Droplet impingement processes

Figure 3.13. Deformation process of a single droplet impinging on a flat surface (Re = 1600, We = 26.7) (a) simulation left), experiment right), and (b) comparison between calculated and measured dimensionless diameter and height ofa flattening droplet. (Photograph Courtesy of Prof. Dr. Jiro Senda at Doshisha University, Japan. Experimental data reprinted with permission from Ref. 334.)...

Figure 3.14. Deformation process of a single droplet impinging on a non-flat surface (Re = 38115, We = 8474, e/D0=0.33, A/D0=2.8). From top to bottom

The first interface occurs between the spray solution and the atmosphere (air) and determines the droplet spectrum, rate of evaporation, drift, etc. The second interface occurs between the liquid droplets and the leaf surface. The droplets impinging on the surface are subject to a number of processes which determine their adhesion, retention, and further spread. The nature of the deposit formed is governed by the rate of droplet evaporation and the concentration gradient of the surfactant across the droplet. 

Because the highest possible interfacial area is desired for the heterogeneous reaction mixture, advances have also been made in the techniques used for mixing the two reaction phases. Several jet impingement reactors have been developed that are especially suited for nitration reactions (14). The process boosts reaction rates and yields. It also reduces the formation of by-products such as mono-, di-, and trinitrophenol by 50%. First Chemical (Pascagoula, Mississippi) uses this process at its plant. Another technique is to atomize the reactant layers by pressure injection through an orifice nozzle into a reaction chamber (15). The technique uses pressures of typically 0.21—0.93 MPa (30—135 psi) and consistendy produces droplets less than 1 p.m in size. The process is economical to build and operate, is safe, and leads to a substantially pure product. 

Flame spraying is no longer the most widely used melt-spraying process. In the power-feed method, powders of relatively uniform size (<44 fim (325 mesh)) are fed at a controlled rate into the flame. The torch, which can be held by hand, is aimed a few cm from the surface. The particles remain in the flame envelope until impingement. Particle velocity is typically 46 m/s, and the particles become at least partially molten. Upon impingement, the particles cool rapidly and soHdify to form a relatively porous, but coherent, polycrystalline layer. In the rod-feed system, the flame impinges on the tip of a rod made of the material to be sprayed. As the rod becomes molten, droplets of material leave the rod with the flame. The rod is fed into the flame at a rate commensurate with melt removal. The torch is held at a distance of ca 8 cm from the object to be coated particle velocities are ca 185 m/s. 

In the atomizing process, a stream of molten zinc is broken into tiny droplets by the force of a pressurized fluid impinging on the stream. The fluid can be any convenient material, although air is normally used. The atomized drops cool and soHdify rapidly in a coUection chamber. The powder is screened to specified sizes. Particulate zinc is also produced by other methods such as electrolytic deposition and spinning-cup techniques, but these are not of commercial importance. 

Another advantage of the radial reacrion turbine is that it can be designed to accept condensation in any amount without efficiency deterioration or erosion. This is possible because there are two forces acring on suspended fog particles, the deceleration force and the centrifugal force, and these two forces can be balanced against each other to prevent the droplets from impinging on specially shaped blades. The process is expl ned as follows ... 

Erosion. The abrasive is likely to be gas borne (as in catalytic cracking units), liquid borne (as in abrasive slurries), or gravity pulled (as in catalyst transfer lines). Because of the association of velocity and kinetic energy, the severity of erosion may increase as some power (usually up to the 3d) of the velocity. The angle of impingement also influences severity. At supersonic speeds, even water droplets can be seriously erosive. There is some evidence that the response of resisting metals is influenced by whether they are ductile or brittle. Probably most abrasion involved with hydrocarbon processing is of the erosive type. 

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