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Why do turbine engines require a compressor section

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Why do turbine engines require a compressor section

Old 21st Feb 2012, 07:03
  #141 (permalink)  
 
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Starting is an entirely separate issue. Claarly, the stable flow/burning process needs to be initiated by persuading everything to flow in the right dierction. However what this thread is all about (I believe - correct me if I'm wrong), is why an increase in compression ratio results in an increase in thermal efficiency.
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Old 21st Feb 2012, 20:42
  #142 (permalink)  
 
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However what this thread is all about (I believe - correct me if I'm wrong), is why an increase in compression ratio results in an increase in thermal efficiency.
The thread is whatever the responders make it

This is so often true in the technical section where the thread starter drops a one-liner subject header and disappears into the nether leaving us to expound all sorts of intelligence that falls on deaf ears.

but it's fun anyway
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Old 22nd Feb 2012, 00:41
  #143 (permalink)  
 
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Keeping the gas going in the "right direction" is everything to do with propulsion.

The starting cycle is a simple way of understanding the need for compression. Without compression, there is no work. How can compression become irrelevant to the process at any time?

Starting is compression, it contains gases in a dynamic flow by directing its passage from a large to small annulus. Without the pipe, there is no power, and without the fan, there is only multidirectional expansion.

Water injection, Open Iris Afterburner, Hypergolic starting, Start carts, Compression is our friend. The difference between early centrifugal compressors and axial flow instructs as to the OP post. PUSH, PACK, POP, off you go.
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Old 22nd Feb 2012, 07:14
  #144 (permalink)  
 
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Lomapaseo – quite right, we do (and should) pick up stuff around the edge of the discussion, however I was addressing what I considered to be the main question. And yes – it is fun.

Lyman, you said “Keeping the gas going in the "right direction" is everything to do with propulsion”.


I’m not disagreeing with you – I was suggesting that the reason the compressor is there is for fundamental cycle effy reasons and not primarily to set the flow direction (it’s perhaps more of a philosophical point I am making here).

You also said : The starting cycle is a simple way of understanding the need for compression. Without compression, there is no work. How can compression become irrelevant to the process at any time?


I’m certainly not saying compression is irrelevant to the process. I’ve established that it is fundamental to getting work out of a machine at a good thermal efficiency. Work is required in the start phase just as in other operational phases (steady-state, accels and decels alike)

But starting is a means to an end, it is all about getting the engine to a stable, self sustaining operation to deliver the required thrust. Starting needs compression, sure, and it needs external cranking power (or windmilling). And yes, compression sets the flow direction, though you have to be especially careful to control the fuel flow (and variable geometry if you have any) otherwise the compressor might get upset (stall). Same comment applies for ‘normal’ (engine-started) operation where reverse flow may occur under some circumstances.

‘Compression is our friend’ – certainly, it keeps me in a job. Compressors however are a nuisance!
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Old 22nd Feb 2012, 11:16
  #145 (permalink)  
 
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Hi jh5speed,

Our local National Trust house has a very old gas turbine installed in the kitchen.

It consists of an open coal fire (at ambient atmospheric pressure) and a very long vertical jet pipe called a chimney. A turbine (set of fan blades) is placed above the fire and its rotation is converted through a suitable system of bell cranks and levers to turn the spit and roast the meat.

It doesn't seem to be very efficient - would a compressor help?
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Old 22nd Feb 2012, 11:50
  #146 (permalink)  
 
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Without a doubt, though probably the extra cost of control-system improvements would outweigh the efficiency gain (however calculated - I guess it would need a labour cost element as well as fuel cost to account for those poor servants who, at present, have to waste valuable time checking that the meat is browning evenly, and fetching in the extra odd bucket of coal). But then, if they weren't doing that, what would they be getting up to?

Lyman will advise you on getting the fire lit without smoking the place out.

Once lit, I reckon if smoke isn't coming back into the room and the meat is turning, you'll probably be better off leaving well alone.

