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Combustion
Published in John B. Heywood, Eran Sher, The Two-Stroke Cycle Engine, 2017
Premixed engine flames become turbulent as they develop, and their propagation rate is then defined by the turbulent flame speed. The flow inside the cylinder during combustion both convects and distorts or wrinkles the thin reaction sheet flame through its combination of bulk or mean flow pattern and local turbulence or random velocity fluctuations. The higher the local turbulence, the faster the flame propagates and becomes wrinkled. If engine flames were not turbulent, with a turbulent flame speed several times the laminar flame speed, engines could not operate satisfactorily because combustion and the pressure rise rate it produces would be much too slow. Because engine turbulence scales with piston speed,1 combustion also scales (almost) with piston speed and so is fast enough throughout the full crankshaft speed range desired for satisfactory engine operation. However, the turbulent flow primarily affects the gross or larger-scale characteristics of the flame (on the scale of several mm), such as its average burning velocity. Locally, the thin reaction zone propagates forward into the adjacent unburned mixture at the laminar flame speed.
Basic Processes of Internal Combustion Engines
Published in K.A. Subramanian, Biofueled Reciprocating Internal Combustion Engines, 2017
The laminar speed can be defined as the velocity at which a flame propagates into a premixed unburned mixture. Flame is the result of a self-sustaining chemical reaction occurring within a region of space called the flame front, where unburned mixture is heated and converted into products. Laminar flame speed depends on only the thermal and chemical properties of the mixture.
Flames
Published in Kenneth M. Bryden, Kenneth W. Ragland, Song-Charng Kong, Combustion Engineering, 2022
Kenneth M. Bryden, Kenneth W. Ragland, Song-Charng Kong
For each premixed fuel–air mixture, there is a unique laminar flame speed that depends on the fuel type, fuel–air mixture ratio, and the initial temperature and pressure of the reactants. Before considering a mathematical model of a laminar flame, some basic data on laminar burning velocities are presented.
Experimental Study on the Combustion of Thermally Cracked Endothermic Hydrocarbon Fuel
Published in Combustion Science and Technology, 2020
Bei-Jing Zhong, Hui-Sheng Peng
In the above brief review, many published studies showed that practical fuels can be cracked into different kinds of species in different thermal environments, resulting in different ignition properties. However, to the extent of the authors’ knowledge, few studies have investigated the differences between the combustion characteristics (e.g., laminar flame speed) of practical fuels and their pyrolysis products. Moreover, the laminar flame speed is not only a fundamental parameter of combustion but also the base of the turbulent burning velocity. It is of practical importance in the design and optimization of internal combustion engines and power plant burners (Milton and Keck, 1984). Therefore, studying the effect of pyrolysis, in which product components vary greatly under different pyrolysis conditions, on the laminar flame speed of practical fuels is a valuable undertaking.
Effects of endothermic chain-branching reaction on spherical flame initiation and propagation
Published in Combustion Theory and Modelling, 2019
Haiyue Li, Huangwei Zhang, Zheng Chen
Due to its simple geometry, propagating spherical flames are popularly used to measure the laminar flame speed of different fuel/air mixtures. Experimentally, static homogeneous combustible mixture in a closed chamber is centrally ignited by an electrical spark or a laser beam which results in an outwardly propagating spherical flame. The flame front history and/or the pressure rise history are/is recorded during the experiment and used to obtain the laminar flame speed. As reviewed in [31–33], there are more than 30 groups which conduct spherical flame experiments. In addition, such experiments were also used to study the ignition process and minimum ignition energy (e.g. [34,35]). However, it is difficult to assess of the effects of endothermic chain-branching reaction on spherical flame initiation and propagation. Therefore, theoretical analysis with simplified chemical model is conducted there.
The pressure dependence of laminar flame speed of 2-methyl-2-butene/air flames in the 0.1–1.0 MPa range
Published in Combustion Science and Technology, 2018
Bei-Jing Zhong, Zhao-Mei Zeng, Hui-Sheng Peng
The laminar flame speed, Su0(φ) was derived from experimental results measured in this work at reference conditions of T0 = 450 K and P0 = 0.1 MPa. To clarify the effect of pressure on laminar flame speed, the laminar flame speeds of 2-methyl-2-butene/air mixtures versus pressure at different equivalence ratios were presented as a log–log graph, as shown in Figure 5. It is found that, in the logarithmic graph, laminar flame speed decreases linearly at each equivalence ratio. In other words, laminar flame speed decreases exponentially with increasing pressure. The experimental and simulated values of the power exponent β(φ) versus equivalence ratios are shown in Figure 6 and listed in Table 2. As shown in Figure 6, β(φ) varies with equivalence ratios as an inverted U curve with a maximum value at around φ = 1.0, indicating that effect of pressure on laminar flame speed of 2-methyl-2-butene/air is greatest at approximately φ = 1.0 and is weaker in fuel-lean conditions. Value of the power exponent β(φ) varies slowly with equivalence ratio in fuel-rich conditions showing that pressure effect maintains a high role in fuel-rich flames. It can be seen from Figure 6 that, in general, the simulation is in good agreement with experimental results, although there is a large error at both ends of fuel-rich and fuel-lean conditions. The equivalence ratio dependence coefficients Su0,i and βi were then calculated by Eqs. (6) and (7) and both summarized in Table 3.