A cold flow model study of turbulent mixing in a secondary combustor

The research conducted was a cold flow model study of the flow and mixing in a secondary combustor. The modeled secondary combustor is installed in the flow train of the Coal-Fired Flow Facility (CFFF) at The University of Tennessee Space Institute. An objective of the investigation was to determine the effects of various operating parameters on the mixing efficiency of this typical industrial combustor.

The secondary combustor is a square cross-sectioned duct having rows of ports on two opposite walls. Air is injected through the ports to mix with the flow coming from the CFFF primary combustor and complete the combustion of gas produced by the fuel-rich primary combustion process. The cold flow model was 1/6 scale, using water as the working fluid to permit experimental conditions of high Reynolds number to be obtained. To perform the experimental flow field study, a dual Bragg cell laser velocimeter was used to obtain detailed two-component velocity data. Measurements of mean velocities, turbulence intensities, turbulent kinetic energy, Reynolds stress, and velocity-time history were obtained.

Five test cases were run with the model. The flow was found to oscillate periodically in all cases. Observations revealed that the oscillation was caused by vortices, produced by interaction of the jets and the incoming bulk flow, which were shed periodically. One recirculation zone always formed upstream of the jets and another recirculation zone usually formed downstream. The size of the downstream zone varied from case to case and it was completely absent when a 50% downstream area reduction was tested.