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Fire Hazards and Associated Terminology
Published in Asim Kumar Roy Choudhury, Flame Retardants for Textile Materials, 2020
A flame is a mixture of reacting gases and solids emitting visible, infrared, and sometimes ultraviolet light, the frequency spectrum of which depends on the chemical composition of the burning material and intermediate reaction products. In many cases, such as the burning of organic matter, for example wood, or the incomplete combustion of gas, incandescent solid particles called soot produce the familiar red-orange glow of fire. This light has a continuous spectrum. Complete combustion of gas has a dim blue color due to the emission of single-wavelength radiation from various electron transitions in the excited molecules formed in the flame. Usually oxygen is involved, but hydrogen burning in chlorine also produces a flame, producing hydrogen chloride (HCl). Among many other possible combinations producing flames are fluorine and hydrogen, and hydrazine and nitrogen tetroxide. Hydrogen and hydrazine/unsymmetrical dimethylhydrazine (UDMH) flames are similarly pale blue, while burning boron and its compounds, evaluated in the mid-20th century as a high-energy fuel for jet and rocket engines, emits intense green flame, leading to its informal nickname of “Green Dragon”.
Combustion in Natural Fires
Published in James G. Quintiere, Principles of FIRE BEHAVIOR, 2016
A chemical reaction is a process that involves the change in a molecule by rearranging its atoms into different molecules. Combustion or fire is a chemical reaction involving the release of energy, some of which can be in the form of light. Combustion and fire are synonymous. A combustion reaction commonly involves the fuel molecule combining with oxygen to produce new molecules as products. A flame is combustion in which a fuel gas reacts with an oxidizer, commonly oxygen in the air. Smoldering is combustion in which a fuel reacts as a solid with oxygen in the air. To define a chemical reaction as fire or combustion, sufficient perceptible energy must be released. The rate of energy release per unit volume of the chemical reaction determines whether that reaction is fire. The size of the flame is not a factor. For a flame produced by a gaseous fuel combining with oxygen in normal air, the energy release is strongly dependent on the temperature of the molecules. Low temperatures produce an imperceptible amount of energy. Let us take a deeper look at this process.
Chemically Reacting Flows at the Microscale
Published in Sushanta K. Mitra, Suman Chakraborty, Fabrication, Implementation, and Applications, 2016
Premixed flames are characterized by propagation of the flame into the unburned mixture at a definite rate. Thus, propagation of the flame is cardinal to the behavior of premixed flames. For measuring the rate of propagation of the flame, an appropriate coordinate system has to be fixed to the propagating flame. The speed of the unburned mixture relative to the flame is known asflame speed, Su (Turns, 2000). For a flame propagating freely through a quiescent mixture, the flame speed is the rate of propagation of the flame through the mixture. On the other hand, for a flat flame stabilized on a burner, this is equal to the rate at which the unburned mixture reaches the flame. Flame speed is influenced by several factors such as flame curvature, flow nonuniformities, heat loss, and ambient pressure and most importantly by the chemical properties of the fuel and the air-fuel ratio (stoichiometry) of the reacting mixture. To isolate the effects of fuel chemistry and stoichiometry, flame speeds are often computed for freely propagating planar adiabatic flames. Flame speeds for such flames are often referred to as laminar burning velocity (Su0).
Visualization of Flame Propagation and Quenching of Methane/Air Mixture in a cubic enclosure with Perforated Plates: Experimental Study
Published in Combustion Science and Technology, 2023
Hadi Younesian, Mohsen Nazari, Mohammad Mohsen Shahmardan
According to the results of Figure 6, by decreasing the hole size of the perforated plates, the velocity of the flame tip decreases after hitting the perforated plates. Therefore, at an initial pressure of 0.964 bar, the perforated plate with a hole size of 5 mm has the highest flame tip speed of 12 m/s. Meanwhile, according to Figure 6b in position A, the flame tip speed at the initial pressure is 0.964 bar and the hole size is 2 mm was only 2.5 m/s. By referring to the results of Figure 4 in ‘frame d,’ the flame propagation from larger holes leads to a formation of a large turbulent vortex. The vortex structures behind the perforated plate are also seen in ‘frame c’ of Figure 4a–c. In the next stage of flame propagation, a larger turbulence can create a much more turbulent mixture which increases the speed of combustion and acceleration of the flame (see frame d of Figure 4a).
The Investigation of Soot Free Length of Jet Flame of Propane and Carbon Dioxide Gas Mixture
Published in Combustion Science and Technology, 2022
Yuling Dou, Changfa Tao, Jingwu Wang, Huaqiang Chu, Yang Hua, Xiaoyong Liu
The turbulent jet flame can be divided into two parts according to the flame color, which is proved to be affected by the soot formation process (Zhang et al. 2017). The length of blue flame is called soot free length (without soot contain), and the yellow flame is called the luminous flame (with soot formation) (Sobiesiak and Wenzell 2005). Moreover, the total flame length () is expressed as the sum of soot free length () and the luminous flame length. The main reason for the difference of flame colors is soot emissions. The blue flame is generally considered to be caused by the radiation of water vapor and CO2, and the yellow flame is considered to be caused by the radiation of water vapor, CO2 and soot. In order to quantify the degree of soot reduction during the flame combustion process, the soot free length fraction () has been proposed which is defined as the ratio of to (Kumar and Mishra 2008; Sobiesiak and Wenzell 2005).
Improvement in performance of CI engine using various techniques with alternative fuel
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2021
Prem Yadav, Naveen Kumar, Raghvendra Gautam
Guide vanes in the intake runner is basically used to direct the airflow before entering into the cylinder. This exhibits a static structure to direct the airflow according to its shape. Normally, this is positioned in the intake system between the air filter and inlet valve. Sun et al. (Sun, Li, and Du 2011) used a swirler and positioned it on the inlet valve in the diesel engine and examined four different configurations of ‘swirler’ where one was straight while the other three wares arc design and trial for actual DI diesel engine. The examination revealed an improvement in fuel economy at an optimized swirl ratio. A significant difference in specific fuel consumption, nearly 11.9% reduced as compared to diesel engine without swirler. A similar study was done by Mahmud et al. (Mahmud, Cho, and Sang-Shin 2009) and they placed a device called ‘variable countercurrent distribution’ just before the intake valve. Simulation results added that device-generated more vorticity as compared to the standard diesel engine at the intake stroke. Moreover, an experiment was performed on a 5-cylinder mercedes benz and ssangyong car and concluded that drastic reduction in soot concentration and nearly 97% faster acceleration were observed than without swirl configuration. The additional turbulence leads to better combustion through increased flame propagation.