Search Results (40)

This article uses FLUENT to construct a two-dimensional axisymmetric numerical model of a cryogenic hydrogen charging pipeline. By loading with initial temperature gradient and transient initial pressure disturbance, the basic characteristics of low-temperature hydrogen Taconis thermoacoustic oscillation are calculated, including temperature, heat flux density distribution, pressure amplitude, and frequency. The instability boundary of hydrogen TAO is also obtained. The results show that (1) the temperature distribution and flow characteristics of the gas inside the pipeline exhibit significant periodic changes. In the first half of the oscillation period, the cold-end gas moves towards the end of the pipeline. Low-viscosity cold hydrogen is easily heated and rapidly expands. In the second half of the cycle, the expanding cold gas pushes the hot-end gas to move towards the cold end, forming a low-pressure zone and causing gas backflow. (2) Thermoacoustic oscillation can also cause additional thermal leakage on the pipeline wall. The average heat flux during one cycle is 1150.1 W/m² for inflow and 1087.7 W/m² for outflow, with a net inflow heat flux of 62.4 W/m². (3) The instability boundary of the system is mainly determined by the temperature ratio of the cold and hot ends α, temperature gradient β, and length ratio of the cold and hot ends ξ. Increasing the pipe diameter and minimizing the pipe length can effectively weaken the amplitude of thermoacoustic oscillations. This study provides theoretical support for predicting thermoacoustic oscillations in low-temperature hydrogen transport pipeline systems and offers insights for system stability control and design verification. Full article

Acoustic oscillations in cryogenic systems can either be imposed intentionally, as in pulse-tube cryocoolers, or occur spontaneously due to Taconis-type thermoacoustic instabilities. To predict the propagation of sound waves in ducts with sudden changes in cross-sectional areas, minor losses associated with such transitions in oscillatory flows must be known. However, the current modeling approaches usually rely on correlations for minor loss coefficients obtained in steady flows, which may not accurately represent minor losses in sound waves. In this study, high-fidelity computational fluid dynamics simulations are undertaken for acoustic oscillations at transitions between tubes of different diameters filled with cryogenic hydrogen. The variable parameters include the tube diameter ratios, temperatures (80 K and 30 K), and acoustic impedances corresponding to standing and traveling waves. Computational simulation results are compared with reduced-order acoustic models to develop corrections for minor loss coefficients that describe transition losses in sound waves more precisely. The present findings can improve the accuracy of design calculations for acoustic cryocoolers and predictions of Taconis instabilities. Full article

This paper is about the characteristics of and a method to recognize the onset of limit cycle thermoacoustic oscillations in a gas turbine-like combustor with a premixed turbulent methane/air flame. Information on the measured time series data of the pressure and the OH* chemiluminescence is acquired and postprocessed. This is performed for a combustor with variation in two parameters: fuel/air equivalence ratio and combustor length. It is of prime importance to acknowledge the nonlinear dynamic nature of these instabilities. A method is studied to interpret thermoacoustic instability phenomena and assess quantitatively the transition of the combustor from a stable to an unstable regime. In this method, three-phase portraits are created on the basis of data retrieved from the measured acoustics and flame intensity in the laboratory-scale test combustor. In the path to limit cycle oscillation, the random distribution in the three-phase portrait contracts to an attractor. The phase portraits obtained when changing operating conditions, moving from the stable to the unstable regime and back, are analyzed. Subsequently, the attractor dimension is determined for quantitative analysis. On the basis of the trajectories from the stable to unstable and back in one run, a study is performed of the hysteresis dynamics in bifurcation diagrams. Finally, the onset of the instability is demonstrated to be recognized by the 0-1 criterion for chaos. The method was developed and demonstrated on a low-power atmospheric methane combustor with the aim to apply it subsequently on a high-power pressurized diesel combustor. Full article

The premixed combustion of a gas turbine is prone to thermoacoustic oscillation, which affects the safety of combustion systems. This study experimentally investigated the suppression mechanism of a stratified jet-in-crossflow on the thermoacoustic instability and nitrogen oxides (NO_x) in an unstable lean-premixed combustor. Two key parameters of the jet-in-crossflow—gas density and jet flow rate—were investigated to elucidate their effect on momentum ratios. The results reveal that the stratified jet-in-crossflow reduces the maximum amplitude of combustion oscillation by 58%, while the NO_x concentration exhibits a high damping ratio of 48.8%. Higher jet flow rates and gas densities enhance the suppression of combustion thermoacoustic oscillations and NO_x emissions. The distribution of flame radicals indicates that an increase in the jet flow rate reduces the intensity of the flame heat release rate, thereby reducing the flame thermoacoustic instability. As the argon/helium volume ratio increases, the mode of thermoacoustic oscillation shifts. As the argon/helium volume ratio gradually increases from 0%/100% to 100%/0%, the main frequency of thermoacoustic oscillations gradually decreases from 267 to 121 Hz. Notably, the transient amino-group radicals in the flame increase with the increasing argon/helium volume ratio, indicating that the jet suppresses NO_x generation. The changes in peak temperature and flame shape after jetting further confirm that the stratified jet-in-crossflow alters the flame structure within the combustion chamber. The effect of the momentum ratio on the suppression of thermoacoustic instability is studied for the first time. This study provides a promising method for suppressing the thermoacoustic oscillations and NO_x emissions in premixed flames, contributing to a safer operation and cleaner emissions in lean-premixed combustors. Full article

