Jet engines and compressors

[Cheif Operations Director]

1 minute ago, ARS said:

Individually, impeller cannot withstand the airspeed where the ramjet operates and ramjet cannot operate efficiently in impeller airspeed. Since impeller is designed mostly for subsonic flight, it'll be ripped apart in ramjet airspeed while ramjet is designed only for supersonic flight. A hybrid design such as SR-71 engine used the shock cone intake adjustment (pushing it forward or backward) to shift between subsonic (where low-speed air entering the turbine) and supersonic mode (where high-speed air directly enters combustion chamber)

I mean who not just use the impeller as the compressor and then the rest of the system is essentially a ramjet in design I. Not saying it is a ram jet. But just a ramjet with an 1 impeller and a diffuser like this

https://images.search.yahoo.com/images/view;_ylt=AwrEwM4LI11bQdcAHSuKnIlQ;_ylu=X3oDMTIzNzZjMnBqBHNlYwNzcgRzbGsDaW1nBG9pZAM2NzQ3NTA3ZWIzMmRiOTQxNjJhN2M2MDIwMWZkODIyNARncG9zAzEzBGl0A2Jpbmc-?.origin=&back=https%3A%2F%2Fimages.search.yahoo.com%2Fsearch%2Fimages%3Fp%3Dimpeller%26fr%3Dipad%26fr2%3Dpiv-web%26tab%3Dorganic%26ri%3D13&w=613&h=500&imgurl=procnc.com%2Fimages%2Fgallery%2Fimpeller_medium.jpg&rurl=http%3A%2F%2Fprocnc.com%2Fportfolio%2Fgallery&size=40.0KB&name=CNC+Equipment+-+Trulife+Engineered+Solutions+-+AS9100%2FISO+...&p=impeller&oid=6747507eb32db94162a7c60201fd8224&fr2=piv-web&fr=ipad&tt=CNC+Equipment+-+Trulife+Engineered+Solutions+-+AS9100%2FISO+...&b=0&ni=21&no=13&ts=&tab=organic&sigr=113ltbber&sigb=12uiab6ka&sigi=11dupv7nv&sigt=11tmpcveo&sign=11tmpcveo&.crumb=0sQN5ahmzm8&fr=ipad&fr2=piv-web

What does it have to be like this 

https://images.search.yahoo.com/images/view;_ylt=AwrEwSxOI11bcf4AoOiKnIlQ;_ylu=X3oDMTIydGVqNXRqBHNlYwNzcgRzbGsDaW1nBG9pZAMwMjY5ODU4NjA1NmMyOGE3NzcwMDJjNmQyNGM5ODc0NQRncG9zAzYEaXQDYmluZw--?.origin=&amp;back=https%3A%2F%2Fimages.search.yahoo.com%2Fsearch%2Fimages%3Fp%3Djet%2Bengine%26fr%3Dipad%26fr2%3Dpiv-web%26tab%3Dorganic%26ri%3D6&amp;w=550&amp;h=413&amp;imgurl=cdn.comsol.com%2Fwordpress%2F2013%2F08%2FGas-turbine-jet-engine-diagram.png&amp;rurl=http%3A%2F%2Fwww.comsol.com%2Fblogs%2Fturbine-stator-blade-cooling-and-aircraft-engines%2F&amp;size=162.7KB&amp;name=Turbine+Stator+Blade+Cooling+and+<b>Aircraft<%2Fb>+<b>Engines<%2Fb>+|+COMSOL+...&amp;p=jet+engine&amp;oid=02698586056c28a777002c6d24c98745&amp;fr2=piv-web&amp;fr=ipad&amp;tt=Turbine+Stator+Blade+Cooling+and+<b>Aircraft<%2Fb>+<b>Engines<%2Fb>+|+COMSOL+...&amp;b=0&amp;ni=21&amp;no=6&amp;ts=&amp;tab=organic&amp;sigr=12ek0adk2&amp;sigb=12vhpe5qu&amp;sigi=12357lmgm&amp;sigt=12c217rcl&amp;sign=12c217rcl&amp;.crumb=0sQN5ahmzm8&amp;fr=ipad&amp;fr2=piv-web

[Cunjo Carl]

Having a single impeller do all the work winds up being thermodynamically lossy unless it's quite long and very -very- well designed. You wind up creating a ton of heat! Having many smaller compression stages helps keep all the energy you put in turn into pressure rather than heat.

