Strategy for improved NH detection in combustion environments using an Alexandrite laser.

A new scheme for NH detection by means of laser-induced fluorescence (LIF) with excitation around wavelength 385nm, accessible using the second harmonic of a solid-state Alexandrite laser, is presented. Detection of NH was confirmed by identification of corresponding lines in fluorescence excitation spectra measured in premixed NH-air flames and on NH radicals generated through NH photolysis in a nonreactive flow at ambient conditions. Moreover, spectral simulations allow for tentative NH line identification. Dispersed fluorescence emission spectra measured in flames and photolysis experiments showed lines attributed to vibrational bands of the NH AA←XB transition but also a continuous structure, which in flame was observed to be dependent on nitrogen added to the fuel, apparently also generated by NH. A general conclusion was that fluorescence interferences need to be carefully considered for NH diagnostics in this spectral region. Excitation for laser irradiances up to 0.2GW/cm did not result in NH fluorescence saturation and allowed for efficient utilization of the available laser power without indication of laser-induced photochemistry. Compared with a previously employed excitation/detection scheme for NH at around 630nm, excitation at 385.7nm showed a factor of ~15 higher NH signal. The improved signal allowed for single-shot NH LIF imaging on centimeter scale in flame with signal-to-noise ratio of 3 for concentrations around 1000ppm, suggesting a detection limit around 700ppm. Thus, the presented approach for NH detection provides enhanced possibilities for characterization of fuel-nitrogen combustion chemistry.