Why do turbine engines require a compressor section
Hi Crabman.
You are on the right track.
What you have to understand here, is that the concepts which others have discussed (flame front speeds, timing and better burning) relate to design parameters of a particular engine using the Otto cycle - not the theoretical thermodynamics of the engine.
Oggers and CW say that faster “flame front speeds” make engine X more efficient at higher compression. I say you can make engine X just as efficient by operating it at low RPM. In fact, this is what the world's most efficient piston engines do - they operate at extremely low RPM.
Oggers and CW say that “better burning” make engine Y more efficient. If this were true, consider what would happen if you increased the compression ratio of an engine at constant fuel flow. Oggers and CW say that you will more completely combust much more of the fuel. This should mean that the exhaust gas temperature should increase. We saw from the linked document in post 41 that in fact, completely the opposite occurs – EGT reduces. Ask them to explain why “better burning” (which obviously they are implying means less unburnt fuel being wasted) causes the EGT to reduce.
You can easisly take their piston engine arguments out of the equation by running a large diesel engine at extremely low RPM (where flame front speeds mean jack sh*t and where the fuel is essentially completely combusted). But why does compression ratio still affect the thermodynamic efficiency?
I can tell you why - because you are adding less energy to the fluid.
As for the practicalities in a car or aircraft of a large, extremely low RPM diesel - I obviously understand this and a multitude of other reasons why for practical purposes there other considerations in making piston engines adaptable and useable over a large range for a particular application. But these are inconsequential to the FUNDAMENTAL concept of the OP.
Not only will Oggers and CWs arguments not apply to a large, efficiently designed and operated piston engine, they also fail to apply to a turbine. This is why they have both failed to answer my question about this so many times – because their reasoning simply can't explain it.
You are on the right track.
What you have to understand here, is that the concepts which others have discussed (flame front speeds, timing and better burning) relate to design parameters of a particular engine using the Otto cycle - not the theoretical thermodynamics of the engine.
Oggers and CW say that faster “flame front speeds” make engine X more efficient at higher compression. I say you can make engine X just as efficient by operating it at low RPM. In fact, this is what the world's most efficient piston engines do - they operate at extremely low RPM.
Oggers and CW say that “better burning” make engine Y more efficient. If this were true, consider what would happen if you increased the compression ratio of an engine at constant fuel flow. Oggers and CW say that you will more completely combust much more of the fuel. This should mean that the exhaust gas temperature should increase. We saw from the linked document in post 41 that in fact, completely the opposite occurs – EGT reduces. Ask them to explain why “better burning” (which obviously they are implying means less unburnt fuel being wasted) causes the EGT to reduce.
You can easisly take their piston engine arguments out of the equation by running a large diesel engine at extremely low RPM (where flame front speeds mean jack sh*t and where the fuel is essentially completely combusted). But why does compression ratio still affect the thermodynamic efficiency?
I can tell you why - because you are adding less energy to the fluid.
As for the practicalities in a car or aircraft of a large, extremely low RPM diesel - I obviously understand this and a multitude of other reasons why for practical purposes there other considerations in making piston engines adaptable and useable over a large range for a particular application. But these are inconsequential to the FUNDAMENTAL concept of the OP.
Not only will Oggers and CWs arguments not apply to a large, efficiently designed and operated piston engine, they also fail to apply to a turbine. This is why they have both failed to answer my question about this so many times – because their reasoning simply can't explain it.
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Hi Slippery Pete,
Er... I'd say that if the fuel was burned completely by the correct moment during the cycle, then more useful work can be extracted from the energy supplied, therefore you would need less fuel compared to the inefficient cycle, therefore the EGT would reduce.
Your engine X may be very efficient at low RPM, because the combustion is completed by the time the piston is at position Z. However it is not producing as much power as a faster revving engine.
As the revs increase, we need faster burning (hence higher compression ratio) so that combustion is complete by the time the piston is at position Z.
Ask them to explain why “better burning” (which obviously they are implying means less unburnt fuel being wasted) causes the EGT to reduce.
Your engine X may be very efficient at low RPM, because the combustion is completed by the time the piston is at position Z. However it is not producing as much power as a faster revving engine.
As the revs increase, we need faster burning (hence higher compression ratio) so that combustion is complete by the time the piston is at position Z.
Second Law
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Slippery Pete,
At no stage have I referred to flame front speed or indeed to "better" burning in this thread, I have never referred solely to piston engines.
I have carefully not considered the kinetics of the hydrocarbon combustion processes as they follow and do not lead in this discussion on "why do we compress?" We can go into the kinetics if you wish presumably to bring some order into this discussion if you wish, the pun is intended.
