Development and Application of Single-Ended Picosecond Laser Diagnostics

Sammanfattning: Light detection and ranging (lidar), a method for nonintrusive, single-ended, and range resolved measurements, has been developed and applied for combustion related diagnostics. Employing picosecond laser pulse durations and detectors providing temporal resolution on the order of picoseconds, lidar has been applied successfully for both quantitative temperature- and species concentration measurements, obtaining an ultimate range resolution of 0.46 cm. The use of a streak camera as detector allows two-dimensional imaging in a radar-like fashion, or the possibility to extract time resolved spectral information. The experimental setup, permitting lidar measurements in the near field (<10 m), has been developed and characterized, yielding detailed knowledge of picosecond lidar (ps-lidar) and its feasibility in combustion studies. Rayleigh scattering thermometry has been conducted in a tube furnace, various flames, and a large scale room fire experiment. The accuracy, under ambient conditions, was within 5% and the measurements were less prone to scattering off dust in the air than traditional perpendicular detection schemes. In premixed ethylene/air flames, the lidar temperatures agree very well with reference data from coherent anti-Stokes Raman Spectroscopy (CARS) measurements up to an equivalence ratio of 1.3. A filtering routine has been developed to be used for applications in which interfering scattering from particles is expected. The filtering routine was applied in the room fire measurements, which resulted in temperature images of two dimensional planes. The temperatures achieved in experiments with a methanol pool fire located in the room agree well with thermocouple measurements and computational fluid dynamics (CFD) simulations, while the signals recorded with a methane gas diffusion flame in the room are dominated by particle scattering, preventing Rayleigh thermometry, but allowing qualitative particle mapping instead. Tunable optical parametric picosecond lasers were used for differential absorption lidar (DIAL), yielding species concentrations of acetone, ammonia, hydroxyl radical (OH), and potassium chloride (KCl). With a range resolution of 15 cm, a fractional absorption of ~5%, for example corresponding to an ammonia concentration of 40 ppm at ambient condition employing the wavelengths 212.2 and 214.5 nm, sets the detection limit. Processes based on non-elastic light-matter interaction have been investigated for use with ps-lidar. Raman scattering from liquid samples was used for demonstration of range resolved species determination of nitromethane and hydrogen peroxide. Laser-induced fluorescence (LIF) was used in a demonstration of hydroxyl radical (OH) detection and mapping in a slot burner flame. A model for evaluation of laser-induced incandescence (LII) signals acquired with ps-lidar has been developed and demonstrated. Measurements of spatial soot volume fraction distributions yielded results that are in excellent agreement with reference measurements carried out with regular right-angle LII. The following major conclusions can be drawn: ps-lidar is suitable for diagnostics in large scale combustion related devices, particularly devices limited to only one optical access, and, hence, it permits measurements that are currently not possible to realize in any other way, making it a very valuable asset for today’s as well as future challenges within energy science.