Non-premixed Jet Flames Research Articles

An experimental study of the characteristics of blended hydrogen-methane non-premixed jet flames

The addition of hydrogen to natural gas promotes the reactivity of fuel and thus increases the jet fire risk from pipeline leakage during the transport process of blended hydrogen-natural gas. Aiming to evaluating the potential jet fire risk, this paper experimentally studied the characteristics of H2/CH4 jet flames at varied heat release rates (HRRs) with H2 volume fraction (fv) ranging from 0% to 90% by two circular nozzles (diameters: 3 and 5 mm). The results show that with the increase of H2 volume fraction, two competing effects on flame height were observed, that is, the increase in mixture molecular diffusivity, radicals pool, and air entrainment decreases the flame height, while the increase in flame temperature and jet velocity of fuel flow with a small flow Reynold number increases the flame height. Comparisons of different previous correlations of flame height with current experiments were conducted to obtain the applicable flame height model for H2/CH4 flames. The ratio of flame width to flame height for two nozzle diameters increases with dimensionless HRR when HRR is small, and then becomes nearly constant (∼ 0.145). The flame width normalized by nozzle diameter has a power dependence on dimensionless HRR, i.e., 0.48 and 0.59 power for the 3 and 5 mm nozzle, respectively. Further theoretical analysis suggested the dimensionless correlations of lift-off height of H2/CH4 flames with fv ≤ 50%.

The influence of a plasma actuator embedded in a bluff-body on the stabilization of nonpremixed jet flames

This study is to experimentally investigate the influence of an annular plasma actuator embedded in a bluff body burner on flame stabilization, extending the previous work on non-reacting jets to propane jet diffusion flames. The objective of the actuator is to produce a vortex ring in the vicinity of the jet exit, subsequently to alter the local flow velocity, and thus to achieve flame stabilization. Results show that repetitive flame reattachment may occur with the presence of the induced vortex ring. Flame reattachment frequency and attachment time are analyzed with fast Fourier transform (FFT) and a probability density function (PDF), respectively, and both results suggest that the degree of interaction between the flame and the vortex ring has strong dependence on the imposed flow momentum, characterized by the momentum ratio of the vortex ring to the main jet. A flame describing function (FDF), characterizing the dynamic flame response to the induced flow, exhibits a trough where the vortex ring momentum is comparable to the jet momentum. Given the constant vorticity independent of the jet momentum, a greater degree of interaction between the vortex ring and the fuel jet leads to more repetitive flame reattachment, resulting in more steady heat release.

Large Eddy Simulation of turbulent nonpremixed sooting flames: Presumed subfilter PDF model for finite-rate oxidation of soot

Modeling soot evolution in turbulent reacting flows using Large Eddy Simulation (LES) is challenging due to the complex subfilter soot-turbulence-chemistry interactions. Soot particles form at fuel-rich mixtures and are subsequently oxidized as they are transported toward fuel-lean mixtures. In previous work, this phenomenology was explicitly encoded into a presumed subfilter PDF model for soot by confining soot strictly to mixtures where growth rates exceed oxidation rates. However, this model implicitly assumed that oxidation is infinitely fast. In this work, a new presumed subfilter PDF model for soot is proposed to account for finite-rate soot oxidation. The distribution of soot with respect to the flame structure (mixture fraction) is modeled by comparing the local relative motion of diffusionless soot particles with respect to mixture fraction iso-contours with the local oxidation rate. When the oxidation rate is suppressed or the transport rate is very fast, soot is allowed to penetrate further into fuel-lean mixtures. This model can allow for soot leakage across the flame, a critical phenomenon in smoking flames. The new model is validated a priori against Direct Numerical Simulation (DNS) databases of turbulent nonpremixed jet flames and then a posteriori against experimental measurements in a laboratory-scale turbulent jet flame. A priori results show remarkably good agreement with filtered DNS data and are shown to correctly allow for soot leakage at low Damköhler number. Finally, LES results of the turbulent sooting jet flame show an improvement of soot prediction using the new soot subfilter model compared to previous works. In this flame, as in experiments, evidence of soot leakage is found near the beginning of the sooting region, and this soot leakage cannot be predicted with previous subfilter models that presume infinitely fast oxidation.

