Flame Lift‐Off and Blow‐Out Stability Limits and Their Application in Gas Burners

Abstract

Flame stability is associated with the phenomena observed as a result of a balance or mismatch of the local flame speed and the local flow velocity. These phenomena include flame attachment to the burner rim, the flame base lifting off the rim and the flame being extinguished as a lifted flame or an attached flame.Flame stability is usually characterized by three terms, namely lift-off velocity, liftoff height, and blow-out velocity. The lift-off velocity is defined as the mean jet velocity at which the flame lifts above the burner rim. If the jet velocity is further increased, the flame moves downstream to a new position where it stabilizes. Consequently, the distance between the base of the lifted flame and the burner mouth is described as the lift-off height. A further increase in the jet velocity results in the flame being pushed further downstream, where reaction may not be sustained and the flame is extinguished. The velocity at which the flame is extinguished is called the blow-out velocity. The features of a lifted flame are illustrated in Figure 5.1 and an image of a lifted flame is shown in Figure 5.2.Flame extinction could also occur in an attached flame where the flame goes out directly without the lift-off stage. This is sometimes called blow-off to differentiate it from blow-out. Reducing the fuel flow rate could also cause stability problems. If the local flame velocity exceeds the local flow velocity, the flame enters and propagates upstream through the burner tube or port without quenching. This phenomenon is called flashback and it is generally a transient event, occurring as the fuel flow is decreased or turned off. For the safety of the fuel system, a fire arrest device is recommended to prevent flash back and protect the fuel tank.Theseparametersdescribethestableandsafe operatingrangeforaparticularburner. Conditions for lift-off and blow-out are important in the development of burners for combustion and process applications. In some industrial applications, burners may be subject to strong cross-wind and therefore the stability and performance of those burners might be influenced by cross-wind. This chapter provides a comprehensive review of the main theories on the stabilization mechanisms for a turbulent jet flame

Experimental Comparison of Working Region, Flame Stability, and Flame Height of LPG, DME, and DME-mixed LPG in an Atmospheric Diffusion Cylindrical Burner

“…Further increasing the fuel-jet velocity moves the flame into the downstream, finding a new position to stabilize. Knowing the fuel-jet velocity at which LO happens is important (Wu, 2010). BO is the phenomenon when high fuel-jet velocity results in an extinguished flame.…”