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Show point. This results in small particles moving faster in the axial direction. These particles (having smaller mass) are influenced more by the fluid motion and are entrapped more easily by the evolving large-scale vortex structures near 2r / d = 0.3. The larger particles have larger aerodynamic response time and are not influenced as much by the fluid motion. Large particles (100 J.Lm diameter) move slowly and are influenced more by radial velocity and start moving upward. These particles then start recirculating once they reach the recirculating zone. This is clearly seen in Fig. 6. The 100 J.Lm size particles (shown in the figure by *) move up and then start recirculating, moving backwards. The 5 J.Lm size particles (shown in the figure by 0) move very fast in the axial direction but are also entrapped by large-scale vortices shedding from the corner of the burner and furnace. The particles that are released at 2r / d = 0.1 move almost in rectilinear motion, while the ones released at 2r /d = 0.3 move with rotation due to the vortices. The 50 J.Lm size particles (shown in the figure by +) show an intermediate behavior. Particle density changes with pyrolysis and this change also influences particle motion. This behavior of small and large particles looks contrary to but confirms the expectation that small particles follow a fluid behavior, and heavy particles follow their own paths and cross the stream lines. This behavior of the particles influences their heating rates and, as a result, their reaction history, as we discuss below. Figures 7 and 8 give particle flight through the imposed temperature and oxygen fields, which are based on the experimental results of reference [3]. The temperature at the entrance is 300 K and rises along the axis to a maximum of 1800 K at 2x / d = 2.0, while oxygen has a maximum value at the entrance and then declines. Heavy particles released at 2r / d = 0.2 and 0.3 do not move downstream very much and, therefore, do not attain high temperatures, although these particles are in a high oxygen area. The particles that are released at radial position 2r / d = 0.1 pass though the high temperature zone. The small particles pass through a high temperature field and high oxygen field. This movement results in high reaction rate and faster burning for small particles. In Fig. 9, the differences between particle and local gas temperatures are shown for four particle sizes. Particle temperature is initially lower than local fluid temperature but 9 |