The impinging wall effect on flame dynamics and heat transfer in non-premixed jet flames

The impinging jet flame is studied experimentally and numerically accounting for the complex flame-wall interactions in practical combustion devices. Flame dynamics and heat transfer with the effect of impinging wall are analyzed. The 3-D large eddy simulation coupled with detailed chemical reaction mechanism and particle image velocimetry experiment based on cross-correlation measurement principle are performed for verification and further analysis. Results show that vortices are generated due to the Kelvin-Helmholtz instability originated from velocity gradient. The 3-D vortex interactions involving vortex rings and spirals are also indicated by vorticity and the convection of streamwise vorticity is responsible for the effect of vortex spirals associated with turbulent flow transition. In addition, results calculated from four wall thermal conditions are compared and analyzed. Dirichlet condition is inferred to be more suitable for the case of wall materials with higher thermal conductivity. It is indicated that wall thermal condition mainly affects the heat transfer in the near-wall region, but has little effect on the momentum transfer. This study provides references for the adoption of wall conditions in numerical simulation and near-wall treatment in combustion systems.

Blowout limits of inclined nonpremixed turbulent jet flames

The blowout behavior of inclined nonpremixed turbulent jet flames is investigated by varying the jet inclined angle in the range of -90° to 90° The critical jet velocity at blow-out limit is quantified experimentally for various nozzle diameters, different fuels and inclined angles. Numerical simulations are performed to emphasize the flow field difference for the positive and negative inclined angles. Physical modeling is conducted to incorporate the effect of the inclined angle on blow-out behavior. Major findings include: (1) The negatively inclined jet flames show more intense yellow luminosity with larger sooting zones than the positively inclined jet flames; (2) The blowout limit decreases appreciably with the jet inclined angle for the negatively inclined flames, while for the positively inclined jet flames, this decrease is relatively small; (3) Physical analysis of the flow development of inclined jets is conducted, indicating the centerline velocity along the jet trajectory decreases faster for the flame with smaller inclined angle. And the decrease rate is relatively larger for the negatively inclined jet flames; (4) Based on the analysis of the flow development as well as the characteristic velocity with the inclined angle variation, a model based on the Damköhler number (Da) accounting for the effect of jet inclined angle is developed to characterize the blowout limits of inclined jet flames. The proposed model successfully correlates the experimental data. The present findings provide new data and a basic scaling law for the blowout limit of nonpremixed inclined turbulent jet flames, revealing the effect of the relative angle between the jet momentum and buoyancy.

A comparison between turbulent non-premixed jet flames of CH4 and the 50%NH3+50%H2 blend

A challenge of application of ammonia in combustion devices is due to its low chemical reactivity. Addition of hydrogen into ammonia can increase drastically the premixed laminar flame speed of the blends. In practical combustion devices, however, turbulent non-premixed jet flames are usually employed, while researches on ammonia jet flames are scarce. In this study, firstly, the premixed laminar flame speeds of CH4 and the 50%NH3+50%H2 blend in volumetric fraction are investigated by the Schlieren imaging. It is confirmed that the laminar flame speeds of the 50%NH3+50%H2 blend are remarkably faster than those of CH4 for equivalence ratios above 1.1, while they are similar for lower equivalence ratios. Then, the Schlieren imaging and planar laser induced fluorescence imaging of hydroxyl of the burning turbulent gas jets are conducted. It is found that the flame propagation speed and OH intensity in the CH4 jets are remarkably higher than those in the 50%NH3+50%H2 jet. To unveil the mechanism, the equivalence ratio and fuel mole fractions along the jet axis are investigated by the laser induced breakdown spectroscopy. The equivalence ratios of the 50%NH3+50%H2 blend are significantly lower than those of CH4. Compared to that of CH4, the volumetric flow rate of 50%NH3+50%H2 blend is higher for the choked flow of the under-expanded jets. Therefore, the lower equivalence ratios are attributed to the higher stoichiometric fuel-air ratio of the 50%NH3+50%H2 blend. Stratification of NH3 and H2 due to their distinct molecule transfer properties is experimentally observed, which probably lowers the flame propagation speed in the jet. These findings are expected to be a valuable reference for development of simulation models and design of practical combustion devices with zero-carbon fuels.

