Originally Posted by
MSOCS
Tartare,
To add to Engines reply, water injection sprayed de-mineralised water, at pressure, onto the HP turbine blades and reduced the Jet Pipe Temp (JPT) by 23-25C. This temperature suppression allows increased fuel flow (and therefore RPM under certain conditions).
This is an almost universally misunderstood field. The following is a standard answer that was prepared by one of the gas turbine design specialists at RR:
Are you sitting comfortably? Good, then we shall begin ....
First of all, gas turbine water injection is a thrust augmentation device. The concept of injecting water into a gas turbine has got sod all to do with cooling the engine as one of the advantages of a gas turbine is that they are self cooling – ponder the extensive use of such beasts in stationary applications, such as on oil rigs, pipelines and for powering ships.
Now we’ve got that one nailed – just how can you increase an engine’s thrust by injecting water? At first glance, it seems an absurd thing to do. Well, it’s simple really, and there are three different ways of doing it:
1) Add the water at the front of the compressor
2) Add the water directly to the combustion chamber
3) Add the water immediately before the turbine section or just before the propelling nozzle.
Taking each one in turn:
1) Injecting water at the compressor face has the effect of lowering the temperature of the inlet air, (assuming the water is at a lower temp than the ambient temp, of course, but seeing as you will generally be using water injection on hot days, that's taken as read). Remember the old maxim of 'It's fookin' difficult to compress cold air and vladimir impossible to compress hot air' and you soon realise that lowering the inlet air temp allows you to get either:
a) the same level of pressure rise as before but from less power offtake or
b) more pressure increase for the same shaft power requirement.
Both of these effects give you greater thrust (via less power offtake or through higher pressure ratio respectively) but option 1b) is usually the one used. In a nutshell, you are fooling the engine into thinking the ambient temperature has suddenly gone down and gas turbines work best at low temperatures. Because you have lowered the inlet air temp then obviously you are lowering the compressor outlet temp as well. This allows you to add more fuel and gives you a greater delta t across the combustor. You are also putting more mass flow through your engine (because you've added the water and water is more dense than air), giving you greater thrust because thrust is directly related to mass flow. Additionally, you can utilise a water/ethanol mix if you so desire, with the ethanol being burnt in the combustor giving you even more bang.
From point 1b) you can see the problems that occur with gas turbines at high ambient air temperatures: Higher air temp = lower compressor efficiency = lower pressure ratio = less efficient combustion = lower resultant thrust because the turbine is using up all the available power to run the compressor = you ain't going nowhere.
2) Adding water directly to the combustion chamber is one for the theoretical physicists. What you are trying to do is induce blockage and temporarily reduce the volume of the chamber, thus increasing the pressure inside the combustion chamber as the efficiency of the combustion process is increased at higher pressure. It also has some other peculiar effects such as increasing the air flow speed which is not detrimental. This type isn't used much as it's difficult to model and understand and can lead to combustion instability, which is a bit of a bad thing (tm)
3) Adding water at the turbine face or just before the nozzle simply works by adding mass flow to the engine's exhaust thus giving you more stuff out the back = greater thrust.
Option 3 is the simplest and most straightforward whereas option 1b will most probably give you the greatest thrust increase. Sometimes you will get a water injection installation that gives you both compressor and nozzle injection to get even more increase at the expense of plumbing complexity.
If anybody ever says that water injection is for 'cooling the engine', just ask them exactly what a couple of gallons of water is supposed to do to a raging inferno at 1,200 degrees centigrade travelling at 200 meters per second. You should get a few blank faces in return...