Individual droplet flames

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. 

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,... 

Emissions of soot on the other hand represent a smaller fraction of the overall emission, but are probably of greater concern from the standpoint of visibility and health effects. It has been suggested that soot emissions from fuel oil flames result from processes occurring in the vicinity of individual droplets (droplet soot) before macroscale mixing of vaporized material, and from reactions in the bulk gas stream (bulk soot) remote from individual droplets. Droplet soot appears to dominate under local fuel lean conditions (1, 2), while bulk soot formation occurs in fuel rich zones. Factors which are known to affect soot formation from liquid fuel flames include local stoichiometry, droplet size, gas-droplet relative velocity and fuel properties (primarily C H ratio). 

In the dispersed mode, the droplet stream was aerodynamically dispersed to permit the sooting behavior of individual droplets to be investigated. The visual appearance of the flame produced was strikingly different from the nondispersed flame. Instead of a sheet of luminous radiation down the center of the burner, the... 

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 quantitative results contained in Figure 3 are quite instructive. For example, if a cloud containing only 10 particles were to bum in a QS-individual flame mode, it would have to have a mean interparticle separation of about 7000 Rp. By comparison, droplets in a spray combustor typically have interparticle separations of (10-100 )Rp, implying that QS clouds with particle spacings of practical interest would never bum in the individual flame mode. This results from the remarkable eflBciency of such cloud particles in preventing oxidizer penetration by diffusion into the cloud. 

Rapid developments have taken place in the fleld of laser anemome-try, and this technique has been applied successfully in a number of studies on measurements in gaseous flames. In these studies, the gas flow was seeded with micron or submicron particles, and the velocity of these particles was taken to be representative of the velocity of the local gas flow. For the study reported here, a laser anemometer was adapted for the special problem of measurements in a spray flame which initially contains a polydisperse cloud of droplets up to 300 /un in diameter. Droplets and carbon particles are present, and seeded particles are added to the annular air flow. For the particles larger than 1 /un, significant differences exist between velocities of particles and surrounding gas. A complete description of the velocity field requires simultaneous measurement of velocity and size of individual particles. This has not yet been achieved, and, for this study, the velocity of all particles passing through the measurement control volume of the laser anemometer are reported. 

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). 

As mentioned earlier, optical atomic spectroscopy is only able to analyze solution sample. As a result, ceramic powders to be tested should be made into solution. The solution is then broken into line droplets and vaporized into individual atoms by heating, which is the step critical to the precision and accuracy of the analysis. Flame is generally used to vaporize the solution, which is therefore also known as flame atomic absorption spectrometry or flame AA. 

For the analysis of ceramic powders by optical atomic specfroscopy, a portion of the powder has to be converted into individual atoms. In practice, this is achieved by dissolving the powder in a liquid to form a solution, which is then broken into fine droplets and vaporized into individual atoms by heating. The precision and accuracy of optical atomic spectroscopy are critically dependent on this step. Vaporization is most commonly achieved by introducing droplets into a flame (referred to as flame atomic absorption spectrometry or flame AA). Key problems with flame AA include incomplete dissociation of the more refractory elements (e.g., B, V, Ta, and W) in the flame and difficulties in determining elements that have resonance lines in the far ultraviolet region (e.g., P, S, and the halogens). While flame AA is rapid, the instruments are rarely automated to permit simultaneous analysis of several elements. 

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