Global Equivalence Ratio Research Articles

Effect of jet momentum flux and heat density on NOx emission in a flameless gas turbine combustor

Experimental testing and validated CFD models have been used to evaluate the performance of the FC4GT flameless gas turbine (GT) combustor operating at a global equivalence ratio of 0.44. The methane-fuelled combustor was tested at atmospheric pressure, modelled at elevated pressures up to 25 bar, and optimised for a combustor pressure of 15 bar. The combustor was analysed for firing rates between 15 kW and 16 MW, which correspond to heat density range between 0.15 MW/m3.bar and 150 MW/m3.bar, respectively. The performance of the combustor was also evaluated with pure hydrogen. Overall, the flow fields while using both fuels, are found to be similar; exhibiting highly uniform spatial (temperature and oxygen) distributions, and low exhaust NOx emissions. Volumetric histograms of temperature and oxygen in the combustor are identified as a powerful analysis tool for quantifying the level of uniformity of the scalar mixing and combustion field, NOx emission, and for determining the combustion regime.The momentum flux of the primary air jet has been identified as a key parameter in controlling NOx emission. Across the power generation mode for GT operation with a heat density of 5–150 MW/m3.bar, the predicted NOx emission (15% O2 dry) for methane was 34–6 ppmv, respectively, and for hydrogen 353–33 ppmv, respectively. At 50 MW/m3.bar, the predicted NOx emission for methane was 17 ppmv, and 146 ppmv for pure hydrogen. This is a significant outcome for a non-premixed combustor considering that the NOx emission of the GE6FA GT engine using partially premixed combustion at 50 MW/m3.bar, is 15 ppmv for methane, and 200 ppmv for 85% hydrogen. Overall, the low NOx emission, for methane and hydrogen, combined with the highly uniform spatial temperature and oxygen distributions, and supported by experimental visualisation provide irrefutable proof that a flameless combustion regime has been achieved. It also demonstrates the plausibility of a non-premixed flameless combustor achieving comparable NOx emission to premixed flames.

Multi-bifurcation behaviors of stability regimes in a centrally staged swirl burner

Multi-bifurcation behaviors of staged swirl flames fueled with methane at atmospheric pressure are experimentally investigated by varying the global equivalence ratio (ϕglo). Based on the characteristics of measured pressure oscillations and the associated results of phase space reconstruction, recurrence plots, and synchrosqueezing-transform-based time-frequency analysis, the multi-bifurcation behaviors of this thermoacoustic system with four different stability regimes have been identified. With incremental increase in ϕglo from 0.55 to 0.79, these four stability regimes follow the sequence of a quiet mode (Regime I), the first limit cycle with moderate oscillations (Regime II), an intermediate state with intermittent bursts of multi-modes (Regime III), and the second limit cycle with much stronger oscillations (Regime IV). The flame dynamics in Regimes II and IV undergoing limit cycle oscillations are compared. The flame structure in Regime II displays an attached twin-flame structure, the same as that observed in Regime I. However, a large-scale periodic convective motion is found in Regime IV, which is identified to be the main thermoacoustic driving factor in the local Rayleigh index maps. Further experiments are carried out by continuously increasing ϕglo to examine the synchrosqueezing-transform spectra of transient processes during the two bifurcations. The present investigation is instrumental in obtaining a fundamental understanding of nonlinearity and multi-bifurcation of thermoacoustic instabilities in centrally staged swirl combustors, which is vital in guiding the early stage design and developing detection/control strategies in practical low-emission combustion systems.

