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ANU physicists have breached an important frontier of supersonic scramjet research, by developing a simple, cheap and effective measurement of fuel combustion, which could help pilots monitor the performance of their engine.
Scramjets, or supersonic combustion ramjets, are a type of jet engine expected to power the next generation of aircraft and promise to deliver flight speeds five times the speed of Concorde.
In rockets, liquid oxygen is mixed with a liquid fuel, typically hydrogen. The two gases combust creating energy, which is channelled to provide the thrust that powers the aircraft. In traditional rockets, tanks of oxygen and hydrogen are carried on board for this process.
Lighter, smaller, faster
In scramjets, air from the atmosphere is compressed by the front of the engine before being mixed with the hydrogen. This eliminates the need to carry tanks of oxygen on board — the status quo in traditional rockets — making the jet lighter, smaller and faster.
ANU PhD student Mr Alan Griffiths has developed a laser sensor to measure the amount of water vapour released by the interaction of scorching hot air and hydrogen in the scramjet — an indicator of engine performance.
“This technique will enable pilots to monitor the efficiency of their engine and will signal whether more or less fuel needs to be used to power the jet at its optimum,” Mr Griffiths says. “More water vapour indicates more combustion in the engine.
“It is the refinement of an important element that will contribute to the eventual development of supersonic combustion aircraft able to fly at 10 times the speed of sound.”
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Scramjets seem an unusual leap for Mr Griffiths, whose honours project was on ocean currents.
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Mr Griffiths is a member of the Aerophysics and Laser Diagnostics Research (ALDiR) Laboratory, led by Dr Frank Houwing, in the Department of Physics, Faculty of Science at ANU.
“Another important aspect of this development is that the technology is small and inexpensive,” Dr Houwing says. “This makes it attractive to researchers around the world conducting scramjet research, who will have the potential to accurately monitor combustion in real flight tests.”
Mr Griffiths refined his technique inside the ANU T3 free-piston shock tunnel, in which a flow of air at 800C travels at four times the speed of sound (equivalent to Mach 8 during flight), through the scramjet, where it mixes with the fuel, which burns, and produces energy.
This process is extremely fast — there are only two milliseconds to record the water vapour levels and temperature before the combustion is over.
In this minuscule timeframe, two tiny lasers — like those used in household CD players — measure the amount of water vapour released as the hydrogen interacts with the hot air in the scramjet, as well as the temperature of combustion, and the results are fed back to a computer.
“The technique, known as tunable diode laser absorption spectroscopy (TDLAS), measures two essential outputs of the fuel combustion — water vapour concentration and temperature — 20 times per millisecond. This is an advance on alternative techniques that can only measure these variables once,” Mr Griffiths says. “That means I get about 40 readings for every test in the shock tunnel.
“We have tested this technique with other fuels, such as ethylene, and have shown it works just as effectively.”
It took three and a half years for the ALDiR team to apply the technique successfully to scramjet fuel measurement. The TDLAS system is also being investigated by research groups overseas for its potential to detect atmospheric pollutants and to develop highly sensitive fire alarms.
Hypersonic research
The T3 shock tunnel was designed and built at ANU and plays a vital role in Australia’s hypersonic research program. The 20 metre long device is capable of simulating the extreme conditions of flight at 10 times the speed of sound.
“We have expertise with other laser-based techniques, but Alan’s TDLAS is special because it will lead to the development of a sensor compact enough to be used in scramjet flight tests,” Dr Houwing says.
The ALDiR team is a member of the Australian Hypersonics Initiative (AHI), which also includes The University of Queensland, The University of New South Wales (ADFA campus) and the Defence Science and Technology Organisation. This group is involved in an international collaboration with the United States to continue scramjet research on the ground and flight-testing.
The ALDiR team hopes their fuel combustion measurement system will be used to bring the flight system closer to reality in future field tests.
“We plan to continue to refine the technique until it is sufficiently compact and robust to be used in actual flight tests,” Dr Houwing says.
“In collaboration with the AHI, we hope to secure funding to build new, more compact, flight-hardened versions of our TDLAS system.
“The technology in these new systems will be another important step towards achieving an on-board TDLAS sensor for a flying scramjet.”
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