液体中空泡收缩过程的数值研究

材料在液体中的空蚀破坏程度会因为液体环境的改变而大相径庭,而液体中空泡的生长、溃灭是造成材料空蚀破坏最根本的原因。为了研究液体环境对空泡运动过程的影响,利用有限差分法计算空泡在不同液体环境中的收缩过程,对不同液体黏性、表面张力条件下不同尺寸空泡的收缩过程进行了分析。计算结果表明:液体黏性会减缓空泡收缩过程中的脉动现象,且黏性越大,空泡收缩过程中脉动现象所持续的时间越短,脉动幅度也越小;表面张力越大,空泡收缩稳定后的半径也越大,且表面张力在空泡收缩过程中表现出的对空泡泡壁的加速效应会增大空泡的脉动频率。与前人的研究相比,研究还发现:在同样的液体环境中,随着空泡初始半径的减小,空泡在收缩过程中呈现出的脉动现象所持续的时间会越来越短,而最终稳定后的量纲一半径则会越来越大,即空泡在收缩过程中被压缩的比例越来越小。计算结果为理解液体中空泡的溃灭行为和由此引起的空蚀破坏提供了理论依据。     

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.     

Experimental investigation on noise of cavitation nozzle and its chaotic behaviour

The researches of cavitation noise mainly focus on the incipiency and developing of cavitation to prevent the cavititation erosion in the hydraulic machinery, while there is few report about the collapse strength of cavitation bubbles produced by water jet through the cavitation nozzle to utilize efficiently the collapse energy of cavitation bubbles. The cavitation noise signals are collected with hydrophones for the cavitation nozzle and general nozzle at the target position and the nozzle exit separately in the conditions of different standoff distance. The features of signal’s frequency spectrum and power spectrum are analyzed for various nozzles by way of classical methods. Meanwhile, based on chaotic theory, phase space reconstruction is processed and the maximum Lyapunov index is calculated separately for each cavitation signal’s time series. The results of chaotic analysis are compared with the one of conventional analysis. The analyzed data show that there are the marked differences at the spectrum between the cavitation nozzle and general nozzle at the target position while the standoff distance is 35 mm, which mainly displays at the high frequency segment (60-120 kHz). The maximum Lyapunov index calculated appear at standoff distance 35 mm, which is an optimum standoff distance for the most bubbles to collapse at the target. At the nozzle exit, the noise signal of cavitation nozzle is different from the general nozzle, which also displays at the high frequency segment. The results demonstrate that the water jet modulated by the cavitation nozzle can produce effectually cavitation, and at the target position the amplitude and energy of noise spectrum in high frequency segment for cavitation nozzle are higher than conventional nozzle and the Lyapunov index of cavitation nozzle is larger than conventional nozzle as the standoff distance is less than 55 mm. The proposed research reveals that the cavitation noise produced by collapse of cavitation bubbles attributes mainly to the high frequency segment     

Performance of environment-friendly hydraulic fluids and material wear in cavitating conditions

The cavitation performance of various metals and hydraulic fluids used in a hydraulic system was evaluated using the vibratory test method. Mineral oil, vegetable oil and oil-in-water emulsions were used in the experiments. The materials were selected based on the general components employed in a hydraulic system—AA 5005 aluminium alloy, ASTM A536-84 ductile spheroidal graphite (SG) cast iron, ASTM A48-83 grey cast iron, AISI 303 stainless steel and BS 1400 LG2 bronze. It was observed that vegetable oil exhibits the best medium for erosion resistance for all metals due to its high viscosity index. Emulsions having higher oil concentration produced lesser erosion damage. It was seen that an increase in viscosity led to a decrease in the rate of growth and collapse of bubbles and hence reduced erosion on the surfaces of the specimens. The experiments also revealed that materials with high hardness had less cavitation damage for all lubricants. A comparison of cavitation performance revealed that materials and hydraulic fluids have a dependent relationship. Results indicate that AISI 303 stainless steel would be the best choice in the construction of a hydraulic system and this is especially the case when using a hydraulic fluid that has a high viscosity index.     

