| OCR Text |
Show At the centerline of the furnace, the internal recirculation zone shows no mean radial velocity, but fairly broad fluctuations. Comparison with Fig. 5 shows that the mean flow is purely axial but.the fluct:uations indicate some sloshing from side to side. As would be expected, the mean radial velocIty component goes to zero at the centerline, except at z = 25 mm. Here the combination of a very broad distribution and unknown bias effects contribute to a slightly negative mean value. Figure 7 shows the tangential velocity component. Positive tangential velocities (counterclockwise flow when viewed looking down at the nozzle) are shown above the baseline and negative (clockwise) are shown below. It is apparent that, in the mean, the entire flowfield rotates with the main air and fuel swirl, but that fluctuations are significant in the flame and droplet regions. In these regions the tails of the distribution extend to negative tangential vel~ties. Overall it is reasonable to think of the flowfield as being twisted from within by the swU'1 of the air and fuel jets, with the velocities tapering off toward zero at the walls. As apparent from the figure, fluctuations dominate the tangential velocity component in much of the flowfield. The mean axial and radial velocity components have been superimposed in Fig. 8 in order to facilitate location of the "stagnation boundaries" in the r-z plane. Of course these are not true stagnation points (tangential velocity is non-zero throughout the flowfield) but positions at which o~e of the mean velocity components is zero. As shown in the figure, one boundary coin.cides WIth the schlieren gradient visible in Fig. 3 between the main air jet and the external r~rculation/entrainment zone. As already pointed out, the flow outside this boundary is pure~y radial. The other boundary is that of the internal recirculation (and entrainment) zone, separattng the fuel jet from the reverse flow of gases at the centerline. As denoted by the bold arrows, flow in this region is purely axial. The mean axial and radial velocity components have been combined into the velocity vectors shown in Fig. 9. Also shown are the stagnation boundaries just defined. The bias regions are marked by a line adjacent to the tail of the velocity vectors. Note that the size of the head of the velocity vectors has been kept constant and only the length of the tail is varied to indicate magnitude. This is done so that the direction of the flow may be discerned even when the magnitude of the vector is small. These data highlight the interactions between the various regions of the flow. It is panicularly interesting to note that the highest velocity magnitude at the entrance plane of the furnace is immediately adjacent to the nozzle and inward at about 45-. Figure 10 gives the mean velocity vectors in the radial-tangential measurement planes. The most prominent feature of these data is the penetration of the fuel jet into the air jet illustrated by the vectors at z = 25 and 50 mm. At z = 25 mm, the air jet has a near uniform tangential velocity profile and little radial velocity. The fuel jet immediately adjacent to it, however, has a strong - radial component. As the flow proceeds downstream, this radial velocity causes the fuel jet to push into (and mix with) the main air jet until at z = 100 mm both streams have acquired similar, strongly radial, velocities. Please note that there are some velocity vectors in the central regions of the flow at z = 75 to 150 nun with peculiar orientations. As shown in figures 6 and 7, this is the region of the flow where both the radial and tangential mean velocity components go to zero. As such, only a very small deviation in the mean velocity is required to rotate the direction of these vectors to any orientation. A comparison of the fluctuations in the three velocity components is shown in Fig. 11. Data from the biased region have been omitted since the usefulness of any conclusions drawn from these data is likely to be overshadowed by uncenainties due to the bias. The fluctuations of the three components are about the same in the external recirculation zone and in the central pan of the internal recirculation zone. However at the boundary of the fuel and air jets (mixing zone) the radial fluctuation in the air jet begins to exceed the fluctuations of the other two components. - 8 - |
