Week 7

May 12, 2016

In week 7 we continued to work towards improving and refining the current heat pipe design.

After completing the analysis of the data from both trials (as shown in the final report and the analysis section of the blog), we noticed that smaller wicks displayed greater capillary action, which allowed for greater rates of heat transfer. However, even with using our smallest available wick (22 hpi), the equilibrium temperature of the cool end of the pipe was roughly 40°F less than the temperature at the warm end of the pipe.

From this we thought of two ways that the pipe could be improved so that the temperatures at both end of the pipe could become roughly equal, at which point the heat pipe would be operating at maximum efficiency.

Possibility 1: change the amount of working fluid present in the pipe

From careful literature study and analysis of the merit numbers (see analysis section) for various types of working fluids, we already knew that water would be the optimal fluid to use in the final design. What we didn't know, however, was how much of it should be used.

Theoretically, we wanted to use just enough so that the heat pipe would strike a balance between condensed fluid and vaporized fluid. We wanted the rate at which water was vaporizing inside the pipe to equal the rate at which it was condensing.

Our initial design was filled 25% of the way with water. This was definitely too little, as our heat pipe did not work as intended. During that trial we noticed that the middle of the pipe was reaching equilibrium, but not all the way to the cooler end. Because of this, we knew that the water was condensing before it reached the other end of the pipe, so we needed to add more.

Our past two trials have been conducted using a heat pipe filled slightly less than 50% of the way with water. While there was now sufficient water for the heat pipe to properly work, the equilibrium temperatures of both ends of the pipe were vastly different.

This led us to begin the week 7 lab by reducing the amount of working fluid in the pipe to 35% of the total pipe volume. We believed that we hit upon the two extremes with our first and second sets of trials, so we chose to fill it to a level in about the middle of that range.

We went through the same process of opening the pipe, draining then refilling the water, heating until vapor was observed, and securing the cap in place. Due to the large number of groups testing their pipes, we did not have time to test out our pipe with this configuration. Next week we plan to test it and analyze the results, to determine if our change in the amount of working fluid positively or negatively impacted the performance of the pipe. From those results we will be able to deliberate on further changes to be made to the design.

Even with the change to the working fluid, we still wondered if that would be enough to allow the heat pipe to operate as efficiently as possible, so we also thought of a second possible change.

Possibility 2: change the exposed surface area of the cooler end of the pipe

When the internal water vapor transports energy in the form of heat from the heated end to the cooler end, the cooler end starts to heat up. Eventually, the cooler end heats to the point where it no longer is cold enough to condense the water vapor.

The solution to this problem would be to increase the surface area of the cooler end of the pipe. An increased surface area would provide more area for the heat to spread out and dissipate into the surrounding environment. Since the cooler end of the pipe will thus stay cooler for longer, it can take on a larger amount of heat, allowing equal temperatures to be established between the heated and cooler ends.

This solution could be implemented by securing metal fins to the cooler end. These fins can be either crude sheets of metal or professionally manufactured. Either way, by securing fins onto the cooler end of the pipe, the surface area will be increased and hopefully a closer equilibrium between the two ends can be established.

For the time being we are focusing on possibility 1, changing the amounts of working fluid. After testing and analyzing our results from next week, we will be more certain as to whether we should continue to make changes to the amount of working fluid or if we should proceed with securing and testing the metal fins.

We look forward to continuing to optimize our heat pipe design with high hopes that we can get it to operate near maximum efficiency.

-- Alec, Tran, Matt, and Shjon