Gas Turbine-Basic

All combustion engines depend upon the flowrate of oxygen through the equipment to develop power, as the power is a function of the type and flowrate of fuel and the flowrate of oxygen. The flowrate of oxygen is a function of pressure, which is a function of altitude.

Injecting water into the intake of an engine can, up to a point, produce more power per unit of fuel consumed as the expanding steam from it can be used to drive the output shaft of the engine. However, too much water will 'blow the candle out'. In the case of turbine engines, liquid water impinging on turbine blades has the capability of introducing wear and distortion, thereby shortening engine service life.

There is a six-stroke reciprocating engine cycle suitable for water injection discussed in an article in Wikipedia. The stroke sequence is suck-squeeze-bang-blow-squirt-chuff, in a nutshell.

Clearly, water injection is impractical when temperatures are below the freezing point of water, and there is an economic break-even point where the increased weight of the components and the additional liquid storage accompanying the engine cannot be justified for the return given by the water injection adaptation; one wouldn't go six-stroke on a 50cc moped, for example.

Thank you to your reply.I am fully agree with you. but I am not pointing the variation of o2 with respect to the elevation. My doubt is, how the water injection at the suction side of a gas turbine (Evaporative cooling, Fogging etc ) will increase the efficiency of GT even though the density of air will decrease with addition of water? Does density plays a role here?

Dear,

When you are spraying water in the inlet air filter, air temperaturte will go down & humidity will increase.

So over all, air density will increase & eventually air mass flow rate will increase.

So gas turbine power output & efficiency will increase.

IMHO Water spray on a GT inlet is done to cool the inlet air, increasing the density, increasing the O2 charge, increasing efficiency. Humidity is a lot less than 100%. I have generally seen this done in desert areas and I don't think it is going to do any damage to the blades. GT's are the standard source of power on every oil platform in the North Sea where it is generally very wet, a lot of the time (and salty) and they run just fine!

ARRRRRRRRRG!

All Gases closely follow PV=nRT, the ideal gas law, look it up it's secondary school physics. So, increasing the temperature, by burning fuel, will increase the pressure.

Putting liquid water into the inlet, means the energy available to heat the gas is less, instead it goes into the liquid-gas phase change. Once it's turned into gas there's nothing magic about steam it too obeys the gas law.

So adding liquid water vapor to a compressor inlet lowers the efficiency.

(Of course, there may be other reasons ie cooling that it is done)

Why is my post off topic? If you disagree with what I say just refute it.

and of course if you google "gas turbine inlet cooling" you'll get all the answers you ever need.

Water/methanol is only used for "High Density Altitude" (High Altitude or High Ambient Temperature) for underpowered engines or close to max. weight during "Take Off Only!" It will increase air density thru the cooling effect from the pressure drop of the fluid being injected, thus "lying" to the fuel control to schedule more fuel, (Which Equals More Power). There is a major price to pay for augmented injection, it cuts your engine cycles almost in 1/2 depending on how often and duration of injection, provide you don't exceed max EGT!! It is not used after V₂ and gear up. I won't even go into EPR. PW Slack is right about a Radial Engine that was complex in operation, here in the States, we were taught, that it was a "Compound Supercharged Radial Engine", much to complex in get into here and it's an entirely different animal than a Gas Turbine. Gas Turbines are are simple compared to one of those beasts!

I need to add, that Gas Turbine fuel controls are also controlled by 2 major probes besides throttle and condition levers, (1) Pitot/Static probe and (2) Temperature probe. Augmentation is introduced before these probes thus tricking the fuel control to schedule more fuel, thus increasing power output. This configuration is more common with Turboprops operating in high temperature climates.

The basic relationship of PV=MRT is at work. Assume M and R are constant and T goes way up so therefore P1V1/T1=P2V2/T2 as this basic relationship shows, P2 (the resulting pressure as T goes up) is significantly greater. I hope this helps.

I always understood that water injection in engines allows higher powers as the water cools important parts that might otherwise be damaged.

It is a misnomer to believe that water alone increases power. (Though many wrongly believe this!)

As correctly pointed out elsewhere here, for engines that "fly" carrying large quantities of water would offset any gains. Water is usually just used for short times when high boost/power is called for.....

This has been discussed already several times on CR4, I am amazed that before posting, people don't simply do a few intelligent searches first.

Let me categorically state that water itself is not a fuel per se.....

But can also be part of a gas turbine cleaning procedure as (pointed out already here) it causes the parts of the engine that you cannot even see, to vibrate at high frequencies and therefore to shake off any products of combustion, which are then expelled at high speed....Whether or not this is still done I cannot say as my experience in this area lies about 40 years ago!

We also used powdered walnut shells as well for cleaning a running turbine.....!!! Also probably not actual data today......

It also happens that in a cogen factory (making your own power from steam) that the first thing that sees the steam is a turbine. It is standard to superheat the steam between 5 to 10 degrees Fahrenheit. After the power generating turbine the superheated steam is sprayed with condensate which converts to more usable steam.

A "steam turbine" is different in operation than a "gas turbine" engine, mainly the steam turbine is lacking the "compressor" directly connected to the turbines wheel(s), which is directly effected by "Density Altitude" and fuel scheduling. Steam turbines are great as a stationary power plant, but are not very efficient as a mobile power plant.

Did you get your temperatures right? That does not read correct to me.....

The steam the RN used in its ship's was used at around 300 PSI and 600°F (315°C).....they were developed between the wars, so what they achieve today I don't know....the RN uses mostly diesel and diesel electric systems today....

If you are really interested look up "Admiralty three-drum boiler" for better infos.....

The early 747 engines had water/methanol injection to simply cool the air and increase the density, thereby increasing the amount of oxygen in the same volume.

Only used as previously stated on high power/max weight take offs, and for a short period of time.

This system was removed when higher powered engines were developed.

#1: Humid air at a given (elevated) temperature has more mass than dry air at this same temperature. The water does not displace the air from the air silly man. This is why moist air is better for gas turbines, since there is more mass flow.

#2 Air does become less dense as the temperature rises, otherwise hot air ballons (and certain congressmen and senators and presidents) would not be bouyant. Since the hot air is less dense, it takes more of it (volume) to arrive at the same final temperature, pressure, and volume flow rate (that is necessary to achieve maximum power output). The turbine compressor can only adjust a small amount to compensate, therefore, even with applying more work to the compressor (reducing work available at the output shaft), there is a lower net power output.

The only way around this is to add a second compressor ahead of the first one, then cool the first stage compressed air, remove liquid moisture, and repeat compression to the final density upstream of the burner cans. See GE LSM 100 for an example.

Even this scheme has its limitations.