Engineers know that gas turbine engines for aircraft and power plants
are more efficient and burn less fuel when they run at temperatures high
enough to melt metal. But how to raise temperatures and efficiencies
without damaging engine parts and pieces?
Iowa State University's Hui Hu and Blake Johnson, working away in a
tight corner behind the university's big wind tunnel, are developing new
technologies to accurately test and improve engine cooling strategies.
Their current focus is to improve the turbine blades spun by the
engine's exhaust. Those blades at the back of the engine drive front
blades that force compressed air into the combustion chamber.
"Right now, the current state of the art for engine combustion is
about 3,000 degrees Fahrenheit," said Hu, an Iowa State professor of
aerospace engineering. "That temperature is above the melting
temperature of all engine materials. If you don't have cooling
technologies, all the material will melt."
One technology is to build hollow turbine blades and blow coolant
through an arrangement of holes in the blades. The holes create a
cooling film between the hot exhaust gases and the turbine blades,
allowing the blades to keep their shape and strength.
But now, as manufacturers experiment with biofuels and efficiency
improvements, Hu said combustion temperatures are heading higher and
higher. And so it's getting more and more important for engineers to
research and develop heat-resistant materials and cooling technologies.
Better cooling can mean fuel savings, longer-lasting parts and
significant cuts in operating costs.
For the past 19 months, Hu and Johnson, an Iowa State post-doctoral
research associate in aerospace engineering, have been working with the
GE Global Research Center in Niskayuna, N.Y., to study turbine blade
cooling.
Rather than trying to replicate the high temperatures inside a jet
engine, the engineers have developed new technologies and
room-temperature tests to study the effectiveness of cooling hole
shapes, arrangements and the cooling film they create over a turbine
blade. They've built an experimental rig that places a model turbine blade
at the bottom of a wind tunnel's test section. Jets of pure nitrogen or
carbon dioxide are blown through the model blade's cooling holes. The
main stream of the wind tunnel blows oxygen-rich air above the test
blade. Using oxygen-sensitive paint on the model blade, an ultraviolet
light source and a digital camera, Hu and Johnson can see if the cooling
film keeps oxygen molecules from the main stream off the model blade. "If we find an oxygen molecule on the model blade, we know that the cooling stream didn't create a barrier," Hu said.
So far, the Iowa State engineers have been working with low-speed
flows. They're now building and testing another experimental rig that
can handle high-speed flows approaching the speed of sound.
They've also been using an advanced flow diagnostic technique called
particle image velocimetry -- seeding the test flows with tiny particles
that can be photographed with a laser and camera -- to record and
measure what happens when gases blow out of the cooling holes.
Those tests provide data about flow structure, thickness of the
cooling film, density ratios, velocity ratios and other measurements
related to cooling effectiveness. "The big goal of this study is to find anything that GE can do to
improve the function of its film cooling system," Johnson said. "Better
cooling equals longer-lasting blades. And that could be worth billions
of dollars across a fleet of engines."
http://www.sciencedaily.com/releases/2013/08/130802094803.htm
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