Lab 7: Entropy, Stirling Engine, Coefficient of Performance, and Bubbles

We started the day talking about the main difference between auto cycle and diesel cycle. Diesel cycle can keep on compressing until the gas ignites, allowing a constant pressure process. Also, it is more simpler and has more effectiveness compared to auto cycle since you can get more work per cycle. Diesel engines try to inject fuel at the very last second in order to achieve instantaneous ignition.

We learnt a new term, entropy, which is the amount of heat energy absorbed over the thermal energy. Also the second and third law of thermodynamics was introduced to us, which define that entropy can be created but not spontaneously destroyed, and it is the entropy (not energy) of a system that goes to zero as the absolute temperature goes to zero, respectively.

We also worked on a stirling engine cycle, which is another real gas cycle. It is used to run very slow. Fast cycles are adiabacs, and slow cycles are isotherms, which are used to generate electricity. First we conduct a experiment in which the hot reservoir was located at the bottom of the apparatus while the cold reservoir was on the top side of the apparatus.

Here is a video that shows how the stirling engine works, when the bottom is hot and the top is cold. The revolutions per second increases as the temperature difference increases. In this case the fan rotates counterclockwise.

Then professor Mason, reverse the temperatures, by using ice to cold the bottom side, and add hot water to the top side of the apparatus. Causing the fan to rotate clockwise.

Here is a video of reversing the temperatures. We can see, when professor Mason tries to make the fan spin counterclockwise, the fan just goes clockwise.
Stirling engines generates double the electricity compared to solar panels.

Here we calculated the efficiency of the stirling engine cycle.

Professor Mason introduced the idea of coefficient of performance, which is the ratio of the heat removed over the work. Also the maximum coefficient of performance is give by the ratio of the temperature of the object over the difference in temperature of the outside and the object. In this example we used the maximum COP to find the heat of the refrigerator.

Here is an example of the effectiveness of a heat engine, which is the ratio between the actual output over the maximum output of energy by reversible process.

This was another working example of effectiveness.

This was a refrigerator freezing ice problem that consisted on finding the maximum COP and the time to freeze 4.2kg of water at 18 degree Celsius.

Then we conducted an experiment about bubbles. First when professor Mason blowed through the apparatus the bubbles fell down.

In this video we can see that when the bubbles are blown by natural gas, then they tend to go up.

This video shows that if you burn the bubbles made of natural gas, they will burn.

Here is professor Mason playing with fire.