# Tag Info

39

Centrifugal compressors only produce a more compact engine at low mass flow, which means low thrust. The amount of thrust an engine can produce is proportional to its intake area times exhaust velocity. Increasing the latter is undesirable, as energy and thus fuel consumption is proportional to velocity squared. So engine designers target mass flow to gain ...

24

Because the LP turbine extracts power for the fan, which requires the most power. The HP and IP turbine only extract power for their connected compressors - the LP turbine extracts power for the fan and the LP compressor. The fan does work on all airflow through the engine, the compressors only on a fraction (10:1 for a high bypass like the Trent 1000). The ...

19

There are a few very practical reasons why turbine engines use inner shafts/spools. First, jet engine spools are very high speed rotating parts, this means they are subjected to high amounts of centripetal force. Centripetal force is calculated by $F=mr\omega ^2$ were F is Force, m is mass, r is radius of gyration and $\omega$ is angular velocity. So ...

19

Jet fuel will not self-ignite when starting a modern turbine engine. This article from the WingMag Aviation Magazine says: As the temperature isn’t quite sufficient to initiate self-ignition (the autoignition temperature of aviation fuel is around 220 degrees Celsius), spark plugs are arranged around the combustion chamber. They generate a spark that ...

17

Axial turbine engines take up a lot of space...lengthwise. Centrifugal compressors are shorter and wider, and are very often used in turboprop and turboshaft engines, for instance the Rolls Royce Dart is a single axis turboprop engine. The photo demonstrates the compactness of the engine...lengthwise. . Can you create a centrifugal air pump that pushes ...

13

That is the drain mast and breather outlet combined. There's some discussion about this in airliners.net (not the same engine, but I sure they are similar): The exhaust from the air/oil separator goes out here. Usually hot air, but when the engine is running at idle you can see smoke coming out. This is engine oil that has got past the seals. The smoke ...

13

A rotating drum will have a higher inertia. When the pilot commands a thrust increase, spooling up the drum will take longer than spooling up a shaft. Also, a big drum is harder to balance well than a thin shaft. However, what you propose is not so far from general practice: The high pressure spool on modern engines is quite big already, but it is still on ...

13

Turbine Blade The turbine blade (part number 1475M35P01) is from the High Pressure Turbine (HPT) a CFM56-3 according to this ATSB report on performance testing of the engine. The report contains several images of blades which look similar to your photos. Compressor blade A search on locatory.com for the part number (B778503) lists the description as C-3 ...

10

The low pressure compressor does not reduce pressure. It increases it, but operates at a lower pressure than the high pressure compressor. Some engines also have an intermediate pressure (IP) stage. https://en.wikipedia.org/wiki/Gas_turbine_engine_compressors https://www.grc.nasa.gov/www/k-12/airplane/compress.html Each stage increases the pressure as ...

10

Assuming we are speaking of axial compressors/turbines Compressor blades are generally thin and straight, and resemble a tiny rectangular wing with low camber thickness. Image source Turbine blades are more curved, often in almost "U" shapes (like the ones seen here). In particularly large and recent engines, where efficiency is critical, turbine blades ...

10

The autoignition temperature of kerosene is 428F. The temperature coming out of the last stage is usually above that at moderate power settings, up to 7-800 F or more at takeoff thrust. However the temperature has dropped somewhat by the time the air gets to the fuel nozzle, and in heavy rain there may be superheated water cooling things down some more, ...

9

Early jet engines had only one set of compressor disks which were sitting on the same shaft and spinning at the same speed. More modern designs use two or even three concentric spools, each spinning at its own speed. All spools have several compressor discs at the forward and one or a few turbine discs at the rear end, and allowing them to run at different ...

9

The most important characteristic for a jet engine compressor is high mass flow rate with light weight and if possible small size. For the same size, the flow rate can be increased by operating the compressor at higher rpm. But sliding components don't allow that. Only the axial and centrifugal compressors can achieve sufficient flow rates, so they are used....

8

The answer provided by Daniel Kiracofe is correct, I just have a couple of additional comments and I don't have enough reputation to add this as a comment to his answer. On high bypass engines, the speed of the LP rotor is usually limited by the tip speed of the fan due to its large diameter. One of the first determinations in choosing between a geared and ...

8

Of course it depends on the engine, and the exact how is proprietary information. That being said, nowadays jet engines are designed to allow faster acceleration than before. So it's not just the fuel rate the FADEC controls. COMPRESSOR AIRFLOW CONTROL To limit compressor surge and to provide good acceleration, the engine is equipped with a ...

