Week 6

May 5, 2016

In week 6 we made substantial progress towards collecting the data we need to write and develop our analysis.

We began by testing our modified heat pipe. If you can recall from the previous week, we increased the amount of working fluid in the pipe, from 25% to 50%, in hopes that it would be sufficient to establish an equilibrium between evaporating and condensing fluid.

If too little fluid was in the pipe, like it was during our first test in week 4, the water would condense before it reached the top end of the pipe, which wouldn't allow for heat transfer over the entire length of the pipe. Too little working fluid would also run the risk of all of the fluid evaporating, which would render the heat pipe entirely ineffective.

If too much fluid was in the pipe, the vapor pressure in the empty pipe portion would be higher, not allowing for much fluid to evaporate. This would also render the heat pipe ineffective, since the fluid cannot evaporate and transfer heat away from the source.

Remember, the goal of our project was to create a working heat pipe then conduct a wick analysis comparing the rate of heat transfer of three differently sized wicks. We were not as concerned about our heat pipe operating at maximum efficiency, we just needed to ensure that it was working before proceeding with other tests on different wicks. Since we only had a limited number of tests we could conduct due to time constraints with the testing apparatus, it was more important to keep fluid levels and pipe orientation constant across all trials (in order to create a legitimate analysis) rather than proceeding with future modifications to improve general efficiency.

Trial 1 was conducted using a wick with 22 holes per inch (22 hpi). The blue curve shows the temperature of the heated end of the pipe as a function of time. The orange curve shows the recorded date for the cooler (higher) end of the pipe).

After the first trial was complete, we replaced the old wick with one of a different size. Fluid was completely drained from the pipe and carefully measured and refilled.

When the old wick was removed, many of its capillaries were saturated with water. We could not possibly know how much water was being lost, so we decided to completely drain the pipe and refill it with new working fluid, to ensure near constant conditions from trial to trial.

Careful note was also made as to the angle of inclination of the pipe during testing, to ensure that it could be returned to this angle for future trials.

We finished off the new pipe by taping the threads with PTFE tape and again heating it from the bottom until vapor exited from the open end. At this point the cap was quickly secured and the new pipe was ready for testing.
Trial 2 was conducted in a similar fashion using a slightly larger wick with 16 holes per inch (16 hpi).

The rate of heat transfer for each wick can be found by analyzing the curved portion of each graph. The linear portion before the curve reflects a period during which most of the water is condensed, and little is vaporized and contributing to the transfer of heat. The linear portion after the curve is a state of equilibrium between the transport of heat to the cooler (higher) end of the pipe and heat loss to the environment. If the experiment were left to continue past 720 seconds, this line would remain nearly horizontal, as the system is in balance between the upwards transfer of heat and the release of heat to the environment.

We observed the curved portion of the graph from trial 1 and noticed that it spanned the range t = 300s to t = 570s. We used the below equation to calculate the average rate of heat transfer.

heat transfer rate = ΔT / Δt

We found that the rate of heat transfer for trial 1 with the 22 hpi wick was 0.085°F/s. The same analysis was done for trial 2 and the 16 hpi wick and we found that the rate of heat transfer was much lower, at 0.052°F/s.

This finding confirmed our hypothesis that a smaller wick size would result in a greater rate of heat transfer. The smaller wick provided better capillary action to efficiently return condensed water to the heated end of the pipe for re-evaporation.

We also noticed that the equilibrium temperature for trial 1 was 102°F, 10 degrees higher than the equilibrium temperature for trial 2, which was 92°F. This bolsters our belief that the smaller wick resulted in a more efficient heat pipe, since a greater amount of heat was able to be transferred to the cooler end of the pipe.

We finished by reflecting on some possible sources of error for our experiment. We immediately went to the temperature probes, and noticed that they did not maintain the same amount of contact from one trial to the next. It was difficult to keep the same surface area of the probe in contact with the curved edge of the pipe, which likely skewed some of the temperature readings. We also thought about the wick, and how it could have become bent or deformed when it was inserted into the pipe. Any tight bends or crimps could have negatively affected the ability of the wick to transport condensed water.

All in all, week 6 was a big success, with a few mistakes that we can learn from and improve upon. We were able to collect data for two different sized wicks and graph temperature vs time to start constructing a relationship between rate of heat transfer and wick size (holes per inch). However, due to a few external variables, conditions were not constant from trial to trial and likely caused our analysis to be slightly skewed.

Now that we are reaching the conclusion of the project, we have reassessed the project budget and materials list and noticed that the flux, solder, pipe cutter, and one roll of screen, as well as some extra caps and fittings were not used for the heat pipe. These additional materials have been returned, and our savings reflected in our finalized budget below.

Category	Cost
0.5 in x 5 ft Copper Type M Pipe	$6.76
Threaded Steel Cap	$1.57
Copper Cap	$0.67
Metal Mesh Screen (3)	$25.43
PTFE (Teflon) Tape	$0.97
Threaded Adapter	$1.42
8% sales tax	$2.95
TOTAL	$39.77

By borrowing many materials that we needed to complete the pipe, we were able to save roughly $80.00 off of our original estimate of $119.54.

Heading into week 7, we look forward to potentially conducting a third test with a larger sized wick, to obtain a better idea of how wick size relates to heat transfer. We hope to learn from our mistakes and keep the external variables nearly constant to the other two trials for this upcoming test. We are eager to get started on drafting our final report and analysis for the upcoming week.

-- Alec, Tran, Matt, and Shjon