Diode laser cavity-based techniques for quantification of trace species in laminar sooting flames

Student thesis: Doctoral Thesis

Abstract

Cavity-enhanced absorption spectroscopy (CEAS) has been applied for the first time to in situ measurements of acetylene in sooting flames using a near-infrared diode laser. This is motivated by the role of acetylene as a major precursor in soot formation and the need for accurate measurements of acetylene to understand soot formation (with the eventual goal of reduced emissions). Vertical profiles of acetylene have been investigated in two flat flame burners with some consideration of the effect of radial profiles on the results. This thesis builds on the novel application of continuous-wave cavity ring-down spectroscopy (cw-CRDS) to in situ flame measurements of acetylene: in this work, refinements are made to the technique, which is developed to the point of generating reliable data in standard flames of interest in studies of soot formation. A key advantage of the cw-CRDS approach is that it is an absolute method, not requiring calibration. Nevertheless, the slow data acquisition led to the consideration of alternative approaches. This resulted in research on CEAS, which forms the bulk of this thesis. The advantages of the CEAS technique over cw-CRDS are highlighted, whilst noting the agreement between the two techniques. The CEAS approach allows spectra to be acquired much more rapidly and with better spectral resolution, as well as having a somewhat simpler experimental set-up. This has enabled the acquisition of a large dataset of broad scans over the full scanning range of the diode lasers employed. This thesis also goes on to explore the application of the CEAS technique to flame measurements of OH radical. The recovered profiles of acetylene concentration show good agreement for both techniques and follow the expected trend of higher acetylene concentration with increasing equivalence ratio. They also show a trend of decreasing acetylene concentration with increasing height above the burner surface. This would be consistent with the consumption of acetylene, including in reactions forming polycyclic aromatic hydrocarbons and ultimately soot. However, this is contrary to model predictions also shown in the thesis.
Date of Award1 Jun 2021
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorIain Burns (Supervisor) & Michael Lengden (Supervisor)

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