Low Emission Combustion Research Articles

Predicting bifurcation and amplitude death characteristics of thermoacoustic instabilities from PINNs-derived van der Pol oscillators

Self-sustained thermoacoustic oscillations as observed in low-emission combustion- involved gas turbines and aero-engines involve complicated thermal fluid–acoustics interaction and rich nonlinear dynamics. Such pulsating oscillations are known as thermoacoustic instability. When it occurs, large-amplitude limit cycle oscillations (LCOs) of thermodynamic parameters are frequently observed. These LCOs could cause overheating, flame flashback, and even engine failures. Thus it is critical to understand and predict the generation mechanisms and nonlinear dynamics behaviours, and then develop corresponding control approaches to prevent or control the onset of such instabilities. In this work, we develop and extend the classical van der Pol oscillators by integrating a physics-informed neural networks (PINNs) algorithm with a modelled nonlinear Rijke-type thermoacoustic combustor. The theoretical Rijke tube system (with Galerkin expansion and modified King's law implemented) and a CFD simulation model are applied to provide ‘training/calibration data’ for the extended van der Pol (EVDP)-PINNs model. The optimized EVDP oscillators are confirmed to be capable of capturing the key nonlinear characteristics by comparing the transient growth behaviours of thermodynamic perturbations and LCO amplitude and frequency. Further investigations are conducted to obtain Hopf bifurcation and amplitude death (AD) characteristics. Comparison is then made to the coupled EVDP systems. Quite similar Hopf bifurcation features, but differences in regions of AD, are observed. In general, we demonstrate an applicable approach to intelligently ‘learn’ a nonlinear thermoacoustic system and to create reliable EVDP oscillator systems, which have great potential to contribute to the development and testing of control approaches, such as the coupling described in this work, which may replace costly experimental tests.

Implementation of Regenerative Thermal Oxidation Device Based on High-Heating Device for Low-Emission Combustion

In this paper, a heating device is implemented by considering two large factors in a 100 cmm RTO design. First, when the combustion chamber is used for a long time with a high temperature of 750–1100 °C depending on the high concentration VOC gas capacity, there is a problem that the combustion chamber explodes or the function of the rotary is stopped due to the fatigue and load of the device. To prevent this, the 100 cmm RTO design with a changed rotary position is improved. Second, an RTO design with a high-heating element is implemented to combust VOC gas discharged from the duct at a stable temperature. Through this, low-emission combustion emissions and energy consumption are reduced. By implementing a high heat generation device, the heat storage combustion oxidation function is improved through the preservation of renewable heat. Over 177 h of demonstration time, we improved the function of 100 cm by discharging 99% of VOC’s removal efficiency, 95.78% of waste heat recovery rate, 21.95% of fuel consumption, and 3.9 ppm of nitrogen oxide concentration.

Combustion of liquid hydrocarbon fuels sprayed into gas generation chamber with superheated steam as low emission technology for energy production

Experimental and numerical studies were conducted to scientifically validate the Steam Injection Method with Gas Generation chamber (SIMGG) as a low-emission combustion technology for energy production in low and medium power boilers using different types of fuels. For the first time, a fully functional prototype of a burner implementing SIMGG was tested under the conditions of a standard low-power boiler installation, and combustion characteristics of fuels with various physical properties (diesel fuel and a mixture of used automotive oils) were obtained under different operating parameters. The results of the studies show that using SIMGG reduces carbon monoxide (CO) and nitrogen oxides (NOx) emissions by up to half or more under closed-combustion conditions for various fuels. The lowest total emissions of CO and NOx are achieved at an air excess coefficient of 1.2 and a steam-to-fuel ratio of greater than 0.66.

