A little demo and discussion about the combustion section of a jet engine. We answer the question: "Why doesn't the hot gas come out the front of the engine?"
I’ve flown jet aircraft for a living for the past 43 years. Despite having a degree in Mechanical Engineering I never had a good intuitive understanding of why the gas didn’t want to reverse flow out the front. Now I do, thanks to your video. Bravo. We’ll done. I just wish I’d had this explanation 43 years ago when I was going through Navy flight training. Cheers.
Nice video. I always think of it along the lines of pressure drop. As long as there is enough pressure drop across the can from the outside to the inside, flow will never go the wrong way. If the pressure drop is too low then you get combustion instability and (buzzing) sounds.
in the same way that burning the fuel in a reciprocating engine increases heat and volume forces the piston down, the turbine increases the volume of the incoming air charge by the heat of combustion+extra combustion gases.
Yes, you can see the tiny gap between liner and casing at turbine end of the combustor that restricts the flow and hence introduces pressure difference. Same goes with all the other small holes that liner has.
Jay- Nice explanation. Your talk caused me to think of an obscure analogy. The military used to have an offensive weapon called a flamethrower. It shot lightly regulated flames from a fuel nozzle causing massive amounts of damage. In a way, combustion chambers are tightly regulated flame throwers that channel flames in a productive way to do work instead of damage.
I love that by subscribing to this channel I get to the see the innards of a Orenda combustion chamber. As for explanation I think it could just be simply said that flow will follow the path of least resistance. To go back out the intake it would need to get past a choke point. Out the exhaust it's an easy open flow, so it takes the exhaust path. I believe this is what you described, and even said in some places, but not in as little words.
like a water leak if the pressure outside is the same as the inside the water takes the path of least resistance as would the air with a giant open hole at one end.
Actually, little leprechauns inside turn the turbines furiously- I've watched all your vids- about 70pct of them, and I'm now designing a jet engine with 100pct efficiency if I can just capture those dang leprechauns. Thx for the great info JZ
Off topic. I found a relatively complete jet engine in the mud at a Fresno wrecking yard. I researched the model/-# and found that it was originally used on a prototype B-66(?). I figure it got here because Fresno used to have a race car enthusiast who build a jet car out of surplus in the 60's. Perhaps the car was scrapped and the engine wound up there. I called Castle Air Museum with details and they didn't want it if it wasn't in the can.
Well! That's a couple of good details. The J79 is too big for a personal restoration project. More importantly, thousands of -11s were made, so to a museum, they are almost literally a dime a dozen. We have about a dozen, and we got them for free.
If it helps - remember the vast majority of the air compressed by the compressor is not combusted in that burner-can. Only a small percentage is actually burnt up with the fuel. All the rest just blows right on by to help cool the turbine and blow out the back. Hot gas rarely can reverse into the compressor. This happens when the compressor surges and the pressure in the combustor exceeds that in compressor. It happens for fractions of a second. If you ever see an airplane taking off and hear a series of bang bang bangs and flames coming out both ends, that was a surge. Engine designers go to great lengths to try and make it hard for that to happen, but because you get your best performance at high blade/stage compression ratios where surge is more likely - it still sometimes occurs, usually at max power, or rapid changes in power settings.
Another fabulous explanation, love your vids. As I was watching it occured to me that something I do not see mentioned often. Near the head of the combuster there is large scale combustion and heat. This causes expansion and acceleration of the gas. The air that is moving around the outside is providing cooling to the liner but is also getting heated and accelerating. At each increment where air is entering that air is also heating and therefore accelerating. By the end of the liner all the available air is inside the outlet, it has been heated to a common point and blended to a place where all of the air, combustion products, and waste are now moving at maximum velocity. One does not need to "contain" the flame portion it is actually being directed.
