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.     

Analysis of blood flow interacted with leaflets in MHV in view of fluid-structure interaction

Interaction of blood flow and leaflet behavior in a bileaflet mechanical heart valve was investigated using computational analysis. Blood flows of a Newtonian fluid and a non-Newtonian fluid with Carreau model were modeled as pulsatile, laminar, and incompressible. A finite volume computational fluid dynamics code and a finite element structure dynamics code were used concurrently to solve the flow and structure equations, respectively, where the two equations were strongly coupled. Physiologic ventricular and aortic pressure waveforms were used as flow boundary conditions. Flow fields, leaflet behaviors, and shear stresses with time were obtained for Newtonian and non-Newtonian fluid cases. At the fully opened phase three jets through the leaflets were found and large vortices were present in the sinus area. At the very final stage of the closing phase, the angular velocity of the leaflet was enormously large. Large shear stress was found on leaflet tips and in the orifice region between two leaflets at the final stage of closing phase. This method using fluid-structure interaction turned out to be a useful tool to analyze the different designs of existing and future bileaflet valves.     

A structural analysis on the leaflet motion induced by the blood flow for design of a bileaflet mechanical heart valve prosthesis

This paper presents a structural analysis on the rigid and deformed motion of the leaflet induced by the blood flow required in the design of a bileaflet mechanical heart valve (MHV) prosthesis. In the study on the design and the mechanical characteristics of a bileaflet mechanical heart valve, the fluid mechanics analysis on the blood flow passing through leaflets, the kinetodynamics analysis on the rigid body motion of the leaflet induced by the pulsatile blood flow, and the structural mechanics analysis on the deformed motion of the leaflet are required sequentially and simultaneously. Fluid forces computed in the previous hemodynamics analysis on the blood flow are used in the kinetodynamics analysis on the rigid body motion of the leaflet. Thereafter, the structural mechanics analysis on the deformed motion of the leaflet follows to predict the structural strength variation of the leaflet as the leaflet thickness changes. Analysis results show that structural deformations and stresses increase as the fluid pressure increases and the leaflet thickness decreases. Analysis results also show that the leaflet becomes structurally weaker and weaker as the leaflet thickness becomes smaller than 0.6 mm.     

In Vitro hydrodynamic evaluation of prosthetic polymer heart valves in steady flow

Hydrodynamic comparison of two polymer valves with two mechanical valves is presented. The valves were perfused in a steady flow system, and comparisons between the valves were made on the transvalvular pressure distribution and drop, opening area, and leakage volume. Particular emphasis was placed on a slit-type bileaflet polymer valve which was newly designed and fabricated in our research group. The results showed that the functional characteristics of a slit-type bileaflet polymer valve compared favorably with that of mechanical valves. This valve may be a viable and inexpensive alternative, especially for short-term use in TAH or VAD systems.     

A new method for quantitative evaluation of perceived sounds from mechanical heart valve prostheses

Closing clicks from mechanical heart valve prostheses are transmitted to the patient's inner ear mainly in two different ways: as acoustically transmitted sound waves, and as vibrations transmitted through bones and vessels. The purpose of this study was to develop a method for quantifying what patients perceive as sound from their mechanical heart valve prostheses via these two routes. In this study, 34 patients with implanted mechanical bileaflet aortic and mitral valves (St Jude Medical and On-X) were included. Measurements were performed in a specially designed sound insulated chamber equipped with microphones, accelerometers, preamplifiers and a loudspeaker. The closing sounds measured with an accelerometer on the patient's chest were delayed 400 ms, amplified and played back to the patient through the loudspeaker. The patient adjusted the feedback sound to the same level as the 'real-time' clicks he or she perceived directly from his or her valve. In this way the feedback sound energy includes both the air- and the bone-transmitted energies. Sound pressure levels (SPLs) were quantified both in dB(A) and in the loudness unit sone according to ISO 532B (the Zwicker method). The mean air-transmitted SPL measured close to the patient's ear was 23 +/- 4 dB(A). The mean air- and bone-transmitted sounds and vibrations were perceived by the patients as an SPL of 34 +/- 5 dB(A). There was no statistically significant difference in the perceived sound from the two investigated bileaflet valves, and no difference between aortic and mitral valves. The study showed that the presented feedback method is capable of quantifying the perceived sounds and vibrations from mechanical heart valves, if the patient's hearing is not too impaired. Patients with implanted mechanical heart valve prostheses seem to perceive the sound from their valve two to three times higher than nearby persons, because of the additional bone-transmitted vibrations.     

