Research About Impacts Of Re-entrant Jet On Cavitation Structure According To Similarity Theory

Cavitation is a phase transition process between liquid and steam which occurs in low pressure region of fluid, especially on the interface of fluid and solid face. It is an unique phenomenon happens only in liquid working medium. The cavitation occurs in hydraulic machinery will cause severe pressure fluctuation and even structure distribution. Since the cavitation phenomenon has been discovered a century ago, it has always been an important aspect in research of hydraulic machinery. In our nation's CAP1400 nuclear power unit, the main coolant pump is one of the key components, excellent cavitation performance is required in the design of the main coolant pump.With the development of cavitation as well as the growth of cavity and changing of cavitation state, re-entrant jet can always be found in the fluid, and the existence of re-entrant jet will strongly influence the structure and development of cavity. In order to investigate the influences of re-entrant jet on periodical evolution of cavity which attached to upper surface of hydrofoil, established several two dimensional meshes around hydrofoil at different attack angle. The distribution of pressure, velocity and phases of flow field with the existence of cavitation was analyzed by computational fluid dynamics method. According to analysis of different method, unsteady simulation with RNG k-? turbulence model and Schnerr-Sauer cavitation model was used to simulate 2-D hydrofoils with different inflow situations which cavitation numbers were 0.9,1.2,1.5 and attack angles were 4-degrees,10-degree and 12-degree.According to the result of unsteady simulation, the impact of re-entrant jet was analyzed from the aspects of motion features and interaction with cavity. The result showed an closed relationship between low pressure region, cavity and re-entrant jet. In the interaction between cavity and re-entrant jet, the vaporize of fluid and the impact of jet to cavity existed at the same time and formed a motion balance. A characteristic number was used to estimate the strength of re-entrant jet based on velocity, characteristic length and other features and put forward a judge basis relay on the characteristic number.The results showed that the strength of re-entrant jet was an important factor which influenced the structure of cavity, and characteristic number's order of magnitudes was correlated with different periodic of cavity and different states of cavity. Large characteristic number represented higher strength of re-entrant jet, the cavity trend to collapse and separate. Small characteristic number represented higher level of vaporization, the cavity trend to keep steady and less influenced by re-entrant jet. The characteristic number was applied to analysis the steady simulation results of 3-dementional pump cavitation. The state of cavity indicated that the result of 3-dementional simulation also met the principles summarized from 2-dementional simulation. The character of unsteady cavitation flow can also be predicated by the characteristic number summarized in paper.