I do however agree that rate (or flame front speed) will show a form of direct proportionality to gas pressure during combustion. It follows from simple rearrangement of the ideal Gas Equation pV = nRT. Pressure can be equated as concentration on rearrangement etc.
What I have stressed is that compression allows more combustion per unit time thereby releasing more energy to do useful work per unit time.
We compress primarily to make the engine more powerful. You can be as efficient as you like in your isolated system engine but if you lack the power to reach Vr what's the point! I believe Oggers said something similar some time ago albeit in a different way.
Your efficiency argument is predicated on thermodynamically closed systems.
Remember that piston or gas turbine engines are more or less fully open systems thermodynamically.
The heating effect due to increased combustion per unit time is orders of magnitude greater than that relating to heating from compression alone.
Beyond a defined threshold, the need to add less combustion energy to an already compressed fluid is relatively trivial in an open system.
Hope this helps!
CW
At no stage have I referred to flame front speed or indeed to "better" burning in this thread, I have never referred solely to piston engines.
I have carefully not considered the kinetics of the hydrocarbon combustion processes as they follow and do not lead in this discussion on "why do we compress?" We can go into the kinetics if you wish presumably to bring some order into this discussion if you wish, the pun is intended.
I do however agree that rate (or flame front speed) will show a form of direct proportionality to gas pressure during combustion. It follows from simple rearrangement of the ideal Gas Equation pV = nRT. Pressure can be equated as concentration on rearrangement etc.
What I have stressed is that compression allows more combustion per unit time thereby releasing more energy to do useful work per unit time.
We compress primarily to make the engine more powerful. You can be as efficient as you like in your isolated system engine but if you lack the power to reach Vr what's the point! I believe Oggers said something similar some time ago albeit in a different way.
Your efficiency argument is predicated on thermodynamically closed systems.
Remember that piston or gas turbine engines are more or less fully open systems thermodynamically.
The heating effect due to increased combustion per unit time is orders of magnitude greater than that relating to heating from compression alone.
Beyond a defined threshold, the need to add less combustion energy to an already compressed fluid is relatively trivial in an open system.
Hope this helps!
CW
Last edited by chris weston; 16th Jan 2012 at 16:00.
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Compression
This is a little off topic but valid.. 3 'planes take off to the north but the route is to the south. (A) takes off, leaves the flaps down, turns south then cleans up and accelerates. (B) takes off, cleans up the flap, remains at 0 flap speed, turns south then accelerates. (C) cleans up, accelerates to 250 Kts IAS, then turns south. Which has the best performance and burns the least fuel?
It's "C". Why? At 250 Kts IAS there is more ram air and therefore more power. "C" was further south and higher on less fuel than "A" & "B". More compression (ram) means more power. This was done in the late '70s.
It's "C". Why? At 250 Kts IAS there is more ram air and therefore more power. "C" was further south and higher on less fuel than "A" & "B". More compression (ram) means more power. This was done in the late '70s.
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It's "C". Why? At 250 Kts IAS there is more ram air and therefore more power. "C" was further south and higher on less fuel than "A" & "B". More compression (ram) means more power.
So the "more power" aspect is fighting the physical facts of more time and distance. And you don't get "more power" except by burning more fuel. Perhaps "C" is correct, but not by much.
(Although - I certainly can sympathize with a Canadian hurrying to fly south...)
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I read all the above and reckon there’s room for another angle on this so I’m going to resurrect this thread and wade in here – this is one of those questions which keeps me awake at night – something that’s generally accepted (e.g. because a computer model or a graph tells you) but there are rarely any good/satisfying answers to be found …
The question was put in terms of gas turbines and also reciprocating engines. I’ll answer in terms of the former because that’s what I know about. I’ll assume that the same principle can be read across to reciprocating engines. That exercise is left to the student...
So why do we compress? I distinctly remember answering this with ‘because it packs more wallop’ at a university interview nearly 30 years ago - not a bad answer really, I got a degree in the end….
Let’s put a few things to bed:
1. It is nothing to do with rate of combustion.
I can show (below) that you can get more thrust (and/or better thermal efficiency) from a turbojet with a 14:1 OPR (overall pressure ratio) than from a turbojet with a 12:1 OPR both designed for the same inlet airflow and operating at the the same fuel flow.
