Mechanism for cavitation phenomenon in mechanical heart valves

Recently, cavitation on the surface of mechanical heart valve has been studied as a cause of fractures occurring in implanted Mechanical Heart Valves (MHVs). It has been conceived that the MHVs mounted in an artificial heart close much faster than in vivo sue, resulting in cavitation bubbles formation. In this study, six different kinds of monoleaflet and bileaflet valves were mounted in the mitral position in an Electro-Hydraulic Total Artificial Heart (EHTAH), and we investigated the mechanisms for MHV cavitation. The valve closing velocity and a high speed video camera were employed to investigate the mechanism for MHV cavitation. The closing velocity of the bileaflet valves was slower than that of the monoleaflet valves. Cavitation bubbles were concentrated on the edge of the valve stop and along the leaflet tip. It was established that squeeze flow holds the key to MHV cavitation in our study. Cavitation intensity increased with an increase in the valve closing velocity and the valve stop area. With regard to squeeze flow, the bileaflet valve with slow valve-closing velocity and small valve stop areas is better able to prevent blood cell damage than the monoleaflet valves.     

Flow instability due to cryogenic cavitation in the downstream of orifice

Flow instability in LRE (liquid rocket engine) occurs due to various reasons such as flow interactions with valve, orifice and venturi, etc. The inception of cavitation, especially in the propellant feeding system, is the primary cause of mass and pressure oscillations because of the cyclic formation and depletion of cavitation. Meanwhile, the main propellant in a liquid rocket engine is the cryogenic fluid, which properties are very sensitive to temperature variation. And the change of propellant properties to temperature variation by thermodynamic effect needs to be properly taken into account in the flow analysis in order to understand basic mechanisms for cryogenic cavitation. The present study focuses on the formation of cryogenic cavitation by using the IDM model suggested by Shyy and coworkers. The flow instability was also numerically investigated in the downstream of orifice with a developed numerical code. Calculation results show that cryogenic cavitation can be a primary source of flow instability, leading to mass fluctuations accompanied by pressure oscillations. The prediction of cavitation in cryogenic fluid is of vital importance in designing a feeding system of an LRE. This paper was recommended for publication in revised form by Associate Editor Jun Sang Park Changjin Lee received his B.S. and M.S. degrees in Aeronautical Engineering from Seoul National University in 1983 and 1985. He then went on to receive his Ph.D. degree from University of Illinois at Urbana- Champaign in 1992. Dr. Lee is currently a Professor at the department of Aerospace Engineering at Konkuk University in SEOUL, Korea. His research interests are in the area of combustion instabilities of hybrid, liquid rocket and jet propulsions. Tae-Seong Roh received his B.S. and M.S. degrees in Aeronautical Engineering from Seoul National University in 1984 and 1986. He then went on to receive his Ph.D. degree from Pennsylvania State University in 1995. Dr. Roh is currently a Professor at the department of Aerospace Engineering at Inha University in Incheon, Korea. His research interests are in the area of combustion instabilities, rocket and jet propulsions, interior ballistics, and gas turbine engine defect diagnostics.     

调压阀流体空化特性仿真及影响因素分析

针对船用二级调压阀空化问题,建立流域瞬态仿真模型,结合Singhal空化模型和标准k-ε湍流模型对调压阀流体空化现象进行数值模拟,通过流场气体体积分数分析,得出了流体空化强度及分布形态的演变规律,通过流体速度场和压力场分析,阐明了空化演变过程调压阀流场特性,进而研究了开度、流量和背压对调压阀流体空化现象的影响规律.结果...     