Investigation of the OH∗ chemiluminescence characteristics in CH4/O2 lifted flames

Premixed and non-premixed flames of CH4/O2 are widely used in industrial fields, including the non-catalytic partial oxidation (NC-POX) of natural gas to produce hydrogen-rich syngas. As a major spontaneous emission present in flames, OH∗ chemiluminescence is often applied in combustion diagnostics to indicate flame structure, reaction mechanism and equivalence ratio. In this study, the flame imaging technique was applied in order to investigate the flame structure, lift-off height, peak OH∗ intensity and core reaction zone for a CH4/O2 lifted flame impinging a plate. The results demonstrated that both the increase in the global equivalence ratio (λ) as well as the reduction in the plate-to-nozzle distance (L/De) can contribute the generation of the lifted flame. In regard to the fixed L/De, the lifted flame exhibited a narrower flame front and lower chemical reaction intensity in the case of a small λ. As L/De decreased, the position of the flame core reaction zone shifted to the flame base. Meanwhile, the core reaction zone of the flame and peak OH∗ intensity was found to increase progressively. In addition, with the increase in λ, the flame lift-off height (H) initially rose gradually, after which it then declined sharply and thereafter increased. As indicated by the flame lift-off characteristic, as λ rose, a circulation zone developed at the flame base, which was located between the flame inner shear layer and the jet of oxidizer. Accordingly, the circulation zone can promote the generation of a lifted flame due to the enhanced mixing of the reactants.

Experimental investigation on burning characteristics of diesel pool fires in naturally ventilated compartment

SummaryIn this study, experiments were performed in a 4 m × 4 m × 4 m compartment having a door size of 2 m × 1 m in the front wall. The pan diameter has been varied from 0.2 to 1.0 m in order to evaluate parameters, such as mass loss rate, CO/CO2 yield, global equivalence ratio (GER), flame height, flame pulsation frequency and total heat flux within the compartment. Experimental mass loss flux was found to be 0.23 times lesser than the mass loss flux obtained with an open fire test. A correlation was developed to estimate the flame height for compartment fire as the function of pool diameter and heat release rate. Pulsation frequency has been well correlated with previous studies for smaller pool sizes, due to no interaction between the flame tip and upper smoke layer. However, for a larger pool, this seems to be deviated due to a larger smoke layer thickness. Heat flux variations were observed to follow power law trends for all the inner surfaces of the compartment with respect to pool diameters. This investigation is also essential for better understanding of diesel (sooty) fuel burning effects with respect to pool size in a compartment having well‐ventilated door opening.

Flame structures and thermoacoustic instabilities of centrally-staged swirl flames operating in different partially-premixed modes

Flame structures and thermoacoustic instabilities of centrally-staged swirl flames fueled with methane at atmospheric pressure are experimentally investigated by varying the stratification ratio (SR) and covering four different partially-premixed modes, namely fully-premixed main and pilot stages, fully-premixed-pilot/partially-premixed-main stages, partially-premixed-pilot/fully-premixed-main stages, and partially-premixed main and pilot stages. The flame structures in the quiet cases at SR = 0–3 are studied, and three basic flame types can be defined with increasing SR: the lifted flame, the attached twin-flame, and the V-shaped flame. The above basic flame structures show deviations in different partially-premixed modes. The links between flame structures and thermoacoustic instabilities are investigated in the unstable cases by exciting thermoacoustic instabilities with higher thermal power at increased global equivalence ratio. The results show that the partial premixing in the main stage excites stronger thermoacoustic instabilities and thus leads to different flame structures. Simplified thermoacoustic network analysis is also conducted to provide an insight into the nature of thermoacoustic instabilities in the three basic flame structures summarized in the unstable cases. The present investigation reveals the effect of partially-premixed modes on flame structures and thermoacoustic instabilities, which can guide the early-stage design and the development of instability control strategies in practical combustion systems.

Effect of ventilation on fire growth in diesel fueled compartment fire

ABSTRACT This paper quantifies experimentally the effect of ventilation in diesel pool fire (d = 0.6 m) in a cubical compartment of 64 m3 with ventilation opening factor in range of 0.088–2.828 . The work is addressed to fire safety in nuclear power plant in which ventilated enclosure are frequently used for chemical processing of flammable fuels. In present work, fuel mass loss rate, global CO concentration, global CO/CO2 yield, flame temperature, global equivalence ratio, doorway temperature with velocity profile, and heat flux are measured to predict the effect of ventilation in compartment fire. Ventilation with test 3 is noted as critical opening i.e. after that mass loss rate suddenly decreased. It is also observed that compartment fire is affected more by varying door height rather than varying door width. Unpredictably, global CO emission are obtained nearly the same regardless of doorway opening. Combustion efficiency is found to be in range of 0.39–0.93. Results showed that reduction in ventilation results in oscillating flame behavior and increase in upper layer gas temperature. Present data correlated well under thermal scaling analysis. The results shown in present work can be used as an input to fire hazard analysis in nuclear power plants.

