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Current Status of the use of Parallel Computing in Turbulent Reacting Flows: Computations Involving Sprays, Scalar Monte Carlo Probability Density Function and Unstructured Grids
Published in W.J. Minkowycz, E.M. Sparrow, Advances in Numerical Heat Transfer, 2018
The gas-turbine combustor flows are often characterized by a complex interaction between various rate-controlling processes associated with turbulent transport, mixing, chemical kinetics, evaporation and spreading rates of spray, convective and radiative heat transfer, among others [1]. The phenomena to be modeled as controlled by these processes often interact with each other at various disparate time and length scales. In particular, turbulence plays an important role in determining the rates of mass and heat transfer, chemical reactions, and liquid-phase evaporation in many practical combustion devices. The influence of turbulence in a diffusion flame manifests itself in several forms, ranging from the so-called wrinkled or stretched flamelets regime to the distributed combustion regime, depending upon how turbulence interacts with various flame scales [2–3].
Combustion in Natural Fires
Published in James G. Quintiere, Principles of FIRE BEHAVIOR, 2016
A diffusion flame is a combustion process in which the fuel gas and oxygen are transported into the reaction zone due to concentration differences. This transport process is called diffusion and is governed by Fick’s Law, which says that a given species will move from a high to low concentration in the mixture. In connection with fire, species are the components of the reaction, for example, oxygen, fuel, CO2, and other components comprising the reactants and products. To understand the nature of diffusion, consider a drop of blue ink in a glass of still water. Eventually, the dye droplet will disperse into the water to give a uniform blue tinge. This motion of the dye into the clear water is diffusion. Oxygen in air will move to the flame zone where it has a concentration of zero since it is consumed in the reaction. Fuel is transported into the opposite side of the flame by the same process. The combustion products diffuse away from the flame in both directions. This process is illustrated in Figure 2.2.
Fire Hazards and Associated Terminology
Published in Asim Kumar Roy Choudhury, Flame Retardants for Textile Materials, 2020
There are different methods of distributing the required components of combustion to a flame. In a diffusion flame, oxygen and fuel diffuse into each other; the flame occurs where they meet. As a result, the flame speed is limited by the rate of diffusion. In a diffusion flame, combustion takes place at the flame surface only, where the fuel meets oxygen in the right concentration; the interior of the flame contains unburnt fuel. This is opposite to combustion in a premixed flame.
The Speed and Temperature of an Edge Flame Stabilized in a Mixing Layer: Dependence on Fuel Properties and Local Mixture Fraction Gradient
Published in Combustion Science and Technology, 2020
Edge flames are also relevant to studies of turbulent combustion. A diffusion flame can be sustained in a turbulent flow as long as the amount of heat diffusing away from the reaction zone is balanced by the heat produced by combustion. Due to velocity fluctuations, local quenching of the flame occurs when velocity gradients are excessive and the residence time in the reaction zone is too short compared to the chemical reaction time, namely when the rate of chemical reactions is not able to keep up with the heat diffusing away from the reaction zone. Through the hole formed on the flame surface, fuel and oxidizer come in contact and a local mixing region is formed. When thermal gradients become well above the quenching limit through turbulent fluctuations, the partially-premixed gas can ignite with combustion reestablished in the form of an edge flame. The edge flame can either propagate inwards into the pocket of fresh gases, reestablishing the flame and reducing the size of the hole, or propagate outwards spreading extinction to other parts of the flame. Flame “extinguishment” or “healing” is accomplished here through the propagation of an edge flame.
Effect of Diluent Addition on Combustion Characteristics of Methane/Nitrous Oxide Inverse Tri-Coflow Diffusion Flames
Published in Combustion Science and Technology, 2020
Yueh-Heng Li, Chun-Han Chen, Mustafa Ilbas
For diffusion flames, the flame can be divided into two types according to the feeding pattern of the oxidizer and the fuel, namely normal diffusion flame (NDF) and inverse diffusion flame (IDF). If the fuel stream is surrounded by the oxidizer stream, it is an NDF. Conversely, if the oxidizer stream is enclosed by the fuel stream, it is an IDF. Mikofski et al. (2006) validated Roper’s theory (Roper 1977) for the suitability of CH4/air and C2H4/air IDFs and modified Roper’s theory by comparing the flame height measured using the OH-planar laser-induced fluorescence (PLIF) measurement system. In addition, the results indicated that the flame height detected using the OH-PLIF measurement system was not the actual height of the flame, because the soot image contaminated the measurement results of the OH radical. The formation of soot in fossil fuel flames is an important concern. For an IDF, soot is generated in the fuel stream and deposited on the flame reaction zone (Kang et al. 1997; Sidebotham and Glassman 1992). Characteristically, the soot particles do not fly across the flame reaction zone (Shaddix et al. 2005). Therefore, the IDF is an excellent flame mode to quantify soot formation. Mikofski et al. (2007) investigated in detail the structure of CH4/air and C2H4/air inverse diffusion flames through OH-PLIF, polycyclic aromatic hydrocarbon (PAH)-PLIF, and soot-planar laser-induced incandescence (PLII), and revealed that for the IDF and NDF, the distances among the OH, PAH, and soot regions were similar, but their relative positions were reversed. Liu and Smallwood (2011) determined that the central air jet stream exerted a considerable influence on the flame structures and sooting characteristics of the flame in a triple port co-annular burner. Johnson and Sobiesiak (2011) studied the hysteresis of inverse methane diffusion flame and suggested that when the local equivalence ratio approached unity upstream of the flame, the partially premixed flame (PPF) propagated upstream and stabilized as an IDF. The behavior of the leading edge of the lifted-off flames was investigated through numerical simulation (Yamamoto et al. 2015). The results showed that an inward radial velocity component toward the central axis at the leading edge of the outer lifted-off flame, resulting in the acceleration of the flow field in front of the inner lifted-off flame. The behavior of the leading edge of the lifted-off flames was investigated through numerical simulation (Yamamoto et al. 2015). The results showed that an inward radial velocity component toward the central axis exists at the leading edge of the outer lifted-off flame, resulting in the acceleration of the flow field in front of the inner lifted-off flame. The behavior of the inner lifted-off flame reattaching to the nozzle can be observed with an increase in the lifted-off height of the inner flame. The edge propagation speed decreased considerably when the lifted-off heights of the inner and outer flames were comparable. However, the effect of the acceleration of the flow field in front of the inner lifted-off flame was mitigated. Therefore, the edge propagation speed and axial velocity played a vital role in the flame reattachment.