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January 3, 2018

New laser imaging system improves combustion diagnostics

Written By: Will Cushman


UW-Madison Mechanical Engineering Associate Professor David Rothamer has designed and built a pulse-burst laser system that opens the door to new research opportunities for imaging turbulent reacting flows.

A faculty member of UW-Madison’s Engine Research Center, Rothamer studies combustion diagnostics and is particularly interested in developing new optical techniques to study combusting flows. Because the flows are dynamic and turbulent, and simultaneously have rapid chemical reactions occurring, typical cameras cannot capture them very well. Instead, researchers use a combination of laser-based imaging techniques and high-speed imaging using natural light emission from the combustion process.

But most conventional laser imaging techniques have a hard time capturing the true nature of a combusting flow. That’s because conventional high-pulse energy laser sources used for such imaging are only available with low pulse repetition rates, only allowing researchers to capture images at a rate of usually 10 images per a second. While that may seem fast, it’s not ideal for understanding fast-paced turbulent combustion in internal combustion engines where the entire combustion event occurs within the space of a few thousandths of a second.

“For a lot of diagnostics that’s okay,” says Rothamer. “Except that when you’re studying flows, generally things are happening much faster than at the rate of 10 times a second. So each image you get is relatively uncorrelated with the previous image.”

On the other hand, researchers have more recently developed laser systems that continuously provide pulses at high repetition rates, but the pulses are limited in energy. The idea Rothamer has leveraged combines the advantages of the two types of systems to generate high-energy pulses at a rapid rate.

“The limitation is that if you did that continuously, your system would require a huge amount of power,” he says.

Instead, Rothamer has devised a system where a train of relatively low-energy laser pulses passes through a series of amplifiers that strengthen them over a limited duration—20 to 40 milliseconds at a time. The system creates high-energy laser pulses over that duration at a high rate, without requiring a huge facility with major energy requirements. In fact, the system could fit in a large suitcase.

“I’m excited about this; it allows for application of laser imaging techniques inside engines at speeds that have never before been achieved,” says Rothamer, who spent two years building the system. “I’m particularly excited about the flexibility of the system. In our labs, the system could replace all of our 10-Hertz laser systems. It’s much more flexible than those other systems and provides significantly more capability.”