Quantitative laser-induced fluorescence of NO in ammonia-hydrogen-nitrogen turbulent jet flames at elevated pressure

Ammonia is a very promising carbon-free fuel, but its combustion is prone to generate a large amount of harmful nitric oxide (NO). Designing NO reduction strategies for ammonia flames requires computational fluid dynamics and accurate kinetic mechanisms. However, there are currently no available experimental data that can be used to validate models describing chemistry-turbulence interactions in ammonia flames. This study introduces two non-premixed turbulent jet flames that emulate some features of the cracked ammonia combustion at 5 bar, relevant to micro gas turbines. These ammonia-hydrogen-nitrogen jet flames feature well-controlled boundary conditions and are particularly amenable to modeling. A one-dimensional NO laser-induced fluorescence (NO-LIF) method was implemented and combined with 1-D Raman spectroscopy to measure the NO mole fraction quantitatively. To avoid laser absorption by ammonia, excitation in the A2Σ+−X2Π (0–1) band near 236 nm was chosen instead of the more conventional (0–0) band near 226 nm. Due to the unsteady nature of turbulent jet flames, offline NO-LIF measurements are not useful, and undesirable interferences from Rayleigh scattering and oxygen LIF were instead quantified and removed using the temperature and major species mole fractions measured with Raman spectroscopy. NO quantification algorithms were optimized and validated first with a laminar NH3–H2–N2 counterflow flame, and then applied to the turbulent jet flames. Results show that a large amount of NO (∼1500 ppm) is produced in the NH3–H2–N2 jet flames. Increasing the ammonia cracking ratio from 14% to 28% reduces the NO concentration in laminar and turbulent NH3–H2–N2 flames. Data also shows that turbulence smear effects of differential diffusion. To the best of our knowledge, this is the first available database featuring quantitative measurements of spatially-resolved NO mole fraction in turbulent NH3–H2–N2 flames at a practically-relevant pressure. This unique database can be used in the future to validate turbulent combustion models for such flames.

A generalized partially stirred reactor model for turbulent closure

A generalized Partially-Stirred Reactor (PaSR) model is presented in this work based on the inclusion of multiple chemical times. The PaSR model has shown promising results at modelling turbulence-chemistry interaction in Large-Eddy Simulations (LES) and Reynolds-Averaged Navier-Stokes (RANS), providing an extension of the well-known Eddy Dissipation Concept (EDC). PaSR model divides the computational domain into reactive and non-reactive parts. The factor defining this partition is expressed as a function of the system characteristic chemical and mixing times. However, the estimation of these factors, particularly the chemical one, is often oversimplified. The approach proposed in this study seeks to include in the PaSR model the whole set of chemical times involved in the reactive system. Besides, the concept of fine structures, first introduced in the EDC and often adopted also in the PaSR model to characterize the evolution of chemistry in the reactive part of the fluid, is here abandoned in favour of direct manipulation of species production rates. The mean source term is formulated according to the new generalized model through a modal decomposition of the Jacobian matrix. The method is validated a priori with DNS data of a syngas non-premixed jet flame, whose filtered data represent the validation benchmark. A good agreement is found between the new PaSR model and the filtered data for all species at different filter widths. Comparison with the single time scale based model clearly shows the limitations of the old standard approach and the necessity of including the whole spectrum of chemical times for a more comprehensive description of turbulence-chemistry interaction. A thorough analysis with the time scale participation index reveals the complexity of reaction rates contributions to the development of a specific time scale, underlying the importance of developing a model able to inherit all kinetic pathways in the turbulent closure.

