Heat Transfer and Thermal Insulation Competition

EG1004 Lab 8: Heat Transfer and Thermal Insulation

1 OBJECTIVES

The experimental objective of this lab is to design and build a container that minimizes the heat lost by an egg placed inside.

Furthermore, you will learn about heat as a form of energy (thermal energy) and how it is transferred. We will also investigate the principles of minimal design. Your design will be entered in a competition against the other containers in your section. The team with the lowest Minimal Design Ratio (insulating capacity*cost) will be declared the winner.

2 OVERVIEW

Heat is a form of energy. Heat transfer is thermal energy that is transferred from one body to another as a result of a temperature difference. Temperature is the measure of average kinetic energy of atomic motion. The faster the atoms are moving, the higher the temperature. In this lab, we will focus on the ways heat is transferred.

The specific mechanisms that convey energy from one location to another are conduction, convection, and radiation. If you put any two objects with different temperatures together, heat will be transferred from the object with the higher temperature to the one with the lower temperature until the system reaches equilibrium. Let's say you placed a hot bowl of soup on the kitchen table. What would happen? What if you placed a glass of ice water on the same table? Our job is to understand how this heat transfer occurs and how to slow the process down.

When there is a temperature difference within a solid body or between two solid bodies in contact with each other, energy (heat) will flow from the region of higher temperature to the region of lower temperature. This is known as conduction. The formula to determine the conductivity in a rod of material is:

In this equation, Q is heat transferred in time t, k is the thermal conductivity constant, A is area, T is temperature and d is the thickness of the barrier. Therefore, increasing d or decreasing A would reduce conduction.

Imagine a metal rod that is heated at one end. The atoms of the rod collide at the point where the temperature differs, transferring heat until the temperature of the rod becomes uniform.

Convection is the transfer of heat within a fluid medium (fluids consist of gasses and liquids). Convection can occur in two different ways. The first way is known as natural convection. Natural convection occurs when the main mechanism for heat transfer is due to the temperature difference either within the fluid or at its boundaries. This difference in temperature causes the molecules in the higher temperature region to expand and rise due to bouyant forces. See Figure 1 for a diagram of natural convection. The second way in which convection occurs is called forced convection. Forced convection occurs when the main mechanism for heat transfer is due to the forced flow or motion of the fluid.

Figure 1: Air circulation diagram

Today we heat our homes through radiators using natural convection. Air flows through a heating element and then is dispersed through the room. Would it be helpful to put bigger fins on the heating element? To decide, use the formula for heat transferred through convection:

In this equation, Q is the heat transferred in time t, ∆T is the difference in temperature, h is the convection coefficient, and A is the surface area.

Suppose you wanted to disperse the heat even faster. In that case you could blow a fan past the heating elements. This is forced convection; it is one of the principles used in air conditioners to have a relatively small unit cool a large space.

According to the Encyclopedia Britannica, radiation is the process by which energy, in the form of electromagnetic radiation, is emitted by a heated surface in all directions. It travels to its point of absorption at the speed of light and does not require an intervening medium to carry it. The heating of the Earth by the Sun is an example of heat transfer by radiation. The formula for heat transferred through radiation:

In this equation, Q is the heat transferred in time t, Ts is the surface absolute temperature, e is the constant of emissivity, T_sur is the surrounding absolute temperature, A is the cross-sectional area, and σ is the Stefan-Boltzmann constant.

How would you feel out in the sun on hot day in a black t-shirt? How would you feel if your shirt were white? A black surface is an excellent absorber of heat, while a white surface is not. Silver is a poor heat absorber; that is why rooftops of apartment houses are often painted silver.

Now that we understand how heat is transferred, we must consider how to slow it down. Understanding how to minimize heat loss is the key to designing a successful insulating container.

The first consideration is the materials you choose. Using materials that are poor conductors of heat will minimize heat loss. Glass is one of these. Fiberglass insulation uses spun glass for its insulating effect.

Plastic is also a poor conductor of heat. Foam cups are made of plastic which has tiny air bubbles suspended in it. Air is among the poorest conductors of heat. It turns out that a vacuum, or the complete lack of air, is the best insulator of all. This is the principle employed in Thermos design.

The other important consideration in creating your container is its cost. Minimal design uses the fewest resources while maintaining the safety and efficacy of your product. Engineers almost never have the luxury of designing products or systems where cost is not a factor. Given unlimited resources, you could most likely come up with a container that would maintain the egg's temperature almost indefinitely, but no one could afford to buy it from you. Balancing cost and effectiveness into your plans is a critical skill you will need later.