It's true what I've been calling pressure for clarity is better called 'head'. In real life the turbine increases the energy of what it's pushing on and that energy can take the form of either pressure or velocity depending on what's going on downstream (namely the cross sectional area of the chamber). I thought jet engines primarily boosted pressure, given the air is already enterring a right fraction of the speed of sound. Was I wrong about that?

[Tonka Crash]

2 hours ago, Cheif Operations Director said:

What I do not understand is why not just use an impeller why the complex stator turbine system instead

Essentially a ramjet with a impeller

You know Wikipedia might be a better source for general info questions not related to the game. But to answer why complex stator turbine sysytemes are used is due to engine fuel efficiency. The higher the compression ratio, the higher the fuel efficiency. A single centrifugal stage (no one calls it an impeller in jet engines) can't compare to a multistage axial compressor for overall compression ratio. 

[wizzlebippi]

17 hours ago, Cheif Operations Director said:

What I do not understand is why not just use an impeller why the complex stator turbine system instead

Essentially a ramjet with a impeller

Because the process of compressing air slows the air down.  The compression/deceleration process starts at the  intake on all jet engines, slowing it to less than 0.3 Mach before it reaches the compressor.  This is because below 0.3 Mach, air can't be aerodynamically compressed. From there, multiple compressor stages mechanically work the air. These can be numerous axial rotor/stator stages, or on some engines, a few axial stages and a large centrifugal stage before the burners. The idea is the same, something speeds air up and forces it into something stationary to increase pressure.

On a ramjet, the intake air is fast enough that aerodynamic compression is enough to operate the engine. No moving parts needed. 

17 hours ago, Cheif Operations Director said:

 

Edited July 29, 2018 by wizzlebippi
Something went wrong while posting

[wizzlebippi]

17 hours ago, Cheif Operations Director said:

 

Edited July 29, 2018 by wizzlebippi
Something went wrong while posting

[MaverickSawyer]

16 hours ago, Cheif Operations Director said:

What I do not understand is why not just use an impeller why the complex stator turbine system instead

Essentially a ramjet with a impeller

Because the compressor requires a source of energy to function, which then turns the engine into a turbine engine with a ram intake.

As for why they use axial flow compressors, they're more efficient and have a smaller frontal cross section, reducing drag.

Yes, ramjet's are seemingly simple, due to the only moving parts being the fuel pump, but, as others have mentioned, there is a minimum airspeed you have to reach to function, and even then, they're not terribly efficient until somewhere above Mach 1.5. And most aircraft don't spend a lot of time going that fast. Missiles, however, do, and there's been a lot of work done on that front. Check out the BrahMos cruise missile, or the MBDA Meteor air-to-air missile. Both use ramjets or variants of same.

Edited July 29, 2018 by MaverickSawyer

[mikegarrison]

16 hours ago, Tonka Crash said:

You know Wikipedia might be a better source for general info questions not related to the game.

Or maybe not....

Some of us here actually have designed jet engines. (Or parts of them, anyway.)

[mikegarrison]

So here's how a turbojet engine works:

There is a shaft through the center of the engine. Ignore the complexity of how to start this, and just think about after it's already running. The shaft is turning the compressor. The typical compressor is a series of rows of little airfoil blades that add energy to the flow. (Don't worry if the energy is in the form of pressure or velocity -- those can be traded back and forth. The key is that each compressor row adds enthalpy, which is the ability to do work.) There may or may not be a centrifugal compressor as the last stage. Usually there is not more than one centrifugal stage because they mainly only make sense as a final stage.

Then this air, which is now rather hot and very highly pressurized, flows into the combustor. Here it mixes with fuel and is burned. This adds even more energy to the flow, mainly in the form of temperature. The combustor air then flows out through the turbine. The turbine is like a reverse compressor, and it extracts enthalpy from the flow. After every stage of the turbine the temperature of the flow drops because energy is being taken out of it. The turbine is attached to the same shaft as the compressor, and it provides the energy needed to run the compressor.

At the end of this, the airflow still has a lot more energy than when it entered the engine, and the nozzle turns this energy into velocity. That velocity is the jet of air out the back that provides the thrust.

A turbofan works the same way, except it has two shafts. Instead of just using the energy at the end of the turbine for thrust, instead the turbofan runs it through another turbine. This turns the second shaft, which turns the fan in the front. The fan pushes air around the side of the engine and out the back, making thrust.

