Individual flame droplet combustion

Godsave examined the burning of drops in unconfincd air. Ignition w as accomplished by momentarily exposing the droplet to a small gas flame. Kumagai and Isoda carried out their tests in essentially the same manner. In some of their later tests they introduced a vibrating air field for investigation of its effect on droplet combustion. Kobayasi and Nishiwaki both studied the combustion of individual drops in a horizontal,... 

In certain practical situations the vahdity of the above dilute spray assumption may be substantially weakened. This is particularly serious for spray combustion exhibiting individual droplet combustion modes. The enveloping flames are close to each other and frequently may even overlap. In fact Chigier and his co-workers (14,15) have asserted repeatedly from their experimental observations that the individual droplet combustion mode cannot exist in the oxygen-starved environment within a spray. Only limited research has been conducted on the combustion of closely spaced droplets (16,17,18,19,20). 

The vapor cloud of evaporated droplets bums like a diffusion flame in the turbulent state rather than as individual droplets. In the core of the spray, where droplets are evaporating, a rich mixture exists and soot formation occurs. Surrounding this core is a rich mixture zone where CO production is high and a flame front exists. Air entrainment completes the combustion, oxidizing CO to CO2 and burning the soot. Soot bumup releases radiant energy and controls flame emissivity. The relatively slow rate of soot burning compared with the rate of oxidation of CO and unbumed hydrocarbons leads to smoke formation. This model of a diffusion-controlled primary flame zone makes it possible to relate fuel chemistry to the behavior of fuels in combustors (7). 

An individual isothermal surface can be traced with the help of laser tomography, also known as laser sheet imaging, where a laser sheet and oil droplets are combined to visualize the instantaneous flame surface in a plane. This technique is ideal when wrinkling of an isoline is of interest besides, typically it shows the area occupied by the combustion products if the instantaneous flame thickness is small, such as a black area in... 

Pulsation in a spray is generated by hydrodynamic instabilities and waves on liquid surfaces, even for continuous supply of liquid and air to the atomizer. Dense clusters of droplets are projected into spray chamber at frequencies very similar to those of the liquid surface waves. The clusters interact with small-scale turbulent structures of the air in the core of the spray, and with large-scale structures of the air in the shear and entrainment layers of outer regions of the spray. The phenomenon of cluster formation accounts for the observation of many flame surfaces rather than a single flame in spray combustion. Each flame surrounds a cluster of droplets, and ignition and combustion appear to occur in configurations of flames surrounding droplet clusters rather than individual droplets. 

G. Chen, A. Gomez Dilute laminar spray diffusion flames near the transition from group combustion to individual droplet burning, Combust. Flame, 110, 392-404 (1997). 

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