Turbine Questions

[uwochris]

Hey guys,

I have some questions regarding turbine engines, and I hope someone can provide some feedback.

1. I do not quite understand the purpose of "stator vanes." I have read that they are mainly used along with axial-flow compressors, and that they simply help to keep the air flowing parallel to the longitudinal axis. Do they have any other purpose? ie) do they aid in compression at all? I have seen pictures where they are used in the compressor AND turbine sections of the engine.

2. I also do not quite understand what a "diffuser" is. I know they are used on a centrifugal-flow compressor. Is the air compressed in the diffuser, or does the diffuser simply direct the air flow to the combustion chamber?
In the Turbine Pilot's Manual, the author explains that "a diffuser is simply a divergent duct that slows the velocity of the impeller's output air, thereby increasing the air pressure before it enters the combustion chamber." This explanation, however, is not very clear to me ( I do not understand how it slows the air, and why the air pressure would need to be increased if it has already been compressed through the high-pressure compressor).

3. EPR is described as the ratio of engine output pressure to engine intake pressure (thrust output/thrust input, or turbine discharge total pressure/compressor inlet total pressure). I am just wondering where are the pressures measured (i.e. where are the probes located). It seems to me that if the engine output pressure is measured where it is discharged, the pressure would be very low (i.e. the exhaust gases pass through the turbine blades, and are accelerated through the nozzle. If the pressure were to be measured here, it would be very low, and thus, the EPR gauge would read low.). I am just thinking that if the air pressure were to be measured at a point where it is accelerating, it would read low, compared to where the air is entering the compressor section.

4. My understanding of a "hot start" is that it occurs when the compressors are not turning fast enough, causing the mixture to pool up and build in the combustion chamber, leading to very high temperatures and pressures. Is this correct? I read that they occur when the expanding gases do not travel aft through the engine.

Thanks in advance for any replies,

Chris.

 

[EagleRJ]

1- You are right that stator vanes straighten the airflow, i.e. keep it parallel to the longitudinal axis. They're needed because the engine's rotating assembly would tend to induce a spiral airflow as the air flowed through the engine. That would cut efficiency, and it would play havoc with combustion in the engine's burner cans (they have a spiral airflow already inside, but they were designed for a straight, stable intake of air).

2- The diffuser duct in the compressor section slows the air before it enters the burner section to aid combustion. The pressure is already much higher than ambient, but it decreases a little as it the diffuser expands it. The pressure then rises again as fuel is introduced and burned.

Many modern engines also have a diffuser on the tail pipe. It is a nozzle with a sawtooth or wavy shape that helps merge the high velocity air coming out of the fan with the even higher velocity air coming out of the core, to reduce noise.

3- I believe it varies from engine to engine. The only engine I'm familiar with that uses EPR is the JT8D, and on that the inlet pressure is measured by a probe in the center of the nose dome and the exhaust pressure is measured by six probes located after the last turbine stage. The ratio is calculated and sent electrically to the EPR gauge.
Most engines nowdays measure power using percent N1, since it's more relevant and user-friendly than EPR.

4- Starting a jet engine is always a race between speed and temperature. The engine needs to accelerate quickly enough to bring in enough cooling air, and if it doesn't and you have too much fuel, you get a hot start. Engine start temperature limits are often close to the material limits of metals inside the engine, so it doesn't take much of a rise over the limit before you start melting stuff.
Once the engine is up to full speed and stable, the temperature drops rapidly to the normal range. It's usually only during start that you have to worry about temperature, except for some full-thrust takeoffs in high ambient temperatures.

 

[avbug]

EPR is generally used in turbjets, while N1 is typcally used for primary power in turbofan arrangements.

The burner cans in a turbine engine are delicate thin ber-can tin arragements that are very susceptible to heat damage. So much so that the flame cannot be allowed to touch the burner can.

The walls of a burner can are louvered with a series of air inlets tha tprevent the flame from reachign the metal on the can. The nozzles are designed typically in such a way that several stages of fuel flow will occur under varying conditions. In the start phase, most nozzles being duplex, a smaller inner cone delivers a delicate fuel spray that's designed to keep from burning up the engine at lesser airflows and lesser engine speeds. At higher fuel flows, a pressurization and dump valve (which takes different forms and the function may be handled by other components such as the fuel controller, flow dividers, etc), sometimes called the P&D valve, provides a secondary fuel flow through outer orfices in the fuel nozzle. At this stage, more airflow is occuring through and around the burner cans.

If too much fuel enters the combustors before adequate engine speed has ben reached, or on other words, before adequate airflow has been established, a hot start may occur. This can happen if the engine doesn't reach a high enough speed during start, if fuel is introduced too early, if a start is attempted before the engine has been allowed to cool from a previous shutdown, if fuel has pooled in the combustors and hasn't drained, if fuel has vaporized in the combustors and the ignitors are turned on before adequate airflow exists, if too much fuel is dumped into the combustors too early by a P&D or fuel controlle error, if a power failure or reduction occurs during the start sequence, or if a low voltage condition exists (such as a low battery during a battery start, or a GPU failure during start).

The universl soloution is to shut off fuel and continue to motor the engine; let airflow take care of the situation and remove the source of the the heat.

