Page 182 - Industrial Wastewater Treatment, Recycling and Reuse

156 Industrial Wastewater Treatment, Recycling, and Reuse

area (a), and the ratio of the throat/orifice diameter to the pipe diameter and
the divergent angle (in the case of a venturi). Different designs of cavitating
devices can be constructed using the variations in these parameters which
can aid in controlling the cavitational intensity.
The cavity inception and the number of cavities being generated depend
on the size and shape of the constriction. To quantify this dependency, two
parameters need to be considered in the analysis: (a) parameter a, which is
defined as the ratio of the throat perimeter to the throat area and (b) b, which
is the ratio of the throat area to the cross-sectional area of the pipe.
(a) Throat size and shape
Changing the shape of the throat, for example, the orifice plate and
venturi, can be a efficient approach with the aim of maximizing a. It has
been observed that for plates having the same flow area, it is advisable to
use a plate with a smaller hole size opening, thereby increasing the
number of holes in order to achieve a larger extent of the shear layer
(Sivakumar and Pandit, 2002). The higher value of a results in the gen-
eration of a higher number of cavities. The value of turbulence pressure
fluctuating frequency (f T ) also increases, leading to a more efficient col-
lapse. On the contrary, for larger hole sizes, the frequency of turbulence
(f T ) is likely to be much lower than the natural oscillation frequency of
the generated cavity, resulting in a lower cavity collapse intensity. Also,
parameter a can be increased by constructing different throat shapes
such as rectangular and elliptical. Also, if there is a choice for the mag-
nitude of the flow area, a lower percentage area should be chosen
because with a decrease in flow area, the intensity of cavitation
increases. The size of the required throat depends on the cavitation
number. The area of the throat can be finalized based on the required
cavitation number for the desired physicochemical transformation and
volume of the effluent to be treated.
(b) Size and shape of the divergent section
Once the cavities are formed, they need to grow to a certain size
before collapse, or in other words, the life of cavity should increase
in order to get maximum cavitational yields. In the case of the orifice
plate, because of the sudden pressure drop, cavities are forced to col-
lapse immediately without attaining maximum size, which results in a
reduced cavitational effect. In the case of the venturi, the pressure
recovers smoothly because of the divergent angle, and cavities get
enough time to grow to a maximum size before collapsing, thereby
allowing cavities to undergo various cycles of expansion and collapse.