Premixed Gas Turbine Model Combustor Research Articles

Analysis of the local equivalence ratio under fuel-feeding excitations in a lean gas turbine model combustor: Large eddy simulation approach

The synchronization process between the fuel-feeding excitation (ff) and the local equivalence ratio (ϕlocal) is a key issue that has not been well studied numerically under lean conditions. Therefore, in this study, a large eddy simulation (LES) with a wall-adapting local eddy-viscosity (WALE) subgrid model was utilized to analyze the interaction between various fuel-feeding excitations and the local equivalence ratio in a lean partially premixed gas turbine model combustor. The model combustor was simulated using five values of fuel-feeding excitation: 0 Hz, 15 Hz, 50 Hz, 100 Hz and 200 Hz. The results demonstrated that the complex interaction between the swirling flow and the fuel-feeding excitation affected the flow structure. These changes enhanced the excitation of toroidal counter-rotating vortices in the region between the fuel jet exit and shear layers, which improved the homogeneity of ϕlocal inside the inner recirculation zone (IRZ). Furthermore, the fuel-feeding excitation modulated the shear layer thickness by increasing the rolling-up of the instantaneous counter-rotating vortices, leading to spatial regularity for ϕlocal inside the IRZ. The coupling process between the fuel-feeding excitation and the precessing vortex core (PVC) significantly increased the fluctuations of ϕlocal around the fuel jet exit and the shear layer region. An analysis of the probability distribution showed a nonlinear relationship between ff and ϕlocal, where ϕlocal was high when ff = 50 Hz, enhancing the uniformity of ϕlocal inside the IRZ. However, ϕlocal was more spread radially when ff = 0 Hz in the downstream direction. The merging of vortex shedding and the PVC increasingly excited the self-oscillation of ϕlocal, while the fuel-feeding excitation dominated the oscillation of the counter-rotating vortices by increasing the subharmonic vortices. Moreover, the fuel-feeding excitation controlled and organized the small vortical structures, leading to stabilization of ϕlocal when ff = 50 Hz, 100 Hz, and 200 Hz.

Effect of fuel-feeding excitation on the isothermal turbulent swirling flow structure in a lean gas turbine model combustor

Fuel-feeding excitation (ff) is an important factor in lean combustors because it controls the combustion process and maintains a stable mixing process. In this work, the structure of turbulent swirling flow is analyzed via large eddy simulation (LES) for a lean partially premixed gas turbine model combustor. The fuel injection is artificially excited at various frequencies of 0 Hz, 15 Hz, 50 Hz, 100 Hz, and 200 Hz. The LES findings show that all fuel-feeding excitation cases have a bubble vortex breakdown as the predominant flow topology. The size of the corner recirculation zone (CRZ) is constant at ff values of 50 Hz, 100 Hz, and 200 Hz, stabilizing the swirling flow in the upstream direction. The fuel-feeding excitation increases the swirl intensity by a rate of 35% and the instantaneous counterrotating vortices spiral further, creating a modulation for the shear layers that increases the rolling-up of small vortices. A mushroom-shaped vortex is formed near the fuel jet, where its position is fixed when ff = 0 Hz and oscillates periodically toward the downstream direction when fuel-feeding excitation is applied. The mushroom-shaped vortex shrinks to a balloon-shaped vortex and merges with the fuel jet when ff = 200 Hz. For all fuel-feeding excitation cases, the precessing vortex core (PVC) considerably increases the velocity fluctuations in the axial directions by 20%. The fuel feeding excitation improves the production of turbulence near the shear layers and the fuel jet exit by rates of 30% and 20%, respectively. In addition, the fuel-feeding excitation enhances the vorticity generation by small structures with a characteristic size that highly depends on the PVC. According to the proper orthogonal decomposition (POD), the first mode has the most energy compared to the other modes due to the prevalence of the PVC and Kelvin-Helmholtz instability in the shear layer region.