节流槽形式对滑阀空化流场的影响

为了提高采煤工作面液压支架推移拉架准确度,降低空化现象对控制滑阀性能的影响,采用Pumplinx建立了不同节流槽形式下滑阀内部流体域动态模型。仿真分析了不同节流槽形式滑阀在不同开度时,压力场和空化分布以及气体体积分数的变化趋势。结果表明:不同节流槽形式对滑阀内部的压力分布和空化分布具有不同的影响;气体体积分数随着阀口开度的增大,呈现先稳定波动然后陡增最后在阀口完全开放后迅速降低的现象;交错分布形节流槽空化剧烈起始位置为4.5 mm,最大气体体积分数约为0.12,相较于其他槽形明显降低。     

2D伺服阀矩形先导控制阀口气穴特性研究

为研究双自由度（2D）伺服阀先导控制阀口处气穴现象的影响因素及对阀芯稳定性的影响,运用Fluent软件中cavitation模型对2D伺服阀矩形先导阀口进行了气穴特性的仿真研究。研究表明,在出口压力低于1 MPa时,阀口处会出现气穴现象,且因气穴指数σ较小,故在弓形感受通道会出现气泡现象,出口压力高于5 MPa时,无明显气泡现象但阀口处的气穴仍然存在;随着入口流速增加,阀口内侧壁和外侧壁处气穴强度和分布范围增加;在出口压力0.1 MPa情况下,随着阀口开度增加气穴现象减弱。结果表明,2D伺服阀正常工作时先导阀口处会产生气穴,对伺服阀阀芯运动的稳定性产生干扰。     

Experimental study on unsteady characteristics of the transient cavitation flow

The characteristics of unsteady cavitation flow under different cavitation numbers are investigated experimentally. An optical test rig is designed to visualize the cavitation flow and the cavity length is defined based on high-speed camera technology. The development of cavitation flow can be divided into inception and fusion region, expansion region and collapse region. Due to the interaction between the re-entrant jet and the cavity interface, the cavitation bubbles near the solid wall collapse earlier than them in the downstream. The cavitation number is also the important factor determining the cavity length pulsation. Further analysis shows that the smaller the cavitation number, the smaller the frequency of cavity length fluctuations and the larger the mean cavity length. The cavitation intensity is stronger at lower cavitation number. Meanwhile, the cycle collapsing frequency of cavitation is around 220–320 Hz. With the decrease of cavitation number, the amplitude of cavitation pulsation increases while the corresponding collapsing frequency decreases.     

Relationship between cavitation structures and cavitation damage

A study of visual and erosion effects of cavitation on simple single hydrofoil configurations in a cavitation tunnel was made. A two-dimensional hydrofoil with circular leading edge was used for the experiments. In addition, the hydrofoil geometry was modified to obtain some three-dimensional cavitation effects. A thin copper foil, applied to the surface of the hydrofoil, was used as an erosion sensor. Cavitation phenomenon above hydrofoils at different flow conditions (system pressure, water gas content) was observed. Images of vapour cavities from above and from side view were taken. A statistical evaluation of cavitation structures was made. Images of damaged copper coated hydrofoil surface were taken under sufficient magnification. A pit-count method, based on computer-aided image processing, was used for direct measurement of the cavitation erosion by evaluating the damage of the surface of the hydrofoil. A relation between characteristics of cavitation structures and cavitation damage was established.     

Experimental study of cavitation phenomenon in a centrifugal blood pump induced by the failure of inlet cannula

Cavitation of centrifugal blood pump is a serious problem accompany with the blocking failure of short inlet cannula. However, hardly any work has been seen in published literature on this complex cavitation phenomenon caused by the coupling effect of inlet cannula blocking and pumps suction. Even for cavitation studies on ordinary centrifugal pumps, similar researches on this issue are rare. In this paper, the roles of throttling, rotation speed and fluid viscosity on bubble inception and intensity in a centrifugal blood pump are studied, on the basis of experimental observations. An adjustable throttle valve installed just upstream blood pump inlet is used to simulate the throttling effect of the narrowed inlet cannula. The rotation speed is adjusted from 2 600 r/rain to 3 200 r/min. Glycerin water solutions are used to investigate the influences of kinetic viscosity. Bubbles are recorded with a high-speed video camera. Direct observation shows that different from cavitation in industrial centrifugal pumps, gas nuclei appears at the nearby of vane leading edges while throttling is light, then moves upstream to the joint position of inlet pipe and pump with the closing of the valve. It＇s found that the critical inlet pressure, obtained when bubbles are first observed, decreases linearly with viscosity and the slope is independent with rotation speeds; the critical inlet pressure and the inlet extreme pressure which is obtained when the throttle valve is nearly closed, fall linearly with rotation speed respectively and the relative pressure between them is independent with rotation speed and fluid viscosity. This paper studies experimentally on cavitation in centrifugal blood pump that caused by the failure of assembled short inlet cannula, which mav beneficial the desima of centrifugal blood Dumo with inlet cannula.     

