Brigham Young University
PHOTOGRAPHIC FLAME CHARACTERIZATION
Visual observations give important insight into the behavior and structure of flames. Photo and video images can be taken to record these visual observations. The laboratory has a 35mm camera and a Hi8 video camera both with zoom lens capability. The video camera is interfaced with a computer which allows video images to be digitized. Film images are digitized using scanners available at BYU. Analysis of these digital images can provide insight into the flow structures and reactions zones of flames.
LASER DOPPLER ANEMOMETRY (LDA)
Instantaneous, in situ gas velocity measurements are important in understanding the fluid dynamics associated with a combustor, and velocity data collected are needed for comparison to computer code predictions. The BYU Combustion Laboratory has a two color LDA system that allows simultaneous and instantaneous measurements of two velocity components (axial and radial, or axial and tangential). Up to 4000 velocity realizations are collected at up to 100 combinations of axial and radial locations within the combustor.
These data sets allow mean and standard deviation velocity contour plots to be created. The contour plots, which are created using conventional computer software (e.g., Spyglass Transform), provide a graphic presentation of the flow fields within the combustor. The tabular data sets also provide specific data for comparison to code predictions. An example of experimental LDA results is presented in figure 1. This figure presents mean axial iso-velocity contours from a practical fuel injector operating with gaseous propane fuel in the laboratory scale gas turbine combustor.
COHERENT ANTI-STOKES RAMAN SPECTROSCOPY (CARS)
A dual Stokes CARS instrument is used in the BYU combustion to obtain gas temperature and limited species concentrations (CO, CO2, O2, and N2). Details of the CARS facility have been documented by Boyack and Hedman, (1990); Hancock, et al., (1991 and 1992). The current dual Stokes CARS instrument is very similar to the folded box-CARS phase matching scheme employed by Boyack and Hedman (1990). However, the combustor is remotely located in the current arrangement and the pump laser beams are directed over a distance of about 15m onto optical bread boards located on either side of the laboratory-scale gas turbine combustor. Stokes lasers and additional optical components are also located on these remote optical breadboards. The CARS signal is returned to the spectrometer with a fiber optic cable. The CARS pump laser and spectrometer are located in an optics room distant from the combustion facilities keep the optical components clean. CARS temperature and species concentration measurements are also taken at up to 100 radial and axial locations in the combustor. Mean and standard deviation measurements of the temperature and species data are determined for each set of about 500 data points which can be compared to code predictions at that location. Iso-contour plots of mean and standard deviation of gas temperature and species concentrations can be created from the data sets using appropriate software (e.g., Spyglass Transform). An example iso-contour plot of mean gas temperature for a practical fuel injector operating on propane is shown in figure 2.
PLANAR LASER INDUCED FLUORESCENCE (PLIF) IMAGING
In PLIF (Planar Laser Induced Fluorescence) imaging, a dye laser is tuned to a resonant frequency which causes the particular combustion radical or molecule to fluoresce at a different frequency. This fluorescence image is recorded with an intensified charge coupled display (ICCD) electronic camera. A capability to obtain PLIF images of combustion intermediates has recently been added to the Combustion Laboratory. Images of OH and CH in representative flames have been obtained, and images of NO, CO, and NO2 are planned. An example PLIF image of OH radical in a premixed, turbulent, swirl stabilized burner operating on natural gas is shown in figure 3.
For this image, the OH radicals were excited with an ultra-violet (ca 283 nm) sheet of laser light produced by a tunable dye laser. This sheet of laser light was passed through the centerline of the laboratory-scale gas turbine combustor. The ICCD camera, located normal to the laser sheet, captured the approximately 100 mm high two-dimensional image (ca 308 nm) of the OH fluorescence. The single image shown illustrates the instantaneous nature of the OH radical in the flame zone. The images are analyzed and enhanced using conventional computer software.
PHASE DOPPLER PARTICLE ANALYZER (PDPA)
To advance ACERC work with liquid fuels, an Aerometrics PDPA (Phase Doppler Particle Analyzer) has been acquired. The instrument at BYU provides simultaneous measurements of droplet size and two-components of the velocity vector in reacting and non reacting two-phase flows. The data collected with this instrument allows the determination of the size-velocity correlation, the turbulent kinetic energy, the shear stresses, as well as the particle size and velocity distributions. Experiments are planned for a laboratory-scale facility to generate information to upgrade a previously developed spray model already used in modeling of full-scale, practical systems.
It is important to be able to monitor in situ the particle dynamics in reacting systems for the experimental and analytical study of particle dispersion. The recent addition of a PDPA system with 2-D capabilities adds significantly to the particle sizing capability in the Combustion Laboratory. The instrument supplies simultaneous non-intrusive measurements of two perpendicular velocity components and particle diameter. Unlike the PCSV instruments the PDPA is limited to sizing spherical particles but provides more detailed velocity information.
Initially, a laser-based Particle-Counter-Sizer-Velocimeter External device (PCSV-E) was purchased from the Insitec Corporation. In order to extend the PCSV-E measurements to large scale system, the PCSV-P (probe version of the PCSV-E system) was also purchased. The PCSV is the only single particle counter to size particles ranging from spherical liquid droplets to solid particles with irregular surfaces such as pulverized coal. Extensive measurements of particle dynamics in both large- and laboratory-scale reactors have been performed using the PCSV-P system. In order to take advantage of the PCSV-E's non intrusive capabilities, a new reactor for the study of particle dispersion has been built. Because of the importance of particle dynamics in two-phase combustion (e.g., pollutant formation, radiation, fouling and slagging, and chemical reaction), this capability is generating strong interdisciplinary interaction with other projects at BYU.