A pathway for implementing ammonia solutions as fuel blends for achieving low-emission combustion in diesel engines

The intriguing exploration of ammonia fuel has captured the spotlight of researchers, heralded for its status as a zero-carbon fuel and acclaimed as a potent inhibitor of NOx emissions in internal combustion engines. Ammonia solution, one of the forms of ammonia fuel, demonstrates its ability to blend seamlessly with conventional fuel. However, the slower combustion reaction of ammonia is a significant challenge to overcome. One of the effective approaches to improve the combustibility of ammonia fuel in diesel engines is to blend ammonia solution with diesel emulsion fuel. So, the focus of this study was to comprehensively analyze the effects of blending ammonia solution at various concentrations with diesel emulsion fuel and to closely examine their effects on combustion sustainability and emission profiles in diesel engines. The ammonia solution used is an aqueous solution containing 28 wt% of ammonia. Herein, five different fuel samples were examined: diesel, water/diesel (W/D), and W/D containing 1, 3, and 5 wt% ammonia solution (A1%-W/D, A3%-W/D, and A5%-W/D, respectively). The tests were carried out with a conventional diesel engine running at a fixed engine speed of 2400 rpm, while the loads were adjusted to a variety of levels. Interestingly, we found that the diesel emulsion containing a 5 wt% ammonia solution is ineffective in diesel engines, leading to performance degradation and unstable combustion. Compared with diesel, the rate of heat release was increased by 17.6 %, 15.3 %, 10.9 % for W/D, A1%-W/D, and A3%-W/D, respectively. Furthermore, the presence of ammonia solution prolonged the ignition delay period and facilitated the reduction of NO emissions. Compared with diesel fuel, the W/D, A1%-W/D, and A3%-W/D fuels reduced NO emissions by 39.3 %, 29.5 %, and 25 %, respectively. The results of this study demonstrated that the introduction of ammonia solution at concentrations up to 3 % by weight blended with diesel emulsion fuel can be effectively combusted in a diesel engine, promoting sustained combustion stability while contributing to a significant reduction in exhaust emissions.

Tomographic Reconstruction Of Combustion Flow Fields From Density Measurements Based On Physics-Informed Neural Networks

Achieving stable, low-emission combustion with green hydrogen is crucial for climate-neutral ground-based power generation in turbomachinery. Lean combustion modes with green hydrogen reduce fuel consumption but increase unsteadiness. Thus, multimodal detection techniques for parameters like density and flow velocity are essential to understand the interconnected behavior of combustion, advection velocity and noise production. We previously introduced a high-speed camera-based laser interferometric vibrometer system to detect thermoacoustic oscillations and record advection velocities, using interferometric detection of density fluctuations and correlation-based velocity estimation. However, this method only estimates integral velocity fields, necessitating the solution of the inverse problem. This is relying on iterative optimization, which is computationally expensive and struggles with high dimensional, noisy or limited data. Physics-Informed Neural Networks offer an innovative and efficient approach to these problems, combining neural network flexibility with physical law constraints to unravel intricate cause-effect relationships. Here an approach for reconstructing local velocity fields from a single projection velocity calculated from integral density data is presented. Using a U-net and model assumptions for coupling local and integral velocity fields, the training minimizes errors between measured integral input fields and the predicted local field integrals. The comparison of measured local velocity and the network prediction achieved a relative mean squared error of 3 %.

Flame stabilization and pollutant emissions of turbulent ammonia and blended ammonia flames: A review of the recent experimental and numerical advances

Compared to traditional hydrocarbon fuels, ammonia presents significant challenges as a fuel, including high ignition energy, low reactivity, slow flame propagation, and high NOx emissions, which hinder its use as a renewable fuel. Blending ammonia with fossil fuels like natural gas improves its combustion reactivity and helps mitigate CO2 emissions. However, there is still much to understand about the complex dynamics of ammonia and its blends with hydrocarbons. Key areas such as reaction kinetics mechanisms, ignition properties, flame propagation behaviors, and methods for controlling combustion performance under various conditions require further elucidation. This paper reviews recent advancements in experiments and numerical simulations aimed at developing stable, and low-emission combustors for ammonia-fired power generation. Recent burner and flame configurations, including non-swirling jets, single-stage swirl burners, two-stage burners, and newly developed double-swirl burners are analyzed for their flame stability and pollutant emission potential when firing ammonia and ammonia blends. Chemical kinetic modeling of ammonia and its blends plays a crucial role in understanding combustion behavior and pollutant emissions, particularly for NOx. However, there are challenges in predicting NOx emissions accurately, with significant disparities among different models. High-fidelity numerical simulations using detailed and skeletal mechanisms, direct numerical simulation, and large eddy simulation, have helped uncover crucial operational conditions affecting combustion and pollutant emissions, such as combustor pressure, air dilution, wall cooling, fuel/air mixing, and fuel blending. Nonetheless, the accuracy of chemical kinetic models and their integration into turbulent flow simulations remain critical limitations for numerical simulations of ammonia combustion.

