EML 4304L Thermal Fluids Lab
Thermal Conduction
Experiment #3
Teaching Group #3: Christopher Hokes, Ryan Prentiss, Chris Bolyard, Fredrick Davidson, and Siye Baker
Home Objectives Background Testing Equipment Pictures Procedure Questions Links
Testing
Equipment
The apparatus for the experiment consists of:
1.
Conduction
elements
2.
Heating
sources
3.
Heat
sinks
4.
Thermocouples
and thermocouple selector
5.
Digital
temperature indicator
6.
A large
graduated cylinder, and a stopwatch
Some of the apparatus for the experiment are
discussed below.
Conduction
Elements
1. For derivation of Fourier's law
Figure
1. Conduction apparatus for Derivation of Fourier’s Law
Figure 2. Dimensions of the tapered rod conduction unit
2. For thermal contact resistance
The apparatus consists of an insulated, tubular-shaped unit containing in the central section a 1850°F tube furnace which heats two axially-aligned, slightly separated, stainless steel bars to one of which is attached end-to-end, left, a copper, then a steel bar; and to the other, right, an aluminum followed by a magnesium bar. Each bar assembly terminates in an immersion-type, fluid cooled heat sink which is used both to regulate and to measure heat flux through the terminal cross-section of the bar. Means for independent control of fluid flow rates through each heat sink is provided. All heating and conducting elements are enclosed in an insulating jacket. A schematic of the apparatus is shown in Figure 3.
Figure
3. Conduction apparatus for the thermal contact resistance experiment
Two stabilized stainless steel bars are used to
conduct heat from the high temperature zone within the furnace into both sides
of the apparatus. The stabilized stainless steel bars maintain a uniform and
durable surface condition which is essential because most of the heat entering
the bar is transmitted from the heating elements directly by radiation, and any
change in the interface conditions of the bar after a number of operations would
affect the measurements. Furthermore,
the two bars extending left and right from the center of the furnace are
slightly separated to prevent either bar acting as an undesired heat sink for
the other.
At the left, attached axially to the stainless steel
bar, there is a bar of conditioned copper with a steel bar attached to it in a
similar manner. At the right,
aluminum and magnesium bars are similarly arranged.
Each set of bars is tightly screwed together. Thus, the bars provide durable heat conduction and at the
same time provide for measurable barrier effects.
Heat Source
1.
For derivation of Fourier's law
Two hot plates are used as the heat source, one each
for the variable area conductor (Unit #3) and for the constant area conductor
(Unit #4). Each hot plate draws 6.1 amps at 120 volts and has its own dial
control knob. The dial knob is a
combination on-off switch and heating rate selector with a built-in thermostat
and an indicator light. With the
knob at the counterclockwise end of the dial scale, the current is switched off.
Turning the knob clockwise starts the flow of current, and allows the
operator to select a desired heat rate up to the maximum of 750 watt.
2.
For thermal contact resistance
The tube furnace is especially oriented to heat two
horizontal stainless steel bars in axial alignment. The left-handed stainless steel bar is coupled to a copper
bar, the copper bar to a steel bar, which in turn is coupled to a copper element
of the heat sink (Unit #1). The
right hand stainless steel bar is coupled to an aluminum bar and the aluminum
bar to a magnesium bar, which is
again coupled to a copper element of the heat sink (Unit #2). Heat flows in series through these units.
Thermocouples located along these axes of the bars are spaced
longitudinally to enable the experimenter to take temperature measurements at
critical points along the entire length of the heat flow.
The furnace is rated at 750 watts and requires 115 V AC.
The furnace base contains a multi-step temperature-controlling
transformer; all internal wiring originates from this point.
To regulate temperature, proper adjustment of the
control on the front panel must be made. To obtain the fastest heating, the
switch should be set on the Number 10 position. However, you must be very
careful to reduce the maximum input at the proper time to prevent over-heating
and possible burnout of the heater units. Maximum safe operating temperature for
the furnace is 1850° F. This corresponds approximately to the Number 6 position
for 120 V AC. operation.
The heat path of each unit ends in a liquid-cooled
heat sink for independent measurement of heat flux. The heat sink is a well in the upper end of the vertical
conductor bar, as shown in Figure 4. For
Units #1 and #2, a copper element, attached to the end of each set of bars,
extends horizontally into the coolant chamber as shown in Figure 5.
The heat sink inlet
manifold has one thermocouple and each heat sink outlet has its own
thermocouple. Locations of heat
sink thermocouples are shown in Figure 6.
Figure 5. Heat sink assembly for Units 1 and 2
Figure
6. Location of heat sink thermocouples
Temperature
Measurement
Glass-insulated
Chromel-Alumel thermocouples are used for all temperature measurements.
Ten thermocouples are mounted at the center of each metal bar at the
positions shown in Figures 7 (for Unit #1) and 8 (for Units #3 and #4)
(Dimensions given in inches). Unit
#2 is configured similar to Unit #1, hence it is not shown. The inlet
temperature to each heat sink is obtained from a single thermocouple mounted in
the inlet manifold to the heat sink. The heat sink outlets have their own thermocouples.
Figure
8. Thermocouple positions for Unit #3 and #4
Thermocouple
Selector
All thermocouple pairs lead into a junction box with
a selector switch bank, which connects any thermocouple to a common terminal
post to allow rapid read-out of all thermocouples using only one digital display
gage. In the selector switch bank, each position on the unit-selector switch is
numbered to its corresponding unit. The
thermocouple number selector switch is designed the same way; each thermocouple
position is numbered to its appropriate thermocouple on the unit. Units #1 and
#2 refer to the appropriate units on the thermal contact resistance unit; Unit
#3 corresponds to the tapered rod, and Unit #4 corresponds to the uniform rod.
The heat sink thermocouples have been grouped for convenience as Unit #5,
when using the Unit Selector switch. Location
of the various thermocouples is shown in Figure 9.
Thermocouple # 5, Unit # 5,corresponds
to the heat sink inlet temperature, while thermocouples #1,#2, #3 and # 4 for
Unit # 5 corresponds to the heat sink outlet temperatures for Unit #1, Unit #2,
Unit #3 and Unit # 4, respectively.
