MECHANICS of FLUIDS LABORATORY - Mechanical Engineering

placed on the torque arm to reposition the motor
to its balanced position. The product of weight
and torque arm length gives the torque input from
motor to fan.
A tachometer is used to measure the
rotational speed of the motor. The product of
torque and rotational speed gives the power input
to the fan:
dW a
dt
= Tω (17.2)
This is the power delivered to the fan from the
motor.
The efficiency of the fan can now be
calculated using Equations 1 and 2:
η = dW/dt
dW a /dt
(17.3)
Thus for one setting of the motor controller, the
following readings should be obtained:
1. An appropriate reading for the flow meter.
2. Weight needed to balance the motor, and its
position on the torque arm.
3. Rotational speed of the fan and motor.
4. The static pressure at section 2.
With these data, the following parameters
can be calculated, again for each setting of the
motor controller:
1. Outlet velocity at section 2: V 2 = Q/A 2 .
2. The power using Equation 17.1.
3. The input power using Equation 17.2.
4. The efficiency using Equation 17.3.
Presentation of Results
On the horizontal axis, plot volume flow
rate. On the vertical axis, graph the power using
Equation 1, and Equation 2, both on the same set of
axes. Also, again on the same set of axes, graph
total pressure ∆p t as a function of flow rate. On a
separate graph, plot efficiency versus flow rate
(horizontal axis).
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