| OCR Text |
Show For this reason, the present study was undertaken to establish the spatial structure of the flow. This is accomplished by imaging a thin slice of the flow in the streamwise and crossstream directions. These images allow us to see the structure of the interface region between the incoming jet and the fluid already in the furnace and hence the length scales of the flow. Laser Doppler velocimetry is also used to establish the time-mean flow characteristics and fluctuations, as well as to establish the inlet conditions of the flow. The data are then used to construct a "mental model" of this flow from the viewpoint of the vorticity fed into the flowfield and its subsequent dynamics. This mental model, while speculative in nature, provides an alternative framework from which to view the flow (as opposed to the traditional time-mean velocity and pressure) and in doing so, provides an explanation for many of the important features of the flow. EXPERIMENT Figure 1 shows the geometry of the Sandia Research Furnace in which the studies were conducted. The furnace is octagonal in cross section and its walls are made up of fused silica window panels, providing complete optical access. The swirling air flow is generated by tangential jets located in the swirler assembly and flows through the annular passage adjacent to the nozzle support tube to the inlet plane of the furnace. The nozzle support tube and nozzle itself form a cylindrical centerbody which terminates at the inlet plane. As the flow enters the furnace it undergoes a sudden expansion of about 6:1 in diameter. The coordinate system referred to in the following discussion is cylindrical (z,r,9) with the origin of the Z coordinate at the entrance plane. The air flowrate for this study was the same as that used under normal burning conditions with a kerosene spray flame. Table I summarizes the flow conditions and gives some of the key dimensions of the system. Figure 2 shows the orientation of the laser sheet optics for observation of the streamwise flow structure. In this orientation the sheet cuts through the center of the furnace and lies in the r-z plane. In the cross-stream orientation the optics are rotated through 90° and the sheet lies in the r-9 plane. The light source for these studies was a frequency-doubled, Nd: Y AG laser (532 nm) with pulse duration of 8 ns. The measured sheet thickness was less than 1.5 mm (FWHM) across the full image field. The interface between the incoming fluid and that in the furnace was visualized by observing the elastic scattering of the laser sheet off of Ti02 particles in the incoming fluid. These particles were formed by introducing a stream of TiCl4 vapor into the swirler assembly and allowing it to react with water vapor in the air to form Ti02 (Ebrahimi and Kleine, 1977). The flowrate of TiCl4 used was approximately l/l000th of the air flow supplied to the furnace and no additional water vapor was required beyond that normally present in the air stream. The size of the particles formed by this process has been estimated to be around 0.2 J..Lm (Witze and Baritaud, 1988). In order to provide contrast between the fluid in the furnace and the incoming jet, the flow of TiCl4 was not steady but was started and stopped impulsively at intervals sufficiently long for - 3 - |
