Diffusion flame

About: Diffusion flame is a research topic. Over the lifetime, 9266 publications have been published within this topic receiving 233522 citations.

The Influence of the Temperature on Extinction and Ignition Limits of Strained Hydrogen-Air Diffusion Flames

Abstract: Structures, extinction and ignition limits of strained diffusion flames are examined. The influence of the oxidizer stream temperature is specifically considered. Calculations are performed for diffusion flames formed by a counterflow of diluted hydrogen and air. The air temperature differs from the hydrogen temperature. The reactive flow equations are solved numerically by employing Newton iterations and adaptive continuation techniques. The model includes detailed transport and complex kinetics. Flame structures, extinction and ignition limits and characteristic flame response curves are determined for different values of the fuel-air ratio and for a set of air stream temperatures. It is shown that this last parameter strongly affects the conditions producing extinction. For an air stream temperature of 100OK, the critical extinction strain rate is ten times that found for an air stream at room temperature It is also found that a temperature exists beyond which combustion always takes place wha...

Stability of the Porous Plug Burner Flame

Abstract: We examine the linear stability of a premixed flame attached to a porous plug burner, using activation energy asymptotics. Limit function-expansions are not an appropriate mathematical framework for this problem, and are avoided. A dispersion relation is obtained which defines the stability boundaries in the wave-, Lewis-number plane, and the movement of these boundaries is followed as the mass flux is reduced below the adiabatic value and the flame moves towards the burner from infinity. Cellular instability is suppressed by the burner, but the pulsating instability usually associated with Lewis numbers greater than 1 is, at first, enhanced. For some parameter values the flame is never stable for all wavenumbers; the Lewis number stability band that exists for the unbounded flame disappears. For sufficiently small values of the stand-off distance the pulsating instability is suppressed.

Visualization of blow-off events in bluff-body stabilized turbulent premixed flames

Abstract: Confined and unconfined turbulent methane-air lean premixed flames stabilized on an axisymmetric bluff-body of diameter d = 25 mm have been examined close to the blow-off limit and during the extinction transient, with OH∗ chemiluminescence, flame tomography and OH-PLIF operated at 5 kHz, allowing a quantification of the duration of the blow-off event. Blow-off was approached by increasing the bulk velocity U b or decreasing the equivalence ratio and the flame shape changed from conical to cylindrical with decreasing length. Close to blow-off, the flame closed on the axis and was about 2 d long, and just before the blow-off condition it took an “M” shape with reaction fronts inside the recirculation zone (RZ). During the blow-off event, fresh reactants entered the RZ from the forward stagnation region and significant fragmentation of the flame occurred, with branches of the flame remaining anchored on the bluff-body edge and separate flame pockets moving randomly inside the RZ. Overall blow-off occurred with the gradual elimination of these flame fragments. The integrated OH∗ emission decreased slowly to zero as the flame surface decreased over a period of about 15 d / U b . The results suggest that the blow-off event in recirculating flames lasts long compared to the residence time in the RZ and the structure of the flame close to extinction supports the underlying assumptions behind well-stirred reactor concepts of blow-off.

Effects of lewis number on extinction behavior of premixed flames in a stagnation flow

Abstract: The combined effects of the velocity gradient and the Lewis number of deficient reactantsin the premixed gas on the behavior and extinction of a premixed flame in a stagnation flow were studied experimentally. The flat twin flames established in the stagnation region of the two dimensional opposed flow of the same mixture were used in order to make an adiabatic and noncatalytic stagnation surface. The flame extinction behavior observed is purely attributed to the flow-transport properties-chemical reaction interaction. The mixture employed was hydrogen, methane, propane, or butane with air. The flame temperature and the distance between the two flames were measured for the wide variations of the fuel concentration and the velocity gradient. As the stagnation velocity gradient was increased by increasing the opposed flow velocity, the flat two flames approached each other, and finally the extinction occurred abruptly. By observing the flame behavior, the extinction process of the flame was classified into two patterns corresponding to the ranges of the Lewis number Le of the deficient reactant in the premixed gas (fuel in the lean mixture and oxygen in the rich mixture). Its ranges are Le 1. Under the condition of Le 1 (lean hydrogen or methane-air and rich propane or butane-air), as the velocity gradient is increased, the flame temperature increases and reaches a maximum point at a certain value of the velocity gradient, beyond which the flame temperature decreases. The flame is extinguished close to the stagnation surface or in contact with the stagnation surface. The results of this experiment are in good agreement with the theoretical predictions.