Correction procedure for a tomographic optical setup employing imaging fiber bundles and intensified cameras.

For reliable tomographic measurements the underlying 2D images from different viewing angles must be matched in terms of signal detection characteristics. Non-linearity effects introduced by intensified cameras and spatial intensity variations induced from inhomogeneous transmission of the optical setup can lead, if not corrected, to a biased tomographic reconstruction result. This paper presents a complete correction procedure consisting of a combination of a non-linearity and flatfield correction for a tomographic optical setup employing imaging fiber bundles and four intensified cameras. Influencing parameters on the camera non-linearity are investigated and discussed. The correction procedure is applied to 3D temperature measurements by two-color pyrometry and compared to results without correction. The present paper may serve as a guideline for an appropriate correction procedure for any type of measurement involving optical tomography and intensified cameras.

Calibration-free imaging thermometry via two-line atomic fluorescence in combustion processes.

In this Letter, we demonstrate, for the first time (to the best of our knowledge), calibration-free imaging of full-frame temperature fields for particle-laden flames using two-line atomic fluorescence (TLAF) of indium. Measurements were carried out in laminar premixed flames with indium precursor aerosol added. The technique is based on the excitation of the 52P3/2 → 62S1/2 and 52P1/2 → 62S1/2 transitions of indium atoms and the detection of the subsequent fluorescence signals. To this end, the transitions were excited by scanning two narrowband external cavity diode lasers (ECDL) over the transition bandwidths. To achieve imaging thermometry, the excitation lasers were formed into a light sheet of 1.5 mm width and 24 mm height. Employing this setup on a laminar, premixed flat-flame burner, temperature distributions were measured for various air:fuel ratios of 0.7, 0.8, and 0.9. The presented results demonstrate the capability of the technique and encourage further developments, e.g., for its future use in flame synthesis of nanoparticles containing indium compounds.

In Situ Determination of Droplet and Nanoparticle Size Distributions in Spray Flame Synthesis by Wide-Angle Light Scattering (WALS).

The investigation of droplet and nanoparticle formation in spray flame synthesis requires sophisticated measurement techniques, as often both are present simultaneously. Here, wide-angle light scattering (WALS) was applied to determine droplet and nanoparticle size distributions in spray flames from a standardized liquid-fed burner setup. Solvents of pure ethanol and a mixture of ethanol and titanium isopropoxide, incepting nanoparticle synthesis, were investigated. A novel method for the evaluation of scattering data from droplets between 2 µm and 50 µm was successfully implemented. Applying this, we could reveal the development of a bimodal droplet size distribution for the solvent/precursor system, probably induced by droplet micro-explosions. To determine nanoparticle size distributions, an appropriate filter and the averaging of single-shot data were applied to ensure scattering from a significant amount of nanoparticles homogeneously distributed in the measurement volume. From the multivariate analysis of the scattering data, the presence of spherical particles and fractal aggregates was derived, which was confirmed by analysis of transmission electron microscopy images. Monte Carlo simulations allowed determining the distribution parameters for both morphological fractions in three heights above the burner. The results showed relatively wide size distributions, especially for the spherical fraction, and indicated an ongoing sintering, from fractal to spherical particles.

4D temperature measurements using tomographic two-color pyrometry.

This work presents a new approach for high-speed four-dimensional (3D + t) thermometry using only two high-speed cameras which are equipped with different band pass filters to capture thermal radiation signals at two narrow wavelength bands. With the help of a customized fiber bundle and a beam splitter, a total number of nine projections at each band were recorded, and the temperature distribution was evaluated by tomographic two-color pyrometry. In order to validate the effectiveness of this method, the 3D temperature distribution of a premixed steady flat flame was evaluated. The determined temperatures were compared to those of other studies, as well as to the results from inverse Abel transform and line-of-sight data. Further, the 3D temperature evolution of a weakly turbulent diffusion flame was observed at a repetition rate of 7.5 kHz. Such 4D temperature measurements are expected to be valuable in understanding turbulent combustion mechanisms especially of practical devices.

Burst-mode OH/CH2O planar laser-induced fluorescence imaging of the heat release zone in an unsteady flame.

