Coaxial Jet Flame Research Articles

Lifting of Transversely Forced Turbulent Nonpremixed Coaxial Jet Flames

The lifting behavior of turbulent nonpremixed coaxial jet flames was investigated experimentally as a function of the center jet Reynolds number, annular-to-center jet velocity ratio, acoustic frequency, and acoustic amplitude, all with and without transverse acoustic disturbances acting on the flames situated at a pressure antinode. Global lifting regimes were mapped and consisted of attached flames, periodically lifted flames, and permanently lifted flames. With one exception, all flames were attached in the absence of acoustics and lifted only because of the applied acoustic excitation. The exception occurred at the highest Reynolds number and velocity ratio, where the flames were unconditionally lifted in all cases. Between these two extremes, a range of lifting regimes was observed. Lower applied frequencies and smaller amplitudes were found to generally promote attachment to the burner. Flow visualizations using high-speed Schlieren and OH* chemiluminescence imaging revealed a lateral spreading effect near the jet exit. The lateral spreading was hypothesized to be caused by an axially oriented counterflow collision between the downstream flowing jets and presumed local upstream portion of the acoustic velocity fluctuations. Physical mechanisms were hypothesized based on this observation, which appear to consistently explain most of the observed behavior.

Investigation of hydrogen/air co-flow jet flame propagation mechanism in supersonic crossflow

The design of fuel injection schemes is crucial for improving the combustion performance of high-Mach number scramjet. To clarify the feasibility of the coaxial jet injection scheme, high fidelity Large Eddy Simulation of the supersonic coaxial jet flame is conducted. The simulations are in good agreement with the experimental results in terms of time-averaged velocity, temperature, and species distribution. Auto-ignition phenomenon and the characteristics of partially premixed flame are well captured. The introduction of co-flow air increases the vorticity magnitude close to the injection port and downstream near-wall region, which results in a 400 K rise in the time-averaged temperature on the downstream near-wall region and a 4% increase in the proportion of premixed combustion near the injection port. Moreover, the instantaneous distribution of hydroxy radical indicates that the spanwise width of the windward reaction shear layer is reduced utilizing the coaxial jet scheme. Chemical kinetic analysis is applied to reveal the propagation mechanism of partially premixed flames. Thermal explosion is the chemical explosion mode for all coaxial jet flame front, which are dominated by a high-temperature reaction path. The low-temperature reaction path mainly exists in the transverse jet injection port, downstream near-wall region of the single transverse jet and co-flow lifted flame base. These significant findings provide valuable insights for the potential engineering application of the coaxial jet injection scheme to a high Mach number scramjet.

Effects of O2 concentration of O2/CO2 co-flow on the flame stability of non-premixed coaxial jet flame

Greenhouse gas emissions should be reduced to stop the advance of global warming. Oxy-fuel combustion is one of the methods to reduce CO2 emissions. This paper aims to study the stabilization of CO2-diluted oxy-methane non-premixed jet flames and compare this with the results of methane-air flames. The lower the co-flow velocity and the higher the oxygen concentration in the co-flow, the higher the stability. If the flame stability is strong enough to withstand flame shape changes from the over-ventilated flame to the under-ventilated flame, the flammable range without blowout increases significantly under the under-ventilated flame. Otherwise, by decreasing the liftoff flame stability, the over-ventilated flame cannot change to the under-ventilated flame and blows out near the globally stoichiometric condition. The flame stability of a CO2-diluted oxy-fuel flame can differ from that of methane-air flames due to the properties of CO2 and N2. The effective velocity proposed by Guiberti et al. shows a good collapse of the dimensionless liftoff height among the O2/CO2 co-flow, but the dimensionless liftoff height under the air co-flow is not on the same line with the O2/CO2 co-flow. This study suggests a modified dimensionless liftoff height equation considering the fuel and co-flow mixture kinematic viscosity instead of the fuel kinematic viscosity and the ratio of the thermal diffusivity of the mixture. The thermal diffusivity ratio does not affect the result of the air co-flow. Additionally, the new coefficient β and turbulent Schmidt number are suggested because these numbers are factors depending on the burner geometry. The modified effective velocity using new coefficients and the modified dimensionless liftoff height show a good collapse for not only O2/CO2 co-flows but also air co-flow. Also, the modified effective velocity shows the possibility of the critical liftoff velocity prediction.

