MECHANICS of FLUIDS LABORATORY - Mechanical Engineering

EXPERIMENT 5
IMPACT OF A JET OF WATER
A jet of fluid striking a stationary object
exerts a force on that object. This force can be
measured when the object is connected to a spring
balance or scale. The force can then be related to
the velocity of the jet of fluid and in turn to the
rate of flow. The force developed by a jet stream
of water is the subject of this experiment.
Impact of a Jet of Liquid
Equipment
Jet Impact Apparatus
Object plates
Figure 5.1 is a schematic of the device used in
this experiment. The device consists of a tank
within a tank. The interior tank is supported on a
pivot and has a lever arm attached to it. As
water enters this inner tank, the lever arm will
reach a balance point. At this time, a stopwatch
is started and a weight is placed on the weight
hanger (e.g., 10 lbf). When enough water has
entered the tank (10 lbf), the lever arm will
again balance. The stopwatch is stopped. The
elapsed time divided into the weight of water
collected gives the weight or mass flow rate of
water through the system (lbf/sec, for example).
The outer tank acts as a support for the table
top as well as a sump tank. Water is pumped from
the outer tank to the apparatus resting on the
table top. As shown in Figure 5.1, the impact
apparatus contains a nozzle that produces a high
velocity jet of water. The jet is aimed at an object
(such as a flat plate or hemisphere). The force
exerted on the plate causes the balance arm to
which the plate is attached to deflect. A weight
is moved on the arm until the arm balances. A
summation of moments about the pivot point of
the arm allows for calculating the force exerted
by the jet.
Water is fed through the nozzle by means of
a centrifugal pump. The nozzle emits the water in
a jet stream whose diameter is constant. After the
water strikes the object, the water is channeled to
the weighing tank inside to obtain the weight or
mass flow rate.
The variables involved in this experiment
are listed and their measurements are described
below:
1. Mass rate of flow–measured with the
weighing tank inside the sump tank. The
volume flow rate is obtained by dividing
mass flow rate by density: Q = m/ρ.
2. Velocity of jet–obtained by dividing volume
flow rate by jet area: V = Q/A. The jet is
cylindrical in shape with a diameter of 0.375
in.
3. Resultant force—found experimentally by
summation of moments about the pivot point
of the balance arm. The theoretical resultant
force is found by use of an equation derived by
applying the momentum equation to a control
volume about the plate.
Impact Force Analysis
The total force exerted by the jet equals the
rate of momentum loss experienced by the jet after
it impacts the object. For a flat plate, the force
equation is:
F = ρQ2
A
For a hemisphere,
F = 2ρQ2
A
(flat plate)
(hemisphere)
For a cone whose included half angle is α,
F = ρQ2
(1 + cos α) (cone)
A
For your report, derive the appropriate
equation for each object you use. Compose a graph
with volume flow rate on the horizontal axis,
and on the vertical axis, plot the actual and
theoretical force. Use care in choosing the
increments for each axis.
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