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System for Imaging Turbulent Combustion Flows

Flow regimes inaccessible to prior instrumentation can now be studied.

The Components of the System enable diverse types of imaging, acquisition of image data, and processing of the data to extract information on multiple aspects of high-speed combustion flows.
Asystem of scientific instrumentation and data-processing equipment has been assembled for research and education in turbulence, mixing, and chemical reactions (especially combustion) in high-speed laminar and turbulent flows. The system is designed to enable study of temporally varying, three- dimensional structures in flow regimes that have been inaccessible to prior instrumentation systems, with emphasis on flow regimes relevant to the operation of ramjet and scramjet engines.

The system (see figure) includes components for planar laser-induced fluorescence (PLIF); flame-speed, ignition, and extinction measurements of laminar flames at variable pressure; tracking flow structures in a high-speed mixing layer using highspeed color schlieren imaging; laserbeam manipulation and volume scanning for three-dimensional turbulence measurements; and an expanded computational infrastructure for processing experimental and numerical- simulation data.

  • The PLIF instrumentation is augmented with an intensified charge-coupled- device image detector and associated data-acquisition components.
  • Flame measurements are made by means of particle-streak velocimetry, using a continuous-wave laser beam modulated with a Pockels cell. The measurements enable the study of details of profiles of such relatively high-speed flames as those encountered in the combustion of ethylene and other rapidly burning fuels, thereby contributing to understanding of flame structures and enabling extensive testing of chemical-kinetic models.
  • The color schlieren imaging is performed by means of a high-speed color camera containing a complementary metal oxide/semiconductor image detector, an associated flashlamp, and other support components. The measurement data extracted from the color schlieren images are providing details of turbulent mixing in important flow geometries for both subsonic internal flows (as in diffusers) and supersonic internal flows (as in scramjet engines).
  • The subsystem for laser-beam manipulation and volume scanning includes mirrors, servoactuators, motion controllers, and an optical scanner. This subsystem enables three-dimensionally spatially resolved measurements of fully developed turbulence as a function of time.
  • The processing and visualization of the data acquired by this system are enhanced through upgrades of the memory circuits, central processing units, and tape backup units of previously available data-acquisition and visualization computers.

This work was done by Paul E. Dimotakis of California Institute of Technology for the Air Force Research Laboratory.

Topics:
Aerodynamics Combustion and combustion processes Data acquisition and handling Engines Imaging and visualization Jet engines Optics Physical Sciences Powertrains Scramjet engines Turbulence
This Brief includes a Technical Support Package (TSP).
Document cover
System for Imaging Turbulent Combustion Flows

(reference AFRL-0040) is currently available for download from the TSP library.

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    Defense Tech Briefs Magazine

    This article first appeared in the October, 2007 issue of Defense Tech Briefs Magazine (Vol. 1 No. 5).

    Read more articles from the archives here.

    Overview

    The document is a final progress report summarizing a research program focused on high-speed digital image data acquisition, processing, and visualization systems for studying turbulent mixing and combustion. The project, led by Principal Investigator P. E. Dimotakis at the California Institute of Technology, was supported by the Air Force Office of Scientific Research (AFOSR) under Grant FA9550-04-1-0253, covering the period from May 1, 2004, to October 31, 2006.

    The report outlines the development and implementation of advanced equipment designed to enhance the understanding of turbulence and combustion phenomena, particularly in high-speed flows. A key innovation in this research is the use of hybrid CCD-CMOS detectors, which significantly improve the signal-to-noise ratio (SNR) of focal plane array (FPA) detectors, allowing for more precise measurements in turbulent environments.

    The system includes various components for conducting experiments, such as Planar Laser Induced Fluorescence (PLIF) for tracking flow structures, flame-speed measurements, and ignition and extinction studies of laminar flames under varying pressure conditions. The research aims to provide insights into the three-dimensional structure and dynamics of turbulence, as well as the mixing processes in both incompressible/subsonic and compressible/supersonic flows.

    The report emphasizes the educational aspect of the project, highlighting its role in training research students in disciplines relevant to the Department of Defense (DOD) mission. The experiments conducted with the developed equipment offer a first look at the large-scale structures of turbulence, contributing to the broader understanding of fluid dynamics and combustion processes.

    In summary, this report encapsulates a significant advancement in the field of turbulence and combustion research, showcasing the integration of high-speed imaging technology and innovative measurement techniques. The findings are expected to have implications for various applications, particularly in aerospace and defense, where understanding high-speed flows is critical. The research not only enhances scientific knowledge but also prepares the next generation of researchers in this vital area of study.

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