Oh - and it might be a good idea to send a child up the chimney with a tub of goose fat to grease all those bellcranks and pulleys (thus increasing mechanical efficiency) ... While he (or she) is up there a decoke of the turbine blades might be a good idea.

Last edited by jh5speed; 22nd Feb 2012 at 12:04.
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Old 22nd Feb 2012, 12:45
  #147 (permalink)  
 
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Hi jh5speed,

Thanks for the tips and advice which I've passed on to the N.T. kitchen turbine department.
Apparently they have tried using a compressor (mechanical bellows of soft leather nailed between flat wooden plates) which increases the kinetics and raised the combustion temperature. The more energetic exhaust flue gas turned the turbine more quickly.

It seems to be a win win situation, but the bellows operator is demanding extra pay because of the increase in thermal efficiency. They are now working on a mechanically driven set of bellows from the extra power from their turbine.
They want to know if it is worth adding a second turbine, downstream of the first to work the bellows?

They are grateful for the advise on the turbine wash and suitable gearbox lubrication.
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Old 22nd Feb 2012, 14:39
  #148 (permalink)  
 
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jh5speed, rudderrat

Hmmm. Save the coal for your children at Christmas, seal the bellows at the top of the flue and have your valet pump the bellows into the chimney in reverse flow. Change the pitch on your fan, and use the gear to drive your abacus.

Do your part for the Planet
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Old 28th Feb 2012, 16:18
  #149 (permalink)  
 
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more thoughts..if I dare

I think this is a new angle and hope it is not in the mind-numbing category (not derogatory but complimentary in this context) of most of the previous posts. It is not in answer to any outstanding question, be it either still to be answered or of great merit.

As already stated the RJ needs (and has) a compressor just as does a jet engine. I suspect the reason for the initial observation that it doesn't have one is the belief that they are fundamentally different machines. The RJ and TF are just different breeds of the same with very similar cycle requirements but obtained from very different looking parts. I don't think there's any great stumbling block to be made of the fact that a conventional RJ doesn't work at zero speed but the TF can.

At zero airspeed the TF does no more than an RJ. It can sit on a test bed running at TO from Monday to Sunday (with slave oil supply) and it has actually done less than nothing. It has moved nothing, except the thrust cradle a few thou, but has cost a weeks worth of fuel/oil/cell occupancy/creep life/etc. Once it starts down the runway though, it has taken the first step to being a ramjet and the turbom/c compressor has taken the first step to being redundant. eg you can't use the SLS 43:1 PR of a big fan in an F-22.

The B777 at cruise has the same subsonic piece of compressor hardware as the pre-SCRJ, a piece of ducting. the only difference is that the rest of the compressor is downstream on the TF but upstream on the RJ.

I find it satisfying to look for underlying similarities rather than thinking there are fundamental differences.

To get the job done both the RJ compressor, with its attendant shock compression, and the TFC, with its supersonic regimes, treat the air with the utmost violence on the one hand, and then gently on the other, with the touchy subsonic diffusion in the duct or rotating blade rows and fixed stator passages.

The degree of brutality which the RJC metes out has always been foisted upon it by the missile cruise requirements. The turbom/c, on the other hand, has increased in brutality from the gentle subsonic compressor of a J79, for example, where the inevitable low stage PR required 17 stages to get about 13:1. You have to thrash a lot more energy into the air if you want a compact HPC where 10 stages give about 20:1. The road to this level began with turbocompressors entering the RJ compressor regime by using supersonic blade relative MN.

Therein lies a similarity. It's high relative MN between air and pieces of metal that give you the makings of a compressor. You don't necessarily need relative motion between the metal of the compressor and its 'mother'.

But the conventional RJ still can't get off the ground!
Bear in mind that the TF only exists to cruise just like the RJ. It needs to be a 'different' machine to get there, in as much as, in its money making regime, it has different ratings, ECS bleeds, turbine clearance bleeds, etc. compared to TO and CL. A B777 won't economically get to 35000ft in its cruise 'config'. The TF is a hybrid.

