Flame lift-off in oil fired pressure jet burners is an unwanted condition in which the

flame A flame () is the visible, gaseous part of a fire. It is caused by a highly exothermic chemical reaction made in a thin zone. When flames are hot enough to have ionized gaseous components of sufficient density, they are then considered plasm ...

and burner become separated. This condition is most commonly created by excessive

combustion air Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...

and often results in the loss of flame as the photoelectric cell fails to register the light of the flame, this in turn results in a safety lockout of the control box.

Other outcomes

Other outcomes may be experienced: – # There may be delayed

ignition Ignition may refer to: Science and technology * Firelighting, the human act of creating a fire for warmth, cooking and other uses * Combustion, an exothermic chemical reaction between a fuel and an oxidant * Fusion ignition, the point at which a ...

as the oil spray is too far forward for the

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...

s to ignite, only when the oil spray has filled the

combustion chamber A combustion chamber is part of an internal combustion engine in which the air–fuel ratio, fuel/air mix is burned. For steam engines, the term has also been used for an extension of the Firebox (steam engine), firebox which is used to allow a mo ...

will the mixture ignite. In this condition it is likely that the excessive volume of unburnt oil will ignite with explosive ignition. # The oil mixture will fail to ignite resulting in safety lockout of the control box. # The oil mixture will ignite but may burn very inefficiently due to excessive air chilling of the oil particles to the point where complete

combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...

of the oil is not possible.

Flame lift-off height

A non- premixed jet flame has a tendency to lift off from the burner

nozzle A nozzle is a device designed to control the direction or characteristics of a fluid flow (specially to increase velocity) as it exits (or enters) an enclosed chamber or pipe (material), pipe. A nozzle is often a pipe or tube of varying cross ...

position when the jet velocity of the flame is over a critical value of

U_c

. With the increasing of the jet velocity, the lifted height will increase and when it's beyond certain critical height and the flame will be blown off. Therefore, the stability of the lifted flame is an important parameter for basic combustor design. Scholefield and Garside's theory claimed that the transition to

turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...

is a prerequisite for the lifted

diffusion flame In combustion, a diffusion flame is a flame in which the oxidizer and fuel are separated before burning. Contrary to its name, a diffusion flame involves both diffusion and convection processes. The name diffusion flame was first suggested by ...

stabilisation and the flame anchors at a point where the flow is turbulent. Gollahalli argued that the flame will tend to stable at the position where the local

flow velocity In continuum mechanics the flow velocity in fluid dynamics, also macroscopic velocity in statistical mechanics, or drift velocity in electromagnetism, is a vector field used to mathematically describe the motion of a continuum. The length of the f ...

balance the normal flame propagation velocity. Navarro-Martinez and Kronenburg have demonstrated that the excessive turbulent stretching at the nozzle leads to the lift-off and they also claimed that auto-ignition can be used to promote the flame stabilisation mechanism. Recently the observation from Kiran and Mishra's visual experiment proved the flame lift-off height varies linearly with jet exit velocity. They presented a semi-empirical correlation between the

normalized Normalization or normalisation refers to a process that makes something more normal or regular. Science * Normalization process theory, a sociological theory of the implementation of new technologies or innovations * Normalization model, used in ...

lift-off height to the nozzle exit diameter.

\frac=1.8*10^*\frac

Where,

H_L

: lift-off height

D_f

:diameter of the fuel tube

U_f

:fuel jet velocity In addition to the velocity effect, The

stoichiometric Stoichiometry () is the relationships between the masses of reactants and products before, during, and following chemical reactions. Stoichiometry is based on the law of conservation of mass; the total mass of reactants must equal the total m ...

burning on the physical mechanism blowout has been investigated by Broadwell et al. and Pitts. According to their study on diffusion flame, the fresh air entrained by the

vortices In fluid dynamics, a vortex (: vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...

structure cools down and over dilutes the flame jet, which leads to the flame extinction.

References

{{Reflist Combustion