A new approach to planar laser-induced fluorescence of the CH radical in combustion systems was developed which provided a one to two order of magnitude increase in signal strengths over previous techniques. Both excitation and observation were applied in the diagonal vibrational band systems of the CH molecule (DX/DC), maximizing the potential fluorescence signal strength of this diagonal molecule. A computer model of the CH fluorescence process was developed as a design tool to determine the optimum excitation wavelength for Diesel engine conditions. CH PLIF was demonstrated in a premixed oxyacetylene welding torch, a premixed propane-air Bunsen burner flame, and a dimethyl-ether high speed jet diffusion flame. Excitation was performed at 425.015 nm in the R-branch of the (0,0) vibrational band, and a holographic notch filter (HNF) was employed to remove elastic scatter of laser light while transmitting fluorescence emission in the Q- and P-branches from 428 nm to 440 nm. Strong spectral interference within the CH emission band was observed, and a detailed investigation of the interference in the oxyacetylene flame showed that fluorescence of polycyclic aromatic hydrocarbons (PAH’s) was the most probable source. The interference was spatially overlapped with the CH fluorescence emission in premixed flames, while it was spatially separated from the CH fluorescence emission the DME diffusion flame. A computer analysis tool was developed to quantify flame curvature, flame thickness, and flame surface density. A cascade of the flame curvature distribution was observed as the flame was probed at greater heights above the burner. The flame reaction zone, as indicated by the CH fluorescence, was thin (200 ?m?500 ?m) for all conditions tested. The wrinkling observed in the inner cone was greatest at mid-height. Due to the increased line broadening and collisional quenching, the signal strength of the technique will be borderline when applied to high pressure Diesel combustion. The strong interference observed in the core of the DME diffusion flame suggests that it will be difficult to identify flame structure in the core of a Diesel fuel spray. However, combustion near the periphery of the plumes should be observable.
Musculus, M. P. Development of a Diagonal excitation/Observation Diagnostic Technique for Planar Laser-Induced Fluorescence of the CH Radical in Diesel Engines. University of Wisconsin-Madison, 1998.