell-shaped, with a maximum value of 3.2 g/kg at Φ = 1.2 for the CAP flame and 3 g/kg at Φ = 2.2 for the CoA flame.

Lean premixture combustion on a coaxial burner

The interaction between inner and external coaxial flames is experimentally investigated in detail. The minimum equivalence ratio t of CH4-air premixtures (test premixtures) for complete combustion is measured as a function of the flow rate by the use of a small pilot flame whose equivalence ratio, p is equal to unity or less. The minimum t is dependent upon p and the flow rate of the test premixture. The effects of radial and axial separations of the two flames are also investigated with the use of three types of burner ports. The existence of diffusive and convective mixing of the two flows on a coaxial burner is proved. The magnitude of the mixing is estimated by a numerical calculation. A small amount of mixing plays an important role in stabilizing the flame in a highspeed flow of the lean premixture. It is concluded that the use of a coaxial burner for the measurements of the flammability limits is invalid. This paper also discusses the merit of simple coaxial burners without any thermal feedback or recirculating flow flow for lean premixture combustions. The overall minimum equivalence ratio attained by the present experiments is 0.6 at a mean flow velocity of 5.3 m/sec.

A comment on the flame angle, double-burner technique of determining flammability limits

The interaction between inner and external coaxial flames is experimentally investigated in detail. The minimum equivalence ratio t of CH4-air premixtures (test premixtures) for complete combustion is measured as a function of the flow rate by the use of a small pilot flame whose equivalence ratio, p is equal to unity or less. The minimum t is dependent upon p and the flow rate of the test premixture. The effects of radial and axial separations of the two flames are also investigated with the use of three types of burner ports. The existence of diffusive and convective mixing of the two flows on a coaxial burner is proved. The magnitude of the mixing is estimated by a numerical calculation. A small amount of mixing plays an important role in stabilizing the flame in a highspeed flow of the lean premixture. It is concluded that the use of a coaxial burner for the measurements of the flammability limits is invalid.This paper also discusses the merit of simple coaxial burners without any thermal feedback or recirculating flow flow for lean premixture combustions. The overall minimum equivalence ratio attained by the present experiments is 0.6 at a mean flow velocity of 5.3 m/sec.