Consistent submodel coupling in hybrid particle/finite volume algorithms for zone-adaptive modelling of turbulent reactive flows

A hybrid particle/finite volume algorithm has been formulated for zone-adaptive modelling of turbulent reactive flows to achieve both high fidelity in predictions and high computational efficiency. Specifically, a computationally economical species transport model via finite volume algorithm is employed as the base model for the whole computational domain, while the advanced transported probability density function (TPDF) method via Lagrangian particle tracking is employed only for regions with intense turbulence-chemistry interaction. The ‘PDF regions’ can be updated dynamically based on local flow and flame characteristics, and are compatible with complex geometric structures such as separated multi blocks, non-convex, and multi-connected regions. A two-way particle/finite volume submodel coupling is formulated to ensure the composition consistency in submodels in the PDF regions and to impose the correct interface conditions for composition and mass flow rate on the boundary of the PDF regions. The spatial partition and particle algorithms for time-varying PDF regions are demonstrated and the convergence characteristics of the adaptive modelling are investigated specifically for the variation of statistical error and bias with the number of particles per cell. The proposed zone-adaptive hybrid particle/finite volume algorithm has been numerically validated in a turbulent hydrogen/air non-premixed jet flame. It is shown that the predictions from zone-adaptive modelling are almost identical to those of stand-alone TPDF, illustrating the preservation of prediction accuracy but with significantly less computational cost.

Relative influence of soot oxidation kinetics and subfilter soot-turbulence interactions on soot evolution in turbulent nonpremixed flames

Oxidation of soot is a critical process in determining the evolution of soot in turbulent reacting flows. On the one hand, the kinetic rates of soot oxidation remain one of the most significant uncertainties in soot models. On the other hand, turbulent transport plays a significant role in the distribution of soot with respect to the flame structure so the location of soot in regions where oxidation can actually occur. The objective of this work is to assess the relative importance of small-scale turbulent transport and oxidation kinetics on the prediction of soot evolution in turbulent nonpremixed flames. A series of Large Eddy Simulations has been performed in a turbulent nonpremixed piloted jet flame of ethylene and air using a series of soot subfilter PDF models (with different assumptions regarding the small-scale turbulent transport of soot) and a series of kinetic rates for soot oxidation (with differences in kinetic rates for both OH and O2 oxidation). Similar to previous work, the results show that accounting for the disappearance of soot in lean mixtures due to soot oxidation is extremely important to avoid excessive “spurious oxidation” of soot otherwise artificially present in lean mixtures. In addition, the sensitivity to soot oxidation kinetics is found to be significantly less important than this effect. However, once this effect is taken into account in the soot subfilter PDF, the sensitivity to soot oxidation kinetics becomes rather significant compared to further refinements to the soot subfilter PDF models. More specifically, the assumption in the soot subfilter PDF models whether soot oxidation is infinitely fast or finite rate compared to small-scale turbulent transport is a less significant model sensitivity compared to the sensitivity to the kinetic rates of soot oxidation. Furthermore, oxidation by OH and O2 are revealed to play fundamentally different roles in soot evolution.

Thermal Runaway Propagation in Li-ion Battery Packs Due to Combustion of Vent Gases

Accurate understanding of propagation of thermal runaway is of much importance for developing safe battery pack designs. Combustion of vent gases emerging from a trigger cell undergoing thermal runaway has not been studied in sufficient detail, even though the additional heat generated during combustion likely plays an important role in thermal runaway propagation. This work presents comprehensive numerical modeling and simulations of thermal runaway propagation in a pack of cylindrical cells. The model accounts for multiple coupled non-linear phenomena, including vent gas flow and combustion, radiation and thermal runaway. Non-premixed combustion of venting gas is modeled using k-ε turbulence model and finite rate chemical kinetics. Simulation results are shown to be in good agreement with experimental data for a benchmark turbulent non-premixed jet flame. Simulations show that hot combustion products are rapidly transported in gaps between cells, potentially leading to self-sustained thermal runaway propagation to adjacent cells. Results demonstrate the critical importance of combustion in determining the nature of propagation of thermal runaway. The vent hole location is identified as an important parameter that influences whether and the extent to which thermal runaway propagation occurs. This work contributes towards the practical understanding of thermal runaway safety of Li-ion battery packs.