Having been responsible for the design of combustion chambers for a while, I prefer to think of the combustion process being "contained". To make the point, let me tell you of an experience I had with a problematic in-service combustion chamber. Most examples of the chamber behaved as expected and had acceptable overhaul lives. However, a small number would hole within a matter of tens of hours and we never did fathom out why. We had to control the problem with boroscope inspections every few hours. To reassure the operator that, if we missed a chamber that was developing a hole, the engine would shut down safely, we tested one to destruction on the bed. We built a holed chamber into an engine with instrumentation and a TV camera focussed on the CCOC wall above the hole. The engine was cycled to accelerate a failure and it was obvious that the flame was not being "contained" or "directed": it was 'torching' the CCOC wall. I recall watching the video of the final few cycles. The CCOC wall was glowing and bulging: it burst, there was a brief flash of flame into the bypass duct, and the engine surged.
PS Earlier today, I spoke to a combustion engineer colleague who was involved with the problem. He told me that an aircraft was lost as a result of a holed combustion chamber burning through the CCOC.
AgentJZ I know you are absolutely correct but it's so counter intuitive it still does my head in even after designing "bean can" jet engines. Just in case you are curious the best I know of for this type of engine is 45 seconds run time at a thrust to weight ratio of just under 0.3. That was using butane (lighter fuel). It's never going to be enough to really do anything but it's good for a giggle.
Tiny pressure differences can have a big effect. Just notice how the wind moves from an area of high pressure (i.e. 30.92 inches of mercury) towards a standard pressure of 29.92". Or how much difference there is in aircraft turn or slip with a very tiny bank angle.
"When velocity increases, pressure decresses." That was very interesting. I didn't know the air going around went back in the chamber through holes. Ingenious.
Yes. The liner helps maintain a protective layer of cooling air, preventing the super hot flame from ever touching the outer case. It also helps that cooling air envelope and eventually mix with the flame.
Good explanation - I guess strictly the reason is because the compressor (section) increases the gas pressure to the max cycle pressure immediately at the exit of the last compressor blade row and the flow will always follow the (negative) pressure gradient. So as long as the compressor is making pressure, the flow will follow the negative pressure gradient between the combustor and the atmosphere through the turbine rather than against the gradient in towards the back of the compressor and out through the compressor. There isnt really the concept of "least resistance" (there will be similar friction loss through the turbine section as there was "up" through the compressor section). The counter proof is that, in the event of compressor stall, where the compressor actually does stop producing pressure the hot gas will come out of the front of the engine. I think the idea of "least resistance" is really confusion relating to the fact that the compressor is imparting energy into the flow (and so increasing the pressure) and the turbine and nozzles are in fact extracting energy (and therefore pressure) from the flow. EDIT: Actually the velocity doesnt increase in the combustor, despite the pressure remaining constant, because the majority of the energy being added is being used to increase the internal energy of the dilution air added incrementally along the combustor length, so on aggregate the combustion temperature remains constant as well. Velocity only starts to increase at the first nozzle.
Good explanation. In my mind I think of it as, the air inlet has a smaller cross sectional area than the outlet, so it's going to flow through the outlet. Hydraulics and injectors use this type of fluid mechanical advantage too. You could calculate the pounds per square inch on the inlet and the outlet, and find the outlet will always be less due to cross sectional area.
I thought I was mechanically astute. I am not, at least not when it comes to combustion chambers...You explained it perfectly! Being the "air", is how I figure out most mechanical problems. (air, electron, atom, etc.)
Whenever you see the guts of a turbojet engines combustion section, it's clear that the pressures couldn't be anywhere near the 600psi of the final stage compressor section. Those soda can looking tubes and housing couldn't hold that kind of pressure like say a cylinder sleeve in a piston engine. That impression alone supports the idea that all the psi becomes velocity after the compressor is done. The turbine is a kinetic wheel catching some of that velocity before the nozzle.
The combustor outer case does indeed hold in CDP. The combustion liner has many holes in it, which allows cooling air to flow inward towards the flame. It is subjected to CDP on both sides, so no real force is there to distort its shape.
@@AgentJayZ I guess my piston engine thinking is used to what holds extreme pressures that rise sharply and then a micro second later are subject to near the opposite. I suppose jet engine pressures aren't meant to surge so can be maintained within apparently lighter looking vessels. Part of your video confused my thinking in that I thought the CDP had already dropped by the increasing velocity again in the burners so I figured the burner casing experiences the same. My bad. So does the CDP only drop after the turbine section?
