Abstract: The liquid-vapor two-phase cavitating flow from the middle section to the end of throat tube is observed and verified to be homogeneous froth flow when jet pump works under the operating limits. Based on the assumption of the steady isothermal horizontal flow and the Wood sound velocity equation, the one-dimensional governing equation of homogeneous froth flow from the middle section to the end of throat tube under the operating limits, is derived. With the aid of the measurement data of pressure distribution along the wall of the jet pump, the Mach number of the liquid-vapor two-phase flow from the middle section to the end of throat tube is calculated by the derived governing equation. It is found that the Mach number increases gradually from the middle section to the end of throat tube, and reaches the maximum value 0.94, very close to 1, at nearby the lowest point of pressure. Further analysis indicates that under the operating limits, the liquid-vapor cavitating flow from the middle section to the end of throat tube performs as critical two-phase flow, and the flow velocity reaches the local sound velocity of the liquid-vapor two-phase flow. The flow is choked and results in the appearance of the operating limits that the entrained flow rate will keep unchanged and not increase with the decrease of outlet pressure under a certain driving pressure. The mechanism of flow within a jet pump under operating limits is revealed and this is of significance to the further research work.

Key words: Cavitation, Critical flow, Jet pump, Liquid-vapor two phase flow, Operating limits, Wood sound velocity equation