Effect of Nozzle Outlet Shape on Cavitation Behavior of Submerged High-Pressure Jet

A submerged high-pressure water jet is usually accompanied by severe cavitation phenomenon. An organ pipe nozzle can greatly improve the cavitation performance of the jet, making use of the self-excited oscillation of the flow. In order to study the effect of organ pipe nozzles of different nozzle outlet shapes on cavitation behavior of submerged high-pressure jet, in this paper we build a high-pressure cavitation jet experiment system and carried out a high-speed photography experiment to study cavitation cloud characteristics of a high-pressure submerged jet. Two organ pipe nozzles with and without a whistle were compared. The dynamic characteristics of the cavitation cloud was extracted through the POD method, it was found that the result effectively reflect the dynamic characteristics of the cavitation jet. The reconstruction coefficients of mode-1 obtained by the POD can better reflect the periodic time-frequency characteristics of cavitation development. The effect of the nozzle outlet shape on the cavitation behavior of organ pipe nozzle was analyzed based on unsteady numerical simulation, and it was found that the jet generated by the nozzle with a divergent whistle had a larger vorticity in the shear layer near the outlet. Further, stronger small-scale vortex and much severe cavitation occurred from the nozzle with a divergent whistle.

Numerical Simulation and Experimental Analysis of Volume Alternate Cavitation (VAC) - A new Cavitation Generation Method

Cavitation generation methods have been applied in multifarious directions due to their diversity. And scholars have carried out numerous researches and discussions on cavitation generation methods. The purpose of this study is to explore the generating mechanism and evolution law of volumetric alternate cavitation (VAC). In the VAC, the liquid water is placed in an airtight container with variable volume. With the volume alternately changes, the liquid water inside the container continues to cavitate. In this study, the mixture turbulence model and in-cylinder dynamic grid model were used to apply computational fluid dynamics (CFD) simulation of volume alternate cavitation. In the simulation, the cloud images at 7 heights on the central axis are monitored, and the phenomenon and mechanism of height and eccentricity are analyzed detailedly. By using the method of cavitation flow visualization (CFV), the generating mechanism and evolution law of cavitation are clarified. The synergistic effects of experiments and high-speed camera capture confirm the simulation. In the experiment, the volume change stroke of the airtight container is 20 mm, the volume change frequency is 18 Hz, and the shooting frequency of the high-speed camera is set to 10000 fps. The results show that the position of occurring cavitation phenomenon has a reasonable law during the whole evolution cycle of the cavitation cloud. It is evident that a cycle of volume alternation corresponds to the generation, development and collapse stages of cavitation bubbles.

Static and dynamic characteristics of a Rayleigh-steps mechanical seal with reverse steps

This paper presents a Rayleigh-steps mechanical seal with reverse steps (RS-MS), and the governing equation was solved by the finite difference method (FDM). The effects of angular misalignment, working condition parameters, and film thickness on sealing performance were discussed, including the opening force, cavitation ratio, leakage rate, frictional torque, stiffness and damping coefficients. The results indicate that the cavitation phenomenon in the reverse step groove can restrain the leakage, while it also affects the stability of the seal. The angular misalignment makes the seal have greater stiffness and damping coefficients. The stiffness and damping coefficients decrease rapidly with the increase of the film thickness, and the dynamic stability of the mechanical seal decreases with the increase of the film thickness, which is not conducive to the stable operation of the seal. The research results can guide the optimization design of mechanical seals.

Investigation the Ultrasonic Vibration on Tin Soldering Welding of Copper Wires and Plates

Ultrasonic-assisted soldering welding is widely applied for joining difficult materials. The cavitation phenomenon in liquid always occurs during the ultrasonic excitation. Base metals are striked by ultrasonic cavitation, creating erosion on the surface. The soft solder materials are penetrated on the rough surface, generated inter-metallic compounds. This work expresses the design of ultrasonic soldering machine using 20 kHz source and steel sonotrode. The curvature of reflecting plates with specific radius and their location are also condidered. The major technological parameters of ultrasonic soldering welding such as ultrasonic exciting time, power and curvature radii of reflecting plate are discussed. Tin soldering material is utilized for joining copper wires and plates are investigated. SEM images on the surface of tin soldering on cooper plates and tensile strength are investigated.

