What now?!

Lizard dance celebration (Photo credit)

This will be the final blog post in this series as I have now graduated as a MSc in engineering design and applied mechanics. Wooop! In this post I will report on the work carried out between handing in and presenting my thesis. I will summarise the feedback that I received and then list some practical advice for those interested. 

Heat pipe with luer and end cap

Final work

In the last 10 days before presenting, I felt it would be worthwhile to try and charge/seal the prototypes and then thermally test them. However, I was never able to evacuate the heat pipes correctly or seal them after the charging attempt. It was pre-mature going ahead with prototyping and it would have been better stopping sooner and writing up on the results, even if it meant that the heat pipes were not created.

Testing set up (HP highlighted in red)

However, regardless of the seal, I still wanted to go ahead with the testing to see what kind of heat conductivity I might get out of the prototypes (stubborn). I set up the apparatus as shown on the right, with the heat pipe clamped gently in the heater (the evaporator end) and a fan set up at the condenser side (to focus the flow, I made a cardboard box element to focus the flow - left image). It was necessary to deflect the forced cool air from the hot end and so a guard (aluminium foil) was placed between both ends. As the HP was not filled correctly, the applied heat only increased the temperature at the hot end and there was an accumulation of heat there. After reaching around 90 °C, it was removed and it was observed that the Si seemed to warp. The results were not conclusive but due to time limits I took some readings using a laser infra-red thermometer which was awkward due to the experimental set up. This was a good example of how not to run an experiment...

Feedback

Although the literature review and presentation was good, I lacked enough analysis and structure relating to the experimental results. I should have focused on the wick structure element and characterised it more thoroughly, perhaps by comparing it to other wick structures currently available. I didn't persuade the examiner that by further investing effort in a lizard inspired wick surface, a more effective and economic solution could be found for the cooling problem. But the biggest rookie mistake I made was that I did not summarise clearly the results of the hypothesis in the summary. These were expensive mistakes...

 

Final words of advice

Here are some final words of advice which might help you out: 

• Create a deadline for when to stop creating new data, finish experiments, develop prototypes etc. Focus on the analysis of the work you have done to that date as you will be more thorough and scientific in your analysis. I would suggest stopping 3 weeks before you hand in the thesis. I found this hard to do...
• Present your supervisor with bullet point lists of what you have accomplished every week in a paragraph. Less is more for these busy people!
• Order parts and components early in the process and make sure you have funds (ask your supervisor). However, I would suggest only ordering parts when you know they will work...or at least are nearly sure they will!
• You have to hand in the thesis in hard copy and electronic. I think the number of hard copies vary depending on the department/supervisor and number of examiners.
• The presentation can last 30 minutes. After this, there is a round of questions which lasts approx. 45 minutes. There can be questions from the examiners (two in my case, one being my supervisor) and I also had a question from a member of the audience. I don't think that inviting guests is of any benefit to your presentation but then again, maybe you might feel more like a rock star. You are then asked to leave where they will make the judgement.
• I had too much to say during the presentation and although I practised it with a friend beforehand, I felt I was rushing through it. The feedback on the slide show was good but I think that I should have reduced the amount of content and chatter. You can find the presentation below (Prezi of course):

 

Thank you/mange tak!

Thanks for reading and keep in touch with me through LinkedIn: 

dk.linkedin.com/in/bryanoregan/ or visit my website: www.suiledesign.com - always interested in collaborating on projects when I can. Good luck! Bryan.

Charging and sealing the prototype before the big day

A selection of the FMHP prototypes

Last week I handed in my thesis one hour before setting off to Cork in Ireland for a family holiday. I now have to send in two hard copies to my supervisor from Ireland. My defence is on Thursday 12 September but before then, I have to get the prototypes working. I return back to Copenhagen on Monday 10 September which leaves very little time to fix some big issues including:

1. Charging and sealing the flat micro heat pipe (FMHP)
2. Setting up the equipment for testing the heat transfer capabilities of the FMHP and working out the overall thermal conductivity of the prototypes
3. Organise the data and compare to literature 
4. Putting together the presentation and other stuff that I hope to use for the defence.

Leak tests

I set up a simple leak test to check whether the glue had worked on the prototype FMHP. I set up a test as shown in the image below, sucking out the air, closing the valve and then dipping the end of the tube in water. Opening the valve allowed the water to be sucked through the tube, hence proof that a vacuum existed. However, there could be slow leaks, given time, would relieve the pressure - to be sure, I added an extra layer of sealant. 
 

Leak test set up (red areas show potential leak points)

Charging and sealing

Each prototype has an inlet consisting of PTFE tubing with 0.6 mm outer diameter. I am going to attempt to fill/charge the FMHP using a syringe vacuum technique. I will try to evacuate air from the FMHP, then use the internal vacuum in the FMHP to suck in a volume of fluid which should wet the wick section. Then I will crimp shut the PTFE tubing.

FMHP charging set up

The biggest headache thus far has been figuring out how to crimp the FMHP shut. I have tried different methods and tested the crimp by flushing air through and watching for bubbles in a water bath.

Needle inserted in PTFE tubing and crimped - FAIL.

So far I have tried three different crimping methods and they have all failed:

1. Metal sleeve crimp (as in the last blog post) - this cracked the PTFE tubing and leaked
2. Needle crimp - I inserted a steel needle into the PTFE tube (applying glue) and then crimped the steel. I tried a number of crimps, at different angles on the needle but it still leaked. Out of five attempts, one seemed to hold but there was still some air bubbles visible at the outlet. One small bubble seemed to just stick at the end of the needle and I am not sure why this happened.
3. Tube bending - I tried to bend the tubing 180 ° and tie the tube in this position but it still leaked.

I am now looking at finding a Teflon sealed adaptor or something that will allow me to inject fluid and then pull out the syringe without losing vacuum pressure and/or fluid. Hopefully someone at DTU Nanotech can help with this.

Testing equipment

Once filled and sealed, I will have FMHP ready for testing. With testing, I will have to apply heat at one end of the FMHP over a small area and then measure the temperature at the other end in order to calculate the over all heat transfer. Practical considerations for the experimental set up will include:

1. Heater with controllable hot plate that can preferably apply a controlled amount of heat to the Si side of the FMHP. This might involve creating a metal shim 1 cm² in area and surrounding it with insulating material so that heat is focused on a particular region (as in a CPU cooling situation). A thermal insulation paste will be needed to create good contact with the FMHP.
2. Thermometers to read the temperatures at different points on the heat pipe. It will be necessary to glue the thermometers which means I need approx 6 per FMHP prototype - 2 at the hot end (evaporator), 2 at the centre (adiabatic region) and 2 at the cold end (condenser). They will have to be glued (i.e. one use only) so I will also need a conductive glue.
3. Prof. Larsen suggested a water cooler at the cold end where the inlet and outlet water temperature can be measured (more thermometers). This will involve a structure design which will allow the end of the FMHP fit in securely and be water tight.

And this all depends on whether these prototypes:
a) Melt - PVC can start to soften at 100 °C so I have to keep the temperatures below that which is the case for CPUs which need a safe operating temperature of approx. 85 °C
b) Leak - Will the glue crack and let air in or will the tubing connections leak?
c) Crack under vacuum - Will the combination of heat and pressure crack the plastic? Are there hidden cracks in the Si from my disastrous labelling method (using a glass cutter on Silicon!)
d) WORK!