Turbulence Characteristics in a Natural Convection Boundary Layer Above a Heated Round Plate

A turbulent natural-convection boundary layer in air near the upper surface of a heated round plate is experimentally investigated. The heated round plate, the diameter of which is 300 mm, is horizontally placed in a test section of chamber. The upper-surface temperature is 473 K. Instantaneous temperature and two-dimensional velocity vectors near the upper surface are measured by using a cold wire and a particle image velocimetry, respectively. The measurements reveal a multi-layer structure near the upper surface. In the lower layer, velocity fluctuation of horizontal component is active compared with that of vertical component. This intermittently causes the incursion of low temperature fluids. On the other hand, in the upper layer, the intensity of vertical velocity fluctuation with high temperature fluid is much larger than that of horizontal velocity fluctuation. This multi-layer structure is mainly generated by the large-scale fluid motions, such as winding of high temperature fluids and entrainment of low temperature fluids.