Cavitation erosion of aluminum matrix sintered composite with AlN dispersoids

The cavitation erosion resistance of P/M aluminum alloy-sintered composite with AlN dispersoids, prepared via the in situ synthesis and the conventional premixing process, was evaluated by using magnetostrictive-vibration type equipment. In situ synthesized AlN particles were effective for the improvement of the erosion resistance of the composite because of their good bonding with the aluminum matrix. The additive AlN by the premixing process were easily detached from the specimen surface due to the insufficient coherence with the matrix, and caused the poor resistance. The cavitation resistance also depended on the porosity of the sintered composite. The continuously opened pores accelerated the wear phenomena by the cavitation due to the high-pressure attack on the primary particle boundaries of sintered materials in the collapse of the bubbles.     

表面形貌与空化噪声的相关性研究

空蚀过程中,材料的表面形貌与噪声均会发生变化。在超声振动空蚀装置中,空泡的溃灭情况并不会随着空蚀阶段的发展而发生变化,即空泡噪声不变。固体材料在空泡溃灭时形成的微射流或冲击波作用下所形成的噪声是引起整体噪声变化的主要因素,并受材料表面形貌的影响。通过相关性分析方法,得出在超声振动空蚀实验装置中,由于空泡溃灭冲击固体表面所形成噪声的特征频率所在频段的中心频率为3.15 kHz与6.3 kHz,并且它们对应的声压级随粗糙度的增大而逐渐减小。     

Mechanisms of Formation of Spheroids Produced by Cavitation Erosion

The prediction, control, and prevention of unscheduled downtime due to wear of industrial equipment is an important engineering problem. The characterization of wear particles and the study of the mechanism of their formation will aid in preventive maintenance. Of particular interest is the mechanism of formation of spherical particles during wear and erosion. It has been suggested that cavitation erosion is a mechanism of formation for these spherical particles. Using both a vibratory apparatus and a jet erosion facility, spherical particles were produced on various materials including aluminum and 52100 steel. Special techniques for isolating and mounting the eroded particles were developed. Spheroids ranging from 0·5 μ (microns) to 30 μ in diameter were observed in the vibratory cavitation method both in distilled water and in SAE 10W nondetergent oil. Much larger spheroids up to 150μ in diameter were observed in clusters with the jet erosion method. Scanning electron microscope studies revealed craters, plastic flow, overflowing lips, and splashing stems. Based on these observations, it is theorized that the high strain rate indentation of the cavitation bubbles leads to the splash of metal into the surrounding liquid where surface tension produces spherical particles. Supporting experimental evidence and calculations are also presented.     

A study of cavitation bubble collapse pressures and erosion part 3: the results in a venturi facility

Cavitation erosion generated in a venturi facility was studied by comparing the erosion loss with the distributions of cavitation bubble collapse pressures (impact loads). The erosion process in the venturi tests is similar to that in the vibratory tests, although its progression is very slow. That is, the surface first deforms and fractures as a result of fatigue with repeated bubble collapse pressures below the threshold pressure needed to form a pit impulsively. By comparing the distributions of impact loads measured using our method with the hypothetical stress-number of cycles curves for fatigue, it is found that the incubation period and the volume loss rate during the stable period follow Miner's law regardless of the venturi, vibratory and cavitation conditions and materials. Therefore we found that we are able to estimate cavitation damage in a flowing system in the same way as damage in the vibratory tests from Miner's law although the distributions of cavitation bubble collapse pressures are markedly different.     

Estimation of cavitation intensity from the time taken for bubbles to develop

In order to estimate the cavitation erosion rate, the time taken for cavitation bubbles to develop and the cavitation erosion intensity were investigated. The cavitation intensity was found to be proportional to the 7th power of the time taken for bubbles to develop. This is a similar dependency to the effect of scaling on cavitation erosion, which shows how the cavitation erosion rate increases with cavitating length.     

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。     

磁致伸缩仪超声空化流的三维非定常数值模拟

采用Singhal完全空化模型和SST k-ω湍流模型结合动网格技术对磁致伸缩仪超声空化流进行三维非定常数值模拟。计算结果表明,由于变幅杆高频振动,在靠近试样表面附近局部流场的压力和空泡体积组分变化具有周期性,压力波动的最低值可达到汽化压力,该局部流场可发生空化。由于空化,试样表面压力波动具有脉冲特征。压力和空泡体积组分在试样表面近似呈环形分布。在同一环形区域内,压力和空泡体积组分存在无规律断续脉动。试样表面中心区域空泡经历两次振荡后溃灭,产生强烈脉冲压力,最大脉冲压力可达约14MPa。脉冲压力在试样表面按间隔环形区域分布,且随试样振动在相邻环形区域上交替出现。在磁致伸缩仪超声空化流场中,试件表面可近似多个声波发生源,各声波传播时相互叠加和干扰。在声波传播的过程中压力衰减很快,只是在距试样表面约20mm内,压力有明显波动。     

Estimation of Incubation Time of Cavitation Erosion for Various Cavitating Conditions

Estimations have been made, resulting in a general method for the prediction of the incubation time for cavitation erosion using various cavitating conditions and materials. From a single erosion test, the incubation time can be estimated for various conditions and materials by plotting the mass loss as a function of exposure time to cavitation on a log–log scale.     

