“Supersonic combustion occurs when you’re trying to burn a fuel in air when the air is travelling faster than the speed of sound… there are several technological problems that have to be solved. One of them is getting the fuel and air to mix and burn before it’s left the engine.”
Designing an aircraft that can travel at many times the speed of sound poses a range of challenges unique to supersonic conditions—one of those is finding a way to ignite the aircraft’s fuel when that fuel is leaving the engine at incredibly high speeds.
This is a problem Dr Sean O’Byrne at UNSW Canberra’s ADFA School of Engineering and Information Technology is trying to solve.
“Supersonic combustion occurs when you’re trying to burn something when you’re travelling faster than the speed of sound,” said Dr O’Byrne.
“When you try and do that really fast there are several technological problems that have to be solved. One of them is getting the fuel and air to mix and burn fast enough before it’s left the engine… If they’re going super fast you don’t have much time to mix them.”
He gave an example, describing an aircraft travelling at eight times the speed of sound. For an aircraft engine 2.5 metres long, the fuel only has one one-thousandth of a second to mix with the air and burn before it travels the entire length of the engine.
“So, our research is about trying to make that process of mixing and burning fuel as efficient as possible, and we do that by studying the behaviour of that fuel and air in engines,” said Dr O’Byrne.
Researchers conduct experiments simulating supersonic speeds using an Australian invention called a free-piston shock tunnel, which compresses air and pushes it out through a nozzle at very high speeds. The rush of high-speed air only lasts a couple of milliseconds, but using laser-based methods the researchers can determine whether and how quickly the fuel and air are able to mix.
There are two main approaches researchers can take to solving the problems of supersonic combustion.
“We can change the geometry of the fuel injector to encourage mixing with the air, and we can look at different methods of igniting the fuel and the air,” Dr O’Byrne said.
To explain the first approach, he described air flowing very fast across the surface of a table, then drilling a hole in the table to create another flow of air (representing the fuel) coming out perpendicular to the air flow across the table. This set-up effectively mixes the fuel and the air, but in the case of an aircraft it also creates drag, slowing it down.
An alternative might be to put the fuel in a cavity that the air can flow across, mixing the two that way—different injection methods have pros and cons.
Dr O’Byrne develops new measurement techniques to determine the effectiveness of the methods used to improve the combustion.
“We’ve come up with a means of laser ignition that was the first of its type in the world. So, we’ve shown that lasers can make things burn in conditions when they wouldn’t usually burn.”