Improving jet engine performance, with help from an MRI

A Stanford University researcher is using the MRI machine to improve the efficiency and performance of jet engines for the military.

A key tool to help improve the efficiency of jet engines may be found in your local hospital.

The magnetic resonance imaging, or MRI, machine is usually used to take three-dimensional pictures of organs and soft tissues to allow doctors to inspect for damage. But one researcher at Stanford University is using the technology for the aerospace industry.

Doctoral researcher Michael Benson discovered that the MRI machine can collect in a few hours as much data on flow and mixing as two or more years of conventional measurement.

That means that the time needed to develop and test new engine designs is greatly reduced.

The technique was pioneered by Stanford researchers Christopher Elkins and John Eaton, who first used it to study coral colonies and turbine blades.

Taking a suggestion from Eaton, Benson -- a lieutenant colonel in the U.S. Army -- applied the technique to jet turbines, analyzing the mixture of hot combustion and cooling gases.

Jet engines are precarious machines that are more efficient when they run hotter. To maximize efficiency, the trailing edges of the blades immediately downstream of the engine's combustor run very close to their melting point. In fact, that's why they're ultra thin -- so they don't melt.

The blades are cooled by diverting incoming air into a series of tiny channels that run through each blade. But at a point, the blades become too thin for this, and cooler air simply runs over the trailing edge.

When it exits the blade, it mixes with the hot air from the combustor, raising the temperature of the blade surface.

That's where the MRI technique comes in: by analyzing how the hot and bypass air mix, Benson hopes to optimize the design to reduce the amount of coolant needed.

Measuring the the temperature and velocity of the hot and bypass air streams as they mix with conventional techniques requires researchers to release fluorescent dyes or oil droplets and illuminate them with a laser, to be captured by a high-speed camera and analyzed by a computer.

But this technique has limitations: the cameras can only look at one small area at a time and have a narrow depth of field. Simply, there's a lot of post-production work to be done before the researcher has a three-dimensional portrait of the engine in question.

The MRI manages to do this is four to eight hours. Why? Because MRIs are designed for three dimensions, using an electromagnetic pulse to disturb the protons in hydrogen molecules and then measuring their locations as they realign with the magnetic field.

For scans, Benson uses water mixed with copper sulfate -- inexpensive, compared to the gadolinium used in medical MRI machines.

Benson says he's already been able to increase surface cooling by 10 percent, the equivalent of bringing a blade's temperature down by 100 to 150 degrees Fahrenheit.

He presented his work on Monday at the 63rd annual American Physical Society Division of Fluid Dynamics meeting in Long Beach, Calif.

Photo: The IAE V2500. (Rolls-Royce)

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