The Diffuser - Turbine Engines: A Closer Look
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- เผยแพร่เมื่อ 19 ก.พ. 2012
- We look at what a diffuser is and I try to explain the way it works in a jet engine. If it seems to not make sense... search for Bernoulli and his equations.
The book with the diagrams I show is the subject of a video of mine called
Jet Questions 96: Books!
It's the thumbnail, actually. - ยานยนต์และพาหนะ
I am currently a 2nd year engineering physics student in Canada and I just wanted to say thank you for making these videos they have really helped me grasp the ideas behind these engines
I used to watch your videos with awe, now I am actually designing jet engines in one of the best companies. Thank you so much for your motivational "booster"s. :D Dont forget that you are inspiring people. Keep the amazing videos coming!!!
Best regards...
Beginning at 3:55, the view of the width of the narrowing stators finally clicked in my head how the physics of the compression really happens. I always understood WHAT happened in compression, now the HOW makes sense.
Thanks! I like it when learning is fun.
...You remember that kid that sat at the back of physics class, but was always asking the teacher questions ? That was me!
Iam an aspiring engineer and these videos teach me more about what am studying and they make me even like the field more. Thank you so much and may God bless you.
Sir, you are more than a book !
Thank You so much for your videos, i use to watch them.
This is a great video to learn!! I couldn't understand in writing but I totally understand through your video. Thank you so much.!!
Was about to go to sleep 3 hours ago... Thanks for all your vids they really are very interesting! The more I find out about this engine the more I am astounded by how logically impressive it actually is. That & the awesome sound of it :D
Smooth and great explanation. Can't thank enough but thank you. Really appreciated!
you really know your stuff! i can loosely follow you on the theories and ideals and mechanics of this stuff, but it is really fun to watch you break it down.
Awesome job! Helped me understand better for my interview! Thank youuuuu
Excellent explanation!
I am truly jealous of your job, working around jet turbines is a childhood dream of mine!
Great video's, many thanks for practical explanations.
Magnificent video. Thank you!
wow! ! ! i was quite confusing about learning the difuser's principle and and function of it but thanks to you i am able to get what exactly it is.
Very nice, that was wonderful explanation. Thanks a lot.
Very well explained! Great video.
As always a very good video. Informative & very interesting to see how the physics translates into engineering practice
great video mate, currently building a tin can gas turbine and this helped me work out what the inside of it should look like to increase pressure for ignition and then to increase velocity :) thanks
sir ,u have explained it much better than my teacher . thanks a lot..
That concept bent my brain around a corner for a while. My brain tell me that by increasing the volume of the gas you would naturally decrease the pressure...
But when you described what's happening with the speed I do actually think I get it. :)
AgentJayZ, u r an excellent teacher. Thanks a lot.
You should be an A&P/AME instructor. I learn a lot more watching your videos that I did in my turbine theory and overhaul classes.
Thanks for your videos!
Ohhh finally my doubt cleared...! thank you
Gracias a usted , estamos aprendiendo mas. Saludos desde Perú
Pressure increases as it enters an area of greater volume (the diffuser in this case) while the air velocity decreases. The latter part of that I would have assumed. The first part however, that's a tough one to wrap my mind around. I'm vaguely familiar with Bernoulli's principle and how it applies to an airfoil (private pilot here) but beyond that, I'm somewhat clueless. Looks like I've got an exciting homework assignment.
Yet another piece of the pie... thanks again, Jay!
I wrote a comment earlier about the diffuser's role, but realized it is positioned BEFORE the combustor. In this context (before the combustor), the design advantages are (assuming subsonic flow): increases in fluid density, temperature. A temp increase would serve as preheating the fluid, so you don't have to use "too much" fuel in combustion, and save fuel costs; inc fluid density would give you more mass per unit of volume of air for combustion, which is always desirable. (CONT.)
VERY COOL EXPLANATION. THANK YOU.
makes alot more sense. i've already watched a few of your videos while studying for my a&p license exam. i just wanted to say thanks for taking your time making videos to help students like me. much appreciated and respect! do you still read these comments?
@deSloleye The highest temperature (pre-combustor) should be at the diffuser exit/combustor entrance.
First view and first comment, Jay. Great video!