2. It is nothing to do with keeping the air going in the right direction.
Mr Whittle figured he’d need a compressor for cycle efficiency reasons and was probably jolly glad that this also kept things going aftwards – so that was something he didn’t have to worry about. I can also show that an engine with hardly any OPR (1.1:1) will develop some thrust but will have a shocking thermal efficiency. You don’t need much pressure ratio to keep things going rearwards – a slightly inclined tube with a fire in it does the job pretty well - think of the Mt Blanc tunnel fire of 1999…
Essentially, the answer is that for the higher OPR engine, the nozzle pressure ratio (NPR) is higher so you get more pressure thrust. Momentum thrust is the same (in our example).
For a convergent nozzle: Gross Thrust = W8.v8 + (Ps8-Pambient).A8
... where 8 is the nozzle throat plane.
W is flow
Ps is static pressure
v is velocity
A is area
Momentum thrust (W8v8) is the same because W8 is the same (we fixed that at the start in the way we set up the comparison for both engines). Exhaust jet velocity v8 is the same because T8 is the same and the nozzle is choked (Mach 1).
But why is T8 the same for the second engine, and indeed, and why is NPR higher i.e. why is nozzle entry pressure (P7) higher?
Furthermore why, for the higher compressor PR, doesn’t the turbine operate at a similarly higher pressure ratio to make the NPR the same as before?
Burning questions indeed ....
Let’s look at two design points as described above. I ran these on a proprietary Gas Turbine simulation program (GasTurb – you can download a free reduced functionality version from the website). This saved me doing the calcs myself. The first engine had 12:1 OPR and the second 14:1 at the same airflow and fuel flow (for this we need a smaller turbine throat area and final nozzle area – these areas fall out of the thermo calcs).
Sure enough – we get a higher NPR in the 14:1 case at much the the same T8 because the extra T we’ve added at the compressor is taken off by the turbine, with about the same combustor heat addition in each case. Thrust is higher (due to increased NPR) and so is thermal efficiency (obviously, because we are getting more thrust at the same fuel flow).
Furthermore, Gasturb allows you to throttle back the second engine (i.e. run slightly slower at fixed geometry using nominal component maps) to get the same thrust as the first engine. Lo and behold, the thermal effy was slightly higher in the second engine case (throttled back – so no longer operating at 14:1).
The 1.1 OPR example gave quite a bit of thrust by virtue of the momentum term (v was significantly increased due to the temp rise) but the cycle was at ~1% thermal effy!
Compare this with Engine 1 at 40% and Engine 2 at 42%.
I haven’t answered those questions yet though - all the GasTurb run showed me is what we know happens, though it does illustrate very clearly that it is not a rate-of-combustion thing...
Essentially the answer is in the relationship:
Pressure Ratio = Temp Ratio ^ (gamma/gamma-1)
Because the turbine has to develop a deltaT (starting at a higher T due to the combustion process) to drive the compressor, the turbine temp ratio is smaller than the compressor temp ratio and consequently the pressure ratio is smaller. For an increase in OPR, the turbine therefore satisfies the extra power requirement with some pressure ‘to spare’ thus NPR increases.
You can do these calcs by hand, just using the equation above – making them very simple by assuming 100% component efficiency and assuming an identical combustor temp rise for each engine (not far from the truth in these two examples – remember we set the airflow and fuel flow to be the same). You can also assume constant gas properties (gamma and Cp) – realities such as varying gamma and Cp just make the sums trickier but don’t change the basic principle.
I realise that I haven’t really explained things all that satisfactorily as my ‘explanation’ still relies on an unchallenged thermodynamic equation (I’m happy to accept it!). But it does illustrate where the benefit comes from, and possibly more clearly than ‘explaining’ things in terms of Temperature-Entropy charts where the pressure lines ‘conveniently’ diverge! (for the same reason of course).
So, a slightly different, and perhaps long-winded - angle on the subject to what’s gone before. And hopefully useful !
The question was put in terms of gas turbines and also reciprocating engines. I’ll answer in terms of the former because that’s what I know about. I’ll assume that the same principle can be read across to reciprocating engines. That exercise is left to the student...
So why do we compress? I distinctly remember answering this with ‘because it packs more wallop’ at a university interview nearly 30 years ago - not a bad answer really, I got a degree in the end….
Let’s put a few things to bed:
1. It is nothing to do with rate of combustion.
I can show (below) that you can get more thrust (and/or better thermal efficiency) from a turbojet with a 14:1 OPR (overall pressure ratio) than from a turbojet with a 12:1 OPR both designed for the same inlet airflow and operating at the the same fuel flow.
