In week 8 we had some great success with our third (and likely final) round of testing. The changes that we made to the pipe over the course of the previous weeks allowed the pipe to operate wit high efficiency, with the temperatures at both ends of the pipe being roughly equal as the system reached a state of equilibrium.
We began week 8 by testing our existing design. This was the design from the previous week, where we filled the pipe 35% of the way with water and used the 22 hpi wick (the smallest of the two).
We learned from trials 1 and 2 that there was a relationship between wick size (hpi) and efficiency of heat transfer. The smaller the wick size, the greater the capillary action the wick could provide to the condensing water, which translates to a more efficient heat pipe.
Additionally, from initial testing we learned that filling 25% of the pipe with working fluid was not enough to transfer heat all of the way to the cool end of the pipe. In trials 1 and 2, the pipe worked with greater efficiency as we filled it 50% of the way with water. However, from additional research, we hypothesized that reducing the amount of working fluid to some "happy medium" between 25% and 50% of the pipe's total volume would allow for maximum efficiency of heat transfer. We chose 35% to start, and planned to adjust this value in either direction as needed after examining the data from trial 3.
Just as a reminder here is a list of the changes made from trial to trial
Initial Testing: 25% water, 22 hpi wick
Trial 1: 50% water, 22 hpi wick
Trial 2: 50% water, 16 hpi wick
Trial 3: 35% water, 22 hpi wick
Below is a picture of the experimental setup. Due to technology errors these were not included in a previous blog post.
The temperature sensor is pictured in yellow. Notice how one probe was collecting the temperature near the heated end and the other probe at the cooler end. The heat pipe was secured in a temperature resistant rubber clamp, and kept at a constant angle across all trials. The hot air blower is shown in red, with the air directed towards the clear heat shield and the fixed cap end of the pipe. Not pictured is the timer and notebook, in which the temperature at each probe was recorded in 30 second intervals.
We initially thought that we would have to adjust the working fluid after Trial 3, however additional adjustments were not needed, as the data from trial 3 showed that the pipe operated near maximum efficiency. Below is the graph in the same format as before, with blue representing the temperature at the heated end of the pipe and orange showing the temperature at the cooler end of the pipe.
Using the same methods of analysis detailed in the week 6 post, the rate of heat transfer was calculated to be 0.136 °F/s. This analysis was done on the curved portion of the graph from t = 390s to t = 780s, which represents the time during which the working fluid was evaporating and condensing to influence heat transfer.
Not only was this rate of heat transfer far greater than any rate calculated from previous trials, but an equilibrium was established very close to the temperature at the heated end of the pipe. The cooler end of the pipe got within 6 °F of the heated end, representative of the success with which the working fluid was able to evaporate and condense using the wick to transfer heat away from the source. While ideally these two temperatures would be equal at equilibrium, we were quite happy with how close we were able to get them in a laboratory setting.
Going forward, we will be continuing with the analysis of trial 3 as well as the comparison between all three of the trials conducted. We also plan to go back and look at how our thought process and hypotheses evolved as we encountered different problems and obstacles from week to week. This reflection should influence our final presentation in week 10, where we can detail not only the results of the project but what we learned that will help us in a future setting at professional engineers.
We look forward to start on the final analysis and presentation going into the last two weeks of the term.
-- Alec, Tran, Matt, and Shjon
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