Researchers have developed an analytical instrument that uses a high-speed laser for precise measurements of temperature and hydrogen concentration. Their new approach could advance the study of greener hydrogen-based fuels for use in spacecraft and aircraft.
“This instrument will provide powerful capabilities for probing dynamic processes such as diffusion, mixing, energy transfer, and chemical reactions,” said research team leader Alexis Bohlin from Luleå University of Technology in Sweden. “Understanding these processes is fundamental to developing more environmentally friendly propulsion engines.”
In Optica Publishing Group Optix ExpressBohlin and colleagues from Delft University of Technology and Vrije Universiteit Amsterdam, both in the Netherlands, describe a new, coherent Raman spectroscopic instrument for studying hydrogen. This is made possible because of a setup that converts broadband light from a laser with short pulses (femtoseconds) into ultrashort pulses, which have a wide range of wavelengths.
The researchers showed that this super-vacuum generation could take place behind the same kind of thick optical windows found in hyperbaric chambers used to study a hydrogen-based engine. This is important because other methods of generating ultra-wideband excitation do not work when these types of optical windows are present.
“Hydrogen-rich fuels, when manufactured from renewable sources, can have a significant impact on reducing emissions and contribute significantly to mitigating human-caused climate change,” Bohlin said. “Our new method can be used to study these fuels under conditions very similar to those found in rocket and aviation engines.”
There is great interest in developing aircraft engines that run on hydrogen-rich renewable fuels. In addition to their sustainability appeal, these fuels have the highest specific thrust achievable — a measure of how efficiently a chemical reaction in an engine is creating thrust. However, it has been very difficult to make hydrogen-based chemical propulsion systems reliable. This is because the increased reactivity of hydrogen-rich fuels significantly changes the combustion properties of the fuel mixture, increasing the flame temperature and reducing ignition delay times. Also, combustion in rocket engines is generally very difficult to control due to the extremely high pressures and high temperatures that are encountered when space travel.
“The progress of technology for sustainable launch and propulsion systems in space depends on a coherent interaction between experiments and modelling,” Bohlin said. “However, many challenges remain in terms of producing reliable quantitative data to validate models.”
One obstacle is that experiments are usually conducted in an enclosed space with limited transmission of light signals in and out through optical windows. This window can cause the supercontinuity pulses needed for coherent Raman spectroscopy to stretch as they pass through the glass. To get around this problem, the researchers developed a method for transmitting a femtosecond pulsed laser through a thick optical window and then used a process called laser-induced filament to convert it into ultra-continuous pulses that stay coherent on the other side.
Hydrogen flame study
To demonstrate the new tool, the researchers created a femtosecond laser beam with optimal properties for supersecond generation. Then they used it to perform coherent Raman spectroscopy by means of exciting hydrogen molecules and to measure their spin shifts. They were able to demonstrate robust measurements of hydrogen gas over a wide range of temperatures and concentrations, and they also analyzed hydrogen/air diffusion flames similar to what can be seen when burning hydrogen-rich fuels.
The researchers are now using their instruments to perform a detailed analysis of turbulent hydrogen flames in the hope of making new discoveries about the combustion process. With the aim of adopting a research and testing method for rocket engines, scientists are exploring the limits of this technology and would like to test it using hydrogen flames in a closed, slightly pressurized shell.