Lifting of Single and Coaxial Nonpremixed Jet Flames Under Transverse Acoustic Forcing

The dynamics of gaseous reacting turbulent nonpremixed single and coaxial jet flames in unforced and acoustically forced environments was investigated at atmospheric pressure. Two burner configurations were used: one having a single fuel jet and one having a coaxial jet of fuel and oxidizer streams. The flames were exposed to transverse acoustic waves tuned to produce pressure antinode acoustic forcing at various frequencies and amplitudes. The flame responses were recorded using high-speed Schlieren and OH* chemiluminescence imaging. The center jet was set to be the same between the single and the coaxial jets at two different Reynolds numbers. The annular flow of the coaxial jet was set to have the same bulk velocity as the ambient coflow, a configuration here called a coaxial single jet. The configuration was so named because it was expected that a coaxial jet having the same annular bulk velocity as the ambient coflow would produce results similar to that of the single jet in the same coflow. It was found instead that the flame lifting behavior was considerably different, even qualitatively. The flow visualizations revealed differences in large-scale structures that explain the different lifting regimes.

Combustion characteristics of O2/CH4 coaxial jet flames in a model combustor through their visualization and the statistical analysis

Combustion characteristics of O2/CH4 coaxial jet flames in a model combustor through their visualization and the statistical analysis

Localized Combustion Phenomena of Inverse Nonpremixed Pure O2/CH4 Coaxial Jet Flames at Near-Limit Conditions

ABSTRACT The stability of inverse nonpremixed oxygen (O2)/methane (CH4) coaxial jet flames near blowout limits in a model combustor is studied by analyzing signals for their local combustion phenomena using high-speed imaging and dynamic pressure. The power spectral analysis and the statistical analysis are conducted to delineate the stability of the inverse nonpremixed flames. Depending on the O2 to CH4 momentum flux ratio ((O/F)mom), two distinguished combustion phenomena which lead to different liftoff mechanisms are observed in the same burner configuration: the partial liftoff related to the aerodynamic mechanism and the local extinction related to the flame–turbulence interaction. The results of the power spectral density (PSD) show that on approaching the near-blowout limits, the local extinction or the partial liftoff occurs not in a specific frequency, but in a time-varying random distribution. The normalized root mean square (NRMS) based on the frame-integrated visible light signal (S VL) is found to be effective for quantifying the local extinction and the partial liftoff and particularly delineating the stability of the present inverse nonpremixed flames with the partial liftoff.

Flame describing function and combustion instability analysis of non-premixed coaxial jet flames

This study examines the effect of nonlinear flame dynamics on combustion instabilities in coaxial jet flames. The flame describing function (FDF) was used to establish this nonlinearity experimentally. The FDF gain was approximately constant despite velocity oscillation changes. However, the phase difference caused constructive interference to be converted into destructive interference as the velocity oscillations increased. With a closed boundary and external forcing, the initial velocity oscillation is damped from 200 to 240 Hz and amplified from 260 to 340 Hz. In the damping frequency range, the magnitude of the initial velocity oscillation decreases as the combustion chamber length decreases. When the velocity oscillates between 260 and 340 Hz, the combustion length that shows the maximum velocity oscillation varies from 200 to 500 mm and also decreases as the forcing frequency increases. This can be related to the resonant frequency that increases as the combustion chamber length decreases. There is no damping or amplification with other excitation frequencies. The self-excited pressure oscillations also occurred at a similar frequency range, which can amplify the initial velocity oscillation, but only for a combustor length of 350 mm. The second resonant frequency calculated via low-order combustion instability simulation, OSCILOS, is within the pressure oscillation range. The growth rate computed via OSCILOS exhibited a negative value, except for a combustor length of 350 mm. The growth rate also became negative as the velocity oscillations increased from 0.15 to 0.2 m/s. This could be related to a phase difference change in the FDF. The phase difference of the FDF is varied at only 300 Hz, and changes from in-phase to out-of-phase as the velocity oscillation increases from 0.1 to 0.15 m/s. The results of this study emphasize the importance of the FDF for combustion instability analysis.

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.

