Joel Kuehner, associate professor of physics and engineering at Washington and Lee University, has received a $300,000 grant from the National Science Foundation (NSF) to continue his research into how two different fluid streams—fuel and air, for example—mix at supersonic speeds.
Kuehner's research uses a laser technique to measure how the temperature varies as the fluids go through the mixing region, from which he can infer how fuel might mix at supersonic speeds.
Unlike automobile engines where fuel is injected into a stream of stagnant air to achieve combustion, problems arise when the air moving is at supersonic speeds. "Once you get the flow going faster than the speed of sound, it's not very receptive anymore to being measured," Kuehner said. "But if we can understand how the two different fluid streams mix, then other people can take that information and maybe make better jet engines that run at supersonic speeds, or make better controls over sound generation in jet engines."
Kuehner hopes to do for the jet engine what researchers did for the basic internal combustion engine in the 1950s and 1960s. "Car engines were horribly inefficient at that time and a lot of that inefficiency came from not understanding the mixing process. Some of the fuel was going out of the tail pipe because it never mixed, it never burned. So the hope is we can do something similar and make supersonic combustion more efficient. The military and aircraft manufacturers such as Boeing and Airbus would love a better understanding of this," he said.
He described the current process for fuel combustion in jet engines as both inefficient and ineffective. "As soon as you try to bring air into a jet engine faster than the speed of sound, you have to slow it down to subsonic speed so that you can mix it with the fuel, burn it, and then get it out the back of the engine. Then you have to reaccelerate to supersonic speeds again," he said. A better understanding of how this mixing works at supersonic speeds could mean that air would not need to be slowed down before it mixes with and burns the fuel.
Another potential application for Kuehner's research is a reduction in the noise of jet engines on takeoff, since much of that noise is generated by the stream coming out of the back of the jet engine and mixing with the surrounding air. While that would help reduce noise at civilian airports, it would also be of interest to the military. "Once it's in flight, a jet plane creates a lot of noise and the military is concerned that if someone knows the type of noise a certain plane makes, it's very easy to track that plane," said Kuehner.
While there is great interest in the potential of this type of research, Kuehner said that the U.S. military and the NSF had given up trying. "They didn’t want to hear about it anymore," he said. "So we had to go a non-traditional route to get funding."
So Kuehner applied for and received a grant from the Thomas F. Jeffress and Kate Miller Jeffress Memorial Trust a few years ago which enabled him to prove that his technique works.
"We made several measurements that indicated how the temperature varied significantly with large fluctuations as the fluids go through the mixing region." he said. "The Jeffress Memorial Trust grant really set us up to go after the NSF grant. Hopefully, now we'll be able to say not just that we can make these measurements, but to actually make them, and once we can show our research is reliable it will be easier to get funding."
With the NSF grant, Kuehner aims to improve the laser technique and apply it to a wider range of conditions, showing how temperature fluctuates in the flow over a range from subsonic to faster than the speed of sound.
The NSF grant will fund three W&L students each summer for the next three years to work with Kuehner on the research. "It will have a great impact," he said, "and will allow us to expand experiments in the fluids lab as well as the fluids course. We'll have new equipment that will permit us to do a lot of different research, not just for this project but for other projects as well."
Kuehner joined Washington and Lee in 2004. He received his B.S. in mechanical engineering from Pennsylvania State University. He received his M.S. and Ph.D. in mechanical engineering from the University of Illinois at Urbana-Champaign.