Would the turbine also have any play on the flow of gases ( working ) on the likes of start up? Took my boy to a model plane club down the road from us I was amazed at how many different small jet units there was and size. Thank you for making the lack of pressure so easy to understand.
I don't know what you are asking. The purpose of the turbine is only one thing: to extract power from the combustion gases so it can turn the compressor. Without it, there is engine, but a fire. In a piston engine, the equivalent part is the piston.
Wow! How many Sabres are still flying? Per Wikipedia, some are listed as "privately owned", I assume they are flyable. But many on static display. How many engines are out there mothballed and which (consumable) parts are getting critical? ie gaskets, seals, filters, etc.
A few Sabres are flying, and more are being restored. No engines are being properly stored, and I am one of the people recuing them. What does critical mean? No parts are being manufactured.
Well I was wondering if there were any upgrades to the design that may have improved efficiency like a singular combustion chamber or something I'll look it up but I like listening to your show
@@AgentJayZ we've been talking about how to accelerate a projectile through space they seem to have settled on these high powered electromagnets to force a more total vacuum and accelerate the ionized particles down the hull I had been trying to build an intake to make hydrogen and then deuterium to ride a thermonuclear event lol I also was thinking about how to utilize a particle accelerator to make water from ionized subatomic particles but how to insulate a passenger from such a field in motion and energy discharge
Exactly and that is why thrust is proportional to T2-T1 where T1 is the inlet temperature and T2 is the eflux temperature (which is Charles's law not Boyles as some have stated). That said with some of the super weird fuels that were experimented with it's also a function of the chemical reaction. IE air can be considered to be roughly 80% nitrogen and 20% oxygen. If your fuel increases the amount of volume as well (nitro-methylene and paraffin mix can) then you get even more thrust. The only problem is that it makes a military engine on full afterburner look frugal. I'm taking this from memory but a standard J-79 is heading for 12,000 LB thrust dry. Zip fuels (For example borane) could either increase this a little (up to the limit of the engine bearings etc) or increase range. Nitro-methylene and paraffin in the right ratio however was nuts. IE in the right combination and engine modifications (vastly higher rated thrust bearings etc) I think NASA got one up to near 30,000lb dry thrust. That said I cant find the data online and I was searching through microfiche near 20 years ago.
That would be research and development of new engine designs. We only test established designs to make sure they perform as they should after we work on them. The testing of new designs is more Graham's area of expertise.
You need to be more clear. A power turbine is not a part of this engine, but it is what the industrial version of this engine is used with. Saying power turbine is a different thing than saying turbine.
more paitence then me JayZ, these days I'd be.. 'Small hole, bIg hole.. positive pressure from small hole, air wants to leave, big hole at end. Positive pressure maintained.. If Pressure raised then compressor has failed and I might want to check if there's a stall. other way to have people understand is to go 'does the fire come out your wood stove's intake vents? unless you mess something up?
Um - I have no jet engine experience, so I've always wondered (espec. when watching the amazing F1 launch vids), what component in the engine is taking the full load of the 'push' from the combustion? Something must be the first part to transfer the load to the vehicle, but it must be handling tonnes of load?
Ola Special AgentJayZed of the Royal Canadian population. Two questions 1) is there a flame holder in the combustion (now I do not know what to call it so I am going to call it) pipe. As the igniter is not continuous, I am assuming that somewhere in the combustion pipe thingy, there must be a flame holder to keep the flame lit? 2) Is there a flame exiting the combustion pipe thingy? Or has the mixed outside air and fuel burned completely so you get high-velocity air entering the power turbine section but the first stage power turbine is not in direct contact with the flame? Thank you very much for the time and effort you spend teaching us Luddites how gas generators and power turbines really work.
1 No flame holder. Not necessary. Flameouts are about one thousand times more rare than in movies. 2 the flame is about 3000 degrees F or so. The purpose of the liner is to fully mix the cooling air with the flame, so that the gases leaving the combustor are about 1000 degrees F. No flame gets into the turbine after the start cycle. 3 Power turbine is a specific thing that is not in this engine. The thing you are thinking of here is the turbine. you could call it the compressor turbine if you really wanted, but since this engine only has one turbine, it is called the turbine. With an industrial Orenda, the complete engine discharges its total output into another, separate turbine, called a power turbine, which turns a shaft to do external work, like run a large pump or electrical generator.
