Student works to improve film cooling in gas turbines

Alex Navin, a University Honors Program student, whose co-ops with GE Energy and GE Aviation laid the groundwork for knowledge of gas turbines, conducted research with film cooling in the Hokie Heat Transfer lab, which provides a step in the right direction to improving the cooling efficiency in gas turbines and engines.

“In the heat transfer lab, you’re always trying to find better ways to cool things and better usually means more efficient; more efficient means saving energy and saving costs,” Navin said. “Particularly for gas turbines, if you’re more efficient, you’re saving gas, and that’s a pretty big deal these days.”

Navin, of Vienna, Va., a senior mechanical engineering major in Virginia Tech's College of Engineering, also pursuing minors in mathematics and economics, conducted research in summer 2008 for graduate student Santosh Abraham’s paper on “Film Cooling Hole Design,” with additional guidance from Srinath Ekkad, an associate professor of mechanical engineering.

   

Graphic of film cooling rig Navin designed and built this test rig to test the efficiency of each of the film cooling plates.

This “research should hopefully move forward the body of knowledge on what we already know about film cooling design,” Navin said. “By showing what new hole designs worked and explaining why it was so, we can potentially improve film cooling in real gas turbine engines.”

Film cooling is a process where a layer of cold air is blown over metal surfaces, creating a barrier to the combustion gasses. This enables gas turbine blades to operate in temperatures in excess of 2,000 degrees Fahrenheit.

“Film cooling is incredibly important for gas turbines, like jet engines and power generating turbines, as the temperatures after combustion are hot enough to melt any metal in existence,” Navin said. “The research will hopefully advance the state of the art of gas turbine design (and) also get the university noticed by industry giants such as Siemens, GE, Solar Turbines, and Pratt and Whitney.”

With what he learned from his two co-ops and from his classes, Navin helped design and build a test rig that allowed him to analyze different hole designs in a wind tunnel.

“The design Alex tested is a new design that will enhance the cooling effectiveness of the cooling holes and (will make it) easier to manufacture, thus reducing upfront costs and also reducing operating costs,” Ekkad said.

Navin was responsible for designing and building the film cooling rig, for ordering needed parts, and for quickly prototyping the film cooling plates using Autodesk Inventor, software that produces an accurate 3-D model that validates a design before it is built.

To compare the results in the experiments to the computer models, Navin used a computational fluid dynamics (CFD) program, ANSYS, to create simulations that mimicked his experiments. The program also allowed him to visualize the flow streams to be able to explain why the new designs are better.

   

Cooling flow streamlines image By using the ANSYS program, Navin and Abraham were able to visualize the flow streams to be able to explain the efficiency of each plate design.

To test the efficiency of the film cooling plates, Navin used a wind tunnel rig with plates with different hole configurations. Cold air was blown through the tunnel, and warm air through the plenum chamber. When the hot air flowed through the chamber, it penetrated the holes in the plates and heated everything downstream. Navin and Abraham used an infrared camera to measure the temperature of the surfaces downstream to test which hole configuration provided the most uniform hot air down stream as possible.

   

Cooling effectiveness image According to Abraham, film cooling effectiveness is dependent on shape, orientation and arrangement of the holes in the plates. He and Navin tested six different plates with variations in geometrical parameters at different blowing ratios.

Navin and Abraham concluded that the film cooling plate with the widest angled holes and longest approach generated the best results because it spread the air more evenly. More narrow angled holes forced the air to go straight out, inefficiently heating the area.

“In doing this research, we hoped to improve the cooling efficiency of existing designs to enhance longer life and performance of components inside engines,” Ekkad said. “It’s important because the materials will melt without cooling.”

Ekkad is currently applying for a patent and is in the process of talking to gas turbine companies about the advantages of this hole design.

“The impact and importance of this research is long term and far reaching,” Ekkad said. “But it is just another small step in building better economical, energy efficient gas turbine engines in the future.”

Navin has completed his testing of the film cooling plates, and is working on building a new test rig designed to analyze the effects of rotation on cooling airflow through ducts, which will simulate the conditions that exist within a gas turbine blade.

Department of Mechanical Engineering Research

Mechanical Engineering is one of the largest departments in the College of Engineering.

About 170 graduate students and 40 faculty members conduct research in six centers and over 10 labs located in over 12 facilities around campus.

Other turbine research at Virginia Tech

The Department of Mechanical Engineering is known for its excellence in research in 40 areas of expertise including biomedical engineering, dynamics and controls, and heat transfer.

Some of the current turbine-related research includes

Did you know?

  • Engineers Sir Frank Whittle, Hans van Ohain, and Max Hahn produced viable gas turbine engines in the 1930s. Whittle designed a machine for jet propulsion, and van Ohain and Hahn designed a German counterpart to the British jet engine.
  • With traditional gas turbines, efficiency is proportional to size (an “economy of scale” system).
  • Gas turbines are a fossil fuel-powered distributed generation technology.
    Consortium on Energy Restructuring, Virginia Tech

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