Galvanic effects on cavitation damage for 1Cr13 stainless steel

Based on the electric-spark principle, a new kind of cavitation damagetesting apparatus was designed, which could produce the bubble. Bubbles collapsed and acted on asmall surface area of the alloy surface, so an intensive electrochemical inhomogenous state would beinduced and lead to the existence of surface galvanic effect on the alloy. The galvanic effect of1Cr13 SS during cavitation damage has been quantitatively investigated in NaCl solution. The resultsshow that intensive galvanic effect of 1Cr13 SS during cavitation damage appears the potential ofthe damaged area shifting negatively with the bubble collapse and simultaneously the galvaniccurrent generates. The mass loss of alloy due to cavitation increases with the bubble quantity, andincreases with the increasing of Cl- content. And both the total mass loss and the mass loss due togalvanic corrosion increase linearly with the area ratio of cavitation damaged to undamaged area.Such surface galvanic effects speed up the dissolution rate of the alloy, which, in turn,accelerates the mechanical damage, and aggravates the mass loss of the alloy. It has been furtherconfirmed that the mass loss due to the surface galvanic effect can be decreased by catholicprotection.

QUANTITATIVE STUDY ON GALVANIC EFFECTS DURING CAVITATION DAMAGE OF 1Cr13 STAINLESS STEEL

The influence of the surface galvanic effects of 1Cr13 SS during cavitation damage has been investigated in NaCl solutions. The results show that the solution concentration and area ratio of bubble unacted surface to the bubble-acted surface greatly influence the cavitation damage of the alloy. The surface galvanic effects can speed up the dissolution rate of the alloy, and the mass loss per area of the alloy increases linearly with the area ratio of bubble unacted surface to the bubble acted surface of the alloy.

EFFECTS OF ENTRAINED AIR MANNER ON CAVITATION DAMAGE

Early in 1953 the experiments by Peterka proved that air entrainment has effects on decreasing cavitation damage. This technology has been widely used in the release works of high dams since the inception of air entrainment in the Grand Goulee Dam in 1960. Behavior, mechanism and application of air entrainment for cavitation damage control have been investigated for over half century. However, severe cavitation damage happened due to complex mechanism of air entrainment. The effects of air entrainment are related to many factors, including geometric parameters, hydraulic parameters and entrained air manners. In the present work an experimental set-up for air entrainment was specially designed, the behavior of reducing cavitation damage was experimentally investigated in the three aspects of entrained air pressure, air tube aera and air tube number. The results show that magnitude of reduction of cavitation damage is closely related to the entrained air tube number as well as entrained air pressure, air tube aera, and that the effect through three air tubes is larger than that through single air tube although the entrained air tubes have the same sum of tube aera, that is, 1 ＋ 1 ＋ 1 〉 3. Therefore, it is important to design an effective manner of air entrainment.

Critical size effect of sand particles on cavitation damage

The critical size of the sand particles in liquid is determined by means of the special vibratory apparatus, and it is related to various effects on the cavitation damage. The increase of the sand size or concentration would aggravate the cavitation damage if their sizes are larger than this critical size, conversely, this damage would be relieved.

STUDIES ON GREY MODEL DYNAMIC PREDICTION TO CAVITATION DAMAGE OF TURBINES

In thus paper, cavitation damage of turbines is firstly analyzed andpredicated from the view point of system engineering by use of the grey system theory. A detaileddescription of modeling, dynamic prediction and accuracy test for the grey system of turbines wasgiven. Two special cases for grey dynamic prediction as well as its accuracy test were illustrated.The results fall into the grade I, the accuracy is satisfactory and its prediction requirements canbe met well. It shows that the grey system theory can be employed to predict the cavitation damageof turbines, and it can provide a scientific basis for timely investigation and maintenance ofturbines.

DISCUSSION ON MECHANISM OF CAVITATION DAMAGE ON SUPER HIGH DAMS

In this short paper the following viewpoints are discussed: it is possible in super high dams that the discharge flow, either in discharge tunnel or in open channel, may be an air water mixtures (two phase flow) in which the sound speed could reduce sharply depending on concentration of air, and the shock wave could be caused somewhere in super high flow while the flow velocity reaches or exceeds the sound speed in it. Besides bubble collapse, the correlation between shock wave and cavitation bubble collapse should be considered as the main mechanism of serious damage on discharge structures of super high dams.

