An experimental study on bubble rise path after the departure from a nucleation site in vertical upflow boiling

Visual study was conducted to elucidate the rise characteristics of vapor bubbles after the departure from a nucleation site in forced convective boiling. The flow direction was vertical upward and nearly saturated water was used as a working fluid. The outer surface of a transparent glass tube of 20 mm in inner diameter was electrically heated to generate vapor bubbles inside of the tube. The number density of bubbles in the test section was kept low to observe the behavior of individual bubbles using high speed cameras. The cross-sectional area-averaged velocity of bulk flow was set at 0.5 and 1.0 m/s. The approximate maximum diameter of each bubble was 1.6-2.9 mm. The following observations were made on the behavior of typical bubbles generated in the present experimental conditions: (i) bubbles grew at a nucleation site for a short period of time less than 1 ms with the shape flattened along the vertical heating surface; (ii) bubbles then slid upward the vertical wall for a few millimeters with the gradual increase in their size; (iii) after that, bubbles were detached from the heating surface and migrate towards the bulk flow but they remained close to the wall; (iv) at the instant of detachment, bubble rise velocity was already comparable with the local velocity of surrounding liquid and the rapid growth seen immediately after the nucleation was already completed; (v) after the detachment, bubble size was decreased in slightly subcooled bulk fluid and some of them were collapsed; (vi) bubbles that did not experience the collapse in subcooled liquid reversed the direction of radial migration and was eventually reattached to the wall; (vii) after the reattachment, bubbles generally slid upward the vertical surface for a long distance with the gradual growth due to the heat from the heating wall, though the bubbles that experienced larger gap with the wall after the first detachment showed rebounding motion with the reduced amplitude. The detachment from the wall and the reattachment to the wall of bubbles were clearly observed as the typical bubble behavior after the departure from a nucleation site in the present experiments. It was expected that the inertia force and shear induced lift force have the important roles in these phenomena.