Combustion instability is one of the prominent and unavoidable problems in the design of high-performance propulsion systems. This study investigates the heat release rate (HRR) responses in a triple-nozzle swirling nonpremixed combustor under various thermoacoustic self-excited instability modes. Dynamic pressure sensors and high-speed imaging were employed to capture the pressure oscillations within the combustion chamber and the characteristics of flame dynamics, respectively. The results reveal nonlinear bifurcations in the self-excited thermoacoustic instabilities at different equivalence ratios. Significant differences in flame dynamics were observed across the instability modes. In lower frequency modes, the fluctuations in flame length contribute to the driving force of thermoacoustic instability. In relatively high-frequency modes, HRR fluctuations are dominated by the rolling up and convective processes of wrinkles on the flame surface. Alternating regions of gain and damping are observed on the flame surface. At even higher frequencies, both aforementioned HRR fluctuation patterns are simultaneously observed. These findings provide a deeper understanding of the complex interactions between flame dynamics and thermoacoustic instabilities, offering new insights into the design and optimization of nonpremixed combustion systems. The study underscores the importance of considering the spatial and temporal variations in flame behavior to effectively predict and control thermoacoustic instabilities. Full article

The transition to low-carbon energy systems has heightened interest in hydrogen and ammonia as sustainable alternatives to traditional hydrocarbon fuels. However, the development and operation of combustors utilizing these fuels, like other combustion systems, are challenged by thermoacoustic instabilities arising from the interaction between unsteady heat release and acoustic wave oscillations. Among many different methods for studying thermoacoustic instabilities, thermoacoustic network models have played an important role in analyzing the essential dynamics of these instabilities in combustors operating with low-carbon fuels. This paper provides a comprehensive review of thermoacoustic network modeling techniques, focusing specifically on their application to hydrogen- and ammonia-based combustion systems. We outline the key mathematical frameworks derived from fundamental equations of motion, along with experimental validations and practical applications documented in existing studies. Furthermore, current research gaps are identified, and future directions are proposed to improve the reliability and effectiveness of thermoacoustic network models, contributing to the advancement of efficient and stable low-carbon combustors. Full article

This paper investigates the role of chaotic analysis and deep learning models in combustion instability predictions. To detect the precursors of impending thermoacoustic instability (TAI) in a swirled combustor with various fuel injection strategies, a data-driven framework is proposed in this study. Based on chaotic analysis, a recurrence matrix derived from combustion system is used in deep learning models, which are able to detect precursors of TAI. More specifically, the ResNet-18 network model is trained to predict the proximity of unstable operation conditions when the combustion system is still stable. The proposed framework achieved state-of-the-art 91.06% accuracy in prediction performance. The framework has potential for practical applications to avoid an unstable operation domain in active combustion control systems and, thus, can offer on-line information on the margin of the combustion instability. Full article

When aiming to cut down on the emission of nitric oxides by gas turbine engines, it is advantageous to have them operate at low combustion temperatures. This is achieved by lean premixed combustion. Although lean premixed combustion is a proven and promising technology, it is also very sensitive to thermoacoustic instabilities. These instabilities occur due to a coupling between the unsteady heat release rate of the flame and the acoustic field inside the combustion chamber. In this paper, this coupling is investigated in detail. Two acoustic design parameters of a swirl-stabilized pressurized preheated air (300 °C)/natural gas combustor are varied, and the occurrence of thermoacoustic limit cycle oscillations is explored. The sensitivity of the acoustic field as a function of combustion chamber length (0.9 m to 1.8 m) and reflection coefficient (0.7 and 0.9) at the exit of the combustor is investigated first using a hybrid numerical and analytical approach. ANSYS CFX is used for Unsteady Reynolds Averaged Navier-Stokes (URANS) numerical simulations, and a one-dimensional acoustic network model is used for the analytical investigation. Subsequently, the effects of a change in the reflection coefficient are validated on a pressurized combustor test rig at 125 kW and 1.5 bar. With the change in reflection coefficient, the combustor switched to limit cycle oscillation as predicted, and reached a sound pressure level of 150 dB. Full article