It's important to understand that airflow through the combustors is primarily used for cooling and insulation. Only about 25% of the air that flows into the engine is actually used to support combustion. I should clarify that by stating that it's 75% of the air flowing through the power section of the engine. When referring to a turbofan engine, the percentage of airflow through the engine that's used for combustion is substantially less.

It's also important to understand that fuel flow and combustion doesn't work the same as it does in a piston engine. In a piston engine, keep introducing more fuel, such as enriching the mixture, and the temperature will drop. But in a turbine engine the more fuel you dump into the engine the hotter it gets. Airflow is necessary to control that temperature and protect the internal components of the engine.

The true victim of hot starts isn't so much the combustors, though they do burn and crack (most combustors crack over the life of the engine, actually; you'd be scared to fly the aircraft if you could se just how cracked they usually are when the engine ets pulld for overhaul or even for a hot section inspection). It's the turbine inlet guide vanes, and the first stage turbine blades. These are victims of temperature, and sulfidation which is closely related to temperature. Sulfidation takes place as a byprocess of temperature and combustion, and is more of a corrosion and erosion issue. An engine that has been overtemped must be torn down and inspected closely; the hot section components may have suffered metalurgical damage, and may require replacement.

The easiest way to think of the diffuser in a turbine engine is to think of a reverse venturi. The purpose of a compressor is to increase air pressure; get as much air at as high a pressure as possible, into the engine. The duct through which the air flows through the various compressor stages narrows; air velocity increases, and pressure is increased mechanically by each stage of the compressor.

As airflow accelerates through a venturi, the pressure decreases; it's energy is in the direction of airflow, rather than pushing out in all directions. You're already familiar with this when you think of what happens above a cambered wing. On top of the wing, the principle we refer to sometimes as "bernoulli's principle" describes a venturi action...air pressure decreasing as the velocity increases.

In the diffuser section, just the opposite happens. The velocity is allowed to slow down, and instead of the airflow being forced through a narrower constriction, it's allowed into an expanding area...the diffuser. This allows the air to rapidly increase in pressure, just before being admitted to the burner or combustor section. In the first part of the engine, the pressure is increased mechanically. In the diffuser section, the pressure is increased aerodynamically. In the the combustor section, the air pressure is increased thermodynamically. In the turbine section, energy is extracted from the exaust airflow, the temperature cools from one end of the burner to the other, and pressure is again traded for velocity.

Pressure and velocity is traded back and forth throughout the engine to maximize the effects of the airflow to it's greatest advantage to accomplish the specific needs of the engine at any given point. In many engines, it also changes direction repeatedly. Changes in velocity vs. pressure help assist in making these directional changes possible, which in turn helps keep the engine small which makes it possible to be kept lighter, and more aerodynamic by reducing the amount of engine that is exposed to the atmosphere, or filling up the aircraft structure internally...that means a smaller aircraft structure, less drag, and more efficiency.

Stator vanes are used to set airflow to the most efficient angle of attack for any given compressor or turbine stage. Each turbine blade, and each compressor blade, is an airfoil; a miniature wing or propeller. Each one is subject to the same rules that apply to the wing of your airplane. It's most efficient at a given angle of attack. Angle of attack varies with speed; the faster that compressor disc turns, the lower the angle of attack.

Some engine use variable stators that adjust the airflow angle to the compressor or turbine, automatically. Others use fixed stators that are designed to set the appropriate efficient angle at normal operating speeds.

 

[mar]

mar and Ebert

Hi Chris.

I read the Turbine Pilot's Flight Manual a long time ago. I'd give it one thumb up and one thumb down.

It does a fair job of explaining basic turbine theory: suck, squeeze, bang, blow and then it all goes to hell after that.

The problem is the authors tried to keep it too general (tried to cover too many different flavors of turbine engine applications: commuter, corporate, transport, etc).

If you really want to get into the guts of turbine engines start with the Pratt & Whitney PT-6 and the Garrett TPE-331.

These are very common and very basic designs. The PT-6 is a good example of a free turbine with reverse flow and the TPE-331 is a good example of a single-shaft turbine (with and without water injection).

After you feel comfortable with the basic vocabulary and theory you can check out bigger turbines like turbojets and hi bypass fans, etc.

You can probably find lots of esoteric material on Amazon. But I have to say, with your attention to detail, you probably be better served with specific examples of real applications than some bubble-gum condensed version with pretty pictures.

Good luck.

 

[bafanguy]

Actually, P & W has produced some outstanding turbine engine theory manuals. A P & W rep gave me mine so I don't know how you get them, but they are out there.

 

[avbug]

Journey through a jet engine...visit this:


http://www.rolls-royce.com/education/schools/journey/flash.html

 

[mzaharis]

2- The diffuser duct in the compressor section slows the air before it enters the burner section to aid combustion. The pressure is already much higher than ambient, but it decreases a little as it the diffuser expands it. The pressure then rises again as fuel is introduced and burned.

One thing regarding the diffuser between the compressor and the combustion chamber. It actually INCREASES pressure of the air as it goes through the diffuser. Remember Bernoulli's law, which says that, when you are not adding or taking away energy from an airflow, dynamic pressure (velocity, essentially) plus static pressure is constant. As the airflow passes the compressor, its static pressure is increased, but so its dynamic pressure (velocity). In the diffuser, it is slowed, in effect converting that velocity/dymamic pressure to static pressure, and allowing more time for combustion to take place.

 

[johnpeace]

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