The oxygen-deficient combustion and its effect on the NOx emission in a localized stratified vortex-tube combustor

The oxygen-deficient combustion characteristics of methane in a localized stratified vortex-tube combustor (LSVC) are studied by diluting combustion air with nitrogen. The influences of oxygen mole fraction (0.13–0.21) on flame configuration, combustion stability, combustion efficiency, and NOx emission characteristics are experimental investigated at the inlet temperature of 300 K. Combined with the numerical simulation method, the NOx generation, and emission mechanisms are analyzed in this combustor. Results show that the LSVC can achieve a wide stability limit, in which the global equivalence ratio can be as low as 0.22 at the lowest oxygen mole fraction (β) of 0.13. To ensure high combustion efficiency, the β should be kept above 0.16 since the oxygen-deficient condition reduces the reaction rate and flame temperature. The combustor can achieve ultra-low NOx emission of below 10 ppm (@ 15 vol% O2) due to low oxygen concentration and flame temperature. Furthermore, part of NOx entrained into the fuel-rich reduction zone by the swirl flow field is reduced by the reductive species (i.e., CO and H2) to further lowering NOx emissions. The results of this paper can guide the development of the LSVC in the high-efficiency and low-emission combustion fields.

A data-driven approach using machine learning for early detection of the lean blowout

A data-driven approach using machine learning is presented for the identification of the critical flame location for the early detection of an incipient lean blowout (LBO) in a realistic gas turbine engine combustor under engine-relevant conditions. This method is demonstrated by utilizing the temperature (T) and the hydroxyl radical mass fraction (YOH) data from high fidelity large eddy simulations (LES) of Jet-A combustion. The fuel flow rate is progressively reduced in numerical simulations with a fixed airflow rate to mimic experimental studies of LBO in the gas turbine combustor. These simulations are the first of their kind for a fully resolved realistic combustor geometry with adaptive mesh refinement and have accurately captured the dynamics of the LBO process and global lean blowout equivalence ratio. Time-series of T and YOH are extracted in the primary zone of the combustor, from stable flame condition to LBO condition, to train the machine learning model. A Support Vector Machine (SVM) model with radial basis function is successfully developed to identify the critical flame location for early detection of incipient LBO condition in a practical combustor for the first time. The performance of the SVM model is quantified using the F-score, and the critical flame location corresponds to the maximum value of the F-score. The critical flame location is found to be in the flame root region and is effective in the early detection of incipient LBO. The conventional statistical measures are compared with the results of the trained machine learning model to assess the feasibility of the latter for online flame health monitoring. The machine learning model successfully prognosticated the LBO approximately 20 ms before the event, and this study has shown significant promise for the use of the SVM model in engine prognostics and health management.

Flame stabilization and local combustion modes in a cavity-based scramjet using different fuel injection schemes

This paper investigates the impact of fuel distribution on flame stabilization and local combustion modes in a cavity-based scramjet. Two injection schemes with a global equivalence ratio of 0.4 are comparatively studied. Ethylene is injected only from upstream of the cavity in the first case, whereas in the second case 75% of the fuel is injected from upstream of the cavity and 25% of the fuel is injected from the cavity floor. The reactive flow in the scramjet is numerically investigated using a hybrid RANS/LES method. The agreement between calculation results and experimentally measured pressure profiles and CH* chemiluminescence images is good. The simulation results are further employed to gain a fundamental understanding of local combustion modes from different aspects: supersonic vs. subsonic, and premixed (fuel-rich vs. fuel-lean) vs. diffusion by using filter functions. In the upstream-only fueling case, it is found that the flame is stabilized by the cavity shear layer and most of the heat is released by supersonic-premixed combustion. Moreover, subsonic-premixed combustion and supersonic-diffusion combustion are significant, and the leading mode changes along the combustor. In the dual fueling case, a cavity assisted jet-wake stabilized flame is observed with higher combustion efficiency. Diffusion combustion becomes the dominant mode, with similar heat release in subsonic and supersonic regions. Nevertheless, the flashback of a premixed flame along the walls plays an important role, leading to diffusion combustion around the fuel jet. The results elucidate the link between the fuel injection schemes, flame stabilization locations, and local combustion modes.