水滴迷宫式调节阀空化特性计算与结构改进

针对超(超)临界机组中水滴迷宫式调节阀在高温高压工况下引起的严重气蚀问题,基于计算流体力学理论和空化机理,选用标准k-ε湍流模型、Mixture模型和Schnerr-Sauer空化模型,比较了改进前后调节阀在典型开度下的压力、速度、气相体积分数等结构性能.计算结果表明:原始碟片结构压降大,最大可至19.95 MPa,流...     

Influence of the flow area around the ball valve on the flow characteristics of the injector control valve

The increase in common rail pressure can lead to increased cavitation inside the injector, resulting in degradation of injector performance and reduced life. The paper investigates the effect of the pressure block structure parameters (initial flow area around the ball valve) on the velocity field, pressure field, fuel gas phase volume fraction and drain rate of the control valve. The relationship between the initial flow area around the ball valve on the cavitation strength and unloading rate inside the valve was revealed. The results show that both the reduction of the flow area around the ball valve and the increase of the cavitation intensity inhibit the rate of oil discharge from the control valve. The reduction of the fuel flow area inhibits the expansion of the low-pressure region (0–1 MPa) within the flow layer, thus limiting the development of cavitation. The reduction of the cavitation area increases the fuel flow rate, however, the increase in flow rate increases the cavitation phenomenon, and these changes form a cycle (Reviewer 5. comment 2). The increase in cavitation inhibits the control valve pressure relief rate more significantly than the decrease in the initial flow area around the ball valve. Based on this, a stepped-pressure block model is proposed. The stepped pressure block model can effectively reduce the cavitation strength near the seal and enhance the oil discharge rate of the control valve. The study can provide a reference for the engineering optimization design of high-pressure common rail injector control valves.     

基于图像灰度统计的调节阀空化分布特性研究

针对调节阀内复杂的空化流动现象,研究调节阀内的空化形态特征和分布特性。建立调节阀空化图像采集试验系统,采用高速相机对空化形态进行捕捉。由于空泡的生长、溃灭等动力学行为使空泡群的密度和数量的不同,捕获的图像上灰度值的变化与空泡的演变过程之间存在相关性。该文通过空化图像中灰度值的变化来识别空化区域,且为提高分析可靠度,引入三个特征量：相似系数R确定所需样本数量、灰度变化率λ来表征空泡的动力学行为变化、平均灰度值ψ表达空化长度。根据这三个特征量,我们准确获得了调节阀内空化产生的位置、分布区域以及形态的变化。调节阀流道内空化附着区域可分为空化形成区(IR)、发展区(DR)和溃灭区(CR)。背压是影响空化附着区域形态的一个重要因素。随着背压的增加,其对空化的抑制作用比较明显,空化形成区的终点也是溃灭区的起点,即空化在流道内的分布形式仅为空化形成、溃灭这两种分布形式。此外,随着背压的增加,空化强度减弱,空化长度亦减小。     