The front end of the combustion chamber is specifically designed to act as a flame holder, with some recirculation in what is termed the primary zone, where the air/fuel ratio near stoichiometric. Downstream of the primary zone, dilution air enters through the ports you can see in those Orenda flame tubes, in order to reduce the temperature to a level acceptable for the turbine.
It's not a good idea to use improper terms, because they just make it harder to understand what's going on. Fan is a specific term that does not exist in a turbojet. The air is increased in pressure by a ten stage compressor, and then it flows into the combustion section.
I've done that. "reverse flow" is a misnomer. The gas path goes around a couple of corners, but schematically, it still goes from compressor to combustor to turbine, just like in every gas turbine engine.
@@AgentJayZ I just checked it out, the part that mixes me up is the “air jacket”. In that pt6 diagram it looks as if the compressor air literally wraps all the way around the combustion chamber and hits a wall. I realize there are pathways where compressor air moves into the combustion chamber but it’s still difficult to wrap my head around
Great videos! What happens if an igniter fails in this engine? Does unburned fuel exit as part of the exhaust gas or will it auto ignite when mixing with the hot air in the turbine section? Also, how is a single ignition fail condition detected/rectified? Fascinatedly, Mike
Same thing as if a spark plug falls into your car engine. ie, has never, ever happened. Not saying it can't ever, but it's just not a thing. Also, you only need one spark to start a fire, and the engine has two on independent circuits, so a complete ignition failure is very unlikely. Starting a jet engine is a lot like starting a candle. Spark until it goes, then stop sparking. Sometimes it takes one second, sometimes three. All normal. If a failure to ignite happens, it's as obvious as when your car doesn't start. The manual says de-energize the ignition systems and the starter, then determine the problem.
Your question suggests you might be thinking that the igniters are firing continuously while the engine is running: they are not. They are only needed to ignite the fuel on starting the engine. You wouldn't keep that little spark going on your gas hob once the gas was lit, would you? An engine will start quite satisfactorily on one igniter. Two igniters are used on every engine that I've worked on for reasons of redundancy. Engines do occasionally flame out in flight and need to be relit. Would you want to rely on a single igniter to get an engine going again? You might be wondering how an engine with six combustion chambers (combustors, if you must) lights 'all round' when it only has two igniters? That's why there are interconnectors between the chambers: one chamber cross-lights another. I recall, as an apprentice, helping with some altitude cross-lighting tests for the Olympus engine in the ill-fated TSR2. The tests were done on a combustion rig, which contained two chambers. The chamber with the igniter would light up then, after a delay, the second would light up, with a 'thump'.
@@AgentJayZ Correct me if I'm wrong but don't most engines of this style have links between the cans so if one fails to ignite or for some reason goes out (very unlikely) it will be ignited by the flame from nearby cans?
There are multiple combustor liners in both "can type' and "can-annular" combustor sections. Those have usually two ignitors. Each combustor liner is connecter to the next one with what are called interconnectors, or crossfire tubes, which allow the flame to travel to all the liners on ignition.
Here we go again: yet another example of English-speaking nations divided by a common language (and terminology). The majority of engines I have worked on have had combustion chambers, not combustors (that's an Americanism). The Olympus engines you have shown us have eight combustion chambers, contained in a combustion chamber outer casing (CCOC). The annular chambers that I have worked on (and I did have responsibility for combustion design at R-R IMD for while) were combustion chambers - not combustors. Having said all this, I do vaguely recall that the terms combustion liners and flame tubes were also used for some of the earlier engines. Whether that was the description of the component on the actual drawing, I couldn't say now.
Hey, you are the expert on your terminology, I'm the so-called expert on mine... Oh, to be a Canadian! I am always forced to choose on various websites, what language I speak. I always have the same two choices: English (UK), or English (US), and so I suffer, knowing that we are not really between, but more the third apex on a sort of linguistic triangle. Or the 4th on a tetrahedron if we include NZ... or the fifth on the pyramid if we include Oz! For a while now, I have been warming up to the idea of saying aluminium (spell check redlined that!) instead of aluminum. Lotta times I pick UK, but I must have YT set on US. I think our culture is closer to US, but our language is closer to UK.
