Fiber optic diagnostics for combustion studies

This study reports the first gas phase Laser-Induced Breakdown Spectroscopy (LIBS) and Laser-Induced Incandescence (LII) measurements using a fiber optic laser pulse delivery. A silver and polymer coated hollow fiber was used to delivered high-energy laser pulses, which were either focused to generate high-energy-density plasmas in ultra-lean methane-air mixtures for LIBS measurements, or delivered inside an ethylene-air sooting flame to heat soot particles for LII measurements. The results were compared with conventional open optics LIBS and LII.;The first part of the LIBS investigation was performed with a conventional open optics setup except the insertion of the hollow fiber in the optical path. The beam delivery through the hollow fiber changed the original Gaussian laser beam profile to a collection of hot spots and increased beam divergence. LIBS analysis showed a linear variation of H/N elemental ratio with equivalence ratio. Then a movable probe that integrated both the beam delivery and collection optics was developed, and was used to examine the effect of delivery fiber curvature on LIBS results. While the curvature was found to alter both the intensity profile and energy of the exiting beam, the LIBS signal linearity and variability were found to be consistent provided the output energy and beam spot diameter on the focusing lens were held constant for each radius. This suggested the influence of curvature was exerted primarily through its affect on output energy. A data selection criteria using Halpha and Hbeta hydrogen emission line ratio was evaluated and found to decrease the LIBS signal standard deviation by an average factor of 3.25, and improved linearity by 10%.;Fiber-delivered and conventional open-optics LII signals were compared using similar laser fluence and detection strategies. The signal magnitudes from the fiber delivered laser pulses were two to 3.5 times greater at the two burner heights examined and decayed slower temporally, but the normalized radial profiles were nearly identical. The intensity differences are thought to be caused by collection optics alignment inconsistencies and the temporal decay differences are thought to be due to beam profile dissimilarities between the two laser delivery methods.