CAVITATION CONTROL BY AERATION AND ITS COMPRESSIBLE CHARACTERISTICS

This paper presents an experimental investigation and a theoretical analysis of cavitation control by aeration and its compressible characteristics at the flow velocity V=20m/s-50m/s. Pressure waveforms with and without aeration in cavitation region were measured. The variation of compression ratio with air concentration was described, and the relation between the least air concentration to prevent cavitation erosion and flow velocity proposed based on our experimental study. The experimental results show that aeration remarkably increases the pressure in cavitation region, and the corresponding pressure wave exhibits a compression wave/shock wave. The pressure increase in cavitation region of high-velocity flow with aeration is due to the fact that the compression waves/shock wave after the flow is aerated. The compression ratio increases with air concentration rising. The relation between flow velocity and least air concentration to prevent cavitation erosion follows a semi-cubical parabola. Also, the speed of sound and Mach number of high-velocity aerated flow were analyzed.

PRESSURE CHARACTERISTICS OF CAVITATION CONTROL BY AERATION

This experimental investigation was systernatically conducted with the aid of a non-circulating water tunnel in the Hydraulics Laboratory at Zhejiang University of Tech nology in China, The test velocity is between 20m/s and 40m/ s. The least air concentration to prevent cavitation erosion lies between 1. 7% and 4. 5%. Pressure waveforms with and without aeration in cavitation and cavitation erosion regions were measured. Time-averaged pressure profiles with and without aeration were compared. Pressure characteristics cotresponding to least air concentration to prevent cavitation erosion in cavitation and cavitation erosion regions were analyzed.

An experimental investigation of transient cavitation control on a hydrofoil using hemispherical vortex generators

Unsteady cavitation causes noise,damage,and performance decline in the marine engineering and fluid machinery systems.Therefore,finding a method to control the cavitation and its destructive effects is important for the industrial applications.In this work,we proposed a passive method to control the unsteady behavior of transient cavitation at the medium Reynolds number.For this aim,we performed an experimental study using a high-speed camera to analyze the effects of hemispherical vortex generators(VGs)on the cavitation dynamics around a hydrofoil surface.In addition,the pressure pulsations induced by the collapse of the cavity structures in the wake region of the hydrofoil were captured with a pressure transducer mounted on the wall downstream of the hydrofoil.The results showed that the instability behaviors of the cavity structures on the hydrofoil were mitigated using the proposed cavitation passive control method.In addition,the pressure pulsations in the wake region of the hydrofoil were reduced significantly.It can be concluded that the suppression of cavitation instabilities can improve the operating life and reliability of the marine and hydraulic systems.

Effect of water injection on the cavitation control:experiments on a NACA66(MOD)hydrofoil

The objective of this work is to investigate experimentally controlling cavitating flow over NACA66(MOD)hydrofoils by means of an active water injection along its suction surface.The continuous water vertically jets out of the chamber inside the hydrofoil through evenly distributed surface holes.Experiments were carried out in cavitation water tunnel.using high-speed visualization technology and the particle image velocimetry(PIV)system to study the sheetlcloud cavity behaviors.We studied the effects of this active control on cavity evolution with four kinds of jet flow at two different jet positions.We analyzed the effect of water injection on the mechanism of the cavitating flow control.The results were all compared with that for the original hydrofoil without jet and show that the active jet can effectively suppress the sheet/cloud cavitation characterized by shrinking the attached cavity size and breaking the large-scaled cloud sheding vortex cavity into small-scaled ones.The optimum effectiveness of cavitation suppression is affected by the jet flow rates and jet positions.The water injection at flow rate coefficient 0.0245 with the jet position of 0.45C reduces the maximum sheet cavity length by 79.4%and the cavity shedding is diminished completely,which gives the most superior effect of sheet cavitation suppression.The jet blocks the re-entrant jet moving upstream and weakens the power of re-entrant jet and thus restrains the cavitation development effectively and stabilizes the flow field.

