Measurements of absolute concentration profiles of C 2 in non-sooting and sooting diffusion flames by coupling cavity ring-down spectroscopy and laser induced fluorescence

Abstract

Coupling between cavity ring down spectroscopy (CRDS) and laser induced fluorescence (LIF) methods has been implemented to measure nascent C 2 (a 3Π u) in both non-sooting and sooting flames stabilised on a Wolfhard-Parker burner at atmospheric pressure. In the non-sooting flame, relative concentration profiles have been recorded by LIF measurements and calibrated by CRDS in absolute concentration (maximum of 3.4 × 10 −9 in mole fraction). In the sooting flame, it is shown that LIF is unusable to measure nascent C 2 in the sooting regions due to the creation of additional C 2 from laser soot vaporisation, even by using extremely low laser energies. It is besides demonstrated that, although CRDS measurements were also affected by PAH/soot interference, relative profile determination and absolute quantification could be satisfactorily obtained. Complete C 2 profiles in the sooting flame are thus obtained by combining LIF and CRDS. Hence, LIF can be advantageously used to measure relative profiles of C 2 in the vicinity of the burner (non-sooting zone) and CRDS can then provide determination of C 2 profiles in the presence of soot. Absolute concentration has been determined by CRDS, providing a peak mole fraction of 6.8 × 10 −9. Based on a comparison with PAH and soot profiles issued from a previous work, the role of C 2 in soot formation mechanism is discussed. A comparison with earlier profiles of CH is also proposed, showing a good correlation between C 2 and CH radical profiles.