The Helmholtz method is developed to predict the self-excited thermoacoustic instabilities in a gas turbine combustor, combining flame describing functions, the measured damping rates under the firing condition, and the non-uniform spatial distributions of the physical parameters. The impact of the hydrodynamic and geometrical parameters on the thermoacoustic instabilities is investigated. The measured damping rates show lower values under a hot condition compared with those in a cold state. The experimental results indicate that the relative errors of the predicted eigenfrequencies and the velocity fluctuation levels are below 10%. The pressure amplitude decreases and the phase increases in the axial direction, indicating a typical 1/4-wavelengh mode. At a higher equivalence ratio, the mode shape in the axial direction becomes steeper due to the elevated fluctuation amplitude at the pressure antinode after enhancing the thermal power. When the air flow rate increases, the discrepancies between the pressure shape on the flame tube side and that on the plenum side are reduced. The velocity fluctuation level increases as the combustor length increases at a constant damping rate. In fact, the velocity fluctuation level first increases and then declines, caused by more significant damping rates when employing longer flame tubes. Self-excited thermoacoustic instabilities can be well predicted using the proposed method. Full article

In combustor systems, thermoacoustic instabilities may occur and must be avoided for reliable operation. An acoustic network model can be used to predict the eigenfrequencies of the instabilities and the growth rate by incorporating the combustion dynamics with a flame transfer function (FTF). The FTF defines the interconnection between burner aerodynamics and the rate of combustion. In the current study, the method to measure the FTF in a pressurized combustor is explored. A siren unit, mounted in the fuel line, induced a fuel flow excitation of variable amplitude and high maximum frequency. This was performed here for pressurized conditions at 1.5 bar and 3 bar and at a thermal power of 125 kW and 250 kW. In addition to the experimental investigation, a 1-D acoustic network model approach is used. In the model, thermoviscous damping effects and reflection coefficients are incorporated. The model results compare well with experimental data, indicating that the proposed method to determine the FTF is reliable. In the approach, a combination of an FTF with a band stop approach and a network modeling approach was applied. The method provides a good match between experimentally observed behavior and an analytical approach and can be used for instability analysis. Full article

Thermoacoustic oscillation is indeed a phenomenon characterized by the symmetric coupling of thermal and acoustic waves. This paper introduces a novel approach for monitoring and predicting thermoacoustic combustion instability using a combination of recurrence quantification analysis (RQA) and an optimized deep belief network (DBN). Six samples of combustion state data were collected using two distinct types of burners to facilitate the training and validation of GA-DBN. The proposed methodology leverages RQA to extract intricate patterns and dynamic features from time series data representing combustion behavior. By quantifying the recurrence plot of specific patterns, the analysis provides valuable insights into the underlying thermoacoustic dynamics. Among three different feature extraction methods, RQA stands out remarkably in performance. These RQA-derived features serve as input to a carefully tuned DBN, which is trained to learn the complex relationships within the combustion process. The classification accuracy of deep belief network optimized by genetic algorithm (GA-DBN) reached an impressive 99.8%. Subsequent multiple comparisons were conducted between GA-DBN, DBN, and support vector machine (SVM), revealing that GA-DBN consistently demonstrated satisfactory classification results. This method holds significant importance in monitoring intricate combustion states. Full article

This study theoretically explored the effects of parameter settings on thermoacoustic oscillations with a low-order model. Three factors were explored—combustor length, inlet gas temperature and thermal power. The research findings indicate that optimizing the parameter settings can yield better thermoacoustic oscillation suppression results. The sound pressure amplitude decreased from 3.2 × 10⁵ Pa to 2.1 × 10⁵ Pa as the combustor length increased from 1.2 m to 6.0 m. The triggering time increased from 0.32 s to 0.91 s when the combustion chamber length increased. The climb rate declined from 23.38 × 10⁵ Pa/s to 3.75 × 10⁵ Pa/s when the combustor length was elongated. The sound pressure amplitude decreased from 3.44 × 10⁵ Pa to 2.4 × 10⁵ Pa as the gas temperature rose from 0 to 100 °C. The triggering time and climb rate variation tendency were similar when the gas temperature increased—both declined as the gas temperature rose. The sound pressure amplitude experienced a slight fluctuation when the thermal power rose. However, the triggering time decreased from 0.26 s to 0.043 s when the thermal power improved. The climb rate increased from 18.72 × 10⁵ Pa/s to 27.65 × 10⁵ Pa/s when the thermal power rose. The oscillation frequency presented was completely different in three cases that had different wavelengths and oscillation intensities. The triggering time and climb rate fluctuated extensively in varying conditions, and the above two factors were interrelated and contradictory to each other when thermoacoustic oscillation was excited. This study explored parameters’ effects on triggering time and climb rate, thereby providing references for constructing a model-based control system for thermoacoustic oscillation feedback control. Full article

