Regenerator material option

[vamoose]

Here is a possible material for regenerator use. Its called 'termimesh' in Australia (and is a termite barrier material used in the construction industry). It is a stainless steel interwoven mesh that is around 300 microns in thickness, with strands 150 microns in diameter and has about 22 strands per 10 mm. It can be cut with heavier duty scissors, its not too rigid and can be manipulated into shape as its quite malleable. I guess it might be available from pest controllers in other countries with termites. I've been informed its not covered in any chemicals but is just a physical penetration barrier.
(I just picked it up off a job site rubbish pile)
vamoose

[Geoff V]

Hi Vamoose
Maybe........... we are about to run some back to back tests with different regenerator materials, 2thou SS foil, Recemat, and mesh of a very similar size to your Termite mesh (bandage wound), will let you know the verdict. The engine is a beta currently running with SS foil at a fill factor of 18%, with very good results, so for what it's worth, my money is on the foil.
GeoffV

[Ian S C]

Hi Geoff/ Vamoose, I don't know if you have Andt Ross's " Making Stirling Engines" book, theres an interesting bit in there about how he ran one of his motors, then for some reason he stripped it, and accidentally put the foil regenerator in back to front, and could not get the origional power from the motor. The foil was dimpled, curious. Be interested to hear your results Geoff. Ian S C

[vamoose]

Hey Guys,
Will be an interesting comparison with the different surface area to mass ratio and increased air turbulence of the mesh. Geoff, are you attempting to use a similar fill factor for both materials? (I guess the best way to confirm this or the proportion is by weight, if they're a similar grade stainless steel).

I remember reading that experience that Andy Ross described in his book. If I recall correctly it had something to do with the foil edges folding in a bit and blocking/affecting the flow when wrapped in the opposite direction. I think the dimpling was to set the gap between the progressive layers.
I wondered if some kind of blockage might also occur with the mesh. Could the threads overlay/inter-mesh and constrict flow? Maybe/maybe not, as the threads are in an interwoven configuration.
None the less I considered cutting the mesh strip on a slight angle to the lace directions. Also if using the mesh in cut layers, rotating them at 30deg or so, on each layer.

Also, had a closer look and it's 15 threads every 10mm, don’t know how I got it so wrong, I must be in need of a monocle ...or 2.
Really keen to know how it turns out Geoff, keep us posted.
vamoose

[Geoff V]

Andy Ross used 0.001" foil which when dimpled and wound in the wrong direction form a curve (stroke wise) which largely blocks the passage way. Wound in the other direction this does not occur, it's caused by stresses in the foil due to the dimpling. We use 0.002" foil which resists the temptation to curve and is not critical as to whether it's dimples in or dimples out.

GeoffV

[Geoff V]

Gentlemen

As promised, an update on the regenerator material tests undertaken by Ian Larque.

The engine is a pressurised beta of 46mm bore with a unique crank mechanism which permits the strokes and phase angle to be varied between runs. The hot heat exchanger is slotted internally and externally and is coaxial with the cylinder, the cool heat exchanger is similarly slotted within a water jacket, between these is an annular regenerator chamber with a 0.002" dimpled, bandage wound, SS foil regenerator matrix at 18% fill factor. By a series of trials, the best oveall performance was established with a phase angle of 51.4deg and a displacement ratio of 0.75:1.

The first regenerator matrix trials were conducted with a stainless steel foam 'Recemat' with a fill factor of just 10% by mass, the engine was slow to start, well down on offload speed, torque and consequently power.

The next trial employed a SS mesh of 80, bandage wound to a fill factor of 30% by mass, the off load speed was much better than the Recemat but less than the 0.002" foil at 18% FF, yet there was a 10-15% increase in torque.

The foil clearly allows the engine to 'breath' more easily, however this may be down to the lower FF or it maybe the passages between the foil are less turbulent than the mesh and hence the pumping losses are lower with the foil.

The engine is currently undergoing a major refit to increase the size of the rod bearings, when completed it is planned to run more trials with the SS foil at increased FF. To achieve an increased FF with the foil, requires a pair of precision rollers to form the dimples to a more uniform and lower height by passing the foil between the rollers before winding the foil into the regenerator matrix.

More to follow.

GeoffV

[vamoose]

Hey Geoff,
Thanks for the update.. interesting results!
That engine with its flexibility sounds like a great test bed to play around with.
I wonder if the increased turbulence from the mesh plays a part in reducing the 'boundary layer' , and if the flow losses are greater or lesser than the potential improved heat exchange in the working gas, between it and the regenerator.
Was also wondering if the increased turbulence (higher Reynolds number) when exiting the regenerator, may have a flow-on effect on heat exchange, when the more turbulent air passes through the hot and cold heat ex-changers.
If anyone is interested in getting some of this mesh, i see that it can bought on ebay, type in 'stainless steel mesh' (obviously). There are different sizes, but it seems a little expensive for what u get though.
vamoose

[Geoff V]

Vamoose

I think the mesh you've found is probably too coarse, we've been using 80 to the inch (FF 30%) yet the early Whispergen regenerators used approx 150 to the inch (FF 40%), unless the mesh is very fine it is too thick to be wound tight enough to get a good fill factor.

Regarding the effects on the boundry layer, because the gaps are so small the flow must surely be inside the BL, so I suspect the foil has lower pumping losses, the next test should throw some light on this aspect.

GeoffV

[Ian S C]

Also as the heat is only in the first thou or so of the thickness of the metal, any greater thickness is just a waste of space. I think, without my own tests, that.001" - .002" foil is proberbly about as good as it gets, may be some more exotic metal might be better, Titanium or something like that, or one of the alloys used in gas turbine engines.
Whisper tec even tryed catolitic converters (in NZ they used to remove them from new imported cars, because with the muck we were using in place of fuel they did'nt work). One day when I was in visiting Dr Don Clucas, I asked him if they had tryed them, he just pointed at a shelf along one wall of the workshop, dozens of them, no good. Ian S C

[johnmaster]

I would bet the thickness of the material they use for the catalytic converter honeycomb was many times thikcer than .002. It would probably breathe well though.