Laser-based Diagnostics Research Articles

Application of Rainbow Thermometry to the Study of Fuel Droplet Heat-Up and Evaporation Characteristics

Advanced, nonintrusive, laser-based diagnostics are being developed for simultaneously measuring the size, velocity, temperature, and instantaneous regression rates of vaporizing/burning fuel droplets in polydisperse flow environments. The size and velocity of the droplets are measured using a conventional phase Doppler particle analyzer (PDPA), and the droplet temperatures are simultaneously measured with a rainbow thermometer. This integrated diagnostic has been applied to the study of fuel droplet heat-up characteristics in a swirl-stabilized kerosene spray flame. It has also been shown that a novel extension of rainbow thermometry can be used additionally to extract the instantaneous droplet vaporization rate. The feasibility of measuring the instantaneous regression rate has also been demonstrated using controlled experiments with a vaporizing/burning stream of ethanol droplets.

5587521 Pipe testing system: Lanasa Douglas Channelview, TX, United States

In the 50 years since its introduction, the gas-turbine engine has become an essential component of our global society. One need only look at the nearest airport to realize its dominance of air transportation. It has also become a significant element of the power-generation industry. In the last decade, power-generating combined-cycle powerplants have increased in thermal efficiency to about 60%, while NOx emissions have been reduced by an order of magnitude, to below 9 ppm (dry, at 15% O2) in some cases. This paper reviews the ongoing transition from science to the needed technologies: new modes of combustion have been introduced in gas turbines, including lean premixed combustion, reheat and axially staged combustion, catalytic combustion, and rich-lean combustion: high-efficiency low emissions performance is being extended to nonpremium fuels such as coal gas and crude oil: new materials such as superalloys thermal harrier coatings, and ceramics have been incorporated into designs: and improved theories greatly dependent on advanced laser-based diagnostics of flame structure have led to design tools of increasing scope. Future challenges—such as viable propulsion for supersonic transports, powerplants fueled byrenewable resources, and extension of gas turbines to micropower applications—can be met only through further progress in the underlying aerothermal and materials sciences.

Range of validity of the Rayleigh–Debye–Gans theory for optics of fractal aggregates

The range of validity of the Rayleigh-Debye-Gans approximation for the optical cross sections of fractal aggregates (RDG-FA) that are formed by uniform small particles was evaluated in comparison with the integral equation formulation for scattering (IEFS), which accounts for the effects of multiple scattering and self-interaction. Numerical simulations were performed to create aggregates that exhibit mass fractallike characteristics with a wide range of particle and aggregate sizes and morphologies, including x(p) = 0.01-1.0, ‖m - 1‖ = 0.1-2.0, N = 16-256, and D(f) = 1.0-3.0. The percent differences between both scattering theories were presented as error contour charts in the ‖m - 1‖x(p) domains for various size aggregates, emphasizing fractal properties representative of diffusion-limited cluster-cluster aggregation. These charts conveniently identified the regions in which the differences were less than 10%, between 10% and 30%, and more than 30% for easy to use general guidelines for suitability of the RDG-FA theory in any scattering applications of interest, such as laser-based particulate diagnostics. Various types of aggregate geometry ranging from straight chains (D(f) ≈ 1.0) to compact clusters (D(f) ≈ 3.0) were also considered for generalization of the findings. For the present computational conditions, the RDG-FA theory yielded accurate predictions to within 10% for ‖m - 1‖ to approximately 1 or more as long as the primary particles in aggregates were within the Rayleigh scattering limit (x(p) ≤ 0.3). Additionally, the effect of fractal dimension on the performance of the RDG-FA was generally found to be insignificant. The results suggested that the RDG-FA theory is a reasonable approximation for optics of a wide range of fractal aggregates, considerably extending its domain of applicability.

Measurement of catalytic recombination coefficients on quartz using laser-induced fluorescence

Advanced laser-based diagnostics have been developed to examine catalytic effects and atom/surface interactions on thermal protection materials. This study establishes the feasibility of using two-photon laser-induced fluorescence for detection of O and N atom loss in a diffusion tube to measure surface catalytic activity. The experimental apparatus is versatile in that it allows fluorescence detection to be used for measuring species selective recombination coefficients and for performing diffusion tube and microwave discharge diagnostics. Many of the potential sources of error in measuring atom recombination coefficients by this method have been identified and taken into account. These include scattered light, detector saturation, sample surface cleanliness, reactor design, gas pressure and composition, and selectivity of the laser probe. Recombination coefficients and their associated errors are reported for N and O atoms on a quartz surface at room temperature.

A shock tube with tunable, pulsed laser-based diagnostics

This article describes a unique shock tube that is interfaced with a short-pulse, Nd:YAG-pumped, tunable dye-laser system to provide real-time information on transient chemical species. The tube’s internal bore (38.1-mm-diam) accomodates 18 optical diagnostic ports. Pressure-driven shock waves are generated in a double-diaphragm driver section and are timed in the driven section by HeNe laser schlieren detectors. Incident shock speeds range between 1000 and 1500 m/s and are reproducible to ±5 m/s. Individual pulses from the laser and the gate for a photodiode array detector are synchronized with the arrival of shock fronts at the viewing station. Laser-induced fluorescence (LIF) imaging experiments demonstrate the diagnostic power of the new apparatus. One-dimensional (1D) LIF images are recorded in shocked mixtures of ethane, oxygen, and argon by propagating the tunable laser beam along the shock tube’s axis and collecting fluorescence from the OH radical on the 1D detector. The detection sensitivity limit for OH by LIF in this shock tube is approximately 4×1011 radicals/cm3 (0.1 ppm).

Planar laser-induced fluorescence imaging

New measurement techniques based on planar (2-D) imaging of laser-induced fluorescence provide a powerful complement to single-point laser-based diagnostics, with significant potential to impact combustion and fluid mechanics research. Though still in an early stage of development, these imaging methods offer prospects for non-invasive, spatially and temporally resolved measurements of species concentrations and mole fractions, temperature, density, velocity and pressure. Extensions of these 2-D techniques to new flowfield variables and species, and to 3-D imaging by rapid scanning of the illumination plane, are already in progress.

Combustion diagnostics: Planar imaging techniques

New measurement techniques based on planar (2-d) imaging of scattered light provide a powerful complement to single-point laser-based diagnostics, with significant potential to impact combustion research. Though still in an early stage of development, these imaging methods offer prospects for non-invasive, spatially and temporally resolved measurements of species concentrations and moler fractions, temperature, density, velocity, and pressure. Imaging processes encompassed in this review include laser-induced fluorescence and Raman, Mie and Rayleigh scattering. Extensions of these 2-d techniques to new flowfield variables and species, and to 3-d imaging by rapid scanning of the illumination plane, are already in progress.