Whenever you get the time, here are some suggestions for future videos:
1. What kind of sensors are on turbine engines and where they are
2. How overspeed control works if the turbines you work on have it
3. TAC Generators: I've looked everywhere, but all I can find is that they convert heat to electricity and some hard-to-understand descriptions
4. Exciter boxes, and
5. My personal favorite: debunking the chemtrail myth :P
So interesting for me, a guy who didn't study science but love science (I was not good enough in math, too bad...)
Thanks a lot from France.
Jay is spot on. Think of it as a venturi in reverse. Bernouli's principal states that an increase in velocity is a decrease in pressure. Here in the diffuser the velocity is decreased and psi increases. Pressure and velocity are inversely proportional.
@deSloleye One reason for having a diffuser is to slow down the air. When air exits the compressor, its velocity is about 500 ft/sec. The speed of burning kerosine at normal mixture ratios is only a few feet per second. So when the air enters the combustion chamber with a velocity of 500 ft/sec, the kerosine would just be blown away.
Thanks mate.... got my answers for my assignment
Thank you for another great video. Just a thought: I guess it is okay to say "that is out of my league", sometimes, when you have got a thorough grasp (like you do) of implicit hurdles, required efforts and capabilities, and need to choose between options or professional pathways, and when you are not afraid but content with building upon a certain choice. An engineer might just as well wish to be more hands on and practical like you can be now. On the other hand, you can just *choose* to learn more about the theory than is required as you like, where an engineering student might just rush it over to pass the exam/module.
@deSloleye If you go to the end of the video, you can pause it when you see the graphs in the book, these show the velocity, temperature and pressure of the air at the various stages of the engine
You are confusing the compressor with the diffuser. The diffuser is a passive aerodynamic device that, through its shape, converts some of the velocity contained in the gas stream into pressure.
The compressor is an active aerodynamic device that adds velocity to the gas stream via the moving rotor blades, and then converts that velocity into pressure in the divergent passage created by the stator vanes. this is accomplished at each stage, and there are 17 of them.
Another very good video... ;)
@343jonny Totally interested in the sensors that go towards controlling these things. That'd be a great video.
Very good!
thanks mr AgentJayZ... a big heard...
Who could not like this?
Hey... I always thought that the diffuser would take the high pressure and slow moving air from the combustion chamber and speed it up while decreasing the pressure. That sped up air on the other hand can then be used to power the turbine fans...
So now I'm slightly confused... what is high pressure and slow moving air good for
@deSloleye they add the diffuser(velocity decrease and pressure increases ) because there will be too much air in the combustion section, because if you let pass that higher air velocity into the combustion chamber, you can have flame out or no flame at all! yes, diffuser has the highest pressure & temp. at the cold section
brillint thank you
You did a great job, bro. (Student Helicopter Pilot)
"So if you want to learn more about how to design a gas turbine engine, you need to become an engineer!"
hahaha love it! Great video!
always love your videos AgentJayZ...what's the title of the book you are using to show the examples?
Any fluids mechanics book, may I suggest White
Thank you !
Bernoulli's principle can only be used on incompressible fluids, it does not apply to compressible fluids. You've got to relate the continuity equation and idealise the movement of the gas (isentropic).. Then you'll find a relationship that relates the speed to the gas to the size of the nozzle to the pressure of the gas. dA/A = (M^2-1)dV/dA. That equation explains the phenomenon.
depends, sometimes it's to reduce the possibility of shock waves occurring, or to increase pressure in the system.
Thanks, I'll try to find it
Ok, so why do you have diffusion? A lot of energy is added to the air in the previous combustion stages, and the air, I take it, gets very fast. What are the desired properties of the air coming from the diffuser that make the engine work? Second question, does this air get hotter in the diffuser or is it hottest at the point of the final compressor? I would think that heat is found where there's high pressure, but I think that idea assumes energy is added.
you are the best
There will be a small decrease in pressure due to loss. The main idea is total pressure is sum of dynamic and static pressure. In a divergent duct, the air slows down so the dynamic pressure is converted to static pressure and vice versa in a convergent duct.
