Variations of Performance and Pressure Pulsation During Cavitation in Centrifugal Pumps with Entrained Air

Most of the research on the cavitation with entrained air has focused on the military direction, but it ,about centrifugal pumps, which is relevant to people's livelihood, is still relatively lacking. In order to study the basic law of the development of cavitation inside centrifugal pumps under aeration conditions, a test bench suitable for cavitation experiments with incoming flow containing gas was obtained. Furthermore, a single-stage single-suction 6-blade centrifugal pump was used as the research object to conduct pressure pulsation experiments under cavitation condition when the incoming flow was 1.0% air viod fraction at 2900r/min-50m3/h. The results showed that: After cavitation happened, the greater aeration content will deteriorate the pump's anti-cavitation performance, but the head curve is more gentle in falling down compared to natural cavitation. Hence aeration has a beneficial effect on the performance degradation of the pump under the cavitation condition. At the same time, before the cavitation number drops to the fracture cavitation number of the pump, aeration has improvement in the efficiency of the pump in different degrees , especially in the situation with the ventilated rate of 1.0%. The main frequency of pressure pulsation at the inlet and outlet of the test pump after aeration is dominated by the blade frequency. The shaft frequency signal at the outlet gradually decreases with the cavitation number lessened. Moreover the amplitude of the blade frequency grows slightly with the reduction of the cavitation number. But it tends to soar when the cavitation number is less than the fracture cavitation number.

Effect of cavitation and free-gas entrainment on the hydraulic performance of a centrifugal pump

An experimental study on gas–liquid two-phase flow characteristics in a low-specific-speed centrifugal pump is presented via employing multiple investigation techniques, such as visualization observation, measurements of acoustic emission and vibration, etc. Specially, three different flow conditions were inspected, namely gas locking initiation, critical cavitation with/without free-gas presence, etc. For gas locking, the drastic deterioration of the pump performance and the disrupted balance of shaft were observed. Especially, at low rotational speeds, the gas locking accompanied with intermittent or churn flow can be triggered by even lower inlet gas volumetric fractions. When it came to the cavitation flow, a small amount of gas entrainment could induce the rapid deterioration of cavitation and stimulate much higher amplitude in low-frequency band of shaft rotation. The relationship between the gas bubble trajectory and the vibration level under the backflow is discussed. The results reveal that the combined effect of the free-gas entrainment and cavitation on the pump instability is much stronger than that under natural cavitation or free-gas entrainment flow, whose fault diagnosis can be determined by the data manifested in different spectral segments.

Spatiotemporal Evolution of Rotational Natural Cavitation in Rotational Supercavitating Evaporator for Desalination

A novel desalination device named rotational supercavitating evaporator (RSCE) has been proposed and designed by utilizing supercavitation effect. With special focus on the spatiotemporal evolution of rotational natural cavitation, the hydrodynamic characteristics of cavitating flows in RSCE under different rotational speeds are studied by the visualization experiments and three-dimensional steady numerical simulations. The results of the visualization experiments show that with increasing rotational speed, the cavity morphology develops from several transient isolated bubbles moving with the blades, to blurred partial cavity, and finally to transparent supercavity with nearly constant size. Numerical simulation can predict the development of the cavity morphology in the experiment qualitatively and quantitatively. Vapor phase structures are shed at the tail of the cavity due to the reentrant jet, which are in the forms of single smaller bubbles and U-shaped vapor phase structures under lower rotational speeds and of cavitation clouds and cavitating filaments containing strings of bubbles under higher rotational speeds. Vortex structure is captured based on Q-criterion and encloses the cavity in the radial direction, wherein the periphery of the cavity is enclosed by a single tip vortex tube which can explain the generation of drifting stripe-shaped cavity under higher rotational speeds due to tip vortex, and the cavity tail is enclosed by two vortex tubes split from the single tip vortex tube. A power-law empirical formula for the dimensionless supercavity length versus the cavitation number considering the effect of rotation is obtained by fitting the experimental data on fully developed supercavitation.

Numerical Study on Ventilated Cavitation Influenced by Injection of Drag-Reducing Solution

The influence of injection of drag-reducing solution on ventilated partial cavitation and supercavitation for an axisymmetric underwater vehicle is analyzed by numerical simulation. Turbulence, cavitation and multiphase models are SST k-ω, Schnerr-Sauer and Mixture models, respectively. The Cross viscosity equation is adopted to represent the fluid property of aqueous solution of drag-reducing additives. First of all, for non-cavitating conditions, the pressure distribution is obtained to determine the positions of injecting drag-reducing solution and ventilation. Then natural cavitation at different cavitation numbers is investigated for acquiring inception cavitation number. Finally, numerical simulations are conducted on the ventilated cavitating flows with and without the injection of drag-reducing solution at the cavitation number slightly smaller than the inception cavitation number (partial cavitation) and much smaller than the inception cavitation number (supercavitation). It is shown that for partial cavitation, the shape of cavity with the injection of drag-reducing solution is larger and the resistance of underwater vehicle decreases in comparison with the case without the injection of drag-reducing solution. However, for supercavitation, just viscous drag force obviously decreases, while cavity shape does not change.

Research on Hydro-Dynamic of Column Body Water-Entry with Six-DOF at High-Speed

By using the commercial CFD software Fluent6.3, the three-phase (water, air, vapour) flow field with natural cavitation was established. The UDF(User Defined Function) was used to resolve and control the missiles movement. It realized the numerical calculation of unsteady high-speed water-entry flow field, which consider natural cavitation as well as the missiles multiple degree of free movement. In this paper, the missile high-speed water-entry flow field with different attitude angle were simulated and analyzed, and the influence of the attitude angle on the flow field were gained.

Meshing Method of Calculating Water-Entry Flow Field at High Speed

An important part of the numerical simulation is the grid which the quality has great influence on the calculation precision, and also the influence often is crucial factor in most of situation. Water-entry at high speed is a complex unsteady process, and its numerical simulation needs to take consider of natural cavitation as well as rotation of the underwater body. In this paper, a new meshing method was given with using the Layering, Smoothing and Remeshing for calculating the unsteady flow field. Numerical simulation shows that the mesh given in this paper has better quality, and can be used to calculate the multi-phase mode of water-entry at the high speed.

Evaluation on the Application of Two-Fluid Multiphase Model to Supercavity

The lubrication of external liquid with supercavity has been the goals of specialists for many years. Either ventilated cavity or natural cavity is firstly related to multiphase flow. In this paper, in order to quantitatively predict the cavitating flow especially the ventilated supercavity and understand the structure of flow field in the cavity, two kinds of multiphase model including the homogeneous flow model and the two-fluid model were adopted separately. Besides, SST and DES turbulence model are used for steady and unsteady simulations. By comparing the simulating results with experimental results in water tunnel, the two-fluid model was proved to have the high accuracy in predicting the ventilated supercavity including the cavity shape and lift coefficient of the vehicle. On the other hand, for natural cavitation, the mixture model and the two-fluid model have little difference in predicting the pressure coefficient, however, the two-fluid model can give more detailed flow field.