Diode-laser tomography for arcjet plume reconstruction

Experimental demonstration of 4D imaging in two-phase flows based on computed tomography at 5 kHz

Turbulent flows inherently feature 3D spatial structures and temporal dynamics. As a result, 4D experimental techniques have been long desired to fully resolve turbulent flows in all three spatial directions and time. This work describes the demonstration of such 4D measurements in two-phase flows using computed tomography and Mie scattering. Demonstration measurements were performed in airflows seeded with water droplets at a 5 kHz frame rate, with an exposure time of 0.2 ms and a measurement volume of 85  mm×85  mm×85  mm discretized into 128×128×128 voxels. Experimental and computational studies have been conducted, with a focus on comparison and validation of the 3D reconstructions against experiments performed in flows with recognizable patterns.

Validation of temperature imaging by H2O absorption spectroscopy using hyperspectral tomography in controlled experiments

This paper describes a preliminary demonstration and validation of temperature imaging using hyperspectral H2O absorption tomography in controlled experiments. Fifteen wavelengths are monitored on each of 30 laser beams to reconstruct the temperature image in a 381 mm × 381 mm square room-temperature plane that contains a 102 mm × 102 mm square zone of lower or higher temperature. The hyperspectral tomography technique attempts to leverage multispectral information to enhance measurement fidelity. The experimental temperature images exhibit average accuracies of 2.3% or better, with pixel-by-pixel standard deviations of less than 1%. In addition, even when the internal zone is only 4 K cooler than the surroundings, its presence is still detectable; statistical analysis of the associated experimental image reveals a 98% confidence that the internal zone is in fact cooler than the surroundings.

Distributed feedback diode laser spectrometer at 2.7 microm for sensitive, spatially resolved H2O vapor detection

A new, compact, spatially scanning, open-path 2.7 microm tunable diode laser absorption spectrometer with short absorption path lengths below 10 cm was developed to analyze the spatiotemporal dynamics of one-dimensional (1D) spatial water vapor gradients. This spectrometer, which is based on a room-temperature distributed feedback diode laser, is capable of measuring absolute, calibration-free, line-of-sight averaged, but laterally resolved 1D H(2)O concentration profiles with a minimum fractional optical resolution of 2.1x10(-3) optical density (OD) (2.5x10(-4) OD after a background subtraction procedure), which permits a signal-to-noise-ratio of 407 (3400) at 10,000 parts in 10(6) (ppm)H(2)O, or normalized sensitivities of 2.6 ppm x m (0.32 ppm m) at 0.5 Hz duty cycle. The spectrometer's lateral spatial resolution (governed by the 500 microm sampling beam diameter) was validated by analyzing a well-defined laminar jet of nitrogen gas in humidified air. This scanning setup was then used to (a) quantitatively investigate for what we believe to be the first time the H(2)O boundary layer from 0.7 to 11 mm beneath the stomatous side of a single, undetached plant leaf, and (b) to study the temporal boundary layer dynamics and its dependence on stepwise light stimulation of the photosynthetic system. In addition the 2.7 microm diode laser was carefully characterized in terms of spectral purity, beam profile, as well as quasi-static and dynamic wavelength tuning coefficients.

Numerical investigation of hyperspectral tomography for simultaneous temperature and concentration imaging

We investigate the simultaneous tomographic reconstruction of temperature and species concentration using hyperspectral absorption spectroscopy. Previous work on absorption tomography has relied on a small number of wavelengths, resulting in the requirement of a large number of projections and limited measurement capability. Here we develop a tomographic inversion method to exploit the increased spectral information content enabled by recent advancement in laser technologies. The simulation results clearly demonstrate that the use of hyperspectral absorption data significantly reduces the number of projections, enables simultaneous mapping of temperature and species concentration, and provides more stable reconstruction compared with traditional absorption tomographic techniques.

Frequency-resolved absorption tomography with tunable diode lasers

A tunable diode laser was used for absorption tomography in an axisymmetric atmospheric pressure flat-flame burner. A rapid tomographic inversion algorithm was used to facilitate the many reconstructions at a relatively sparse set of projections typical of laser absorption tomography. Profiles of temperature and CO2 mole fraction were measured simultaneously in methane-air flames. Absorption measurements were made near the R-branch bandhead at 4.17 microm to minimize interferences with other species, while providing good temperature and concentration sensitivity at flame conditions. The procedure showed the advantage of reconstructing detailed spectra at each radial node.

Rapid wavelength scans over one octave and application to laser-induced fluorescence

Rapid excitation scans of laser-induced fluorescence (LIF) have been demonstrated. Broadband light was generated in a photonic crystal fiber and transmitted through a long fiber. Due to group-velocity dispersion in the long fiber, a wavelength scan emerged from the fiber in time. The wavelength was swept over approximately one octave in approximately 150 ns. The generated light was used to excite LD 700 Perchlorate diluted in methanol. The LIF excitation scan had a spectral resolution of approximately 15 nm, and the integrated fluorescence spectrum was found to be within 7% of the integrated absorption spectrum of the dye molecule. The method presented makes possible spatially and spectrally resolved LIF excitation scans with scanning speeds up to the limits set by the excited-state lifetime of the dye molecule.