Cavitation damage prediction

New results from cavitating venturi water tests were used to reinforce the concept of cavitation erosion efficiency previously developed from tests in a vibratory facility with both water and sodium. The concept emerges from a technique which allows a priori prediction of eventual cavitation erosion rates in flow machines. Bubble collapse pulse height spectra obtained from submerged microprobes are correlated with measured erosion rates in given laboratory and/or field devices to allow this prediction. Preliminary results from such correlations are presented together with other measurements of the effects of gas content, velocity and cavitation condition upon the mechanical cavitation intensity as measured by the pulse height spectra.New results from vibratory facility tests in tap water and synthetic seawater upon three materials of variable corrodability (304 stainless steel, 1018 carbon steel and 1100-0 aluminum) are presented. The ratio between maximum erosion rates for the saltwater and freshwater tests were found to increase toward unity as the mechanical cavitation intensity is increased, i.e. increased mean depth to penetration (MDPR), as expected on theoretical grounds.The relation between the incubation period and MDPR_max was examined from the vibratory test results, and was found to depend upon the material properties as well as the fluid flow conditions.     

MICRO-COURSE OF CAVITATION EROSION

By cavitation tests and scanning electron microscope （SEM） microanalysis, the micro-appearance of cavitation samples is studied. It is the first time that the micro-appearance of metals is pursued successfully. According to the changing course of the micro-appearance of metals, the damaging course of cavitation erosion is determined. The destructive way of collapsing bubbles on the metal surface is known. Firstly cavitation pinholes appear on the metal surface, then cracks generate and grow under the action of collapsing bubbles. When cracks connect each other, small pieces are removed from pinhole wall and pinholes develop into cavitation pits. When the previous surface is removed completely, new pinholes are produced again on the new surface. A pinhole is the result of the powerful striking of a micro-liquid jet ejected by a large collapsed bubble near the surface. At some stages, cracks grow in the way of fatigue. The corrosion phenomenon is observed during the cavitation erosion. The cavitation pattern can be used to explain the cavitation pregnancy and the changing regulation of sample surface.     

三相磨料射流作用下材料的破坏机理研究

针对一种新型磨料射流形式，从摩擦学角度出发，分析材料在其作用下的破坏机理。结果表明材料磨蚀主要由冲蚀磨损、气蚀破坏以及它们的共混蚀损作用造成的。特别是共混蚀损，它是三相流中一种独特的现象，相互制约，相互促进，加速了材料的破坏。     

Experimental Study on Influences of Surface Materials on Cavitation Flow Around Hydrofoils

In order to resist on the cavitation erosion, many researchers try to change the solidity and tenacity of the coatings,but ignore the influence of surface characteristics of materials on cavitation flow and the interaction with each other.In this paper, high speed visualization system is used to observe the cavitation flow patterns in di erent stage. After comparing the characteristics of cavitation flow around hydrofoils made of aluminum(Foil A), stainless steel(Foil B)and the hydrofoil painted with epoxy coating(Foil C), the study shows that material has a significant e ect on the cavitation flow. Firstly, when the incipient cavitation occurs, cavitation number of Foil A is highest among three hydrofoils, generating horseshoe vortex randomly. For Foil B and Foil C, it shows in the form of free bubbles. When the sheet cavitation occurs, Foil A has the highest cavitation number and shortest period, which is contrary to Foil C. And cavity consists of lots of small finger-like cavities. For Foil B and Foil C, it both constitutes with many bubbles. Compared with the high-density and small-scale cavities over surface of Foil C, the cavity of Foil B has larger scale and less density,which causes a minimal scope of influence of the re-entrant jet and strong randomness. When the cloud cavitation occurs, Foil C has the lowest cavitation number and shortest period. Secondly, compared with aluminum, both of stainless steel and epoxy coating restrains the occurrence and development of cavitation, and stainless steel and epoxy coating performs better than aluminum. For inception and sheet cavitation, stainless steel performs better than epoxy coating and aluminum. For cloud cavitation, epoxy coating performs better than stainless steel and aluminum.The objective of this paper is applied experimental method to investigate the e ect of surface materials on cavitation around Clark-Y hydrofoils.     