Review on the Effect of the Phenomenon of Cavitation in Combustion Efficiency and the Role of Biofuels as a Solution against Cavitation

Concerning the problem of wanting the performance of heat engines used in the automotive, aeronautics, and aerospace industries, researchers and engineers are working on various possibilities for improving combustion efficiency, including the reduction of gases such as CO, NOx, and SOx. Such improvements would also help reduce greenhouse gases. For this, research and development has focused on one factor that has a significant impact on the performance of these engines: the phenomenon of cavitation. In fact, most high-performance heat engines are fitted with a high-speed fuel supply system. These high speeds lead to the formation of the phenomenon of cavitation generating instabilities in the flow and subsequently causing disturbances in the combustion process and in the efficiency of the engine. In this review article, it is a question of making a state-of-the-art review on the various studies which have dealt with the characterization of the phenomenon of cavitation and addressing the possible means that can be put in place to reduce its effects. The bibliographic study was carried out based on five editors who are very involved in this theme. From the census carried out, it has been shown that there are many works which deal with the means of optimization that must be implemented in order to fight against the phenomenon of cavitation. Among these solutions, there is the optimization of the geometry of the injector in which the fuel flows and there is the type of fuel used. Indeed, it is shown that the use of a biofuel, which, by its higher viscosity, decreases the effects of cavitation. Most of these jobs are performed under cold fluidic conditions; however, there is little or no work that directly addresses the effect of cavitation on the combustion process. Consequently, this review article highlights the importance of carrying out research work, with the objective of characterizing the effect of cavitation on the combustion process and the need to use a biofuel as an inhibitor solution on the cavitation phenomenon and as a means of energy transition.

Cavitation on top of collision breaks the cover of coronary plaques and triggers acute coronary syndrome

Background Coronary injuries are hypothesized to be caused by the cavitation phenomenon (explosion of air bubbles) which is seen frequently in industrial pipes. Based on hydraulics principles applied to the coronary circulation. during distal negative suctioning in diastole, if the coronary static pressure decreases below the vapor pressure (VP), bubbles will form. They explode when the coronary static pressure recovers > the VP during systole. These explosions create jet waves weakening and rupturing the cover of the coronary plaques, triggering acute coronary syndrome (ACS). How could these events be observed, recorded and compared? Methods Coronary angiograms of patients with ACS and stable coronary artery disease (CAD) (control) were selected. The arteries were recorded at 15 frames per second and saved in the electronic health records and reviewed image by image. After the index artery was completely filled with contrast, the following images showed the blood in white moving in on a background of black contrast. The flow could be laminar, turbulent (mixing of blood in white and contrast in black), antegrade or RETROGRADE (black column traveling backward). At the same time, an artificial intelligence (AI) program was used to detect and identify the flow. Results There were 104 patients with ACS enrolled and 20 patients with stable CAD as control. First, in the ACS group, 84 lesions (80%) were in the end of the proximal segment of the left anterior descending artery (LAD) and mid-segment of the right coronary artery (RCA). 20 lesions (19%) were at the distal RCA. Second, during diastole, 95% of the flow were laminar. The flow became turbulent at the beginning of systole. The turbulence was caused by the COLLISION of the antegrade flow (end of diastole) and the retrograde flow (at the beginning of systole). These collisions were seen in 95% at the location of vulnerable plaques of patients with ACS. In the control patients, there were only 2 cases (10%) with collision. Third, in the 20 patients with lesions at the distal RCA, the lesions were seen to be located at the areas of recirculating flow, at the ostium of the posterior descending artery (PDA) or proximal to the origin of the PDA. The cause of turbulence was most likely due to cavitation on top of collision. The cavitation happened because of continuous steady forward flow (of the PDA) in the myocardium during systole, while at the proximal RCA the blood flew forward more slowly. (Fig.1) The DSICREPANCY of velocities at the proximal and distal RCA allowed the formation of an empty gap (bubble of air). When the flow reversed during systole, this retrograde flow slammed on the bubble which collapsed violently, injured, ruptured the cover of the plaque and started ACS. Conclusions Rupture of bubbles (cavitation) on top of collision was most likely the cause of injury to the cover of vulnerable plaques, triggering ACS. Understanding the mechanism will help to better manage ACS. FUNDunding Acknowledgement Type of funding sources: None. Cavity formation and collision Formation of cavitation at the PDA

