Iowa State University’s Hui Hu and Blake Johnsonare 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.
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.
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.
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.