Heat-release dynamics in a doubly-transcritical LO2/LCH4 cryogenic coaxial jet flame subjected to fuel inflow acoustic modulation

Large Eddy Simulations are used to study the heat-release response to fuel inflow acoustic harmonic oscillations, in a coaxial jet flame where both reactants (O2 and CH4) are injected in a dense cryogenic state. The geometry is that of the academic test rig Mascotte, operated by ONERA (France), which high pressure operating conditions are relevant for the characterization of flame dynamics in Liquid Rocket Engines (LREs). The simulations, which are carried out with a real-gas fluid solver using a detailed kinetic scheme for CH4 high-pressure oxycombustion, provide a thorough insight into the flame response for a wide range of forcing frequencies, spanning from approximately 1kHz to 20kHz. Local Flame Transfer Functions (FTF) are computed and analyzed: regions of preferential heat-release response are observed to be highly dependent on the forcing frequency. An analysis based on a flame sheet assumption is conducted to distinguish the main sources of heat release fluctuations: the primary contribution comes from the species diffusivity oscillations, while the density variations have a negligible effect. The second largest contributor is either the mixture fraction gradient or the flame surface area, depending on the forcing frequency. The FTFs are expected to be useful for thermoacoustic Low-Order Models or Helmholtz solvers, and the subsequent analysis has the potential to guide future development of analytical models for flame dynamics in LREs.

Active control of coaxial jet flames under different fuel compositions through manipulation of mixing process with miniature jet actuators

Active control of coaxial jet flames under different fuel compositions through manipulation of mixing process with miniature jet actuators

Combustion stability of inverse oxygen/hydrogen coaxial jet flames at high pressure

Turbulence-chemistry interaction of inverse gaseous oxygen/hydrogen coaxial jet flames in a test model combustor at elevated and high pressures (up to 23.6 bar) has been studied experimentally. Since it is difficult to experimentally determine the Damkohler number (Da), an important parameter representing the turbulence-chemistry interaction, for turbulent nonpremixed flames, a new method to estimate Da using the flame visualization by OH planer laser induced fluorescence and OH∗ chemiluminescence is suggested. The proposed modified Da that is based on the flame base location where the maximum OH∗ intensity is observed and the mean OH layer thickness provides the limits for the occurrence of combustion instability leading to local flame extinction. Thus, it is found to be useful for representing the regimes that involve the local flame extinction, particularly when considering a wide range of pressure conditions where thermochemical and transport properties simultaneously vary and thus complicated phenomena are observed. The combustion instability is observed only for Da ≤ 0.4, and it is found that the OH layer thickness generally decreases, the disconnected OH reaction zone is reduced, the occurrence of the local flame extinction is suppressed and the maximum value of flame surface density is enhanced with increasing combustion pressure.

Numerical Investigation on the Detailed Structure of a Coaxial Coal Jet Flame Using Large-Eddy Simulation with Elementary Reactions

A numerical simulation of a pulverized coal jet flame was performed to investigate the physics and flame structure in detail by means of large-eddy simulation with an elementary reaction mechanism....

Effect of air jet momentum on the topological features of turbulent CNG inverse jet flame

Inverse jet flame (IJF) configuration is a variant of coaxial nonpremixed jet flame which can produce a nonluminous compact blue flame with independent control of air and fuel jet momentum. In the present work, the effect of central air jet velocity on visible topological features such as, visible flame height and base flame height is investigated experimentally. Also the fluid dynamics behind the evolution of base flame in IJF configuration is unraveled in this study. Notably, buoyancy induced oscillations of IJF, which is relatively unexplored in open literature is studied in this work. Moreover, a systematic classification of IJF based on the evolution of its visible flame features with variation in the central air jet and annular fuel jet velocities is reported. In addition, a semi-empirical correlation for visible flame height of IJF with air-fuel momentum ratio is arrived in the present work which can be useful in the design of IJF based burners utilized for impingement heat transfer applications.

