Planar laser-induced fluorescence of HCO for instantaneous flame front imaging in hydrocarbon flames

Abstract

The detection of the formyl radical (HCO) is of great interest in the field of combustion research since it can provide information about the local heat release (HR) rate which is a key parameter in the understanding of combustion processes. Unfortunately, due to the low signal level, HCO planar laser-induced fluorescence (PLIF) has so far not been applicable for single-shot imaging, which is necessary in particular for studying turbulent flames. In the present paper for the first time, to the best of our knowledge, single-shot PLIF imaging of HCO in flames is demonstrated by employing a multimode frequency-tripled alexandrite laser. On the basis of long pulse duration along with a spectrally broad bandwidth enabling multi-line excitation a sufficient signal-to-noise ratio is achieved. In detail, excitation in the B–X system around 258.69nm is applied and subsequent fluorescence detection in the spectral range 300–400nm is performed. A series of experiments concerning spectral interferences, saturation behavior and the influence of flame stoichiometry (in the range ϕ=0.6–2) have been conducted. Two typical fuels have been employed: methane (CH4) as conventional hydrocarbon as well as dimethyl ether (DME) as modern bio fuel. Single-shot HCO imaging is finally demonstrated in a laminar DME/air Bunsen flame and in a slightly turbulent methane/oxygen welding torch flame. The results indicate that the developed HCO PLIF technique offers a strong potential for improved flame studies particularly in turbulent flames. In general, the approach to employ laser sources with reasonably broader bandwidth and longer pulse duration compared to conventional Nd:YAG pumped systems for PLIF will open up new possibilities in the field of combustion diagnostics.