A numerical analysis on the curved bileaflet mechanical heart valve (mhv): leaflet motion and blood flow in an elastic blood vessel

In blood flow passing through the mechanical heart valve (MHV) and elastic blood vessel, hemolysis and platelet activation causing thrombus formation can be seen owing to the shear stress in the blood. Also, fracture and deformation of leaflets can be observed depending on the shape and material properties of the leaflets which is opened and closed in a cycle. Hence, comprehensive study is needed on the hemodynamics which is associated with the motion of leaflet and elastic blood vessel in terms of fluid-structure interaction. In this paper, a numerical analysis has been performed for a three-dimensional pulsatile blood flow associated with the elastic blood vessel and curved bileaflet for multiple cycles in light of fluid-structure interaction. From this analysis fluttering phenomenon and rebound of the leaflet have been observed and recirculation and regurgitation have been found in the flow fields of the blood. Also, the pressure distribution and the radial displacement of the elastic blood vessel have been obtained. The motion of the leaflet and flow fields of the blood have shown similar tendency compared with the previous experiments carried out in other studies. The present study can contribute to the design methodology for the curved bileaflet mechanical heart valve. Furthermore, the proposed fluid-structure interaction method will be effectively used in various fields where the interaction between fluid flow and structure are involved.     

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...     

水压锥阀空化射流流场结构与流量特性的相关性研究

为了研究水压锥阀空化流场与流量特性的相关性,对两种阀座结构的水压锥阀内部的空化射流开展了三维动态流场仿真.结果表明,直角型锥阀和倒角型锥阀均在阀芯后沿存在分离流诱发的附着型空化,在阀口下游有漩涡空化;此外,倒角阀座流道内亦存在分离流现象并形成附着型空化.倒角型流道入口处的分离流造成流体的局部加速,对于0.6 mm开口度...     

Cavitation reduction in the globe valve using oblique perforated cages: A numerical investigation

Globe valves are one of the oldest and most widely used flow control valves. However, cavitation due to high-pressure drop is a common phenomenon in such valves, reducing valve lifetime and increasing replacement costs. Recently, a series of perforated cages have been used in globe valves to reduce cavitation damage (especially in the body and disk). In this study, a globe valve is numerically investigated under four different structures, i.e., without the cage, with a perforated cage whose holes are radial, with a perforated cage whose holes are facing upwards, and with a perforated cage whose holes are facing downwards. The effects of the valve opening, inlet velocity, and holes direction on the pressure drop and cavitation incipient/intensity are investigated. The results show that cavitation is observed with the decrease of the valve opening even at the low velocities. And with increasing the velocity inlet, the pressure drop and cavitation intensity increase significantly. It can also be concluded that in the globe valve using a perforated cage with holes downwards, less cavitation is observed than in other models. Therefore, this configuration is proposed to protect the inner components of the valve (such as seat, seal, and plug) from cavitation damage.     

Instability measurement of cavitation conditions in a spool valve through the definition of a cavitation instability index

Cavitation phenomenon is commonly known as a cause of deterioration of hydraulic components and a source of noise. In a proportional spool control valve for fluid power applications, the reduction of the cross section area at small openings can lead to the growth of gas bubbles inside the fluid phase. This phenomenon modifies the operating efficiency of the valve and can damage the material over time. The oil flow has been visualised through an experimental activity by using a 2-way 2-position spool valve with a plexiglass case and analysed by a CMOS type camera, with a high sampling rate. The recorded images focus the interaction between gas and liquid phases and highlights how the shape of cavitation bubbles is modified when the surface stress increases. In the present work, the interaction between liquid and gas phase has been studied by applying a non-linear method on the numerical matrices, derived from the acquired images. A cavitation instability index has been proposed, which is able to describe the cavitation behaviour and to define the instability of the equilibrium between the two phases. The trend of this index gives information about the two phase flow conditions, in order to reduce the cavitation phenomenon.     

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。     

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.     

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.     

挤压油膜阻尼器内气液均相流模型

在挤压油膜阻尼器内气穴现象实验研究的基础上，本文对挤压油膜因气穴现象而发生物理特性的变化进行了讨论。以气液均相流模型作为挤压油膜的近似描述，导出了上述模型的密度和粘度与油膜压力之间的关系式。分析了当挤压油膜采用这一可压缩模型时，其压力分布的描述更能与实际接近，对阻尼器的力学特性的预测有重要的参考价值。     

