Naked Science Forum
Non Life Sciences => Technology => Topic started by: peppercorn on 25/06/2008 19:18:12
-
Taken from
http://en.wikipedia.org/wiki/Steam_turbine#Turbine_Efficiency
[... Most modern steam turbines are a combination of the reaction and impulse design. Typically, higher pressure sections are impulse type and lower pressure stages are reaction type.]
Can anyone tell me why this is?
-
Here's a suggestion.
The very first stage has to have fixed nozzles, so you may as well have that stage, at least, as an impulse stage.
As the gas progresses through the engine, the pressure drops and the velocity of the gas increases. Reaction engines work better as the propellant velocity increases - which may explain why the low pressure stages use reaction. The earlier stages, possibly have to use impulse because the velocity is too low for reaction to work well.
AFAIK water turbines use impulse - the velocity is very low in comparison with gas turbines.
Well, that's a start. Let's see what other people think.
-
Can't say I've ever really looked into steam turbines but it surprised me a bit to find that most modern steam turbines are a combination of impulse and reaction.
I can see how having an impulse stage at the front of the turbine might help in starting the turbine from rest, but once it's spinning at it's operating rate I wouldn't expect the impulse stage to be very efficient at all.
Heh - it's strange to think of the gas entering the turbine cooling & contracting instead of being heated and expanding, as in a gas turbine engine, where the low pressure stages come before the high pressure stages.
-
it's strange to think of the gas entering the turbine cooling & contracting
I don't see how it can - and do any work.
Doesn't it have to start at high pressure and hot and end up (expanded) at low pressure and cooler? The PV diagram wouldn't have a positive area under it to represent the work done.
The condenser in a steam turbine will produce a useful low pressure at the exit, tho' but that is due to a state change. Is that what you meant?
-
Once the steam has entered the turbine it can only cool - it certainly can't gain heat without energy being added - and as it cools it must contract. The ratio between the amount of energy lost to cooling and the energy output by the turbine shaft must be the efficiency ratio - umm... or perhaps not - the steam existing the turbine will still probably have some energy above that of the source water, so the total efficiency, from heating to exhaust, will be lower. The KE of the steam can only come from the heating, otherwise it will cost energy to accelerate the steam to give it the KE to transfer to the impulse stage.
In principle, it should operate backwards, when compared with a gas turbine engine.
I think... like I said, I'm not a steam turbine scientist [;)] - Ok - I didn't actually say that, but it's the fort that counts.
-
and as it cools it must contract.
That would only be correct if it started off at ambient pressure. The reason it cools is because it is transferring kinetic energy to the turbine blades (or whatever) and the average KE of the molecules (or Temperature) will go down. All heat engines work that way - steam engines - steam expands - IC engines - gases expand and they get cooler as it happens.
The input to a steam turbine is High Pressure and High Temperature (super heated). It comes out at Low Temperature and Low pressure and very much higher volume.
If this didn't happen, there could be no work done on the turbine blades/ pistons etc..
A cumulus cloud works in the same way. It starts off near the ground and work is done in raising it up. The temperature drops and the volume expands as it rises. This is really fundamental stuff.
If you filled a balloon with hot gas and just let it cool then, of course, it would contract. But no useful work would be done and it couldn't be regarded as a heat engine.
In a heat engine there are Adiabatic changes and the energy is transferred as work.
Trust me - would I let you down?
-
Thank you peeps!
Here's a suggestion.
The very first stage has to have fixed nozzles, so you may as well have that stage, at least, as an impulse stage.
I see... would it be inefficient to devise nozzles with a venturi to increase velocity at the cost of pressure? - making all stages reactive.
-
I think what you're saying about the cooling due to imparting kinetic energy may true for impulse turbines but I'm not sure about it applying to reaction turbines. With a reaction turbine, the drive is derived from aerodynamic principles i.e. achieving a pressure differential across the aerofoil, where speed and not temperature is the most important factor.
-
I think what you're saying about the cooling due to imparting kinetic energy may true for impulse turbines but I'm not sure about it applying to reaction turbines. With a reaction turbine, the drive is derived from aerodynamic principles i.e. achieving a pressure differential across the aerofoil, where speed and not temperature is the most important factor.
You will always get cooling if you extract energy from a mass. The aerofoil is just a means of achieving a pressure difference. Reaction is what you get when you eject some mass at a velocity - the resulting momentum change results in an energy transfer. A pressure difference is not, per se, an energy transfer. It's force times distance or pressure times volume change which gives you the actual energy transfer. If you have reservations about this because it 'doesn't seem to make sense', then you have to examine some basics. A jet engine works by reaction - a gas is expelled at high velocity and the rocket goes forward because of momentum considerations. With a reaction turbine, the same happens - you can't divorce speed from temperature; speed is bulk KE and temperature is average KE. That's how heat engines work - by transferring one to the other.
peppercorn - if you are going to introduce steam into the turbine you have to have holes / nozzles. The actual CSA of the nozzles may or may not be the same as the CSA of the inlet pipe. The actual velocity of the steam jet which hits the turbine blades is a 'detail', determined by the designer to get the optimum performance. I'm sure efficiency is quite high on the list of criteria but there can't be any useful reaction from fixed nozzles - that would just turn the body of the turbine + world. The very first energy transfer would have to be impulsive as a jet of gas would be hitting a movable blade.
According to all the refs I have seen, this gas is then redirected onto a further set, or sets, of blades in the same way. This is the impulse stage.
After a while, the gas has got faster and the subsequent stages have moving blades with constrictions between them and the emerging gas is going very fast - producing reaction forces and efficient energy transfer, that way.
It now makes sense to me (having read about it for the first time yesterday in Wikkers and about ten other sources - and so I am now a 'World Expert').
It seems that the gas is flowing fairly slowly in the early stages and faster in the later stages. This is why they are divided into the two different types of turbine.
-
Thanks SC.
...In future I will come straight to your door with all my steam turbine quandaries!! - what with you now being a world expert & all!!! Ho Ho!
...[steam velocity hitting] the turbine blades is a 'detail', determined by the designer to get the optimum performance. I'm sure efficiency is quite high on the list of criteria but there can't be any useful reaction from fixed nozzles...
I somehow feel efficiency (determined by matching to load, eg. a power station alternator) should be the number one priority for a designer in almost all cases.
I'm confused though. Are you saying the nozzles are not fixed?
I suppose they could be rotating, but I'm sure that's not what you're suggesting...
-
Having gone back & looked at the diagram on wikipedia I've noticed it does indeed specify a rotating nozzle... Ooops!
This is easy to visualise for something like Hero's aeolipile - with nozzles spinning round, but I don't know how this would work in modern designs...
-
Thanks SC.
...In future I will come straight to your door with all my steam turbine quandaries!! - what with you now being a world expert & all!!! Ho Ho!
I am expecting a phone call from Brown Brothers any time now.
-
It is all about using entropy in a micro and the second law of thermodynamics, It is a reactive system.
Old steam engines were impulse engines.
Alan