Dissertations / Theses on the topic 'Reactive premixted flow'

Les restrictions environnementales abordent la réduction de l’utilisation des sources d’énergie primaires, motivant la recherche pour progresser vers des technologies améliorées. Parallèlement à ces efforts, la fiabilité et les performances doivent être assurées, en particulier dans des conditions délicates de pression et de température, c’est-à-dire en haute altitude. Dans les moteurs à turbine à gaz, ces deux éléments sont cruciaux pour offrir des produits qui répondent aux besoins et aux attentes fixés par le scénario actuel. L’allumage est un processus multiphysique constitué de plusieurs phases et événements qui concourent en couvrant une gamme diversifiée d’échelles de temps caractéristiques. La résolution numérique de la phase d’allumage précoce, pour laquelle des informations fines et détaillées font défaut, est étudiée dans cette étude. L’efficacité de l’allumeur est estimée par calorimétrie dans de l’air sec, ce qui montre que les variations de pression initiale ont une influence sur l’efficacité. La même étude a révélé que la température (20 ° C; - 20 ° C) par ailleurs a un effet négligeable. Les propriétés physiques du noyau en termes de volume, surface, surface de projection, rayon de l’arc électrique établi dans la cavité, sont estimées en adoptant différents diagnostics optiques, notamment de l’imagerie ultra rapide, strioscopie et ombroscopie à 1 MHz. Des calculs sont effectués pour obtenir une évolution temporelle pendant le temps de dépôt d’énergie (130 μs). Un effet de la pression initiale est observé sur les propriétés du noyau de telle sorte que avec la réduction de la pression initiale le volume du noyau augmente. De plus, des visualisations directes filtrées de la cavité de l’allumeur montrent qu’un effet de pression est discerné à partir de 20 μs. La taille du noyau est également mesurée pour des prémélanges de méthane pour différentes richesses. Cela vise à déterminer l’influence de la variation de la composition par rapport à un cas de référence dans du N2 pur qui est comparé aux mesures dans des prémélanges gazeux (à la fois de nature inerte CH4 / N2 et réactive CH4 / O2 / N2). Une comparaison entre les cas inertes et réactifs expose des réactions de combustion actives déjà pendant le dépôt d’énergie. Pour étudier l’exposition aux éléments d’un environnement réel, l’impact d’un écoulement transverse dans des conditions ambiantes est étudié dans une soufflerie. Cela a été adapté pour simuler l’effet combiné de l’écoulement transverse et de l’air de refroidissement auquel l’allumeur est exposé en étant monté dans une douille. L’effet de la douille sur la projection du noyau est étudié, révélant un impact sur la projection et la déformation du noyau en fonction de la vitesse imposée. La génération du noyau est examinée dans un écoulement réactif prémélangé à 0,45 et 1 bar. Le champ de vitesse a été étudié au préalable par PIV pour connaître la vitesse à proximité de l’allumeur et dans le domaine spatial où le noyau est projeté. Trois conditions de vitesse sont retenues pour effectuer la décharge. Il est observé que la pression initiale influence la déformation subie par le noyau en fonction de la vitesse initiale. En effet, à 1 bar, le noyau est préservé plus longtemps. Un effet secondaire de richesse est trouvé. Une étude préliminaire est réalisée pour explorer l’interaction entre le noyau et un spray de gouttes à 0,45 bar et 1 bar. Le fuel utilisé est du décane. L’ombroscopie à fort grossissement est le diagnostique utilisée pour effectuer des statistiques sur une fenêtre spatiale de 2 x 2 cm où des gouttelettes sont observées se déposer sur les électrodes. Des variations de leurs propriétés sont détectées en fonction de la synchronisation avec la décharge. Des visualisations par strioscopie sont ensuite réalisées pour observer qualitativement les phénomènes apparaissant dans une fenêtre temporelle de 1 ms. Le modèle existant de Taylor-Sedov est testé pour déterminer les capacités prédictives
Environmental restrictions tackle the reduction of the use of primary sources of energy motivating research to advance towards upgraded technologies. Alongside with these efforts, reliability and performance need to be ensured, especially for detrimental conditions of pressure and temperature, i.e. high altitude. In gas turbine engines, both these elements are crucial to offer products that fit to both the needs and expectations set by the present scenario. Ignition is a multiphase process constituted by several phases and events that span a diversified range of characteristic time scales. The numerical resolution of the early ignition phase, for which fine and detailed information is lacking, is investigated in this study. The efficiency of the igniter is estimated through calorimetry in pure air, which shows that variations of initial pressure have an influence on efficiency. The same investigation revealed that temperature (20° C; - 20°C) has a negligible effect. Physical properties of the kernel in terms of volume, surface, projection surface, radius of the