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Join Date: May 2017
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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?
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?
So what causes a turbine engine to leave a trail of black smoke
http://www.dtic.mil/dtic/tr/fulltext/u2/a227750.pdf
Fuel Cooling
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?
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 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.
In a word: "Fuel Cooling". On my Airforce Engine Mechanic's course, I was told
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
That's a fantastic book, thanks for posting the link, Megan.
My bad ...
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 :-)
I hate being wrong