Apparent mass torque coefficients for fluctuations of the flow rate and angular velocity are determined experimentally for two-dimensional centrifugal impellers. Nearly sinusoidal fluctuations of the flow rate and angular velocity are given by using crank mechanisms, and the resulting torque on the impeller is measured. The torque is divided into components in-phase and out-of-phase with the displacements. The in-phase component is attributed to the apparent mass effect and is represented by the apparent mass torque coefficients. Drag torque coefficients are defined and used to represent the out-of-phase components. The apparent mass coefficients are compared with theoretical values obtained under the assumption of a two-dimensional potential flow. The experimental values are 5 to 20 per cent larger than the theoretical ones, and no appreciable effects of the frequency and the amplitude are observed within the range varied in the experiments.
This paper addresses the rotordynamic instability of an overhung rotor caused by a hydrodynamic moment due to whirling motion through the structural coupling between whirl and precession modes. First, the possibility of instability is discussed based on a vibration model in which the hydrodynamic forces and moments are assumed to be smaller than structural forces with the structural coupling being represented by a structural influence factor. Then, the fundamental characteristics of rotordynamic moment on the backshroud of a Francis turbine runner under whirling motion were studied using model tests and numerical calculations. The runner is modeled by a disk positioned close to a casing with a small radial clearance at the outer periphery. The moment is caused by an inward leakage flow that is produced by an external pump in the model test. The experiments were designed to measure the rotordynamic fluid force moments under various leakage flow rates with various preswirl velocities and various axial clearances between the backshroud and casing. The computation was carried out based on a bulk flow model. It was found that the fluid force moment is generally destabilizing, except for a small region of positive whirling speed ratios.
This paper describes a new time marching calculation of blade surface cavitation based on a linearized free streamline theory using a singularity method. In this calculation, closed cavity models for partial and super cavities are combined to simulate the transitional cavity oscillation between partial and super cavities. The results for an isolated hydrofoil located in a 2-D channel are presented. Although the re-entrant jet is not taken into account, the transitional cavity oscillation with large amplitude, which is known to occur when the cavity length exceeds 75 percent of the chord length, was simulated fairly well. The partial cavity oscillation with relatively high frequency was simulated as damping oscillations. The frequency of the damping oscillation agrees with that of a stability analysis and of experiments. The present calculation can be easily extended to simulate other cavity instabilities in pumps or cascades.
A three-dimensional linear analysis of rotating stall is carried out to clarify the effects of spanwise distribution of cascade characteristics. A semi-actuator disk model of annular cascade is employed. Linear spanwise distributions of the local performance slope are considered to examine the effects of the three-dimensionality in the cascade characteristics. Many eigenvalues were found corresponding to various kinds of radial mode. It was found that the 0th order radial mode is more unstable than higher order modes. It was also found that uneven spanwise distribution of cascade characteristics makes the 0th mode more unstable, and makes the amplitude of the disturbance larger at radial locations where the positive slope of the performance is larger.
The effect of blade tip geometry on unsteady cavitation was investigated in a 3-bladed inducer. Experiments in inducers with and without the blade tip rounding on the pressure side were carried out and the results were compared. Through the measurement of inlet pressure fluctuation, it is found that unsteady cavitation is suppressed in the inducer with rounded tip compared with that with flat tip at relatively high flow rates. Backflow in each inducer was also investigated for various cavitation numbers because backflow may be also affected by tip geometry. It is clarified that backflow in the inducer with rounded tip is a little greater than that with flat tip and the backflow becomes weaker with the decrease of cavitation number.
Abstract Cavitation instabilities such as rotating cavitation and cavitation surge often occur in high speed turbopumps. In the present study, numerical simulation of cavitation surge, which numerically solves the one-dimensional momentum and continuity equations with modelled dynamic cavitation characteristics, is conducted. The phase lag of cavity response against the inlet pressure and the suction flow rate variations is modeled in the form of the first-order lag system considering that the frequency of cavitation surge is small. Differently from linear stability analysis made in our previous study, the equations are solved in time domain with retaining some of non-linear terms. The method is validated through the comparison with the stability analysis. Then the effect of phase lags in dynamic cavitation characteristics is examined.
The present paper treats the backflow vortex structure observed at the turbomachinery inlet at reduced flow rate. It is caused by the roll-up of the shear layer between swirling backflow and axial main flow. In order to verify this, a simple model test was carried out in which the effect of impeller was represented by an axisymmetric swirling backflow. In the present paper, the flow field of a simplified model test is simulated by using LES calculations to investigate detailed flow structure. The computed results are compared with experimental results.
The present paper describes about the effects of the inequality of blade spacing on the steady cavitation and its stability.The development process of the cavity in the cascade with unequallyspaced blades is more complex in comparison with that in the cascade with equally-spaced blades.The development process can be reasonably explained by the interaction between the flow near cavity trailing edge and the leading edge of adjacent blade.The minimum cavitation number of the region where the cavitation is stable can be decreased by unequalizing the blade spacing.Various unstable cavitation appears in the low cavitation number where the cavity is longer than the 65% of blade spacing.
