Some jet engine theory (Lupin)

[Guest HavaJava]

Hi,

Lupin, I deleted your e-mail address by accident...my written project went very well though (87%). For everyone else, I'm preparing for my oral exam in my Flight Propulsion class at Seneca College. Every student had to prepare a report on a different engine and mine was the General Electric CF-34-3A1.

I know of a couple of the questions that I will be asked in this exam and I wanted to get some feedback from the experts.

First, the main point of my oral exam will be to discuss the shape of the flow channels from the fan to the compressor and from the combustion chamber to the turbine. The duct gets smaller in radius as it comes from the fan to the high pressure compressor. The duct from the C.C. (and high pressure turbine) to the low pressure turbine gets larger in radius. I figure this has something to do with helping to compress the entering air but I'm not sure. I'm also thinking that this is done in order to keep the high pressure compressor blades short. I'm kind of confused about the theory behind this though.

The other questions I know of are

- Why does the spinner rotate along with the fan? (is there an aerodynamic reason or is it just for ease of construction?)

- The reasons for trailing edge cooling of turbine blades.

Thanks in advance for any help any one can give me.

Andrew

[Guest lupin]

I can be reached at mrlupin10@hotmail.com

Lupin

[Guest lupin]

Umm...bernouilli's theory explains alot of the diferent shapes and channels inside the engine but I think I can get it down much simpler then that.

The nominal pressure for the CF34-3a is 14.5 ,that means that the pressure of the air at the compressor discharge point in the engine will be 14.5 times higher then inlet pressure.(think of absolute pressures)

The reason the air duct gets smaller is that the air is getting compressed.The compressor takes a volume of air and compresses it before adding vaporized fuel to it and letting it expand again to then extract energie from the expanding gases.Therefore the smallest path in the engine should be tour last stage of compressor blades.(where you will find peak pressure in the engine)Every stage of compression actually compresses the air by a ration of 1.44 to 1 so as the air is compressed,the air path gets smaller because the same volume of air now takes less space.

Now if you want to get even more technical about it,between each compressor stage you will find a set of stators.They are set up to make the air path slighly diverging (the air is compressed by the compressor then allowed to expand slightly)That phenemenon is explained by bernouilli's theory.By opening up the path slighly you lose velocity,but you gain pressure(what you are ultimately seeking).It also makes it easier to compress the air if its not travelling so fast,the compressor is more efficient that way.This happens at every stator stage in the compressor.

When you get to the turbines....the air has passed its point of highest pressure.Contrary to popular beleif...the highest pressure point in the engine is not when the fuel/air mixture is burned but slightly before.The compressor discharge is the highest pressure point in the engine. When the mixture is burned....the expansion has started,you are now getting increased velocity (therefor lower pressure)of the gases and this energy will be mostly taken away by the multiple stages of turbine.

For your lenght of the blades question.....The path which the air takes is like a ventury.big at the front,narow in the middle,big at the back.The blade lenght is set up so that the stage of compressor blade or turbine blade is optimum for compressing or extracting energie from the gases at that particular point.As the air gets compressed in the compressor it takes less space.its path is smaller therefore the turbine blades will have to be shorter.Hope that explains it for you....

for your spinner question.....the engine is a dual spool engine.you have a high pressure section and a low pressure section.the low pressure spool is the longuer one,and the high pressure spool is shorter .the two shafts that conect the compressor to turbines are one over the other.The high pressure shaft is hollow and bigger then the low pressure shaft.The long low pressure shaft actually runs inside the high pressure shaft,allowing them to have diferent speeds.Have a look inside your books,you should easily be able to get a diagram explaining how the two shafts are set up.With that you should be able to understand why the spinner rotates with the fan,it would be nearly impossible (to my knoledge)to get an engine to have a stationary spinner.Its like the spinner on a prop engine... it has to spin,it's attached to the prop.The same is applicable to the turbine engine...the spinner is attached to the fan.

And the reason for the cooling of turbines blades and stators?It allows the engine to run at higher temperatures when you cool the blades and stators with bleed air.The hot bleed air is much cooler then the gases running over the blades.Cooled stator vanes (turbine nozzles)and turbine blades can operate at higher temperatures without melting .Operating at higher temperatures allow you to generate more power with less fuel.As to why they are cooled on the trailing edge??I imagine that letting the bleed air exit the blade in the trailing edge section allows the entire blade to be cooled.

Lupin