Self-excited thermoacoustic instability (SETAI) is an undesirable and dangerous phenomenon in combustion systems. However, its control is difficult, thus greatly limiting the development of combustion technology. Our previous works clarified how the premixed chamber length (L_P) and equivalence ratio (φ) influence SETAI behavior in a symmetrical hedge premixed combustion system. On real-world sites, however, the supply structure or combustion condition in a multi-flame system could be asymmetric due to space limitations or combustion adjustment needs. This paper aims to clarify the SETAI behavior of a combustion system with an asymmetric supply structure or an asymmetric combustion condition. The results indicate that the sound pressure amplitude under strong oscillation can reach 160 dB, which is about 5% of the total pressure. The SETAI state under the asymmetric condition is determined by the coupling between the heat release oscillation and sound pressure oscillation on each side and their cooperation. The asymmetric supply structure leads to asynchronous heat release oscillations between the two sides; it may be that one promotes oscillation and that the other suppresses it, or that both have a promotion effect but with asynchronous action, thus partly canceling each other out to lower the system’s oscillation intensity. This brings an advantage for controlling SETAI, which can be achieved by only changing one side of the structure. The oscillation amplitude can be reduced by 80–90% by appropriately changing one L_P only by ~20%. Under an asymmetric combustion condition with φ differing between the two sides, the heat release oscillation on each side is dependent on the local φ but not the global φ. Consequently, SETAI can also be controlled by changing the distribution but maintaining a constant fuel feeding rate and φ. The concepts identified in this paper demonstrate that SETAI can be effectively controlled by adopting an asymmetric φ distribution or an asymmetric structure of the supply system. This provides a convenient SETAI control approach without affecting the equipment’s thermal performance. Full article

Non-premixed swirl combustion has been widely used in pieces of industrial combustion equipment such as industrial boilers, furnaces, and certain specific gas turbine combustors. In recent years, the combustion instability of non-premixed swirl flames has begun receiving attention, yet there is still a lack of related research in academia. Therefore, in this study, we conducted experimental research on a swirl stabilized gas flame model combustor and studied the heat release response characteristics of the swirl combustor through the flame transfer function. Firstly, the flame transfer function (FTF) was measured under different inlet velocities and equivalence ratios, and the experimental results showed that the FTF gain curve of the non-premixed swirl flame exhibited a significant “bimodal” shape, with the gain peaks located around 230 Hz and 330 Hz, respectively. Secondly, two oscillation modes of the flame near the two gain peaks were identified (the acoustic induced vortex mode M_v and the thermoacoustic oscillation mode M_a), which have not been reported in previous studies on swirl non-premixed flames. In addition, we comprehensively analyzed the flame pulsation characteristics under the two oscillation modes. Finally, the coupling degrees between velocity fluctuations, fuel pressure fluctuations, and heat release fluctuations were analyzed using the Rayleigh Index (RI), and it was found that in the acoustic-induced vortex mode, a complete feedback loop was not formed between the combustor and the fuel pipeline, which was the main reason for the significant difference in the pressure fluctuation amplitude near 230 Hz and 330 Hz. Full article

The propagation mechanism of flow disturbance under acoustic excitations plays a crucial role in thermoacoustic instability, especially when considering the effect of non-premixed combustion on heat release due to reactant mixing and diffusion. This relationship leads to a complex coupling between the spatial distribution of the equivalence ratio and the propagation mechanism of flow disturbance. In the present study, the response of a methane-air non-premixed swirling flame to low-frequency acoustic excitations was investigated experimentally. By applying Proper Orthogonal Decomposition (POD) analysis to CH* chemiluminescence images, the harmonic flame response was revealed. Large Eddy Simulation (LES) was utilized to analyze the correlation between the vortex motion within the shear layers and the harmonic response under non-reacting conditions at excitation frequencies of 20 Hz, 50 Hz, and 150 Hz. The results showed that the harmonic flame response was mainly due to the harmonic velocity pulsations within the shear layers. The acoustically induced vortices within the shear layer exhibited motion patterns susceptible to harmonic interference, with spatial distribution characteristics closely related to the oscillation modes of the non-premixed combustion. Full article