Combustion characteristics of gaseous inverse O2/H2 coaxial jet flames in a single-element model combustor

To effectively design hydrogen (H2)/oxygen (O2) liquid rocket engines through understanding the combustion characteristics of H2/O2 bipropellants, fundamental studies for the bipropellants in different phases are needed. This study is focused on the combustion characteristics of inverse gaseous O2/H2 coaxial jet flames in a single-element model combustor as a preliminary step for succeeding studies of injection at different phases in the combustor, visualizing flame structure by direct imaging, OH∗ chemiluminescence and OH planar laser-induced fluorescence. With increasing Reynolds number (Re), the frequency of occurrence of the local flame extinction increases and the length of the disconnected OH reaction zone is extended. The OH layer thickness increases downstream, while it is almost constant where the local flame extinction occurs and decreases with increasing Re due to the enhanced strain and scalar-dissipation rates. The excessive flame wrinkledness increases the local flame strain rate and results in the local flame extinction, exhibiting the tendency of increasing and then decreasing flame surface density with increasing Re. The probability density function of OH intensity quantifies the fluctuation intensity of OH radicals and the possibility of the local flame extinction. A useful database is provided for modeling the combustion of H2/O2 bipropellants under different phases.

Flame structure of methane/oxygen shear coaxial jet with velocity ratio using high-speed imaging and OH*, CH* chemiluminescence

In this study, the effects of gaseous methane/oxygen injection velocity ratio on the shear coaxial jet flame structure are analyzed using high-speed imaging along with OH* and CH* chemiluminescence. The images show that, as the velocity ratio is increased, the visual flame length increases and wrinkles of the flame front are developed further downstream. The region near the equivalence ratio 1 condition in the flame could be identified by the maximum OH* position, and this region is located further downstream as the velocity ratio is increased. The dominant CH* chemiluminescence is found in the near-injector region. As the velocity ratio is decreased, the signal intensity is higher at the same downstream distance in each flame. From the results, as the velocity ratio is decreased, there is increased entrainment of the external jet, the mixing of the two jets is enhanced, the region near the stoichiometric mixture condition is located further upstream, and consequently, the flame length decreases.

Numerical Investigation of Supercritical Combustion of H2–O2

This study investigates GH2/LOX coaxial jet flame at trans- and supercritical conditions using the Reynolds averaged Navier–Stokes approach. Four two-equation-turbulence models, three real equation of states, two chemical mechanisms, and three different chamber pressures are examined. Predictions show good agreement with measurements qualitatively and quantitatively. Based on the results, the predictions of the Soave–Redlich–Kwong equation of state (EOS) are closer to the experiment, while the Aungier–Redlich–Kwong EOS has more deviation than the others. Moreover, the k–ω shear stress transport model has better performance than the other turbulence models. It is also found that the flow field is controlled by two vortices which resulted from extreme expansion of the oxygen dense core and high velocity of inlet gaseous hydrogen into the chamber. The chamber pressure increment delays transcritical conditions and also increases flame length and the length of the secondary vortex and decreases the expansion z...

Combustion stability of gaseous CH4/O2 and H2/O2 coaxial jet flames in a single-element combustor

In order to understand the combustion stability of a methane (CH4)/oxygen (O2) bipropellant as a next-generation rocket liquid propellant, the combustion stability limits and morphology of gaseous CH4/O2 (GCH4/GO2) coaxial jet flames, among various phases, in a single-element combustor are experimentally studied compared with the gaseous hydrogen/O2 (GH2/GO2) coaxial jet flames. Only the stably attached flame and blowoff regimes are observed for both the GCH4/GO2 and GH2/GO2 flames, showing the flame thickness smaller than the injector lip thickness. Although the combustion stability limits of the GCH4/GO2 flames are narrower than the GH2/GO2 flames, practical use of CH4 in rocket engine applications seems to be acceptable since the fuel-rich CH4/O2 flames show very stabilized and intensified burning. For the GCH4/GO2 flames, the outer flame generated by the recirculating O2 is relatively weak and OH∗ is distributed up to the downstream. With increasing O2 injection velocity the length of the GCH4/GO2 flames and the location at the maximum OH∗ intensity increase even under turbulent combustion condition, due to the saturated enhancement of CH4-O2 diffusivity and the strong burning of pure O2 near the injector lip. The present results provide a useful database to model combustion of CH4/O2 bipropellants under various phases.

