Partially Premixed Flames Research Articles

Analysis of experimental flame shapes and blowout velocities of partially-premixed methane-air jet flames

Air/fuel partially-premixed flames may contribute to the suppression of the soot formation and thus, to reduction in the level of pollutant emissions. However, the combustion process may no longer be supported, depending the air/fuel ratio, hence leading to the blowout. This phenomenon can entail several industrial risks with severe consequences. This paper reports an experimental study of partially-premixed methane-air jet flames. Using broadband imaging, the flame-geometry descriptors and blowout occurrence are measured for different ranges of fuel flow rates and air-to-fuel volume ratios. Dimensionless correlations are developed for the geometrical features and the air/methane fuel ratios leading to blowout.

Numerical Study on the Morphology of a Re-Ignited Laminar Partially Premixed Flame with a Co-Axial Pilot Flame

Re-ignited partially premixed flame (PPF) is a quite extensive flame type in real applications, which is directly relevant to the local and global extinction and re-ignition phenomenon. The authors designed a model burner to establish laminar re-ignited PPFs. Numerical simulations were carried out to reveal the morphology of laminar re-ignited PPF. Based on the distributions of temperature, heat release and radicals, the morphologies of re-ignited flames were explored. W-shaped flames were formed under pilot-lean conditions. Line-shaped and y-shaped flames were formed under pilot-rich conditions. Both w-shaped and y-shaped flames had a triple-flame structure. The re-ignited flames can stand beyond the rich flammability limit. Additionally, OH distributions indicated both pilot flame and re-ignited flame well as it rapidly increased near the flame front. OH concentration did not increase visibly while CH2O concentration mildly increased during the mild re-ignition process in the pre-zone of the re-ignited PPF. According to the results of 0-D simulations using closed homogeneous reactor, both OH and CH2O reduced ignition time significantly. The results of this work are helpful for understanding re-ignited PPF more closely.

Flame stabilization mechanism in reacting jets in swirling vitiated crossflow

Flame stabilization in reacting jet in crossflow (RJICF) is often affected by interaction between different mechanisms, involving auto-ignition, partially-premixed flame behavior, and vortex-induced recirculation zones. These flame stabilization mechanisms depend on the local flow dynamics involving the interaction between the jet fluid and the crossflow. In this research a staged combustion system featuring a reacting jet in crossflow as means of secondary combustion zone is investigated. The fuel jet is injected into the crossflow through a contoured nozzle which is protruded into the crossflow to avoid flame contact with the combustor wall. with a top-hat velocity profile. The ignition dynamics of the reacting jet is discussed based on high-repetition-rate OH-PLIF measurements along the RJICF trajectory. The recirculation region in the leeward side of the jet promotes fuel–air mixing and provides conditions favorable for a sustained flame kernel which leads to complete ignition of the jet. There is a difference in the flame structure of a non-premixed H2/N2 flame as compared to a premixed natural gas flame indicating a difference in flame stabilization mechanism. The H2 flame is primarily stabilized at the stagnation plane between the crossflow and the windward shear layer of the jet. Due to the higher propensity of strain rate induced extinctions of premixed natural gas (NG)/air jet flame, a stable reaction zone is seen only in the jet recirculation region. Conditional statistics of the simultaneously computed velocity magnitude, vorticity magnitude and in-plane strain rate were extracted from the location of the flame front in the windward and leeward side of the jet. The conditional statistics show that the mean strain rate plays an important role in flame stabilization particularly for premixed NG/air jets which is found to be susceptible to strain rate induced extinction. Measurements at a plane half jet diameter below the injection location of the jet indicate that there is a continuous entrainment of PIV particles as well as reaction products and unburnt fuel-air mixture. In this study high-repetition-rate simultaneous PIV/OH-PLIF measurements along the jet cross-section are discussed.

