Numerical investigation of flame–vortex interactions in laminar cross-flow non-premixed flames in the presence of bluff bodies

Experimental study of flame characteristics and stability regimes of biogas – Air cross flow non-premixed flames

Abstract: Biogas is an alternative fuel that typically contains around 45% carbon-dioxide by volume, besides methane. Due to the inherent content of carbon-dioxide, it is necessary to study the flame characteristics and stability limits in cross-flow non-premixed burners. In this study, cross-flow non-premixed flames, where biogas is injected through a horizontal porous plate and air is blown parallel to the fuel injector, are studied systematically. In order to increase the stable operating regime, devices such as backward facing steps and cylindrical bluff-bodies are commonly employed. Different step-heights and locations from leading edge of the fuel injector are considered for the cases with backward facing steps. A rectangular cylindrical bluff-body is also used as a flame stabilizing obstacle. Baseline cases are studied without any backward facing step or cylindrical bluff-body. Volume flow rate of biogas is varied from 36 liter per hour to 360 liter per hour. Air velocity is varied in the range of 0.2 m/s to 3.0 m/s. For a given fuel velocity, air velocity is gradually increased in order to record the transition of flame from one regime to another. Flame stabilization is carefully assessed by monitoring the high definition direct flame photographs captured from front and top views, for all the cases. The cases are repeated at least three times to ensure repeatability. Stability maps are plotted as a function of fuel velocity and air velocity for all the cases. For cases with backward facing steps, both step height and its location play an important role in delineating the boundaries of the flame regimes. Parametric variations show interesting features. Bluff-body flames become quite oscillatory and three dimensional at higher air velocities. For this case, stability maps of flames from biogas and pure methane are compared.

Numerical Simulation of Vortex Shedding from a Cylindrical Bluff-Body Flame Stabilizer

Abstract: A two-dimensional, laminar transient flow past a cylindrical bluff body, with methane injection perpendicular to the direction of the free stream flow, i.e. the cross-flow arrangement, is numerically studied. An unstructured grid finite volume method is used and simulations were carried out. The methane mass fraction and the injection velocity of methane injected from the slotted cylinder are altered simultaneously, and their effects on the combustion, flame characteristics, and fluid mechanics are investigated. The flame is anchored right in front of the cylinder and is stabilized by the wake of the bluff body. The current investigation illustrates the qualitative aspects of the vortex shedding phenomena. A particular case of injection velocity and mass fraction is studied in detail and its vortex shedding phenomena are analysed minutely. The non-reacting flow exhibits 2P mode of vortex shedding while the reacting flow exhibits the more common 2S mode. Fast Fourier transform analysis of the temporally fluctuating lift coefficient is performed for the different cases carried out in the present study.

On the identification of a vortex

Abstract: Considerable confusion surrounds the longstanding question of what constitutes a vortex, especially in a turbulent flow. This question, frequently misunderstood as academic, has recently acquired particular significance since coherent structures (CS) in turbulent flows are now commonly regarded as vortices. An objective definition of a vortex should permit the use of vortex dynamics concepts to educe CS, to explain formation and evolutionary dynamics of CS, to explore the role of CS in turbulence phenomena, and to develop viable turbulence models and control strategies for turbulence phenomena. We propose a definition of a vortex in an incompressible flow in terms of the eigenvalues of the symmetric tensor ${\bm {\cal S}}^2 + {\bm \Omega}^2$ are respectively the symmetric and antisymmetric parts of the velocity gradient tensor ${\bm \Delta}{\bm u}$. This definition captures the pressure minimum in a plane perpendicular to the vortex axis at high Reynolds numbers, and also accurately defines vortex cores at low Reynolds numbers, unlike a pressure-minimum criterion. We compare our definition with prior schemes/definitions using exact and numerical solutions of the Euler and Navier–Stokes equations for a variety of laminar and turbulent flows. In contrast to definitions based on the positive second invariant of ${\bm \Delta}{\bm u}$ or the complex eigenvalues of ${\bm \Delta}{\bm u}$, our definition accurately identifies the vortex core in flows where the vortex geometry is intuitively clear.

