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Flame theory and turbulent combustion
Published in J. F. Griffiths, J. A. Barnard, Flame and Combustion, 2019
J. F. Griffiths, J. A. Barnard
The fundamental relationships shown in Fig. 4.2 [44] for combustion in premixed fuel + oxidant under flowing conditions are between the ratio of the ‘turbulence intensity’/laminar burning velocity (u′/S1) on the ordinate and the ratio of the ‘integral length scale’/laminar flame thickness (L/δ1) on the abscissa. These terms require some explanation. The laminar burning velocity, S1, is defined as the rate of propagation of a planar flame into the unburned gas under laminar flow conditions, and is a fundamental parameter of a particular fuel + oxidant composition at a given temperature and pressure. The distinction from the burning velocity, Su, will emerge later in connection with stretched flames. The laminar flame thickness, δ1, confers a three-dimensionality to the flame insofar that it represents the depth of the reaction zone in which the bulk of the heat is released. A correction term, called the Markstein number [45], is applied sometimes to δ1 in order to take an effect of flame curvature into account. The laminar flame thickness is often approximated to υ/S1 [46], where υ is the kinematic viscosity. The kinematic viscosity relates to transport of momentum and is given by the dynamic viscosity of the fluid divided by its density (η/σ).
Global Consumption Speed of Jet Fuel/Air Flames and the Impact of Fuel Chemistry
Published in Combustion Science and Technology, 2023
Jonathan Bonebrake, David L. Blunck
Here, SL,b is the burned laminar flame speed evaluated at the strain rate k, and SLb0 is the unstretched burned laminar flame speed. Laminar opposed twin flames were performed using Cantera (Goodwin, Moffat, and Speth 2017) and the HyChem mechanisms for jet-A, C5, and C1 (Wang et al. 2018; K. Wang et al. 2018; Xu et al. 2018). SLb was evaluated at the location of peak heat release in the strained laminar flame, and SLb0 was determined by extrapolating SLb to zero-stretch. The Markstein numbers for A2, C1 and C5 are presented in Figure 10. The Markstein numbers are consistent with values reported previously for other large hydrocarbon fuels (Duva, Chance, and Toulson 2020). At stoichiometric conditions, the Markstein number for C1 is the largest. As the equivalence ratio decreases the magnitude of the Markstein number increases for the three fuels. The Markstein numbers for C1 and C5 become more similar for the intermediate equivalence ratios evaluated. At the lowest equivalence ratio, the Markstein numbers for C5 and A2 approach each other. The nonlinear relationship between changes in equivalence ratio and Markstein numbers for the various fuels is consistent with the behavior of Lb of other fuels (e.g., dimethyl ether-air (Yu et al. 2015)). These results suggest that C1 is the most sensitive to flame stretch, in general, while A2 is the least sensitive to flame stretch across the equivalence ratios of interest in this study
Kinetic modeling investigation on the NH3/C2H5OH/air laminar premixed burning characteristics at different equivalence ratios
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2021
The appearance of flame instability will wrinkle the flame front, thus causing undesirable changes in burning efficiency and the total pollutant emissions (Kwon and Law 2002; LFM etal. 2021; Sun etal. 2012). Hence, studying the flame instabilities of premixed NH3/C2H5OH blended fuels are necessary. Thermal expansion (σ) and laminar flame thickness (δ) are two leading parameters associated with the intensity of hydrodynamic instability (Kwon and Law 2002; LFM etal. 2021; Sun etal. 2012). Higher σ and thinner δ correspond to the greater intensity of hydrodynamic instability. The intensity of the diffusional-thermal instability can be evaluated by the effective Lewis number (). The larger , the lower diffusional-thermal instability (Kwon and Law 2002; LFM etal. 2021; Sun etal. 2012). The Markstein number () can reflect the influence of thermal-diffusion and hydrodynamic instability on flame front (Oppong etal. 2021). The larger , the lower flame instability. σ, δ, , and are defined as follows: