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Advanced Oxidation Technologies for Wastewater Treatment: An Overview 151

Flow

Orifice Vena contracta
(a)

P 1

Pressure P 2

P v
(b) Distance downstream to orifice
Figure 3.2 Fluid flow and pressure variation in a hydrodynamic cavitation (HC) setup.

in the medium) which reaches its maximum size at the lowest pressure.
Subsequent increase (pressure recovery) in the pressure compresses this fully
grown cavity and subsequently to collapse in near adiabatic fashion, thus
generating extreme pressure and temperature conditions.
A dimensionless number known as the cavitation number (C V ) is used to
relate the flow conditions to the cavitational intensity. The cavitation num-
ber is given by the following equation:

p p v
2
C V ¼ (3.9)
1 pv 2
2 0
where p 2 is the fully recovered downstream pressure, p v is the vapor pressure
of the liquid, and v 0 is the velocity at the throat of the cavitating constriction.
The number at which the inception of cavitation occurs is known as the
cavitation inception number C vi . Ideally speaking, cavitation inception
occurs at C vi equal to 1, and there are significant cavitational effects at C v
values of less than 1. In the earlier work by Gogate and Pandit (2000b),it
has been shown that cavities oscillate under the influence of a fluctuating
pressure field, and the magnitudes of pressure pulses generated are much less,
insignificant or too small to bring about a desired chemical change for the
case where C v values are greater than 1. However, cavitation has been found
to occur at higher cavitation numbers also, possibly due to the presence of