cavitation

1. Fluid Lab
khogr kamal
Cavitation -
Name: Khogr Kamal Ibrahim
Group: 2
Fluid lab.

2. Fluid Lab
khogr kamal
Experiment No. 5
Experiment Name : - Cavitation
Objectives
To demonstrate the appearance and sound of cavitation in a hydraulic system.
To demonstrate the conditions for cavitation to occur (liquid at its vapour
pressure). To show how cavitation can be prevented by raising the static
pressure of a liquid above its vapour pressure
Introduction
Cavitation is the occurrence of small bubbles in a hydraulic system due to a pressure
decrease such that the pressure in the water is less than vapor pressure. When
water pressure is lower than vapor pressure, the water vaporizes within the liquid an
small bubbles are formed. Pressure of the liquid around the bubbles causes the
bubbles to then collapse which releases energy. In a hydraulic system, cavitation
releases so much energy that damage and erosion can occur on the solid aspects of
the system. We can see the negative results of cavitation in pumps, turbines and
dams.For any hydraulic system, cavitation must be taken into consideration both
during thedesign process as well as after the system is in place. Engineers need to
routinely check for cavitation damage during the life of the system. If cavitation
occurs and is not caught quickly colossal damage can occur. An example of this is
outlined in the tunnel collapse of 1974 during the construction process of the Tarbela
Dam in Pakistan. Prevention of cavitation is essential for the success and continued
operation of any hydraulic system. An outline of theory, failures and prevention is
detailed in the following report.Flow meters are used in the industry to measure
the volumetric flow rate of fluids. Differential pressure type flow meters ( Head
flow meters)measure flow rate by introducing a constriction in the flow. The
pressure difference caused by the constriction is correlated to the flow rate
using Bernoulli's theorem. If a constriction is placed in a pipe carrying a stream
of fuid,there will be an increase in velocity,and hence an increase in kinetic
energy ,at the point of constriction.From an energy balance as given by
Bernoulli’s theorem,there must be a corresponding reduction in pressure.Rate

3. Fluid Lab
khogr kamal
Theory
Cavitation is a phenomenon that occurs when the pressure within a flowing fluid
reaches the vapor pressure of the fluid, resulting in the formation of vapor bubbles. It
is generally characterized by a loud crackling noise and a “cloud” of vapor bubbles
that form where the cavitation is initiated (see Figure 1). These audible and visible
signs, along with damage to surfaces within the pipe, are often the most
recognizable aspects of cavitation. There are two different types of cavitation,
depending on the properties of the vapor voids: vaporous cavitation when the
bubbles consist of water vapor and gaseous cavitation when the bubbles
contain gasses other than water vapor. Cavitation occurs due to a pressure drop asa
resultfrom an increase in the velocity of the fluid through a specific zone, often as
aresult of a decrease in the cross sectional area of the flow. Since pressure
decreases at higher altitudes
cavitation can also occur with an increase in the elevation of the hydraulic system.
This
concept is explicitly expressed by Bernoulli’s Equation (Eqn. 1), in which it is evident
that
an increase in velocity or elevation on either side of the equation will result in a
decrease in
the associated pressure in order to maintain equilibrium (constant total head).

4. Fluid Lab
khogr kamal
Calculation

5. Fluid Lab
khogr kamal
Discussion:-
The graphs should clearly show how the pressure at the throat falls as the flow
/ velocity of the water is increased as predicted by the Bernoulli equation. It
should also show that the pressure reaches a minimum value that cannot be
exceeded despite increasing water velocity. Consider the effect of cavitation if
allowed to occur in a hydraulic system. The exercise shows that the cavitation
can be prevented by increasing the static pressure of the fluid. However, this
technique can only be applied to delay the effect / a larger pump is required to
overcome the additional losses in the system. Cavitation is therefore best
avoided by careful system design to eliminate any high velocities, low
pressures or high temperatures that could lead to