The paper presents simultaneous high-speed (7.5 kHz) planar laser-induced fluorescence (PLIF) of formaldehyde (CH2O) and the hydroxyl-radical (OH) for visualization of the flame structure and heat release zone in a non-premixed unsteady CH4/O2/N2 flame. For this purpose, a dye laser designed for high-speed operation is pumped by the second-harmonic 532 nm output of a Nd:YAG burst-mode laser to produce a tunable, 566 nm beam. After frequency doubling a high-energy kHz-rate narrowband pulse train of approximately 2.2 mJ/pulse at 283 nm is used for excitation of the OH radical. Simultaneously, CH2O is excited by the frequency-tripled output of the same Nd:YAG laser, providing a high-frequency pulse train over 10 ms in duration at high pulse energies (>100 mJ/pulse). The excitation energies enable signal-to-noise ratios (SNRs) of ~10 and ~60 for CH2O and OH PLIF, respectively, using a single high-speed intensified CMOS camera equipped with an image doubler. This allows sufficient SNR for investigation of the temporal evolution of the primary heat release zone and the local flame structure at kHz rates from the spatial overlap of the OH- and CH2O-PLIF signals.

Analysis of LIF and Mie signals from single micrometric droplets for instantaneous droplet sizing in sprays.

Planar droplet sizing (PDS) is a technique relying on the assumption that laser-induced fluorescence (LIF) and Mie scattering optical signals from spherical droplets depend on their volume and surface area, respectively. In this article, we verify the validity of this assumption by experimentally analyzing the light intensity of the LIF and Mie optical signals from micrometric droplets as a function of their diameter. The size of the droplets is controlled using a new flow-focusing monodisperse droplet generator capable of producing droplets of the desired size in the range of 21 µm to 60 µm. Ethanol droplets doped with eosin dye and excited at 532 nm are considered in this study, and the individual droplets were imaged simultaneously at microscopic and macroscopic scale. The effects of laser power, dye concentration, and temperature variation are systematically studied as a function of LIF/Mie ratio in the whole range of droplet sizes. Finally, a calibration curve at tracer concentration of 0.5 vol% is deduced and used to extract the droplet Sauter mean diameter (SMD) from instantaneous images of a transient ethanol spray. This droplet size mapping is done using structured laser illumination planar imaging (SLIPI), in order to suppress the artifacts induced by multiple light scattering.

A mobile system for a comprehensive online-characterization of nanoparticle aggregates based on wide-angle light scattering and laser-induced incandescence.

A mobile demonstrator for the comprehensive online-characterization of gas-borne nanoparticle aggregates is presented. Two optical measurement techniques are combined, both utilizing a pulsed Nd:YAG laser as light source. Aggregate size and fractal dimension are measured by Wide-Angle Light Scattering (WALS). An ellipsoidal mirror images elastically scattered light from scattering angles between 10° and 165° onto a CCD-camera chip resulting in an almost complete scattering diagram with high angular resolution. Primary particle size and volume fraction are measured by time-resolved Laser-Induced Incandescence (TiRe-LII). Here, particles are heated up to about 3000 K by the short laser pulse, the enhanced thermal radiation signal is detected with gated photomultiplier tubes. Analysis of the signal decay time and maximum LII-signal allows for the determination of primary particle diameter and volume fraction. The performance of the system is demonstrated by combined measurements on soot nanoparticle aggregates from a soot aerosol generator. Particle and aggregate sizes are varied by using different equivalence ratios of the combustion in the generator. Soot volume fraction can be adjusted by different levels of dilution with air. Online-measurements were carried out demonstrating the favorable performance of the system and the potential for industrial applications such as process control and product development. The particle properties obtained are confirmed through transmission electron microscopy analysis on representative samples.

Supercontinuum high-speed cavity-enhanced absorption spectroscopy for sensitive multispecies detection.

Cavity-enhanced absorption spectroscopy is promising for many applications requiring a very high concentration sensitivity but often accompanied by low temporal resolution. In this Letter, we demonstrate a broadband cavity-enhanced absorption spectrometer capable of detection rates of up to 50 kHz, based on a spatially coherent supercontinuum (SC) light source and an in-house-built, high-speed near-infrared spectrograph. The SC spectrometer allows for the simultaneous quantitative detection of CO2, C2H2, and H2O within a spectral range from 1420 to 1570 nm. Using cavity mirrors with a specified reflectivity of R=98.0±0.3% a minimal spectrally averaged absorption coefficient of αmin=1·10-5 cm-1 can be detected at a repetition rate of 50 kHz.