Cavitation-induced damage model of soft materials in exposure to high-intensity focused ultrasound

High-intensity focused ultrasound can drive cavitation bubbles that damage soft materials.A cavitation-induced damage model is developed to study the mechanical damage of soft materials by high-intensity focused ulrasound.Based on the thermodynamics framework with the acoustic radiation stress theory and the nonlinear elasticity theory,an internal variable which reflects the mechanical damage caused by cavitation bubbles is incorporated into the Helmholtz free energy.Thus.the evolution of material damages for tissue-mimicking gel is established,which is related to the cavitation process and the mechanical dissipation from a macroscopic viewpoint.The theoretical model is then applied to analyze the material damaged area observed in the experiment and the theoretical prediction is found in agreement with the existing experimental results,which shows that the damage induced by cavitation bubbles develops rapidly,then slow down and finally reaches a plateau.

ON NECESSITY OF PLACING AN AERATOR IN THE BOTTOM DISCHARGE TUNNEL AT THE LONGTAN HYDROPOWER STATION

The air entrainment for avoiding cavitation damage has been widely used in long free flow tunnels. It is crucial to determine whether an aerator is needed for shorter tunnels. In this article, the bottom discharge tunnel at the Longtan Hydropower Station was involved, for which the free flow tunnel section was only 50.00 m long. The cavitation in the tunnel with and without the aerator was investigated using the physical models of the scale 1/30, through the measurements of cavitation noise. The experimental results show that it is necessary to place the aerator at the inlet of the free flow section for higher reservoir level to protect this tunnel from cavitation damage.

EMERGENCE ANGLE OF FLOW OVER AN AERATOR

Aerator is an important device for release works of hydraulic structures with high-speed flow in order to protect them from cavitation damage. This kind of protecting effect is related closely to cavity length below the aerator, while the cavity length is dominated by the emergence angle over the aerator. Therefore it is crucial to determine this angle accurately. In the present paper the affecting intensities of flow depth and the fluctuating velocity on this angle were analyzed through two introduced parameters. Furthermore, the improved expressions of emergence angle estimation, for both ramp-type and step-type aerators, were presented by means of 68 sets of experimental data from 6 projects based on error theory. The results showed that the present method has higher accuracy than the previously reported methods.

DETACHED EDDY SIMULATION OF HYDRAULIC CHARACTERISTICS ALONG THE SIDE-WALL AFTER A NEW ARRANGEMENT-SCHEME OF THE SUDDEN LATERAL ENLARGEMENT AND THE VERTICAL DROP

In order to study the cavitation damage in a side-wall when a sudden lateral enlargement and a vertical drop are imposed at the radial gate, a new arrangement-scheme is proposed, where the sudden lateral enlargement and the vertical drop can be imposed at the outlet of the gate chamber. The hydraulic characteristics along the side-wall are simulated by the detached eddy simulation and the Volume Of Fluid （VOF） method. The numerical results agree well with those of experiment. The experimental and numerical results show that the flow condition is smooth with only a weak water-wing appearing behind the lateral cavity, and the length of the lateral cavity becomes longer and is mainly affected by the size of the lateral enlargement and the zone of negative pressure after the water impacts the side-wall would disappear. The hydraulic characteristics of the new arrangement-scheme are beneficial to the prevention of the cavitation damage in the side-wall and the Detached Eddy Simulation （DES） with the VOF method can well predict the hydraulic characteristics after the new arrangement-scheme of the sudden lateral enlargement and the vertical drop.

RESEARCH ON A MORE RATIONAL CONFIGURATION OF THE INVERT CURVE OF A SPILLWAY

This paper is concerned with the research on the rational curve for thebucket of a spillway. A new type of rational curve against cavitation damage to spillway has beenproposed; and the numerical results show that the proposed curve (gradually and continuously variedcurvature curve, abbreviated as GCVC curve) can greatly raise the minimum cavitation number andcause the distribution of water pressure on the curved surface more reasonable. The proposed curve(GCVC curve) is simple, and can be conveniently used in practical hydraulic engineering.