Turbine Cooling Air in a Jet Engine 

PS - great video!

The second reason, and also as extra protection from a possible hot start.

Yes, the Orenda pre-dates that tech. What I call modern engines feature Active Cooling Control.

In this engine, air can only leak from three places: The external lines, which is cause by failed gaskets, and is easily found and remedied. The mating split lines between cases, which doesn't really ever happen due to width, flatness, and enormous clamping pressure. The rear seal of the compressor, shown here. It needs to be critically adjusted. If it was too tight, the turbine would get insufficient cooling air. If it was too big a gap, you would lose a couple % of power produced. Also, your last point is true. Honestly, if you had a half inch hole drilled in the compressor case anywhere... air would blast out with a surprising roaring fury... and the engine would be just fine.

We do not increase the fuel flow or EGT. The coating is to protect and extend the service life of the blades.

There are different methods used. Sometimes EDM, sometimes lasers, and I'm sure other proprietary secrets.

Fair enough. Thank you! @@AgentJayZ

@@Marr_SC As a kid, my dad was one of the lead engineers on the TF-39 before he moved over to CF-6. We frequently got to tour GE's Evendale plant. I remember once seeing the facility where they were drilling those holes using an early (circa 1970) variant of EDM. Lots of machines lined up side by side because the process was so slow, and there are a *lot* of holes to drill.

You need to be careful about the terminology here. I would use the term 'cooling passages' for the internal cavities in the blade. Those 'passages' are typically formed during the casting process using ceramic cores, which are then leached out chemically. The tiny holes you see emerging on the surface of the blade are usually referred to as 'film cooling holes' or 'effusion cooling holes', depending on their precise function. The ones emerging at the trailing edge of the blade are usually referred to as 'trailing edge ejection holes'. Unlike the other cooling holes they are not producing an insulating layer of air on the surface of the blade - obviously! Their function is to cool the trailing edge internally. EDM was commonly used in my day, 20 plus years ago.

This engine was developed before ceramic coatings. the only other thing we do is spend more time balancing the rotating assembly to a smaller final imbalance value than the factory did.

In this engine. Each design is different.

@@AgentJayZ yeah .. but you're absolved in my mind as far as how you mention the patreon video goes, my understanding is the question was specifically regarding the turbine disk. All reconciliatory

Ceramics are like rock, which is not as good at conducting heat as metal is. For both heat and electricity, metals are conductors, and ceramics are insulators.

It's a small percentage of total mass flow. Only the engineers who designed the engine would know the amount.

That's a very good question, and one with many possible answers. These engines all had a military service history, after which they were sold to collectors without any documentation so engine logs were started at the time of sale. ... several decades later ... I start with a good candidate for restoration, and I use that data plate. I may change out a great number of parts and assemblies. We keep the original serial number, and start a new log book with zero TSO, or time since overhaul. I have spoken with several operators of these aircraft, and they have told me this is acceptable to them.

Thanks for the reply. A fan of your channel since the first time I've seen your videos.

Many racing piston engines use thermal barrier coatings on the piston crown. Not sure about the valves, but I would expect to work, and also the surface of the combustion chamber in the head.

@@AgentJayZ thank for replying 😁 do you think that the piston tbc is the same stuff used in jet shops? I read up online how some specialist builders use plasma stuff to put on the ceramic on the pistons and i saw you mention plasma gun so i was wondering

Component Repair Engineer here. In theory, TBC would protect the plastic internals for a brief period, no study has been performed (that I am aware of) as to the efficacy of TBC protected polymers. The problem I see would be the application of the TBC. It is applied through a process that would be beyond the glass transition temperature of most of the 3D printed polymers (EBPVD, APS, HVOF, etc... *google the processes*). It would be prohibitively expensive for the home gamer, assuming a supplier would even bother to quote such a thing.... It would be less expensive to use the 3D print as the basis for a casting of your choice of material.

The ceramic is applied using a plasma torch spray. If you've got a plastic than can handle 5000 degree heat, you'll be a billionaire very soon.