滑阀V形阀口漩涡空化数值仿真分析

使用大涡模拟法(LES)模型、Mixture多相流模型及Schnerr-Sauer空化模型对滑阀V形阀口空化流场进行了数值仿真模拟。分析了V形滑阀在不同出口压力下的速度云图、压力云图和空穴形状,讨论了其中空化产生机制。结果表明:V形滑阀中空化主要分布在阀口上半部分和阀口后的阀腔中,阀口中的空化由流体漩涡运动的低压引起,阀腔中的空化由流体漩涡运动造成的压力脉动引起;无空化时阀腔中各处压力脉动主频一致,主频随入口压力降低而增大,空化出现后会破坏压力脉动的周期性。     

Examination of cavitation-induced surface erosion pitting of a mechanical heart valve using a solenoid-actuated apparatus

Several factors, including peak dp/dt of the ventricular pressure and maximum closing velocity of leaflet have been studied as indices of the cavitation threshold. In the present study, just before closing velocity of the leaflet has been studied as indices of the cavitation threshold, and cavitation erosion on the surface of a mechanical valve was examined by focusing on squeeze flow and the water hammer phenomenon during the closing period of the valve. A simple solenoid-actuated test device that can directly control the valve closing velocity was developed, and opening-closing tests of 3,000 and 40,000 cycles were performed at various closing velocities. There was a closing velocity threshold to occur erosion pitting of valve surface, and its value was about 0.4 m/s in this study. Cavitation-induced erosion pits were observed only in regions where squeeze flow occurred immediately before valve closure. On the other hand, the number of the pits was found to be closely related to an area of water hammer-induced pressure wave below the critical pressure defined by water vapor pressure. Therefore, it was concluded that cavitation is initiated and augmented by the two pressure drops due to squeeze flow and water hammer phenomenon, respectively.     

The cavitation behavior with short length blades in centrifugal pump

A CFD code with 2-D cascade model was developed to predict the cavitation behavior around the impeller blades of impeller in a centrifugal pump. The governing equations are the two-phase Reynolds Averaged Navier-Stokes equations in a homogeneous form in which both liquid and vapor phases are treated as incompressible fluid. To close the model, a standard k-ɛ turbulence model is introduced. And the mass transfer rates between liquid and vapor phases are implemented as well. The validations are carried out by comparing with reference data in impeller of a centrifugal pump impeller. The cavitation characteristics of current centrifugal pumps is tested at an ondesign point (V=8 m/s) and two off-design points (V=20 m/s and V=30 m/s), respectively. The criteria of cavitation and flow instability around blades are presented. The results show that the current centrifugal pump can safely operate without cavitation at on-design point. Also, the simulation shows cavitation develops inhomogeneously among the blades at off-design points. Moreover, the effects of additional blades in the impeller are studied as well. From the numerical results, it is expected that a half-length blade is the optimum configuration as additional blades in cavitation point of view.     

不同开口度下的煤矿水液压安全阀的气蚀特性

针对煤矿水液压安全阀存在的严重气蚀问题,建立双排阀口的CFD模型,分析了不同启动次序和开口度的阀腔气相体积分数和压力分布云图,并得到其影响规律及最佳的阀口开启次序.研究表明:3种开启方式下,开口度为0.8～1.2 mm时气蚀均较为严重;同步开启时,综合抗气蚀效果最差,开口度为0.8～1.2 mm时极易形成涡旋,且对气蚀...     

基于节流阀阀内流场的管道振动分析

建立了板式节流阀的三维模型,运用CFD仿真软件对节流阀内部流场进行了数值模拟,得到了该型号节流阀内流场的三维可视化结果,结果表明:阀芯处的气蚀以及阀出口处的漩涡流是诱发阀芯和阀体振动的主要原因,因此优化阀内部流道对于减小管道振动具有积极的意义。     

小开度节流阀流场特征及空化流动的数值分析

基于两相空化流动的控制方程和湍流模型,对节流阀在小开度下的流场特征及空化流动进行数值分析。结果表明:流体在流经节流口时,流速急剧增加,压力迅速降低至液体的饱和蒸汽压以下,形成空化。当节流阀出入口压差增大时,出口边界流速明显提高,出口两侧的流速差异更加明显,且在低速流一侧形成涡流。并且,出入口压力差的增加、阀门开度的减小会导致空化区域扩大,强度增加。研究成果可为节流阀的后续优化设计和操作提供理论依据。     

