Tests keep turbines running above blade melting point

Iowa State University’s Hui Hu and Blake Johnson are developing new technologies and room-temperature tests to improve cooling strategies in gas turbines.

Their current focus is to improve the turbine blades spun by the engine’s exhaust.

‘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.

However, temperatures are rising as manufacturers experiment with biofuels and efficiency improvements, expediting the need for heat-resistant materials and cooling technologies that can lead to 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, New York 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.

According to a statement, 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 and 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.’