Wei Zhang , Hongwei Zang , Shuo Wang , Junyan Chen , Helong Li , Huailiang Xu , Ruxin Li
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引用次数: 0
Abstract
Laser ignition (LI) is promising for green combustion of lean-fuel mixtures with controllable ignition timing and location. It was recently discovered that despite the inferior energy deposition and low thermal temperature in femtosecond (fs) laser-induced plasma, fs laser pulses can achieve a robust ignition of lean-fuel mixture through forming a “line” kernel by filamentation. Here, to clarify fs-LI mechanism, we investigated a dual-color (DC: 800 nm at 1.5 mJ and 400 nm at 0.43 mJ, ∼50 fs) fs-LI of a lean methane/air mixture with an equivalence ratio of φ = 0.87. An optical emission spectroscopy study was conducted to probe the N2+ and OH emissions and characterize the ignition success rate. It was demonstrated that fs-LI can be achieved at a lower minimum ignition energy (MIE) (<0.46 mJ) by the DC scheme than that (>0.7 mJ) by a single-color (SC: 800 nm at 2.0 and 2.4 mJ, ∼50 fs) scheme, indicating a strong wavelength effect on the successful ignition. A pump-probe measurement was carried out to reveal the effect of the ionization enhancement on the successful ignition. It was found that only when the two-color fs pulses are temporally overlapped, the OH yield is strongly enhanced and the MIE is decreased. By comparing the variation trend of the fluorescence intensity of OH with that of the direct ionization product N2+, we ascribed fs-LI to a non-resonant photochemical ignition mechanism, in which the enhancement in the multiphoton/tunnel ionization of the lean-fuel mixture by the high-energy 400-nm photon can increase the yields of the reactive radicals through various dissociation and chain reaction pathways, and thus result in the successful ignition at the micro-joule level. This work unravels the essential role of the non-resonant photochemical ignition mechanism in fs-LI, and provides a promising route for the ignition of lean-fuel engines by compact ultrashort-pulsed lasers in the filamentation regime.
期刊介绍:
The mission of the journal is to publish high quality work from experimental, theoretical, and computational investigations on the fundamentals of combustion phenomena and closely allied matters. While submissions in all pertinent areas are welcomed, past and recent focus of the journal has been on:
Development and validation of reaction kinetics, reduction of reaction mechanisms and modeling of combustion systems, including:
Conventional, alternative and surrogate fuels;
Pollutants;
Particulate and aerosol formation and abatement;
Heterogeneous processes.
Experimental, theoretical, and computational studies of laminar and turbulent combustion phenomena, including:
Premixed and non-premixed flames;
Ignition and extinction phenomena;
Flame propagation;
Flame structure;
Instabilities and swirl;
Flame spread;
Multi-phase reactants.
Advances in diagnostic and computational methods in combustion, including:
Measurement and simulation of scalar and vector properties;
Novel techniques;
State-of-the art applications.
Fundamental investigations of combustion technologies and systems, including:
Internal combustion engines;
Gas turbines;
Small- and large-scale stationary combustion and power generation;
Catalytic combustion;
Combustion synthesis;
Combustion under extreme conditions;
New concepts.