Pulsating Heat Pipes

I’d started off on a bit of ‘recreational thinking’ (the first time in a week or two… as the brain has been consumed with learning a new job from a dead halt to productive in 5 days… then polishing it for a week…) about Regime Change in heat pipes.

I didn’t get very far into it before I was pulled into another direction.

I’m still interested in pursuit of Regime Change (the idea being that, at some very low temperature, the working fluid would start to freeze and heat transfer would slow, most likely quite a bit. While at very high temperatures, there might also be “issues” as the return flow in the wick of the sidewalls would tend to evaporate prior to reaching the hot end, restricting max heat transfer rates to the ‘sweet spot’ in the middle. (So my thesis goes… still to be investigated, then generalized to our Heat Pipe Earth…)

But along the way I ran into a Very Odd Gadget. Seems like about the 1990’s, a new kind of heat pipe was invented. The Pulsating Heat Pipe. It is generally made of small diameter tubing such that there is not much room for a wick, but the liquid will form capillary ‘plugs’ of liquid mixed with vapor ‘slugs’ in between. As heat is applied, the ‘plugs and slugs’ have interesting dynamics… That is, nobody really knows what happens, but a lot of evaporation, condensation, liquid mass transfer and rapid heat transfer all goes on in very complex ways. Rather, I might add, like the way the earth tosses tons of water up in the air, sometimes to fall as snow, some times as rain or hail, sometimes to just stay there for a few weeks as it goes to the other side of the planet…

Lots of dynamics, oscillations, and very effective heat transport. (Heat Pipes are more thermally conductive than solid metals…)

OK, back at the Pulsating Heat Pipe, or PHP. It’s “different” in that the liquid does not do a nice ‘evaporate here, condense there, wick back’ (or, in the case of vertical large diameter heat pipes, fall as rain back to the hot end.) No, anytime much fluid condenses, it makes a capillary plug. Clearly that is NOT like what happens on the earth. Yet, there are parallels.

Since the PHP is a sealed system, pressure changes in one place influence the rest of the system, as does mass flow. Seen from space, our atmosphere is also a ‘sealed system’ of sorts. Stuff held in by a constant 1 G of gravity. Push here, it wobbles over there…

Similarly, because of those teleconnections, the dynamics are prone to oscillations and instabilities along with very complex behaviours. I have to think that a PHP is a reasonable ‘test subject’ for trying to understand such oscillating closed hydro/vapor dynamic heat flow driven systems. So what can we say about this very simplified “toy world” model of a single pipe with a single working fluid with a single phase change with a single hot pole / cold pole? How hard can it be to “model” and understand?

http://www.electronics-cooling.com/2003/05/an-introduction-to-pulsating-heat-pipes/

says:

Although grouped as a subclass of the overall family of heat pipes, the subtle complexity of internal thermo-fluidic transport phenomena is quite unique, justifying the need for a completely different research outlook. A comprehensive theory of operation and a reliable database or tools for the design of PHPs according to a given microelectronics-cooling requirement still remain unrealized . Nevertheless, the prospects are too promising to be ignored.

Oh. Can’t quite do it yet. I see…

The wiki on heat pipes hardly even hints at them, with the only even partly relevant reference being something they call a Loop Heat Pipe that sounds similar, yet different and in perhaps even more complex ways…

At any rate, that first link has some very nice pictures (that I can’t put here as they are not open source) so you will have to hit the link to see what one of these things looks like. They come in several types and the article does a very nice job of explaining what they are and what they are good at. Even if you don’t think you are interested in the physics of it, realize that these things are going to be popping up in electronics all over the place…

Although a plethora of designs of classical heat pipes are available, recent industry trends have frequently shown the limitations of these conventional designs. This has led to the evolution of novel concepts fitting the needs of present industry demands. A relatively new and emerging technology, Pulsating or Loop-type Heat Pipes (PHP), as proposed by Akachi [3, 4], represent one such field of investigation. This range of devices is projected to meet all present and possibly future specific requirements of the electronics cooling industry, owing to favorable operational characteristics coupled with relatively cheaper costs.