@@AgentJayZ Ahh, aluminium vs aluminum.... There are strong linguistic and scientific reasons for one and not the other. The person that discovered it went through several very different names before settling on one of the two, but in the meantime someone else had named it the other... Do we go with the first, or the more legitimate? Then there us the question of how does its name fit in with uraninium?
Vest vs Waistcoat. I stumbled on that one time. I was in London from the US first time on banking business. Old movies had me believe that everyone would be wearing 3-piece suits, so I did. Arriving, I found everyone wearing only 2-piece suites. On the weekend, I remarked to a gentleman in a pub: "I've been in London an entire week and I've yet to see a man in a vest". My remark gathered some odd looks. We had a good laugh when the difference was explained to me: In London, the 3rd piece of a suit is referred to as a Waistcoat. A Vest is what in the US is referred to as an Undershirt. You can imagine what he initially thought I'd said. Terminology matters! 🙂
@@AgentJayZ While I've been laid low by Covid, I've watched some 'Battlebots' TV shows from the USA, dating back to 2018. There was a bot from the UK that the commentator had some fun with, because of the inevitable language differences, which didn't do well. There was another bot from Canada and the commentator couldn't resist adding an 'Eh?' to his introduction. However, I digress. Having tried a gentle wind-up over the issue of 'combustor' versus 'combustion chamber', I was reminded that I still have some reduced size prints of project study design schemes for combustion chambers, intended for the industrial version of the Olympus 593, which was cancelled in 1979. The production standard combustion chamber in the Concorde engine had T-vaporisers, an evolution of the Armstrong Siddeley 'hockey stick' or 'candy cane' vaporisers, as used in the Iroquois. However, a major redesign would be necessary , if we were going to be able to offer gas and dual-fuelled versions of the engine One of my team worked up versions of the production annular chamber with swirlers and different arrangements of the combustion chamber head. In parallel with his work, I also did some sketches based on the eight combustion chamber arrangement of the 593B development engine, derived from the 200 series Olympus. Well, I've now found the prints and I have to tell you that some of the scheme titles include the word "combustor"! Oh, the shame of it!
Also answering your other comment: as engines have continuously been developed and improved over the years, they gave progressed from can type combustors like the Orenda, to can-annular, like the J79, to what is now the modern standard, the full annular combustor section. So, short answer: no. All of these types of combustors are illustrated in my videos about combustor liners.
It's the same as compressor discharge pressure, or CDP. Most manufacturers and Wiki pages don't list that, but they often list overall compressor ratio, often incorrectly called compression ratio. This engine was designed in the late 1940's and was for a short time the most powerful jet engine in the world. It has a CDP of about 70 psig at max power. So if sea level is 14.7 psia, the the pressure ratio of this engine would be (70+14.7)/14.7 or about 5.75 to 1 A modern airliner engine can have a compressor ratio of over 40:1
The pressure drop across the combustion chamber walls is designed to be around 5percent of the compressor delivery pressure. If it was much less than this, the penetration and mixing of the dilution air would be inadequate.
Yeah, I was worried about that. I'm ashamed to be even distantly connected to a bunch of cowardly losers who are running from a much smaller group of real fighters, and have decided to shoot rockets from their armored military vehicles, targeting schools, hospitals and apartment buildings. They are too scared to be soldiers, yet too arrogant to admit their true status as criminals and losers. May the Ukraine Armed Forces completely consume the Russian Terrorist Forces. As of today, those assholes have had another shocking surprise. I do not mind that the letter Z is becoming an identifier of those who died through their own incompetence. " Russian warship, go fuck yourself ! "
Maybe a little chat about the combined gas law wouldn't go amiss. at least the simple stuff relating pressure/volume/temperature. or maybe you've already done it and i've missed it (likely) Remember kids, point the hot end AWAY from face before ignition!