A regularized deconvolution method for turbulent closure modeling in implicitly filtered large-eddy simulation

Turbulence closure models that are based on a regularized deconvolution method (RDM) are proposed for application to implicitly filtered Large-Eddy Simulation (LES). This method reconstructs sub-grid scale contributions by approximately recovering filtered quantities through an optimization approach. Physical principles on the boundedness and conservation of reconstructed quantities are preserved by constraining the optimization problem. To account for effects of scales that are not resolved in implicitly filtered LES, a reconstruction method is proposed that combines sub-grid projection and energy-similarity to enrich unresolved scales. With this method, closure models are developed for turbulent scalar fluxes and turbulence–chemistry interaction that are intrinsically consistent with the LES formulation. These closure models are evaluated in application to a piloted turbulent jet flame with inhomogeneous inlet composition, in which combustion is represented by a two-scalar manifold model. The RDM-formulation is benchmarked against simulations obtained using a presumed PDF method and eddy diffusivity closure model. In partially-premixed flame regions, it is observed that RDM improves the prediction of turbulent mixing, and faster grid convergence is observed for results with RDM compared to simulations employing a presumed PDF method.

Ensemble-LES analysis of perturbation response of turbulent partially-premixed flames

The response to small perturbations of the Sandia CH4/air flame series is studied using a dynamical systems formulation. Here, the Sandia D and E turbulent partially-premixed flames are computed using large eddy simulation (LES) with a flamelet/progress variable approach. Using 300 simultaneous LES computations of each flame, the partial Lyapunov spectrum of the flow is obtained, which provides a set of Lyapunov exponents (LEs) and Lyapunov vectors (LVs). Special numerical procedures to handle such large data sets and perform matrix manipulations on-the-fly are discussed. From the LEs, the dimension of the attractor for the flames was estimated, for the first time, to be at least 5000 (flame D) and 10500 (flame E). This is a significant result indicating that the chaoticity of the turbulent flame is confined to a volume of phase-space that is quantified. The growth of perturbations occurs at time-scales that are much larger than the smallest strain-rate based times scales, but smaller than the integral scales. Further, the Lyapunov vectors that determine the perturbation response show that the strongest perturbations are aligned with the jet breakdown region, while other LVs are aligned with the shear layer. It is found that both the extinction/re-ignition process and the turbulence-induced jet motion strongly contribute to the flame chaoticity but are not equally important for all the LVs. These results provide new insights into the perturbation response of turbulent flames. More broadly, this Lyapunov approach allows a full characterization of the chaotic dynamics associated with turbulent flames, which have applications for both modeling and control of such complex flows.

H2/CO/air premixed and partially premixed flame structure at different pressures based on reaction limit analysis

Premixed and partially premixed flames (PPFs) of H2/CO/air syngas are studied numerically to investigate the effect of pressure on syngas PPF structure. Chemical characteristics of the syngas flame at different pressures are investigated based on reaction limit analysis using a one-dimensional configuration. The results show that CO affects the syngas reaction limits through both physical effects that consist mainly in dilution and chemical effects that are related to both R23 (CO + OH = CO2 + H) and HCO pathway. In particular, the HCO pathway weakens the flame at low pressures due to the chain-terminating effect of R25 (HCO + O2 = CO + HO2) and R26 (HCO + H = CO + H2), and enhances the flame at high pressures because of the contribution of R25 to the HO2 chain-branching process. These CO chemical characteristics are also observed in the premixed zone of 50% H2 + 50% CO syngas PPFs whereas only R23 is important in the non-premixed zone.

