La ACES Student Ballooning Course
Ballooning Unit
Activity 4 – Thermal Calculations & Measurements
In this activity will use the simple steady state thermal balance equation described in
Ballooning Unit Lecture 3 to calculate the expected temperature inside a payload box and then compare these calculations with measured values. The payload box interior will contain a heat source consisting of a battery operated ~1 watt light bulb and calculations / measurements will be done with and without an insulation layer. The students will need to account for why their calculations and measurements differ. An additional activity would have the students measure the power output of various batteries as a function of temperature.
1.
Thermal chamber consisting of a large ice chest (50 qt) filled with dry ice. One may also want a second, somewhat smaller ice chest for temporary storage of dry ice removed from the thermal chamber.
2.
Mattboard payload boxes constructed during Activity 2
3.
Heat source consisting of a battery, battery holder, switch, ~1 W lamp, lamp holder and connecting wire.
4.
(Optional) An light bulb circuit identical to item 3 to be operated at room temperature for comparison.
5.
Supply of insulation material sufficient to fill the 2 cm gap between the outer and inner payload boxes.
6.
HOBO data acquisition device with internal and external temperature probes
1.
Using the steady state thermal balance equation calculate the expected temperature within your inner payload box under the following conditions and assumptions a.
The inner box contains a ~1 W heat source. You should use the actual power dissipation expected for your choice of lamp and battery. b.
The environment temperature is that of dry ice (i.e. the payload is radiating to –
78.5
o C). c.
Neglect convection. d.
Determine the required thermal conductivity and radiative properties of mattboard and your chosen insulation from manufacturer specifications. e.
Assume each wall of your cube is identical. f.
Do calculations for with and without 2 cm of insulation between your inner and outer box.
2.
Construct your payload heat source by wiring the battery holder, switch and lamp holder together and installing the battery and lamp.
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La ACES Student Ballooning Course
Ballooning Unit
Activity 4 – Thermal Calculations & Measurements
3.
Install the HOBO external temperature probe by “drilling” a hole through the inner and outer boxes, threading the temperature probe through these holes and then taping the probe to the exterior of the outer box.
4.
Prepare the thermal chamber by filling it with crushed dry ice.
5.
Connect the HOBO to your controlling computer and set the data acquisition to record both internal and external temperatures. The sampling rate should be set so that you can measure the temperatures over an ~4 hour time span.
6.
Set the HOBO to begin data collection, install it within your inner box and connect it to the external temperature probe.
7.
Turn the heat source lamp on and install it within your inner box as well. You will want to arrange the HOBO and lamp so that they are not in immediate contact. Secure the lamp, lamp battery and HOBO and seal the inner box.
8.
Install the inner box within your outer box without any insulation between the boxes and seal the lid on the outer box.
9.
“Bury” your payload in the thermal chamber so that there is roughly the same amount of dry ice covering all sides. You may want to be careful to arrange the dry ice so that none of it is in direct contact with the external temperature probe.
10.
Cold soak your payload for about three hours then remove your box from the thermal chamber being careful not to “burn” yourself on any of the cold surfaces.
11.
Remove and open the inner box and observe the light bulb. Is there any visible difference in the brightness of the bulb from before the cold soak? (As an option you could construct a second, identical light bulb circuit and operate it at room temperature during the cold soak.)
12.
Remove the HOBO download the data to the control computer and analyze your results while waiting for your payload to come back to room temperature.
13.
With the payload back to room temperature, reset the HOBO and begin data collecting.
Reinstall the inner box within the outer box, but this time include insulation between the inner and outer box walls.
14.
Again cold soak your payload for about three hours, remove the HOBO and download the data.
15.
From the analysis of your data address the following issues: a.
Did your payload reach steady-state? If not, why not? Is there anything you might alter in your experiment? b.
Do your external and internal steady-state (or extrapolated to steady-state) temperatures agree with your calculations? If not, what are some possible sources of the discrepancy? c.
Calculate the emissivity of your box external surface, the conductivity of your payload structure and the conductivity of your insulation necessary to correctly
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La ACES Student Ballooning Course
Ballooning Unit
Activity 4 – Thermal Calculations & Measurements predict the temperatures you measured. Would it be reasonable to assume these values for future calculations? If so, why? If not, why not? d.
Using the temperature requirements table from Activity 3 compare your inner box temperatures with the lower limits on the survival and operating ranges. What components would not operate correctly in a payload without insulation? …with insulation? Are there any components that would not “survive”?
1.
A quantitative feeling for heat flow and how the steady-state thermal equation might be applied.
2.
Understanding the temperature extreme that a balloon payload will experience during flight and the implications for choosing appropriate components.
3.
Establish procedures for thermal testing of a balloon payload and components.
1.
Emphasize what is meant by steady-state as opposed to thermal equilibrium.
2.
Some students will likely have difficulty understanding the concept of heat, or energy, flow (as opposed to temperature changes) that provides the basis for the steady-state thermal equation.
3.
This activity may require long periods for the cold soak and return to room temperature and this will generate a scheduling problem. Possible solutions include duplicating the experimental setup (i.e. multiple thermal chambers, multiple HOBO, etc.) or having the activity done as a team effort with only one or two payload boxes being tested.
4.
An alternative to the HOBO would be to use the SkeeterSat device constructed and calibrated during the Electronics Unit. One would need to slightly modify the SkeeterSat to extend the speaker wires so the temperature dependent frequency could be monitored outside the thermal chamber. This would have the advantage that multiple payloads could be tested at the same time providing another solution to the scheduling problem mentioned above. In this configuration one might want to use two SkeeterSats per payload to monitor both the internal and external temperature.
5.
One might be able to enhance the thermal chamber by burying a thin-walled metal box
(like a stainless darkroom photographic plate development can) in the dry ice, mounting the payload on a few insulating standoffs within the box and covering the lid of the box with dry ice. This would keep the payload from having direct contact with the dry ice and provide a better approximation of radiative heat transfer from the payload to the cold surface.
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