We're all kids to some extent! Apart from those that made the terrible mistake of losing the part of them that plays with toys, or uses their imagination for fun. and none of us like those guys anyway! Maybe I should re-word my comment; I'd enjoy seeing your take on P*V/T, and what it means inside a turbine engine. it's fairly intuitive, but that doesn't always mean the implications are obvious (not to me anyway). Sure, some of the Citizens are VERY knowledgable, have a degree in aerospace engineering and 30 years of experience, But many (like me) simply enjoy figuring out how this stuff works. Stay playful Kids!
Technically I would say you increase the energy of the air. Which means all the molecule things move faster. This energy has to go somewhere. It increases the volume. If it can't increase as its in a closed space like a piston and cylinder. The pressure has to increase. Just like if you had put in a larger volume of air to start with. If it is to escape out a nozzle of some sort like in a jet. Then the larger volume can only do so in the given time by going faster. If it can't then pressure would go up and try to go back in to the compressor. Also the velocity a particular gas will reach exiting a nozzle depends on temperature. Another thing is the mass flow in and out of the combustion chamber remain the same. The increased volume is less dense however. Just like the air in a hot air balloon. Basically the energy has to go somewhere. And depending on the situation you put the air on it will do different things. This was a bit rambling.
Force = Mass * Acceleration. Mass hasn't changed (air-fuel mixture), but acceleration does. For the life of me I can't figure out why the increase in force is unidirectional. How do you control that, even if it's tied into the compressor stage up front? Sum of all forward forces = sum of all backwards forces + resulting propulsion. That polarity shows that adding fuel is the only input into the system, along with air that is introduced by the system. It has no movement to begin with and is induced by the compressor stage. So much confuse. PV = nRT I understand.
Isnt the difference in volume per revolution of the compressor vs turbine what's the key? To me it became clear when you imagine it like 2 piston engines coupled together. one functioning as a compressor and one as a air motor. Put a combustor inbetween and nothing will happen, but when you change the air motor size to double its original size it makes sense that the gasflow will drive the air motor size more than the resistance its generating on the compressor side.
I presume you are suggesting 'combustion tube' as yet another term for the component? The industry has been at it for years. We have come up with flame tube, combustion chamber, combustion liner, combustor and even 'can' as a colloquialism.
But we are taught by the manufacturers that it is indeed a chamber - I get the thinking behind not describing it as a chamber but technically it is surely.
The compressor provides the pressure to spin the turbine; the combustor supplies the volume. The volume of gas expelled from the turbine is many times that sucked into the compressor. You forgot to mention that in the case of a malfunction that would cause either a sudden drop off pressure supplied to the combustor or an obstruction in the turbine would cause the pressure in the combustor to be higher than the compressor resulting in a stall.
That's an unusual way of putting things. "An obstruction in the turbine": does not happen. "A malfunction that would cause a drop off in pressure to a combustor" ... would not "cause" a stall. It IS a stall. But I will assign you a passing grade.
G'day Jay (Zed) Yay Team ! Well, that's a perfectly delectable Video, absolutely scrumptious. I enjoyed every bit of it... Now, can you tell me how to go about getting it to run on Hydrogen..., sourced from Windmill or Photovoltaic driven Electrolosis...? The latest IPCC Report, which dropped on April 4 this year says that to have a 50% chance of holding Global Warming below 1.5°C then Global CO-2 Emissions have to Peak by 2025..., and by 2030 they have to be 43% lower than they were in 2019 (and in 2021 Emissions increased by 5.1%...). I posted a Video about it on Sunday, two New Scientist magazine articles and a WarbleRant..., titled "Global Warming Sit.Rep., Ukraine War Factor Effect ; The Prognostication...!" It's probably not what you're used to thinking about, but if you check it out with a view to seeing how wrong i might be ; then maybe you'll get a giggle out of it, and maybe you'll find that I'm not as silly as I look (?) ! But, either way, I figure that whomsoever figures out how to run one of your Flying Fireplaces on Green Hydrogen, they will thus get be the last Jet Engine Mechanics, working, on the Planet... If you can take a tip from the Fool on the Hill (?), then that's my best one this week... Such is life, Have a good one... Stay safe. ;-p Ciao !
