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A quick question on compressors

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A quick question on compressors

Old 16th May 2014, 07:58
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maf
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A quick question on compressors

Reading ATPL-theory on gas turbines, Im trying to understand how the flow through the engine works. (I do understand it, just have a problem understanding how it is sustainable)

When you start your engine on a calm day, standing still at the tarmac, What makes the air flow so easily through, say your CFM56?

Air acts like current. It wants to balance inequal pressure and go from areas of higher pressure to areas of lower pressure until equal pressure exists. (taking the easiest way out)

So, once you start your engine and it achieves idle speed and is self sustainable,
the pressure in the compressor is higher than ambient atmospheric pressure on the outside. What makes it not flow the easiest way out?

Is it the velocity of air coming in the duct?
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Old 16th May 2014, 08:19
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A quick question on compressors

No iT is the compressor sucking iT in which is driven by turbine which is driven by combustion exaust gasses.
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Old 16th May 2014, 08:42
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Is it the velocity of air coming in the duct?
Correct. The dynamic pressure exceeds the static pressure ensuring gas flow from front to back.

Page 9 http://www.rolls-royce.com/Images/ga...tcm92-4977.pdf
"The blades accelerate the air increasing its dynamic pressure, and then the vanes decelerate the air transferring kinetic energy into static pressure rises."
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Old 16th May 2014, 08:51
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Thanks Goldenrivett.

Smoothflight: Im not shure what you mean. I know how the engine is driven, but was unsure how to explain the self-sustainability of it.
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Old 16th May 2014, 09:09
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You start the engine by turning the compressor with a starter. This creates a flow from front of the engine throught the compressor, combustion chamber and turbine to the exhaust. Starter turns the engine to the point where the engine is able to sustain itself.

If you have flow through the compressor, and you have fuel (and ignition or sufficient compression for the fuel to ignite), the turbine will get energy and will be able to run the compressor, which will then be able to "suck" new air from the front of the engine, and so on, and so on.
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Old 16th May 2014, 13:27
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So, once you start your engine and it achieves idle speed and is self sustainable,
the pressure in the compressor is higher than ambient atmospheric pressure on the outside. What makes it not flow the easiest way out?
Like a window fan, the blades in the front beat the air back before it can escape forward again
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Old 16th May 2014, 17:47
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Although an axial compressor has a few things in common with a window fan, the analogy is pretty wide of the mark.

A jet engine is a gas turbine. How does that gas turbine get the compressed air it needs to operate? It drives a shaft coupled to a compressor. Most compressors used to be centrifugal types, but now they're almost exclusively axial. If you want to know how an axial compressor works, do a Google search on "how does an axial compressor work".

In fact, if you want to know anything, do a Google search on it and click on the Wikipedia entry. There you are bound to find what you're looking for and you won't get a lot of guesses about blades window fan beating the air.

As far as the gas escaping the "wrong" way, that's exactly what happens during a compressor stall. Axial compressors rely on the orderly flow of air from front to back. High efficiency compressors are not far from disaster much of the time and dirty blades, a bad fuel control unit, damage to the blades, or simply feeding air into the inlet other than head-on can produce a compressor stall.

I used to fly a high performance fighter and, as all of us who understand what pushing the envelop really means, I have a little bit of high speed flight time traveling backwards with fire shooting out of the intake. The old J-57 was not prone to compressor stall most of the time, but it certainly didn't take kindly to going backwards at speeds.

Last edited by Mozella; 23rd May 2014 at 03:32.
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Old 17th May 2014, 05:12
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Mozella,

I think if you do a bit of research on this site, you might find that lomapaseo knows quite a bit about aircraft engines and turbine technology.
You might even learn something new even if you were a fighter jock.
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Old 18th May 2014, 14:39
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I will attempt to to reduce this question to the simple, because I am a simpleton...

In A&P School and advanced flight training I would get stuck on just actually what happens inside a jet engine. A&P books had a simple formula called 2+2. There were two formulas of jet thrust. They are F=MA + P x A That's how a jet engine works. Force = Mass times acceleration and at the exit nozzle it's pressure x area.

Now as to pressure inside the engine it is not immediately apparent but the highest pressure in a jet engine is at the outlet of the last stage of compression. This is the entrance to the combustion section.

Now we all know that if we build a fire in a closed are the pressure will go up. So how come the pressure ahead of the combustion chamber is higher than the pressure in the combustion are itself? It's because the area of the combustion chamber is larger than the minimal area of the outlet of the last stage of compression and therefore the pressure actually falls slightly.

At the other end of the burner section are the wide open spaces of the turbine section. There may be a flow guide or stator but the design is such that there is a rapid area increase in the engine which aids in accelerating the gas flow. This also facilitates pressure drop in this section of the engine. There is extra energy in the flow stream from the fuel being burned. The heat energy is translated directly into velocity. The trick of jet engine fuel control is to have a big fire int he engine without making to too big so that the pressure in the burner can gets higher than the outlet of the compressor. If that happens the engine will surge or stall or essentially "fart" out the front, instead of moving air from front to back.