Blowout characteristics of the partially premixed flame during the condition transition in a supersonic combustor

The blowout characteristics of partially premixed flame during the condition transition in a cavity-stabilized scramjet combustor are investigated by the experimental method. More attention is placed on the unsteady evolution of the turbulent jet flame, as well as its dynamic response with the initial reactive flow states. The Mach number of flow at the entrance of the combustor is 2.52, the total pressure and the stagnation temperature of which are 1.6 MPa and 1486 K. The cavity-stabilized jet flame is blown out once the injection pressure drop reaches a critical value. During the flame blowout process, the most strongly reactive region gradually transits from vicinity of the cavity shear layer to downstream. Accompanied the sharply decrease of combustion intensity, the light intensity drops rapidly until the flame is completely extinguished. The experimental results indicate that the dynamic process of flame blowout depends on the initial reactive flowfields. The jet flame lasts longer and more intermediate products with high temperature reside within the combustor, which is beneficial to ignite the combustible fuel mixed in the air. As the increase of initial combustion intensity, the flame can be maintained in the combustor at a lower limit of global equivalence ratio after the condition transition. The results on the flame blowout limits are of great significance for the design of the condition transition in a scramjet.

A comparison of partially premixed methane/air combustion in confined vane-swirl and jet-swirl combustors

ABSTRACT The characteristics of partially premixed methane/air combustion in aconfined geometry are experimentally investigated in two types of swirling flows introduced by using avane swirl (VS) and tangential jet swirl (TJS), respectively. To make acomparison, acoaxial jet (CJ) burner is also employed. The fuel and air are individually injected through their own inlets, partially premixed in amixing chamber, and then burned in aconfined cylindrical combustor. In VS and CJ combustors, the fuel is axially injected from the center; while in the TJS combustor the fuel is injected through either acentral inlet or tangential inlet. The geometry swirl number varies from 0 to 3.38. The flame structures, extinction limits, temperature distribution, NOx, CO concentrations and flow velocities are systematically examined to compare the flame characteristics. The detailed observations show that the swirl plays asignificant role in the fuel mixing, flame stabilization, and flame structures. As fuel is axially injected, ayellow flame is established at low airflow rate regardless of the swirl number values. As the airflow rate increases, ablue flame is obtained; the flame extinguishes at avery low global equivalence ratio of 0.25 in the VS and TJS combustors. When fuel and air are tangentially injected, ablue flame similar to premixed one is established; asteady flame is obtained from lean to rich limits (equivalence ratio of 0.5 to 1.68). The temperature is distributed more uniformly in the high swirl TJS combustor, particularly for the one adopts pure tangential jets of fuel and air, in which the measured NOx and CO concentrations are the lowest. The differences were mainly attributed to different mixing modes.

The effect of premixed stratification on the wave dynamics of a rotating detonation combustor

Typical injection schemes of rotating detonation combustors inject fuel locally into the combustion channel, creating stratified fuel-rich and fuel-lean mixing regions. In this study, premixed hydrogen and air rotating detonations are explored in a rotating detonation combustor through premixing part of the fuel into the oxidizer flow. The objective is to investigate the effect of premixing on the operation of the combustor. Three premixing schemes are examined where the detonation wave speeds are analyzed. The results show that in premixing, the fuel-lean regions became more favorable for continuous detonation propagation when premixed with the bypass fuel, resulting in higher detonation wave speeds. This phenomenon is shown to be independent of the global fuel-air equivalence ratio and the amount of fuel premixed into the oxidizer. As such, combustor performance and the operational regime could be improved with lean hydrogen premixing amounts in the main flow oxidizer.

