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This paper describes the design and experimental application of an optical system to perform schlieren measurements in the curved geometry of the cylinder of an optically accessible internal combustion engine. Key features of the system are a pair of cylindrical positive meniscus lenses, which keep the beam collimated while passing through the unmodified, thick-walled optical cylinder, and a pulsed, high-power light-emitting diode with narrow spectral width. In combination with a high-speed CMOS camera, the system is used to visualize the fuel jet after injection of hydrogen fuel directly into the cylinder from a high-pressure injector. Residual aberrations, which limit the system’s sensitivity, are characterized experimentally and are compared to the predictions of ray-tracing software.

The effects of collisional quenching on photofragment emission (PFE) detection of vapor-phase HgCl_2 in combustion flue gas constituents are investigated. Exciting HgCl_2 via the 1Π_u1<--1Σ_g+1 transition, time-resolved measurements of emission from the Hg(6^3P_1) daughter in buffer-gas mixtures of N_2, O_2, and CO_2 indicate that the fragmentation pathway passes through a long-lived intermediate species, which we assign to Hg(6^3P_2). Total quenching rate coefficients of Hg(6^3P_1) by N_2, O_2, and CO_2 are consistent with values reported in the literature. In addition, total quenching rate coefficients for the intermediate Hg(6^3P_2) state are determined to be 1.72(±0.08)×10^−10 cm^3 molecule^−1 s^−1 and 2.90(±0.37)×10^−10 cm^3 molecule^−1 s^−1 for N_2 and O_2, respectively. An analysis of the impact of the collisionally dependent energy-transfer process that precedes the formation of Hg(6^3P_1) on the use of PFE to measure HgCl_2 concentration is presented.

A real-time, noninvasive approach for detecting trace amounts of vapor-phase mercuric chloride (HgCl_2) in combustion flue gas is demonstrated using a near-infrared pulsed fiber amplifier that is frequency converted to the ultraviolet. Excitation of the HgCl_2(1∏1_u ? 1∑1g+) transition at 213 nm generates 253.7 nm emission from the Hg (6^3P_1) photoproduct that is proportional to the concentration of HgCl_2. A measured quadratic dependence of the HgCl_2 photofragment emission (PFE) signal on the laser irradiance indicates that the photodissociation process involves two-photon excitation. Additionally, low concentrations of HgCl_2 are detected with the PFE approach in an environment characteristic of coal-fired power-plant flue gas using this compact solid-state laser source. A detection limit of 0.7 ppb is extrapolated from these results.

The viability of pulsed laser photofragment emission (PFE) is evaluated for the in situ measurement of vapor-phase mercuric chloride (HgCl_2) concentration in combustion flue gas. Dispersed emissions from both the Hg (6^3P_1) and HgCl (B^2Σ^+) photoproducts are presented, and the dependence of the HgCl2 PFE signal originating from Hg (6^3P_1) on the collisional environment is examined for buffer-gas mixtures of N_2, O_2, and CO_2. Integrated PFE intensity measurements as a function of buffer gas pressure support the assumption that the primary effect of the relevant flue gas constituents is to quench emission from Hg (6^3P_1). The quenching rate constants for PFE from HgCl_2 were measured to be 1.37 (±0.16)×10^5 Torr^-1 s^-1 for N_2, 9.35 (±0.25)×10^6 Torr^-1 s^-1 for O_2, and 1.49 (±0.29)×10^6 Torr^-1 s^-1 for CO_2. These values are in good accord with literature values for the quenching of Hg (6^3P_1). The emission cross section for Hg (6^3P_1) generated by photodissociation of HgCl_2 in 760 Torr N_2 is found to be 1.0 (±0.2)×10^-25 m^2 by comparing the PFE signal to N_2 Raman scattering.