New laser-based instrument designed to boost hydrogen research

Optics Express (2022). DOI: 10.1364/OE.465817″ width=”800″ height=”469″/>

(a) Energy and Feynman diagrams of a resonant (left) and non-resonant (right) CRS pathway. (b) Polarization angles of resonant (blue line) and non-resonant (red line) CRS signals, β and γ, shown as elevation angle on unit sphere as a function of relative polarization angle (azimuth angle ) of the pump/Stokes and probe fields, α. (c) Schematic of polarization-sensitive coherent imaging spectrometer. OW, optical window; SL, spherical lenses; M, mirror; BPF, band pass filter; PBS, polarization beam splitter; FR, Fresnel diamond; BS, beam off. Inset: probe volume. The probe passes through the ultra-wideband/Stokes pump beam ∼2mm past the end of the filament. Increasing input energy causes the filament to lengthen towards the focusing optic (direction of arrow) (d) Measurement points on the H2/air flame front, the dotted red line identifies the burner edge location at y=9.5mm. Credit: Express Optics (2022). DOI: 10.1364/OE.465817

Researchers have developed an analytical instrument that uses an ultrafast laser for precise measurements of temperature and hydrogen concentration. Their new approach could help advance the study of greener hydrogen-based fuels for use in spacecraft and aircraft.

“This instrument will provide powerful capabilities to probe dynamical processes such as diffusion, mixing, energy transfer and chemical reactions,” said research team leader Alexis Bohlin from the University of Technology in Luleå in Sweden. “Understanding these processes is fundamental to developing more environmentally friendly propulsion engines.”

In Express Optics, Bohlin and colleagues at Delft University of Technology and Vrije Universiteit Amsterdam, both in the Netherlands, describe their new coherent Raman spectroscopy instrument for the study of hydrogen. This was made possible by a setup that converts broadband light from a laser with short pulses (femtoseconds) into extremely short supercontinuum pulses, which contain a wide range of wavelengths.

The researchers demonstrated that this supercontinuum generation could be achieved behind the same type of thick optical window found on high-pressure chambers used to study a hydrogen 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 fuel, when made from renewable resources, could have a huge impact on reducing emissions and make a significant contribution to mitigating anthropogenic climate change,” Bohlin said. “Our new method could be used to study these fuels under conditions that closely resemble those of rocket and aerospace engines.”

Bring in the light

There is a lot of interest in developing aerospace engines that run on hydrogen-rich renewable fuels. In addition to their durability appeal, these fuels have one of the highest specific impulses possible, a measure of how efficiently the chemical reaction in an engine creates thrust. However, it has been very difficult to make the hydrogen-based chemical propulsion systems reliable. This is because the increased reactivity of hydrogen-rich fuels dramatically alters the combustion properties of the fuel mixture, which increases flame temperature and decreases ignition delay times. Additionally, combustion in rocket engines is typically very difficult to control due to the extremely high pressures and temperatures encountered during space travel.

“The advancement of technology for sustainable aerospace launch and propulsion systems relies on a consistent interplay between experiments and modeling,” Bohlin said. “However, several challenges still exist in terms of producing reliable quantitative data to validate the models.”

One of the obstacles is that experiments are usually conducted in an enclosed space with limited transmission of incoming and outgoing optical signals through optical windows. This window can cause the supercontinuum pulses needed for coherent Raman spectroscopy to stretch as they pass through the glass. To overcome this problem, the researchers developed a way to transmit a femtosecond pulsed laser through a thick optical window, then used a process called laser-induced filamentation to turn it into supercontinuum pulses that remain coherent on the other side.

Study of a hydrogen flame

To demonstrate the new instrument, the researchers set up a femtosecond laser beam with the ideal properties for supercontinuum generation. They then used it to perform coherent Raman spectroscopy by exciting hydrogen molecules and measuring their rotational transitions. They were able to demonstrate robust measurements of hydrogen gas over a wide range of temperatures and concentrations and also analyzed a hydrogen/air diffusion flame similar to what one would see when a hydrogen-rich fuel is burned.

The researchers are now using their instrument to perform detailed analysis in a turbulent hydrogen flame in hopes of making new discoveries about the combustion process. In an effort to adopt the method of researching and testing rocket engines, scientists are exploring the limits of the technique and would like to test it with hydrogen flames in a lightly pressurized closed box.

New technique for measuring temperatures in combustion flames could lead to cleaner biofuels

More information:
Francesco Mazza et al, Coherent Raman spectroscopy on hydrogen with in situ generation, in situ use and in situ referencing of ultrawideband excitation, Express Optics (2022). DOI: 10.1364/OE.465817

Quote: New laser-based instrument designed to boost hydrogen research (September 13, 2022) Retrieved September 13, 2022 from .html

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