Femtosecond laser-induced plasma spectroscopy for combustion diagnostics in premixed ammonia/air flames.

Ammonia (NH3) is a sustainable fuel with excellent emission characteristics. Hence, it is important to develop diagnostic techniques for NH3 combustion. In this paper, femtosecond laser-induced plasma spectroscopy (FLIPS) was performed in premixed NH3/air flames. The plasma emission spectra induced by the femtosecond laser in the flame and the chemiluminescence spectra of the flame itself were both measured. Through calibration, we found that the spectral intensity ratio of NH (336 nm)/N2 (337 nm) could be used for equivalence ratio measurements in NH3 combustion flow fields. This work is the first attempt at using a femtosecond laser-based technique for NH3 combustion diagnostics.

Ammonia measurements with femtosecond laser-induced plasma spectroscopy.

Femtosecond laser-induced plasma spectroscopy for in situ ammonia (NH3) measurements was demonstrated in NH3/N2 mixtures. When a femtosecond laser at 800 nm was focused at the flow field, the parent NH3 molecules would be photolyzed to generate electronics excited NH fragments, and then indirect measurements of NH3 could be realized by detecting the NH fluorescence (A3Π-X3Σ-) at 336 nm. A detection limit of 205 ppm was achieved. This work is the first attempt, to the best of our knowledge, for ammonia measurements with a femtosecond laser, and the results are useful for the development of ammonia diagnostics.

Enhancement of femtosecond laser-induced plasma fluorescence using a nanosecond laser.

We demonstrate the enhancement of femtosecond (fs) laser-induced filaments in air and nitrogen flow fields using a nanosecond (ns) laser. With the ns laser being imposed on the filaments, the length and the emission intensity of the filaments were largely increased. Temporally resolved spectra of the enhanced filaments were obtained. The results show that the ns laser enhanced the short-lifetime fluorescence of nitrogen, which comes from the transition processes of N2 +(B2Σu + - X2Σg +), N2(B3Пg - A3Σu +) and N2(C3Пu - B3Пg). However, it had little effect on the long-lifetime chemiluminescence, which mainly comes from reactions such as N2(A3Σu +) + N2(A3Σu +) → N2(X1Σg +, v = 0) + N2(B3Пg). A possible explanation of this phenomenon is given, and this phenomenon might have potential applications in instantaneous one-dimensional measurements of various species in gas flow fields.