Droplet flames clouds

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). 

After vessel rupture, the superheated liquid vaporized in a white cloud consisting of vapor and fine droplets. After ignition, the flame propagated through the cloud, forming a fireball. Fireball size increased as combustion proceeded, and the fireball was lifted by gravitational buoyancy forces. 

Flame atomization and excitation can be divided into a number of stages. Firstly, the heat of the flame evaporates solvent from the droplets of sample aerosol leaving a cloud of small particles of the solid compounds originally present in the solution. These are then vaporized and molecular associations broken down releasing free atoms (atomization) some of which... 

Research on water explosion inhibiting systems is providing an avenue of future protection possibilities against vapor cloud explosions. British Gas experimentation on the mitigation of explosions by water sprays, shows that flame speeds of an explosion may be reduced by this method. The British Gas research indicates that small droplet spray systems can act to reduce the rate of flame speed acceleration and therefore the consequential damage that could be produced. Normal water deluge systems appear to produce too large a droplet size to be effective in explosion flame speed retardation and may increase the air turbulence in the areas. 

Isolated droplet combustion obviously is the condition for a separate flame envelope for each droplet. Typically, a group number less that 10 2 is required. Internal group combustion involves a core with a cloud where vaporization exists such that the core is totally surrounded by flame. This condition occurs... 

Both diffusional flame calculations and detailed spatial mapping indicate that the nondispersed injection mode produces a vapor cloud that is characterized by diffusionally controlled combustion and bulk heating while subjecting the droplets to near isothermal conditions. The soot produced in this cloud is strongly influenced by bulk diffusion limitations and as such represents a bulk soot formation extreme. It was found that fuel changes had little effect on the overall soot yield due to this diffusion control. Lower gas temperatures and richer conditions were found to favor soot formation under bulk sooting conditions, probably due to a decrease in the oxidation rate of the soot. 

In the analysis considered in the preceding section, it was assumed that a uniform spray had been established initially and that once ignited, each droplet burned with an envelope flame around it. These conditions have been achieved reasonably well in the laboratory for various fuel-lean sprays [65]. However, in practical systems the sprays are not uniform, the manner in which the spray penetrates the oxidizing gas is important, and a cloudburning mode of combustion (in which diffusion flames surround groups of droplets, see the last paragraph of Section 3.3.6) may occur [2], [79]. These realities motivate studies of spray penetration and cloud combustion. 

Literature on theories of collective effects of droplet interactions was cited in Section 3.3.6. An approximate criterion for the occurrence of a cloud-burning mode with diffusion flames surrounding droplet clouds is readily stated. The overall fuel-gas ratio v (where v is the stoichiometric mass ratio and is the equivalence ratio) is (/>v From... 

Approach. The continuum total group combustion criterion is established by asking when will the cloud bum as a large pseudo-droplet with the flame located just outside of the cloud droplet region That is, when will the evaporating particles inside the cloud provide sufficient vapor so that fuel and oxidizer mix in stoichiometric amounts at the cloud boundary ... 

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. 

Propane Fuel—Results and Discussion. Onuma (12) showed that in a kerosene spray flame, there is no evidence of droplet burning. The vapor cloud formed by evaporation of the droplets bums like a turbulent diffusion flame. A close relationship between kerosene spray flame and gaseous diffusion flames (using propane as the fuel) was provided. The results reported in this section are those obtained from the modulated swirl combustor using propane as the fuel. 

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. 

To be detected by AAS, the analyte must be presented to the optical beam of the instrument as free atoms. The process of converting analyte ions/molecules, dissolved in a suitable solvent, to gaseous atoms is accomplished by the nebuliser flame assembly. The nebuhser (from the Latin nebula meaning cloud) creates an aerosol (a fine mist) of the hquid sample which is mixed with an oxidant gas and a fuel gas (to support the flame combustion). The mixture is ignited above the burner assembly. The liquid droplets are desolvated, the resulting microcrystals are melted and vaporised and finally the gaseous products are thermally dissociated to produce free atoms. The combustion speed of most flames is such that the conversion from liquid droplet to free atoms must be accomplished within a few milhseconds. 

In the premix burner, the sample, in solution form, is first aspirated into a nebulizer where it forms an aerosol or spray. An impact bead or flow spoiler is used to break the droplets from the nebulizer into even smaller droplets. Larger droplets coalesce on the sides of the spray chamber and drain away. Smaller droplets and vapor are swept into the base of the flame in the form of a cloud. An important feature of this burner is that only a small portion (about 5%) of the aspirated sample reaches the flame. The droplets that reach the flame are, however, very small and easily decomposed. This results in an efficient atomization of the sample in the flame. The high atomization efficiency leads... 

Once formed, the aerosol passes into the burner or spray chamber (sometimes also called the cloud chamber). The role of the spray chamber is to homogenize both the aerosol and gases that tend to dampen fluctuations in nebulizer efficiency, and to remove any large droplets before they reach the flame. Large droplets (diameter > 10 pm) collect on the sides of the chamber and then drain to waste. Spoilers and baffles placed at the end of the spray chamber aid this process. Because the spray chamber will fill with flammable gas, modern instruments will also incorporate some form of antiflashback protection from the flame. 

Another method of gas explosion suppression is dispersal of small droplets of liquid in the mixture (water cloud). It was shown in [16] that H2 + air mixture combustion limits are narrowed when dispersed water is added. The explosion suppression efficiency depends on the droplets size. Large droplets of 100-500 pm have not affected the lower limit of flame propagation and induced turbulence in the combustion. 

In the case of a combustible gas s, the ignition results in a flame (combustion wave) propagation through the mixture, and the cloud of micro-droplets acts as a volumetric-surface inhibitor due to the heat loss by the droplet evaporation at the flame front. However, the partial condensation caused by the sudden expansion of the combustible mixture + water fog system near the combustion limit creates preconditions for transforming the incombustible system into a combustible one. It is a situation where the role of the fog droplets in flame expansion is crucial. 

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