Effect of humidity on engines

[shumway76]

I have been reading the FAA A&P handbook on powerplant, and somewhere there it's mentioned that humidity does no affect turbine engines (or has insignificant effect) compared to piston engines.
Why is this so?

Is it because turbine engines have automatic compensation or something like that in it's fuel metering system, unlike piston engines which only has a carburettor which does nothing about humidity (or even density, unless mixture lever is adjusted) for that effect?

[flame_bringer]

Ok here is what I know:
When there is humidity in jet engines then the air density decreases hence less air to move and burn at the combustion chamber which will degrade the performance.
However the amount of degradation is not very noticable in that most combustion chambers are designed to work with average humidities expected in humid areas (they are tested and thier design is made such that they can withstand certain amount of water droplets), Also on modern engines since most of them are turbofan and most of the thrust is produced at the bypass (around 70% of the air intake goes through the bypass) and not the exhaust the effect is even less since the combustion chamber takes less air.
However heavy rain or water ingestion can cause an engine flamout if its on idle power.
Piston engines rely 100% at the combustion chamber output (unlike turbofan engines which rely on the air discharge at the bypass) to produce the pistons mechanical movements and if the fuel air mixture gets disrupted with water which is a foreign matter the combustion chamber perofomance gets degraded hence the effect of engine performance is more since the combustion chamber reliance is more.

[Bite OK]

i do agree with you as humidity increase density decreases and that will have an impact in turbine engines as its works in the principle of thermal compression and expansions.
But turbine engines are designed to be over effecient on ground and idle powers so it dumps out all the air overboard.
during cruising altitude with lean air, it uses all the air compressed in its compressor stages, but it is still the combusion chamber and its turbine stage which does take all the load to propel the fan stage with its compressor. so it is also 100% driven by the turbine and combusion and any effect of humidity hence density would have an effect in the total performance.
variouse pressure and tempreture sensors fitted in the turbine engine which calculate and schedule the air and fuel mixture (air mixture is managed by altering the variable vanes in the compressor stages - fuel managed by fuel meetering unit) so the principle is the same for piston engines aircraft which shoud be fitted with turbo chargers to compensate for the decreasing air density.

thanks

[flame_bringer]

The variable stator vanes at the compressor dont alter the fuel and air mixture they are there to protect the engine from stalling at high power...

[Flightmech]

The variable stator vanes at the compressor dont alter the fuel and air mixture they are there to protect the engine from stalling at high power...

Actually VSV's and CIT amongst many other P&T sensors have input to fuel scheduling. Operated by master/slave actuators using servo fuel pressure from the HMU/MEC. Obviously if they are off schedule/out-of-rig an aerodynamic stall is possible but the excess airflow into the compressor is offloaded by the VBV system under normal operation. The VSVs ensure a smooth airflow into the compressor and subsequently for combustion under variable power settings.

[lomapaseo]

Actually VSV's and CIT amongst many other P&T sensors have input to fuel scheduling

Regarding the VSV's Isn't it the other way arround :confused:

Is the dog wagging the tail or is the tail wagging the dog ?

[Flightmech]

Actually thinking on you're right. VSV scheduling is a function of N2 and CIT. That sounds better

[flame_bringer]

flightmech
VBV and VSV prime purpose is to protect the engine from stalling at low and high powers they are not there to alter the air intake to compensate for humidity power loss:confused:.
VBV dumps excessive air at low powers to match the air intake off the booster stage with what the HP compressor can take and vise versa for the VSV.
All the respective sensors are there to either monitor the engine performance or to schedule the operation of these systems and I have not heard of a sensor being there for sensing humidity or air density and alter the air/fuel mixture for that reason.:confused::confused:

[Flightmech]

VBV and VSV prime purpose is to protect the engine from stalling at low and high powers they are not there to alter the air intake to compensate for humidity power loss.


Read again. The thread had drifted a little and i didn't mention that VBVs or VSVs were anything to do with "humidity power loss". Where did you read that. I simply implied that VSVs maintained the airflow into the compressor at variable power settings and VBVs could off-load the excess? Like you say, ultimately to maintain efficient engine operation and prevent aerodynamic stalls.

[Flightmech]

By the way, I haven't heard of a humidity sensor in an engine operation either but air density would be a factor of a pressure sensor wouldn't it?:hmm:

[flame_bringer]

Sorry I thought you concured with bit ok thats why I had to post that reply. Water is incompressible so if there is humidity the pressure becomes less at the compressor output.

[Piper19]

I thought that water certainly had an effect on jet engines. It's even in the basic working principle of it. The amount off mass (air) going out of the engine is a direct measure for your thrust. That's why we have a fan that creates 80% of thrust. Hence, if I add water in the compressor (e.g.rain in the air), I have more mass to displace, and thus more thrust created, no?
Of coarse, there is a limit to water as it also cools the flame in the combuster.
Why did 707's and such types have water injection in the past for takeoff power... More thrust, and more cooling for turbine blades. (and a black smoke trail)

[flame_bringer]

Piper19
The amount of oxygen to burn at the combustion chamber becomes less with humidity thats what I'm on about.
However I do agree with you on the cooling effect at the turbines which could
Be somewhat of an advantage.

[lomapaseo]

However I do agree with you on the cooling effect at the turbines which could
Be somewhat of an advantage.

:confused:

Lower gas temperature out of the combustor mean less energy in the turbine. Not good for performance

[Beeline]

More a question than a statement.

I remember when doing my engines module something about water actually displacing the air molecules and having a detrimental effect on thrust.

Water cooling is a way of creating cooler denser air increasing mass/weight of air but by means of performance still out-weighs the negatives of the water/air displacement??

[shumway76]

I think there is a difference between humidity & water injection.

Correct me if I'm wrong - (relative) humidity which is what this topic is talking about is the percentage of water that is in the air below saturation point.
Water injection on the other hand is to decrease the air temperature. A decrease in air temperature causes an increase in air density.

So why does turbine engine not be affected by humidity? From the answers above, I think the answer could be that turbine engine compressors will turn at the speed required to produce the requested "pressure". So if humidity is high (less dense air) the compressors will end up turning faster to produce the same amount pressure required.
Or did I get it all wrong there?

Whereas in a piston engine, the piston just sucks is air and burns it. The less dense the air, the less power produced.
Did I get it wrong again?

[Slasher]

Shum at 100% RH, water comprises 3% of the air by volume.
So since a turbine donk demands mass airflow, any apparent
increase of it would result in higher power for a given EGT. A
bloke needed an RH component to calculate TO & GA thrust in
the earlier jet engines so he wouldn't blow the s**t out of 'em.

Watermeth injection was simply to cool the air thus making it
denser (a la Rolly Rip Dart etc). The water did the cooling and
the methanol as the extra 'fuel' to make up the difference the
FCU pulled back on.

All based on memory as I haven't operated these engines in
the last million years.

[flame_bringer]

A turbine engine will try to spin at a certain speed in each power setting and if there is a loss of power somewhere (bleed leak for example) you will notice an increase in the EGT because it puts on more fuel to produce the same amount of power, however thats not the case in the humidity effect It's as I said before, Humidity does affect the performance but the influence is negligible because of the way the combustion chambers are designed and the fact that most of the air goes through the bypass.

[Beeline]

Cheers Shum.

So humidity could have a negative effect, more with overspeed margins than power/performance if compounded with higher temp air?

Or is the humidity part negligible?

[Beeline]

Just re-read slashers post and got my answers.

Thanks