基于应力均匀化的超纯水隔膜阀隔膜优化

超纯水隔膜阀具有结构简单、耐化学腐蚀、低离子析出等优点,多用于半导体行业,隔膜的疲劳破损是影响隔膜阀寿命和设备可靠性的关键性能指标。通过COMSOL模拟了隔膜启闭过程中的应力变化情况,并详细分析了隔膜工作时的受力特点。结果表明:隔膜启闭过程中受到的应力与阀口开度成线性关系,且阀口开度最大时出现峰值;对于小流量、低流速的超纯水隔膜阀,隔膜失效的主要因素是隔膜启闭过程中机械拉伸引起的疲劳损坏;优化后的隔膜在工作时的应力趋于均匀化,对进一步提高其工作寿命具有良好的工程指导意义。     

基于节流-分流耦合的高压高速节流口空化抑制方法

开展高压高速节流口的空化抑制方法研究是提升阀的寿命和可靠性的关键环节。针对高压高速节流口空化破坏严重的问题,提出了一种基于节流-分流耦合的空化抑制方法。该方法采用多级节流的方式,实现阀口压降的多级承担,有效减小阀口压力梯度并降低流速;通过在阀出口采取多排孔分流的方式,改善流线布局,减少流体冲击。以电磁卸荷阀为例,分析卸荷阀动态性能,获得高压高速节流口实际工况,开展高压高速工况下节流口流体仿真。仿真结果显示,相较多级节流方式和多孔分流方式,所提出的方法可显著改善流场的压力和流速分布,实现了阀口空化现象的有效抑制。     

Cavitation erosion of a flat surface near a rotating shaft

To clarify the phenomenon of cavitation erosion in the sliding bearings of internal combustion engines, an apparatus was prepared and tests were conducted on cavitation in the oil between the cylindrical face of a rotating shaft and the tip of a horn attached to an ultrasonic oscillator. This apparatus produces an oil flow and a plus-minus oil pressure between a shaft and a horn tip to simulate erosion and its distribution on bearing surfaces.From patterns of cavitation erosion on the tip of a horn made of an Al-Sn alloy and the pressure distribution on the wedge oil film, it was determined that the region where cavitation bubbles occur and the region where erosion occurs owing to the collapse of the bubbles do not necessarily coincide.Cavitation erosion was found to occur in both the plus and the minus oil film pressure regions and erosion due to bubble collapse occurs in the regions where pressure increases in the direction of oil flow.The test results allow the postulation of the mechanism and the reasons for the occurrence of cavitation erosion on actual sliding bearing surfaces.     

双密封导轨阀结构设计

介绍了双密封导轨阀的结构设计、工作原理与性能特点。双密封导轨阀是一种新型的截断类阀门，它以独特的斜楔形燕尾导轨结构将阀门启闭过程中密封件间的摩擦消除，实现了阀门密封的零泄漏，长寿命。     

Numerical analysis of 3-D flow through LNG marine control valves for their advanced design

A numerical analysis of three dimensional incompressible turbulent flows through high pressure drop control valves was carried out by using a CFD-ACE code to develop anti-cavitation control valve used in LNG marine system. For this, numerical simulation was performed on several models of control valve that have different orifice diameters of anti-trim and the size of valve discharge. In this study, flow characteristics of control valves with complex flow fields including pressure drop, cavitation effect and variation of flow coefficient as well as correlation of discharge coefficient were investigated and analyzed. Comparing with conventional control valves, newly designed valves by using the CFD analysis showed an improved flow pattern with reduced cavitation and an anticipated performance characteristic. This paper was presented at the 9^th Asian International Conference on Fluid Machinery (AICFM9), Jeju, Korea, October 16–19, 2007.     

Pulsatile blood flows through a bileaflet mechanical heart valve with different approach methods of numerical analysis; pulsatile flows with fixed leaflets and interacted with moving leaflets

Many researchers have investigated the blood flow characteristics through bileaflet mechanical heart valves using computational fluid dynamics (CFD) models. Their numerical approach methods can be classified into three types; steady flow analysis, pulsatile flow analysis with fixed leaflets, and pulsatile flow analysis with moving leaflets. The first and second methods have been generally employed for two-dimensional and three-dimensional calculations. The pulsatile flow analysis interacted with moving leaflets has been recently introduced and tried only in two-dimensional analysis because this approach method has difficulty in considering simultaneously two physics of blood flow and leaflet behavior interacted with blood flow. In this publication, numerical calculation for pulsatile flow with moving leaflets using a fluid-structure interaction method has been performed in a three-dimensional geometry. Also, pulsatile flow with fixed leaflets has been analyzed for comparison with the case with moving leaflets. The calculated results using the fluid-structure interaction model have shown good agreements with results visualized by previous experiments. In peak systole, calculations with the two approach methods have predicted similar flow fields. However, the model with fixed leaflets has not been able to predict the flow fields during opening and closing phases. Therefore, the model with moving leaflets is rigorously required for advanced analysis of flow fields.