Spinning Disk Aerosol Generator

The spinning-disk method of droplet generation is based on the breakup of ligaments of liquid created at the periphery of a rapidly rotating disk when liquid is fed slowly to the center and top of the disk. Under appropriate conditions of stable liquid feed and rotation, the ligament thrown out by the rotating disk breaks up into a primary (large) and a satellite (small) droplet. This is illustrated in Fig. 7. The satellite droplet is deflected away by an airflow and is usually rejected.

Figure 7 Illustration of a spinning-disk aerosol generator.

The primary stream of droplets is monodisperse. Walton and Prewett [49] demonstrated this principle of uniform droplet generation. An improved version with a spinning top was developed by May [50,51]. Further improved versions have been developed since then, and droplet sizes that are produced range from 15 to 150 mm (e.g., see Ref. [52]). A recent version produced monodisperse droplets between 10 and 60 mm in diameter [53]. Similar particles can be obtained using solutions. Toivonen and Bailey produced solid particles up to 40 mm in diameter [54]. Rotation speeds used are up to 70,000 rpm. Improvements in particle concentrations have been obtained by Cheah and Davies [55] but are still nominally less than 100 cm"3. A detailed study of the mechanism of droplet formation was provided recently by Davies and Cheah [56]. Eisner and Martonen [57] demonstrated that if the primary and satellite droplets can be effectively separated, then two monodisperse aerosol streams can be generated simultaneously.

The particle diameter derived from the primary droplets can be predicted with the following expression [49]:

where K is an empirical constant dependent on several factors, including the roughness of the spinning disk or top, v is the radial frequency of the spinning top, T is the surface tension of the liquid, D is the diameter of the spinning top, pL is the density of the liquid or solution, c is the concentration of the solution, and pP the density of the particle. The value of K has been determined to vary from 2.3 to 7.0 with several different solutions for a spinning-top generator used by Mitchell [52].

The spinning-disk aerosol generator suffers from the low concentrations of particles achievable and the need for checking the particle size for a given set of operating conditions, in comparison to the vibrating-orifice aerosol generator. However, this method of generation is superior to that of the vibrating-orifice method in that it is not subject to the problems of orifice plugging. In addition, suspensions can easily be used, in contrast to the vibrating-orifice method, where stable operation cannot be expected, particularly for small orifices. The problem of particle morphology from the evaporation of solution droplets also applies here as in the previous method of aerosol generation.

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