Only someone like Danesh Tafti, who's been taking advantage of NCSA's computers for almost 15 years, would consider his work of three years ago starting out small. But consider these details about his models of the airflow around tiny ribs inside gas turbine blades.

Back then, he was using 32 processors per run. Now, he's averaging about 150. Then, he was looking at three ribs. Now, he's up to 10. Then, the models were covered in a grid with two million individual zones. Now, we're talking about 10 or 20 million. Then, the calculations were taking a month on a Pentium- based cluster. Now, he blasts through much larger calculations in a week on NCSA's Itanium-based TeraGrid cluster.

“We've made a really big jump,” says Tafti, who is an associate professor of mechanical engineering at Virginia Tech. They aren't big runs for the sake of big runs, though. “We've taken prediction technology in gas turbines a step higher–maybe more than a step. This is a big jump in resolution and quality.” He and graduate students Evan Sewall, Aroon Viswanathan, and Samer Abdel-Wahab work with engineers at General Electric and the South Carolina Institute for Energy Studies at Clemson University, which is supported by the Department of Energy. Together, they make sure these improved simulations translate into better turbines in the real world.

“While we do use massively parallel computations of external flows and in other areas of the engine, very little simulation is currently done with these advanced techniques when it comes to internal turbine passages,” according to Andy Smith, a mechanical engineer for GE Global Research's fluid mechanics lab in Niskayuna, NY. “But turbine seizure and failure happen at the local level,” so the degree of resolution and quality delivered by the team is highly valuable.

Birth of the cool The temperature of the gas driving a turbine engine often exceeds 1,300 degrees Celsius. That figure is headed nowhere but northward because the hotter the gas the more efficient and powerful the turbine can be. Higher temperatures, however, cause additional wear and tear. They can even destroy the turbine entirely. For example, if the temperature of the blade is increased by less than 10 percent, according to many estimates, the engine will need to be overhauled twice as often.

Systems for cooling the blades are a crucial part of the turbine design process. Typically, cool air bleeds from a compressor into channels that snake back and forth through a blade's hollow interior. Through convection, heat from the metal blade is transferred to the air. Placid airflow produces a relatively low heat transfer rate; a turbulent flow boosts the rate. Thus, designers typically stud the channel walls with turbulence inducers such as pock marks and tiny ribs.

Tafti likens the process to stirring a cup of tea. “With your spoon, you generate random motion in the fluid and mix in the milk [more rapidly and thoroughly]. Ribs increase the mixing and the ability of the coolant to conduct heat away from the blade surface.”

Turbulence isn't all milk and honey, though. The churning air can accelerate or unexpectedly slow down–or sometimes break into multiple flows. Meanwhile, “dead regions” can develop. These areas have the same already-heated air passing them again and again, reducing the heat transfer from the blade.

Friction is also a concern. Created as the air moves past the turbulence inducers, it causes the air pressure to drop. Pressure in the blade has to remain higher than the pressure outside. Otherwise, the 1,300 degree gas shooting through the turbine rushes into the blade instead of coolant inside the blade benignly streaming out.

You spin me right round As with the pressure difference between the inside and outside of the blade, there's always a balance. Whether it's between performance and wear or between the benefit of turbulence induced and the downside of friction created, the balance is critical. The team explores the finest points of these features. Using GenIDLEST, a fluid dynamics code that Tafti has been developing for more than a decade, they model a series of ribs in a channel and the air that flows past them. Currently, they focus on nine or 10 ribs, watching the air movement both as it develops and once it has settled into a stable, though still turbulent, flow.

Altering the number, orientation, and design of the ribs shows them differences that crop up in pressure at various points in the channel, average pressure drops over time, and heat transfer rates. It also reveals split flows and dead regions. With this information in hand, designers can refine plans and look for ways to improve efficiency without causing new problems.

Every change influences every feature. A rib that has rounded upper corners, for example, eliminates an unwanted recirculation area behind the rib. Unfortunately, it also reduces the heat transfer rate.

“[The flows that result from] a rib that sits at 45 degrees and one that sits at 90 degrees are completely different,” Tafti says.

Some of the team's most recent models have further complicated matters by adding Coriolis forces and centrifugal buoyancy. These forces account for the rotating of the blade and the impact that action has on the airflow within the blade. “It's extremely difficult to determine these things in the lab,” Tafti says. A large rig is spun at high velocity, and very exacting measurements are taken. “Few can really do this,” he says. And even those who can are only able to gauge the aggregate effect, not each force's separate impact.

The Coriolis force is of particular interest to designers because it tends to repress turbulence and thus heat transfer, especially at the bends in the serpentine channels. “One of the purposes of the bends is simply to channel the flow throughout the blades so it can act as as efficient a heat exchanger as possible. But the act of turning the flow, in and of itself, promotes turbulence, which is a good thing,” GE's Smith says.

Initial results of these simulations show that the Coriolis force can increase heat transfer by 50 to 60 percent on one side of the channel but can decrease heat transfer by as much as 50 percent on the other side–when compared to simulations in which the blade's rotation is not accounted for. Centrifugal force, meanwhile, can complement or oppose the influence of the Coriolis force, depending on whether the flow is moving toward or away from the turbine's center.

These results, along with others that discuss issues like rib orientation, were presented in June at the American Society of Mechanical Engineers' Turbo Expo 2004. They can be found in the conference's recently published proceedings.

This research is supported by the Office of Fossil Energy at the U.S. Department of Energy's National Energy Technology Laboratory and the South Carolina Institute for Energy Studies.

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