For Those Interested

[lo_lyf]

So what causes a turbine engine to leave a trail of black smoke? We know that when a piston engine blows black smoke its running too rich.
Doesn't a turbine engine follow the principles of lean-of-peak. As in, its got way more air than required to burn the fuel, so shouldn't all of the fuel burn cleanly?

[megan]

So what causes a turbine engine to leave a trail of black smoke

Combustor design is complex, and in the early days the computer systems were not available to do the number crunching or flow simulations. An improved combustor was introduced on the Allison T56 engine fitted to P-3 which reduced smoke.

http://www.dtic.mil/dtic/tr/fulltext/u2/a227750.pdf

[JohnMcGhie]

Fuel Cooling

Originally Posted by lo_lyf

So what causes a turbine engine to leave a trail of black smoke? We know that when a piston engine blows black smoke its running too rich.
Doesn't a turbine engine follow the principles of lean-of-peak. As in, its got way more air than required to burn the fuel, so shouldn't all of the fuel burn cleanly?

In a word: "Fuel Cooling". On my Airforce Engine Mechanic's course, I was told:

In the early days, Rolls Royce held the patents on most of the alloys capable of holding their strength in the high temperatures developed in the hot section of a turbojet engine. Other manufacturers, particularly the American ones, had to make do with lower-temperature-limited alloys. To prevent the engine melting its turbine blades off, they were "fuel-cooled" at take-off by injecting sufficient fuel to develop a massively rich mixture. Hence the "signature" soot trails.

With the development of better alloys, ceramic coatings, and hollow turbine blades allowing cold air to flow through them (and in the latest versions, to be pushed out of the blade leading edge, forming a protective layer of cold air over the blade) engine manufacturers have been able to raise their turbine inlet temperatures, and hence, to lean their mixtures out. A lot!

And of course, as Megan says, improved combustor design enables the fuel to be more completely vaporized and thus burned before entering the turbine.

[amberale]

The Caravelle that used to leave Brissy to the east was the dirtiest regular operator I saw.
ANA with #2 on fire was worse.

[megan]

In a word: "Fuel Cooling". On my Airforce Engine Mechanic's course, I was told

Youse was told wrong John. The only turbine I've heard of that uses fuel rich for cooling is the theoretical turbo rocket engine where combustion temperatures are of the order of 3,500°C, versus 1,800 to 2,000°C in your regular turbine.

The pollutants found in the exhaust gases are created within the combustor. Use of an airspray nozzle to inject fuel, which carries a proportion of the primary combustion air with the injected fuel, aerates the spray, reducing both carbon formation and exhaust smoke. Other nozzle types produce localised fuel rich concentrations and increase pollutants. There are four main pollutants which are legislatively controlled; unburnt hydrocarbons (unburnt fuel), smoke (carbon particles), carbon monoxide and oxides of nitrogen. The principal conditions which affect the formation of pollutants are pressure, temperature and time. With all other things being equal, the temperature of the gases impinging the turbine is controlled by what percentage of the compressor output is used for combustion compared to that used for cooling. Better turbine blades allows a reduction in the percentage devoted to cooling air, thus increasing temperature and efficiency.

A good book on the subject.

http://airspot.ru/book/file/485/1668...nnyy_dviga.pdf

[josephfeatherweight]

That's a fantastic book, thanks for posting the link, Megan.

[JohnMcGhie]

My bad ...

Originally Posted by megan

Youse was told wrong John. The only turbine I've heard of that uses fuel rich for cooling is the theoretical turbo rocket engine where combustion temperatures are of the order of 3,500°C, versus 1,800 to 2,000°C in your regular turbine.

I sit corrected: thanks Megan! I can't think why I didn't look that up in 1966 when I was told it, I hate being wrong ...

Mind you, I was training on cutting edge technology: Halford H1 (later and more widely known as the de Havilland Goblin) out of the De Havilland Vampire, and the Rolls Royce Avon in the Canberra... OK, they had been in service for more than ten years by the time I got there: but I hadn't :-)

[megan]

I hate being wrong

Not you wrong John, was your lecturer. The JT8 was a smoker originally and new combustors were successfully trialled on a 727, similarly with the Olympus on the Concorde, the prototype was a smoker but new combustors were introduced.