Scientists develop new way to measure water in hydrogen flames
Researchers have created a laser-based diagnostic tool that can precisely track water vapor levels during hydrogen combustion—a capability that could accelerate the development of cleaner hydrogen fuel systems. The technique, demonstrated in laboratory flames, offers manufacturers and energy companies a new way to optimize hydrogen combustion efficiency and safety.
Originaltitel: Water vapor in hydrogen flames measured by time-resolved collisional dephasing of the pure-rotational N<sub>2</sub> CARS signal
<p>We present a novel diagnostic technique to probe water vapor (H<sub>2</sub>O) concentration in hydrogen (H<sub>2</sub>) combustion environments via the time-resolved measurement of the collisional dephasing of the pure-rotational coherent anti-Stokes Raman scattering (CARS) signal of nitrogen (N<sub>2</sub>). The rotational Raman coherence of the N<sub>2</sub> molecules, induced by the interaction with the pump and Stokes laser fields, dephases on a timescale of hundreds of picoseconds (ps), mostly due to inelastic collisions with other molecules in atmospheric flames. In the spatial region of H<sub>2</sub> flames where H<sub>2</sub>O is present in appreciable amount, it introduces a faster dephasing of the N<sub>2</sub> coherence than the other major combustion species do: we use time-resolved femtosecond/picosecond (fs/ps) CARS to deduce the H<sub>2</sub>O mole fraction from the dephasing effect of its inelastic collisions with N<sub>2</sub>. The proof-of-principle is demonstrated in a laminar H<sub>2</sub>/air diffusion flame, performing sequential measurements of the collisional dephasing of the N2 CARS signal up to 360 ps. We measure the temperature and the relative O<sub>2</sub>/N<sub>2</sub> and H<sub>2</sub>/N<sub>2</sub> concentrations at a short probe delay, and input the results in the time-domain model to extract the H<sub>2</sub>O mole fraction from the signal decay, thus measuring the whole scalar flow fields across the flame front. We furthermore present single-shot simultaneous thermometry and absolute concentration measurements in the turbulent TU Darmstadt/DLR Stuttgart canonical ‘H3 flame’ performed by dual-probe CARS measurements obtained with a polarization separation approach. This allows us to probe the molecular coherence simultaneously at ∼20 and ∼250 ps on the basis of a single-laser-shot, and record the resulting signals in two distinct detection channels of our unique polarization-sensitive coherent imaging spectrometer. The proposed technique allows for measuring the absolute concentrations of all the major species of H<sub>2</sub> flames, thus providing a full characterization of the flow composition, as well as of the temperature field.</p>