Thanx for video it is so much helpful... But sir I have one doubt... The textbook diagram you have shown in that diagram the space is getting narrow after diffuser in combustion chamber and as you said in diffuser velocity of air get convert into pressure because of increase in volume so what happens after diffuser does it's pressure again decrease because of small volume....
"the space is getting narrow after diffuser "... no, it's the area coloured yellow, and it's diverging (getting wider) in the diagram.
Nice vid
agentJayZ, i know this must've been asked before but i haven't come across it yet: Are you an engineer or a mechanic with a serious amount of knowledge? thanks for making these videos. after watching just a few i've learned quite a lot about jet engines.
where is the turbo engine is in the asphalt plant....or what does it do in that plant ...?
What's the name of the "introductory textbook" that you reference at the beginning of the video? You pointed to a diagram in this book. Thanks.
From a while back: It's featured in my vid called Jet Questions 96: Books!
It's the thumbnail, actually.
Hi AgentJayZ. On a J79/LM1500, what is the approximate pressure difference from the higher velocity low pressure entrance side of the diffuser vs the lower velocity higher pressure exit to the combustion chamber? I'm very grateful for all the time you have taken to share so much about the inner workings of turbine engines. Thanks for all of your videos, I enjoy watching them immensely
The pressure change through the diffuser is not something we measure, so I don't know. We measure CDP, but the exact position of the sensing port along the diffusion path is not something I'm aware of.
Thanks AgentJayz. Always learning with your videos!!
I believe marek0086 refers to the illustration appearing in at 6:30, between letters D-D. There is a misunderstanding: Indeed the cross section of the white, central part decreases with distance. However, the air doesn't enter the white section. It goes outside of it, in the yellow section.
(CONT) Lastly, Inc pressure is your last gain, which serves a purpose related to the preheating effect of inc t. Steam cycles use the same objective of increasing temp and pressure before the heat addition stage to inc fuel efficiency. There are 2 differential equations found in any thermodynamics book under compressible flow that dictate diffuser shape, and fluid property changes.
Thnx, AJZ
Nice Jay!! the stator of axial compressor compress the air like a diffuser or the stator just direct the air to next rotor stage?
+Giorgi Lucas
Stators form a diffusion passage. They also feed the next stage blades at the appropriate angle
Tanks Jay you are the best in jet city!!
From your experience working on engines; has there ever been a need to account for the compressibility of air within a diffuser? Or do engineers usually design it so that the air is always subsonic/incompressible within the diffuser?
Air is not incompressible. It behaves that way only if unconstrained.
All piston engines, and all gas turbine engines exploit the compressibility of air for their basic function.
The process of aerodynamic diffusion is trading kinetic energy for pressure... the increased pressure makes a given mass of air smaller.
Why do turbine engines need a diffuser? Is it to slow down the velocity of the compressed air so as not to blow out the flame?
@343jonny Maybe the first one. Not the others.
@343jonny You are indeed the first !
It's a story... basically I had to clean up a nickname that I was given - one that actually was fairly close to that really famous guy who does the music thing and is married to that really hot chick who used to be in that group...
What will happen to the flame in the combustion chamber if there was no diffuser?
I'm tempted to say that its the static pressure that increases. The dynamic pressure (or ram pressure as agentjayz calls it) decreases because the velocity decreases. Its possible that the total pressure does indeed increase but it would mean that the static pressure increase would far outweigh the dynamic pressure decrease to cause that to occur, since Ps + Pd = Pt
thanks man
Thanks🌹🌹
dose that meen thet once in the combuster the presure rises?more than it is
Thanks a lot, i got it
We all need to remember that our opinions on what is happening don't matter. The way this works was all worked out over 250 years ago.
No debate necessary...
No need to scratch your head and come up with an explanation... just look up Bernoulli in your favorite search engine... or library !
True but that equation can only account for subsonic flow where the one I stated(even though I made a slight error in it *dV/V not dV/dA) describes the flow even with supersonic flow. You are right though the basic principles for sub sonic flow are governed by that equation. You are talking about flow through a nozzle here and not compression aren't you? Didn't mean to attack though. I appreciate all of your videos. It's nice to see a hands on approach.
