Week 4

April 21, 2016

In week 4 we made big strides in constructing the heat pipe. We started by using the pipe cutter to cut off a 2 ft. long piece of copper pipe to be used as the heat pipe. We chose to use this length because we felt that it would be sufficiently large to serve as a model or prototype for other heat pipes. We were not as much concerned with the pipe itself as we were with conducting a thorough wick analysis.

We then cleaned both ends of the pipe and coated it in flux. The cap piece was placed on one end and soldered, and the adapter on the other end and soldered. Both pieces were soldered securely and the connections were waterproof.

We then went to insert the wick, and encountered a bit of a problem. Initially, we thought that we would simply roll the screen a few times over and slide it right into the pipe. However, this was more difficult than anticipated, since the screen rolled unevenly and had a tendency to crease if bent. In addition, the free ends of the screen had wires poking out in many directions, which made it difficult to simply slide the screen into the pipe.

We did manage to find a solution to this problem. We rolled the screen around a long wooden dowel, which helped to provide structure and prevent creasing. This gave us a tight, uniform roll of screen. By creasing a piece of the screen over the end of the dowel, we could then use the dowel as a sort of ramrod to push the screen role all the way in the pipe, removing the dowel at the end.

A wooden dowel was used to provide structure to the rolled screen

PTFE tape was then wrapped around the threaded part of the adapter. This would ensure a secure, watertight seal when the cap was tightened on top. A 1/2 ft. piece of copper pipe was cut off (from scrap pipe) and filled completely with water. This acted as a measuring device so we could fill about 1/4 of the heat pipe with water. We then capped the pipe and inverted it many times to ensure that the entire wick was moist. It is crucial that all parts of the wick are moist before building up pressure in the pipe to ensure that the vapor will condense properly during operation.

A piece of copper pipe was used as a measuring tool to fill the heat pipe

Now came the crucial part of the heat pipe construction. We removed the cap, and securing the pipe vertically, heated the bottom end. As soon as vapors were observed at the open end of the pipe, we quickly secured the cap and tightened it with a wrench. The heat pipe was now complete, but we would have to test it to check if it was working properly.

A test rig was assembled as follows. A plate stand was placed on the table top, and a rubber clamp attachment secured to the vertical support rod. The heat pipe was secured in the rubber clamp and tilted to make a roughly 45 degree angle with the horizontal. A hot air blower was placed on the table adjacent to the bottom end of the heat pipe. A glass shield was placed on the other side to prevent the hot air from directly blowing onto the wall behind. One probe of a temperature sensor was placed in contact with the non heated (higher) end of the heat pipe, and secured next to the pipe with electrical tape. The other sensor was left free to be periodically placed in contact with the heated (bottom) end of the heat pipe.

The hot air blower was then turned on and the waiting began. If the heat pipe was working properly, we would expect to see similar temperatures at both the heated and non-heated ends of the pipe. This would signal that the working fluid is effectively transferring heat away from the heated end.

However, our heat pipe was not working as intended. The heated end of the pipe reached roughly 500 degrees F, but the non-heated end never made it above 100 degrees F. But an important observation was made by moving the probe in contact with the heated end to different parts of the pipe. We recognized that when the heated end was roughly 500 degrees F, the middle of the pipe was around 300 degrees F. This meant that our heat pipe was working properly, but the working fluid was condensing well before it reached the top of the pipe.

We recognized that one of two things could be done to remedy this problem. Our first option was to shorten the pipe to roughly half of it's original size. That way the top of the pipe would become the point at which the fluid was condensing. This option would require a bit of work, since the cap and adapter were already soldered to either ends.

Our second option was to insert more working fluid into the pipe. That way, there would be more liquid to vaporize, and it would remain in the vapor stage long enough to reach the top of the pipe before condensing. This was clearly the better option, as we would only have to add more water and recreate the partial vacuum. Unfortunately, we ran out of time, and did not get a chance to test the pipe with additional water added. Over the course of the next week we will be doing research to try to discover if there is an equation that will allow us to find the exact percentage of the pipe that needs to be filled with working fluid for maximum heat transfer.

Hopefully by next lab period, with these changes instituted, we will have a working heat pipe prototype, at which point the analysis of different wicks can begin.

-- Alec, Tran, Matt, and Shjon