Experimental investigation of the free convection velocity boundary layer and plume formation region for a heated horizontal cylinder

摘要

Particle image velocimetry measurements were obtained in the free convection water flow around a heated horizontal cylinder for a Rayleigh number of 1.33 × 10~6 and a Prandtl number of 5.98. Radial and circumferential velocity were investigated within and just outside the velocity boundary layer in a vertical plane perpendicular to the cylinder axis. Radial and axial velocity were observed in the early plume development just above the top of the cylinder in a vertical plane parallel to and passing through the cylinder axis. The submersion depth of the top of the cylinder below the free water surface normalized by cylinder diameter ranged from H/D = 2 to 8, which includes depths for which the plume sways and meanders in periodic motion. The effects of plume swaying and meandering on the velocity boundary layer and plume formation were investigated. It was found that the approach flow outside the boundary layer was affected by penetrative convection caused by swaying and meandering of the plume even though there is no horizontal confinement. Measurements within the boundary layer as it nears the plume formation region and within the plume formation region itself reveal that penetrative zconvection in the approach flow altered the boundary layer and these effects propagated downstream into the plume formation region. The data therefore identified a feedback loop for which the plume fluid returns to the boundary layer sustaining the swayine motion. A direct cause of meandering was not determined.