Experimental studies on mixing of multiple turbulent jets in non-reacting and reacting flows.

摘要

In this thesis, the interaction of separately aligned multiple turbulent jets in non-reacting flow was analyzed and compared to that in reacting flow, and the MILD combustion condition in a furnace configured with the separated fuel and oxidizer jets evolving downward was determined.; In the first part, experimental research was performed on the scalar mixing characteristics of multiple turbulent jets in isothermal, incompressible and non-reacting flow. In the aligned three jets configuration, the concentration and its fluctuation distribution of the center jet was measured using planar laser induced fluorescence in the range of the downstream distance 35d from the virtual origin of the jet with nozzle diameter d. It was found that the mixing and dilution characteristics of the separated multiple jets can be analogously explained by the concept of large-scale eddy structure. In addition, for the change of the separation distance and the jet velocities, it is possible that there exists a common minimum of the half width of the radial concentration profile of the center jet.; In the second part, a mid-sized natural gas furnace with separated fuel and oxidizer jets without preheating was constructed. Local temperature and gas composition distributions were measured and analyzed for various oxidizer nozzle diameters, oxygen enrichments of the oxidizer and overall equivalence ratios. UV emission intensities from flame zone were also measured and compared with visual images. It was found that in the MILD combustion conditions the temporal UV emission was uniform with low intensity and the flame was invisible. The MILD combustion was found to be strongly affected by the flow momentum of the oxidizer such that the flame transitions to the MILD combustion regime by reducing the nozzle diameter of the oxidizer or by decreasing the oxygen enrichment of the oxidizer. In addition, it was found that in the range of the overall equivalence ratio phioverall = 0.8--1.1 the MILD combustion was easier to achieve for the higher phi overall.