Now go and find a cross-sectional drawing (not a simplified diagram) of a Rolls-Royce Avon, or a Tyne/Spey/Conway. You will see that the compressor discs are connected to a central shaft by means of 'hairpin' shaped extensions from the disc, which have splined couplings with the shaft. Similarly, If you can find a cross-sectional drawing of an Armstrong Siddeley Sapphire, or a Viper/Mamba, you will see that the compressor discs also have a central shaft. Other engines of that era had various means of joining the compressor discs together, but without a central shaft. For example, Bristol used both a rim bolted arrangement and long bolts through the webs of the discs: take a look at an Olympus or a Pegasus. With the development of laser welding, more recent engines have dispensed with mechanical fasteners (nuts and bolts to you), and the discs are welded together at (or near) their rims. It results in a lighter construction, with an altogether better mechanical integrity. A central shaft is no longer necessary: the welded 'drum' forms a much larger and stiffer shaft.

@@grahamj9101 thanks for the answer!

Perhaps the subject wasn’t a turboprop 🤷‍♂️.

The previous owner was abused by the Allison T-56/501 engine.

Never assume that graffiti or vandalism has any meaning... Or... maybe the previous owner did not understand what the word meant. Assuming competence is behind the action of others can be a dangerous thing to do. I've been going through these comments for 30 minutes this morning, and I have deleted two, because they contained ignorance presented as knowledge by people who did not understand what they were talking about. That's about average for any day, but I'm not done yet. Cheers.

@@AgentJayZ Perhaps he was he something of a pedant in relation to gas turbine engine terminology? Could he have possibly preferred the now almost defunct term 'propeller turbine' engine? Much of the terminology we now use has become commonplace and accepted because of custom and practice. Question: why should the term 'turbojet' refer solely to a jet engine that doesn't have a bypass flow.? All it describes is an engine with a turbine that produces a propulsive jet. Answer: because the term was applied as shorthand term to the early jet engines, which were 'straight through' engines. Incidentally, Frank Whittle wanted to call his jet engine a 'gyrone'. Originally, Rolls-Royce used the term 'bypass turbojet engine' for engines such as the Conway and the Spey (it's there in early editions of the 'Jet Engine' book). In principle, there's nothing wrong it as a descriptive term, but 'turbofan' is now commonplace. However, it's actually just another shorthand term. which was invented by our American cousins and is now accepted - even by R-R.

@@grahamj9101 A direct translation of the Swedish terms are "single stream engine" (turbojet) and "double stream engine" (turbofan), but I think it is old terminology and most people say "turbofläkt" (turbofan) now.

Perhaps it was the first US engine to have cooled turbine blades, with drilled radial cooling passages. However, both BMW and Junkers used cooled turbine blades in their engines towards the end of WWII. They were relatively crude and fabricated from sheet metal, with the cooling used to provide an acceptable, but still relatively short life, using (not very) heat resistant steels.

The J79 originally had inefficient combustors and fuel nozzles. Upgrades on the later models eliminated the smoke trail. The earlier jets all had a slightly less prominent trace of black smoke to the exhaust. Progress in the design of combustion has eliminated that from all turbine engines.

@@AgentJayZ..Thanks

Did water injection add to the black smoke?

@@mattyh2180 Very definitely! That's why B-52Hs still produce a lot of smoke on take-off. When they eventually get their new Rolls-Royce F130 engines and become B-52Js, they will not have water injection and should be smoke free. Also, if you have ever seen a Harrier at an air show, you may have noticed that, in the hover, the exhaust from the 'hot' rear nozzles can suddenly become much smokier. The Pegasus has water injection, intended for a 'short lift wet' take-off rating at maximum weight, but I think that display pilots sometimes use it for effect.

@grahamj9101 I wonder if the harrier uses it to reduce the risk of hot air ingestion at lower altitude close to the ground.

There would be some effect like that, but I would not say that it's working like a centrifugal compressor. It's working like a centrifuge. Difference? No actual gas path, no impeller, and no diffuser.

There are two common methods (with variations) of feeding cooling air to the turbine blades. The first method uses a 'mini-disc' or 'coverplate', which is usually on the front face of the turbine disc . The cooling air is introduced at a low radius into the space between the coverplate and the disc, to flow outwards and into the roots of the turbine blades. There is definitely a pumping effect and some coverplates do actually have radial vanes on their inner faces to enhance the effect. The second method uses pre-swirl nozzles at the radius of the blade roots. These are effectively miniature nozzle guide vanes, which accelerate the cooling air circumferentially, so that it enters the spaces between the blade roots at the correct angle. It does mean that the cooling air is entering the blades at a lower pressure as compared to the first method, so that there is relatively less pressure to drive the flow through the internal passages and out through the cooling holes. However, it does have the advantage of providing a cooling air supply at a relatively lower temperature, and it was R-R's preferred method for the RB211 engines. However, looking at various illustrations, it appears that they have gone for the coverplate method in the Trent engines.

