176.
Temperature and velocity measurements are performed to clarify the effects of sub-millimeter-bubble injection on the transition
to turbulence in the natural convection boundary layer along a vertical plate in water. In particular, we focus on the relationship
between the bubble injection position
L and the transition to turbulence in the natural convection boundary layer. The bubble injection positions used in our experiments
are
L = 1.6 and 3.6 mm. Bubble injection at
L = 1.6 mm delays the transition to turbulence in the natural convection boundary layer, while that at
L = 3.6 mm accelerates the transition to turbulence in the boundary layer. In the case of
L = 1.6 mm, the appearance region of the liquid velocity fluctuation in the bubble-induced upward flow in the upstream unheated
section is restricted to near the wall, although the peak of the liquid velocity fluctuation is high. In contrast, in the
case of
L = 3.6 mm, the relatively large liquid velocity fluctuation is distributed widely over the laminar boundary layer width. These
results suggest that the effect of the liquid velocity fluctuation on the laminar boundary layer is quite different between
L = 1.6 and 3.6 mm. It is therefore expected that the transition to turbulence in the natural convection boundary layer for
the case with bubble injection is dependent on the magnitude and appearance region of the liquid velocity fluctuation in the
bubble-induced upward flow in the upstream unheated section.
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