Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Turbulence around heat transport

04.12.2009
Not only in the Earth's mantle, in the atmosphere and in the outer layers of the Sun, but also in a chemical reactor, the exchange of heat may not be as effective as originally thought.

There, because warm fluid rises and hence induces movement, the turbulent convection can be 100 billion times stronger than in the typical cooking pot. Hot fluids mix turbulently with warm fluids.

As the temperature difference between the cold and warm sides increases, the heat transport increases exponentially. When the turbulence is very strong, the exponential growth decreases twofold. Physicists from the Max Planck Institute for Dynamics and Self-Organization, University of California at Santa Barbara, and the French Centre National de la Recherche Scientifique in Nancy report this discovery in the current issule of New Journal of Physics.

The long standing theory for turbulent convective heat transport from 1962 had predicted that the exponential growth would increase. Now, the theory will need to be reconsidered. (New Journal of Physics, December 1st, 2009).

In some respects the experimental apparatus of Eberhard Bodenschatz and colleagues is similar to a gigantic pressure cooker - even if the Director at the Max Planck Institute for Dynamics and Self-Organization calls it (due to its shape) the "Göttingen submarine". In the hermetically sealed submarine, a two-metre high container of one-metre in diameter is heated from below and cooled from above. In-between, a pressurized gas is mixed by turbulent convection, where hot water rises from the hot plate and sinks from the cool one. The main difference is that the convection in the "Göttingen submarine" is a million times stronger than in a cooking pot. With this, the scientists want to learn about turbulence in the Earth mantle, in the atmosphere and in the outer layers of the Sun, where the convection is yet another 100,000 times stronger.

"We have measured the heat transport of very strong convection and found that it is completely different from what we expect on the basis of previously established theory", says Eberhard Bodenschatz. The stronger the turbulence mixes the hot and cold gas, the stronger the heat transport from the hot bottom to the cold top will be - in essence the heat transport increases exponentially. The team measured this increase and found, surprisingly, that the exponent in the law decreases by the power of two. For a given temperature difference, not only one but two states were observed; once where the exponent falls from 0.308 to 0.253, and, sometimes, for a second time to 0.17. In 1962, the American physicist Robert Kraichnan predicted that the exponent should increase from 0.3 to 0.4 and then should be almost constant in this ultimate regime of thermal turbulence. "In the meantime we have conducted more measurements at the highest turbulence levels and found yet another state with possibly another exponent" says Eberhard Bodenschatz: "This time it may be the predicted Kraichnan regime. The multiplicity of states and the exponents baffles us, as the physical processes are yet to be understood".

To understand this better let's take a closer look at the "cooking pot" in the submarine. At the bottom and top plates, the heat is conducted through a few hundred micron thick thermal boundary layer into the gas. Here, a thermal plume develops which carries hot or cold gas into the interior of the vessel. It is well known that plumes of this type form a lava lamp - for yet unknown reasons however, rising and falling plumes merge to create one large circulation that flows up one side and falls on the other. According to Kraichnan's theory, this circulation should lead the boundary layer to become turbulent. From this point on, the heat conduction should increase more rapidly. "Instead the efficiency decreases and we find two states instead of one" says Eberhard Bodenschatz: "Somehow the boundary layers are changing, but we do not know how".

To investigate the heat transport in a planet like Earth or a star like the Sun is ultimately difficult. Even if scientists only want to investigate the turbulence itself, the conditions are difficult to achieve in the laboratory. Therefore the known experimental data are very limited. "Recently, with the submarine we were able to reach very high turbulence levels by using a two metre high container and sulfur hexafluoride (SF6) at 20 times atmospheric pressure" says Eberhard Bodenschatz.

The experimental data from Guenter Ahlers, Denis Funfschilling, and Eberhard Bodenschatz poses a riddle that will challenge theorists and experimentalist alike. The international team is already on its way to designing an experiment that can resolve the fine scales of the boundary layer. Results will give deeper insights into convective processes in the Earth, the atmosphere and the Sun, as well as the potential to optimize heat transfer in industrial reactors.

Original work:

Guenter Ahlers, Denis Funfschilling and Eberhard Bodenschatz

Transitions in heat transport by turbulent convection for Pr = 0.8 and 1011 ≤ Ra ≤ 1015

(2009 New J. Phys. 11 123001, free to read at http://www.iop.org/EJ/abstract/1367-2630/11/12/123001)

Prof. Dr. Eberhard Bodenschatz | EurekAlert!
Further information:
http://www.ds.mpg.de

More articles from Earth Sciences:

nachricht NASA examines newly formed Tropical Depression 3W in 3-D
26.04.2017 | NASA/Goddard Space Flight Center

nachricht Early organic carbon got deep burial in mantle
25.04.2017 | Rice University

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Making lightweight construction suitable for series production

More and more automobile companies are focusing on body parts made of carbon fiber reinforced plastics (CFRP). However, manufacturing and repair costs must be further reduced in order to make CFRP more economical in use. Together with the Volkswagen AG and five other partners in the project HolQueSt 3D, the Laser Zentrum Hannover e.V. (LZH) has developed laser processes for the automatic trimming, drilling and repair of three-dimensional components.

Automated manufacturing processes are the basis for ultimately establishing the series production of CFRP components. In the project HolQueSt 3D, the LZH has...

Im Focus: Wonder material? Novel nanotube structure strengthens thin films for flexible electronics

Reflecting the structure of composites found in nature and the ancient world, researchers at the University of Illinois at Urbana-Champaign have synthesized thin carbon nanotube (CNT) textiles that exhibit both high electrical conductivity and a level of toughness that is about fifty times higher than copper films, currently used in electronics.

"The structural robustness of thin metal films has significant importance for the reliable operation of smart skin and flexible electronics including...

Im Focus: Deep inside Galaxy M87

The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.

Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...

Im Focus: A Quantum Low Pass for Photons

Physicists in Garching observe novel quantum effect that limits the number of emitted photons.

The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...

Im Focus: Microprocessors based on a layer of just three atoms

Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.

Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Expert meeting “Health Business Connect” will connect international medical technology companies

20.04.2017 | Event News

Wenn der Computer das Gehirn austrickst

18.04.2017 | Event News

7th International Conference on Crystalline Silicon Photovoltaics in Freiburg on April 3-5, 2017

03.04.2017 | Event News

 
Latest News

Link Discovered between Immune System, Brain Structure and Memory

26.04.2017 | Life Sciences

New survey hints at exotic origin for the Cold Spot

26.04.2017 | Physics and Astronomy

NASA examines newly formed Tropical Depression 3W in 3-D

26.04.2017 | Earth Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>