arc channel in the cavity, are estimated adopting different optical diagnostics, including schlieren and shadowgraphy imaging at 1 MHz. Calculations are done to obtain a temporal evolution during energy depositing time (130 μs). An effect of initial pressure is observed on kernel properties such that reducing the initial pressure, kernel volume increases. Furthermore, filtered direct visualizations of the igniter cavity show that an effect of pressure is discerned from 20 μs. Kernel size is also measured for methane premixed mixtures of different equivalence ratios. This is intended to determine the influence of composition variation with respect to a reference case in pure N2 which is compared to measurements in gaseous premixed mixtures (both of inert CH4 / N2 and reactive CH4 / O2 / N2 nature). A comparison between inert and reactive cases exposes active combustion reactions already during energy deposition. To investigate the exposure to real life environment elements, the impact of a transverse flow at ambient conditions is studied in a wind tunnel. This was adapted to simulate the combined effect of a transverse flow and cooling air spilled from the liner that the igniter is exposed to by being mounted in a sleeve. The effect of the sleeve on kernel projection is investigated, which reveales an impact on projection and kernel deformation depending on the imposed velocity. The generation of the kernel is examined in a reactive premixed swirled mixture at 0.45 and 1 bar. The velocity field have been studied beforehand by PIV to know the velocity in the vicinity of the igniter and in the spatial domain where the kernel is projected. Three velocity conditions are retained to perform the discharge. Initial pressure is observed to influence the deformation the kernel undergoes depending on initial velocity. At 1 bar, the kernel appears to be preserved for longer. A secondary effect of equivalence ratio is found. The existing model of Taylor-Sedov is tested to predict kernel properties and compare them to experimental measurements. A preliminary study is performed to explore the interaction between the kernel and a spray at 0.45 bar and 1 bar. High magnification shadowgraphy is used to run statistics on a spatial window of 2 x 2 cm where droplets are observed impinging on the electrodes. Properties variations are detected depending on the synchronization with the discharge. Schlieren visualizations are further performed to observe phenomena to qualitatively explore the dynamics appearing in a time window of 1 ms

The aims of this thesis are to apply the Large Eddy Simulation (LES) and beta Probability Density Function (β- PDF) for the simulation of turbulent non-premixed reacting flow, in particularly for the predictions of soot and NO production, and to investigate the radiative heat transfer during combustion process applying Discrete Ordinates Method (DOM). LES seeks the solution by separating the flow field into large-scale eddies, which carry the majority of the energy and are resolved directly, and small-scale eddies, which have been modelled via Smagorinsky model with constant Cs (Smagorinsky model constant) as well as its dynamic calibration. This separation has been made by applying a filtering approach to the governing equations describing the turbulent reacting flow. Firstly, LES technique is applied to investigate the turbulent flow, temperature and species concentrations during the combustion process within an axi-symmetric model cylindrical combustion chamber. Gaseous propane (C3H8) and preheated air of 773K are injected into this cylindrical combustion chamber. The non-premixed combustion process is modelled through the conserved scalar approach with the laminar flamelet model. A detailed chemical mechanism is taken into account to generate the flamelet. The turbulent combustion inside the chamber takes place under a fuel-rich condition for which the overall equivalence ratio of 1.6 is used, the same condition was used by Nishida and Mukohara [1] in their experiment. Secondly, the soot formation in the same flame is investigated by using the LES technique. In this thesis, the soot formation is included through the balance equations for soot mass fraction and soot particle number density with finite rate kinetic source terms to account for soot inception/nucleation, surface growth, agglomeration and oxidation. Thirdly, the NO formation in the flame is studied by applying the LES. The formation of NO is modelled via the extended Zeldovich (thermal) reaction mechanism. A transport equation for NO mass fraction is coupled with the flow and composition fields. Finaly, the radiative heat transfer in the flame is investigated. Both the luminous and non-luminous radiations are modelled through the Radiative Transfer Equation (RTE). The RTE is solved using the Discrete Ordinates Method (DOM/Sn) combining with the LES of the flow, temperature, combustion species and soot formation. The computed results are compared with the available experimental results and the level of agreement between measurements and computations is quite good.

