Enginuity 2011

Film cooling

Helping turbine-engine blades stay cool

Engines run better when they stay cool. And when it comes to turbine-engine blades, the cooler the better.

Xian Chang LiXian Chang Li, an assistant professor in the Department of Mechanical Engineering, is the principal investigator on two National Science Foundation grants dealing with turbine-engine blades. He needed the first grant to work on the second. The first is titled "MRI: Acquisition of 2D PIV with GSV for Research Activities in Alternative Energy, Advanced Thermal and Mechanical Systems." The $124,000 grant enabled him to acquire the equipment to study the mechanics of turbine blades. That $167,451 grant is titled "BRIGE: Fundamental Study on Fluid Flow and Heat Transfer of Film Cooling with Backward Injection." His study objective explores the fundamental mechanism of film cooling with backward coolant injection, including the optimum design conditions.

“The NSF research grant on turbine blades will take advantage of this equipment,” Li said. “We are working on it now. In addition, the equipment has also been used in other projects.”

As a BRIGE grant—BRIGE stands for National Science Foundation Broadening Participation Research Initiation Grants in Engineering—Li will involve underrepresented groups. For broader participation of people from underrepresented groups in the engineering disciplines, Li plans to offer presentations to high-school students on general engineering concepts and advanced technologies. He also plans to host an engineering seminar for high-school science teachers so that the teachers can share engineering concepts with their classes. In this way, more students will be introduced to engineering fields, he said.

For decades, aircraft and power generation gas-turbine designers have tried to increase the combustor exit and high-pressure turbine stage inlet temperatures. With higher combustor exit temperatures, improved efficiency and reduced fuel consumption can be achieved. When applied to aircraft, the higher temperatures lead to increased thrust. The problem: The higher temperatures have jeopardized the integrity of the high-pressure turbine components, specifically the turbine blades.

Modern turbine stage inlet temperatures exceed the melting-point temperatures of turbine-blade materials. To fight and avoid failure of turbine blades in gas-turbine engines resulting from these excessive operating temperatures, film cooling has been incorporated into blade designs. In film cooling, cool air is taken from the compressor stage, ducted to the internal chambers of the turbine blades and discharged through small holes in the blade walls. This air provides a thin, cool, insulating blanket along the external surface of the turbine blade.

Xian Chang LiLi is studying film cooling with backward coolant injection as opposed to forward coolant injection, which is currently used. “The cooling coverage is poor, especially in the cross-flow direction,” he said of forward coolant injection. “The poor coverage forces the designers to increase the cooling flow, which will lower the turbine’s efficiency.”

He believes that backward injection is the answer. “The backward injection is proposed to overcome the poor cooling coverage of the forward injection,” he said. “The hypothesis is that the reduced momentum in the flow direction will make the diffusion in the cross-flow direction relatively stronger.”

The research will have a broad impact. According to Li, the fundamental research has a strong background in industrial applications. Its success will directly benefit many related industrial companies and eventually benefit even more gas-turbine users such as power-generation and aircraft companies.

His theory is proving to be correct as the numerical simulation so far proves the proposed hypothesis. “A much more uniform cooling coverage can be achieved when the backward injection is used,” he said. “More studies are needed for experimental validation as well as the potential challenge from this proposed concept.”