2. It is nothing to do with keeping the air going in the right direction.
Mr Whittle figured he’d need a compressor for cycle efficiency reasons and was probably jolly glad that this also kept things going aftwards – so that was something he didn’t have to worry about. I can also show that an engine with hardly any OPR (1.1:1) will develop some thrust but will have a shocking thermal efficiency. You don’t need much pressure ratio to keep things going rearwards – a slightly inclined tube with a fire in it does the job pretty well - think of the Mt Blanc tunnel fire of 1999…
Essentially, the answer is that for the higher OPR engine, the nozzle pressure ratio (NPR) is higher so you get more pressure thrust. Momentum thrust is the same (in our example).
For a convergent nozzle: Gross Thrust = W8.v8 + (Ps8-Pambient).A8
... where 8 is the nozzle throat plane.
W is flow
Ps is static pressure
v is velocity
A is area
Momentum thrust (W8v8) is the same because W8 is the same (we fixed that at the start in the way we set up the comparison for both engines). Exhaust jet velocity v8 is the same because T8 is the same and the nozzle is choked (Mach 1).
But why is T8 the same for the second engine, and indeed, and why is NPR higher i.e. why is nozzle entry pressure (P7) higher?
Furthermore why, for the higher compressor PR, doesn’t the turbine operate at a similarly higher pressure ratio to make the NPR the same as before?
Burning questions indeed ....
Let’s look at two design points as described above. I ran these on a proprietary Gas Turbine simulation program (GasTurb – you can download a free reduced functionality version from the website). This saved me doing the calcs myself. The first engine had 12:1 OPR and the second 14:1 at the same airflow and fuel flow (for this we need a smaller turbine throat area and final nozzle area – these areas fall out of the thermo calcs).
Sure enough – we get a higher NPR in the 14:1 case at much the the same T8 because the extra T we’ve added at the compressor is taken off by the turbine, with about the same combustor heat addition in each case. Thrust is higher (due to increased NPR) and so is thermal efficiency (obviously, because we are getting more thrust at the same fuel flow).
Furthermore, Gasturb allows you to throttle back the second engine (i.e. run slightly slower at fixed geometry using nominal component maps) to get the same thrust as the first engine. Lo and behold, the thermal effy was slightly higher in the second engine case (throttled back – so no longer operating at 14:1).
The 1.1 OPR example gave quite a bit of thrust by virtue of the momentum term (v was significantly increased due to the temp rise) but the cycle was at ~1% thermal effy!
Compare this with Engine 1 at 40% and Engine 2 at 42%.
I haven’t answered those questions yet though - all the GasTurb run showed me is what we know happens, though it does illustrate very clearly that it is not a rate-of-combustion thing...
Essentially the answer is in the relationship:
Pressure Ratio = Temp Ratio ^ (gamma/gamma-1)
Because the turbine has to develop a deltaT (starting at a higher T due to the combustion process) to drive the compressor, the turbine temp ratio is smaller than the compressor temp ratio and consequently the pressure ratio is smaller. For an increase in OPR, the turbine therefore satisfies the extra power requirement with some pressure ‘to spare’ thus NPR increases.
You can do these calcs by hand, just using the equation above – making them very simple by assuming 100% component efficiency and assuming an identical combustor temp rise for each engine (not far from the truth in these two examples – remember we set the airflow and fuel flow to be the same). You can also assume constant gas properties (gamma and Cp) – realities such as varying gamma and Cp just make the sums trickier but don’t change the basic principle.
I realise that I haven’t really explained things all that satisfactorily as my ‘explanation’ still relies on an unchallenged thermodynamic equation (I’m happy to accept it!). But it does illustrate where the benefit comes from, and possibly more clearly than ‘explaining’ things in terms of Temperature-Entropy charts where the pressure lines ‘conveniently’ diverge! (for the same reason of course).
So, a slightly different, and perhaps long-winded - angle on the subject to what’s gone before. And hopefully useful !
2. It is nothing to do with keeping the air going in the right direction
But the subject did mention Gas Turbine
From a standing start how do you expect a rotational compressor to turn if the gas doesn't move in the direction of a turbine somewhere downstream of the combustor or are we getting into a related subject like a ram-jet ?
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I like Chris Weston's comment about compression. Isn't it the reason computers, geting smaller, also get faster? The Physics is similar? Faster is better. And faster implies proximity, yes?
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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.
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.
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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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.
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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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!
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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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?
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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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.
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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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.
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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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
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
Why do turbine engines require a compressor section
Apologies if anyone has already mentioned the real reason . . .
...............................
. . which is, of course, to absorb the power developed by the turbine and prevent it from overspeeding - simples!
...............................
. . which is, of course, to absorb the power developed by the turbine and prevent it from overspeeding - simples!
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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.
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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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.
