Thermalization dynamics in a pulsed microwave plasma-enhanced laminar flame

Energy transfer in a pulsed-microwave enhanced flame is investigated using hybrid fs/ps coherent antiStokes Raman scattering (CARS) to monitor both vibrational and rotational temperatures of nitrogen in an atmospheric pressure laminar premixed natural gas/air stagnation flame. Temperatures were measured throughout the laminar flame structure following a 30-kW peak power, 2 mu s duration, 3 GHz microwave pulse in a resonant waveguide cavity. CARS measurements show a delayed increase in vibrational temperature, indicating energy loading via electron impact and subsequent energy cascade. Vibrational energy thermalization was observed over timescales faster than transport through the flame zone, but slower than predicted by known vibrational-translational rates, suggesting a long-lived pathway for increased vibrational temperature. Peak vibrational temperature increases of 100 K were observed and thermalize over 100' s of microseconds, resulting in a measurable increase in the rotational temperature over the same time interval. The magnitude of vibrational excitation and rate of thermalization in such plasmaassisted combustion environments is critical for applications including combustion ignition and control, and hybrid fs/ps CARS measurements provide the necessary detail on vibrational-translational relaxation processes of ground state nitrogen. (c) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

The energy transfer process in a pulsed-microwave enhanced flame was investigated by monitoring the vibrational and rotational temperatures of nitrogen. Results showed a delayed increase in vibrational temperature, indicating a long-lived pathway for energy loading. The measurements provide detailed information on vibrational-translational relaxation processes of ground state nitrogen in plasma-assisted combustion environments.