Experimental study on effects of ammonia enrichment on the thermoacoustic instability of lean premixed swirling methane flames

Ammonia (NH3) has recently emerged as a promising carbon-free energy carrier. Further development and application of NH3 as fuel in the gas turbine industry can significantly reduce the emissions of carbon dioxide (CO2) and contribute to the achievement of a carbon–neutral society. This study experimentally examined the thermoacoustic instability characteristics of a laboratory-scaled lean premixed gas turbine model combustor operated with different NH3 blending ratios with methane (CH4). Experiments conducted under a wide range of inlet velocities and equivalence ratios suggest that NH3 concentration is critical in determining the characteristics of the instability. Specifically, when the NH3 proportion is less than 50 %, the addition of NH3 causes a mode transition of the instability. However, when the content of NH3 is greater than 50 %, it is shown that the instabilities are suppressed, indicating that the addition of a certain amount of NH3 can enhance the stability of CH4 flames. Additional analysis of flame dynamics reveals that the introduction of NH3 causes the lengthening of the flame front and weakens heat release rate fluctuations in the flame root regions. Further Proper Orthogonal Decomposition (POD) analysis of the flow field shows that the instability modes are strongly coupled with periodic vortex motions of the flow dynamics along the shear layers. Finally, the mode shifting phenomena is successfully predicted by low-order thermoacoustic network modeling. It is suggested that the change in convective time delay caused by NH3 addition is responsible for such transitions.

Experimental and numerical studies of the effects of the contraction ratios on the swirling flow characteristics of the model combustor outlet in lean gas turbines

Large Eddy Simulation (LES) is utilized in this work to study isothermal turbulent swirling flow characteristics for a lean partially premixed gas turbine model combustor. The goal is to investigate the influence of contraction ratio (CR) variation on the vortex breakdown phenomena features and the turbulence structure in these combustors. Particle image velocimetry (2D-PIV) measurements were carried out on a single-case model combustor using (air-air) phase to perform the mean flow behavior and the validation process under atmospheric conditions with CR equal to 0 (largest diameter, Dinlet = Doutlet), the equivalence ratio (φ) of 0.3, swirl number of (Sn) = 0.66 and Reynolds number of (Re) = 42,000. While five test cases with various CRs were applied in LES under the same conditions using (air-CH4) phase to predict the vortical structure and the precession vortex core (PVC). The CRs ranged as follows: 0, 50 %, 60 %, 70 % and 80 % (smallest diameter). Experimental and LES results show that there was a reasonable agreement between the PIV and LES results with maximum deviations of 2.8 % and 4.65 % for the normalized mean axial and radial velocities, respectively. As the CR was increased, the pressure drop and the mean axial velocity increased linearly at the contraction part. In addition, the downstream stagnation point and the size of the central toroidal recirculation zone (CTRZ) stabilized when CR = 60 %, 70 %, and 80 %. The swirl level was further enhanced by 40 % when CR = 60 % and 80 %. The presence of CR successfully suppressed the pressure and axial velocity fluctuations generated in the shear layer zones by 35 %. LES data was analyzed by Fast Fourier Transformation (FFT) and Proper Orthogonal Decomposition (POD) to investigate the PVC dynamics. The FFT showed four dominant frequencies as a result of the appearance of the PVC and vortex breakdown oscillation. The frequency of PVC (fpvc) decreased linearly with an increase in the CR where the fpvc was 20.8 Hz when CR = 0 and became 12.5 Hz at CR = 80 %. In contrast, the other frequencies were still constant even with increased CR at 187 Hz, 562 Hz, and 979 Hz. The POD analysis showed the first mode was more energetic and had 40 % of the total pressure fluctuations.

Large eddy simulation/thickened flame model simulations of a lean partially premixed gas turbine model combustor

Large-eddy simulation with the thickened flame model (LES/TFM) is conducted to simulate a three-dimensional dual swirl partially premixed methane/air gas turbine model combustor. Finite-rate chemistry is described by a skeletal chemical mechanism consisting of 16 species and 41 reactions. Flame sensors based on formyl radical (HCO) and chemical explosive mode analysis (CEMA) are proposed and implemented within the TFM framework. The two sensors are designed for multi-step chemical kinetic models to avoid thickening the low-intensity heat release rate (HRR) region. One-dimensional freely-propagating laminar premixed flames are first employed to assess the two new sensors. The HCO-based sensor can successfully avoid the low-intensity HRR region for lean flames, but can fail under fuel-rich conditions. The CEMA-based sensor can robustly avoid low-intensity HRR regions under both fuel-lean and fuel-rich conditions. A second test case using a hydrogen/air reheat burner further demonstrates the robustness of the CEMA-based sensor. The two new sensors are subsequently applied to the gas turbine model combustor, and effects of different flame sensors are studied. Baseline results from the HCO-based sensor are first compared with experimental measurements to validate the LES/TFM solver. The mean and r.m.s. velocity, temperature, and mass fractions of O and CO agree reasonably well with the experiment, although the mixture fraction within the inner recirculation zone (IRZ) is under-predicted. The predicted mean and r.m.s. temperature and species profiles are comparable at most locations, except near the IRZ where the CEMA-based sensor predicts the largest fluctuations by only thickening the chemically explosive regions. After optimisation, only 15% of overhead in computational cost is imposed when the CEMA sensor is employed on-the-fly. Future work includes further reduction of the computational cost of the CEMA-based sensor.

