This study characterizes the central recirculation zones formed under combustion conditions, with natural gas as fuel with different geometries and degrees of premixing using a swirl combustor firing into a confinement representative of gas turbine combustors. Phase-locked particle image velocimetry is used as the main method of characterization. The technique enables characterization of the time-dependent behavior of the central recirculation zone and a three-dimensional reconstruction of its boundaries. The central recirculation zone typically had an asymmetric lobed shape and precessed about the central axis. Partially premixed combustion at near-stoichiometric equivalence ratios reduced the coherence of the central recirculation zone and often caused it to nearly disappear, although a small remnant of recirculation could still be found in the three-dimensional space. Lower equivalence ratios strengthened the central recirculation zone considerably, for both non-premixed and partially premixed combustion. Although the central recirculation zone was asymmetric in shape and precessed about the central axis, the precessing vortex core, commonly found in these flows, was found to be significantly suppressed especially when central fuel injectors were used. Its occurrence then became intermittent and irregular. A quarl exit nozzle with a divergent lip was found to reduce flame attachment to the fuel injector.
Combustion instabilities are considered one of the most serious challenges for developing combustion systems through the years. Undesirable issues linked to these phenomena represent a risk for such systems especially in gas turbines and propulsion devices where the propagation of these instabilities can even lead to considerable damages. Flame flashback from the combustion chamber into premixer represents one of the most important combustion instability issue in swirl combustors used in gas turbines. This study proposes an experimental and numerical approach to validate the use of a central air injection in swirl combustors to reduce flame flashback propensity via controlling the turbulence generation at the tip of the flame while pushing the CRZ, thus retarding the appearance of the CIVB, to mitigate the progression of combustion into the system. Results showed the potential of this technique to affect turbulence generation and pushing back the flame into the combustion chamber, increasing operability limits. Very good agreement was achieved between experimental and numerical results, demonstrating that the use of injection through the central core of the system not only controls the position of the recirculation zone but also affects turbulence and mitigates other forms of flame flashback.
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ResumenEntre las tecnologías más utilizadas en la generación de energía y propulsión de aviones para la reducción de emisiones se utilizan los fl ujos giratorios, los cuales han demostrado durante varias décadas su alto desempeño al facilitar el anclaje de la llama en los sistemas de combustión.Esto, aunado al uso de reactivos premezclados en las antecámaras, proporciona uno de los métodos más innovadores para la reducción de partículas de alto efecto invernadero, como son los óxidos de nitrógeno.Sin embargo, el entendimiento de estos fl ujos es aún precario y se requiere mayor investigación sobre el tema para poder esclarecer conceptos tan complejos como el rol que tienen las estructuras coherentes dentro del sistema de combustión.El presente artículo explica diversos fenómenos ocasionados por algunas de las estructuras coherentes observadas
Swirl stabilized flows are the most widely deployed technology used to stabilize gas turbine combustion systems. However, there are some coherent structures that appear in these flows close to the nozzle whose occurrence and stability are still poorly understood during transition. The external recirculation zone and the Precessing Vortex Core to/from the Coanda effect are some of them. Thus, in this paper the transition of an Open Jet Flow-Medium Swirl flow pattern to/from a Coanda jet flow is studied using various geometries at a fixed Swirl number. Phase Locked Stereo Particle Image Velocimetry and High Speed Photography experiments were conducted to determine fundamental characteristics of the phenomenon. It was observed that the coherent structures in the field experience a complete annihilation during transition, with no dependency between the structures formed in each of the flow states. Moreover, transition occurs at a particular normalized step size whilst some acoustic shifts in the frequencies of the system were noticed, a phenomenon related to the strength of the vortical structures and vortices convection. It is concluded that a transient, precessing, Coanda Vortex Breakdown is formed, changing flow dynamics. The structure progresses to a less coherent Trapped Vortex between the two states. During the phenomenon there are different interactions between structures such as the Central Recirculation Zone, the High Momentum Flow Region and the Precessing Vortex Core that were also documented.
This paper presents a series of experiments and numerical simulations using commercial software (ANSYS) to determine the behaviour and impact on the blowoff process with various geometries and simulated syngas compositions at fixed power outputs. Experiments were performed using a generic premixed swirl burner. The Central Recirculation Zone and the associated turbulent structure contained within it were obtained through CFD analyses providing details of the structures and the Damkolher Number (Da) close to blowoff limits. The results show how the strength and size of the recirculation zone are highly influenced by the blend, with a shift of Da and turbulence based on carbon-hydrogen ratio, shearing flows and Reynolds number. Instabilities such as thermoacoustics, flashback, autoignition and blowoff are highly affected by the flow structures and chemical reactions/diffusivity. Moreover, it has been observed that turbulence close to the boundaries of the central recirculation zone, a region of high stability for swirling flows, is highly altered by the chemical characteristics of the fuel blends. In terms of blowoff, the phenomenon is still not entirely understood. As the process occurs, its theoretical limits do not match its real behaviour. Therefore, one possibility could be the difference in turbulence and Da numbers across the flame, being critical at the base of the flame where the system is stabilized.
