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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.
Gas Installation, Components and Controls
Published in Fred Hall, Roger Greeno, Building Services Handbook, 2017
For correct combustion of natural gas, burner design must allow for the velocity of the gas-air mixture to be about the same as the flame velocity. Natural gas has a very slow burning velocity, therefore there is a tendency for a flame to lift-off the burner. This must be prevented as it will allow gas to escape, possibly exploding elsewhere! Correct combustion will occur when the gas pressure and injector bore are correct and sufficient air is drawn in, provided the gas-air velocity is not too high to encourage lift-off. Some control over lift-off can be achieved by a retention flame fitted to the burner. Flame lift-off may also be prevented by increasing the number of burner ports to effect a decrease in the velocity of the gas-air mixture. A box-type burner tray is used for this purpose.
GAS INSTALLATION, COMPONENTS AND CONTROLS
Published in Fred Hall, Roger Greeno, Building Services Handbook, 2011
For correct combustion of natural gas, burner design must allow for the velocity of the gasair mixture to be about the same as the flame velocity. Natural gas has a very slow burning velocity, therefore there is a tendency for a flame to lift-off the burner. This must be prevented as it will allow gas to escape, possibly exploding elsewhere! Correct combustion will occur when the gas pressure and injector bore are correct and sufficient air is drawn in, provided the gasair velocity is not too high to encourage lift-off. Some control over lift-off can be achieved by a retention flame fitted to the burner. Flame lift-off may also be prevented by increasing the number of burner ports to effect a decrease in the velocity of the gasair mixture. A box-type of burner tray is used for this purpose.
Polydisperse effects in jet spray flames
Published in Combustion Theory and Modelling, 2018
Noam Weinberg, J. Barry Greenberg
The lift-off of jet spray flames has received mainly experimental attention in the literature [1–4]. Marley et al. [1–3] carried out an extensive study of the characteristics of lifted methane gas and ethanol spray turbulent jet flames and compared the two in terms of the leading-edge flame structure. Reddy et al. [4] performed an experimental study on the effect of co-flow conditions on lifted kerosene spray flames. They discovered that the flame lift-off height is proportional to the co-flow velocity and the injection velocity and decreases with an increase in the fuel flow rate. They also looked into the influence of the Sauter mean diameter (SMD) of the spray and found that as it increases it causes fluctuations in the stabilisation point of the lifted flame. This is due to the longer time required by larger droplets to evaporate so that the formation of a combustible fuel vapour/air mixture takes longer in comparison to the local chemical time for reaction. Other, rather sparse, relevant literature on jet spray flames is reviewed in some detail in [5]. Interestingly, the theoretical prediction of jet spray flames has received very little attention in the literature. Sanchez et al. [6] gave a comprehensive overview of the fundamental multiscale processes that occur in spray combustion. However, realistic polydisperse sprays are not dealt with.