Some of that velocity is given up against the turbines and the extra is thrust at the exhaust nozzle.

At the nozzle outlet there is some design criteria to get a slight pressure rise to aid in the 2+2 formula by varying the area of the outlet.

Now that we are all confused at a higher level, what was the original question?
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Old 19th May 2014, 13:07
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When you start your engine on a calm day, standing still at the tarmac, What makes the air flow so easily through, say your CFM56?
The compressor rotor blades are small aerofoils, so they generate a lift force when the engine is rotating. They also generate what in a wing we would call downwash. The lift force acts in a forward direction, so the resulting downwash, acts in a rearward direction. This force the air to move rearwards through the compressor.

Air acts like current. It wants to balance inequal pressure and go from areas of higher pressure to areas of lower pressure until equal pressure exists. (taking the easiest way out)
You are correct in saying that air (and all other fluids) tends to flow from area of high pressure to areas of lower pressure. When the engine is running the pressure increases as the air flows from the front stages of the compressor to the rear stages. This suggests that the air will rend to flow forwards through the compressor.

We can see why this does not happen (most of the time) by imagining that we have taken a very thin slice of the compressor at right angles to the drive shaft. The slice is very thin so the static pressure (acting in a forward direction) on the rear face of the slice, will be equal and opposite to the static pressure (acting in a rearward direction) on the forward face of the slice. If the air is moving from front to rear we will also have a certain amount of dynamic pressure. But dynamic pressure acts only downstream in the direction of flow. So it will exert a rearward acting force on the front face of the slice, but no force on the rearward face. So we will have static pressure plus dynamic pressure acting rearwards and only static pressure acting forwards. So the air will tend to flow from the area of high total pressure at the front face, towards the lower total pressure at the rear face. As long as these conditions persist, the airflow will be from the front of the compressor to the rear.

In order to produce a flow reversal we need something to reverse the pressure difference across our slice of compressor. One such factor could be a sudden large increase in throttle setting. This would increase the fuel flow, which would in turn create an increase in static pressure in the combustion chamber. If this increase is sufficiently large, the (forward acting) static pressure in the combustion chamber will become greater than the (rearward acting) sum of static and dynamic pressure at the compressor outlet. This will cause the air to flow from the combustion chamber into the rear of the compressor. This reverse flow will then cause the blades of the rear stages of the compressor to stall, thereby permitting the reverse flow to continue to move forward. There are of course a number of other factors that can cause the airflow to reverse. Most of these involve aerodynamic stalling of the compressor blades (due to excessive angles of attack), but in all such cases the reverse flow is ultimately due to pressure differences.


So, once you start your engine and it achieves idle speed and is self sustainable, the pressure in the compressor is higher than ambient atmospheric pressure on the outside. What makes it not flow the easiest way out?

Is it the velocity of air coming in the duct?
It does always follow the easiest way out, but it may not be obvious that this is what it is doing.
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Old 19th May 2014, 14:00
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Look at the pressure v. Velocity diagram here. Pressure declines in the burner but velocity goes up almost vertically. Remember F = M x A. Force = Mass times acceleration. The pressure drops due to acceleration and pressure loss across the turbine. Note the gradual pressure drop in the burner section. This is a critical design area. You need pressure drop but you also need a big fire.

One picture is worth a thousand explanations.
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Old 20th May 2014, 11:15
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You are of course correct in saying that pictures are very useful. But we must take great care in how we interpret them.

The blue line in your top diagram is not static pressure, but is total pressure.

The area immediately aft of the HP compressor outlet is called the Diffuser. It is a divergent duct and its purpose is to reduce the air velocity to a level that will enable the flame to remain in the primary zone of the combustion chamber, without being blown aft. The flow through the divergent duct of the diffuser is subsonic, so its velocity decreases. This causes its dynamic pressure to decrease and its static pressure to increase. But there is an overall reduction in total pressure because of friction losses. All of this causes the highest static pressure to occur, not at the outlet of the compressor, but at the outlet of the diffuser. It is the highest total pressure which occurs at the outlet of the compressor.


Look at the pressure v. Velocity diagram here. Pressure declines in the burner but velocity goes up almost vertically. Remember F = M x A. Force = Mass times acceleration. The pressure drops due to acceleration and pressure loss across the turbine. Note the gradual pressure drop in the burner section. This is a critical design area. You need pressure drop but you also need a big fire.
In theory the gas turbine employs a constant pressure combustion process. Ideally we would like the static pressure to remain constant. To achieve this we allow the air to expand aft as it is heated. But friction and losses of heat through the engine casing cause some loss of energy and this causes reductions in velocity and total pressure. We do not actually need the pressure drop in the combustion chamber, but we must avoid getting any significant static pressure increase, because this would tend to cause reverse flow.

