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.

Instantaneous one-dimensional equivalence ratio measurements in methane/air mixtures using femtosecond laser-induced plasma spectroscopy.

Equivalence ratio is one of the most significant parameters in combustion flow fields. In this paper, femtosecond laser-induced plasma spectroscopy (FLIPS) technique for instantaneous one-dimensional local equivalence ratio measurements were performed. By measuring the spatially resolved spectra of FLIPS, we found that the spectral peak area ratios of CH (431 nm)/N2 (337 nm), CH (431 nm)/N2 (357 nm), and CH (431 nm)/O (777 nm) can be utilized to achieve one-dimensional local equivalence ratio measurements. Among them, the CH peak at ~431 nm and the O peak at ~777 nm are strong enough to be used to achieve single-shot measurements, which is important to turbulent flow fields. Furthermore, systematic experiments were performed by using FLIPS in both laminar and turbulent flow fields. The FLIPS technique features the abilities of instantaneous one-dimensional quantitative measurements, high spatial resolution, and no Bremsstrahlung interference.

Filamentary anemometry using femtosecond laser-extended electric discharge - FALED.

We demonstrate a non-contact spatiotemporally resolved comprehensive method for gas flow velocity field measurement: Filamentary Anemometry using femtosecond Laser-extended Electric Discharge (FALED). A faint thin plasma channel was generated in ambient air by focusing an 800-nm laser beam of 45 fs, which was used to ignite a pulsed electric discharge between two electrodes separated over 10 mm. The power supplier provided a maximum voltage up to 5 kV and was operated at a burst mode with a current duration of less than 20 ns and a pulse-to-pulse separation of 40 µs. The laser-guided thin filamentary discharge plasma column was blowing up perpendicularly by an air jet placed beneath in-between the two electrodes. Although the discharge pulse was short, the conductivity of the plasma channel was observed to sustain much longer, so that a sequence of discharge filaments was generated as the plasma channel being blown up by the jet flow. The sequential bright thin discharge filaments can be photographed using a household camera to calculate the flow velocity distribution of the jet flow. For a direct comparison, a flow field measurement using FLEET [Appl. Opt. 50, 5158 (2011)] was also performed. The results indicate that the FALED technique can provide instantaneous nonintrusive flow field velocity measurement with good accuracy.

Applicability of Femtosecond Laser Electronic Excitation Tagging in Combustion Flow Field Velocity Measurements.

Femtosecond laser electronic excitation tagging (FLEET) is a molecular tagging velocimetry technique that can be applied in combustion flow fields, although detailed studies of its application in combustion are still needed. We report the applicability of FLEET in premixed CH4-air flames. We found that FLEET can be applied in all of the combustion areas (e.g., the unburned region, the burned region and the reaction zone). The FLEET signal in the unburned region is significantly higher than that in the burned region. This technique is suitable for both lean and rich CH4-air combustion flow fields and its performance in lean flames is better than that in rich flames.

Comprehensive CO detection in flames using femtosecond two-photon laser-induced fluorescence.

We demonstrate a femtosecond two-photon laser-induced fluorescence (fs-TPLIF) technique for sensitive CO detection, using a 230 nm pulse of 9 µJ and 45 fs. The advantages of fs-TPLIF in excitation of molecular species were analyzed. Spectra of CO fs-TPLIF were recorded in stable laminar flames spatially resolved across the flame front. A hot band (1, n) together with the conventional band (0, n) of the B→A transitions were observed in the burned zone and attributed to the broadband nature of the fs excitation. The CO fs-TPLIF signal recorded across the focal point of the excitation beam shows a relatively flat intensity distribution despite of the steep laser intensity variation, which is beneficial for CO imaging in contrast to nanosecond and picosecond TPLIF. This phenomenon can be explained by photoionization, which over the short pulse duration dominates the population depletion of the excited B state due to the high peak power, but only contributes in total a negligible X state depletion due to the low pulse energy. Single-shot CO fs-TPLIF images in methane/air flames were recorded by imaging the broadband fluorescence. The results indicate that fs-TPLIF is a promising tool for CO imaging in flames.