Basic Gas Turbine - A Couple of Q's

[kiwi_pilot_12]

Hi all,

I am sitting BGT tomorrow and have a couple of questions that have popped up with contradicting answers:

How is exhaust noise reduced in gas turbines?

Am I right in thinking that ideally you want to reduce the airflow velocity; therefore it will have less shear between stationary (atmospheric) air and the exhaust jetstream; therefore a divergent duct is favoured (in a diverging duct velocity will decrease, temp and pressure increase). Is this correct?
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Why do Pressure and Temperature increase in a compressor?

a) Potential and Kinetic Energy increase
b) Volume Increases
c) Total internal energy decreases
d) Compressor is a convergent duct
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3) Which of these affects the efficiency of a turboprop in a negative way compared to a turbojet?

a) TAS
b) IAS
c) Density
d) Pressure
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What does the EEC do?

a) Fine tune fuel to nozzles
b) Automatically prevent engine from exceeding its parameters


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What factor taken alone will increase thrust?

a) Increased Pressure
b) Increase Temperature
c) Increase Density

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What two (2) factors determine/affect a Turbojets propulsive efficiency

a) Pressure and RPM
b) Density and Speed
c) Temperature and Speed
d) Temperature and RPM




Any help would be sincerely appreciated!

[kiwi_pilot_12]

Anybody? :(

My exam is tomorrow...

[Username here]

Ill do my best mate, I'm studying for systems myself....



How is exhaust noise reduced in gas turbines?

By mixing cooler air with the exhaust to increase the frequency of the exhaust. A turbofan mixes the fan air with the exhaust prior to leaving the engine. The exhaust is usually a convergent duct to impart a final acceleration to the air.
__________________________________________________

Why do Pressure and Temperature increase in a compressor?


a) Potential and Kinetic Energy increase (my best guess - don't quote me!)
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3) Which of these affects the efficiency of a turboprop in a negative way compared to a turbojet?

b) IAS

As IAS increases the tip speed reaches a speed where compressibility rapidly decreases the efficiency of the turbo-prop. It's around 350Kts I think)
__________________________________________________

What does the EEC do?

a) Fine tune fuel to nozzles

__________________________________________________

What factor taken alone will increase thrust?

a) Increased Pressure
__________________________________________________

What two (2) factors determine/affect a Turbojets propulsive efficiency

b) Density and Speed

Propulsive efficiency is the greatest when the engine imparts a small acceleration to a large parcel of air. When the aircraft is flying at the speed of its exhaust propulsive efficiency is greates, soooo my guess is b....


Hope this helps! If anyone sees that I have lead him astray let me know - I've got ATPL systems next week so I'd be happy to be corrected myself. (It could be the blind leading the blind here! :})

[VH-FTS]

As IAS increases the tip speed reaches a speed where compressibility rapidly decreases the efficiency of the turbo-prop. It's around 350Kts I think)

Should that be TAS? No many turboprops will get anywhere near 300kts IAS.

[kiwi_pilot_12]

Hey mate, cheers for the reply!

However I have just spoken to my lecturer and he is telling me a couple of other things....CONFUSING!!

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1) He told me that you want to ideally SLOW the exhaust gases, creating less shear between the two air streams; therefore reducing the noise.

You're saying the opposite, correct? Haha

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3) EEC - He is telling me that b) Automatically prevent engine from exceeding its parameters, is the more correct answers. What do you think?

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5) What two (2) factors determine/affect a Turbojets propulsive efficiency?

A guy at my flight school put that as his answer today and got it wrong...not sure what to put!!!


:(

[TURIN]

I'll have a go.

1. The size of the shear interaction boundary needs to be greater. Have a look at all the old jet nozzles, Lots of different ideas from corrugated nozzles to multiple small pipes exiting the exhaust. The 787 also has a chevroned exhaust to increase the boundary between static and jet airflow.

2. D is correct, a is also partially correct, B & C are nonsense.

3. A

4. B is more correct. Although A is also true.

5. C, The more dense the air is the more mass flow you will get. Think about the problems big jets have at 'hot & high' airfields.

6. B. See above, also at higher speeds the turbojet benefits from a ram effect. (Up to transonic speeds, then it all goes haywire. have a look at the Concorde thread in Tech log for a complete explanation).

Hope this helps. Good luck. :ok:


If I have cocked up here, someone please put it right before I ruin this guys life!:uhoh:

[kiwi_pilot_12]

Surely IAS is more apparent than TAS as its what the propeller will "feel?

I am so confused with the factors affecting the turbojet...... :(

Also for question 2, compressors are diverging aren't they!? Therefore they increase pressure?

