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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).
Internal Combustion Engines
Published in D. Yogi Goswami, Frank Kreith, Energy Conversion, 2017
David E. Klett, Elsayed M. Afify, Kalyan K. Srinivasan, Timothy J. Jacobs
Second, the increase in the temperature and pressure of the burned gases behind the flame front will cause it to expand and progressively create thermal compression of the remaining unburned mixture ahead of the flame front. The flame speed will be slow at the start of combustion, then reach a maximum at about half the flame travel, and finally decrease near the end of combustion. Overall, the flame speed is strongly influenced by the level of turbulence in the combustion chamber; the shape of the combustion chamber; the mixture strength; the type of fuel; and the engine speed.
Flames
Published in Kenneth M. Bryden, Kenneth W. Ragland, Song-Charng Kong, Combustion Engineering, 2022
Kenneth M. Bryden, Kenneth W. Ragland, Song-Charng Kong
Combustion in burners and engines uses turbulence to increase the flame speed and heat release rate per unit volume. Turbulence can increase the flame speed 5–50 times the laminar flame speed. Turbulent premixed flames in burners are steady open flames, whereas flames in internal combustion engines are propagating enclosed flames. The flame front interacts with the turbulent eddies, which may have fluctuating speeds of tens of m/s and sizes ranging from a few millimeters to a meter or more.
A Computational Study on the Influence of the Hydrous Ethanol Water Content in Spark-Ignition Engine Performance and in Flame Development
Published in Heat Transfer Engineering, 2023
Fabiano Alves dos Santos, Albino José Kalab Leiroz
One of the determining fuel characteristics for developing more efficient and powerful internal combustion engines is the flame speed inside the cylinder [55]. High-performance internal combustion engines, which work at high speeds, require fuels with high flame propagation speeds [14]. Once Eqs. (1–4) were solved, Eqs. (7) and (9) were used to calculate the local and instantaneous laminar and turbulent flame speeds, respectively. The average laminar and turbulent flame speeds and the cycle maximum turbulent flame speed for MFB varying from 10% to 90% are shown in Table 3. The crankshaft angles for 2%, 10%, 50%, and 90% of MFB were also calculated from the chemical kinetic model [44] and are also shown in Table 3. For MFB between 10% and 90%, an average turbulent flame speed of 6.35 m/s for ethanol E95.7W4.3 and an increase to 7.50 m/s as the fuel water content increases to 10% were observed. The increase in flame velocity as the fuel water content varies from 4.3% to 10% can be related to the previously mentioned rise in air flow to the engine and to an improvement of the combustion process also observed in Figure 9d. Results in Table 3 for fuels with water contents above 10% indicate lower flame turbulent speeds that can be related to high amount of water vapor and excess air inside the engine cylinder. In the present study, the highest calculated turbulent flame speed was 8.41 m/s for E90W10, and the lowest was 6.74 m/s for the E80W20 as shown in Table 3. The maximum difference in flame speed calculated between the studied mixtures was 1.67 m/s.
Investigation of flame propagation in autoignitive blends of n-heptane and methane fuel
Published in Combustion Theory and Modelling, 2019
Bruno S. Soriano, Edward S. Richardson
The effects of pre-ignition chemistry on laminar flame speed in autoignitive methane/n-heptane fuel blends are investigated using premixed laminar flame simulations. Pre-ignition reactions cause the speed of the flame to increase. Fuels that exhibit two-stage ignition behaviour, such as n-heptane, also exhibit a two-stage increase in the speed of the reaction front as the reactant residence time increases. The increase in flame speed is due to distinct contributions of heat release, reactant consumption, and enhanced reactivity ahead of the flame. Addition of methane to n-heptane-air mixtures retards and reduces the first-stage increase in flame speed, in part due to dilution of the more-reactive n-heptane fuel, and in part due to consumption of radical species by the methane chemistry. As the residence time of the reactants approaches the ignition delay time, the reaction front transitions into a pure ignition front, in which diffusive transport is negligible.
Comprehensive influence of uncertainty propagation of chemical kinetic parameters on laminar flame speed prediction: a case study of dimethyl ether
Published in Combustion Theory and Modelling, 2023
Yachao Chang, Pengzhi Wang, Shuai Huang, Xu Han, Ming Jia
The artificial neural network (ANN) is an efficient prediction method without the requirement of understanding the relationship between variables in advance. Therefore, it is employed to predict the relationship between the parameters of the chemical kinetic model (i.e. reaction rate coefficients, thermodynamics data, and transport data) and the calculated laminar flame speed. Compared with the computational cost using the numerical solver (i.e. Chemkin and Cantera), it only requires a low computational cost because the calculation of the laminar flame speed using the ANN model does not need to solve ordinary differential equations (ODEs).