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

A New Architecture for Burst-Mode Planar Imaging of Turbulent Flames

Nitric Oxide Planar Laser-Induced Fluorescence (NO PLIF) imaging is demonstrated at a 10 kHz repetition rate in the Calspan-University at Buffalo Research Center’s (CUBRC) 48-inch Mach 9 hypervelocity shock tunnel using a pulse burst laser–based high frame rate imaging system. Sequences of up to ten images are obtained internal to a supersonic combustor model, located within the shock tunnel, during a single ~10millisecond duration run of the ground test facility. This represents over an order of magnitude improvement in data rate from previous PLIF-based diagnostic approaches. Comparison with a preliminary CFD simulation shows good overall qualitative agreement between the prediction of the mean NO density field and the observed PLIF image intensity, averaged over forty individual images obtained during several facility runs.

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NO PLIF Imaging in the CUBRC 48" Shock Tunnel

We report on a source for MHz-rate, pulse-burst, tunable high-energy UV radiation and its application to high-speed imaging and temperature measurements of combustion based on planar laser-induced fluorescence (PLIF) spectroscopy of the OH radical. The system, comprised of a high-speed injection-seeded optical-parametric oscillator (OPO) pumped by a frequency-doubled MHz-rate pulse-burst Nd:YAG laser, was characterized in a laminar hydrogen-air diffusion flame stabilized over a near-adiabatic burner. High-speed OH PLIF was demonstrated at rates up to 50 kHz in a sonic hydrogen-air diffusion flame, showing the capability to capture turbulent fluid-flame interactions. In addition to high-speed OH PLIF at a fixed excitation wavelength, methods for planar spectroscopy are discussed, including wavelength scanning during a pulse burst as well as dual-wavelength injection seeding. Excitation scans of the UV radiation during a pulse burst can be achieved using various cavity stabilization techniques, such as discrete linearization of the OPO cavity modes. Preliminary work on an alternative two-line approach is also presented, with wavelength switching times as low as 20 ns. The OPO in this case is pumped by a double-pulsed burstmode pump laser with inter-pulse separation as low as 25 ns and pulse energies of 30mJ at 532nm. Feasibility studies of high-speed planar OH scanning and two-line spectroscopy are planned in laminar and turbulent flames.

http://enu.kz/repository/2009/AIAA-2009-523.pdf

Planar Laser-Induced Fluorescence of the OH Radical using a Tunable MHz-Rate UV Source

Hybrid femtosecond/picosecond (fs/ps) coherent anti-Stokes Raman scattering (CARS) is demonstrated for high-speed thermometry in unsteady high-temperature flames, including successful comparisons with a timeand frequency-resolved theoretical model. After excitation of the N2 vibrational manifold with 100-fs broadband pump and Stokes beams, the Raman coherence is probed using a frequency-narrowed 2.5-ps probe beam that is time delayed to suppress nonresonant background by two orders of magnitude. Experimental spectra were obtained at 500 Hz in steady and pulsed H2–air flames and exhibit a temperature precision of 2.2% and an accuracy of 3.3% up to 2400 K. Strategies for realtime gas-phase thermometry in high-temperature flames are also discussed, along with implications for kHz-rate measurements in practical combustion systems.

High-Speed Thermometry in Reacting Flows Using Hybrid fs/ps CARS

Hybrid fs/ps CARS is employed for time- and frequency-domain spectroscopy in reacting flows. Advantages include nonresonant background suppression, chemical specificity, detection at high pressures, potential for kHz-rate thermometry, and ability to resolve energy transfer phenomena.

Joseph D. Miller

Papers

A New Architecture for Burst-Mode Planar Imaging of Turbulent Flames

NO PLIF Imaging in the CUBRC 48" Shock Tunnel

Planar Laser-Induced Fluorescence of the OH Radical using a Tunable MHz-Rate UV Source

High-Speed Thermometry in Reacting Flows Using Hybrid fs/ps CARS

Hybrid fs/ps Coherent Anti-Stokes Raman Scattering for Time- and Frequency-Domain Spectroscopy in Flames