In view of the increasing importance of hydrogen as a fuel, we are presenting a novel experimental technique that allows for two-dimensional (2D), spontaneous Raman hydrogen concentration measurements in optically accessible enclosures. The novelty of the technique lies in that the actual experiment takes place in a laser cavity, which significantly enhances the inherently weak hydrogen Raman signal. Quantitative, 2D measurements of hydrogen number density were performed at elevated pressure by inserting a pressurised vessel in the cavity of a flashlamp-pumped dye laser. Particular emphasis is placed on the determination of the data accuracy of this technique, which is inherently noisy because of the weakness of the Raman signal. The measurements are compared with the results of direct numerical simulation for a laminar jet. The problems of determining accurately the position and the intensity profile of the laser sheet are addressed. A minimum number density of hydrogen that can be detected with an acceptable signal-to-noise ratio, for a 520-m resolution, was determined.
Keywords: hydrogen number density, Raman imaging, laser diagnostics, laminar jet, intra-cavity, alternative propulsion, alternative fuels, Raman hydrogen concentration, combustion, data accuracy