Zero-carbon fuels such as hydrogen and ammonia play a pivotal role in the energy transition by offering cleaner alternatives to natural gas (methane), especially in industrial combustion systems. Binary and ternary blends of these fuels offer a transitionary, low-carbon solution in the near future. Laminar burning velocity (LBV), as a fundamental combustion property, is significantly different for ammonia, hydrogen, and methane. Although the LBV of binary blends of these fuels is well-studied, ternary blends have not been extensively studied. In this study, the primary objective is to employ a simple ensemble learning method to predict the LBV of ternary ammonia/hydrogen/methane/air mixtures. The training dataset consists of experimental data sourced from a large number of publications (3846 data points), as well as synthetic data generated by 1D freely propagating premixed flame simulations in Cantera using a detailed chemical kinetic model. Three machine learning algorithms, namely artificial neural networks, gaussian process regression, and extreme gradient boosting trees are trained and optimised. Then, a simple ensemble averaging method is used to reduce overfitting and improve robustness. The ensemble model achieves a coefficient of R2 = 0.991 with an inference time that is approximately 8,000 times faster than the 1D simulation run time. The ensemble model is capable of predict ing LBVs of ammonia/hydrogen/methane/air mixtures for T = [295−756K], P = [1 − 10bar], ϕ = [0.5 − 1.8] across all possible blending ratios.
The reduction of greenhouse gases emissions from the aviation sector is focused on better engine efficiency or optimized flight pathways. However, the most relevant is probably the use of sustainable biofuels. In order to meet the strict jet fuelspecifications for commercial flights, these biofuels(drop-in fuels) must contain only paraffinic hydrocarbons, without heteroatoms. Several renewable aviation fuels have already been certified by ASTM, others are under examination. Anew promising route consists in the thermochemical conversion of lignin, the main co-product from 2nd generation ethanol. The EU FP7 BIOREFLY project will develop a first industrial pre-commercial lignin-to-jet fuel 2000 ty-1demonstration plant. The present work describes strategies, equipment and R&D lines of BIOREFLY, which aims at evaluating the properties of this bio-jet fuel and its blends in view of future ASTM certification. Injection features and the combustion properties of aviation engines will be investigated in an optical combustor rig. Combustion parameters, emissions and chemiluminescence provide fundamental data to understand the combustion behavior for different hydrocarbons species. Tests in micro-gas-turbines (i.e. power generation and APU-derivative units) will assess the effect of fuels in terms of emissions and evaluating their performances.
Swirl stabilized combustion is one of the most successful technologies for flame and nitrogen oxides control in gas turbines. However, complex fluid dynamics and lean conditions pose a problem for stabilization of the flame. The problem is even more acute when alternative fuels are used for flexible operation. Although there is active research on the topic, there are still various gaps in the understanding of how interaction of large coherent structures during the process affect flame stabilization and related phenomena. Thus, this paper approaches the phenomenon of lean premixed swirl combustion of CH4/H2/CO blends to understand the impacts of these fuels on flame blowoff. An atmospheric pressure generic swirl burner was operated at ambient inlet conditions. Different exhaust nozzles were used to alter the Central Recirculation Zone and observe the impacts caused by various fuel blends on the structure and the blowoff phenomenon. Methane content in the fuel was decreased from 50% to 10% (by volume) with the remaining amount split equally between carbon monoxide and hydrogen. Experimental trials were performed using Phase Locked PIV. The Central Recirculation Zone and its velocity profiles were measured and correlated providing details of the structure close to blowoff. The results show how the strength and size of the recirculation zone are highly influenced by the fuel blend, changing stability based on the carbon-hydrogen ratios. Nozzle effects on the shear flow and Re numbers were also observed. Modelling was carried out using the k-ω SST CFD model which provided more information about the impact of the CRZ and the flame nature close to blowoff limit. It was observed that the model under-predicts coherent structure interactions at high methane fuel content, with an over-prediction of pressure decay at low methane content when correlated to the experimental results. Thus, complex interactions between structures need to be included for adequate power prediction when using very fast/slow syngas blends under lean conditions.
The hydrodynamic behaviour of gas-solid mixtures inside Bed Fluidised Beds (BFB) gasifiers has a major impact on the gasification process due to particles - gas and particle - particle contact mechanisms. The Discrete Phase Model DPM with Multiphase Particle-in-Cell method MPPIC was used as a CFD approach to study the hydrodynamic behaviour of an 800 height x 83 mm φ prototype fluidised bed gasifier with 4 different perforated plate distributors. In terms of bubble forming, pressure drop and superficial velocity, the type D distributor, i.e. triangular, had the best performance among the other types and in turn better bed height and bed movement, thus allowing best fluidisation performance as a consequence of better flow distribution.
There is a perceived need for multi-fuel burner geometries capable of operating with variable composition fuels from diverse sources to achieve fuel flexibility in gas turbines. The objective of the research covered herein is a comparison study between two liquid fuels, a biodiesel (in a pure form) and the biodiesel as a saturated mixture with a pyrolysis by-product; these two fuels were compared against a standard kerosene as a baseline. The research methodology involved two stages: firstly atomization patterns and injection regimes were obtained using a high speed imaging method, secondly a combustion test campaign was undertaken using a swirl burner to quantify the operational behaviour, species production and exhaust gas compositions of the fuels. Emissions, flame stability trends and power outputs were measured at gas turbine relevant equivalence ratios. Excess oxygen and atomization trends in the biodiesel seem to be playing a major role in the production of emissions and flame stability when compared to kerosene. Also, heavy organics seem to be acting as catalytic substances for OH production close to the burner mouth. In terms of stability and combustion, it is proposed that the saturated blend would be a viable candidate for power generation.
