# Turbine question

1. Jul 5, 2016

### Maylis

I know this will sound stupid, but what would happen if you just fed air into a turbine? Doesn't it derive energy from the rotation? Why do people run steam through a turbine? Sounds like a wind mill.

2. Jul 5, 2016

### Simon Bridge

If you boil water for steam in a closed vessel with a small opening, then the steam rushes out the opening and through the turbine or whatever you want to turn or push. You are quite correct that you can use any gas with a turbine - you just have to get it moving.

3. Jul 5, 2016

### Ketch22

It may not be as elementary as stated. Strictly speaking a turbine is a device that extracts force from a moving fluid and produces torque on a shaft to be used in another form. The form of a steam turbine is optimized for the flow conditions experienced. A water turbine from a hydroelectric plant is also optimized for it's expected conditions. Even a windmill is a form of a turbine, modern ones being likewise optimized.
The turbine portion of a Gas turbine engine is running in a very hot high velocity gas. If you could recreate the same conditions in air it would work equally well. However if you have no ready source for your fluid creating the appropriate flow can be very energy intensive.

The energy is not derived from the rotation. The Turbine must stay rotating at the designed speed to be efficient and continue to rotate. One can only extract the power over and above that needed to continue rotation. Conversely if you need a specific amount of power the flow of fluid must be increased until a state of equilibrium is reached.

4. Jul 6, 2016

### Jobrag

A turbine extracts energy from its working fluid, you need to put energy into the working fluid, a boiler is a fairly simple way to putt energy into water. If you wanted to use air as your working fluid you'd have to compress it with machines driven by another machine. Imagine an air compressor attached to an internal combustion engine using the air to turn a turbine attached to a generator, you might as well attach the ic engine straight onto the generator.

5. Jul 6, 2016

### RonL

I would think steam carries a larger energy transfer potential, but air only will produce a much simpler system. If you will allow, I might be willing to suggest a design that is what I think you might be trying to realize, I would need your skills and maybe ChesterMiller to help evaluate it's potential (if any) :)
Thanks
RonL

6. Jul 6, 2016

### Staff: Mentor

Clearly, people do run air through wind turbines...

They run water through hydroelectric turbines...

Steam turbines are different in that they are full thermodynamic cycles: heat is added on one side to carry energy via the phase change. Otherwise, you'd just have a water pump supplying a water turbine.

7. Jul 6, 2016

### jack action

It is.

When the wind hits the blades of a wind mill, it is loaded with energy, in this particular case, kinetic energy (½mv²). With the proper fan geometry, this energy is converted to pressure which rotates the fan.

The question is: If we don't have wind, how do we put energy in air standing still? The answer is often by heating up the air, like in a gas turbine. But you can replace the hot air by any other hot gas (like steam, for example) and it will work too.

Any gas turbine is of the internal combustion type, so the air entering the turbine is actually converted to exhaust gas before hitting the turbine's blades. But it works also with an external combustion, meaning only air is the medium within the turbine. But it is usually very difficult to heat air externally fast enough to create high level of power. Steam turbine is a kind of external combustion engine, but because the medium can change phase (liquid to gas), it gives some thermodynamics advantages over simply heating a gas.

8. Jul 6, 2016

9. Jul 6, 2016

### OCR

Like in a turbocharger, which also works with external * combustion... namely, an internal combustion engine's exhaust gas.

* External combustion might not be, exactly, the right term... however, no combustion actually takes place
within the turbocharger, itself... it's all within the engine, or should be.

Last edited: Jul 6, 2016
10. Jul 7, 2016

### Staff: Mentor

It boils down to design and operating considerations. Think about designing a turbine with air as the working fluid versus steam/water (both operating in a cycle). Think about the volumes of materials that must be handled (outside the turbine), the size of the equipment, the heat transfer area required, the ease of pumping water vs air, etc.

Chet