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 Novel method for investigating pore geometry in rocks
18.06.2018 | Kyushu University, I2CNER

nachricht Decades of satellite monitoring reveal Antarctic ice loss
14.06.2018 | University of Maryland

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

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

Im Focus: AchemAsia 2019 will take place in Shanghai

Moving into its fourth decade, AchemAsia is setting out for new horizons: The International Expo and Innovation Forum for Sustainable Chemical Production will take place from 21-23 May 2019 in Shanghai, China. With an updated event profile, the eleventh edition focusses on topics that are especially relevant for the Chinese process industry, putting a strong emphasis on sustainability and innovation.

Founded in 1989 as a spin-off of ACHEMA to cater to the needs of China’s then developing industry, AchemAsia has since grown into a platform where the latest...

Im Focus: First real-time test of Li-Fi utilization for the industrial Internet of Things

The BMBF-funded OWICELLS project was successfully completed with a final presentation at the BMW plant in Munich. The presentation demonstrated a Li-Fi communication with a mobile robot, while the robot carried out usual production processes (welding, moving and testing parts) in a 5x5m² production cell. The robust, optical wireless transmission is based on spatial diversity; in other words, data is sent and received simultaneously by several LEDs and several photodiodes. The system can transmit data at more than 100 Mbit/s and five milliseconds latency.

Modern production technologies in the automobile industry must become more flexible in order to fulfil individual customer requirements.

Im Focus: Sharp images with flexible fibers

An international team of scientists has discovered a new way to transfer image information through multimodal fibers with almost no distortion - even if the fiber is bent. The results of the study, to which scientist from the Leibniz-Institute of Photonic Technology Jena (Leibniz IPHT) contributed, were published on 6thJune in the highly-cited journal Physical Review Letters.

Endoscopes allow doctors to see into a patient’s body like through a keyhole. Typically, the images are transmitted via a bundle of several hundreds of optical...

Im Focus: Photoexcited graphene puzzle solved

A boost for graphene-based light detectors

Light detection and control lies at the heart of many modern device applications, such as smartphone cameras. Using graphene as a light-sensitive material for...

Im Focus: Water is not the same as water

Water molecules exist in two different forms with almost identical physical properties. For the first time, researchers have succeeded in separating the two forms to show that they can exhibit different chemical reactivities. These results were reported by researchers from the University of Basel and their colleagues in Hamburg in the scientific journal Nature Communications.

From a chemical perspective, water is a molecule in which a single oxygen atom is linked to two hydrogen atoms. It is less well known that water exists in two...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Munich conference on asteroid detection, tracking and defense

13.06.2018 | Event News

2nd International Baltic Earth Conference in Denmark: “The Baltic Sea region in Transition”

08.06.2018 | Event News

ISEKI_Food 2018: Conference with Holistic View of Food Production

05.06.2018 | Event News

 
Latest News

Novel method for investigating pore geometry in rocks

18.06.2018 | Earth Sciences

Diamond watch components

18.06.2018 | Process Engineering

New type of photosynthesis discovered

18.06.2018 | Life Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>