Onsite Research
Laser Spectroscopy Laboratory
NETL's
Laser Spectroscopy Laboratory uses optical diagnostic techniques to improve combustion
processes including those for advanced gas turbine engines. Highly efficient
combustion of fossil fuels is essential to achieving clean power generation,
a national strategic goal. Since many power plants use gas turbines to generate
electricity, the next generation of gas turbines must be capable of satisfying
increasingly stringent emissions requirements. In addition, properties of natural
gas vary based on geographical location, and fuel supplies may be piped to a
facility from a number of locations. Therefore, the engine must be able to compensate
for variations in fuel properties that would otherwise cause it to produce excessively
high levels of regulated emissions.
The use of laser diagnostics is an excellent mechanism to study fluid mechanics
and combustion processes in place, while avoiding unwanted disturbances to the
system being studied. Researchers seek to isolate particular phenomena that contribute
to fluctuations in heat release, by studying combustion flows in simplified systems
that are smaller than commercial-sized units but that have similar combustion
processes and characteristics.
Currently, the lab is using a Rijke tube combustor to simulate gas turbine
combustion properties while excluding variable phenomena such as swirl and
complex nozzle and exhaust geometries. The oscillating flame in the Rijke combustor
is similar to an unstable oscillating flame in a gas turbine combustor. The
combustor is capable of sustaining both stable and unstable combustion, depending
on the operating conditions. A loudspeaker is placed at the base of the combustor
to allow for controlled acoustic stimulation of the flame and convective flow
entering at the base, resulting in small amplitude oscillations of the flame
surface.
The construction of the Rijke tube permits visual access into the ultraviolet
and features variable fuel mixture control. This burner enables researchers
to investigate the combustion coupling mechanism between the flame conditions
and the acoustic pressure variation of the chimney tube surrounding the model.
The system also is being used to develop diagnostic methods to be subsequently
applied to pressurized combustion systems such as gas turbine combustors.
Flame chemiluminescence is used to evaluate heat release mechanisms by recording
the flame surface and the heat release rate in the visible spectrum and in the
ultraviolet, respectively.
Chemiluminescence is recorded in the ultraviolet by a filtered photomultiplier
tube and in the visible spectrum by a high-speed motion analyzer. Images are
collected at phased intervals over a cycle and are processed through an algorithm
to provide a mathematical description of the flame surface, which in turn is
used to determine the flame surface area.
The lab also is using the simplified Rijke burner to investigate mechanisms
involved in overcoming thermal and acoustic instabilities in gas turbine engines
utilizing lean pre-mixed combustion (LPM), which has become a recognized means
of reducing thermal NOx production by lowering peak combustion temperatures.
Conventional power plants remain a major source of emissions that form ozone,
such as NO x , although significant reductions have been achieved in the quantity
of environmentally harmful gases emitted into the atmosphere. Unlike conventional
gas turbine engines that utilize diffusion flames in which fuel and air are
mixed in the reaction zone at the flame, LPM mixes the air and the fuel upstream
of the reaction zone. This eliminates the combustion of rich pockets of fuel
that subsequently increase combustion temperature and thus thermal NOx.
Various experimental techniques being used for these investigations include:
High-speed digital imaging, planar laser induced fluorescence (PLIF), particle
image velocimetry (PIV), flame spectroscopy, pressure and sound measurements,
acoustic flame excitation, and varying fuel composition. Results are used to
aid in the development of models capable of accurately predicting the response
of the flame to changes in operating conditions. Additionally, the models provide
a means of evaluating the stability margin of the combustion process. Use of
this or similar models will provide manufacturers with a tool to predict when
thermal and acoustic instabilities may occur, and to estimate their respective
strengths.
Equipment and resources include the following:
Laser Spectroscopy
- Nd-YAG Laser – Provides tunable laser light from the
near infrared to the ultraviolet
- Dye Laser – Used for a wider range of wavelengths
Optical Detection Systems
- SPEX Spectrometer – 0.5 meter with a gated, intensified
diode array
- Kodak EktaPro – High-speed video camera with a 2-dimensional
image intensifier
For more information contact Steve
Woodruff
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