Flame dynamics and oscillation characteristics based on statistical analysis in a combustor with prefilming and non-prefilming atomization

Suppressing thermoacoustic oscillations in low-emission combustors during their operation poses a major challenge. In this study, we report dynamic flame tests on combustors with prefilming (S1) and non-prefilming (S2) airblast atomizers. We used the image fast Fourier transform, proper orthogonal decomposition, dynamic mode decomposition, and continuous wavelet transform to obtain the spatial distribution of pulsations and spectral characteristics of the flames. The results reveal that the flames of S1 and S2 were significantly different. The combustor of S1 had a dome-attached flame confined to the primary combustion zone, while S2 had a lifted flame that filled the entire combustor. As the rate of airflow at the inlet increased, the flame of S1 exhibited oscillatory combustion, while the flame of S2 remained stable under all tested conditions, which was consistent with observations of its dynamic images. No characteristic peak was observed in the spectra of S2 under all operating conditions, and under rates of inlet airflow of 40 and 60 g/s for S1. However, pulsations in the chemiluminescence signals of the flame had a primary frequency of 116.4 Hz and secondary harmonic at 232.4 Hz at 80 g/s for S1. At 100 g/s, the S1 flame exhibited a primary frequency of 142.9 Hz, secondary harmonic at 285.4 Hz, and tertiary harmonic at 428.3 Hz. Minor adjustments to the geometry of the airblast atomizer can thus significantly alter the mode of spray–wall interactions and impact flame dynamics. Consequently, this study proposes a new control technique for instability suppression.

Fuel-air ratio effect on hydrogen-methane flames in a high pressure burner for gas turbines

This numerical simulation studied the effect of H2-CH4 flame equivalence ratio on turbulent premixed combustion at 50%-50% concentration (by volume). The equivalence ratio was varied from 0.45 to 1.0 in 0.5 increments for 126 kW operating power, matching a 3.3 bar inlet reactant pressure. Tests utilized a gas turbine combustor. The thermal field, flow field, and pollutant emissions (NOx and CO) underwent rigorous analysis. The modelling framework applied steady Reynolds-Averaged Navier-Stokes (RANS) equations coupled to a probability density function (PDF) approach for turbulence-chemistry interactions and a NOx formation model. Results showed increasing equivalence ratio from 0.45 to 1.0 elevated temperature approximately 900 K, significantly promoting NOx up to 1600 ppm and CO beyond 1900 ppm. However, equivalence ratio changes minimally impacted the overall flow field, maintaining stabilized flames. These findings provide new insight on thermochemical effects and flame stability in gas turbine (G.T) combustors across a range of equivalence ratios relevant for clean, high-pressure H2-CH4 combustion. The combined RANS-PDF methodology enables predictive simulation of turbulence, kinetics, emissions, and flame stability to guide optimal fuel-air ratio selection and low-emission combustor design.

New Trends in Catalytic Reaction for High-Temperature and Low-Emission Combustion Technologies

With the continuous rise in global energy demand and the increasing awareness of environmental protection, high-temperature low-emission combustion technology has become a research hotspot in the field of combustion science [...]

A comparative study of effects of methane and hydrogen on ammonia combustion in air via reactive molecular dynamics

Ammonia (NH3) is a promising substitute for fossil fuels to achieve the net-zero emission targets. To improve the combustion performance of NH3, reactive fuels are added to the NH3 combustion. In the present study, the combustions of NH3, CH4/NH3, and H2/NH3 in air conditions were simulated using a reactive molecular dynamics method. Results suggest that whether the addition of CH4 and H2 could accelerate the consumption of NH3 and reduce NOX emissions depends on the fuel ratio of the reactive gas (CH4/H2) and NH3 molecules. The acceleration of NH3 consumption is significant at low H2/NH3 fuel ratios and high CH4/NH3 fuel ratios, as the addition of H2 and CH4 lowers the activation energy for NH3 combustion. NOX emissions are mitigated at low H2/NH3 and CH4/NH3 fuel ratios but increase at high fuel ratios. The pathways for NOx generation and de-NOx reactions were identified. This study unveiled the mechanism via which the reactive fuels affect the NH3 combustion performance from an atomic perspective. The findings will be useful for the design of high-efficiency and low-emission combustion chambers in ammonia-powered energy systems.

