Femtosecond Two-photon Laser-induced Fluorescence for Temperature and Chemical Species Imaging in Flames
Abstract
In recent years, non-intrusive, laser-based combustion diagnostic techniques utilizing ultrashort-pulse, femtosecond (fs) duration laser sources have evolved as promising tools for time-resolved imaging of fast physical processes and complex chemical reactions. The high peak power in fs-duration pulses is ideal for nonlinear spectroscopic techniques involving atoms and molecules. Additionally, the broad spectral bandwidth and high repetition rate of fs sources are advantageous over conventional nanosecond (ns) laser methods in detecting important intermediate chemical species during combustion. However, fundamental laser-matter interactions and signal generation processes during fs laser diagnostics at high-pressure practical combustion conditions are still largely unknown. Hence, this research aims to develop the fs laser-induced fluorescence (LIF) technique for practical gas turbine operating conditions, in particular high pressures and temperatures, utilizing a home-built, high-efficient frequency conversion system based on harmonic generation. This thesis consists of five research tasks highlighting the importance of intermediate species detection using optical-based combustion diagnostics and providing a basis for an environmentally sustainable and prosperous energy future.
In the first task, high-pressure combustion diagnostics is demonstrated by imaging highly reactive intermediate species, atomic hydrogen (H), in methane-air hydrocarbon flames. A comprehensive fs two-photon LIF (TPLIF) imaging study of H-atom was performed at elevated pressures (1–10 bar) for a range of flame conditions.
In the second part of this research, the same fs-TPLIF scheme was extended for flame thermometry up to 10 bar pressure using Krypton (Kr) as an inert gas tracer. Subsequently, both H & Kr were excited simultaneously at an intermediary wavelength of 204.6 nm using the broadband, fs pulses having linewidths in excess of 400 cm^-1.
In the next part, the progress on kHz-rate imaging of methylidyne (CH) radical in flames using the fs planar LIF (PLIF) technique was studied for the first time. In the last task, a detailed spectroscopic and imaging study was extended to a molecular species (i.e., molecular nitrogen, N2) to investigate the complex spectroscopy in N2 excitation.
Overall, the above developments and the associated reacting flow diagnostics are significant steps forward in utilizing the ultrashort, broadband, high-repetition-rate, fs-duration laser pulses for practically relevant combustion diagnostics.
Subject
Femtosecond LaserLaser-induced Fluorescence
Temperature Measurements
Hydrogen Atom
Krypton Atom
CH Radical
Combustion Diagnostics
High-pressure Flames
Citation
Parajuli, Pradeep (2023). Femtosecond Two-photon Laser-induced Fluorescence for Temperature and Chemical Species Imaging in Flames. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /198857.