Experimental Investigation of Turbulence Transition of Natural Convection Boundary Layer Along a Vertical Plate in Water With Sub-Millimeter-Bubble Injection

This paper presents an experimental investigation of the turbulence transition of the natural convection boundary layer along a vertical plate in water with sub-millimeter-bubble injection. In this study, we focus on the relationship between the bubble injection position L and the turbulence transition of the boundary layer. Temperature and velocity measurements show that sub-millimeter-bubble injection for L = 1.6 mm suppresses the turbulence transition of the natural convection boundary layer, while that for L = 3.6 mm enhances the turbulence transition of the boundary layer. For L = 1.6 mm, the appearance region of the bubble-induced liquid velocity fluctuation at the upstream unheated section is restricted near the wall, though the peak value of the liquid velocity fluctuation is high. In contrast, in the case of L = 3.6 mm, the relatively large liquid velocity fluctuation induced by bubbles at the upstream unheated section distributes widely over the laminar boundary layer width. Therefore, we expect that the turbulence transition of the natural convection boundary layer for the case with bubble injection depends on the magnitude and appearance region of the bubble-induced liquid velocity fluctuation at the upstream unheated section.