Experimental and theoretical analysis of carbon driven detonation waves in a heterogeneously premixed Rotating Detonation Engine

Coal dust explosions can be hazardous; however, they can also generate a significant rise in stagnation pressure if adequately harnessed. Rotating detonation combustors seek to take advantage of the stagnation pressure rise phenomenon in a more sustained and controlled manner via confinement to a physical annulus, leading to increased overall thermodynamic efficiency. This investigation presents an analysis of detonations fueled by Carbon Black, a solid particulate consisting of virtually pure carbon molecules and lean Hydrogen-Air mixtures. It is realized that with the addition of Carbon Black, an increase of lean mixture detonability and detonation velocities extending the operating limit over that of a pure hydrogen-air mixture is experienced. For all testing conditions, the total equivalence ratio is held at φ = 1, while the fuel mixture's carbon mass fraction is increased from 0 to 0.7 while the hydrogen is decreased. Detonation wave velocities are extracted from high-speed imaging through applying a Discrete Fourier Transform algorithm to determine changes to the wave speed as Carbon Black particles are introduced. As a result, due to the addition of Carbon Black as an auxiliary fuel source, detonations were formed instead of deflagrations in operating conditions where one would expect deflagrations at the same hydrogen-air equivalence ratios without Carbon Black addition. The detonation formation provides evidence that the coal particles are reacting within the detonation wave in a large enough capacity to support a detonation wave within the annulus. Furthermore, the wave speed is shown to increase with the additional of carbon particles. At a constant global equivalence ratio, the detonation wave velocities were found to decrease with hydrogen's incremental replacement with coal particles. Whereby, through a theoretical comparison of the heat of combustion as computed from the experimentally derived detonation wave velocities, a linear relationship of the two was shown to exist. Therefore, the heat of combustion has the potential to describe an operational limit to sustaining a detonation wave.

Interaction of equivalence ratio fluctuations and flow fluctuations in acoustically forced swirl flames

The generation and turbulent transport of temporal equivalence ratio fluctuations in a swirl combustor are experimentally investigated and compared to a one-dimensional transport model. These fluctuations are generated by acoustic perturbations at the fuel injector and play a crucial role in the feedback loop leading to thermoacoustic instabilities. The focus of this investigation lies on the interplay between fuel fluctuations and coherent vortical structures that are both affected by the acoustic forcing. To this end, optical diagnostics are applied inside the mixing duct and in the combustion chamber, housing a turbulent swirl flame. The flame was acoustically perturbed to obtain phase-averaged spatially resolved flow and equivalence ratio fluctuations, which allow the determination of flux-based local and global mixing transfer functions. Measurements show that the mode-conversion model that predicts the generation of equivalence ratio fluctuations at the injector holds for linear acoustic forcing amplitudes, but it fails for non-linear amplitudes. The global (radially integrated) transport of fuel fluctuations from the injector to the flame is reasonably well approximated by a one-dimensional transport model with an effective diffusivity that accounts for turbulent diffusion and dispersion. This approach however, fails to recover critical details of the mixing transfer function, which is caused by non-local interaction of flow and fuel fluctuations. This effect becomes even more pronounced for non-linear forcing amplitudes where strong coherent fluctuations induce a non-trivial frequency dependence of the mixing process. The mechanisms resolved in this study suggest that non-local interference of fuel fluctuations and coherent flow fluctuations is significant for the transport of global equivalence ratio fluctuations at linear acoustic amplitudes and crucial for non-linear amplitudes. To improve future predictions and facilitate a satisfactory modelling, a non-local, two-dimensional approach is necessary.

