Stirlingengineforum.com

Hi All.

I'm a new poster here, mature student who works as a precision engineer. So i have access to a good range of equipment. I'm doing a project to design and build and build a Stirling engine. I've been set a challenge to make 100 watts! Not easy at all by what i have seen / read. I have researched various calculators online to help size the engine design. From what i can see these seriously over estimate outputs due to being based on theory.

I've settled on a Gamma arrangement with the power cylinder below and slightly offset from the displacer (trying to minimise dead volume) . Engine will be built with a closed crank case to allow pressure up to 10 bar, initially with air as the working fluid. The crank will have an eccentric to give me the 90 deg phase angle between my cylinders.

At the moment i have in mind a power cylinder of 50mm, square stroke of 50mm, swept volume of 98.2cc.

The displacer cylinder etc will be sized according to the ratios you guys discuss here with a volume ratio of 1.5:1 as it will be running with a high temperature differential.

What do you feel about the approx 100cc swept volume, is this enough? The calculators i have looked at suggest up to 500 watts at 10 bar for my numbers, but i'm not sure if i should even expect 20% of what they suggest.

The displacer cylinder i plan on forming from 3 parts that will stack together and be clamped by long studs to the base plate. I plan on machining these with finned inner surfaces on all sections. The heater would also have a finned outer wall to aid heat collection from my gas flame. The cooler will be water cooled. I had in mind of having the centre section as a re-generator, so finned internally and insulated externally. Do you think it is worth trying to incorporate a regenerator like this or should i leave it out? I'm thinking that the usual ratios say 2/3 heater and a 1/3 cooler??? I could increase the length of the displacer and cylinder to give me some extra room for the regenerator (i.e. make it 4/3 long so i can have a 1/3 for my regen??). As the inner surfaces of the cylinder will be finned i am thinking of keeping the gap down to a max of 1mm as i figure i have surface area in the fins for the air to flow down.

I have seen various engines using thin wall tubes forming the heater. What i cant find is some drawings showing how these tubes are attached to the heater. In my mind as the displacer moves through its stroke it should force the air around the tubes. If there isnt a good flow they would just be dead volume with hot expanded air in them. I use a lot of thin wall 316 tube at work so if i can employ this i would like to if i can be confident of making a gain from it.

Your thoughts would be gratefully received.

From what I'v read on the subject of tubular heat exchangers the tubes are brazed into the hot cap, can't help with design on this part, I'm low tech.
I use the form of GAMMA motor that you suggest, it is just a step(sideways) from a BETA motor without the trouble of putting a rod through the power piston.
It's worth (free)down loading Andy Ross's book "Making Stirling Engines".
Ian S C

I have no idea of the tubular heat exchanges either, apparently they do seem to work very well. What Ian is saying about the Gamma design is good. It is only a small way from the Displacer rod and makes it easy to fit, however, I have found if you are worried about dead space, it may be worth using 1:7 or even 2:1 to make up for it as this dead space can still be utilised this way. Not sure what Ian thinks of this idea.
Trevor

Hi Rich. I believe the tubular exchangers are typically employed where the displacer fits more like an ordinary piston and forces the air through the tubes back and forth in the chamber. The slotted cylinder does the same thing but note that where the slots are uncovered at either end of the stroke, the flow will migrate into the chamber and the uncovered slot becomes dead space. Lots of compromises. It seems it’d be easier to maximize surface area with slots, while round tubes would hold more pressure with the same wall thickness.

Bumpkin

Found an image of what I mean. Of course it could be done myriad other ways.



Bumpkin

Having been absent for a while. Seen some interesting activity here. How is everyone doing?
Tubular design is not that efficient. With the small diameter of the tube, the effective contact area is very small. Too increase the contact area, either bigger tubes or more tubes can be use, but that also increase dead volume. Some new technology like 3D DMLS came online recently and it can be taken advantage to produce a heater that has very large contact area and at the same time much smaller dead volume.

Hi Aviator168, long time no see, must have a look at this new tech.
Ian S C

Look at the way he created the slots. It can be done with a DMLS. The slots can be much narrower and more slots can be made on the heater without increase of dead volume.
https://www.youtube.com/watch?v=lo0opKo5_TQ&t=11s

I can't remember his name, but there was a German gentleman on this forum sometime in the last couple of years who used a vertical slotting machine to create fins on the interior of the hot cap of his motor.
Andy Ross machined the slots on flat metal, then rolled it, and brazed it inside the hot cap.
Ian S C

for you !
https://www.youtube.com/watch?v=z7lUzKfdHd4

I have been following Kirk's stirling development for years. It would be nice for him to show us more information on his engine such as the size of the slot, dead space volume, sniffing valves, and etc so we can do a compete analysis on it. Most of he stirling engines I have seen have undefined fluid flow paths; especially in the heater, except the ones that use heating tubes. At the same time, no one has put thermal/pressure couplers in both hot and cold cylinders to measure the exact temperature/pressure. If they would've done that, a better understanding of the reason that their engines did not achieve the desire output can be obtained.

Shouldn't the hot and cold sides of the displacer cylinder be electrically insulated?? That would reduce conductance of heat...

electrically insulated

?

Heat can be transferred from one place to another by three methods: conduction (metal), convection of fluids (gas), and radiation.

I am theorizing that in the displacer cylinder sweep area (gamma or beta), one wants to keep the hot side hot, and the cold side cold. The only energy transfer you want between the two areas is convection via the displacer. However note that with in the hot area, you do want a exchange of energy to quickly warm up the air. Here you would want to use conduction as much as possible which is how economizer or regenerator works. In the cold area you want a regenerator or economizer as well. With all that said, if heat is transferred via conduction between the hot area in the cold area will actually reduce the performance of the cold area over time since you only want energy to be transferred via the gas medium.

One way to test for low conduction of heat is to make sure that the two areas are electrically isolated. Since the conduction of electricity and heat are highly correlated.

A simple way to do that in a design is to use a gasket between the two areas. Gaskets could be used in the form of washers below the nuts and the bolts and an insulator could wrap the bolt.

On carbureted engines for example, it is common to have an insulator beneath the carb:

https://www.opgi.com/monte-carlo/CH2379 ... wAodra0OeQ

Interestingly, thinking about sterling engine models: The glass test tube which is commonly used on the hot side of the displacer, it's an insulator which would increase the runtime* of the engine as it would keep the colder side cold. (*some models after they heat up become a hot lump and the performance decreases...)

Thoughts?

One way to test for low conduction of heat is to make sure that the two areas are electrically isolated. Since the conduction of electricity and heat are highly correlated.

OK. You mean thermal isolation.
That's easily said than done as the temperature we are dealing with can be quite high. Another issue is that in beta and gamma engines, the displacer might be large and has to be very light in weight. Such a part can be difficult to build for pressurized engines.