Thermoacoustic Instability Characteristics and Flame/Flow Dynamics in a Multinozzle Lean Premixed Gas Turbine Model Combustor Operated with High Carbon Number Hydrocarbon Fuels

The impact of fuel variations on thermoacoustic instability characteristics and flame/flow dynamics in a lean premixed prevaporized, multinozzle gas turbine model combustor was experimentally inves...

Effect of hydrogen enrichment on the dynamics of a lean technically premixed elevated pressure flame

Abstract The heat release distribution, combustion instability characteristics and flow dynamics of lean swirl-stabilized flames of hydrogen enriched natural gas were studied using time-resolved (10 kHz) stereo PIV, OH* chemiluminescence imaging and acoustic pressure measurements. The technically premixed gas turbine model combustor was operated at elevated pressure up to 5 bar with preheated air. The H2 vol. fraction in the fuel was varied up to 50%. An M-shaped aerodynamically stabilized flame persisted at 1 bar up to low H2 enrichment ratios. Increasing H2 enrichment caused the M-flame to first transition to a bistable flame then to a shear layer stabilized V-flame. Increasing pressure and H2 enrichment both decreased the flame length. The combustion instability frequency and thermoacoustic amplitude varied and were mapped across operating conditions. The effect of H2 enrichment on the thermoacoustic amplitude was explained by examining the convective coupling between the Helmholtz acoustics and heat release. H2 enrichment was shown to increase the phase delay between the pressure and heat release by decreasing the pressure wave travel times in the plenum and swirler sections of the burner and by decreasing the flame length while simultaneously increasing the pressure wave speed within the burner. Thus H2 enrichment increased the thermoacoustic amplitude when the phase delay at the initial condition was negative, and decreased it when the initial phase delay was positive. Flow dynamics were studied by characterizing the precessing vortex core frequency and energy. The PVC structure existed across all conditions, and its frequency reduced to a different Strouhal number for the M-flame and V-flame. H2 enrichment consistently slightly decreased the PVC energy.

Experimental study on combustion stability characteristics in liquid-fueled gas turbine model combustor: Fuel sensitivities and flame/flow dynamics

In this paper, combustion stability characteristics of a laboratory-scaled lean premixed prevaporized gas turbine model combustor operated with different liquid fuels were experimentally examined. Test fuels involved one linear alkane (n-dodecane), one branched alkane (iso-octane), one cyclic alkane (methylcyclohexane (MCH)) and practical multi-component RP-3 jet fuel. Tests conducted under a wide range of equivalence ratios (∅) and inlet air velocities suggested that n-dodecane flame featured the smallest (∅=0.34) lean blow out (LBO) limit, while iso-octane flame was characterized by the highest (∅=0.39). On the other hand, n-dodecane flame shifted from stable to unstable combustion at the highest equivalence ratio (∅=0.52), which is ~18% higher than the lowest case RP-3 (∅=0.44). Additional flame and flow dynamics were characterized by simultaneous high-speed OH* chemiluminescence (CL) imaging and planar particle image velocimetry (PIV) measurements. Results suggested that the instability frequency and amplitude were dependent on the fuel type and could be well related to their fundamental combustion properties. Moreover, both phase-averaged and time-resolved OH* CL imaging and PIV results demonstrated that the global unsteady flame and flow dynamics were similar for different fuels. However, a time-lag behavior was observed when iso-octane and MCH flames were subject to similar acoustic pressure oscillations in the combustor. This was speculated to be dominantly caused by their distinct ignition delay times (IDTs), which resulted in different phase lags between heat release rate and acoustic pressure oscillations. Such findings were further supported by proper orthogonal decomposition (POD) analysis performed on the simultaneously recorded OH* CL and PIV measurements.

