Digital-Image Data Acquisition System for Turbulent Mixing and Combustion

This system enables visualization of subsonic and supersonic high-speed flows.

An equipment and instrumentation system for high-speed digital image data acquisition and processing includes components for Planar Laser Induced Fluorescence (PLIF), flame-speed, and ignition and extinction measurements of laminar flames at variable pressure; tracking flow structures in a high-speed mixing layer using high-speed color schlieren; laser-beam manipulation and volume scanning for three-dimensional turbulence measurements; and an expanded infrastructure capability for processing experimental and numerical-simulation data.

Flame measurements rely on an extension of Particle Streak Velocimetry (PSV), using a continuous-wave laser modulated with a Pockels cell. The measurements allow the study of laminar-flame profile details in higher-speed flames, such as encountered in ethylene and other fast fuels. The color Schlieren measurements rely on a high-speed color CMOS camera and associated light source and other support components. These measurements provide details of turbulent mixing in flow geometries for both subsonic (diffusers) and supersonic (SCRAMJET) internal flows. The laser-beam manipulation and volume-scanning capability is provided through a network of mirrors, servo actuators, motion controllers, and an optical scanner.

The system enables resolved three-dimensional measurements of fully developed turbulence as a function of time. The processing and visualization of the data acquired are enhanced through memory, CPU, and tape-backup upgrades to previously available data acquisition and visualization computers.

To perform PSV measurements, a continuous-wave Argon-Ion laser operating at 2.5 W was used. The laser beam was chopped at a 50% duty cycle and at a maximal frequency of 2.4 kHz to illuminate the particles for a specified amount of time. The exposure on the CCD imager then produced streaks that were used to determine the local flow velocity. PSV has the crucial advantage of significantly lower particle-loading requirements in this environment (2-3 orders of magnitude). Low particle loading is critical in archival combustion measurements, where large particle loading may alter flame properties. Therefore, a new particle tracking velocimetry (PTV) technique was developed that keeps the low particle-loading advantage of PSV and features a higher spatio-temporal resolution, thanks to the use of both a new illumination source (Nd:YLF 527-nm diode-pumped Q-switched laser with double-pulse option), a new imaging system (4008 × 2672 pix 2, low-noise), 14-bit dynamic range, and a CCD camera with a maximum framing rate of 5 fps at full resolution.

In order to be able to store data continuously to the disk at the same rate as the maximum framing rate of the camera, a designated Datawulf computer system is used that is equipped with fast raid controller with nine hard drives. A camera interface board was chosen to transfer data quickly from the camera to the computer local RAM memory, and software is used to manage data streaming.

With the PTV technique, investigations of pure ethylene flames and blends of hydrocarbon fuel and hydrogen flames at atmospheric, as well as moderately high pressures, are now possible.

This work was done by Paul E. Dimotakis of the California Institute of Technology for the Air Force Office of Scientific Research.
AFRL-0104

This Brief includes a Technical Support Package (TSP).

Digital-Image Data Acquisition System for Turbulent Mixing and Combustion (reference AFRL-0104) is currently available for download from the TSP library.

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