Digital particle image velocimetry (DPIV) is a non-intrusive analysis technique that is very popular for mapping flows quantitatively. To get accurate results, in particular in complex flow fields, a number of challenges have to be faced and solved: The quality of the flow measurements is affected by computational details such as image pre-conditioning, sub-pixel peak estimators, data validation procedures, interpolation algorithms and smoothing methods. The accuracy of several algorithms was determined and the best performing methods were implemented in a user-friendly open-source tool for performing DPIV flow analysis in Matlab.

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PIVlab – Towards User-friendly, Affordable and Accurate Digital Particle Image Velocimetry in MATLAB

Lean blowoff of bluff body stabilized flames: Scaling and dynamics

The editors have assembled a world-class group of contributors who address the questions the combustion diagnostic community faces. They are chemists who identify the species to be measured and the interfering substances that may be present; physicists, who push the limits of laser spectroscopy and laser devices and who conceive suitable measuremen

Applied Combustion Diagnostics

Laser diagnostic techniques play a large and growing role in combustion research and development. Here we highlight three areas where quantitative sensing based on laser absorption has had strong influence: chemical kinetics, propulsion, and practical energy systems. In the area of chemical kinetics, measurements in shock tubes of high-temperature reaction rate coefficients using species-specific laser absorption techniques have provided new and accurate answers to questions about combustion chemical processes. In the area of propulsion, wide-bandwidth measurements of flow temperatures, species concentrations, and velocity have provided engine designers with the necessary information to improve operation and performance. In the area of practical energy systems, real-time measurements of combustor operating conditions and emissions have enabled needed incremental improvements in large power plants and improved safety of operation. Yet, there is still more to be done, and opportunities for new applications will grow as laser sensors evolve. This review seeks to provide an overview of the current power and future potential of these modern diagnostic tools.

Applications of quantitative laser sensors to kinetics, propulsion and practical energy systems

Blowoff dynamics of bluff body stabilized turbulent premixed flames

We describe the further development and application of a dual-injection-seeded optical parametric oscillator and associated kHz-rate burst-mode pump laser for high-speed OH planar laser-induced fluorescence in unsteady flames. A detailed overview of the novel double-pulse configuration of the burst-mode laser is given, indicating the potential for use in systems where both high energy and high repetition rate are necessary. Dual-wavelength injection-seeded operation of the optical parametric oscillator is demonstrated and preliminary OH PLIF line switching in a well-characterized natural-gas/air laminar diffusion flame is presented. Applications to high-speed nearly-instantaneous background capture for the elimination of laser scatter and background contamination from polycyclic aromatic hydrocarbons in fuel-rich hydrocarbon flames are also discussed and preliminary data are shown. Lastly, the potential for high-speed two-line planar OH thermometry is described including advantages of utilizing the burst-mode laser and multi-wavelength OPO for simplifying high-speed experimental systems.

High-Speed Multi-Line OH Planar Laser-Induced Fluorescence in Unsteady Flames

Advances in Nd:YAG OPOs have brought about high-repetition-rate, broadly wavelength-tunable ultraviolet laser systems that produce pulses with enough energy for PLIF imaging of turbulent gas flows.

Tunable ultraviolet burst-mode laser system produces high-energy pulses

The Structure and Dynamics of a Bluff-Body Stabilized Premixed Reacting Flow

: While planar laser-induced-fluorescence (PLIF) imaging of aerodynamic and combustion flows has developed enormously since its inception in the early 1980s, the ability to directly capture the dynamics of unsteady and/or turbulent phenomena at high speed has been constrained by the difficulty of generating high-repetition-rate, broadly wavelength-tunable ultraviolet radiation with sufficient individual pulse energy. However, sources with all these characteristics have now been made possible by advances in the development of Nd:YAG-pumped optical-parametric-oscillator (OPO) systems operated in what has come to be known as "pulse-burst" mode. In combination with sum-frequency mixing, such systems have produced demonstrated output of around 0.5 or 1.0 mJ per pulse at 226 or 313 nm, respectively - sufficient to enable instantaneous nitric oxide (NO) and hydroxyl (OH) imaging at repetition rates as high as 250 kHz. While reported burst-mode Nd:YAG systems differ in detail, they share certain basic characteristics. Specifically, reported systems typically use a low-power (on the order of 100 mW) master oscillator (typically Nd:YAG) that is "sliced" into a burst train using either a pair of electro-optic Pockels cells or an acousto-optic deflector. In one exception, a repetitively Q-switched diode-pumped Nd:YVO4 laser is used as the master oscillator. The burst train is then amplified in a series of flashlamp-pumped Nd:YAG amplifiers. Typical reported burst sequences consist of between 8 and 40 pulses, with interpulse spacing as low as 1 microseconds, and individual pulse duration of between 6 and 25 ns.

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High-Power UV Lasers: Tunable Ultraviolet Burst-Mode Laser System Produces High-Energy Pulses

Nanoenergetic materials can provide a significant enhancement in the rate of energy release as compared with microscale materials. The energy-release rate is strongly dependent not only on the primary particle size but also on the level of agglomeration, which is of particular interest for the inclusion of nanoenergetics in practical systems where agglomeration is desired or difficult to avoid. Unlike studies of nanoparticles or nanometer-size aggregates, which can be conducted with ultrafast or nanosecond lasers assuming uniform heating, microscale aggregates of nanoparticles are more sensitive to the thermophysical time scale of the heating process. To allow control over the rate of energy deposition during laser initiation studies, a custom, temporally tailored, continuously variable-pulse-width (VPW) laser was employed for radiative heating of nanoenergetic materials. The laser consisted of a continuous-wave master oscillator, which could be sliced into desired pulses, and a chain of amplifiers to reach high peak power. The slicer allowed control over the time profile of the pulses via the combination of an arbitrary waveform generator and acousto-optic modulator (AOM). The effects of utilizing flat-top or ramped laser pulses with durations from 100ns to 150ls and energies up to 20 mJ at 1064nm were investigated, along with a broad range of heating rates for single particles or nanoparticle aggregates up to 100-lm diameter. In combination with an optical microscope, laser heating of aggregates consisting of 70-nm diameter Al nanoparticles in a Teflon matrix showed significant dependence on the heating profile due to the sensitivity of nanoenergetic materials to heating rate. The ability to control the temporal pulse-intensity profile leads to greater control over the effects of ablative heating and the resulting shockwave propagation. Hence, flexible laser-pulse profiles allow the investigation of energetic properties for a wide size range of metal/metal-oxide nanoparticles, aggregates, and composites. [DOI: 10.1115/1.4007887]

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1162&context=me_pubs

Joseph D. Miller

Papers

High-Speed Multi-Line OH Planar Laser-Induced Fluorescence in Unsteady Flames

Tunable ultraviolet burst-mode laser system produces high-energy pulses

The Structure and Dynamics of a Bluff-Body Stabilized Premixed Reacting Flow

High-Power UV Lasers: Tunable Ultraviolet Burst-Mode Laser System Produces High-Energy Pulses

Micro-Optical Initiation of Nanoenergetic Materials Using a Temporally Tailored Variable-Pulse-Width Laser