Droplets rapid vaporization

Superheated liquids and associated rapid phase changes appear in a variety of physical phenomena. A simple laboratory experiment is to heat a small droplet of fluid by immersing it in a column of denser fluid which is heated at the top and cooled at the bottom, a bubble column . When the droplet temperature approaches the superheat limit, the droplet rapidly vaporizes (in less than 100 ps for a 1 mm diameter hydrocarbon droplet) with an audible pop. Detailed investigations (Shepherd and Sturtevant [1], Frost and Sturtevant [2]), have shown that under certain conditions (low ambient pressure, large superheat) the vaporization has an explosive character. In explosive... 

An efficient feed injection system produces extremely small droplets that vaporize quickly. Rapid vaporization minimizes the amount of non-vaporized hydrocarbons that block the active sites. An effective feed nozzle system must instantaneously vaporize and crack asphaltenes and polynuclear aromatics to lower boiling entities. 

The droplet number density presented in Fig. 16.4 indicates the solid-cone nature of the spray except in the immediate vicinity downstream of the nozzle exit. On the spray centerline at 2 = 10 mm, steam provides a lower number density as compared to the two air cases. This is due to the expansion of the spray jet at a relatively lower Reynolds number with steam and rapid vaporization of smaller sized droplets. At increased radial positions and 2 = 10 mm, a peak in the number density corresponds to the spray cone boundary. This peak shifts radially outwards with an increase in axial distance due to the expansion of the spray cone. Similar phenomena are observed for the normal and preheated air cases except that droplet number density for the preheated air case is much higher on the spray central axis (at r = 0). This is attributed to the effect of preheated air on atomization (i.e., larger mean droplet size and smaller number density with normal air as compared to that for heated atomization... 

The equilibrium data presented clearly shows the shale oil to behave differently from the petroleum fuels. However, under conditions of rapid vaporization in an inert atmosphere a 150 ym droplet array of these fuels exhibited trends not disclosed by the equilibrium data. Figures 3, 4 and 5 display the non-equilibrium evolution of fuel nitrogen with respect to mass vaporized for shale oil, Indo-Malaysian petroleum, and Gulf petroleum respectively. 

SPRAY DRYERS. In a spray dryer a slurry or liquid solution is dispersed into a stream of hot gas in the form of a mist of fine droplets. Moisture is rapidly vaporized from the droplets, leaving residual particles of dry solid, which are then separated from the gas stream. The flow of liquid and gas may be cocurrent, countercurrent, or a combination of both in the same unit. 

HPN), which achieves nebulization of the liquid sample plus rapid vaporization of the droplets in a heated region prior to the corona discharge. APCI primarily results in the formation of [A + H]+ or [A-H] ions as a result of the complex ion-molecule chemistry that occurs in the plasma formed following the discharge. Because of the requisite heating, the technique is best suited to analytes that possess some thermal stability (not as demanding as for El and Cl) and are of moderate polarity (and thus volatility). 

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. 

The simplest desolvation chambers consist simply of a tube heated to about 150°C through which the spray of droplets passes. During passage through this heated region, solvent evaporates rapidly from the droplets and forms vapor. The mixed vapor and residual small droplets or particulates of sample matter are swept by argon through a second cooled tube, which allows vapor to... 

Suffice it to say at this stage that the surfaces of most solids subjected to such laser heating will be heated rapidly to very high temperatures and will vaporize as a mix of gas, molten droplets, and small particulate matter. For ICP/MS, it is then only necessary to sweep the ablated aerosol into the plasma flame using a flow of argon gas this is the basis of an ablation cell. It is usual to include a TV monitor and small camera to view the sample and to help direct the laser beam to where it is needed on the surface of the sample. 

The strong localized heating causes the liquid to vaporize very rapidly, forming a supersonic jet that leaves the end of the capillary as a mist of fine droplets mixed with vapor. 

Carbon monoxide and dioxide oxidize zinc vapor below 1100—1300°C although only the carbon dioxide reaction is significant. Rapid condensation of the zinc vapor avoids the formation of ziac-oxide-coated droplets, so-called blue powder. 

A third screening smoke-type is white phosphoms [7723-14-0] (WP), P (see Phosphorus and THE phosphides), which reacts spontaneously with air and water vapor to produce a dense cloud of phosphoms pentoxide [1314-56-3]. An effective screen is obtained as the P2O5 hydrolyzes to form droplets of dilute phosphoric acid aerosol. WP produces smoke in great quantity, but it has certain disadvantages. Because WP has such a high heat of combustion, the smoke it produces from bulk-filled munitions has a tendency to rise in pillarlike mass. This behavior too often nullifies the screening effect, particularly in stiU air. Also, WP is very brittle, and the exploding munitions in which it is used break it into very small particles that bum rapidly. 

Another theory of liquid-liquid explosion comes from Board et al. (1975). They noticed that when an initial disturbance, for example, at the vapor-liquid interface, causes a shock wave, some of the liquid is atomized, thus enhancing rapid heat transfer to the droplets. This action produces further expansion and atomization. When the droplets are heated to a temperature equal to the superheat temperature limit, rapid evaporation (flashing liquid) may cause an explosion. In fact, this theory resembles the theory of Reid (1979), except that only droplets, and not bulk liquid, have to be at the superheat temperature limit of atmospheric pressure (McDevitt et al. 1987). 

The formation of droplets and their rapid, efficient vaporization is the reason that there is more vapor in the cloud than the amount which flashed off originally. Schmidli et al. (1990) determined that 5 to 50% of the mass of the original fuel can be found in droplets. This value depends upon initial mass and degree of superheat, that is, amount by which the fuel s temperature exceeds its boiling point. 

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