Heat transfer characteristics of an impinging swirling inverse diffusion butane/air flame jet

This paper presents an experimental study to explore the feasibility of utilizing a swirling inverse diffusion flame (IDF) jet for impingement heating. The investigation concentrates on the impinging flame structure and impingement heat transfer. The effects of air swirling, the air jet Reynolds number, Rea, the global equivalence ratio, Φ, and the non-dimensional burner-to-plate distance, H/da, on the impinging flame shape and impingement heat transfer are investigated experimentally. The Experiments show that introduction of air swirling can either enhance or reduce the heat transfer rate depending on operating condition. Under-swirling produces no evident heat transfer enhancement due to insufficient air/fuel mixing, while over-swirling reduces heat transfer rate because of the dilution of the entrained cold ambient air. Heat transfer enhancement is achieved with the appropriate level of air swirling when the complete combustion occurs in the inner reaction zone around a stoichiometric condition. The investigation shows that the impinging inverse flame jet with an appropriate level of air swirling possesses the advantages of both premixed and diffusion flame jets with blue appearance, ultra-low NOx emission, high flame temperature, good self-stabilization capability, and enhanced heat transfer rate. Therefore, it is a desirable option for impingement heating applications.

Investigation of Reverse Flow Slinger Combustor With Jet A-1 and Methanol

Abstract This paper describes an experimental investigation of the reverse flow slinger (RFS) combustor that has been developed in order to attain high flame stability and low emissions in gas turbine (GT) engines. The combustor employs centrifugal fuel injection through a rotary atomizer and performs flame stabilization at the stagnation zone generated by reverse flow configuration. The design facilitates entrainment of hot product gases and internal preheating of inlet air, which enhances flame stability and permits stable lean operation for low NOx. Moreover, a rotary atomizer eliminates the need for high injection pressures, resulting in a compact and lightweight design. Atmospheric pressure combustion was performed with liquid fuels, Jet A-1 and methanol, at ultralean fuel–air ratios (FARs) with thermal intensity varying from 30 to 52 MW/m3 atm. Combustor performance was evaluated by analyzing the lean blowout, emissions, and combustion efficiency. A very low lean blowout corresponding to global equivalence ratio of 0.1 was observed, which showed the combustor's high flame stability. Sustained and stable combustion at low heat release was attained, and NOx emissions as low as 0.4 g/kg and 0.1 g/kg were achieved with Jet A-1 and methanol, respectively. Combustion efficiency of around 55% and 90% was obtained in operation with Jet A-1 and methanol. The overall combustor performance was significantly better with methanol in terms of emissions and efficiency.

Experimental Investigation Into the Role of Mean Flame Stabilization on the Combustion Dynamics of High-Hydrogen Fuels in a Turbulent Combustor

Abstract A bluff-body turbulent combustor is mapped for its thermo-acoustic stability across variation in airflow rate, nondimensionalized as the Reynolds number (Re) and fuel composition. The combustor stability is evaluated for three fuels, namely, pure hydrogen (PH), synthesis natural gas (SNG), and syngas (SG). The combustion dynamics display markedly different behavior across the fuels, in the extent of the unstable region, as well as the observed dominant Eigenvalues. At low Re, SNG displays stable combustion, while SG exhibits high amplitude oscillations at the fundamental duct acoustic mode. As the Re is increased, SNG displays very high amplitude oscillations at the duct acoustic mode, while SG exhibit relatively low amplitude oscillations at the third harmonic. In the case of PH, high amplitude oscillations observed at higher Re at the first harmonic. These peculiarities are investigated in light of the role of mean flame stabilization. The combustion dynamics of the fuels is influenced by the global equivalence ratio, as well as the jet momentum ratio. These effects significantly demarcate the dynamics of SNG and SG combustion. This is seen manifested in the mean flame structure of flame at high amplitude oscillations, whereby result in SNG flame to be present in the wake, while the SG flame resides in the shear layer. The driving by the flame because of their mean stabilization is quantified by a spatial Rayleigh index. It confirms the presence of large driving regions for SNG compared to that of SG, results in the observed differences in amplitude of the oscillations.