It can be a hard concept to accept that increased velocity = decreased pressure. Consider the hose end attachment for spraying weed killer or fertilizer for the lawn. The velocity of the water passing through the attachment "sucks" the fertilizer from the small tank. But it doesn't suck the fertilizer, the velocity of the water passing through the attachment creates a zone of low pressure such that the relatively higher pressure in the tank pushes the fertilizer up into the water stream.
Wish I could remember all of it after graduation from university the past year.
Thanks J
can you tell me if these are welded brazed on or what holds them on--old man thanks you---
"these" is a word that leaves great uncertainty about just what part you are asking about. A bit of a description, or at least a time stamp would help me out.
What is the advantage of having a static pressure rise both ahead of a compressor and after the combustion chamber?
The first makes the compressor more efficient, and the second does not exist.
my question is: when the air reaches the end of the diffuser, the space is bigger, so there's more volume, if you have more volume, and the same air, why doesen't the pressure decrease?
The density of the fluid doesn't change dramatically (subsonic flow) then to mantain a steady mass flow the velocity at the exit od the diffuser MUST be lower than in the entrance because you have a larger cross-Area. This means that the fluid has decreased significantly it's kinetic energy (it depends on the square of V). As AgentJayZ said you can't dissapear energy, and taking into account there are no height changes and no significant heat exchange then the "Pressure energy" wil rise,
To the second question, while the diffuser increases T, the hottest part of the engine is probably combustion or right after it, which is why the temp. at which the turbine blade melts is an extremely important design factor.
nice one! anyone who knows a little bit of aerodynamics (not my field either) will have spotted your 'subsonic v' on the diagram :)
I'm currently confused about the difuser and combustion chamber
Does someone know what textbook is displayed at 1:43 ?
It's featured in my vid called Jet Questions 96: Books!
It's the thumbnail, actually.
Thanks
Tnx man.
i really not sure how pressure is increased. i am automotive master mechanic and in ic engines the term diffusion means to distribute a substance, either fuel or exhaust gases evenly throughout a larger mass, usually air. i can see how the drop in velocity would occur in the diffuser wich would also create lateral turbulence greatly improving the dispersal of fuel as it is introduced into the airflow. my question is how does the drop in velocity increase pressure?
Diffusion has a different meaning in aerodynamics.
Love the lettering on the coveralls! Can I be the first to ask why you call yourself "Agent" Jay -Z ?
Nice video as always, though I like the nuts-and-bolts videos a little better. Incidentally, the "one-in-one-out-theory-system-idea" is more commonly known as the conservation of mass. ;)
The compressor narrows down towards the rear not to compress the gas stream, but to maintain its overall velocity through the engine. It is already compressed, and therefore takes up less space. In order to keep moving a smaller volume at the same speed through a passage, the passage needs to get smaller as well.
This video was made to discuss the diffuser section, not the compressor, which you have incorrectly called the inlet.
Thanks eng🌹
Could you please share the title, author, etc. of the book used? Thanks
+Lahfyn No. I've got bad news for you: if you can't find a book on the subject you are interested in, you're not going to make it.
+Lahfyn The book doesn't matter much as much as the topic. For you to understand what is happening all you need to look up is Bernoulli's equation regarding static and dynamic pressure along with knowing what continuity is. Both of these topics should be in any Fluid Dynamic book as they are the fundamentals. If you are more interested about specifics of turbines, Mechanics and thermodynamics of Propulsion by Hill and Peterson is an excellent choice but it is condensed specifically with just the mechanics and thermodynamics, it is assumed that if you are reading the book, you understand fluid dynamics. Have fun learning!
+Sophrosyne
Thanks a lot for your reply. Excellent explanation.
Regards....
That diffusor looks like a rocket engine nozzle! How does it work that the same shape transforms a fast, low pressure airflow into a slow, high pressure one and at the same time transforms a slow, high pressure airflow into a fast, low pressure one? Isn't that contradicting?
+Micha Hermann
The rocket nozzle is a supersonic device.
The axial compressor diffuser is a subsonic device.
You have quite a bit of reading to do.
AgentJayZ Thanks for your quick answer! I didn't realize that air behaves in such different ways just because it's supersonic! Well I guess I'll learn the difference when I'm studying aerospace engineering
@deSloleye You need to do some research into the whys.
There's no way I could give a lecture on physics, especially on TH-cam.