Effect of Gas Volume Fraction on the Energy Loss Characteristics of Multiphase Pumps at Each Cavitation Stage

In the process of conveying a medium, when the inlet pressure is low, the cavitation phenomenon easily occurs in the pump, especially in the gas–liquid two-phase working condition. The occurrence of the cavitation phenomenon has a great impact on the performance of the multiphase pump. In this paper, the SST (sheard stress transport) k-ω turbulence model and ZGB (Zwart–Gerber–Belamri) cavitation model were used to simulate the helical axial flow multiphase pump (hereinafter referred to as the multiphase pump), and the experimental verification was carried out. The effect of gas volume fraction (GVF) on the energy loss characteristics in each cavitation stage of the multiphase pump is analyzed in detail. The study shows that the critical cavitation coefficient of the multiphase pump gradually decreases with the increase in GVF, which depresses the evolution of cavitation, and the cavitation performance of the multiphase hump is improved. The ratio of total loss and friction loss to total flow loss in the impeller fluid domain gradually increases with the development of cavitation, and the pressurization performance of the multiphase pump gradually decreases with the development of cavitation. The results of the study can provide theoretical guidance for the improvement of the performance of the multiphase pump.

The CFD Analysis of Cavitation Erosion and Structural Optimization for an Unloading Valve

The unloading valve is selected as the key part of pressure control and overload protection system in the emulsion pump station (EPS). However, severe cavitation erosion occurs on the inner wall of the valve sleeve located in a downstream outlet, which leads to the performance deterioration, instability, and even failure of the unloading valve. This paper aims to reduce the cavitation phenomenon by optimizing the structure of the spool and improving the streamline direction of the unloading valve. The change in internal structure can reduce cavitation cloud formation along with the internal mechanism of the valve. The working conditions of its flow versus time-dependent data’s are obtained by AMESim, and considered as the boundary condition in Fluent. In addition, the changes in flow behavior and input data’s are analyze and validate before the structural is optimized. After an analysis, the variations in velocities and vapor phase, indicating the higher in pressure drop at its minimum size of valve opening and further consideration for design and development stages. Two innovative structures of valve throttling spool are proposed as a simplified design stage and then carefully curved based on internal geometry to uniformly distribute the velocity vector and streamlines. Subsequently, the post-CFD results reveal the presence of factors affecting properties such as pressure loss, high velocity in the outlets, and concentrated of the vapor volume fraction are decreased under the modified throttling structure and effectively lowers the intensity of cavitation phenomenon.

Investigation of Pressure Oscillation and Cavitation Characteristics for Submerged Self-Resonating Waterjet

The cavitation phenomenon of the self-resonating waterjet for the modulation of erosion characteristics is investigated in this paper. A three-dimensional computational fluid dynamics (CFD) model was developed to analyze the unsteady characteristics of the self-resonating jet. The numerical model employs the mixture two-phase model, coupling the realizable turbulence model and Schnerr–Sauer cavitation model. Collected data from experimental tests were used to validate the model. Results of numerical simulations and experimental data frequency bands obtained by the Fast Fourier transform (FFT) method were in very good agreement. For better understanding the physical phenomena, the velocity, the pressure distributions, and the cavitation characteristics were investigated. The obtained results show that the sudden change of the flow velocity at the outlet of the nozzle leads to the forms of the low-pressure zone. When the pressure at the low-pressure zone is lower than the vapor pressure, the cavitation occurs. The flow field structure of the waterjet can be directly perceived through simulation, which can provide theoretical support for realizing the modulation of the erosion characteristics, optimizing nozzle structure.