Spinning Disc Reactor for Green Processing and Synthesis - Process Intensification for Green Chemistry - page 67

compared to the flow on a smooth disc: metal sprayed, reentry grooved and normal grooved
discs. It was found that for a given flow rate and rotational speed, the tailored surfaces,
especially the grooved types, produced a larger number of waves of small amplitude and
short wavelength at the film surface than did the smooth disc. The normal grooved disc was
found to be more efficient at high speeds of rotation, since it generated more waves and
hence more instabilities at the free surface of the film than did the reentry grooved disc.
The effect of these textured surfaces on the heat-transfer process was also studied and will
be discussed in Section 3.3.2.
Matar and coworkers [21-24] have presented extensive modelling studies for thin film
flow over a rotating surface, with a particular focus on predicting the surface instabilities
under various operating parameters and their effects on mass transfer. The models are in
good agreement with experimentally validated data for wave formation and propagation,
obtained by Woods [18], and for gas-liquid mass transfer characteristics, measured by
Aoune and Ramshaw [19].

3.3.2 Heat and Mass Transfer

Very high heat and mass transfer rates are associated with the thin films on a rotating disc
surface. As shown in Figures 3.8 and 3.9, local film heat-transfer coefficients exceeding
25 kW/m 2 Khave been achieved for water flowing at the edge of a 0.5mdiameter smooth disc
spinning at a relatively modest disc speed of 60 rad/s, while local mass-transfer coefficients
in the liquid phase have been measured in the thinnest part of the film at the disc periphery to
be as high as 1 10 3 m/s for the liquid phase [19]. Increases in disc speed and liquid flow

rate have generally been shown to lead to higher transfer coefficients [19,25].
The high heat- and mass-transfer coefficients are attributed to the thin films resulting
in reduced path lengths, across which conduction and diffusion occur more rapidly.

Figure 3.8 Comparison of measured and predicted local heat-transfer coefficient on a smooth
rotating disc for low- and high-viscosity liquid feeds. Reprinted from [ref 19]

1999, with

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permission from Elsevier.