Interaction between self-excited oscillations and fuel–air mixing in a dual swirl combustor

A partially premixed gas turbine model combustor close to an industrial design is investigated using Large Eddy Simulation (LES). Two flames, one stable and another unstable with self-excited oscillations are computed. In particular, this study addresses the previously unexplained transition of flame shape in the experiments, from V-shaped to flat when the flame becomes acoustically unstable, suggesting a notable change of the important convective delay in the thermoacoustic feedback loop. The LES results show good agreement with the measured velocities, temperature and mass fractions. The acoustic power spectral density (PSD) obtained from the LES of the unstable flame also agrees well with the measured amplitudes in the air plenum and combustion chamber, and reasonably captures the frequency with a slight under-prediction. A comparison of the stable and unstable cases shows different mixing and reaction behaviours despite similar mean velocity fields. Further detailed analysis shows that the different mixing behaviour is driven by the significantly varying air mass split between the two air passages during a thermoacoustic oscillation cycle. This variation is due to the different impedances experienced by the pressure oscillations propagating through the two swirling injector passages with different internal geometries. This causes a periodic variation of the radial momentum of the fuel jets injected between the two swirling air flows. The resulting flapping of the fuel jets creates an enhanced radial fuel–air mixing that leads to a flattened flame in the unstable case. This provides a new physical explanation for the transitions of flame shape observed in the experiments.

Nonlinear determinism in degenerated combustion instability in a gas-turbine model combustor

We present a detailed study on the dynamic behavior of pressure fluctuations in degenerated combustion instability [Gotoda et al. Phys. Rev. E 92 (2015) 052906] in a lean premixed gas-turbine model combustor in terms of symbolic dynamics, statistical complexity, complex networks, and nonlinear forecasting. The permutation spectrum test enables us to extract nonlinear determinism in degenerated combustion instability. The possible presence of chaotic dynamics in degenerated combustion instability is reasonably shown by considering the multiscale complexity–entropy causality plane incorporating a scale-dependent approach.

Dynamical Properties of Combustion Instability in a Laboratory-Scale Gas-Turbine Model Combustor

We present an experimental study on the nonlinear dynamics of combustion instability in a lean premixed gas-turbine model combustor with a swirl-stabilized turbulent flame. Intermittent combustion oscillations switching irregularly back and forth between burst and pseudo-periodic oscillations exhibit the deterministic nature of chaos. This is clearly demonstrated by considering two nonlinear forecasting methods: an extended version (Gotoda et al., 2015, “Nonlinear Forecasting of the Generalized Kuramoto-Sivashinsky Equation,” Int. J. Bifurcation Chaos, 25, p. 1530015) of the Sugihara and May algorithm (Sugihara and May, 1990, “Nonlinear Forecasting as a Way of Distinguishing Chaos From Measurement Error in Time Series,” Nature, 344, pp. 734–741) as a local predictor, and a generalized radial basis function (GRBF) network as a global predictor (Gotoda et al., 2012, “Characterization of Complexities in Combustion Instability in a Lean Premixed Gas-Turbine Model Combustor,” Chaos, 22, p. 043128; Gotoda et al., 2016 (unpublished)). The former enables us to extract the short-term predictability and long-term unpredictability of chaos, while the latter can produce surrogate data to test for determinism by a free-running approach. The permutation entropy based on a symbolic sequence approach is estimated for the surrogate data to test for determinism and is also used as an online detector to prevent lean blowout.

Effects of nitrogen dilution on the NOx and CO emission of H2/CO/CH4 syngases in a partially-premixed gas turbine model combustor

