An experimental investigation of confined, multiple, turbulent jet mixing with recirculation

An experimental Investigation was conducted of confined, multiple, turbulent jet mixing with recirculation in an axisymmetric duct that simulated a combustor configuration. The objective was to experimentally examine the flow fields in combustors that utilize injector plates to establish turbulent jet mixing of fuels and oxidizers. Five injector plates were tested; one contained an annular hole or slit, three plates had eight discrete holes on a ring and the remaining plate had thirty two holes on two rings. The last plate was a scaled model of injector plates that have been used in pulverized coal combustors in The United States Department of Energy's Coal Fired Flow Facility at The University of Tennessee Space Institute.

Flow fields downstream of the plates were investigated using flow visualization and laser velocimetry. Qualitative data for flows created by rings of discrete jets were obtained with dye injection and hydrogen bubble generation flow visualization techniques. Wall static pressure distributions were measured with a sensitive pressure transducer. Detailed quantitative descriptions of the velocity and turbulence fields were obtained using a Bragg-diffracted, two-component Laser Doppler Velocimeter (LV). LV data obtained included mean and fluctuating components of velocities, turbulence intensities, turbulent kinetic energies and Reynolds stresses. The locations and extents of recirculation zones and jet expansion rates were deduced from the LV data.

Measurements show that the flow due to an annular jet is two dimensional but those due to discrete jets are fully three dimensional. The results for the annular jet were compared with calculations made using an algebraic viscosity model of turbulence. For injector plates with discrete holes, the ratio of the radius of the ring of holes to the duct radius (r_c/R), and the spacing between adjacent holes on a ring, are found to be very important parameters in deter mining resultant flow patterns. For a single ring of discrete holes, there exists a critical value for r_c/R, being between 0.5 and 0.67. The fundamental nature of the flow downstream of a single ring of jets depends on whether this ratio is above or below its critical value. The flow due to two closely-spaced rings of jets is found to be similar to that due to a single ring of jets, when a geometric constraint is satisfied. For one case tested, the flow field was insensitive to the jet Reynolds number in the range 33,620 to 18,677.