Low-frequency combustion instabilities of an airblast swirl injector in a liquid-fuel combustor

ABSTRACT Low-frequency combustion dynamics of a prefilming airblast injector were experimentally investigated in a laboratory-scale liquid-fuel combustor operated with Jet A-1 fuel and air at atmospheric pressure and elevated temperatures. Our measurements reveal that multiple modes – ranging from 45 to 292 Hz – can be excited in the non-premixed combustion system, including the Helmholtz, longitudinal, and hydrodynamic instabilities. The system's propensity for mode selection depends strongly on combustor length and pilot equivalence ratio. A strong Helmholtz mode with a peak-to-peak amplitude of ∼14 kPa occurs, provided that the combustor length is relatively short and the pilot equivalence ratio is high. A high-amplitude, intermittent burst at approximately 12 Hz always develops when the system is on the verge of transition to the Helmholtz instability. With a longer combustor length, on the other hand, the system transitions to the ¾-wave longitudinal mode via a discontinuous mode-hopping process. The system undergoes well-defined, limit cycle oscillations with an extremely large pressure amplitude of ∼30 kPa, but the global OH* fluctuation amplitude is limited to merely 8.3%. Phase-resolved flame imaging measurements demonstrate that a ring-like partially-premixed flame emerges periodically in the dump region, when the non-premixed flame wrapping around the central recirculation zone is lifted off the fuel nozzle. The mutual interaction between the partially-premixed and non-premixed reaction zones is the dominant mechanism for intense sound generation. Under certain conditions, superposition of the Helmholtz and longitudinal modes occurs, leading to nonlinear interactions manifested by additional spectral peaks at the sum and difference of the two frequencies. In contrast to the other two cases, this superimposed tone is not characterized by limit cycle behavior, but by a noise-driven unsteadiness.

Measurement and simulation of partially-premixed cellular tubular flames

Experimental and numerical simulation results are reported of partially-premixed cellular tubular flames. Parametric measurements across stretch rate and equivalence ratio are taken by chemiluminescent imaging and are presented for the first time. Select hybrid cases with both cellular and non-cellular flame structures are examined with laser-induced spontaneous Raman scattering. Results are spatially resolved in two dimensions and radial interpolations of reaction and extinction zones are compared to numerical simulations using multicomponent transport and detailed chemical kinetics. Experimental cell structures and extinction zones are well predicted by numerical simulation, with discrepancies of temperature and H2O and temperature primarily observed in locations with moderate and high mole fractions of CO2. A novel cellular structure, denoted as a “split-cell” flame, is reported for the first time with both chemiluminescent imaging and Raman scattering. Results indicate that partially-premixed flames are valuable as experimental and numerical benchmarks to advance fundamental combustion research.

Experimental and computational investigation of partially-premixed methoxymethane flames

Experimental and computational studies are carried out to elucidate the structure and extinction of laminar partially-premixed flames employing the counterflow configuration. The fuel is methoxymethane (DME). The formulation considers two laminar streams that flow toward a stagnation plane. One stream called the fuel-rich stream is made up of DME (CH3OCH3), and nitrogen (N2) with small amounts of oxygen (O2) and the other stream called the fuel-lean stream is made up of O2, and N2 with small amounts of CH3OCH3. The level of partial premixing is characterized by the equivalence ratio defined as the ratio of the mass of methoxymethane to the mass of oxygen normalized by the corresponding stoichiometric value of this ratio. The equivalence ratio of the fuel-rich stream is ϕr and that of the fuel-lean stream is ϕl. Previous studies have established that the scalar dissipation rate at extinction depends on the stoichiometric mixture fraction, ξst, and the adiabatic flame temperature, Tst. To clarify the chemical influences of partial premixing on extinction, studies are carried at fixed values of ξst and Tst and for various values of ϕl and ϕr. Use of this procedure separates the chemical influences from thermal effects. A previously developed Burke–Schumann (flame-sheet) formulation is employed to estimate the boundary values of the mass fractions of the reactants. Two sets of experiments are conducted, in one set ϕr−1=0, and measurements are made for various selected values of ϕl, in the other set ϕl=0 and measurements are made for various selected values of ϕr. The computations are carried out using the San Diego mechanism that was recently updated to include kinetic steps describing combustion of methoxymethane. For DME addition to the fuel-lean stream, experiments and predictions show that the value of the strain rate at extinction, increases with increasing ϕl. For O2 addition to the fuel-rich stream, experiments and predictions show very little changes in the values of the strain rate at extinction with increasing ϕr−1. The key observation is that addition of DME to the fuel-lean stream enhances the overall reactivity while addition of oxygen to the fuel-rich stream has little influence on the overall reactivity