As the air flows through the turbines a great deal of energy is extracted from it to drive the engine. It is this extraction of energy that causes the reduction in both static pressure and total pressure.

But the real problem with your employment of this diagram in this thread, is that it does not actually address the original question. IE "Why does the air not flow from the high pressure area at the back of the compressor to the lower pressure area at the front?"


maf, in answer to your statement that:

I know how the engine is driven, but was unsure how to explain the self-sustainability of it.
The process of driving the compressor and accessories at any given rpm requires a certain amount of power. This power is provided by the turbines which extract energy from the hot gasses. Self-sustaining rpm is the lowest rpm at which the turbines can provided sufficient power to keep the compressor running at that rpm.

At speeds below self-sustaining rpm, the turbines are unable to provide sufficient power to keep the engine running. So the starter motor must be used to assist the turbines to keep the engine running and also to provide the additional power that is required to accelerate the engine to higher rpm. If you switch off the starter at any rpm lower than self-sustaining, there will be insufficient power available to keep it running, so the engine will run down and stop.

If you switch off the starter at exactly self-sustaining rpm there will be just sufficient power to keep the engine running at that rpm, but not sufficient to enable it to accelerate to higher rpm. This will cause the rpm to stagnate in what is termed a “hung start”. Unless the pilot acts quickly to shut the throttle, the temperature limits of the turbines will be exceeded.

If you switch off the starter at any rpm above self-sustaining, the turbines will provide sufficient power to accelerate the engine to higher rpm. So a standard start sequence involves running the starter until the engine has reached some specified rpm, which is above self-sustaining speed.

Last edited by keith williams; 20th May 2014 at 14:33.
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Old 21st May 2014, 09:32
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I have question also:

While air leaving compressor is not cooled as it done with almost all piston engines that use compressor/turbine?
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Old 21st May 2014, 11:31
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The short answer is that they are too big and heavy for use on gas turbine engines. The rather longer answer is below.

Turbochargers are used in some high performance piston engines to increase the mass of air flowing into the inlet cylinders. This increases both the mass flow rate of air passing through the engine, and the overall compression ratio. The overall effect is an increase in power output.

But the turbocharging process also increase the temperature of the air going into the cylinders. This can reduce combustion efficiency, cause detonation, excessive wear and heat damage to the cylinders.

Intercoolers are sometime fitted between the turbocharger outlet and the engine inlet manifold, to reduce the temperature of the incoming air. They are simply heat exchangers in which heat is transferred from the charge air to some form of cooling agent such as air or liquid.

The main disadvantages of using intercoolers is increased weight and loss of pressure due to friction losses within the heat exchangers. They also waste fuel in that the heat taken from the charge air is usually discharged into the atmosphere.


In piston engines virtually all of the charge air is used for combustion, so the problem of excess charge air temperature is particularly severe. But in gas turbine engines only a tiny fraction (often less that 5%)) of the air is used for combustion. This leaves a large volume of air available for internal cooling. So the problem of high charge air temperature is less severe. This factor coupled, with the weight penalty involved in fitting intercoolers has meant that aircraft gas turbines do not use them.

Many modern ships employ gas turbine engines as their main propulsion systems. In these cases the availability of extra space (compared to aircraft) and the need for maximum fuel efficiency have led to the use of combined intercooling-recuperation systems.


The description below is taken from a Rolls Royce website describing the 25MW-WR-21, which is based on the Trent engine.

Compressor intercooling
The intercooler, located between the compressor sections, cools the intermediate-pressure compressor (IPC) air before it enters the high-pressure compressor (HPC). This reduces the HPC inlet temperature and therefore HPC work to deliver a significant boost in engine power. The intercooler also enhances recuperator effectiveness, as the inlet temperature is reduced thereby increasing exhaust heat recovery.

Exhaust energy recovery
The recuperator recovers and transfers heat energy from the hot exhaust, which is used to preheat combustion air, therefore much less fuel is required to reach the same power turbine entry temperature (PTET). As a result less fuel is used to achieve the same power.
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Old 21st May 2014, 12:39
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When you talk about the turbine being able to self-sustain, perhaps an analogy will help. Think about a single-cylinder piston engine.

The intake and compression strokes provide a charge of fuel-air mixture into the cylinder, compressed, and ready for ignition. The spark plug ignites the mixture, and the power stroke turns the crankshaft, and the exhaust stroke empties the burned charge.

But what drives the machine into the next cycle? What provides the energy for the next compression stroke?

It's the stored energy - sometimes an external flywheel (or a mower blade, etc.) - that pushes the crank around through the first two strokes of the next cycle.

