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Flames
Published in Kenneth M. Bryden, Kenneth W. Ragland, Song-Charng Kong, Combustion Engineering, 2022
Kenneth M. Bryden, Kenneth W. Ragland, Song-Charng Kong
Premixed flames arise from the combustion of gaseous reactants (e.g., fuel and oxidizer) that are well mixed prior to combustion. Diffusion flames arise from the combustion of separate gaseous fuel and oxidizer streams that combust rapidly as they mix. When the gas flow is laminar, the flame is laminar and has a smooth flame front. When the gas flow is turbulent, the flame is turbulent, and the flame front is wrinkled or broken up. Laminar-premixed flames have a unique flame speed for a given fuel–oxidizer mixture that is less than a meter per second for hydrocarbon–air mixtures at ambient pressure and temperature. Turbulence increases the flame speed. For diffusion flames, the rate of mixing of the reactants determines the flame speed, and the reaction takes place at the interface between the fuel and the oxidizer. Stationary flames are stabilized by flow in a burner or by flow into a stagnation region. In a quiescent combustible mixture, an ignition source initiates a flame that propagates outward from the ignition source at the laminar flame speed. Given sufficient volume, a flame will transition into a detonation, which propagates at more than 1000 m/s and creates a pressure rise (see Chapter 8).
Laminar Premixed Flames
Published in Achintya Mukhopadhyay, Swarnendu Sen, Fundamentals of Combustion Engineering, 2019
Achintya Mukhopadhyay, Swarnendu Sen
Premixed flames suffer from serious hazards like possibilities of different kinds of instabilities like flashback, lift-off and blowout. Flashback refers to a condition when the flame propagates towards the unburned mixture. In the case of burner flames, this condition implies entry of the flame into the burner tube carrying reacting mixtures and propagation upstream along the tube. This condition not only disturbs the stability of the flame stabilised on the burner but is also a potential safety hazard as the flame can move upstream and ignite a large volume of reactants in the mixing chamber where the fuel and air are mixed. This can lead to explosion. Since flashback occurs when the flame propagates into the unburned mixture, the flame propagation velocity exceeds the local flow velocity. Thus, mixtures with high flame speed are more prone to flashback.
Combustion in Natural Fires
Published in James G. Quintiere, Principles of FIRE BEHAVIOR, 2016
A premixed flame is the second type of combustion we can call fire. A premixed flame is a combustion process in which the fuel gas and air (or oxygen) are first mixed before ignition and propagation occur. In a diffusion flame, both laminar and turbulent, the fuel is supplied in a distinct stream, and air is then mixed into it as it burns. In a premixed flame, the fuel and oxygen, or air, are mixed, then burning occurs within that mixture. In a confined space, such a combustion process can cause a rapid pressure increase that manifests itself as an explosion. If sufficient pressure builds up behind the propagating flame, it will accelerate the flame. At a higher speed, turbulence will ensue and increase the reaction rate. Eventually, the flame speed can reach the speed of sound, and with further increases, a shock wave will form ahead of the flame. That shock wave introduces now higher temperature and pressure for the fuel and oxygen supplied into the flame. We know that the chemical reaction rate is increased as temperature and pressure rise. This leads to a feedback effect. The speed of the flame increases, and a stronger shock is formed with yet higher temperature and pressure across it to feed the flame. The end result will produce an extremely high flame speed with a shock wave preceding it. This event is then called a detonation. Such an event has great damage potential; however, even the nondetonating (deflagration) premixed flame in a confined space can cause significant damage due to pressure rise. Both events will be perceived as an explosion. While we are considering flames here, premixed combustion systems can exist as solids (e.g., dynamite, rocket propellants, etc.). The dynamics of premixed flames can be used to explain these systems as well.
Mixing Tube Length Effect on the Stability of Confined Swirling Partially Premixed Methane Flames off a Concentric Flow Conical Nozzle Burner
Published in Combustion Science and Technology, 2021
Mahmoud M.A. Ahmed, Madjid Birouk
Partially premixed flames (PPFs) are used in numerous practical combustion applications such as gas turbine combustors (e.g., Meier, Duan, Weigand 2005), internal combustion engines (e.g., Dec 2009), and industrial burners (e.g., Biagioli, Guthe, Schuermans 2008). These flames are characterized by a triple flame structure which is the main controlling mechanism (e.g., Domingo and Vervisch 1996; Peters 2000). Triple flames can be found in the reaction zone of PPFs where lean and rich pockets along with diffusion flames can manifest due to inhomogeneous mixtures (e.g., Mansour 2002). Partially premixed combustion mode can be accompanied by other combustion modes (i.e., premixed or diffusion mode) under certain operating conditions. For instance, partial premixing is inherent in the liftoff region of diffusion flames (Lee and Chung 1997), spray combustion and local extinction (Vervisch 2000). PPFs can produce NOx emissions comparable to lean premixed flames (Xue and Aggarwal 2003), and better flame stability compared to both diffusion and premixed flames (Mansour, Elbaz, Samy 2012).
Combustion with Multiple Flames under High Strain Rates
Published in Combustion Science and Technology, 2021
The component of velocity has a maximum value and a nearly flat profile between the two flames. The consequences of a reaction rate which is nearly second order are seen when we consider an effect of varying the parameter which is itself monotonically increasing with pressure. As pressure increases, little change in flame speed for the fuel-lean premixed flame is seen. However, the mass burning rate reflected through the negative of does increase with increasing or pressure. The and values peak closer to the interface again. Advection is definitely significant in this region between the two flames. The normal strain becomes substantially larger than ambient values close to each of the two flames.
On the concentration fluctuation moments of gas species in turbulent combustion
Published in Numerical Heat Transfer, Part B: Fundamentals, 2023
The homogeneous combustion in most engineering devices is turbulent combustion. It is well known that turbulence imposes influences on chemical reactions in turbulent combustion. Turbulence promotes the momentum, heat, and mass transfer and enhances the mixing between gas species. The reaction rates are altered and usually remarkably increased by turbulent fluctuations. As a result, the turbulent premixed flame speed of gas mixture becomes large compared to the laminar premixed flame speed. The length for turbulent jet diffusion flame is different from that for laminar jet diffusion flame. It is nearly not relevant to the jet exit velocity [1].