Building a Lamina Flow Engine

Also called 'Stirling engine', 'Heat engine' or just 'Resonating engine' (11/11/98) (11/11/2008) What I was thinking?

 

This is the simplest heat engine that can be made with a few parts: A tube, Piston, Regenerator (wire gauze) and Heat source. It is also known as Lamina Flow Beta Stirling engine. but I believe it cannot be called "Stirling" as it features only a piston. Stirling engines have two pistons, one in the cold side and the other one at the hot side. This engine does have only one piston, two cold sides and one hot side.

 

Lamina Flow Engine or Resonating engine

 

In fact, several websites talks about this engine like a "SASER Engine" (Sound Amplification Stimulated by Emmision of Radiation), also as "Accoustic Engine", "Resonating Engine", "Laminar Engine" or simple "Heat Engine". In additions, appears that no one knows exactly how it works, because I found no website with a good explanation about it.

 

I believe the reason why this engine works is the same principle as the Rijke Tube. Here is a brief explanation about it:

 

Rijke-tube.gif Rijke Tube "P. L. Rijke was a professor of physics at the University of Leyden in the Netherlands when, in 1859, he discovered a way of using heat to sustain a sound in a cylindrical tube open at both ends. He used a glass tube, about 0.8 m long and 3.5 cm in diameter. Inside it, about 20 cm from one end, he placed a disc of wire gauze as shown in the figure at the right side. Friction with the walls of the tube is sufficient to keep the gauze in position. With the tube vertical and the gauze in the lower half, he heated the gauze with a flame until it was glowing red hot. Upon removing the flame, he obtained a loud sound from the tube which lasted until the gauze cooled down (about 10 s). It is safer in modern reproductions of this experiment to use a Pyrex tube or, better still, one made of metal.

 

Instead of heating the gauze with a flame, Rijke also tried electrical heating. Making the gauze with electrical resistance wire causes it to glow red when a sufficiently large current is passed. With the heat being continuously supplied, the sound is also continuous and rather loud. Rijke seems to have received complaints from his university colleagues because he reports that the sound could be easily heard three rooms away from his laboratory. The electrical power required to achieve this is about 1 kW.

 

Lord Rayleigh, who wrote the definitive textbook on sound in 1878, recommends this as a very effective lecture demonstration. He used a cast iron pipe 1.5 m long and 12 cm diameter with two layers of gauze made from iron wire inserted about quarter of the way up the tube. The extra gauze is to retain more heat, which makes the sound longer lasting. He reports in his book that the sound rises to such an intensity as to shake the room!
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Why this setup creates sound? All air flowing past the gauze is heated to the temperature of the gauze and any transfer of heat to the air will increase its pressure according to the gas law.

 

As the air flows upwards past the gauze most of it will already be hot because it has just come downwards past the gauze during the previous half cycle. However, just before the pressure maximum, a small quantity of cool air comes into contact with the gauze and its pressure is suddenly increased. This increases the pressure maximum, so reinforcing the vibration. During the other half cycle, when the pressure is decreasing, the air above the gauze is forced downwards past the gauze again. Since it is already hot, no pressure change due to the gauze takes place, since there is no transfer of heat. The sound wave is therefore reinforced once every vibration cycle and it quickly builds up to a very large amplitude.

 

If the tube is placed horizontal, the sound dissapears. It only works when the air can go up because the convection. I believe when the tube is placed in a horizontal position there is no sound because the air flow is reduced and the frequency slows down. We can't hear any frequency below 60Hz, for that reason we can't hear it still working in some way.

 

Let's see a short video of this engine working:

 

 

The heat should be applied exactly where the mesh ends inside the tube. It's better to reproduce this engine with a glass tube (pyrex) to locate the "sweet point". When the air inside the tube heats, it expands and cools down, then it heats again and the cycle repeats. At the beginning, the air is not hot enough so the oscillation is slow and weak. When the frequency of the oscillation is matched by the flyweel, it starts rotating.

 

When more heat is applied, the frequency in the engine increases. As you can see on the video, the RPM of the engine increases, also the power. There was a guy that used a 1-pound flyweel without any problem.

 

As simple it is, the power can be increased with two or more engines connected in any configuration:

 

1Source: Wikipedia.

 

 

 


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