超声空化对细微流道中固体颗粒的运动特性研究

针对细微流道中的多相流调控问题,研究超声空化效应对微流道内固体颗粒运动特性的影响。利用Fluent有限元技术对空化气泡溃灭的过程进行模拟,得在超声声压作用于近壁区的气泡时,气泡凹陷、破壁、溃灭的演变过程,并对其周围流场的速度矢量分布进行研究,仿真结果显示,空化气泡溃灭能够产生射向壁面的高速微射流,其最大速度达到28m/s,进而确定细微流场观测实验的最优参数;利用高速摄像机对细微流道中气泡的演变过程进行观测实验,并与仿真实验结果进行对照,观测实验结果表明,利用超声空化效应能够实现对固体颗粒向流道壁面运动的有效引导,为实现细微流道的流场调控、提高加工精度等问题提供理论支持。     

超声振动珩磨单空化泡溃灭温度研究

为了研究超声振动珩磨作用下的空化效应,基于超声空化动力学和热力学基本定律,建立了超声振动珩磨单空化泡热力学方程;数值模拟了各珩磨参数对空化泡运动半径的影响,间接分析了各参数对单空化泡溃灭温度的影响。结果显示,珩磨参数对空化泡溃灭温度影响微弱,单空化泡的溃灭温度大约为300～1400K;在超声钻床上,以铝箔纸为试验材料进行了超声空化试验,试验结果与理论分析基本一致。     

冲蚀与空蚀交互磨损三相流场仿真与试验研究

基于计算流体力学方法,数值研究在模拟水轮机工况下,冲蚀与空蚀交互作用时,试件表面上汽液固三相流场的动力学特性(压力场、速度场、汽相体积比分布),然后在转盘式磨损装置上,进行汽液固三相冲蚀与空蚀交互磨损试验,并对试件微观形貌进行分析。结果表明:数值分析得到试件表面最小和最大压力值、气泡速度最大值和汽相比例最大值均出现在空化孔附近,其他位置基本不变,说明空化孔附近交互磨损比单一空蚀磨损严重。就某一空化孔而言,顺着转盘旋转方向的孔边某一夹角展开区域汽相比例较大,并且出现最小和最大压力;从试件微观磨痕分析,其上存在短程犁沟和空蚀孔,磨痕呈现规律性。仿真的气泡轨迹和试件磨痕基本一致,数值计算结果和试验结果吻合得较好,从而证明了数值分析的正确性和合理性。上述数值仿真为揭示流体机械过流部件冲蚀与空蚀交互作用磨损机理奠定了基础。     

Cavitation erosion resistance of Cr–N coating deposited on stainless steel

Results of investigation on cavitation-erosion resistance of Cr–N coating deposited on stainless steel X6CrNiTi18-10 (1H18N9T) by means of the cathodic-arc method are presented. The evaluation of Cr–N coating resistance to cavitation erosion is based on the investigation performed in a cavitation tunnel with a slot cavitator and tap water as a medium. The investigation was performed at variable-cavitation intensity and the estimated cavitation resistance parameters of coatings were the incubation period of damage and the instantaneous erosion rate after exposure of specified duration. It has been confirmed that the incubation period of the Cr–N coating damage is approximately 50% longer than that of the uncoated X6CrNiTi18-10 steel, and the instantaneous erosion rate after exposure of specified duration is comparable in both cases. The scanning microscope analysis indicates that the damage of Cr–N coating is due mainly to its delamination, while the erosion of deeper parts of the coating is of minor importance. The character of the coating and substrate damage in multiple locations indicates that the hard coating microparticles torn-off during the cavitation bubbles implosion hit against the coating and the revealed areas of substrate. As a result, the coating and especially the substrate of relatively low hardness are subject to cavitation erosion and to solid particle erosion with the hard torn-off microparticles of coating. The results of the investigation and the analysis indicate that the factors mainly responsible for a long incubation period and low cavitation erosion rate of the steel substrate/hard coating systems are the gained high hardness of substrate and high level of coating adhesion.