Thermoacoustic engine restrictors

[dickonthelangwang]

I've been searching around to try and find an explanation for the need for the restrictor in a TEE & it's optimum size but without much luck ....... the main impression I get is something between black magic & sheer joss !! Looking at it from the point of view of a player of bagpipes & microwave engineer (retired) I can see how the tube, from the closed end to the restrictor, forms an acoustic resonant cavity. If there was no restriction then the resonant frequency would depend on the total distance to the end of the piston & would change as the piston moved - depending on how long it takes to establish and collapse the standing wave associated with a particular single resonant frequency then would efficiency drop and/or engine simply not work ?? If we have a restrictor in a fixed position then the predominant resonant frequency would be fixed (ignoring the minor (?) effect of the varying distance from other end of restrictor to piston) but what then determines the optimum internal diameter of the restriction ? Too big & we're looking at something similar to a 'no restrictor' situation & too small do we start running into a 'choked flow' condition ?

My head is beginning to hurt & I haven't even had the courage to start looking at the maths involved ...........

Has anyone any thoughts/explanations on the matter and/or can just tell me I've got totally the wrong end of the stick ....... ??

Dick

[Ian S C]

Hi Dick, There is a bit about thermoacoustic motors on the site from people who include Jan Ridders, type thermoacoustic motors into the search window.
look at http://www.scientiareview.org/pdfs/168.pdf Optimization of Stirling Engine Power output through variation of Choke Point diameter, andExpansion space volume. By Anna Brill of Massachusetts Academy of Maths and Science. Ian S C ps try lamina flow, and any of the other names used for these motors.

[nickh]

While there are thermoacoustic analogues of the Stirling engine http://en.wikipedia.org/wiki/Thermoacoustics I have yet to be convinced that there is any acoustic resonance basis to the operation of the single piston engines which often carry that title. I have no idea what 'lamina' is supposed to mean in this context, so 'thermal lag' seems to me the most appropriate name, implying that the essential asymmetry which engenders a conversion of thermal to mechanical energy is brought about by the differing rates of heat transfer during the compression and expansion phases.

As usual Allan Organ is you man for the 'technical bits' and you will find an analysis in his book 'The air engine'.

NHH

[derwood]

Hey Dick, I have hundreds of hours testing this type of engine. Many times I have had to walk away from testing for months at a time. Too many variables begin to consume my thoughts to the point where I dream about it. Be careful! With that said I can confidently say that acoustics have nothing to do with it. I have built several large thermal lag engines, some I have made public on this site and some newer versions that I have not yet made public. In my opinion, all heat difference engines experience a thermal lag. I refer to it as a thermal lag or lamina flow. Lamina flow does play a major role. Straightened air flow is very important, so aerodynamics play a major role too. Turbulence greatly reduces power and can even prevent one from running at all, even if the proportions are correct. I have found that restriction controls the timing, it creates a compression/contraction delay and is very important. Although air is being cooled in the cylinder, It is not responsible for drawing the piston back in. When compressed cool air is heated it expands violently and forces the piston out. Remember that air has mass and can gain momentum. After the expansion takes place the momentum stretches the air like a rubber band and low pressure forms in the heat tube. momentum from the piston also aids in the stretching. This is where lamina flow increases efficiency. At this point the piston is fully extended and the air in the cylinder is cooled. The stretched air draws the piston back in. Now you have cooled air being compressed back into the heat tube. The piston's momentum causes this compression and the process starts over. Part of this process is very similar to a pulse jet. I could go on about proportions and compression ratio but I am already starting to get a headache. LOL!


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