Kriging surrogate model for optimizing outlet temperature distribution in low-emission combustors without dilution holes

Designing advanced combustors that operate at high temperatures and produce little pollution, especially in the absence of primary and dilution holes, is a difficult task that may bring significant challenges. In this regard, this paper introduces a Kriging surrogate model approach to optimize the outlet temperature distribution of the combustor to achieve such advanced low-pollution combustors. Building upon previous research, this study explores the effects of the swirler blade installation angle on the outlet temperature distribution of the combustor without primary or dilution holes. Traditional methods, such as the control variable method using computational fluid dynamics (CFD) for numerical simulation, are limited in application due to the complex coupling of flow, heat transfer, mass transfer, and combustion processes. In contrast, surrogate models, especially the Kriging model, offer a rapid and efficient alternative to extensive CFD simulations that provide accurate predictions and error estimates for the solution of the problem. In summary, this paper details the process of generating sample points through three-dimensional numerical simulations, develops a Kriging surrogate model through Latin hypercube sampling, and optimizes the model to identify the most uniform outlet temperature distribution achievable by adjusting the installation angle of the swirl blade. The optimal design parameters, which are quickly obtained through the Kriging model, showed a significant reduction in the overall temperature distribution function and the radial temperature distribution function by 21% and 27.14%, respectively.

Investigation on efficient and clean combustion pre-injection strategy of a diesel/methanol dual direct-injection marine engine under full load

Diesel ignition in-cylinder direct-injection methanol marine engine has shown enormous application potential due to its superior fuel economy and lower carbon emission. However, with high vaporization latent heat in methanol and increasingly strict emission regulations, new injection strategy urgently need to be studied to further improve the engine performance. To explore the feasibility of achieving high-efficiency and low-emission combustion by using pre-injection strategy in a diesel/methanol dual direct-injection marine engine, a numerical investigation is conducted to understand the combustion development, the mechanism of forming unregulated emissions, and the influence of methanol pre-injection timing (SOMIpre) and methanol pre-injection ratio (MPR) on engine performance. The results reveal that too early SOMIpre leads to a wet wall, while too late SOMIpre leads to insufficient mixing, thereby deteriorating combustion stability. The optimal fuel economy characterizing an indicated thermal efficiency (ITE) of 49.58% can be achieved when methanol is pre-injected at 35° crank angle (CA) before the top dead center (BTDC). As SOMIpre advances, the peak in-cylinder pressure (Pmax), peak heat release rate (HRRmax), and indicated mean effective pressure (IMEP) are decreased, along with increased soot, CO, and HC emissions, while the emissions of NOX and CO2 remain essentially unchanged. Additionally, the optimal methanol pre-injection strategy is yielded with an SOMIpre of 35°CA BTDC and an MPR of 15% when adopting a diesel injection timing (SODI) of 20°CA BTDC, increasing ITE by 19.9% and 2.7% and cutting CO2 emissions by 19.4% and 2.9% compared to the prototype (pure-diesel mode) and single-injection strategy. The results of this study provide new direction and data support for the research and development of efficient and clean combustion marine methanol engines.

Neural network controller of a gas turbine plant low emission combustor

Neural network controller of a gas turbine plant low emission combustor

Experimental investigation of the characteristics of combustion in the full hydrogen blending range of methane-hydrogen mixtures in a MILD model combustor

