HTGL Laser Diagnostics
Advanced Laser-Based Diagnostics
- Combustion Diagnostics
This is a continuously evolving
area in which we are developing new and improved diagnostic techniques for applications in combustion research. The techniques are primarily optical, employing a variety of laser sourc
es. The quantities of interest include gaseous species concentrations, density, temperature, velocity and particle size. Techniques receiving particular emphasis are: tunable laser absorption spectroscopy (infrared and ultraviolet), laser-induced fluore
scence, Mie scattering, and particularly 2-d digital flowfield imaging using planar laser-induced fluorescence (PLIF), a technique pioneered in our laboratory and now in use throughout the world.
- Combustion and Plasma Gas Spectroscopy
Spectroscopy forms the
basis of an increasing number of advanced measurement techniques employed in combustion and plasma research. Examples are various forms of tunable laser absorption, fluorescence and Rama
n scattering techniques for measurements of species concentration and temperature. In order to implement these techniques, there is an expanding need for fundamental spectroscopic data relevant to combustion species and conditions. A portion of our prog
ram is thus devoted to measurements of fundamental quantities such as oscillator strengths and collision line-broadening parameters in high-temperature gases prepared by combustion, shock wave heating, or plasma heating. Temperatures up to 14,000K and pr
essures to 500 atm are utilized.
- Lasers, Cameras and Image Processing
An essential aspect of a
successful program on advanced diagnostics is to utilize the best available technology in laser sources, detectors, and data processing equipment. Accordingly, a portion of our research
effort is invested in improving capabilities of our apparatus, through acquisition of new equipment or modification of existing devices. For example, we continually seek to upgrade our laser sources, CCD cameras and our image processing hardware and soft
ware. These activities provide valuable research training with state-of-the-art equipment and procedures.
- Hypersonic Flow Diagnostics
This is a new effort aimed at
tunable laser absorption and fluorescence-imaging diagnostics suitable for application in nonequilibrium hypersonic flows. Research in these facilities is motivated in part by t
he National Aerospace Plane project, and involves collaborations with researchers at NASA Ames Research Center and Calspan in Buffalo, New York. One recent project involves development of PLIF imaging of velocity and temperature in a hypersonic shock tun
nel. Another project successfully provided instantaneous 2-D images of temperature and flame zones in a model scramjet combustor. Current work is aimed at implementing these diagnostics in large shock tunnels located at NASA Ames Research Center and at
- Gasdynamics Parameters
This work involves development of laser
for sensing velocity, density, temperature and pressure in high speed flows. The sensing strategies, based on absorption or fluorescence of molecular constituents of air, utiliz
e rapidly tunable lasers (such as semiconductor diode lasers) which have potential application in both flight and ground-based flow facilities. As an example, we have recently demonstrated a scheme for non-intrusive measurements of mass flux in a high-sp
eed air stream using laser-wavelength-modulation of a diode laser near 760 nm, the location of the A-band absorption spectrum of molecular oxygen. In an extension of these ideas, we have demonstrated a non-intrusive measurement of thrust at the exit of a
supersonic propulsion system via spectrally resolved measurements of H2O in the 1.4-micron combination vibration-rotation band.
- Diode Laser Sensors for Process Monitoring and Control
rapidly-growing activity in our diagnostics program which seeks to capitalize on the virtues of diode laser sources as spectroscopic sensors for process monitoring and control. Diode
laser sources are essentially tunable monochromatic light sources which can be used to monitor (via absorption or laser-induced fluorescence) species concentrations and gasdynamic properties such as temperature, pressure and velocity. Because these sourc
es are compact, economical, rugged and compatible with fiberoptics, they offer exciting prospects for use in remote detection schemes and in control of engineering systems. Such sensors offer the important advantages of in situ detection with excellent s
pecies specificity, as may be needed for example in monitoring trace hazardous or pollutant compounds. At the present time we are developing sensors for monitoring and control of hazardous waste incinerators, but we anticipate that the work will soon be
extended to other processes such as semiconductor processing. One novel aspect of our work will be to extend the wavelength operating range of diode lasers from that which is presently available commercially (620 nm to 1.6 microns) to both the ultraviole
t and infrared regions through use of nonlinear wave-mixing techniques. This will led to significantly enhanced detection capability for several important species (e.g., NO, NO2, HCl, CO, CO2, H2O) which absorb more strongly in these wavelength regions.
An important recent contribution has been the development of wavelength-division-multiplexing schemes which allow simultaneous, common-path absorption measurements at multiple wavelengths (e.g., for multiple species and temperature). Applications of the
se techniques to stabilize and control combustion flows are in progress.