Proper orthogonal decomposition and its applications—part i: theory

Abstract: In view of the increasing popularity of the application of proper orthogonal decomposition (POD) methods in engineering fields and the loose description of connections among the POD methods, the purpose of this paper is to give a summary of the POD methods and to show the connections among these methods. Firstly, the derivation and the performance of the three POD methods: Karhunen–Loeve decomposition (KLD), principal component analysis (PCA), and singular value decomposition (SVD) are summarized, then the equivalence problem is discussed via a theoretical comparison among the three methods. The equivalence of the matrices for processing, the objective functions, the optimal basis vectors, the mean-square errors, and the asymptotic connections of the three methods are demonstrated and proved when the methods are used to handle the POD of discrete random vectors.

Reduced Kinetic Mechanisms for Applications in Combustion Systems

Abstract: This book is an introduction to the up-to-date technology used in reducing kinetic mechanisms especially for combustion systems. However, it can also be used as a handbook, in that it presents the most recent methods for modelling a variety of systems. In addition it presents existing software for reducing mechanisms and flame calcualtions for mixed and pre-mixed flames and fuels ranging from hydrogen to propane, and it defines a format for general flamelet libraries. Details are thoroughly compared with experimental data. This volume addresses scientists, graduate students and chemical engineers in industry and environmental research.

Scalar profiles and NO formation in laminar opposed-flow partially premixed methane/air flames

Abstract: Measurements of temperature, the major species (N2, O2, CH4, CO2, H2O, CO, and H2), OH, and NO are obtained in steady laminar opposed-flow partially premixed flames of methane and air, using the non-intrusive techniques of Raman scattering and laser-induced fluorescence. Flames having fuel-side equivalence ratios of φ = 3.17, 2.17, and 1.8 are stabilized on a porous cylindrical burner (Tsuji burner) in a low-velocity flow of air. Results are compared with calculations using a version of the Sandia laminar flame code that is formulated for the Tsuji geometry and includes an optically thin treatment of radiation. Because velocity profiles are not measured, the strain rate in each calculation is adjusted to match the measured profile of the mixture fraction. Measured profiles of temperature and species mass fractions are then compared with results of calculations using the GRI-Mech 2.11 and 3.0 chemical mechanisms, as well as a detailed mechanism from Miller. All three mechanisms give agreement with experimental results for the major species that is generally within experimental uncertainty. With regard to NO formation, the relative performance of the three mechanisms depends on the fuel-side equivalence ratio. GRI-Mech 2.11 gives reasonably good agreement with measured NO levels in lean and near-stoichiometric conditions, but it under predicts NO levels in fuel-rich conditions. GRI-Mech 3.0 significantly over predicts the peak NO levels the φ = 3.17 and 2.17 flames, but it yields relatively good agreement with measurements in the φ = 1.8 flame. The Miller mechanism gives good agreement with measured NO levels in the φ = 3.17 flame, but it progressively over predicts peak NO levels in the leaner flames. Comparisons of adiabatic and radiative calculations show that radiation can have a significant effect on the width and structure of partially premixed flames, as well as on the levels of NO produced.

Dynamics of flame/vortex interactions

Abstract: Vortex interactions with flames play a key role in many practical combustion applications. Such interactions drive a large class of combustion instabilities, they control to a great extent the structure of turbulent flames and the corresponding rates of reaction, they occur under transient operations or when flames travel in ducts containing obstacles. Vortices of various types are often used to enhance mixing, organize the flame region, and improve the flame stabilization process. The analysis of flame/vortex interactions has value in the development of our understanding of basic mechanisms in turbulent combustion and combustion instability. The problem has been extensively investigated in recent years. Progress accomplished in theoretical, numerical and experimental investigations on flame/vortex interactions is reviewed in this article.