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Spray combustion processes can be studied, thanks to a device
built to measure fuel droplet size distributions by bathing them
in laser light.
Aerometrics, Incorporated of Sunnyvale, California, first
developed, through a Small Business Innovation Research (SBIR)
contract with Lewis Research Center, a particle analyzer using
a Phase Doppler technique. This laser instrument provided a non-disruptive
method of determining particle size, number density, flow velocity,
diameter versus velocity, trajectory, turbulence intensity, and
more.
A follow-on NASA SBIR contract with Aerometrics has now built
upon that earlier product. The new technology can be integrated
with the company's Phase Doppler Particle Analyzer (PDPA) systems,
or used independently.
This latest device from Aerometrics--a rainbow refractometer--is
keyed to measuring the refractive index of a droplet by monitoring,
in a non-intrusive manner, the main rainbow location with a Charge
Coupled Device (CCD) linear array. Refraction is the bending
of a ray of light at the boundary of two mediums of dissimilar
nature. When droplets are illuminated by laser light, they give
rise to a characteristic rainbow pattern in the backscatter direction.
This pattern can be recorded using a CCD line camera and the
data processed to extract the refractive index information, which
is a function of droplet temperature, for a given droplet composition.
The refractive index of particles as small as 20 microns in diameter
can be measured. For particles less than 150 microns, the Doppler
particle analyzer and the refraction instrument can be used in
conjunction.
Some applications of the technology include:
- Spray Flame Characterization Fuel droplet size and
velocity in complex spray flames can be measured. No other instrument
is capable of measuring individual fuel droplet temperatures
in a spray flame. By measuring droplet size, velocity, and temperature,
data can be used to validate advanced theoretical spray combustion
models that are routinely used in the design of fuel efficient
combustors.
- Droplet Combustion Studies in Microgravity A compact
diode laser-based rainbow refracto- meter system is under development
by Aerometrics. This instrument can be used for obtaining the
temporal evolution of fuel droplet size, internal temperature
profile/gradient, and droplet regression rate in single droplet
combustion experiments. Those tests are to be carried out aboard
NASA's microgravity producing aircraft, created by parabolic
flight trajectories of the plane.
- Rocket Engine Mixing Studies In liquid rocket engine
injectors using more than one liquid propellant, adequate and
uniform mixing of the fuel with an oxidizer is essential if efficient
combustion is to be realized. Tasking the combined Phase Doppler/rainbow
refraction device provides a useful means for studying liquid-liquid
mixing even in a reactive environ- ment. Individual droplets
within propellant sprays can be sorted out based on the measured
droplet refractive index. This information can subsequently be
used to compute the spatial and temporal variations of the liquid
propellant mixture ratio.

| Aerometrics'
instrument can inspect, via laser light, individual droplets
of fuel for combustion research. |
Devices built by Aerometrics are expanding the boundaries
of particle diagnostics technology. Not only research oriented
work by the government, industry, and educational institutions
can benefit by using the laser equipment.
"Everyone benefits from research geared toward furthering
the quality of life," says marketing specialist for Aerometrics,
Kristi Altier. "Our rainbow refraction instrument will make
important contributions toward important chemical and combustion
research," she says.
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