So what if an RJ also needs a bit of hybridization to get to cruise.
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Old 28th Feb 2012, 23:20
  #150 (permalink)  
 
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Barit 1 A car with a turbo charger (or supercharger) produces more power due to the compressed air being forced into the intake but does not burn more fuel. Hence compressing the intake air (more volume) makes a smaller engine produce the same power as a non turbo charged larger engine. Why do jet engines require a compressor? Because they wouldn't work otherwise. Simple. Don't make a mountain out of a mole hill.
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Old 28th Feb 2012, 23:31
  #151 (permalink)  
 
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the rate of any

Hmm. Reverse flow (architecture) supercharging packs gobs more fuel and air into its engine.

Those whose purpose it is to go fast aren't always the huggy fluffs who insist on good economy. Increasing the rate of any exothermia produces more excitement, hence adding more oxygen and/or fuel per event. PACK/POP.

Or push/pop. Choose wisely.

'pull/bang' is for sissies

thermostat. It may be semantic, but your engine will burn more fuel due its increased supply of O2. There will be more power, and more RPM, and more fuel burned. You will have to rate the engine to keep fuel burn equal to pre boost value, No?

Last edited by Lyman; 28th Feb 2012 at 23:44.
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Old 29th Feb 2012, 01:37
  #152 (permalink)  
 
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thermostat:

Recip or turbine, more air requires more fuel. The exact ratio will vary slightly (but ONLY slightly) as component efficiencies migrate, but if you've found a way to boost airflow without increasing fuel flow, better patent it REAL QUICK 'cuz you'll soon be a billionaire!
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Old 29th Feb 2012, 15:13
  #153 (permalink)  
 
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That's darn good advice. Thanks, you're a good man.
My mechanic says yes, it will burn more fuel. However because of the increased power and better performance you may actually get better fuel mileage. It's like the plane in 'C'; clean up flaps (get rid of the drag as you get rid of lift) increase to 250 kias for more ram and climb higher, faster, on less fuel.
Remember that drag increases with the square of lift; increase lift by 3 increases drag by 9. Get rid of the flap drag ASAP etc.
Back in the 70's the fuel prices increased 6 times, so the airlines were forced to find ways to save fuel. That led to: landing with less flap, shutting down an engine after landing, reducing the flap retraction altitude after T.O., cruising at M0.80 instead of M0.082, planning your descent at idle power until the gear was lowered. We even started the last engine 3 mins before take off. Landing flap taken at 1000ft agl instead of at the FAF. These are procedures that most newer pilots know nothing of. Saving fuel just may keep your airline alive.
Happy landings.
T

Last edited by thermostat; 29th Feb 2012 at 15:36.
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Old 3rd Mar 2012, 01:55
  #154 (permalink)  
 
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Quite OK, thermostat, I've managed to learn a thing or two after 70 years around planes and engines, and if I'm able to pass some of it along, I'm flattered.

But my original point was that when making a 180, speed is not always your friend. It increases the time and distance required to complete the maneuver; the extreme case being an SR-71 requiring three states and 15 minutes to reverse direction at Mach 3. It's Newtonian mechanics, pure and simple.


"You've never been lost until you've been lost at Mach 3." -- Paul Crickmore
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Old 3rd Mar 2012, 16:44
  #155 (permalink)  
 
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the airlines were forced to find ways to save fuel. That led to: landing with less flap, shutting down an engine after landing, reducing the flap retraction altitude after T.O., cruising at M0.80 instead of M0.082, planning your descent at idle power until the gear was lowered. We even started the last engine 3 mins before take off. Landing flap taken at 1000ft agl instead of at the FAF. These are procedures that most newer pilots know nothing of. Saving fuel just may keep your airline alive.

regrettably the SOP/Training minions have made this harder to do. I have also had things like "you must be at ref, fully configured at the FAF", "climb at V2+10 up to 1500'", "lack of descent/climb planning ie: descending at 1500 regardless of groundspeed, or climbing best rate into a 100kt headwind"

Common Sense, isn't all that Common
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