A turboprop is like a turbofan, except instead of powering a fan, the second turbine powers a propeller (usually going through a gearbox to slow it down).

Basically, turbojets work best at high speeds, turboprops work best at low speeds, and turbofans work like a blend of a turbojet and a turboprop. Turbojets are very inefficient at takeoff, but they do work there. That's different than ramjets, that can't work unless you are already moving at very high speed.

16 hours ago, Cunjo Carl said:

It's true what I've been calling pressure for clarity is better called 'head'.

"head" is usually used for water or other incompressible fluids. For compressible fluids like air, were usually talk about total pressure and total temperature.

In an engine cycle deck, enthalpy is used as the measure of the energy added by each stage of the compressor (and the energy extracted by each stage of the turbine).

[mikegarrison]

On 7/28/2018 at 8:20 PM, Cunjo Carl said:

I thought jet engines primarily boosted pressure, given the air is already enterring a right fraction of the speed of sound. Was I wrong about that?

It gets tricky. Air always wants to go from an area of higher static pressure to an area of lower static pressure. When it's moving it can have enough momentum to keep going even into a higher static pressure, but if you lose that momentum you get what is called a "compressor stall". With a compressor stall, the air no longer wants to move through the compressor because there isn't enough momentum to overcome the static pressure gradient. The air then actually moves backward out the front of the engine and often causes a lot of damage.

So after every stage the airflow has to be balanced between velocity and pressure so that as it gains static pressure it also continues to have enough velocity to make sure the momentum of the air will push through this adverse static pressure gradient.

Also, remember that mass is conserved. As the air goes through the compressor it gets denser and denser. Leaving aside the bleeds (where some of the air is extracted partway through the compression process, the same mass flow has to go through each stage. So density*velocity has to be the same before and after each stage. As density increases, velocity decreases.

Edited July 30, 2018 by mikegarrison

[Cunjo Carl]

51 minutes ago, mikegarrison said:

It gets tricky. Air always wants to go from an area of higher static pressure to an area of lower static pressure. When it's moving it can have enough momentum to keep going even into a higher static pressure, but if you lose that momentum you get what is called a "compressor stall". With a compressor stall, the air no longer wants to move through the compressor because there isn't enough momentum to overcome the static pressure gradient. The air then actually moves backward out the front of the engine and often causes a lot of damage.

So after every stage the airflow has to be balanced between velocity and pressure so that as it gains static pressure it also continues to have enough velocity to make sure the momentum of the air will push through this adverse static pressure gradient.

Yeah, understood. I had mentioned early on that the purpose of each stage of the compressor was to incrementally boost the pressure and someone corrected me saying each stage was instead for incrementally accelerating the flow (speed) in preparation for a single sudden pressure boost just before the flow enters the combustion chamber. Given continuity and the influx conditions that sounded unusual (like a good recipe for choked flow), but it's been a good while since I did pump/compressor design so I was asking. Thanks!

It's kinda funny, when we were learning compressor design in chemical engineering, we were taught that axial compressors were so expensive that you could optimize the cost of your entire plant by just considering them alone. Aerospace is quite the industry, strapping 2 of these onto a chunk of metal and just tossing them up into the air!

 

 

Edited July 29, 2018 by Cunjo Carl
missed a word

[mikegarrison]

1 hour ago, Cunjo Carl said:

Yeah, understood. I had mentioned early on that the purpose of each stage of the compressor was to incrementally boost the pressure and someone corrected me saying each stage was instead for incrementally accelerating the flow (speed) in preparation for a single sudden pressure boost just before the flow enters the combustion chamber. Given continuity and the influx conditions that sounded unusual (like a good recipe for choked flow), but it's been a good while since I did pump/compressor design so I was asking. Thanks!

It's kinda funny, when we were learning compressor design in chemical engineering, we were taught that axial compressors were so expensive that you could optimize the cost of your entire plant by just considering them alone. Aerospace is quite the industry, strapping 2 of these onto a chunk of metal and just tossing them up into the air!

Well, the engines are the most expensive part of any airplane.

I think it more accurate to say that the compressors both increase the static pressure and maintain the velocity. As you note, if they simply increased the velocity it would soon choke.

Edited July 29, 2018 by mikegarrison