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Show Figure 12 shows a comparison of three test cases with duct area reductions of 0% (no reduction), 25% and 50%. All test conditons other than the size of the area reduction were the same for the three cases. The decay of jet velocity near the injection point was similar for all three cases. Substantial recirculation existed downstream of the injection plane in the 0% restriction flow but was somewhat shorter downstream of the injection plane in the 25% restriction flow. Remarkably, there was no indicated recirculation downstream of the jet injection plane in the 50% area restriction case. Turbulence data indicates that the flow was very uniformly mixed by the time it left the restriction in the 50% case, but was still mixing in the 0% and 25% cases. A reasonable conclusion is that a restriction downstream of the jet injection point shortens the extent of the downstream recirculation zone. A 25% area reduction causes some shortening in the recirculation, but a 50% area reduction is more effective and would probably allow for a shorter secondary combustor with less gas-to-corabustor-wall heat loss. SUMMARY A 1/6 scale water flow model of the Low Mass Flow secondary combustor was tested in order to determine the effects of combustor geometry and combustion air injection techniques on the flow and mixing in the secondary combustor. A dual Bragg cell laser velocimeter was used to obtain detailed two-component velocity data for five model configurations. In the study air flow injection angles of 90° and 120° (relative to the bulk flow direction), velocity ratios of 20/1 and 10/1, and cross-sectional area reductions of 50%, 25% and 0% were tested. The following conclusions were drawn from the results of this study: 1) A bulk velocity ratio of 20 between the secondary jets and the primary stream provides a shorter, more intense mixing region with more uniform mixing across the duct than a velocity ratio of 10 for secondary combustor operation. 2) For the cross flow injection type combustor considered, test results did not indicate clear-cut advantages for either 120° or 90° injection. Injection at 120° results in greater turbulence near the combustor walls while 90° injection provides greater turbulence near the combustor centerline; hence, 90° injection could provide a less severe environment for the walls of the combustor. 16 |