Effect of aromatic fuels and premixing on aromatic species and soot distributions in laminar, co-flow flames at atmospheric pressure

The aim of the present study is to investigate the effects of aromatic fuel structure on the aromatic species and the soot volume fraction in laminar, co-flow flames at atmospheric pressure. Both non-premixed and partially-premixed flames (jet equivalence ratios of 24 and 6) are studied. The four fuels consist of binary mixtures of n-dodecane with one of four aromatic species: n-propylbenzene, toluene, m-xylene, and 1,3,5-trimethylbenzene. For all flame conditions, the total carbon flow rate of the fuel mixture is kept constant. Furthermore, in each binary mixture, the carbon mole fraction from the n-dodecane and the aromatic component is kept constant. Laser-induced incandescence (LII) and laser extinction are used to obtain two-dimensional soot volume fraction in the flames. Laser-induced fluorescence (LIF) is used to obtain the two-dimensional aromatic species distribution in the flames. The experimental results indicate that the 1,3,5-trimethylbenzene/n-dodecane flame has the highest peak soot volume fraction amongst the four fuels; the other three fuel mixtures, within measurement uncertainty, have similar peak soot volume fraction. In addition, premixing changes the spatial distribution of polycyclic aromatic hydrocarbons and soot in the flames. In the non-premixed cases, the peak soot volume fraction is located in an annular region near the flame sheet, whereas the soot field is more uniform across the cross-section of the flame at a jet equivalence ratio of 6. A published aromatic fuel chemical mechanism is used to understand the differences in the pathways to soot precursors among the aromatic fuels. The combined 2D LIF and LII result provide a unique dataset to validate soot and chemical numerical models for the four aromatic fuels: n-propylbenzene, toluene, m-xylene, and 1,3,5-trimethylbenzene.

BEHAVIOUR OF LAMINAR PREMIXED FLAME EXPOSED TO ALTERNATING ELECTRIC FIELD

Combustion in the lean regime is an efficient way to improve combustion efficiency, minimize the emission of pollutants and fuel consumption. The drawback is that instabilities appear which may lead to extinction of the flame or blow-out. Among the different methods for improving the flame characteristics at lean mixtures is the application of electric field. The purpose of the present work is to elucidate experimentally the effect of electric field on laminar partial premixed flame characteristics. Temperature distribution inside flame, flame geometry (height and width), and reaction zone thickness were measured at different mixture strengths with and without the application of electric field and its orientation. The results showed that under the effect of electric field, the flame height decreases with enriching the mixture in the lean side, while the width increases with a higher rate. The minimum difference between change of the flame height and flame width occurs at slightly rich mixture of equivalence ratio equals to 1.03. The reaction zone thickness increases as the electric field is applied on the flame. The change in temperature inside the flame when the electric field is applied is marginal. Electric field makes flame stabilized until reaching a lean flammability limit (ϕ = 0.67). Horizontal electrodes give a little effect on the flame characteristics represented by temperature.

Formation of Soot in Ethylene–Air Partially Premixed Flames Over a Wide Range of Premixedness