Similarly in a turbine engine, there is stored energy is the compressed air, and the compressor designer's job is to prevent a stall (surge) - i.e. that air is not back-flowing out the inlet. He makes sure each airfoil sees its incoming flow at a safe angle of attack, and when this rule is satisfied, the cycle happily continues.
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Old 22nd May 2014, 22:13
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Hot air is less dense than cool air. In in a piston engine, higher temperature means fewer molecules of air (and therefore oxygen) can fit in the cylinder at a given pressure, less fuel can be burned, and less power can be created. Increasing the pressure to compensate would only compound the detonation and wear issues from the high temperature itself.

I don't know whether there's a parallel effect with turbine engines.
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Old 23rd May 2014, 00:51
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Temperature has a definite effect on life. Rule of thumb IIRC is +20⁰C. => 50% life reduction.

Pressure has an effect too, but it's counted more as stress cycles (shutdown > TO > shutdown), and termed LCF or Low Cycle Fatigue. Pressure vessels like casings are certified for a safe life, after which they are scrapped.
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Old 23rd May 2014, 01:31
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Pressure has an effect too, but it's counted more as stress cycles (shutdown > TO > shutdown), and termed LCF or Low Cycle Fatigue. Pressure vessels like casings are certified for a safe life, after which they are scrapped.
I not aware of pressure vessels being certified for safe life under a regulation. Quite a few combustion chambers blew up after numerous weld repairs beyond 50,000 cycles.

Of course it helps if the user volunteers to scraps them as often as he does rotor disks.
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Old 23rd May 2014, 03:51
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How do you get one going backwards at high speed?
Here's one recipe:

First get going at high speed at high altitude; 40,000 feet will do.

Then call "Turn in, Fight's on" and start a head on pass directly toward your adversary, usually one of your squadron mates. Pass as close to him as you dare and the instant you pass roll into a 60 degree (more or less) bank and pull back on the stick as hard as you dare to extract the maximum aerodynamic performance of your aircraft. The idea is to make as tight a turn as possible. If you do this while climbing your speed will decrease and allow a tighter turning radius.

At this moment you are right at the edge of the performance envelope which is a diagram with speed on the X axis and g on the Y axis. You airplane will be g limited by stall at lower speeds and structural limits at higher speeds and when you plot these limits the boundary looks like an envelope with the flap open. Hence the term "pushing the envelope" and all that.

Anyway, if you continue to pull back on the stick and go over that edge you will experience a "departure"; i.e. you will depart from controlled flight. It's hard to predict what might happen next but one option is that the airplane will "swap ends" and do a bit of backwards flying for a moment. Turbojets intensely dislike air being rammed up the tailpipe at high mach numbers and will frequently demonstrate that dislike by producing a compressor stall. This can be accompanied by loud and very scary noises, belching fire out the intake, and the poor pilot having another of those all too frequent chats with God.
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Old 23rd May 2014, 09:14
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"You can tell how good a compressor is by counting the number of stages that have variable pitch stator vanes. They're a mechanical complexity that you'd only ever use to maintain adequate operating margins (and prevent disaster!) in pursuit of efficiencies not achievable by other means."
I hear what you're saying, but first you gotta' define "good".

Let's compare two widely used engines with which I'm familiar, having flown several different aircraft using these engines in various forms. The P&W J-57 which was used with an afterburner in the F-8 Crusader and F-100 Super Saber and without AB in aircraft like the B-52, Boeing 707, and many others. And the GE J-79 used most famously in the F-4 Phantom, the F-104 Starfighter and even in Graig Breedlove's Spirit of America.

The J-79 was a single spool engine featuring a compressor loaded with variable pitch stators. It was light weight and efficient with absolutely fantastic throttle response. The 17 stage compressor produced a 13.5 to one compression ratio. You could argue that the variable stators made the engine resistant to stall, but it's equally as correct to say that they were necessary to avoid engine stall because of the high compression ratio. Of course, there are trade offs. The J-79 smoked badly and if you so much as spit down the the intake, the thing would come apart often taking out the adjacent engine as well. (standing by for hostile fire from ex-F-4 types in spite of the fact I'm one too)

The J-57, on the other hand, was a twin spool engine with fixed stators producing only an 11.5 to one compression ratio. Its thrust to weight ratio wasn't quite as good as the J-79 either, but it was naturally resistant to compressor stall. Plus you could toss a Yugo down the intake and it might belch and fart a little bit, but it would grind up most anything and wouldn't quit; a feature greatly appreciated by we who prefer single engine aircraft.

So yes it's true, as you say, counting variable stators is one way to measure a compressor's "goodness" especially if you're talking about efficiency or thrust to weight ratio. But when you factor in purchase cost, maintainance cost, complexity, and reliability (especially in combat) I would argue that the measure of overall engine "goodness" involves more than counting variable stators.
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