Thesis (M.S.)--University of Cincinnati, 2005.
Title from electronic thesis title page (viewed Apr. 18, 2006). Includes abstract. Keywords: Large Eddy Simulation; LES; Lean Pre-mixed; LPM; Gas Turbine Combustor; Combustion; Reacting Flows. Includes bibliographical references.

This thesis aims at improving our knowledge on swirl combustors. The work presented here is based on Large Eddy Simulations (LES) coupled to an advanced combustion model: the Conditional Moment Closure (CMC). Numerical predictions have been systematically compared and validated with detailed experimental datasets. In order to analyze further the physics underlying the large numerical datasets, Proper Orthogonal Decomposition (POD) has also been used throughout the thesis. Various aspects of the aerodynamics of swirling flames are investigated, such as precession or vortex formation caused by flow oscillations, as well as various combustion aspects such as localized extinctions and flame lift-off. All the above affect flame stabilization in different ways and are explored through focused simulations. The first study investigates isothermal air flows behind an enclosed bluff body, with the incoming flow being pulsated. These flows have strong similarities to flows found in combustors experiencing self-excited oscillations and can therefore be considered as canonical problems. At high enough forcing frequencies, double ring vortices are shed from the air pipe exit. Various harmonics of the pulsating frequency are observed in the spectra and their relation with the vortex shedding is investigated through POD. The second study explores the structure of the Delft III piloted turbulent non-premixed flame. The simple configuration allows to analyze further key combustion aspects of combustors, with further insights provided on the dynamics of localized extinctions and re-ignition, as well as the pollutants emissions. The third study presents a comprehensive analysis of the aerodynamics of swirl flows based on the TECFLAM confined non-premixed S09c configuration. A periodic component inside the air inlet pipe and around the central bluff body is observed, for both the inert and reactive flows. POD shows that these flow oscillations are due to single and double helical vortices, similar to Precessing Vortex Cores (PVC), that develop inside the air inlet pipe and whose axes rotate around the burner. The combustion process is found to affect the swirl flow aerodynamics. Finally, the fourth study investigates the TECFLAM configuration again, but here attention is given to the flame lift-off evident in experiments and reproduced by the LES-CMC formulation. The stabilization process and the pollutants emission of the flame are investigated in detail.