So it’s worth a read of the article just to know what those ‘bare wires’ are doing wandering around between the chips in some of your equipment…

The description of their internal activities is somewhat reminiscent of things we see in the atmosphere too:

The tube is first evacuated and then filled partially with a working fluid, which distributes itself naturally in the form of liquid-vapor plugs and slugs inside the capillary tube. One end of this tube bundle receives heat, transferring it to the other end by a pulsating action of the liquid-vapor/bubble-slug system. There may exist an optional adiabatic zone in between.

Water distributes itself naturally around the planet in the form of liquid puddles and vapor wads, but also as water droplets of various sizes and bits of frozen droplets, large frozen chunks, and the occasional slushy bits… SO a bit more complex. As one end of the earth heats, transferring it to the other end in pulsating actions of the liquid-vapor / puddle-cloud system. And there are plenty of adiabatic zones in between… Oh, and when the electrostatics get mixed in it can be VERY highly pulsating… not to mention shocking and deafening with the thunderclaps and lightning.

So similar in some ways, but devilishly more complex.

Any other interesting bits? There is an approachable discussion of how the thermodynamic imbalances drive the system. Complete with a nice pressure / enthalpy graph. Best I can do without permission to copy the graph is to provide the caption:

Figure 3. Typical pressure-enthalpy diagram.In this way a sustained non-equilibrium state exists between the driving thermal potentials and the natural causality, which tries to equalize the pressure in the system. Thus, a self-sustained thermally driven oscillating flow is obtained in a PHP. Note that no “classical steady state”‘ occurs in PHP operation. Instead pressure waves and pulsations are generated in each of the individual tubes which interact with each other possibly generating secondary and ternary reflections with perturbations.

Gee… “self sustaining thermally driven oscillating flow”… sounds rather like a lot of weather systems to me… and some of those climate systems like the PDO / AMO et. al. too… And yes, no “classical steady state” to hang your theories on. Pressure waves and pulsations reminds me of a variety of odd waves found in weather. Yes, the more I read that article, the more I see reflected in it the same kinds of drivers found on Earth in weather. And, through it, in climate. And the more I look at steady state arguments about radiative heat transfer the more I wonder “Have they ever seen snow? Or a thunderstorm?” Perhaps we ought to start sending them PHP examples (if they are cheap enough to cool chips, they are pretty cheap…) and simply ask: “If you can’t model this, what makes you think you can model the Earth?” And if it works BECAUSE of those waves, oscillations, mass flow, and dynamical behaviours, what makes it OK to ignore them in the Earth System?

But surely it can’t be THAT hard to understand a small tube partly filled with liquid?

Note that if a flow regime changes from slug to annular, the respective roles of the latent and sensible heat transport mechanism may change considerably. This aspect requires further investigation [10].So, the performance not only depends on a large number of parameters described above, but also on the flow pattern. This makes it all the more difficult to undertake mathematical modeling using conventional techniques.

Hmmm…. “Regime Change”… Seems all the vogue these days (though not yet in Libya ;-) and very hard to predict in all cases…

Oh, and gotta love that “difficult to undertake mathematical modeling” part…

OK, now just make it a sphere of a few thousand miles radius, have several working fluids, multiple phase changes, and add some external variable heat flux with variable spectrum, and some external variable electrical flux, along with changes of absorption of that flux (albedo change of ice and clouds). Oh, and don’t forget to spin the whole thing too…

No wonder the “Climate Clown Car Scientists” like to just assume all that stuff away and play with CO2.

Under a picture of what looks like a miniature role of chain link fencing, there is a caption that I just love. It’s talking about performance. Now, just remember that these same evaporative and mass flow activities are happening in the sky. What kind of heat transfer quality is available?

been applied for cooling multi-chip modules [3]. The thermal performance data is also included. The advantages of such systems can be summarized as follows:
Compared to solid metal fins, this type of fin structure is certainly light weight.
If optimal operation is achieved, this fin structure is thermally an order of magnitude better than equivalent solid fins even with an air cooling option.
As the tube diameter is reduced, thermal performance of wicked conventional heat pipes is drastically reduced, while there is a parallel increase in manufacturing complexity and cost.

Got that? Not just a tiny bit better heat conductivity than solid metal, an “order of magnitude better”. IR is pointless in a system like that.

