Measurement of void fraction and observation of bubble dynamics were carried out to investigate the mechanisms causing the net vapor generation (NVG) in subcooled upward flow boiling. The working fluid was distilled water and the heated surface of rather low contact angle was used in the present experiment Since no bubbles could stay at their nucleation sites even at the onset of nucleate boiling (ONB), departure of bubbles from the nucleation sites was not the cause of NVG in the present experimental setup. Following the departure from the nucleation sites, bubbles were lifted off the heated surface to be propelled into the subcooled bulk liquid at low pressures whilst they slid along the vertical heated surface at elevated pressures. As a result, the bubble life time at ONB was considerably shorter in the experiments conducted under low pressures. The distinct difference of bubble dynamics suggested that the mechanisms of NVG are dependent on the system pressure. At low pressures, some bubbles could be reattached to the surface after the lift-off at sufficiently low liquid subcooling Since the bubble life time was markedly elongated, the reattachment of bubbles to the vertical heated surface was considered to trigger the NVG. It was also found that the bubble reattachment usually took place following the coalescence of bubbles produced successively at the same nucleation site. The stochastic bubble motion induced by coalescence was hence considered to be a key phenomenon causing NVG. The NVG mechanism at high pressures is investigated in future studies.