Large eddy simulation of a turbulent supercritical hydrothermal flame with a novel direct moment closure model

A novel direct moment closure (DMC) model has been developed in the context of large eddy simulation (LES) and applied to simulate a supercritical flame. To evaluate the performance of the model, three-dimensional direct numerical simulation (DNS) of a supercritical hydrothermal non-premixed jet flame is conducted. The characteristics of the ignition and propagation of the supercritical hydrothermal flame are analyzed, and then the a-priori analysis of the DMC model is carried out based on the DNS model data. It is found that the prediction of reaction rate can be remarkably improved by the DMC model compared with the laminar chemistry closure (LCC) model. In the a-posteriori study, LES of the turbulent supercritical hydrothermal flame with the DMC and LCC models have been performed and compared with the DNS data respectively. It turns out that the LES-DMC model can successfully reproduce the complex behaviors of the turbulent supercritical hydrothermal flame, and the predicted mean and rms values of velocities, temperature, and species are more accurate than those of the LES-LCC model.

Conditional analysis of temperature and strain rate effects on dissipation structure in turbulent non-premixed jet flames

This work presents results from simultaneous high-resolution temperature and velocity measurements in a series of turbulent non-premixed jet flames. The filtered Rayleigh scattering (FRS)-based temperature measurements demonstrate sufficient signal-to-noise (SNR) and spatial resolution to estimate the smallest scalar length scales and accurately determine dissipation rate fields. A comprehensive set of conditional statistics are used to characterize the small-scale structure, including the dependence of dissipation layer widths on Reynolds number, temperature, and dissipation magnitude. In general, the dissipation layer thickness decrease with increasing Reynolds number and increase with increasing temperature. However, dissipation layer widths show two distinct behaviors with respect to dissipation magnitude. For small dissipation values, increases in magnitude results in broadening of the dissipation layer, while for larger magnitude values of dissipation, the layer widths are thinned, highlighting the complexity of small-scale turbulent mixing. Additionally, measured ratios of the dissipation layer width to the Batchelor length scale are consistent across all Reynolds numbers and agree with previous studies in non-reacting flows. The unique aspect about the current set of measurements is the ability to examine the interaction of dissipation structure with turbulent flow parameters for the first time in turbulent non-premixed flames. Particularly, the strain rate/dissipation relationship is examined and compared to previous studies in non-reacting flows. It is found that the dissipation layers tend to align normal to the principal compressive strain axis and this tendency increases with increasing Reynolds number. For the lowest Reynolds number case, no dependence of the dissipation layer width nor dissipation rate magnitude on strain rate is found. However, for higher Reynolds numbers, a strong dependence of the dissipation layer width and dissipation rate magnitude on the principal compressive strain rate is observed. These results indicate the direct role of the compressive strain rate field on small-scale mixing structure in reacting flows.

Mechanism insight of shielded methane non-premixed jet flame extinction with water mist: OH-PLIF visualization and quantitative analysis of critical fire extinguishing

Natural gas leaks from pipeline systems or storage sites may highly lead to a fire safety incident as well as a potential major hazard in industries or residential. The leaked jet flame commonly sheltered by an obstacle is more difficult to be extinguished as the result of a reduction in effective extinguishing agent flux and potential re-ignition. Especially in cases where the flame is heavily obscured by obstacles, the interaction of the extinguishing agent and flame can be very different from the unobstructed situation. This study aims to determine the critical characteristics of a water mist to extinguish a shielded non-premixed jet flame with the flame structure visualization of hydroxide planar laser-induced fluorescence (OH-PLIF) and quantitative parametric analysis. The effect of obstacles on water mist and a jet flame is firstly quantitatively evaluated by flame obscuration ratio kf and maximum effective mist ratio k in critical fire-extinguishing cases. Flame dynamical behavior from the base of the flame upwards to extinction is found when the flame Froude number Frf > 0.74 and kf > 0.75. Finally, the preferred water mist nozzles with larger spray angles might have more effective fire extinguishment ability typically under the cases of k < 0.5 and kf > 1.0. This study would be valuable for the optimum design of water mist systems in natural gas leakage incidents under shielded conditions.

