. : Ultra low speed gas turbine : .

Here we will r_a_p_e the idea of gas turbine complicatedness. Don't expect to see a large and usefull engine here. Its just a model, kinda alike those tin can Stirling engines, filling the net media.

SO WHY?

It is said that a turbojet is extremely hard to make at home. So hard that only one human on the Earth was able to do that (Some Deutch guy, namely Shreckling) and any other successfull homemade turbines are just reproduction of his design. (For me it looks just like an importunate advertisment, but You still can find such a statement in many places over the internet.) Others say that it is hard even to obain a self sustained mode of operation. (One can easily find the proof of this in comments on YouTube. - Almost any homemade turbine build is accused of being non self sustainable.)

Besides the basic idea of gas turbine engine is one of the most simple in the world. It has no cylinders, no pistons, no crank, and no timing valves. It has only one moving part (the shaft with turbine and compressor wheels) and even its motion is just rotation.

The only difficulty is that turbines are thought to have very high revolution rates. People got used to numbers like 70 000 rpm or more when speaking about gas turbines. Indeed, at those revs one needs a carefull design, strong materials, high precisions and so on.

Any book, analyzing Brayton cycle or gas turbine operation, eventually comes to the conclusion that there is no rpm threshold due to gas dynamic or thermodynamic properties. The need for the high rpm arises only from the necessity to overcome friction. And if we could keep it low enough...

SO HOW?

Three key principles are the next:

• The turbine should be larger than the compressor. This provides that turbine driving torque is higher than the compressor braking torque in tremendously wide range of conditions including the lowest possible efficiencies.
• The gas flow from the compressor to the turbine should not be choked. There should be no waists in the path. Moreover, because the gas tends to expand when being heated in the combustion chamber, the constant section flow path can also be treated as "waist". In order to avoid this the flow channel should actually be widening. And the area of its section at the approach to the turbine should be K times larger, than the area of its section near the compressor. The K ratio should be slightly less than the ratio of the exhaust temperature to the inlet temperature. (Absolute temperatures are meant.) Slightly less - for some flow acceleration, but not too much - to avoid risk of choking. The good initial approximation: K=2..2.5.
• And finally the friction should be as low as possible. The wide spread solution - to use an old bike axle in place of shaft and bearings won't suit if we want to keep rpm not higher than 1..2 thousands. Another old good solution - the use of pinpoint bearings (pin-to-pit support) - have proven itself to give fairly good results (from any point of view but reliability).

Three additional principles are not the key, but You should attend them to make the life easier:

• The turbine blades are set as so the angle between them and the axis of rotation is 1.2 radians. It corresponds to the angle between the blade plane and the turbine wheel blade equal to 21 degree. The optimal angle depends to the behaviour of Lift to Drag ratio for the particular blade profile, You get when curving the blades of Your turbine. A planar blade reaches a reasonable (I got 80% by numeric calculations in the inviscous flow.) efficiency for the angle mentioned above. And it tends to grow for smaller angles (between the wheel plane and the blade plane). On the other hand, too shallow angle means too low torque, so here is the subject for compromise.
• The angle of compressor rotor blades was found to be good when in range from 30 to 45 degrees (again between the wheel plane and the blade plane). At lower angles the turbine tends to attain higher rpm and power, but harder to start. At higher angles the engine starts more easily (sometimes self-start is possiple when the tip bearings and balance are good enough), but the top achievable rpm and power are lower.
• As we are trying to set no obstacles in the flow path there are no stators. In the conventional turbojet the first stator rectifies the flow and forces its energy to become pressure. After the combustion chamber the last (second) stator spins the flow again. And in the same direction as it was rotating after leaving the compressor. So why not to let the flow to spin freely and to deliver the torsion energy just back to the shaft by driving the turbine?

The design is clear from the pictures below.

The shaft is made of a 5 mm taping arrow. The wheels are cut from a can metal (thin steel plate). Their blades are cut and then turned by pliers. I tried to apply some wing-like curvature to the blades and succeeded... to some extent. The depth of camber is 3 mm for the turbine blades and 1 mm for the compressor ones. Compressor has 65 mm diameter, turbine has 95 mm diameter.

Since the taping arrow appeared to be unable to be hardened, the pins were made of some drill bit pieces. The pieces were annealed, then sharpended, then hardened again. Then they were installed into the specially foreseen holes at the ends of the shaft.

The casing is made of can metal (spotwelded). At the ends of the casing there are two straddles, with shaft keeping bolts. The bolts have pits that can hold the pins of the shaft. The bolts are needed for fine tuning of the device. The casing also contains a shaft screening tube - just a piece of a steel tube, placed along the axis of the device. The tube is needed to prevent overheat of the shaft. (which will be red hot in less than 30 seconds if not screened.) The engine operates successfully even without the screening tube, but in a short time it fails due to the shaft bending.

The thing is fed with propane gas from an LPG tank equipped with a pressure regulator. The fuel sprayer is a ring of copper pipe, carrying four orifices, each 0.5 mm in diameter.

Starter is any suitable fan. No need to have a powerfull one. Correctly built and finely tuned engine can be started from an average computer cooler.

Ignition system is any suitable butane blowtorch. Just open the fuel flow and fire the gases at the turbine exhaust end. At some point the flame will blowback into the combustion chamber and no more external ignition will be longer needed.

It works and it is pretty self-sustained. What else did You want for the price?

<< HOME PAGE