• 作者：Yinghe He
• 关键词：Particle formation ; Microstructure ; Microencapsulation ; Flow focusing ; Laminar jet break-up
• 刊名：Chemical Engineering Science
• 出版年：2008
• 期刊代码：18_00092509
• 类别：ce
• 出版时间：May 2008
• 卷：63
• 期：9
• 页码：2500-2507
• 文件大小：371 K

METHOD

The Dantec Particle Analysis System was used to measure the droplet size and velocity. This system consists of a combination of a light scattering system to measure the droplet size and a Laser-Doppler system to measure the axial velocity. The lower detection limit of the system in our case was equal to 3 μm. Ethylene glycol, ethanol and acetone were sprayed in the Cone-Jet mode at various experimental conditions. A nozzle-to-plate spraying configuration was used. The nozzle diameter was 5 mm, it was a cylindrically shaped nozzle with a flap tip, the inner diameter was 0.25 mm. The nozzle to plate distance was 12.1 cm.

The above-mentioned effects, like for instance the size segregation effect and the acceleration of the surrounding gas, were observed for each liquid at various flow rates. When liquids are sprayed, a possibly polymodal but at least bimodal size distribution is produced. However, when using ethylene glycol, the average size of the first mode of satellites is relatively large. For instance, at a flow rate of 1.39 10⁻⁹ m³/s, the average main droplet size is about 18 μm and the average size of the largest mode of satellites is equal to 8 μm. Combined with the low volatility of ethylene glycol, these relatively large satellites allow more precise investigation of the above-mentioned effects.

The measurement results were compared to results of a Lagrangian model, which is an improvement of Gañán-Calvo's model (1994), Hartman (1996). It is a physical model which describes the movement of highly charged droplets in an electric field. The model takes the drag force, the electric force due to the applied electric field, the electric forces due to electrical interaction between charged droplets, and the droplet inertia into account.

RESULTS

Due to the strong gradient of the electric field in an area within a few centimetres from the droplet production area, electric force and drag force are not in balance, so the droplet inertia has to be taken into account when calculating the droplet velocity.

Comparison between measurements and simulations shows the already known droplet size segregation effect in the spray. The model predicts the droplet size as function of the radial position and the concentration profile in the spray well, but it underestimated the radial dispersion of the droplets in the spray. The underestimation of the radial electric field and neglecting dispersion due to flow of the surrounding air are thought to be the main reasons for this underestimation.

Mutual electric interaction of charged droplets and differences in inertia are found to be the main reason for the size segregation effect.

Calculations showed that the electrical interaction between the charged droplets influences also the axial velocity of these droplets, Fast highly charged droplets are slowed down by slower lower charged droplets. However, the effect is limited.

Comparison between measurements and calculations showed that fast ethylene glycol droplets, 25 m/s, can accelerate the surrounding gas. In the spray centre, at one centimetre from the droplet production area, gas velocities can reach up to approximately 8 m/s.

Taking all the mentioned effects in account, a relation between droplet size and droplet charge for ethylene glycol could be established. This relation was found to be equal toq g border=0 src="http://www.sciencedirect.com/scidirimg/entities/223c.gif" alt="not, vert, similar" title="not, vert, similar"> d^1.6 Due to the above-mentioned effects, which in some cases have to be estimated, the value of the power of this relation is also estimated. However, taking extreme assumptions showed that the absolute accuracy of the obtained power, 1.6, is lower than 0.6. This value of 1.6 means that the surface charge density on the smaller droplets is slightly higher than on the larger droplets. This can be explained qualitatively from the jet break-up mechanism when the time constant of the moving charge on the jet is considerably smaller than the time constant of jet break-up. However, more research is needed for a more quantitative result.