This paper reports an investigation of soot formation in ethylene–air partially premixed flames (PPFs) over a wide range of premixedness. An axisymmetric co-flow configuration is chosen to establish PPFs from the fully nonpremixed to fully premixed conditions. Reducing the fuel flow rate as a percentage of the maximum from the core stream and supplying the same to the annular stream leads to stratification of the reactant concentrations. The thermal power, overall equivalence ratio, and the average velocity in both the streams are maintained constant under all conditions. The soot volume fraction is estimated by light attenuation method, and laser-induced incandescence (LII) is performed to map the soot distribution in the flow field. The soot volume fraction is observed to exhibit an “S”-type trend as the conditions are traversed from near the premixed to the nonpremixed regimes. That is, when traversing from the nonpremixed to near-premixed regime, below 60% fuel flow rate in core, the soot volume fraction drops drastically. The onset of sooting in the PPFs is clearly seen to be at the tip of the rich-premixed flame (RPF) branch of their triple flame structure, which advances upstream toward the base of the flame as the premixing is reduced. The S-type variation is clearly the effect of partial premixing, more specifically due to the presence of the lean premixed flame (LPF) branch of the triple flame. LII intensities are insufficient to capture the upstream advance of the soot onset with decreased premixedness. So, a quick and inexpensive technique to isolate soot luminescence through flame imaging is presented in the paper involving quasi-simultaneous imaging with a 650 nm and a BG-3 filter using a normal color camera.

Study on flame structures and emissions of CO and NO in Various CH4/O2/N2–O2/N2 counterflow premixed flames

An oxygen-diluted partially premixed/oxygen-enriched supplemental combustion (ODPP/OESC) counterflow flame is studied in this paper. Flame images are obtained through experiments and numerical simulations with the GRI-Mech 3.0 chemistry. The oxygen dilution effects are revealed by comparing the flame structures and emissions with those of a premixed flame and partially premixed flame (PPF) at the same equivalence ratio (ϕΣ = 0.95 and ϕ f = 1.4). The results show that both PPF and ODPP/OESC flames have distinct double flame structures; however, the location of the premixed combustion zone and the distance between premixed/nonpremixed combustion zone are significantly different for these two cases. For the ODPP/OESC flame, the temperature in the premixed combustion zone is lower and the premixed zone itself is located farther downstream from the fuel nozzle, which leads to reduction of NO and CO emissions, as compared to those of the PPF. Therefore, by adjusting the distribution of the oxygen concentration in the premixed and nonpremixed combustion zones, the ODPP/OESC can effectively balance the chemical reaction rate in the entire combustion zone and, consequently, reduce emissions.

Effect of Multilateral Jet Mixing on Stability and Structure of Turbulent Partially-Premixed Flames

An experimental study was conducted to investigate the effects of multilateral jet mixing, using both three and four side-jets, on the structure and stability of turbulent partially-premixed flames. Particle Image Velocimetry and OH*-chemiluminescence were used to study the effects of geometry and operating conditions on the resulting flow-field and reaction zone structures, respectively. These effects were compared under varying ratios of side-jet to primary flow momentum, whilst keeping the bulk flow constant. It was found that the mixing regimes upstream of the nozzle exit affect the flame characteristics, i.e. an impinging regime is likely to generate a lifted flame whilst a backflow regime is likely to generate an attached flame. Unlike the 4 side-jets cases, the OH* images and v r m s profiles for the 3 side-jets cases show distinct asymmetry, with intense OH* and low velocity fluctuations on the opposite sides of the fuel injection. It was also found that the flow and scalar fields become independent of the upstream conditions, for both 3 and 4 side-jets, after one diameter downstream of the nozzle exit.

Multi-environment probability density function approach for turbulent partially-premixed methane/air flame with inhomogeneous inlets

