Characteristic Behavior of Boiling Bubble Initiation Under High Pressure Conditions
This report describes the experimental results of vapor and liquid two-phase flow characteristics focusing on the bubble diameter dependence on the pressure. Though two-phase flow research has been conducted associated with nuclear power technology, the results are limited to low pressure conditions, and actual plant behaviors, under operational pressure conditions as high as 7MPa, have not been clarified. This study is intended to clarify the higher pressure two-phase flow phenomenologically. To reach this goal, the boiling behaviors are directly visualized. The originality of this report is that it deals with high pressure boiling phenomena with forced flow, which have not been previously reported. To make direct visualization easily, the authors developed unique test apparatus with higher pressure glass windows. The pressure vessel was made of stainless steel (I.D. 0.1m and 0.3m height). The non-direct heater rod (O.D 10mm and 100mm heated length) was submerged vertically inside the transparent water jacket and heater power was increased so as to visualize the initiation boiling through the window. Pressure as the main test parameter was varied from 1 to 7MP by supplying nitrogen . Other important test parameters were flow conditions (i.e. stagnant flow, natural circulation, and forced flow). The 7MPa bubble behaviors could be successfully visualized. The bubble diameter was confirmed to be reduced monotonically with pressure. However, its decrement was not constant with pressure. Under a lower pressure range within 1MPa (i.e. between 0.1 and 1MPa), the diameter was reduced from 1mm to 0.3mm, whose behavior seemed to be consistent with that of ideal (i.e. pv=const). After 1 MPa, in contrast, the bubble diameter seemed to be kept constant and become asymptotic to 0.16mm. Moreover, with convective flow, bubble diameter became somewhat smaller than those without flow, while the difference was quite small. The study further proceeded to clarify the mechanistic model of the bubble onset by referring to Fritz and Levy’s model. From the balance of three forces (buoyancy, drag, and surface tension), the bubble diameter was found to be adequately correlated. It is concluded that, under nuclear operational condition around 7MPa, bubble diameter was as tiny as 0.16mm, which was almost constant from about 2MPa. Conversely, real plant two-phase flow behaviors may be simulated by the 2MPa flow.