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Effect of the Submerged Vortex Cavitation Occurred in Pump Suction Intake on Hydraulic Forces of Mixed Flow Pump Impeller
No full textThe effect of submerged vortices on the hydraulic forces of a mixed flow pump impeller was studied using a model pump with an intake sump. The experiments comprise observation of submerged vortices reaching the impeller with a high-speed video camera and measurement of the velocity distribution around the submerged vortex by PTV (particle tracking velocimetry) to obtain the strength of the vortex. Measurement results are compared with CFD (computational fluid dynamics) calculation to evaluate their accuracy. Hydraulic forces on the model pump when the submerged vortex reached the pump impeller was measured with load cells. The relationship between the strength of the submerged vortex and the effect of the vortex on the fluctuation of the hydraulic forces are describe
Tip-Leakage Vortex Inception on a Ducted Rotor
No full textThe tip-leakage vortex occurring on a ducted rotor was examined using both three component Laser Doppler Velocimetry (LDV) and planar Particle Imaging Velocimetry (PIV). The vortex strength and core size were examined for different vortex cross sections downstream of the blade trailing edge. The variability of these quantities are observed with PIV and the average quantities are compared between LDV and PIV. Developed cavitation is also examined for the leakage vortex. The implication of vortex variability on cavitation inception is discussed
A Numerical Study of Bubbles Captured by a Gaussian Vortex
No full textThe capture mechanics of cavitation nuclei by a Gaussian Vortex is explored. The capture process is examined by both Direct Numerical Simulation of the Navier Stokes equations and a Particle Tracking model. We also investigate the parameters that effect the capture and bubble growth process, and finally we compare the results of the two approaches. The DNS results show that the shape of the bubbles can be significantly distorted as they are captured by a vortex. This distortion can lead to the creation of lift forces that shorten the capture time of the bubbles. The capture times computed with the two methods varied over less than an order of magnitude for most cases, and this suggests that one might derive and average capture time. The exact results of the particle tracking models can vary widely depending on the selected values of the lift and drag coefficients and nominal Weber number of the captured nucleus. For model scale flows, the Weber number may vary from 1 to 100. Thus, it is possible that a particle-tracking model should have variable lift and drag coefficients that better take into account the estimated instantaneous bubble deformation. The drag coefficient by Haberman and Morton (1953) yielded a trajectory that was closer to the DNS trajectory than when using the drag coefficient given by Clift et al.(1978). It was determined that the lift coefficients given by Dandy and Dwyer (1990) and Auton et al. (1988) yielded capture times that were much bigger than the DNS results and that lift coefficient between the those given by Saffman (1965) and Dandy and Dwyer (1990) would give results that are closer to the DNS results
Dynamics of a Cavitating Cascade
No full textA method for solving nonlinear problems on unsteady cavity flows within the framework of the model of viscous fluid is proposed. A solution to the nonlinear problem on the unsteady cavity flow past a cascade is presented. Based on linearization of the completely nonlinear solution, a system of equations for calculating the inducer transfer matrix can be obtained
Energy Evaluation of Shock Wave Emission and Bubble Generation by Laser Focusing in Liquid Nitrogen.
No full textWhen the energy level of a laser beam at the focus exceeds an irradiance threshold the optical breakdown occurs, followed by a series of high-speed phenomena such as plasma formation, shock wave emission and vapor bubble generation. In this paper the energy evaluation associated with these phenomena has been made for laser focusing in liquid nitrogen. It is found that the threshold laser energy capable of bubble formation has an extent with some probability, tending to decrease with increasing the applied pressure. The mechanical energies of shock wave emission and bubble generation have been investigated
Comparison of Two Computational Methods for High-Velocity Cavitating Flows Around Conical Projectiles:OTi-HULL Hydrocode and Two-Phase Flow Method
No full textWe present a comparison between two CFD methods for the axisymmetric case: the hydrocode named OTi-HULL and the two-phase flow model presented in the Cocchi's thesis. These methods are applied to the calculation of a test case: a biconical body (nose cone with a cylindrical part and rear cone) at a velocity of 3000 m/s in water.
