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.

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