A few years ago, I became interested in cooling systems by technology Contour Heat Pipes (KTT). They have significantly lower thermal resistance than conventional heat pipes, can operate over long distances, and do not have a large performance degradation depending on orientation. Then I was confused whether ammonia as a heat carrier, or steel materials, or aerospace applications. Everything changed when I came across video about “dry water” and decided to return to KTT to find out more.

My name is Oleksandr – I am a lover of compact PCs. Therefore, it will be about the design of a low-profile cooler using KTT technology. I managed to find two interesting materials on which I (mainly) relied when developing the design. The first, in the magazine “Electronics” from 2017 No. 6by authorship Yu. Maidanika – one of the inventors of this technology. It is about theory and practical examples. Secondby authorship Patrik Nemec on the intechopen.com resource. There are very interesting results of practical experiments in search of truth. Next, I will refer to these two articles.

The working principle of KTT

1. Capillary pump. The porous structure of the Wick inside the Evaporator absorbs the incoming liquid and pumps it to the Heat Sink, creating a force of capillary pressure and passively moving the liquid inside the system.

2. Evaporation. The liquid is evaporated by the heat exchanger, which is in contact with the heat source. Through the steam removal channels of the wick, the steam is directed towards the condenser through the steam pipe.

3. Condensation. In the condenser, due to the temperature difference, the vapor condenses again into a liquid. The liquid is then returned to the Evaporator.

4. Groundhog Day. As long as heat is supplied to the heat sink, and the condenser has a lower temperature than the heat source, this process continues indefinitely. The system is very similar to an all-in-one water cooling kit. Only instead of a pump – an evaporator.

Evaporator

Like the impeller in the water cooling pump and in the Evaporator, the Wick is the main element of pumping the liquid. What will happen if the edge of a piece of sugar is dipped in hot tea? Everything is correct – it will instantly fill with water. This is exactly how capillary pressure works. Different liquids require their own optimal capillary structure of the wick. The main parameters of the capillary structure are its porosity and effective pore radius. Porosity is how much liquid it can hold, and the radius of the pores determines the size of the particles from which the Wick is baked. Powders of metals and their alloys can be used for this, as well as composite materials – and this again depends on the liquid used.

In his work Patrik Nemec (section 4) used specific examples to investigate the dependence of the Evaporator efficiency on the porosity and pore radius of the Wick. The conditions for its baking are also described there. As a result of the experiments, it was established that the optimal capillary structure for water as a heat carrier is a wick made of copper powder with a grain size of 100 μm and a porosity of 55%. The wick was sintered at a temperature of 950°C for 30 minutes.

in Article Yu. Maidanika on page 126, an experiment with KTT is described, where a cylindrical evaporator with a diameter of 10 mm with a length of the active evaporation zone of 40 mm and a condenser cooled by running water was used. In this experiment, it was possible to divert up to 300W of thermal energy. I extrapolated the specified evaporator to a horizontal plane and used the resulting dimensions for the wick. They fit almost perfectly into the dimensions of the usual heat sink (shoe) from the CPU cooler.

The main characteristics of the Vaporizer:
The material is copper
Wick size – 35x30x3mm (sintered copper powder 100μm)
Number of steam removal channels – 8 (1x1x27mm each)
The size of the heat sink in contact with the heat source is 40x38mm
Mounts for various CPU sockets are standard
Overall dimensions – 58x38x12mm
Probable output is 250W+

Capacitor

Another advantage of KTT is the ability to evenly distribute the cooling tubes over the entire area of ​​the Condenser, thereby increasing the cooling efficiency. When developing the design, I used two symmetrical snake-like contours, each 460 mm long. It seems to me that it will be more fun to pump the liquid in contrast to one circuit, which is almost a meter long.

The figure shows two versions of the capacitor. Option A – serpentine type, B – collector type. It is also possible to combine all 10 tubes into one collector in one circuit. What is better is not yet clear, but you can find out experimentally.

Main characteristics of the Condenser:
The material of the tubes is copper
The inner diameter of the tubes is 2.2 mm
The length of the condensation circuits is 2×460 mm
Steam pipe – 2×40 mm
Condensate pipe – 2×70 mm
The material of the cooling fins is aluminum
The total area of ​​cooling lamellas (on both sides) is 1,580 square meters.
Support for cooling fans – 92x92x15mm, 92x92x25mm
Fan mounting clips are standard
The overall dimensions are 94x94x19.5 mm
Probable output is 120W+

Heat carrier

As mentioned earlier, distilled water can be used as a coolant. In my opinion, it is more promising to use the so-called “dry water”. Example – “3M Novec 649 Engineered Fluid“. This liquid has almost four times better specific heat than water. It is half as viscous and boils already at a temperature of 49°C (1 atm), and freezes at -108°C. It (from the word at all) does not interact with copper, aluminum, tin and other metals. This liquid does not conduct electricity. It is non-toxic in such small quantities, although you certainly cannot drink it.

In his work Patrik Nemec (section 4.4) concluded that the optimal amount of liquid for his test bench was 60% of the internal volume of the entire system. This parameter is affected by both the length of steam-conducting and condensate-conducting channels. Therefore, it is better to establish the optimal amount of liquid experimentally.

Thoughts aloud

I suspect that in Contour Heat Pipes has the potential to create more efficient coolers for desktop PCs compared to existing heat pipe solutions. If it is possible to increase the efficiency by 20-25%, it will be quite a significant result. At the same time, the design itself is not much more complicated than ordinary coolers. And the phrase “Aerospace technology in your PC” – sounds very tempting.

Resources used:

1. Yu. Maydanik. “Contour heat pipes – highly efficient heat transfer devices for electronics cooling systems”. Magazine “Electronics” from 2017 No. 6.

2. Patrik Nemec “Porous Structures in Heat Pipes” at intechopen.com.

3. Elvis Liu from the company ID-Cooling (Help in the development of a cooling radiator).

4. Channel Thoisoi on YouTube (video about “dry water“).