Page 147 - Industrial Wastewater Treatment, Recycling and Reuse

Advanced Physico-chemical Methods of Treatment for Industrial Wastewaters 121

Although cavitation can be realized in variety of ways, acoustic cavitation
and hydrodynamic cavitation are widely used in practice. Acoustic cavita-
tion, based on piezo-electric ultrasound horns, is more suitable for specialty
and small-scale applications. Higher energy requirements and difficulties
in scale-up associated with acoustic cavitation do not make it attractive
for effluent treatment applications. Hydrodynamic cavitation, however, is
inherently suitable for effluent treatment applications.
Hydrodynamic cavitation is realized by creating low pressure regions in
the flow domain. There are mainly the following two ways with which such
low pressure regions can be generated:
• Increasing the linear velocity of liquid by forcing it to flow through con-
strictions such as orifices or venturi. The constrictions are designed in
such a way that velocity at the throat (smallest flow area zone) is large
enough to generate cavities. These cavities will collapse further down-
stream of the constriction, resulting in the desired effect.
• Increasing the tangential velocity of a liquid by forcing it to flow through
a device like a vortex diode, which has an outlet port from the center of
the vortex (tangential flow). The dimensions of the diode and outlet port
are designed in such a way that a low pressure region is generated because
the highly swirling flow in the diode chamber is adequate to generate
cavities. The cavities escape the diode chamber via the outlet port and
then collapse as they enter the high-pressure region.
The efficacy of cavitation essentially depends on the number density of gen-
erated cavities and the intensity of the collapse of generated cavities. The
cavitation process and its various applications are discussed in detail in
Chapter 3. In order to avoid duplication, here we restrict the scope of
the discussion to the type of cavitation (cavitation realized by tangential
flow) not covered in Chapter 3.

2.5.1 Cavitation Using Tangential Flow/Vortex Diodes

Recently, Ranade et al. (2008, 2013) have developed a cavitation device in
the form of a vortex diode for effluent treatment and other applications. A
vortex diode is a disk-shaped chamber with a tangential port and a cylindrical
axial port. As compared to conventional cavitating devices that depend on
constriction for pressure changes, the vortex diode relies on fluid vortex
phenomena for its operation. The chamber is characterized by its diameter
and height along with curved surface, with or without internals, which
decide the chamber volume. The flow entering the device through the