An investigation of cavitation control using a porous material on a hemispherical cylinder at various cavitation numbers

In this paper,a passive control method based on a porous material is applied to the surface of a hemispherical cylinder to control a cavitating flow,and the control effect of this method at different cavitation numbers(σ)is evaluated through the cavity morphology and volume,which is important for the application in engineering.The results indicate that the control effect is improved with a reduction in the cavitation number,for the reduction of vapor volume increases from 22%–50%withσdecreasing from 0.50–0.20.Further investigation indicates that the cavity inception at different cavitation numbers is still induced by the Kelvin-Helmholtz instability,while the spatial distribution of the vapor changes significantly.Moreover,the porous material suppresses the cavitating flow in the front region but enhances it downstream at large cavitation numbers.Whenσ=0.20,the cavitating flow is controlled in both the front and rear regions.

Passive control of cavitating flow around an axisymmetric projectile by using a trip bar

Quasi-periodical evolutions such as shedding and collapsing of unsteady cloud cavitating flow, induce strong pressure fluctuations, what may deteriorate maneuvering stability and corrode surfaces of underwater vehicles. This paper analyzed effects on cavitation stability of a trip bar arranged on high-speed underwater projectile. Small scale water tank experiment and large eddy simulation using the open source software Open FOAM were used, and the results agree well with each other. Results also indicate that trip bar can obstruct downstream re-entrant jet and pressure wave propagation caused by collapse, resulting in a relatively stable sheet cavity between trip bar and shoulder of projectiles.

Experimental and CFD investigations of choked cavitation characteristics of the gap flow in the valve lintel of navigation locks

The cavitation is ubiquitous in the water delivery system of high hydraulic head navigation locks.This paper studies the choked cavitation characteristics of the gap flows in the valve lintel of the navigation locks and analyzes the critical self-aeration conditions.The cavitation gap flow in the valve lintel is experimentally and numerically investigated.A visualized 1:1 full-scale slicing model is designed,with a high-speed camera,the details of the cavitation flow is captured without the reduced scale effect.Moreover,the numerical simulations are conducted to reveal the flow structures in the gap.The experimental results show that the flow pattern of the gap flow in the valve lintel could be separated into four models,namely,the incipient(1)the developing,(2),the intensive,(3),and the choked(4)cavitation models.The numerical simulation results are consistent with the experimental data.The choked cavitation conditions are crucial to the gap flow in the valve lintel.When the choked cavitation occurs,the gap is entirely occupied by two cavitation cloud sheets.The gap pressure then decreases sharply to the saturated water vapor pressure at the operating temperature.This water vapor pressure is the ultimate negative pressure in the gap that remains unchanged with the continuous decrease of the downstream pressure.The volumetric flow rate reaches a peak,then remains constant,with the further decrease of the pressure ratio or the cavitation number.At the choking point,the volumetric flow rate is proportional to the root mean square of the difference between the upstream pressure(absolute pressure)and the saturated pressure of the water.Moreover,the pressure ratio is linearly correlated with the downstream cavitation number with a slope of(1+ζc).

Experimental study on effects of air injection on cavitation pressure pulsation and vibration in a centrifugal pump with inducer

Cavitation commonly induces performance deterioration and system vibration in many engineering applications.This paper aims to investigate the effects of air injection on cavitation evolution,pressure pulsation and vibration in a centrifugal pump with inducer.In this paper,the high-speed camera is used to capture the gas flow pattern and cavitation evolution process in the inducer.The impacts of air injection on the inlet pressure pulsation and vibration are also investigated.The results show that the cavitation development in the inducer undergoes four patterns:incipient cavitation,sheet cavitation,cloud cavitation and super cavitation.During the development of cavitation,the main frequency of the pressure pulsation shifts to lower frequencies,and the amplitude of the vibration increases.In addition,air injection promotes the incipient cavitation but delays the cavitation development.A small amount of air can effectively decrease amplitudes of pressure pulsation and vibration.But as the air content increases,the fluctuations and amplitudes of pressure pulsation and vibration increase.