In this study, the nitrogen dilution effect in H2/CO/CH4 syngas flames on the characteristics of NOx and CO emissions were investigated in a partially premixed gas turbine model combustor. Nitrogen was diluted into the fuel side and combustor liner temperature, and the flame structures were observed in order to find information that was critical for analyzing the results of the NOx and CO emissions. The addition of nonflammable N2 was effective in decreasing the combustion temperature, which was attributed to the reduction in the reaction rate of syngas oxidation as well as the increase in the overall heat capacity of the air-fuel mixture at a constant heat input. NOx suppression was successfully achieved up to the one digit ppmv level for all heat input and all fuel composition by the 60%-fuel side dilution of nitrogen. However, increases in nitrogen dilution increased the possibility of CO emission; therefore, extremely high CO was emitted in a 100%-CO flame at a low load with 60%-nitrogen dilution reaching 10,000 ppmv. The dilution of nitrogen in syngas considerably changed the flame structure, such as flame length, thickness, angle, location, and intensity. These flame structures provided valuable clues for the analysis of the NOx and CO emission results in relation to the flame temperature results. Based on the integration of the test results, a cause-and-effect diagram of NOx and CO emissions was plotted, and the root causes of NOx suppression were determined to be the decreases in both temperature and residence time in the hot combustion zone, and the critical path of these root causes was clarified. Although the nitrogen dilution technique has been investigated in detail, and the effects are fully understood, the careful monitoring of CO and flame detachment from the fuel nozzle is recommended when applying the nitrogen dilution technique to reduce NOx emission, especially in cases of low load operation. A practical dataset was collected from the combustion test at gas turbine relevant conditions for a wide range of fuel compositions. These results and the knowledge gained from this study could be usefully applied for the environmentally friendly and safe operation of integrated gasification combined cycle and synthetic natural gas firing gas turbine power plants.

Development of NOx response model of H2/CO/CH4 syngases with nitrogen dilution in a gas turbine model combustor

The NOx prediction model was developed by utilizing the NOx emission data obtained from various compositions of H2/CO/CH4 syngases with and without nitrogen dilution at 40 kWth and 50 kWth. The knowledge obtained from the test results of a partially premixed gas turbine model combustor clarified that the Zeldovich mechanism dominated NOx formation in which flame temperature and residence time were the two vital factors. The ANOVA general linear model was applied to obtain the correlation between the input parameters (fuel mole concentration and heat input) and NOx emissions after two facts were verified: 1) the flame temperature could be expressed in terms of only input parameters; 2) the residence time and input parameters were highly correlated with OH∗ chemiluminescence images from which the flame volume could be calculated by depicting the primary combustion zone.The new empirical correlation showed a good match with the experimental data. The correlation coefficient between the measured and the predicted NOx was 89%, and most of the predicted NOx was within the 95% z-confidential interval. Although the developed model was fitted to the tested combustor, the comparison of the results of the developed model with other models indicated that a model developed for only one combustor had better precision than the generalized model. It was also noted that only the fuel composition and the heat input with the diluents ratio, if used, were enough to establish the NOx prediction model in developing the NOx model of a gas turbine combustor for power generation because the air flow rate and the combustor pressure were kept constant during the operation.The developed NOx prediction model could be efficiently utilized in the reduction of NOx emissions during the design and operation of syngas turbines because its application covers a wide range of H2/CO/CH4 syngas compositions with a wide range of nitrogen dilution ratios. Moreover, because it does not need complex computational fluid dynamics simulation or additional experimentation, the developed model could potentially reduce the costs of such tasks.

F234 希薄予混合ガスタービンモデル燃焼器で発生する燃焼不安定と酸素濃度の関係(OS-03:革新的技術のための燃焼研究(5))

F234 希薄予混合ガスタービンモデル燃焼器で発生する燃焼不安定と酸素濃度の関係(OS-03:革新的技術のための燃焼研究(5))

Detection and prevention of blowout in a lean premixed gas-turbine model combustor using the concept of dynamical system theory

We propose a novel online method of detecting lean blowout (LBO) in a laboratory-scale premixed-type gas-turbine model combustor from the viewpoint of dynamical system theory that enables us to quantify the complexity in combustion instability. The multiscale entropy method and nonlinear forecasting method, which have not been previously adopted for the analysis of combustion instability, demonstrate that combustion instability close to LBO exhibits much more complicated stochastic dynamics than fractional Brownian motion. The permutation entropy, which can be estimated by quantifying the rank order pattern of components in phase space vectors constructed from the short time-series data of pressure fluctuations, is used as a control variable to prevent LBO. The online method proposed in this work has the potential to control the complex combustion state close to LBO.

