ChEN 202
Lab #1 – Conversion of Pump Work into Heat (Joule Heating)
Introduction
In this experiment, you will demonstrate that there are different forms of energy and that these
forms can be interconverted. It is important to note that the efficiency of converting the different
forms of energy into each other varies widely. You will collect experimental data to estimate the
efficiency of converting pump work into enthalpy or internal energy for water pumped through
an open system and closed system, respectively.
Theory and Principles
In this experiment, water will be circulated through a pump. The amount of energy added to the
fluid by the pump motor will be determined and compared to the amount of energy that has been
transferred to the water via frictional heating. A schematic of the process is shown in Figure 1.
Figure 1: Schematic of experimental setup
The rate of work done by the motor will be determined from knowing the torque and angular
velocity of the motor. From physics
& = τ ⋅ω
W
(1)
where τ is the torque and ω is the angular velocity (in radians per second). The angular velocity
will be determined using a tachometer, which simply measures the shaft speed of the motor in
revolutions per minute [this must be converted to radians per second to use Equation (1)]. The
torque will be determined from
1
τ = F× d
(2)
where F is the force applied at a certain point on a lever arm attached to the motor and “d” is the
length of the lever arm. The motor is set up such that the force vector acts perpendicular to the
distance vector (see Figure 2), which means the magnitude of the torque is F ⋅ d . The force will
be determined from a top loading balance.
Figure 2: Determination of torque from motor
In this experiment, both an open system and a closed system will be analyzed. For open systems,
& , the pump power will be required for the analysis; however, for closed
only values of W
systems, the rate of work should be converted into the total work input into the system.
Assuming the rate of work is constant,
& ⋅t
W= W
& is the power, and ‘t” is the total
where W is the total amount of work input into the system, W
elapsed time that the motor and pump have been running.
2
(3)
For closed systems, the internal energy change is
∆U t = mc v ∆T
(4)
where m is the mass of the water, cv is the heat capacity of the water (4.184 J/g o C), ∆T is the
temperature change in the water, and ∆Ut is the change in the internal energy of the water.
For the open system, the rate of change in the energy of the water must be calculated. This
change in energy is simply
& t = mc
& p ∆T
∆H
(5)
where m
& is the mass flow rate, ∆H& t is the rate of change in enthalpy of the water, and cp and ∆T
are defined above.
Procedure
This lab will be performed by groups of 3 students. The closed system will be analyzed first,
followed by the open system.
Operation of Pumping Apparatus
1.
Make sure the motor is turned off (switch in middle position) and that the motor speed is
set to 0.
2.
The 3-way valve should be set so that flow will return to the feed water bucket. There is
an arrow on the valve handle that indicates the direction of flow. Also make sure the ball
valve that will be used to control flow for the open system is completely closed (valve
handle should be perpendicular to the inlet and outlet). Fill the bucket to the line marked
"Closed System" with water (this corresponds to a volume of 0.345 ft3 ).
3.
Turn on the motor so it operates in the Clockwise direction. Very Slowly turn the motor
speed up to approximately 50 on the speed control. At this time start your stopwatch and
record the temperature of the water in the bucket. You should also record the top loading
balance readout and photo-tachometer reading. The tachometer readout is in rpm, so you
will need to convert this to radians/second in order to accurately perform the necessary
calculations. In addition, you should also record the force required to keep the pump
stationary (from the balance readout).
4.
Continue to record the balance readout, the tachometer reading, and the temperature of
the water every 3 minutes for a total of 12-15 minutes.
5.
Turn the motor speed to 0 and turn the motor off (switch in middle position). Record the
final balance value (which should be close to the initial balance value).
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6.
Using the 2nd bucket, fill the original tank to the open system fill line. Place the 2nd
bucket below the outlet of the open system exit stream to catch the water leaving the
system.
7.
Open the ball valve that is used to control the flow rate of water out of the open system
approximately 1/2 way. Change the 3-way valve so the outlet is now to the 2nd bucket
(arrow on valve handle should point to the open system line).
8.
Record the initial temperature of the water in the original bucket and the initial balance
reading.
9.
Turn the motor on (Clockwise Direction) and very slowly adjust the speed to
approximately 40. At this point water should be flowing into the 2nd bucket. You will
want to control the flow of water out of the system by using the ball valve. Slow the flow
of water out of the system to the point of a slow, but continuous, stream.
10.
After 2 minutes have elapsed, record the temperature of the outlet water stream by
collecting 20-40 mL in the cup provided and placing the thermocouple rod in the water.
You will want to repeat this procedure every 3 minutes in order to determine if the
system is at steady state (the system is at steady-state when the temperature of the outlet
water stream is constant). Also record the balance readout and the angular velocity from
the tachometer provided.
11.
Once steady-state has been reached (when the outlet temperature is constant), determine
the outlet flow rate by using the “bucket and stopwatch” method. You will want to
collect the outlet water for 1-2 minutes and determine the mass of the water collected.
The flow rate is the mass of water collected divided by the time it took to collect the
water.
12.
Turn the motor speed down to 0 and turn the motor off. Record the final balance reading.
Close the ball valve used to control the outlet flow rate of water in the open system and
switch the 3-way valve so that the water flows back to the original bucket.
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Calculations and Analysis
Closed System
1.
What is the rate of work done by the motor? In calculating the rate of work, you will
need to know the force applied. In order to more accurately determine the applied force,
several measurements were recorded from the balance. You will need to average these
measurements. You should also average the angular velocity readings taken from the
tachometer.
2.
What is the total amount of work done by the motor?
3.
What is the total internal energy change of the water in the tank?
4.
How much heat is transferred during the closed system experiment? Based on your
results, is heat transferred into or out of the system? Does the direction of heat transfer
make sense? Why or why not?
Open System
1.
What is the rate of work done by the motor?
2.
What is the enthalpy change of the water flowing through the pump at steady-state?
3.
How much heat is transferred during the open system experiment at steady-state? Based
on your results, is heat transferred into or out of the system? Does the direction of heat
transfer make sense? Why or why not?
4.
Plot a temperature vs. time profile for this system. Why does the temperature initially
rise, then flatten out and become constant? What does the constant temperature mean?
5.
Compare the magnitude of energy lost in the open and closed system experiments.
Which is larger? Why?
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