The present study aims to systematically evaluate the capability of a multi-environment PDF approach using non-premixed and premixed tabulated chemistry to predict the fundamental characteristics of turbulent partially-premixed piloted flames with near-homogeneous and inhomogeneous inlets. For the near-homogeneous case, the non-premixed manifold yields better agreement with measurements in terms of conditional mean, unconditional mean, and rms scalars; however, the premixed manifold generates a narrower hot flame zone, which is mainly attributed to its inability to account for diffusion in the mixture fraction space. For the inhomogeneous flame, the premixed and non-premixed manifolds are limited in their ability to reproduce the measured flame at upstream locations with multiple combustion modes, while the non-premixed manifold is reasonably good at predicting the unconditional and conditional profiles of all scalars in the downstream regions with dominant non-premixed combustion. In terms of the predicted environment-conditioned scalar profiles, the present multi-environment PDF approach demonstrated the ability to realistically predict the near-vertical transition from the fuel-rich flammable mixture condition to the stoichiometric pilot condition. Unlike the near-homogeneous case, the inhomogeneous case generates a distinctly different distribution of CO conditional means, where the peak conditional CO level is leaning to the richer side along the downstream region. This tendency is a clear indication of the flame transition from a premixed-dominated flame to a diffusion-dominated flame. Based on our numerical results, detailed discussions about the essential features of turbulent partially-premixed flames with multiple combustion modes are presented, as well as the limitations of the proposed approach.

On defining progress variable for Raman/Rayleigh experiments in partially-premixed methane flames

Progress variable is a central concept in the theory and modeling of partially-premixed flames. However, there has been no consensus in the literature on its definition, particularly in the context of experiments. Several possible definitions of progress variable are considered, based on mass fractions of the seven major species that can be measured by combined Raman/Rayleigh scattering methods applied to turbulent methane flames. The behavior of three candidate progress variables is evaluated using laminar opposed-flow flame calculations. The effects of normalizing by the fully-burnt state rather than the more commonly used equilibrium state are considered. A progress variable, cO, defined as the mass of oxygen bound in the products CO2, CO and H2O, divided by that which would be bound if the sample were taken to its fully-burnt state, is argued to offer some advantages over the other definitions considered. It is shown that inclusion of a larger number of species in calculating cO (18 versus 7) makes little difference, such that the seven major species accessible to Raman/Rayleigh laser diagnostics are sufficient to quantify reaction progress in partially-premixed methane flames at atmospheric pressure. The measured joint statistics of mixture fraction, Z, and progress variable, cO, are presented for two turbulent piloted partially-premixed methane–air jet flames that have significantly different mixture fraction profiles at the jet exit. Favre average radial profiles of first and second moments, as well as the correlation coefficient, RZ, c, highlight differences in near-field scalar structure between the two flames. Examples of joint Z, cO histograms, sampled from 0.3-mm segments centered at the radial location where Z˜ = 0.065 in each profile, are presented. This mixture fraction value corresponds roughly to the location of peak heat release rate on the fuel-rich side in the laminar flame calculations. A clear conclusion is that the common modeling assumption of statistical independence of mixture fraction and progress variable is contradicted by the turbulent flame measurements. The proposed progress variable can be easily calculated from simulations as a post-processing step, independent of the modeling approach, allowing for consistent comparison of measured and modeled turbulent flame results.

LES-CMC of a Partially Premixed, Turbulent Dimethyl Ether Jet Diffusion Flame

Large-eddy simulations have been coupled with a conservative formulation of the conditional moment closure (CMC) approach for the computation of a turbulent, partially-premixed dimethyl-ether jet flame. Two different numerical setups and 3 different detailed chemical mechanisms were investigated. The results are compared with measurements of velocity, temperature, and major and intermediate species. The general agreement between simulations and experiments is very good, and differences between the different mechanisms are limited to the predicted concentrations of intermediates only. Larger differences can be observed if the CMC grid size is reduced. This is due to reduced averaging effects on the conditionally averaged dissipation rates that allow to better capture high dissipation events that lead to larger deviations from a fully burning solution. A high CMC resolution provides excellent agreement with experiments throughout the flame and the results demonstrate CMC’s capability to accurately predict turbulence-chemistry interactions in partially-premixed flames involving complex chemistry.

