Optical measurements of the effects on a boundary layer of puffing in a large scale buoyant plume.

摘要

An experimental study has been performed on a buoyant helium plume with a 1 meter base diameter. A cone was immersed in the plume to determine the effect of plume puffing on the boundary layer along the cone. The plume with an immersed cone was also compared to an unobstructed plume. Two regions of two-dimensional velocity fields were measured using particle image velocimetry (PIV) simultaneously. Helium mass fraction was determined by seeding the helium with acetone vapor for planar laser-induced fluorescence (PLIF). Each test includes 7 to 11 puff cycles.; Several experimental and analytical techniques were developed. A cone was designed that could effectively transmit the UV laser sheet needed for whole field PIV measurements. A 12 cm by 8 cm boundary layer PIV region was situated within a 1 m by 1 m PIV whole field region, and both PIV regions were recorded simultaneously using the same light sheet, seed particles, and frame rate. Concentration field measurements were obtained simultaneously with the velocity field measurements using PLIF for a region 0.8 m high and 1 m wide. Whole field velocity fields were measured with centimeter resolution in two spatial dimensions with temporal resolution of 0.005 second for a multi-second test. Techniques were developed to produce PIV velocity vectors for a highly accelerating flow with large velocity gradients, to register each image to correct for distortion, and to align each image relative to another eliminating a major source of velocity vector error.; While Reynolds-averaged turbulent statistics indicate that the presence of the cone does not affect the overall plume behavior, mixing in a puffing plume with an immersed cone is suppressed relative to that in an unobstructed puffing plume. No separation of the boundary layer flow along the cone was observed given the spatial resolution of 0.3 cm. As the toroidal (puff) vortex propagates upward, measurements suggest that the boundary layer thins and freestream velocity increases at the vertical location of the vortex. The plume with an immersed cone revealed splitting toroidal vortices and smaller scale vortices near the cone surface. These vortices may contribute as much to boundary layer unsteadiness as the large-scale periodic toroidal vortices. The boundary layer within a buoyant plume is turbulent with the mean profile shape and thickness varying considerably over time. Additional tests of finer spatial resolution in the boundary layer PIV region that are synchronized with whole field measurements are needed to further quantify the boundary layer variance.