MILD combustion technology is expected to enable safe and stable low-emission combustion of hydrogen-doped fuels, or pure hydrogen fuels, in gas turbines designed for zero-carbon emissions. To improve combustion stability at high hydrogen content while maintaining low NOx emissions, a MILD model combustor was modified, specifically the upstream fuel/air distribution and mixing method. And the combustion stability of the modified MILD combustor was significantly extended. Here, the combustion characteristics of the modified MILD combustor are experimentally investigated at 0–100% hydrogen contents. The investigation results demonstrate that unlike the combustion stability region at low hydrogen concentrations, a narrow thermoacoustic instability region occurs at 60% hydrogen content. As the hydrogen content increases, the instability region becomes narrower while shifting towards lower equivalence ratios. The change in hydrogen content is accompanied by a significant peak energy migration of the dynamic pressure. The OH* chemiluminescence images showed that the heat release zone became more concentrated with increasing hydrogen content. And the dominance of high-frequency peak energy correlates to lateral motion, whereas the dominance of low-frequency peak energy corresponds to axial motion. This transition in flame dynamics occurs not only for different hydrogen contents but also for different equivalence ratios of pure hydrogen combustion. In contrast, NOx emissions are not sensitive to hydrogen content. At the same time, the MILD model combustor produces low emissions, with NOx emissions of less than 10 ppm@15%O2 at adiabatic flame temperatures ranging from 1200 K to 2100 K. These results show that the MILD model combustor has favorable fuel suitability.

Моделирование работы газотурбинной установки SGT5-4000F с исследованием влияния режимных и климатических факторов на устойчивость горения в камере сгорания

When regulating gas turbine units, it is necessary to be confident in the stable combustion process in the combustion chambers, considering the dynamics of the processes and changes of external climatic factors. Many scientific studies are devoted to the problems of experimental research and mathematical modeling of processes in combustion chambers. The possibilities of flame failure, expansion of the lower limit of combustion, determination of the stable position of the flame front and other issues have been assessed. Even though the problem of design of mathematical models and their use remains relevant. The purpose of this research is to model and study the influence of operation and climatic factors on the stability of combustion in the combustion chamber of the SGT5-4000F gas turbine unit. The subject of the research is the SGT5-4000F gas turbine unit with a low-emission combustion chamber. The simulation model of the gas turbine unit has been developed in the environment of dynamic modeling of technical systems SimInTech. The research is carried out using theoretical methods of calculating fuel combustion, thermodynamic foundations of the theory of gas turbine engines, methods of mathematical and simulation modeling, as well as data from the archive of automated process control systems. A simulation model of SGT5-4000F gas turbine unit has been developed. It is distinguished by the ability to assess the proximity of the compressor operating mode to the stability limit and determine the boundaries of stable combustion procedure in the combustion chamber, considering environmental climatic factors. The authors have studied the stability of the compressor and combustion chamber. The results of the experimental studies of a gas turbine unit in the operating load range from 113 to 282 MW and in the range of outside air temperature from –12 to 30 °C have shown that the combustion process in the combustion chamber is unvarying over the entire operational stress spectrum. The results obtained are verified by comparing model values with technological parameters of a gas turbine unit taken from the archive of the power plant control system. The authors have assessed the influence of electrical load on the performance indicators of a gas turbine unit at outdoor temperatures of +30, +15 and –12 °C. According to the results of simulation modeling, the model of SGT5-4000F gas turbine unit is found to be adequate. The developed model can be used as a tool at the stage of functional and technological design of automated control systems for the development of effective automatic control systems.

A novel method for quantifying variations in NO formation pathways using a sub-mechanism accounting and reaction tracing algorithm

This work presents a new implementation of reaction path analysis that traces molar fluxes attributable to various sub-mechanisms from inception through to their final product. The post-processing procedure involves creating a network of these fluxes and quantifying the contributions made by the specified formation and destruction mechanisms through an iterative process starting from the individual initiation reactions. A key distinction of this approach is that the reaction fluxes are directly correlated to their initiation reaction, which is then followed throughout the entire reaction network. This allows for the distinction of contribution to the formation of a target species from an arbitrary number of sources. The effectiveness of this approach is demonstrated by examining the formation of NO in three different case studies: (1) the impact of equivalence ratio on methane/air premixed counterflow flames, (2) the effect of hydrogen addition on ultra-lean methane/air premixed counter flames, and (3) the influence of pressure elevation in partially premixed methane/air flames. The results are presented in emission indexes, NO production rates throughout the flame, and multidigraphs illustrating the allocated flows between the species. Compared to replicated analysis methods previously used for these case studies, this new approach provides a more comprehensive and detailed analysis for several reasons. Firstly, the method does not require duplicated simulations with modified kinetic mechanisms, but is applied purely post-processing. This avoids the previous problems of needing to re-simulate the flame multiple times and allows for the fully coupled chemistry set. Secondly, the method allows for an arbitrarily defined set of initiation pathways which propagate through all possible routes within the network. This allows for the identification of spatial/temporal variability in the sub-mechanisms and thereby yields more information than the integral values of previous methods.Novelty and significance statementUnderstanding reaction pathways that lead to the formation of pollutants such as NO is an essential aspect in the development of new sustainable and low-emission combustion concepts. However, evaluating the different underlying sub-mechanisms is difficult due to the high degree of interconnectedness and spatial/temporal overlap of the reaction pathways. This work, therefore, presents a new algorithm that can be used to investigate the contributions of an arbitrary number of sub-mechanisms to the formation or decomposition of a species. The results can be expressed not only as integral values, but also as spatially and/or temporally resolved production rates and reaction path figures and graphs, leading to a more comprehensive and detailed analysis. The approach is also applied as a purely post-processing technique, which does not require multiple simulations with different chemical kinetic mechanisms as has previously be applied.

