QUANTITATIVE FLUORESCENCE MEASUREMENTS OF ATOMIC HYDROGEN IN FLAMES VIA TWO-PHOTON ABSORPTION (LASER COMBUSTION, DIAGNOSTICS)

JOSEPH THADDEUS SALMON, Purdue University

Abstract

An experimental protocol, using photoionization controlled-loss spectroscopy (PICLS), has been developed for obtaining absolute number densities of atomic hydrogen in flames. PICLS uses two-photon absorption to excite hydrogen atoms from the ground state to the second excited state and then strongly photoionizes the excited atoms by introducing a second beam into the probe volume. Strong photoionization makes the fluorescence measurements independent of quenching, thus overcoming the inherent limitation of conventional fluorescence techniques. PICLS inherently requires that the photoionization rate be approximately ten times the quenching rate, which effectively attenuates the fluorescence signal by a factor of ten. Addressing this problem, a model has been developed that reduces this requirement and extends the range of viability of PICLS to the point where the photoionization rate is equal to the quenching rate. At this point the fluorescence is attenuated by only one-half. Thus, with the model PICLS can either extend the range of flame conditions where PICLS is viable or lower the detection limit of the measurement. PICLS was verified in fuel-lean, stoichiometric, and fuel-rich flat premixed H(,2)/O(,2)/N(,2) flames at pressures of 20 Torr and 72 Torr. After calibrating in the post-flame zone of one flame by partial-equilibrium calculations, the ratios of number densities obtained from PICLS to those calculated from partial-equilibrium in the other flames are constant to within 20%. Most of the error comes from the sensitivity of the partial-equilibrium calculations to small uncertainties in both the flow rates of the fuel and oxidizer and the measured OH concentration, which are inputs to the calculations. The model for PICLS also yields the relative change in quenching as a function of position within each flame. Absolute number densities of atomic hydrogen were obtained from two-photon fluorescence measurements in 72-Torr C(,2)/H(,4)/O(,2)/Ar flames under stoichiometric and fuel-rich conditions. Although evidence of photochemical effects was observed, no adverse effect on H-atom measurements was detected.

Degree

Ph.D.

Subject Area

Mechanical engineering

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