Experiment # 10: FREE & FORCED CONVECTION HEAT TRANSFER 10.0 Objectives The objectives of this experiment are:
i.
To demonstrate the relationship between power input and surface temperature in
free convection
ii.
To demonstrate the relationship between power input and surface temperature in
forced convection.
10.1 Equipment Description 10.1.1 Introduction Heat transfer by simultaneous conduction and convection, whether free or forced,
forms the basis of most industrial heat exchangers and related equipment. The
measurement and prediction of heat transfer coefficients for such circumstances by
studying the temperature profiles and heat flux in an air duct with associated flat and
extended transfer surfaces. The vertical duct is so constructed that the air temperature and
velocity can be readily measured, and a variety of “plug-in” modules of heated solid
surfaces of known dimensions can be presented to the air stream for detailed study. A fan
situated at the top of the duct provides the air stream for forced convection experiments.
A Control Panel contains temperature measurement, power control, and fan speed
control circuits with appropriate instrumentation. Temperature measurement, to a
resolution of 0.1 is effected using PTD sensors with direct digital read-out in oC,
Air velocity is measured with portable anemometer mounted on the duct.
The power control circuit provides a continuously variable, electrical output of 0100 watts with a direct read-out in watts. Using the instrumentation provided, free and
forced convective heat transfer coefficients may be determined for:
-
1.
A flat surface
2.
An array of cylinders (pinned heat sink)
3.
An array of fins (finned heat sink)
Each module may be used independently on the bench, to establish free
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convection coefficients for horizontal orientation.
10.1.2 Detailed DESCRIPTION The equipment consists of a vertical rectangular duct supported by stand and a
control panel. A flat plate pinned or finned exchanger may be installed in the duct
and secured by a quick-release catch on each side. Each exchanger incorporates an
electric heating element with thermostatic protection against overheating. The
temperature at the base of each exchanger is monitored by a thermistor sensor with
connecting lead.
The exchanger in use may be viewed through an acrylic window in the wall of the
duct. An upward flow of air may be generated in the duct with a variable speed fan
mounted at the top of rectangular duct.
Air velocity in the duct, whether natural or forced, is indicated on a portable
anemometer held in a bracket on the duct wall. The anemometer sensor s inserted through
the wall of the duct.
Temperature sensors are provided for the measurement of the in-going and outgoing air temperatures together with surface temperatures of exchanger pins and fins. A
digital readout indicates the temperature using a thermistor probe connected to a flexible
lead.
These temperatures are determined by inserting the probe through access holes in the
duct wall.
A control panel incorporates a variable power regulator with a digital readout to control
and indicate power supplied to the exchanger on test. The exchanger is connected to the panel
via the supply lead. A variable low voltage D.C. supply is provided for the fan via the supply
lead. Power is supplied to the equipment via a supply lead connected to control panel.
10.1.3 Theoretical Background 10.1.3.1 Free Convection A heated surface dissipates heat primarily through a process called convection. Heat is
also dissipated by conduction and radiation, however these effects are not considered in this
experiment. Air in contact with the hot surface is heated by the surface and rises due to a
reduction in density. The heated air is replaced by cooler air which is in turn heated by the
surface and rises. This process is called free convection.
The hotter the temperature of the surface, the greater the convective currents and more
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heat (power) will be dissipated. If more power is supplied to a surface, the temperature of the
surface must rise to dissipate this power.
10.1.3.2 Forced Convection In free convection the heat transfer rate from the surface is limited by the small
movements of air generated by this heat. More heat Is transferred if the air velocity is increased
over the heated surface. This process of assisting the movement of air over the heated surface is
called Forced Convection. Therefore, a heated surface experiencing forced convection will have
a lower surface temperature than that of the same surface in tree convection, for the same power
input.
10.2. Experimental Procedures 10.2.1 1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Start­up Procedures Connect the mains input power supply plug to a nearest single-phase electrical supply.
Turn the heater power control knob and fan speed control knob fully anti-clockwise.
Connect the fan supply lead to the socket at the side of the control panel.
Connect the temperature probe lead to the socket beneath the control panel.
Clamp the flat plate heat exchanger into the duct using the two toggle clamps and connect
the heater power supply lead to the socket on the cover.
Connect the plate temperature (TH) connector to the socket on the heat exchanger.
Place the meter into the bracket situated on the side of the duct. Then, connect the cable to
the connection beneath the control panel.
Ensure that the sensor hole is aligned with the direction of the airflow when inserting the
probe through the wall of the duct.
Switch on the main power.
Check that the LED temperature meter and watt-meter are illuminated. Check that the
temperature meter indicates ambient temperature.
Increase the heater power in the exchanger by rotating the power control knob clockwise.
The power supplied to the exchanger should be shown in wafts on the meter.
Switch on the fan and increase the speed by rotating the fan speed control knob clockwise.
Observe that the air velocity is indicated on the indicator.
Check that the plate temperature (Tk) increases.
Set the heater power control and fan speed control knob to minimum.
Now you are ready for the following experiments.
10.2.2 Shut­down Procedures Switch off the heater and turn the power control knob fully anti-clockwise. Set the fan speed
control knob to maximum to cool down the hot plate heat exchanger.
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Turn off the main power supply after plate heat exchanger has cooled down to room temperature.
10.3 EXPERIMENT 10a: Free Convection Heat Transfer Procedures:
1.
2.
3.
4.
5.
Remove the fan assembly from the top of the duct.
Place the tinned heat exchanger into the test duct.
Record the ambient air temperature (Ta).
Set the heater power control to 20 Watts (clockwise).
Allow sufficient time to achieve steady state conditions before noting the heated plate
temperature (Tn) into the table below.
6. Repeat this procedure at 40, 60 and 80 Watts.
7. Plot a graph of power against temperature (Tv-Ta).
Ambient air temperature (Ta) =
Input Power Input, W
20
40
60
80
Plate Temperature (TH) oC
(TH – Ta) oC
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10.4 EXPERIMENT10b: Forced Convection Heat Transfer 1.
2.
3.
4.
Place the fan assembly on to the top of the duct.
Place the finned heat exchanger into the duct.
Note the ambient air temperature (tA).
Set the heater power control to 50 Watts (clockwise). Allow sufficient time to achieve
steady state conditions before noting the heated plate temperature (TH).
5. Set the fan speed control to give a reading of 0.5 m/s on the thermal anemometer, allow
sufficient time to achieve steady state conditions. Record heated plate temperature.
6. Repeat this procedure at 1 m/s and 1.5 m/s.
7. Plot a graph of air velocity against temperature. (TH —Ta)
Ambient air temperature (Ta) =
Power input
Air velocity m/s
0
0.5
1.0
1.5
= 50 W
Plate Temperature (TH) oC
(TH – Ta) oC
Report the previous results, sample calculations, graphs and the discussions in a technical
report format.
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