博士
國立臺灣大學
機械工程學研究所
106
Abstract Syngas, a clean and alternative fuel, has a great potential to replace hydrocarbon fuels in combustion applications. An impinging flow field has attracted interest in the investigation of its mixing characteristics of fuel and oxidant in many fuel-injection systems, but up to now the research on jet-impinging flames has been focused mainly on diffusion flames with hydrocarbon fuels. For a practical application, we therefore propose a concept of clean combustion through combining the advantages of syngas and an impinging burner. Furthermore, the varied proportions of H2 and CO are the crucial causing a variation in the fuel mixing and combustion reaction when using syngas as a principal fuel. We performed experimental measurements of particle image velocimetry (PIV), chemiluminescence of free radicals, flame temperature, and CO emission to examined how and why the varied proportions of H2 and CO affected the fuel mixing and combustion reaction of a syngas premixed impinging flame. For a C3H8 premixed impinging flame on the V-shaped burner, its flame propagation speed increased with the addition of H2 and CO into the fuel mixture, which expanded its lean flammability. The addition of H2 in the fuel mixture enhanced the reaction intensity of flame sheet, but, decreased the reaction intensity of flame tip, which shows that the reaction zone was dominated by strong mass diffusivity. The temperature of flame sheet hence increased, and the temperature of flame tip decreased with increasing H2 proportion. Although the mass diffusivity of reaction zone on the flame sheet became weaker when CO presented a large proportion of fuel, the fuel mixture conducted the second reaction within the impinging zone through the well preheating and deceleration. The reaction intensity of impinging zone hence increased, and the emission of CO decreased. We further examined the characteristics of fuel mixing and reaction of CH4/syngas/air impinging flame with H2/CO in varied proportions using a multi-way impinging burner. The results showed that a deceleration area in the main flow formed through the mutual impingement of two jet flows, which enhanced the mixing of fuel and air because of an increased momentum transfer. The deceleration area expanded with an increased CO proportion, which indicated that the mixing of fuel and air also increased with the increased CO proportion. CO provided in the syngas hence participated readily in the reaction of the CH4/syngas/air premixed impinging flames when the syngas contained CO in a large proportion. Our examination of the OH* chemiluminescence demonstrated that its intensity increased with increased CO proportion, which showed that the reaction between fuel and air accordingly increased. Finally, to enhance the reaction intensity, we introduce a central air jet injecting into a CH4/syngas/air impinging flame. For a fuel-rich CH4/syngas/air impinging flame, the added central air jet caused no acceleration of the fuel mixture flowing toward downstream when ratio UC/UF was less than 1.0. The fuel mixture obtained additional oxidant from the central air jet, which increased its reaction intensity; the CO emission hence decreased and the flame temperature increased when the UC/UF ratio was less than 1.0. When UC/UF exceeded 1.5, however, the central air jet caused the fuel mixture to accelerate in its escape downstream because of the increased upward momentum; the reaction intensity thus exhibited a decreasing trend and the CO emission greatly increased. The results shown in our work provide a significant reference and a prospective concept for the utilization of syngas, which improves the feasibility of fuel-injection systems using syngas as an alternative fuel.

Combustion phenomena are complex in theory and expensive to test, analysis techniques
provide handles with which we may describe them. Just as simultaneous experimental tech-
niques provide complementary descriptions of flame behavior, one might assume that no
analysis technique for any kind of flame measurement would cover the full description of
the flame. To this end, the search continues for complementary descriptions of engineering
flames that capture enough information for the engine designer to make informed decisions.
The kinds of flames I have encountered are high pressure transverse jet flames issuing into a
vitiated crossflow which is itself generated from combustion of a gaseous fuel and oxidizer.
Summarizing the behavior of these flames has required my understanding of experimen-
tal techniques such as Planar Laser Induced Fluorescence of a reaction intermediate -OH,
Particle Image Velocimetry of a passive tracer in the flame and OH * chemiluminescence of
another reaction intermediate. The analysis tools applied to these measurements must reveal
as much information as is laden in these measurements.
In this work I have also used wavelet optical flow to track flow features in the visualization
of combustion intermediates using OH * chemiluminescence. There are many limitations to
the application of this technique to engineering flames especially due to the interpretation
of the data as a 2-D motion field in 3-D world. The interpretation of such motion fields
as generated by scalar fields is one subject matter discussed in this dissertation. Some
inferences from the topology of the ensuing velocity field has provided insight to the behavior
of reacting turbulent flows which appear attached to an injector in the mean field. It gives
some understanding to the robustness of the attachment mechanism when such flames are
located near walls.