Manipulation of Large-Scale Vortical Structures and Mixing in a Coaxial Jet with Miniature Jet Actuators Toward Active Combustion Control

Manipulation of large-scale vortical structures and associated mixing in a methane-air coaxial jet is carried out by using miniature jet actuators installed on the inner surface of the annular nozzle. The periodic radial miniature jet injections are achieved with a rapid-response servo-valve. The spatio-temporal primary jet structures are investigated through phase-locked 2C-PIV (2 Component Particle Image Velocimetry) and stereoscopic-PIV. It is found that intense ring-like vortices are produced perfectly in phase with the periodic miniature jet injections regardless of the valve-driven frequency fv examined. When the Strouhal number Stv, which is defined with fv, is larger than unity, the ring-like vortices are densely formed and thus methane/air mixing is prompted with low periodic fluctuation. The diameter of the vortices becomes small as Stv is increased, so that the transport range of the inner methane and outer air fluids can be controlled by changing Stv. In addition, the evolution of counter-rotating vortex pair is also observed in the cross-sectional plane. These streamwise vortices are directly formed as a result of the radial miniature jet injection, which leads to entrainment of the ambient fluid near the primary jet shear layer, and they also contribute to the mixing enhancement. Moreover, it is demonstrated that coaxial jet flame characteristics such as carbon monoxide (CO) emission and flame holding can be drastically improved under different equivalence ratios by the present jet control scheme.

Experimental study of the lifting characteristics of the leading-edge of an attached non-premixed jet-flame: Air-side or fuel-side dilution

The impact of air-side and methane-side dilution (CO2, N2 and Ar) on the lifting process of attached non-premixed methane/air coaxial jet flames is studied over a wide range of aerodynamic conditions. The study of the competition between aerodynamics and dilution has allowed to discriminate and quantify the different phenomena involved in the lifting process. First, two effects, only dependent on the amount of the added diluent, contribute to promoting flame detachment (liftoff): a fluid mechanical effect that causes the bulk velocity of the reactants to increase; a mixing effect that changes the mixture fraction spatial distribution. They are significant for the methane-side dilution but negligible for the air-side dilution. Then, the main mechanism and the dimensionless numbers characterizing flame liftoff are identified. The attached-flame stability is analyzed on the basis of the lifting limits measured by the critical flow rate ratios, (Qd/Qf)lift and (Qd/Qox)lift when the diluent is added either to the methane or the air stream. These limits follow self-similarity relationships based on the fuel and oxidant Peclet numbers of the diluted streams, satisfied whatever the diluents. Results using PIV and CH* measurements are interpreted through a flame-leading-edge approach, where CH4/air/diluent are mixed locally at the flame base. The flame propagation velocity SL which balances the incoming gas velocity, is shown to be described by self-similarity relationships based on the molar fraction at the leading-edge reduced by values at liftoff, Xd/Xliftd. To confirm the leading-edge propagation characteristics, the flame attachment height Ha and radius Ra are investigated at the attached-flame base. Ra is representative of the mixing and mass effects induced by the pure dilution. Contrary to Ra, Ha is piloted by SL, and evolves according to a unique law dictated by Xd/Xliftd. Xliftd is a self-similar parameter highlighting the propagation nature of the leading-edge.

Combustion properties of gaseous CH4/O2 coaxial jet flames in a single-element combustor

Fundamental combustion properties of gaseous methane (CH4)/oxygen (O2) coaxial jet flames in a single-element combustor are experimentally evaluated as a preliminary step for subsequent studies of injection at very low temperature or using liquid O2 for CH4/O2 bipropellants, recently appearing to be the oncoming liquid bipropellant. A combustion chamber with quartz windows, a single shear coaxial injector and an exhaust nozzle on the downstream of the chamber is considered for the present study. Focusing on the measurements of the ignition and combustion stability limits of the coaxial jet flames in the chamber, flame visualization is also conducted by OH∗ chemiluminescence, schlieren imaging and direct imaging. Results show the ignition limits restricted than the combustion stability limits. Flame behaviors are largely classified into two, the stably attached flame and the oscillating, liftoff (near-blowout) flame. Due to cooling effects on the wall of the chamber, stably liftoff flame is not observed. The stability of the flame is greatest at fuel-rich condition (based on the injected amounts of CH4 and O2). From the flame visualization flame thickness is found to be smaller than the injector lip thickness and insensitive to injection conditions. The laminar-flow behavior near the injector exit due to the strong burning of pure O2 is observed even for high Reynolds numbers (Re). The flame visualization also exhibits the recirculating O2 that enhances burning in the combustor through the reaction with the outer fuel jet. The results of ignition limits, combustion stability limits and flame visualization can be used as a database for researches of modeling the gaseous CH4/O2 jet flames in the combustion chamber.