Effect of total phosphorous pollution on cavitation pressure characteristics of working water used in turbines

Many experiments have proven that with increasing silt and gas(out-of-phase media)content in water,the cavitation characteristics change and the cavitation pressure increases.Currently,many large reservoirs in China are polluted by total phosphorus(TP)and other chemical contaminants because of the use of phosphate fertilizer runoff from agriculture.However,research regarding how chemical pollutants(in the form of out-of-phase media)affect the cavitation pressure characteristics of water is sparse.In this paper,the Goupitan Hydropower Station,the largest hydropower reservoir on the Wujiang River,which is heavily polluted by TP,is taken as an example to evaluate the effects of chemical pollution on water cavitation pressure characteristics.In this study,the cavitation pressure characteristics of polluted and clean water are compared.The results show that the cavitation pressure of water polluted by chemicals is larger than that of clean water.In a hydraulic power generation system,cavitation and cavitation erosion are likely to occur earlier in runners when the fluid is polluted.These results are of great importance to further studies of cavitation theory and can directly influence the arrangement of turbines in practical engineering.

Biosorption effects of copper ions on Candida utilis under negative pressure cavitation

Under the optimal condition of copper ions adsorption on yeast,we found some different effects among static adsorption, shaking adsorption and negative pressure cavitation adsorption, and the methods of yeast with different pretreatments also affect adsorption of copper ions. At the same time, the change of intercellular pH before and after adsorption of copper with BCECF was studied. The copper distribution was located by using PhenGreen （dipotassium salt and diacetate）, and the surface of yeast was observed by an atomic force microscope. The results showed that negative pressure cavitation can improve bioadsorption capacity of copper ions on yeast. However, the yeasts＇ pretreatment has a higher effect on bioadsorption. It indicates that heavy metal bioadsorption on yeast has much relation with its cellular molecule basis. With the adsorping, the intercellular pH of yeast increased gradually and changed from acidity to alkalescence. These results may suggest that negative pressure cavitation can compel heavy metals to transfer from the cell surface into inside cell and make the surface of yeast coarse.

The Relationship of Cavitation to the Negative Acoustic Pressure Amplitude in Ultrasonic Therapy

The relationship between the cavitation and acoustic peak negative pressure in the high-intensity focused ultrasound （HIFU） field is analyzed in water and tissue phantom. The peak negative pressure at the focus is determined by a hybrid approach combining the measurement with the simulation. The spheroidal beam equation is utilized to describe the nonlinear acoustic propagation. The waveform at the focus is measured by a fiber optic probe hydrophone in water. The relationship between the source pressure amplitude and the excitation voltage is determined by fitting the measured ratio of the second harmonic to the fundamental component at the focus, based on the model simulation. Then the focal negative pressure is calculated for arbitrary voltage excita- tion in water and tissue phantom. A portable B-mode ultrasound scanner is applied to monitor HIFU-indueed cavitation in real time, and a passive cavitation detection （PCD） system is used to acquire the bubble scattering signals in the HIFU focal volume for the cavitation quantification. The results show that： （1） unstable cavitation starts to appear in degassed water when the peak negative pressure of HIFU signals reaches 13.5 MPa; and （2） the cavitation activity can be detected in tissue phantom by B-mode images and in the PCD system with HIFU peak negative pressures of 9.0 MPa and 7.8 MPa, respectively, which suggests that real-time B-mode images could be used to monitor the cavitation activity in two dimensions, while PCD systems are more sensitive to detect scattering and emission signals from cavitation bubbles.

Numerical Simulation of Droplets Interacting with a Microcolumnar Solid Structure

The VOF method is used to simulate the dynamics of a droplet interacting with a structure consisting of an array of microcolumns mounted on a flat surface.Such a specific configuration is intended to mimic the typical properties of lotus leaves,which typically display regularly arranged micron-scale papillary structures.After setting the initial velocity of the simulated droplet on the basis of practical considerations,an analysis is conducted about the effect of the characteristic size of the microstructure on the apparent contact angle.The pressure variation in the microstructure caves is also examined.The simulation results show that the change of the contact angle of the droplet in the quasi-static state is consistent with theoretical calculations and the contact angle in the dynamic state is smaller than that in the quasi-static state.Moreover,under dynamic conditions,the pressure fluctuation in the microstructure cave depends essentially on the characteristic size of the microstructure.

Effects of Area Discontinuity at Nozzle Inlet on the Characteristics of Self-resonating Cavitating Waterjet

The current research on self-resonating cavitating waterjet（SRCW） mainly focuses on the generation mechanism and structure optimization.Researches relating to the influences of disturbances at nozzle inlet on the characteristics of the jet are rarely available.In order to further improve the performance of SRCW,effects of area discontinuity（enlargement and contraction） are experimentally investigated using three organ-pipe nozzles.Axial pressure oscillation peak and amplitude as well as aggressive erosion intensity of the jet are used to evaluate the effects.The results reveal that area enlargement and contraction affect the peak differently,depending on the inlet pressure,nozzle geometry,and standoff distance;while area contraction always improves the amplitude regardless of these factors.At inlet pressures of 10 MPa and 20 MPa,area discontinuity improves the peak at almost all the testing standoff distances,while this only happens at smaller standoff distances with the inlet pressure increased to 30 MPa.The capability of area discontinuity for improving the amplitude is enhancing with increasing inlet pressure.Moreover,the cavitation erosion ability of the jet can be largely enhanced around the optimum standoff distance,depending on the type of area discontinuity and nozzle geometry.A preliminary analysis of the influence of area discontinuity on the disturbance waves in the flow is also performed.The proposed research provides a new method for effectively enhancing the performance of SRCW.

SIMULATION AND EXPERIMENT OF BUBBLY FLOW INSIDE THROTTLING GROOVE

The relationship between pressure distribution and cavitation （noise） inside throttling groove is investigated by numerical simulation and experimental method. A valve pocket with several transducers is performed to detect the pressure distributions inside the valve chamber, and the results fit quite well with the computational fluid dynamics（CFD） analysis. High-speed imaging techniques are employed to investigate the cavitation mechanisms, in particular bubble inception and cluster formation near the throttling groove. A spectrum analyzer is used to measure the sound pressure level of noise generated by the bubble flow. It is found that the pressure distributions inside the groove are sensitive to the valve port configuration and back pressure. The pressure distribution determines the bubble size and number passing through the valve grooves and the sound pressure level of noise induced by collapsing bubbles. The inlet pressure mainly affects the saturation degree of bubbly flow inside the groove and the intensity of sound nressure level accordingly.

THE INFLUENCE OF PRESSURE FLUCTUATION AND PRURE GRADIENT ON CAVITATION INCEPTION OF IRREGULARITIES

An experimental investigation was conducted to study the cavitation inception of six isolated irregularities for negative and positive pressure gradients.The relationship between the root-mean-square of pressure fluctuation and the oscillating radius of gas bubble was determined in this paper.The results of the experiment show that the incipi- ent cavitation numbers for the same protrusions are higher and the intensities of pressure fluctuation are lower in negative gradient flows than that in positive pressure gradient flows.The peaks of pressure fluctuation are seven to twelve times that of the root-mean-square values of pressure fluctuation.