Simulation of turbulent combustion in a self-aerated domestic gas oven

Characteristics of the partially-premixed flames are numerically studied in a self-aerated domestic gas oven. The fuel (methane) and air are introduced into the premixing chamber from different inlets and, therefore, the non-premixed formulation of the reactive scalars is possible to use. The Reynolds-averaged transport equations are solved with the k-ε turbulence model to assess the mixing efficiencies of the primary and the secondary aeration processes in the burner and combustion chamber with relevance to the flashback and flame blow-off issues. The results reveal that the most important flow feature in the chamber is the vortical motion above the fuel jets, which recirculates hot flue gases into the flame zone and reduces the burning intensity and the peak flame temperature. The concentration of flue gases passing to the compartment appears to be sufficiently diluted by the secondary aeration so that the contamination of food by the exhausts is prevented. The positions of the intake and outlet slots of the combustion chamber are also discussed for the appraisal of the effectiveness of the secondary aeration, the characteristics of the air streams driving the flow in the chamber as well as the uniformity of the temperature of the cooking compartment and flue gases.

Combustion instability characteristics of H2/CO/CH4 syngases and synthetic natural gases in a partially-premixed gas turbine combustor: Part I—Frequency and mode analysis

This paper presents the results of combustion instability characteristics of H2/CO/CH4 synthetic gases and synthetic natural gases by testing a scale-downed industrial GE7EA gas turbine that features a partially-premixed swirl-stabilized combustion. H2/CO/CH4 syngases ranging from 0% to 100% and synthetic natural gases containing H2 from 0% to 30% were tested and analyzed in view of H2, which has distinctive combustion characteristics of fast burning and high flame temperature. Interestingly, as features of the partially-premixed flames that were adapted to inhibit flashback from fast-burning H2-containing syngas, high multi-mode combustion instabilities such as 2nd, 3rd, 4th, and over were observed without fundamental longitudinal mode. To investigate these high multi-mode combustion instabilities, combustion properties such as the laminar burning velocity, adiabatic flame temperature and ignition delay time were calculated for the all tested fuels and their dependence within them and dependence on instability frequencies were closely analyzed. Test results demonstrated that H2 contents are the root cause of the high-order instability mode and shifting within modes, since fast burning H2 affects the chemical time and flame temperature of syngas more than that of CO and CH4. The relationships between the combustion properties have also illustrated that the sensitive change of combustion properties are responsible for generating flame oscillations of high multi-mode and sensible frequency and mode shifting. Therefore, the laminar burning velocity, adiabatic flame temperature and ignition delay time should be considered as important parameters when designing a gas turbine combustor; moreover, deciding operating conditions for new fuels and careful management of fuel quality, particularly in view of H2, is required when operating integrated gasification combined cycle plants and synthetic natural gas plants.

Experimental investigation of combustion instabilities in lean swirl-stabilized partially-premixed flames in single- and multiple-burner setup

In the present work, combustion instabilities of a modular combustor are investigated. The combustor operates with partially premixed, swirl-stabilized flames and can be operated in single- and different multiple-burner setups. The design parameters of the combustor prevent large-scale flame–flame interactions in the multiple-burner arrangements. The objective is to investigate how the interaction of the swirl jets affects the thermoacoustic stability of the combustor. Results of measurements of pressure oscillations and high-speed OH*-chemiluminescence imaging for the single-burner setup and two multiple-burner setups are discussed. Additionally, results of investigations with different flame characteristics are presented. These are achieved by varying the ratio of the mass flow rates through the swirlers of the double-concentric swirl nozzle. Several unstable modes with high pressure amplitudes are observed in the single-burner setup as well as in the multiple-burner setups. Numerical studies of the acoustic behavior of the combustor setups were performed that indicate that the different geometries show similar acoustic behaviors. The results lead to the conclusion that the interaction of the swirl jets in the multiple-burner setups affects the thermoacoustic response spectrum of the flame even in the absence of large-scale flame–flame interactions. Based on the findings in earlier studies, it is concluded that the differences in the flame response characteristics are induced by the reduction of the swirl intensity in the multiple-burner arrangements, which is caused by the exchange of momentum between the adjacent swirl jets.