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Laminar Premixed Flames
Published in Achintya Mukhopadhyay, Swarnendu Sen, Fundamentals of Combustion Engineering, 2019
Achintya Mukhopadhyay, Swarnendu Sen
Lift-off, on the other hand, refers to a condition where the flame detaches itself from the burner and moves downstream away from the burner. In lift-off, the flames are stabilised at some distance above the burner and are known as lifted flame. Lifted flames can be undesirable in practical applications for several reasons [4]. First, with the flame stabilised away from the burner, some fuel may escape unburned. Second, as it is difficult to stabilise the flame at a specific location, poor heat transfer may result from the flame. These flames can also be noisy and susceptible to external perturbations. Flame lift-off occurs when the flame propagates in the direction of the fluid motion. Hence, in these situations, local flow velocity exceeds the flame speed. In case of jet flames, as one moves away from the burner rim, the jet expands and its velocity decreases. The lifted flame positions itself at a height where the flame speed balances the local flow velocity. As the flow velocity at the burner exit increases, the flame has to position itself at a greater distance from the burner. Beyond a certain distance, the flame can no longer be stabilised and it blows off completely. This phenomenon is known as blowout. Blowout is an undesirable phenomenon as it can lead to shutdown or fatal accidents. Since flame speed decreases as one moves away from stoichiometric mixture towards lean or rich side, lean mixtures used for low emission characteristics are often susceptible to blowout. Such blowout is called lean blowout.
Development of an Industrial Burner Accommodating Methane-Air Triple Flames
Published in Combustion Science and Technology, 2021
A. E. Hussin, A. M. Hamed, M. M. Kamal, A. R. Elbaz
Inasmuch as the levels of preheating across the two flame wings depend on the strength of each mixture, the triple flame propagation speed depends on the transverse mixture fraction gradient (Dold 1989; Azzoni et al., 1999). Many researches were devoted to the triple flames when their fuel-rich and fuel-lean mixtures develop due to a diffusion flame lift-off (Muniz and Mungal 1997) or when there is a single mixture which becomes associated with a partially premixed flame lift-off (Lock et al. 2005). On the other hand, the research works on triple flames which develop due to feeding of two fuel-air mixtures into the burner ports were highlighted by Kamal (2007). There is still more space for involving triple flames in novel burner designs which benefit from the large burning capacities of triple flames in comparison to other flame modes.
Static Flame Stability of Circumferentially Arranged Fuel Port Inverse Jet Non-Premixed Flame Burner
Published in Combustion Science and Technology, 2020
Vishnu Hariharan, Debi Prasad Mishra
The hysteresis effects in normal diffusion jet flame are widely reported (Akbarzadeh and Birouk, 2015; Lin et al., 1993), whereas such studies for inverse diffusion flames has not been reported in the open literature to the best of authors knowledge. The hysteresis effect is initiated due to change in mass entrainment effects at the onset of flame liftoff. Figure 9a depicts the hysteresis observed for various fixed air velocity range from 11.8 to 32.4 m/s. The fuel flow velocity required for the onset of flame lift-off is denoted as VfL, and Vf is increased further to achieve flame lift-off. Once CAFP IJF lifts off from the burner rim and attains a maximum liftoff height, the fuel velocity is then reduced slowly to the reattachment of the flame base to the burner rim. This velocity for which the base flame reattaches to its base of the burner and is termed as reattachment velocity VfR . Hysteresis phenomenon is marginally observed at a lower central air velocity which corresponds to lower MFR particularly when the flame behaves like a NJDF. The percentage of hysteresis observed at a lower air jet velocity regime of 11.8 and 14.7m/s is found to be 10% and 8.4%, respectively. It can be noted that for the higher range of air velocities (23.6 to 35.4 m/s), the hysteresis percentage is within the experimental error due to a formation of stronger base flame anchored to burner rim at higher MFR.
Modelling compression ignition engines by incorporation of the flamelet generated manifolds combustion closure
Published in Combustion Theory and Modelling, 2019
Amin Maghbouli, Berşan Akkurt, Tommaso Lucchini, Gianluca D'Errico, Niels G. Deen, Bart Somers
Transient flame lift-off results are compared to the experimental values in Figure 14. The figure also shows simulated iso-surfaces of 2% of the maximum OH as a marker of the diffusion flame. It should be mentioned that the experimental mean flame lift-off values were determined during a small time duration when diffusion combustion was dominant [41]. Figure 14 shows that numerically larger flame lift-off values are obtained at the start of diffusion combustion and then as the time progresses flame lift-off becomes shorter. Iso-contours of 2% of the maximum OH show that by increasing the ambient temperature at SOI, flame lift-off becomes shorter. This is due to accelerated fuel evaporation and mixing as well as faster chemical heat release at higher temperatures.