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Show Figure 7 is an example of images obtained using this technique. The photographs show the interface between the incoming combustion air seeded with Ti02 and the fluid in the furnace using the IFRF 300 kW fuel staged burner [3] at a swirl number of .56, with and without staging. Photograph A is of the baseline condition: no secondary fuel. One can see that the turbulence scale between the incoming air and the fluid in the furnace is small compared to the diameter of the quarl. Photograph B is of the staged condition with 60% secondary fuel. Although one can see the secondary fuel jets mixing with the incoming seeded air just above the jet exits, no increase of the turbulence scale is noted. This indicates that the secondary fuel is being slowly entrained in the combustion air by these small scale structures while traveling downstream. Since the hot combustion products are on the opposite side of the incoming air from the secondary fuel jets, combustion of the secondary fuel is delayed until the hot products, air, and fuel have had sufficient time to mix. This results in a lengthened flame with lower peak temperatures and reduced NOx emission (46 ppm for baseline compared to 23 ppm for staged with low heat extraction boundary conditions). An example of a study of the structure of a swirling jet using planar Mie scattering can be found in [4]. 3.2.2 Laser Doppler Velocimetry Laser Doppler velocimetry (LDV) is used to non-intrusively measure fluid velocity. In this technique, two laser beams of the same wavelength are focused and crossed. When seed particles which are introduced with either the air or gas streams pass through the beam crossing, they scatter the laser light. This scattered light is focused onto a detector and, from the frequency of the light scattered on the detector, the velocity can be deduced (see [5] for more detail). Figure 8 shows the layout of the two component LDV system used in BERL. The light source is a 7W argon ion laser. The laser output is routed into the Aerometrics "Fiber Drive" where the colors are separated; one color for each of the two velocity components. Beams of each color are "split" into two beams of equal intensity. Four beams emerge from the delivery head which focuses and crosses these beams into a probe volume 120llm in diameter and 60 mm long. The collection head is tilted at 3° above the axis of the delivery optics. This facilitates measurements near the floor of the furnace and, since a 100llm pinhole is used for the detector aperture, tilting the collection head also decreases the effective length of the probe volume to 20 mm. The light scattered from the probe volume is gathered by the collection optics where the two colors (velocity components) are separated and sent into photomultiplier tubes. The output of the photomultiplier tubes is then analyzed by the Fourier Transform Burst Detector. Velocity histograms, average and RMS velocities for each component are displayed on the computer monitor. Figure 9 is an example of LDV measurements taken under the same conditions as the Mie scattering images (Fig. 7). The figure shows the mean ± RMS velocity at five axial locations with both staged and un-staged combustion. -11- |
