Exploitable results
A method has been developed for imaging parent fuel fraction in nonpremixed gas fuelled flames, based on fluorescence from acetaldehyde. The method requires a high level of seeding in turbulent flames to overcome competing signals from polycyclic aromatic hydrocarbon (PAH) fluorescence and laser induced incandescence (LII). Photolytically generated carbon(2) following multiphoton absorption by fuel molecules was found to be a problem for short wavelength excitation around 248 nm but it was not significant for excitation at 308 nm.
The parent fuel fraction measurements were all performed with a xenon chloride Excimer laser operating at a wavelength of 308 nm and with laser fluences in the laser sheet of circa 10 to 30 MW cm{-2}. Images of parent fuel fraction have been successfully measured in both a laminar Wolfhard-Parker flame and a simple turbulent jet flame; in the latter case both average and single pulse images were recorded.
It is shown that the seed species, acetaldehyde, behaves in a chemically similar fashion to methane, from an analysis of a full kinetic calculation of a 1-dimensional counter flow nonpremixed flame. For undiluted nonpremixed hydrocarbon combustion, the method is best suited to the study of pure aldehyde flames because of the requirement for high seeding levels of aldehyde to dominate competing signals from LII and PAH fluorescence.
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