[TURIN]

The intake is divergent, as you say increasing pressure due to ram effect.
The Compressor gets steadily narrower, convergent. The compressor blades reduce in length the further through the engine you go.

http://www.zamandayolculuk.com/cetinbal/FJ/JetEngineCutaway.jpg

[muffman]

3: Definitely TAS not IAS. IAS has little relevance outside of aerofoil performance and does not really relate to propeller efficiency.

The reduction in efficiency with a turboprop occurs as you go faster through the air (measured by TAS), not as the mass of air flow increases (measured by IAS).

This is because the efficiency of a turboprop is in its ability to accelerate a large mass of air by a small amount, compared to a turbojet. As TAS increases, it would need to accelerate that large mass of air by a larger and larger amount in order to keep going faster, reducing its efficiency compared to a turbojet.

[Username here]

Yep, just check my AFT notes and its KTAS on the propulsive efficency graph...

[Bounceferret]

Can someone recommend a good BGT theory/study book?
Preferably similar to BT study guides (Practice questions etc)

I've already read the RR Jet Engine book.

Cheers.

[Lindstrim]

Waypoints one worked well.

Vol 11: Basic Turbine Knowledge - Pilot Books (http://www.waypoints.co.nz/books/basic-turbine-knowledge.html)

[WhyByFlier]

How is exhaust noise reduced in gas turbines?

Using a mixed exhaust. As an example look at the back of the two types of A320 engines - the IAE and CFM. The IAE has a mixed exhaust which mixes the cold air from the fan and the hot air from the combustion before expelling out of the back. The CFM has an unmixed exhaust with several layers of different temperature air. Noise, as has been mentioned above is created by the temperature gradient and is thus reduced by mixing the gases. The benefits of the unmixed exhaust are obviously higher thrust.

[john_tullamarine]

Exhaust jet noise ...

Probably a bit more complicated than temperature gradient. Principal contribution is jet velocity with noise usually correlating with velocity to a power of 6 to 8, typically quoted is the eighth power.

Plenty of useful tech links on the net, a typical paper being this one (http://adg.stanford.edu/aa241/noise/noise.html).

Keep in mind that the usual pilot jet engine references are broad brush .. one needs to refer to something a bit more directed for specific considerations.

Mind you, ask any knuck and the answer will be "the sound of freedom".

[OZvandriver]

RR Jet Engine book is an amazing resource for all this sort of stuff.

[Captain Nomad]

Jeppesen Aircraft Gas Turbine Powerplants is also a good text.

[A Squared]

Surely IAS is more apparent than TAS as its what the propeller will "feel?

As some others have said, it's TAS which is most relevant. The propeller efficiency is degrades when the prop tip speed approaches the speed of sound.

Try this thought experiment:

A airplane flying at the speed of sound at 50,000 ft will have a CAS of approximately 250 knots. Now imagine that you had a plane which could climb at a constant 250 knots IAS. IAS is generally within a few knots of CAS, so if you maintained a 250 knot IAS climb indefinitely, somewhere around 50,000 ft, you will exceed the speed of sound, even though your IAS hasn't changed. Now obviously, the speed of the prop tips though the air is going to be faster the speed of the aircraft through the air, (TAS) so sometime before you reach 50,000 ft, your prop tips will have reached the speed of sound and lost efficiency, even though your IAS has been constant.

Follow?

[FlareHighLandLong]

I think a couple of the responses about prop efficiency are relating a little bit to 'jet thinking' - i.e. mass airflow. Remember a prop blade is just like a wing. It produces lift by increasing AOA onto the airflow. As you accelerate, the angle of incidence of the air onto the prop 'disc' increases. In order to maintain the thrust, the blades must coarsen to maintain the same AOA. As this happens, the lift and drag vectors off each blade tilt. The tilting of the lift vector reduces the forward component, and you need to use a percentage of your torque simply to fight the lift your own prop blades are making. There is now also a component of the drag off each blade directly adding to the aircraft's total drag. Use a digital camera to see how coarse a prop blade is when at high alititude (with both high IAS and TAS). This is why increasing TAS and IAS are both equally bad for a prop. I would suggest that the most correct answer is B (TAS) Increasing TAS is bad for a prop, but invisible to a jet (I think - can you tell I've done some prop time?), whereas increasing IAS is good for a jet (ramrise), but also bad for a prop. Therefore the contrast is greater for increasing IAS, and the most (but not only) correct answer. Standing by to be smashed by the aero eng experts here.

Prop tip speed is generally more of an issue with regards to maximum airspeed for the aircraft, rather than being an issue for engine power.