NO emission characteristics of air coflowed non-premixed ammonia jet flame at elevated ambient temperatures and with N2 dilution

Ammonia is promising carbon-free alternative fuel, while the poor combustion performance and severe NO emission hinder its application. In this work, a strategy combined with preheating and dilution was discussed, and NO emission characteristics of air coflowed non-premixed ammonia jet flame were experimentally investigated over 550–700 °C and with N2 dilution, using a novel preheating facility. Results show that the general excess air coefficient is significant for NO emission, but not the unique factor. NO emission monotonically increases with the general excess air coefficient, while the slope decreases. Ambient temperature is also of a remarkable effect, that NO emission decreases by about 100 ppm (15% O2, dry) when the ambient temperature rises from 550 °C to 700 °C. Besides, effects of dilution were considered with three specific strategies. Air coflow dilution is of a negative influence on NO emission, while, NH3 jet dilution promotes NO emission in contrast. The interaction between the two single-dilution strategies is limited, and the effect of double dilution seems to be a simple sum of that of the other two single-dilution strategies. In addition, flame liftoff was observed in experiments and a considerable decline of NO emission was detected with the flame regime transition. According to the experimental results, the combination of elevated ambient temperature and air dilution is evaluated to be a promising low NO emission combustion method of ammonia.

Low-Emission Combustion Chambers of GTU: Modern Trends, Diagnostics, and Optimization (Review)

Low-Emission Combustion Chambers of GTU: Modern Trends, Diagnostics, and Optimization (Review)

Study of the combustion process of liquid off-design fuel in a spray burner device with a controlled supply of primary air

This work studies the process of combustion of liquid off-design fuel (using waste oil as an example) in a jet of superheated water vapor in a spray burner device with a controlled supply of primary air. The average temperature along the vertical axis of the burner device was measured, the composition of intermediate and final combustion products, and heat release were studied when the parameters in the gas generation chamber were varied. It has been shown that changing the primary air flow rate when spraying liquid off-design fuel with a jet of superheated water vapor can additionally influence the flame structure and the level of emissions. And the technology itself can be used to create low-emission burner devices.

The effects of swirler vane configurations on the vortex-mixing interaction and combustion stability in a low-emission combustor

Particle Image Velocimetry (PIV) measurements and Large Eddy Simulation (LES) simulations are conducted for non-reacting and reacting flows to investigate the effects of swirler vane configurations on the vortex-mixing interaction and combustion stability in a low-emission tower-type coaxial-staged combustor (LETCC). Results show that the LES simulation results agree well with the experimental data. The combustor without vane lobes exhibits large mixture fraction bubbles in the inner shear layer (ISL) in non-reacting flow, and shows unstable combustion in reacting flow. Adding lobes to the first main stage swirler vanes eliminates instability factors in both non-reacting flow and reacting flow. Adding lobes to the second main stage swirler vanes suppresses mixture fraction bubbles in the ISL, but forms large areas with low mixture fraction in the outer shear layer (OSL) and outer recirculation zone (ORZ) in non-reacting flow, and induces localized flame extinction and re-ignition within the ORZ in reacting flow, leading to combustion instability.