Study on the influence of structural parameters on the flow and cavitation characteristics of tandem multi-stage pressure-reducing valves

Tandem multi-stage pressure-reducing valves (TMSPRV) are widely used for piping systems in the process industry. The flow coefficient is a central factor in valve design. The cavitation was caused by the local pressure of the fluid passing through the pressure-reducing valve being lower than the saturated steam pressure. Would cause serious damage to the pipeline system. Therefore, it is important to investigate systematically the effect of throttling structure parameters on the flow and cavitation characteristics of valves. In this paper, a combination of experimental and numerical simulations was used to study the effect of different structural parameters of valves on the flow coefficient. The results showed that increasing the flow channel inclination is beneficial to enlarging the flow coefficient. Meanwhile, the effects of different structural parameters on pressure and velocity of pressure reducing valves are discussed, the results indicated that increasing the inclination of the flow channel would reduce the vortex volume at the outlet. With the increase of the chamfer, the low-pressure area caused by the vortex in the near-wall surface decreases. Numerical simulations are conducted to investigate the effect of different structural parameters on the cavitation characteristics of valves. The numerical results showed that the flow channel inclination angle is 60° and the flow channel chamfer is less than 6 mm as the optimal value. In summary, considering the influence of structural parameters on flow coefficient, flow characteristics, and cavitation characteristics. The runner inclination angle is 60°, and the runner chamfer is 4 mm as the best value. The research work in this paper could provide technical support to achieve a better fluid pressure reducing and flow state of the TMSPRV under severe working conditions.     

Interaction mechanism between nanoparticles and ultra-smooth surface under effect of cavitation

During the nanoparticle colloid jet polishing process to obtain ultra-smooth surface, cavitation is easy to occur because of the intense fluid turbulence. Cavitation interacts with nanoparticles in the colloid solution, which has a great influence on the processing quality of the ultra-smooth surface. In this paper, the collision between nanoparticles and ultra-smooth surfaces under the micro-jet effect formed by cavitation is studied. Molecular dynamics simulation result shows that the collision of nanoparticles changes the atomic position of the collision region of workpiece, resulting in lattice distortion, coordination number and atomic potential energy increased. The ultrasonic cavitation system is used to prepare the monocrystalline silicon workpiece that effected by nanoparticle colloid cavitation. After the process, the dark spots could be observed on the surface of workpiece by SEM. The dark spots are the traces of the collision between the nanoparticle and workpiece, which verify the molecular dynamics simulation results. XPS detection of the surface of monocrystalline silicon workpiece processed by nanoparticle colloid cavitation has been done and the result is slightly different from the workpiece that have not been processed by cavitation. The difference illustrates that the chemical bonding between nanoparticles and monocrystalline silicon surfaces has formed. The simulation and experimental results show that the cavitation is beneficial to the removal of the material of the workpiece by nanoparticles, but it has an adverse effect on the formation of ultra-smooth surface.     

Cavitation effect on flow resistance of sleeve regulating valve

Sleeve regulating valve is the key component of the industrial system. As the main cause of vibration and noise inside the sleeve regulating valve, cavitation also increases the energy loss when fluid flows through the valve. In this paper, in order to quantitatively analyze the influence of cavitation in the sleeve regulating valve, the effects of throttling windows number, pressure difference, valve opening and flow direction on the resistance coefficient are investigated with the application of cavitation model. By defining the cavitation influencing factor of the resistance coefficient, the calculation formula of the resistance coefficient when cavitation occurs in the sleeve regulating valve is proposed. The results show that when the fluid flows forward and the aggravation degree of cavitation increases, the energy loss also increases. And in the backward flow, the effect of cavitation on the resistance coefficient is related to the valve opening and the throttling windows number. This work is of significance for the optimization and design of sleeve regulating valve and pipeline system.