3 reasons why Pulsating Heat Pipes (PHPs) offer an opportunity for design engineers

Let's start with their operation.

PHPs (or Oscillating Heat Pipes - OHPs) are typically composed of a continuous capillary tube or channel formed into a serpentine pattern, partially filled with a working fluid. The operation relies on the following principles:

1. Heat Absorption and Vapor Formation: At the heat source, the liquid working fluid absorbs heat and vaporizes, expanding vapor plugs within the capillary tube.
2. Heat Rejection and Condensation: At the cold source, the vapor condenses back into liquid, expanding liquid slugs. The heat is dissipated through conduction/convection in this zone.
3. Pressure-Driven Oscillations: The expansion and contraction of these vapor plugs and liquid slugs creates pressure differences, pushing the fluid alternately throughout the continuous tube. This pulsating or oscillating motion moves heat away from the source.

Scientific research has been evolving to support the increased interest in the technology.

Traditional research following the invention of PHPs was focused on experimentation, typically looking at how design parameters alter performance - understandably given the lack of widespread knowledge of the operation and therefore design.

As with any new technology, particularly two-phase efforts were made to model the physics in order to predict performance and improve design. However, modelling any kind of two-phase system is not so easy - as we all know. For PHPs, the especially chaotic fluid behaviour make it even more difficult to model compared to other forms of heat pipes. There is also no widely-used commercial software to design PHPs, which does exist for other types of heat pipes.

In the past decade, more and more research has focused on PHP applications. This is great to see as this will provide a path for this technology to commercialized for real-world applications. As this progresses understanding will improve and in tandem performance and ease of design will also move forward.

Other recent trends have investigated the microscopic physical phenomena within PHPs, such as liquid thin film behaviour on the inner walls of the tubes/channels. It is always tricky to study these aspects given that creating PHPs with transparent materials where you can observe/record the vaporization/condensation affects is extremely challenging given the need for a closed loop.

Here is a couple of recent papers my colleagues noted on the topic:

• Innovations in pulsating heat pipes: From origins to future perspectives
• Physical principles and state-of-the-art of modeling of the pulsating heat pipe: A review
• Applications of pulsating heat pipe (PHP) as an efficient heat transfer device: a review of recent developments

The challenge of industry adoption

While the topic is increasingly popular there is still limited adoption in terms of actual production. Calyos has a number of ongoing research projects that are working in this direction but they are not there yet.

There are a few obvious reasons for this:

1. Prototyping a continuous serpentine is not easy and often requires techniques like 3D printing, not ideal for mass production.
2. Modelling as I have already mentioned is a particularly acute challenge given the complexity in doing so.
3. Awareness, knowledge and expertise are all limited - hopefully we can do something about this.

#5 Tips to design better PHP systems

To optimize PHP performance, keep these aspects in mind.

• Fluid selection

The fluid's properties must be matched to the operating temperature range and compatibility with chosen materials. There are a host of different fluids that can be used inside PHPs as with any two-phase system.

• Internal Tube Diameter

It is best to utilize the largest possible diameter within the PHP’s critical range for the working fluid. Obviously the mechanical constraints of the application are also a limiting factor here.

• Evaporator configuration

Ensure sufficient heat load is important for both starting the oscillation and  sustaining it. Obviously this also needs to be managed with other design choices to avoid dry-out.

• Number of turns

Maximizing the number turns provides better performance, especially for horizontal or anti-gravity applications. Obviously this is a trade off with size and ease of production.

• Filling ratio‍

Different filling ratio’s will yield different performance. It is important to test prototypes at different ratio’s in order to find what is optimal for your application - in general the optimum lies around ~50%.

If it is nothing but challenges, why consider PHPs?

Firstly, they bring with them what all passive two-phase technologies bring; reliability thanks to no moving mechanical parts, native passive operation and great thermal performance. Of course all of these factors are determined by a well executed design, with good material selection and mechanical design. Not to mention the solution must fit your application.

So what is unique about PHPs in contrast to other two-phase technologies, well let's revisit the list I mentioned in the introduction and expand on this a little:

• Unique Form Factors
  Given their continuous serpentine, they can be embedded inside flat plates or 3D mechanical parts (think fins). This is quite different to the tubular traditional heat pipes and the two-phase loops with dedicated evaporators. 3D designs in particular offer the opportunity to enhance the performance of fins, transporting heat throughout them - something that could be particularly advantageous for natural convection applications.
• Gravity/Accelerations
  Through effective design, Calyos and others have been able to produce systems capable of transporting heat against gravity. Just as with our LHP systems this offers the potential, when combined with a new form factor, to really target unique applications that before were not viable. However further research is required to determine the true effectiveness and corresponding design constraints to achieve this.
• Low cost, wick less production design
  The actual design of a system once defined is often relatively straightforward and offers the potential to be easily mass producing using CNC milling and friction stir welding. I should note that this would be for flat plate designs, while 3D designs remain more complex. In our experience systems also do not need a wick structure inside, this removes a complicated part of the manufacturing process.

3 Promising PHP Applications

1. 3D-Printed Heat Spreader
Calyos developed the world’s first 3D-printed copper PHP heat spreader. The project highlights a centralized evaporator design with precisely manufactured spiral-shaped channels, with an ability to dissipate a heat flux up to 50 W/cm². The optimal performance was reached with a working fluid filling ratio between 39-59%.

‍2. Networked PHPs for Satellites
Researchers proposed a satellite structure integrating "networked PHPs" to manage thermal loads efficiently. This innovation improves performance while reducing weight, crucial for space applications​.

Read the paper titled: Thermal and Structural Performance of a Small Satellite with Networked Oscillating Heat Pipes

3. Heat Lane by TS Heatronics

In the beginning of the 2000s,  a flat aluminum PHP was designed by H.Akachi, the inventor of PHPs, and sold by his company TS Heatronics under the trade name “Heatlane” for the heatsink of computers. To the best of our knowledge, this is the earliest (and maybe the only) commercialized PHP for non-space applications.

Image Source: Frosty Tech

The Future of PHPs in Thermal Management

The future of PHPs lies in advancements in material science and simulation tools, enabling better performance and scalability. Ongoing R&D efforts include integrating PHPs with nanofluids and exploring modular designs for diverse applications. Despite hurdles like cost and production scaling, PHPs remain poised to revolutionize thermal management across industries​​.

A small thank you.

When putting this together I relied heavily on two of my colleagues, Naoko and Vincent. Without them this article would have certainly lacked the depth of information I was able to include.

This week Vincent has been at the Joint 22nd International Heat Pipe Conference and the 16th International Heat Pipe Symposium in Thailand, where he has presented three papers on the topic:

• Experimental study of pulsating heat pipes for cooling the stator of an electrical machine
• Heat Transfer Performance of a Six-turn Pulsating Heat Pipe for Aeronautical Application under Vibration Environment
• Experimental investigation of the effect of the vibration on the operation of the direct contact condensation “box” component of a Capillary Jet Loop

More information on these papers will be released in due course, please check out the publications page on our website for the latest updates.

Naoko has a history working with the Japanese Space Agency (JAXA) and studied in both Japan and Italy.

➜ Research Gate Profile

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Vincent has been with us since the start, and also with Euro Heat Pipes before that.

➜ Research Gate Profile

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