Turbo Restrictors and Air Cavitation....

[takai]

SL666, true, i was thinking on carriers, where they do normally launch into the wind so that the planes can get the maximum amount of lift off the short launch.

[Pube]

true as well, aircraft takeoff and land into the wind for maximum lift at lower speeds but as for static testing it is very important that the engines face into the wind to minimise the risk of surging

[Billzilla]

Heres a bit of useless info, gas turbine engines can cavitate and destroy engines. However in the aircraft industry we call it a surge. I have seen it happen during engine testing where the aicraft is stationary on the ground and max takeoff power is applied and the aircraft is not facing directly into the wind. As the engine pulls air into the intake, the air can simply run out as seen by the intake begining the cloud up. As there is no more air in the front of the engine, the fan blades draw air fom behind the blades. This can cause an immediate stall and damage to the compressor blades and also a loud bang. Thats why it is important to face a gas turbine into the wind when running. Anyway thats my story for the day.

It's generally not that bad. In 16 years of operating jets I'ev only had two compressor stalls, or surges. Both were on landing in the 747 and all that happened was a loud 'BANG' and the EGT's went way high. But they came down over a minute or so with no damage.
But yes, in some rare cases it's possible to get a stall so bad it'll spit some blades out.
They'er pretty insensitive to airflow direction, we ignore it for engine start. It's very occasionally a problem when setting takeoff power at low speeds but as I mentioned I've never seen one. They used to be moer common than they are now though.

[SL666]

and all that happened was a loud 'BANG' and the EGT's went way high

not supposed to use nawz on a 747 zilla

[Billzilla]

not supposed to use nawz on a 747 zilla

But I was doing a G-Tec run bro!!!

(Actually I have, it was about 72,000hp)

[Pube]

Probably very rare on engine startup but when we carry out engine runs after maintenance we sometimes (depending on what was serviced) go to takeoff power for a few minutes with the wheels chocked and brakes applied. I saw if the other week with a 767 with Rolls Royce engines and the wind was light when it swung slightly across the intake, just a few bangs before they backed the fuel off. Apparently the Rollers are quite robust and wont break blades in most cases but the General Electric are a bit more sensitive and apparently the old Pratts were even more sensitive.

I guess on engine startup you cannot always face into the wind especially at airports when you push back you dont have a choice which way you will face or what taxiway you will be using.

[oldcorollas]

just to add another thought

turbo molecular pumps are commonly used for high vaccum devices... like.... argon sputterers, electron microscopes etc...
you use it after a rough rotary pump, and before a diffusion pump (which is a work of art in it's simplicity as well!)

the turbo molecular is basically like the axial compressor of a jet engine, has a diameter of about 3-4", and spins? 100? 200,000rpm?...

it's working in virtually NO air, and it's purpose is to pull out gas molecules and keep them out (vacuum, is a weird thing )

the blades are not in "stall or surge" as there is no air to do that

aiui, the restrictor is only an issue because of supersonic speeds and the pressure waves generated at that point cause havoc with airflow... and effectively places a limit

technically, cavitation occurs in fluids, and is when the pressure on the back of a blade decreases below the partial pressure of the fluids gas phase at that temp... whent hat happens, the fluid turns into a gas and forms a bubble.. or cavity...
the reason it is bad is not because the bubbles form.. but because the bubbles collapse and this can lead to severe erosion problems as the impact the fuid occurs at high speeds etc etc...

whilst gas is a fluid, it cannot cavitate, but as said the turbine can overspeed and lose it's "grip" on the air....

as for the bang.... is that caused by the blades going "relatively supersonic" in the low pressure air? (since speed of sound decreases in low pressure)

[kingmick]

i thought 'cavitation' was to do with water.. specifically it boiling?

i would asusume that turbo's might surge pretty badly if they run out of air.. but im not even sure that would happen.. NFI....

it is from turbulent water. lol i only said cavitation because taki said it so i thought it is how he understood it. i think he would have understood when i said" pulled threw" choked.

[kingmick]

Heres a bit of useless info, gas turbine engines can cavitate and destroy engines. However in the aircraft industry we call it a surge. I have seen it happen during engine testing where the aicraft is stationary on the ground and max takeoff power is applied and the aircraft is not facing directly into the wind. As the engine pulls air into the intake, the air can simply run out as seen by the intake begining the cloud up. As there is no more air in the front of the engine, the fan blades draw air fom behind the blades. This can cause an immediate stall and damage to the compressor blades and also a loud bang. Thats why it is important to face a gas turbine into the wind when running. Anyway thats my story for the day.

wasnt useless at all!

[kingmick]

small jets engines like the citations are fine! ive never had it on start up and its usually beside the hangar on startup which works as a nice wind chute. On the 747 i can understand it because of the size and the airbus's must get it more often than the 7's
lol of topic

[oldcorollas]

From some further thinking i have done the solution seems to be to start bleeding off exhaust through the wastegate as the RPM rises, but let the poundage fall, so that the shaft speed of the turbo stays the same.

Man thats going to be a pain in the arse to figure out.

thats pretty much it..... but it all depends on the compressor map..

the restrictor will limit the MASS of air going thru.. more velocity = lower pressure = higher pressure ratio in turbo = closer to surge line (there was some thread on PF i read regarding that also.. datman??)

so as well as the pressure decrease/supersonic thing in the restrictor itself.. you are changing the turbo inlet pressure and changing it's pressure ratio and efficiency..

basically you want to run the pressure ratio of the compressor either just below the surge line (or a bit below to get more efficiency) across the rev range.... however, knowing that, and having a compressor map... if you used one of the smarter boost controllers where you can program boost vs rpm and TPS, it should be relatively easy to work it out... and if you use a differential pressure sensor between the compressor inlet and outlet (so you are ONLY going off the pressure ratio.. instead of maximum pressure)

that make some sense?

[takai]

Yep, that makes sense there. I only had to read it twice to get it to make sense.
Keep this thread going, its a good explanation for restrictors atm.

Btw, i do know its just the compressor "over spinning" but didnt really connect it with "pulling through". And yes, cavitation is the wrong word, but a lot of the workshops i talk to sort of look at you with a huh when you talk about the turbo blades going supersonic etc. They know the word cavitation and so i generally use it.

[kingmick]

same here mate! when in rome.

[Billzilla]

How about a little backfire, like this?

http://www.micom.net/oops/Fire%20photo4.jpg


Okay, no more thread hijacking from me.
Interesting info Pube, thanks.

[kingmick]

just keep throwing fuel at it, it will relight.hahahahahahaha

new i shouldnt have filled it with avgas, need a bloody diffrent size filler neck on the avtur.