Development and validation of a combustion large-eddy-simulation solver based on fully compressible formulation and tabulated chemistry

This study concerns the development and validation of a high-fidelity CFD solver for large-eddy simulations (LES) of combustion and reacting flows. The solver is built upon a high-resolution numerical scheme and leverages a compressible flamelet formulation, so that it can capture both the turbulent combustion and thermoacoustic effects simultaneously. The validation study is first performed by considering a scalar-mixing case in homogeneous turbulence, and then extended to LES of a non-premixed jet flame. The solver accuracy is assessed by comparing the numerical solutions with the available experimental data. It is found that the newly developed solver is able to accurately predict the time-averaged combustion fields as well as the fluctuation quantities. The predictive accuracy is comparable to that of the state-of-the-art low-Mach solvers in literature.

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.

The partially stirred reactor model for combustion closure in large eddy simulations: Physical principles, sub-models for the cell reacting fraction, and open challenges

For their ability to account for finite-rate chemistry, reactor-based models are well suited Turbulence–Chemistry Interactions (TCI) Sub-Grid Scale (SGS) closures for Large Eddy Simulations (LES). The SGS closure in the Partially Stirred Reactor (PaSR) model relies on the determination of the reacting fraction of each computational cell, whose definition is based on estimates of the characteristic mixing and chemical time scales. Direct Numerical Simulations (DNS) of turbulent combustion can supply key information on TCI for the development, validation, and comparison of combustion models. In particular, a priori testing allows the direct validation of model assumptions. In the present work, an a priori assessment of the PaSR model is conducted. Its ability to reconstruct thermo-chemical quantities of interest is investigated along with model assumptions. Sub-grid quantities are extracted from the DNS to investigate the role of the cell reacting fraction. Various definitions are then proposed to estimate the characteristic chemical timescale in the PaSR model. Modeled chemical source terms and heat release rates are compared against the filtered quantities from DNS data of a two-dimensional, spatially developing, turbulent nonpremixed jet flame with detailed kinetics. The results demonstrate the importance of accounting for the fine structures quantities in the context of reactor-based models. A new formulation of the chemical timescale is proposed and provides improved overall predictions. Several issues are raised in the discussion, representing realistic prospects for further developments of the PaSR model as a SGS combustion closure for LES.

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.

Experimental and Numerical Study on the Combustion Characteristics of a Laminar Non-Premixed Methane Jet Flame in Oxygen/Carbon Dioxide Coflow

The combustion characteristics of laminar non-premixed CH4 jet flame in an O2/CO2 coflows with different oxygen mole fractions were studied experimentally. The flame heights at different oxygen concentrations and fuel jet velocity were obtained. The experimental observation shows that the luminosity of the CH4 jet flame in O2/CO2 coflow is different from that of the flame in air stream. A two-dimensional numerical study of a laminar non-premixed CH4 jet flame in the O2/CO2 coflow with the O2 mole fraction of 0.35 was conducted to analyze the effects of CO2 dilution on the flame. The distribution of OH radicals in the flame was measured experimentally using planar laser-induced fluorescence (PLIF) to validate the computational method adopted in this work, and the computational and experimental results of the OH distributions showed good consistency at various fuel flow velocities. Three artificial species were created in the numerical experiment to analyze the effects of the chemical reactions, third-body collisions, and transport properties of CO2 on the height, width, and temperature distribution of the flame. The results showed that CO2 participation in chemical reactions exerts significant effects on the flame. However, the influences of the third-body effects and transport properties of CO2 on the jet flame are unremarkable. The global reaction pathways and distributions of important species in the laminar non-premixed CH4 jet flame were analyzed in detail to investigate the influence mechanisms of CO2 on the flame height and temperature. The entire flame can be divided into two oxidation parts, which separated by the boundary of the HCCO. The H, O, and OH concentrations and distributions in different parts of the flame were influenced by CO2 dilution, resulting in different flame heights and temperature distributions.