Multidimensional Numerical Modeling of Combustion Dynamics in a Non-Premixed Rotating Detonation Engine With Adaptive Mesh Refinement

Abstract In the present work, a novel computational fluid dynamics (CFD) methodology was developed to simulate full-scale non-premixed rotating detonation engines (RDEs). A unique feature of the modeling approach was the incorporation of adaptive mesh refinement (AMR) to achieve a good trade-off between model accuracy and computational expense. Unsteady Reynolds-averaged Navier–Stokes (RANS) simulations were performed for an Air Force Research Laboratory (AFRL) non-premixed RDE configuration with hydrogen as fuel and air as the oxidizer. The finite-rate chemistry model, along with a ten-species detailed kinetic mechanism, was employed to describe the H2-Air combustion chemistry. Three distinct operating conditions were simulated, corresponding to the same global equivalence ratio of unity but different fuel/air mass flowrates. For all conditions, the capability of the model to capture essential detonation wave dynamics was assessed. An exhaustive verification and validation study was performed against experimental data in terms of a number of waves, wave frequency, wave height, reactant fill height, oblique shock angle, axial pressure distribution in the channel, and fuel/air plenum pressure. The CFD model was demonstrated to accurately predict the sensitivity of these wave characteristics to the operating conditions, both qualitatively and quantitatively. A comprehensive heat release analysis was also conducted to quantify detonative versus deflagrative burning for the three simulated cases. The present CFD model offers a potential capability to perform rapid design space exploration and/or performance optimization studies for realistic full-scale RDE configurations.

Experimental investigation of equivalence ratio fluctuations in a lean premixed kerosene combustor

In this study a measurement technique for determination of equivalence ratio fluctuations from flame chemiluminescence in a kerosene-fuelled lean premixed combustor under atmospheric conditions is presented. Firstly, fundamental investigations into the relationship between the ratio of different chemiluminescence signals and the equivalence ratio are carried out using an imaging spectrometer. The chemiluminescence intensity is recorded for a wide range of equivalence ratios and fuel mass flows during steady state operation. The spectra show that the CH*/OH* ratio depends linearly on the equivalence ratio and is independent of the mass flow in the investigated range. Moreover, the background radiation has no influence on the monotonous trend of the CH*/OH* ratio for kerosene combustion. This interesting finding opens up new possibilities for passive optical measurement of the equivalence ratio in kerosene flames. Bandpass-filtered phase-correlated images of OH* and CH* chemiluminescence of an acoustically excited flame are taken simultaneously on one camera chip using an image doubler. After distortion correction, the image pair is used to calculate the global equivalence ratio from the CH*/OH* ratio. Based on the calibration chart derived in stationary operation, phase-resolved equivalence ratio perturbations are determined during acoustic excitation. The presented technique allows a quantitative measurement of equivalence ratio fluctuations in spray combustion and can therefore provide a better understanding of the fundamental mechanisms of thermoacoustic instabilities triggered by equivalence ratio fluctuations.Graphic abstract

The Effect of the Degree of Premixedness on Self-Excited Combustion Instability

Abstract The use of lean, premixed fuel and air mixtures is a common strategy to reduce NOx emissions in gas turbine combustors. However, this strategy causes an increased susceptibility to self-excited instability, which manifests as fluctuations in heat release rate, flow velocity, and combustor acoustics that oscillate in-phase in a feedback loop. This study considers the effect of the level of premixedness on the self-excited instability in a single-nozzle combustor. In this system, the fuel can be injected inside the nozzle to create a partially-premixed mixture or far upstream to create a fully-premixed mixture, varying the level of premixedness of the fuel and air entering the combustor. When global equivalence ratio is held constant, the cases with higher levels of premixing have higher instability amplitudes. High-speed CH* chemiluminescence imaging shows that the flame for these cases is more compact and the distribution of the heat release rate oscillations is more concentrated near the corner of the combustor in the outer recirculation zones. Rayleigh index images, which are a metric for the relative phase of pressure and heat release rate oscillations, suggest that vortex rollup in the corner region is primarily responsible for determining instability characteristics when premixedness is varied. This finding is further supported through analysis of local flame edge dynamics.