Cocchi's results are comparable to the HULL ones outside the cavity, but inside the cavity the HULL results are wrong in temperature (too high) and in density (too low) compared to Cocchi's.
For the cavity width, the results show an acceptable agreement between the twm methods for the cones with the half-angles of 45° and 30°. But for the 15° cone there are some discrepancies: the cavity is 10% larger for Cocchi's results in which the phase change of water in taken into account. These results have to be confirmed by experiments
Partial Cavity Instabilities and Re-Entrant Jet
No full textThe purpose of this talk is to give an overview of different instabilities which can affect partial cavitation. We shall focus on the case of a unique partial cavity attached to an isolated hydrofoil or to the throat of a Venturi. Another paper presented by Tsujimoto (2001) in this conference is dedicated to cavitation instabilities in turbomachinery, where several cavities are simultaneously present and can interact. In this paper, we have deliberately chosen to emphasize some particular aspects of partial cavity instabilities, especially the re-entrant jet instability and the associated cloud cavitation. Some discussions reflect our personal point of view on a subject still in progress
Evaluation of Cavitation Erosion based on Erosion Particles
No full textCavitation erosion mechanisms were studied through the observation of removed particles for annealed S15C (equivalent to AISI 1015) steel and heat-treated S55C (AISI 1055) steels. In the initial and the incubation stages, single impact loads removed many small sharply-edged particles. During the acceleration and the maximum rate stages, large striated particles were observed due to cyclic loads. The volume fraction of particles exhibiting fatigue fracture in these stages amounts to 70 or 80% irrespective of the material including pure copper and pure aluminum. The exponent of the crack growth rate determined from the fracture is almost the same as that obtained from a regular fatigue test. The fatigue crack growth rate for many metals is inversely proportional to the square of Young's modulus, E^(-2). The particles fall off from the protrusive surface and their sizes depend on the unevenness in relation to the hardness of the material. The average diameter of erosion particles decreases inversely with the square root of Vickers hardness, HV^(-1/2). Therefore, the volume is proportional to HV^(-3/2). Thus, the dependence of the volume loss rate in the maximum rate stage is well described by HV^(-3/2)•E^(-2). The conclusion is that cavitation erosion can be evaluated in terms of the hardness of the material and the fatigue crack growth rate
A Generalized Compressible Cavitation Model
No full textA new multi-phase model for low speed gas/liquid mixtures is presented; it does not require ad-hoc closure models for the variation of mixture density with pressure and yields thermodynamically correct acoustic propagation for multi-phase mixtures. The solution procedure has an interface-capturing scheme that incorporates an additional scalar transport equation for the gas void fraction. Cavitation is modeled via a finite rate source term that initiates phase change when liquid pressure drops below its saturation value. The numerical procedure has been implemented within a multi-element unstructured framework CRUNCH that permits the grid to be locally refined in the interface region. The solution technique incorporates a parallel, domain decomposition strategy for efficient 3D computations. Detailed results are presented for sheet cavitation over a cylindrical headform and a NACA 66 hydrofoil
Similarities and Geometrical Effects on Rotating Cavitation In Two Scalted Centrifugal Pumps
No full textTwo scaled centrifugal pumps with vaneless radial diffuser running at identical Reynolds-Numbers are in the focus
of experiments at Technical University of Braunschweig and Darmstadt University of Technology in the frame of a
cooperation hosted by the DFG (Deutsche Forschungsgemeinschaft). Both pumps can hold different runner
geometries as well as different leading edge geometries within the same runner. This paper describes experimental
investigations of different configurations in both pumps. All configurations show the occurrence of a periodic
cavitation state called "rotating cavitation" in a wide range of part load conditions, which onset can be characterized
by an almost constant value of the dimensionless parameter ó/2á. Comparison of the main characteristics of both
pumps as well as optical investigations to determine the dynamic properties in cavitating conditions have been carried
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