3 YOUR ASSIGNMENT

(One report per team-check the “Technical Communication�? section of this manual)

The following discussion points are to be addressed in the appropriate section of your lab report and presentation:

• Describe the rules of the competition in your introduction. What consequences did the rules have for your design decisions? Use the appropriate equations in your answer. You may do this in a numbered list, but use full sentences please.
• Explain heat transfer and all the mechanisms that perform heat transfer. Discuss which of these mechanisms applied to your design.
• Discuss minimal design and its importance.
• Describe your container's design. Explain the choices you made. Make sure you include a discussion of the materials you chose and why. Talk about your team's strategy for winning the competition.
• What changes would have increased/decreased your MDA or IC?
• Should the temperature vs. time graph be smooth or should it have spikes? Explain how closely your curve approximates the ideal.
• Describe how your design succeeded or failed. Discuss design improvements.

Note: It is not unusual to experience instrumentation errors in this lab, leading to incorrect temperatures being recorded. Be sure to read "How to Handle Unusual Data" in the online manual to learn how to handle this.

4 MATERIALS AND EQUIPMENT

Materials with Price List

• Large foam cup - $0.50
• Lid - $0.25
• Paper cup - $0.40
• Styrofoam pieces - $0.05/6 pieces
• Tape - $0.10/ft
• Aluminum foil - $0.30/ft²
• Plastic wrap - $0.02/ft²

"NO RETURN" POLICY: When calculating cost, you must include ALL materials you request, even if you end up not using them. Make your selections carefully so you don't end up with cost that hurts your performance, but has no benefit.

Equipment Used

• Lab PC with DAQ Board and LabVIEW Program “Therm VI�? Installed

5 COMPETITION RULES

The following rules must be observed at all times during the competition. Violation of any of these rules will result in the disqualification.

• All materials must be inside the container.
• The container may not be larger than the largest cup provided.
• The egg may not be returned to the water.
• The container may not be held or covered during temperature readings.
• External heat sources are prohibited.
• The egg may not be out of the water and not be within the container for more than 30 seconds.

6 PROCEDURE

Insulating Container Design

1. Analyze your materials and consider your design options, keeping in mind the lab's specifications. Make sure you take notes and make preliminary sketches during this process.
2. Now, sketch your design in pencil using the graph paper provided on the EG website (http://eg.poly.edu). Label your drawings clearly. Prepare a price-list for your insulating container based on the materials you have chosen. Have your TA sign the sketches and the price-list.
3. Build your insulating container based on the sketch you just completed. Your TA will provide the materials needed for your design. If you decide to modify your design during the construction of your insulating container, note the changes and describe the reasons for them. If the modifications required more materials to be used, make sure you update your price list and your TA approves it.
4. Ask your TA to take a picture of your container.

Competition

1. Before receiving your egg, make sure that the yellow wire of the thermocouple is connected to pin 3 on the DAQ board and the orange wire of the thermocouple to pin 4. Secure a length of cellophane tape, large enough to encircle the egg, to the other end of the thermocouple.

Note: The thermocouple wire must remain in constant contact with the egg throughout the 15-minute measurement period. Ensure that the thermocouple wire is attached securely.

2. The TA will bring the egg to you once you are ready to receive it. Attach the other end of the thermocouple to the egg using the tape you prepared. Be sure the ends of the tape overlap. Make sure the container is closed tightly.

Warning: Be careful! The egg is very hot.

3. Start the LabVIEW program (see Figure 2). Your TA will already have calibrated the Therm VI.

Warning: Make sure the program is set to Run. This will allow the thermal VI to record 30 temperature readings to an Excel spreadsheet. They will be your source data for performing necessary calculations.

Figure 2: Therm VI window

Data Analysis

1. Create two graphs using the X, Y Scatter template. For each of them, name the x-axis Time, and plot it in minutes with thirty second intervals. Name the y-axis Temperature. You may select the intervals based on your results. The first graph includes all fifteen minutes of testing. The second graph is just the last five minutes of testing. When you are finished, your graphs should look similar to Figures 3 and 4.



Figure 3: Temperature vs. Time diagram of 15 minute VI run



Figure 4: Temperature vs. Time diagram of Last 5 minutes of VI run

2. Calculate the Insulating Capacity (IC) of your design. IC is the value of the slope of the last five minutes of data. To do this, right click on any data point on your second graph and select Add Trendline, then Options. Check the box labeled Display equation on chart and click OK. Click on the equation and drag it to an area on your chart where it will be legible.
3. Print out the graphs and the table from LabVIEW and have your TA initial them. Have your TA write in your lab notes "Data/graph done. (signed) TA". This way only the lab notes have to be scanned in to complete the electronic lab report since the data/graph already exists in electronic form.
4. Calculate the Minimal Design Ratio (MDR) for your design:

MDR=|IC|*cost

The team with the lowest MDR wins.

Your lab work is now complete. Please clean up your workstation. Return all unused materials to your TA. Refer to section 3 Your Assignment for the instructions you need to prepare your lab report.

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