Investigation into the cause of high multi-mode combustion instability of H2/CO/CH4 syngas in a partially premixed gas turbine model combustor

In this paper, the fuel composition effects of H2/CO/CH4 syngas (0–100% for each component gas) on the self-excited high multi-mode (mainly the 3rd and 4th with their harmonics) combustion instability (CI) characteristics have been studied in a partially premixed swirl-stabilized gas turbine model combustor by investigating phase-resolved high-speed OH∗ planar laser induced fluorescence (OH-PLIF) and OH∗ chemiluminescence at a rate of 12.5kHz and analyzing the proper orthogonal decomposition (POD), spatio-temporal Rayleigh index (RI), and time-lag. Phase-synchronized OH-PLIF images suggested critical clues of CI driving mechanisms, including the periodic alternation of flame attachment/detachment and vortex coupling with everlasting flames at the outer recirculation zone due to high hydrogen fuels’ high reactivity. These PLIF results also demonstrated that the relatively short mixing length and highly fuel-dependent flame length are the root causes of a high instability-mode and sensible mode-shift. Thus, PLIF images were used to calculate the flame length by obtaining the intensity-weighted centroid for the more precise time-lag analysis. The spatio-temporal RI results indicated that the fuel composition affects the location and intensity of CI driving/damping, RI frequency, and instability mode and frequency. POD analysis from high-speed OH∗ images showed that the distinct coherent structures and large roll-up of flames are responsible for generating flame oscillations for each mode. High cross-correlation between the POD modes showed the convection of these coherent structures and axially-alternative swirl-like flame motions. At some particular compositions of H2/CH4/CO, high multi-mode CI was observed (e.g., the 3rd, 4th, and 6th modes appear simultaneously) and the original time-lag model could not be applicable, since the acoustic pressure wave is not simply sinusoidal but largely distorted. Thus, a new time-lag model using skewness time (τskew) was proposed to reflect the distortion from multi-mode CI and the accuracy of this model was verified using the experimental data.

Detection and control of combustion instability based on the concept of dynamical system theory.

We propose an online method of detecting combustion instability based on the concept of dynamical system theory, including the characterization of the dynamic behavior of combustion instability. As an important case study relevant to combustion instability encountered in fundamental and practical combustion systems, we deal with the combustion dynamics close to lean blowout (LBO) in a premixed gas-turbine model combustor. The relatively regular pressure fluctuations generated by thermoacoustic oscillations transit to low-dimensional intermittent chaos owing to the intermittent appearance of burst with decreasing equivalence ratio. The translation error, which is characterized by quantifying the degree of parallelism of trajectories in the phase space, can be used as a control variable to prevent LBO.

Characterization of complexities in combustion instability in a lean premixed gas-turbine model combustor

We characterize complexities in combustion instability in a lean premixed gas-turbine model combustor by nonlinear time series analysis to evaluate permutation entropy, fractal dimensions, and short-term predictability. The dynamic behavior in combustion instability near lean blowout exhibits a self-affine structure and is ascribed to fractional Brownian motion. It undergoes chaos by the onset of combustion oscillations with slow amplitude modulation. Our results indicate that nonlinear time series analysis is capable of characterizing complexities in combustion instability close to lean blowout.

High-speed laser diagnostics for the study of flame dynamics in a lean premixed gas turbine model combustor

A series of measurements was taken on two technically premixed, swirl-stabilized methane-air flames (at overall equivalence ratios of ϕ = 0.73 and 0.83) in an optically accessible gas turbine model combustor. The primary diagnostics used were combined planar laser-induced fluorescence of the OH radical and stereoscopic particle image velocimetry (PIV) with simultaneous repetition rates of 10 kHz and a measurement duration of 0.8 s. Also measured were acoustic pulsations and OH chemiluminescence. Analysis revealed strong local periodicity in the thermoacoustically self-excited (or ‘noisy’) flame (ϕ = 0.73) in the regions of the flow corresponding to the inner shear layer and the jet-inflow. This periodicity appears to be the result of a helical precessing vortex core (PVC) present in that region of the combustor. The PVC has a precession frequency double (at 570 Hz) that of the thermo-acoustic pulsation (at 288 Hz). A comparison of the various data sets and analysis techniques applied to each flame suggests a strong coupling between the PVC and the thermo-acoustic pulsation in the noisy flame. Measurements of the stable (‘quiet’) flame (ϕ = 0.83) revealed a global fluctuation in both velocity and heat-release around 364 Hz, but no clear evidence of a PVC.