8

The part number on the turbine blade shows it comes from a CFM56-3 series (Boeing 737 classic). No hits on the compressor blade, but it looks similar to one from the same engine.

7

Because the constant pressure lines are diverging with increasing enthalpy. Have a look at the right most diagram. The two lines are constant pressure lines, with the higher line indicating the higher pressure, and as you can see, they are diverging. 1 is the beginning of the cycle, thus at ambient temperature From 1 to 2, we increase the temperature, and ...

6

Here's an even more interesting question. Take a look at the P&W geared turbofan cross section (e.g. this article). It only has three LPT stages! So how come the GTF has 3 and all of the other engines like the RR Trent 900 have a lot more? (e.g. a GE90 has like 6 or 7). The answer is this: fans are best when they run slowly. LPTs are most efficient ...

6

Some aircraft may have a special "ETOPS APU" that can be purchased, but many aircraft simply have THE APU that they come with. It is maintained to the standards required in the approved ETOPS program, and is either run during the ETOPS portion of the flight, or else has some means to ensure that it will reliably start even when cold-soaked. (For instance, at ...

6

The statement about the rotor blades ("the rotors increase the speed of the air and descrease the pressure") is not correct, because Bernoulli's Equation does not apply in this situation. Hence, the statement "as the flow of a fluid increases, the pressure decreases, and vice versa" isn't appropriate for the rotating compresser blades. Bernouilli's equation ...

6

Jet engine designers have to find compromises all the time, and the one that counts here is between lower complexity and higher efficiency. In the 1960s, Rolls-Royce fell behind in large commercial jet engines as competitors introduced large bypass ratio fans such as the GE CF6 and the P&W JT9D, which powered the new generation of wide body jets being ...

5

You seem to be describing an Exoskeletal Turbine design. I'm no expert, only vaguely associated with the industry. They're attractive in part because your rotor materials only need to be strong in compression, so you can use e.g. ceramics. Further, the hollow inner volume can be reused for an alternative flow-path or engine type, which opens up options for ...

5

How is this managed when there are several shafts connecting several compressors/turbines? The different shaft will rotate at different speed from each other, but the relative compressor/turbine pairs will rotate at the same speed. Expanding a little, in the most complex case that I heard of, there are 3 shafts: High Pressure (HP), Intermediate-Pressure (...

5

As a 25 year gas turbine veteran across many different engines, including engine strip and assembly, it goes like this - The discs and blades are balanced independently to start with. The disc/hub arrives pre balanced, the technician then has to make a set of blades up with roughly similar weight. Each blade's weight is then inputted into a programme which ...

5

This article covers the issue (and is a pretty interesting read) but does not really explain how they do it. It just says After machine balancing, the part can then be manipulated (mass added or removed) and retested on the machine until satisfactory balance tolerance is achieved. In answer to this question, And what method do they use to ...

5

are compressor blades actually a barrier to the air flow, or is the airflow accelerated by the compressor These are not exhaustive alternatives. Keep in mind that subsonic fluid flow when constricted accelerates and pressure decreases (Bernoulli's principle). What the compressor does is increase the pressure. The speed decreases slightly. are compressor ...

5

In a C20-250 engine the compressor section is driven by the high pressure turbine (Ng spool, not N1) independent of the power turbine. The power turbine drives the gearbox and subsequently the output shaft via a concentric driveshaft to the gas core spool. I believe you’re confusing the configuration of this engine by assuming that all free turbine ...

4

Jet engine axial type compressors rotor blades do accelerate airflow. It is important to note that the net speed change in velocity is usually negative, that is, airflow through the compressor section taken as a whole is usually slowed. Additionally, the engine intake generally slows airflow as well while increasing pressure. This occurs within the airflow ...

4

The flow enters the compressor at approximately Mach 0.4 to 0.6 in order to ensure a high mass flow with still subsonic compressor blade speeds. Compressing the air allows to reduce flow speed, and between compressor exit and combustion chamber is a diffusor to reduce flow speed even more. Why? To ensure a high degree of combustion! The longer the reactants ...

4

Contra-rotating spools are a very recent development, apart from the Bristol-Siddeley (later: Rolls-Royce) Pegasus engine for the Harrier. The Rolls-Royce Trent 900 started the trend when its high pressure spool was made to rotate in the opposite direction to the other two spools. From Wikipedia: It is also the first member of the Trent family to feature ...

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