Thank you, it must have been the cover plate version I saw in some image.

Yellow/orange smoke is caused by the formation of Oxides of Nitrogen (NOx). In that specific case, its caused by the high temperature oxidation of atmospheric nitrogen (known as Thermal NOx) - when the burner is turned off, excess O2 which was being consumed when in AB is allowed to react at the high temperatures of the AB section to form orange NOx until temps cool down. There’s also Fuel NOx but that’s another case.

@@GlutenEruption It's true that nitrogen oxides can be coloured - it is specifically nitrogen dioxide which has the orange-red colour, all the other nitrogen oxides are colourless. Hence RFNA (red fuming nitric acid), which gets its colour from having excess nitrogen dioxide dissolved in it. However, the quantities of nitrogen oxides produced in an engine are not likely to be enough to see the colour of it in the exhaust. It takes several percent at minimum to see the colour, whereas the concentration in engine exhaust is going to be in the parts per million range, a tiny fraction of a percent at most. This is because the vast majority of the oxygen in the air will be consumed by burning the fuel, leaving hardly any left over to react with nitrogen. Gas turbines do of course use most of the air they draw in as cooling air or dilution air, but once the combustion products from the flame have been diluted, the temperature is not going to be high enough for the production of nitrogen oxides.

All of the engines I work on were at one time in service with the RCAF, but I have always been a civilian.

Alright, I give. This is the second time in two days you have beat me. More on that in the the next video. Jeez... you're such an engineer. I still say the turbine engine community needs to fight the Oxford English Dictionary... and Common Sense will be in the audience. Edit: I should say, sir that although it seems like we are arguing on this minor point, you are effectively a partner on this channel. The gate keeper robots are extremely strict, and often flag comments with the word good, because they mistake it for god. Anyway, they hold those comments for my review. There are many each day, and I carefully read each before approving them for release. Your comments, whenever they are held in such a way, I approve without even reading, because I know you only deal in facts and information. Such is my respect for your input. Nobody else gets that treatment. It's important to me that you know that.

@@AgentJayZ Of course, I had no idea that I was getting preferential treatment, nor have I ever expected it. Thank you for telling me: I will now ensure that I respect it at all times. I might have been brought up in the motor trade here in the UK (US = auto repair business), with 'hands on' experience at an early age, but I had a career lifetime in gas turbine engine design. That career began in Coventry, the city of Frank Whittle's birth, which was also Auto City UK when I was a young man. That experience will be in my blood for the rest of my days (I'm now in my 80th year). I have a huge respect for your 'hands-on' knowledge and experience, which, of course, I could only get 'second hand' after my basic training. I recall an event during a placement in the Rocket Department at Ansty. We were doing a flow check in the engine build shop on something (I can't remember what) and we could see bubbles in a sight glass, which puzzled us. I suddenly realised that a union nut was probably loose and picked up a spanner/wrench to tighten it. The fitter I was working with grabbed my wrist extremely firmly to stop me. As I was a technical apprentice and not a craft apprentice, the union rules prohibited me from doing so. I could have caused a strike!

@@AgentJayZ PS I've had some light-hearted fun over the years with the "two nations divided by a common language" issue. After all, it has happened (and sometimes still happens) between Bristol and Derby. For example: snubber/clapper, thrust bearing/location bearing: (turbine annulus) flare/hade: blow-off valve/bleed valve - are some that immediately come to mind. However, being more serious for a moment, all I'm asking for is that long-standing and different gas turbine engine terminology on this side of the pond is at least recognised and accepted as being valid. A few years ago, I had a row on your channel with a guy who rejected the fact that, here in the UK, we have long used the term 'angle of incidence' for the difference between the air/gas angle and the geometric angle at the leading edge of an aerofoil (you say airfoil, we say aerofoil). I even quoted the fact that this had been recognised back in 1944 by Langewiesche in his book 'Stick and Rudder', which may still be in print. Similarly, I was ridiculed by another guy for telling him that British engines had reheat, not afterburning. I pointed out the fact that the engines I'd worked on (including the Olympus 593) had Reheat Fuel Control Units: he just would not accept this. I found those comments offensive, insulting and unacceptable, both personally and for the engineers who have gone before me.

Hurray the current thing 🤖