Oh, but the earth is much much larger, so surly that makes it different and not as effective? Well, larger yes… But the next diagram shows the change of thermal flow in conventional heat pipes with pipe diameter. As it gets bigger, heat flow improves… The limit tends to be mass flow back to the ‘hot pole’. As the Earth can do this by dropping a ton of water out of the sky in a second or two, it can have massively better heat flow in that direction…

I can do no better than to quote their conclusion verbatim:

Pulsating Heat Pipes, apparently simple and very promising cooling devices, are very intriguing for theoretical and experimental investigations alike. They are attractive heat transfer elements, which due to their simple design, cost effectiveness, and excellent thermal performance may find wide applications. Since their invention in the early nineties, they have so far found market niches in electronics equipment cooling. Their complex operational behavior, which is not yet fully understood, has raised an ever growing academic interest. Until now, it has not been possible to simulate the PHP performances and there exists no complete engineering design tools.

So despite not being able to simulate one, we can make and use them, and they are better than solid metal at moving heat.

Another Paper

I also ran into another paper, but it is a bit deep in the academic doo for me to read as I prepare to go to bed for 5+ hours and start work again. Perhaps on the weekend… For now, I’ll just point to a pdf download site and quote the abstract:

Download site: http://home.iitk.ac.in/~samkhan/Bio_data/publications/Khandekar_3.pdf

Originally found at:

http://academic.research.microsoft.com/Paper/4845142

Abstract:

Understanding operational regimes of closed loop pulsating heat pipes: an experimental study
(Citations: 5)
Sameer Khandekar, Nicolas Dollinger, Manfred Groll
Increasing performance of electronic components is resulting in higher heat flux dissipation. Two-phase passive devices are proven solutions for modern microelectronics thermal management. In this context, heat pipe research is being continuously pursued evolving newer solutions to suit present requirements. Pulsating heat pipes (PHPs), a relatively new and emerging technology is one such field of investigation. The operating mechanism of PHP is not well understood and the present state of the art cannot predict required design parameters for a given task. The aim of research work presented in this paper is to better understand these mechanisms through experimental investigations. Experiments were conducted on a PHP made of copper capillary tube of 2-mm inner diameter. Three different working fluids viz. water, ethanol and R-123 were employed. The PHP was tested in vertical (bottom heat mode) and horizontal orientation. The results strongly demonstrate the effect of input heat flux and volumetric filling ratio of the working fluid on the thermal performance of the device. Important insight into the operational regimes of the device has been gained. � 2003 Elsevier Science Ltd. All rights reserved.
Journal: Applied Thermal Engineering – APPL THERM ENG , vol. 23, no. 6, pp. 707-719, 2003

There’s more out there, but that will have to wait for another day. For now it’s enough to just look at a small tube and marvel at what it does, and that there are still some incredibly simple things in the world that we just don’t have the skills to model. Yet.

So, for now, I’m off to bed. To dream again of spherical heat pipes and a thermal transfer earth with heat flux better than copper… and just how pointless CO2 is to such a paradigm.

Oh, and two minor postscripts:

1) After turning off a bunch of “gesture” bits on the laptop I can now use it well without the occasional errant thumb causing “strange things” to happen. I’m starting to like it much more now… (If only I can find how to make the windows go to their old solid form and stop being semitransparent… not so good when you have a colorful wallpaper…)

2) France and Germany decided that since the PIIGS have been successful at tapping them out for largess, then F & G will just have to rope more folks in the Socialism Ponzi Scheme too, so proposed a tax on all financial transactions. Markets, of course, immediately tanked. Euro Socialism and new financial taxes are sure to do that. If only we could just muzzle all the politicians and set tax rates at “Maximum 10% of GDP PERIOD… but you can play with who and how…” we MIGHT be able to get the economy moving again… For now, it’s Sovereign Risk writ large and the Brazilian Real continues to rise along with the Swiss Franc. Be afraid, be very afraid, when Euro Socialists are trying to find new things to tax… The US Socialist / Liberal / Progressive / {whatever sheepskin du jour they hide under} can’t be far behind in their “tax envy”… For now, no investment in Euro Land can be considered safe. Too much “talking greedy heads” risk.

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