Influence of temperature on erosion by a cavitating liquid jet

The influence of water temperature on cavitation erosion has previously been studied using a vibratory apparatus, but no researches have been conducted at a constant cavitation number in flow condition. This study deals with the influence of the water temperature on cavitation erosion using a cavitating jet apparatus. The optimum stand-off distance at 25 °C was 11, 15, 21 and 25 mm at cavitation number of σ = 0.03, 0.025, 0.02 and 0.015, respectively, and was almost the same as that of a guideline in the ASTM G134 standard. The optimum stand-off distance at 75 °C is similar to that at 25 °C. The erosion rate increases with the liquid temperature and reaches a peak, followed by a decrease. The relative temperature was defined as 0 °C for freezing temperature and 100 °C for boiling temperature of pressurized water. The peak appears at the approximate average of freezing and boiling relative temperatures. The erosion rate increases by 1%/ °C between 5 and 45 °C of relative temperatures, and decreases by 2%/ °C between 45 and 80 °C.     

Effect of geometrical parameters on submerged cavitation jet discharged from profiled central-body nozzle

The flow characteristics of cavitation jets are essential issues among relevant studies. The physical properties of the jet are largely determined by the geometrical parameters of the nozzle. The structure and cavitation jets characteristics of the angular-nozzle and the self-resonating cavitation nozzle have been extensively studied, but little research is conducted in the central-body cavitation nozzle mainly because of its hard processing and the cavitation jet effect not satisfactory. In this paper, a novel central-body nozzle (a non-plunger central-body nozzle with square outlet) is studied to solve above problems. Submerged jets discharged from the novel central-body nozzle are simulated, employing the full cavitation model. The impact of nozzle configuration on jet properties is analyzed. The analysis results indicate that when central-body relative diameter keeps constant, there is an optimal contraction degree of nozzle’s outlet, which can induce intense cavitation in the jet. The central-body relative diameter also affects jet profiles. In the case of large central-body relative diameter, most of the bubbles settle in the jet core. On the contrary, a smaller relative diameter makes bubbles concentrate in the interface between the jet and its surrounding fluid. Moreover, the shorter outlet part allows the cavitation zone further extend in both the axial and racial directions. The research results further consummate the study on the central-body nozzles and the correlation between cavitation jet and the structure, and elementarily reveal the mechanism of cavitation jet produced in a non-plunger novel central-body nozzle and the effect of the structure parameters on the cavitation jet, moreover, provide the theoretical basis for the optimal design of the nozzle.     

Effect of the stress level on the cavitation erosion rate

The effect of stress on the cavitation erosion rates of cast iron and mild steel was studied using a specially developed ultrasonic test rig. The mechanism of erosion experienced in the test rig is discussed.     

Investigation of cavitation erosion using X-ray residual stress analysis

In this paper it will be shown that by X-ray residual stress analysis it is possible to obtain information on the cavitation process after only short cavitation times. To clarify the correlation between cavitation erosion and changes in residual stress, samples of various steels were tested in a flow cavitation device. The residual stress measurements showed that cavitation leads to the development of compressive residual stresses in the surface of the samples. The rate at which these residual stresses develop depends on the intensity of cavitation. Cavitation-induced changes in residual stress may also be observed in samples with pre-existing residual stresses due to handling, e. g. mechanical working, heat treatment etc. After very short cavitation times the local variation in cavitation intensity may be very clearly seen in the residual stress distribution in the surface. After a longer period of cavitation, compressive residual stresses reach a limiting value which is not exceeded even at points of highest cavitation intensity.     

Comparison of cavitation erosion test results from venturi and vibratory facilities

A detailed comparison of cavitation erosion performance in tap water for five alloys in a vibratory (no-flow) system and a Venturi (flow) system was made. The effects of temperature variation (80 – 200 °F), Venturi throat velocity (34 – 49 m s^?1) and vibratory horn double amplitude were studied. Correlations between maximum erosion rate (maximum mean depth of penetration rate (MDPR_max)) and incubation period IP, and the material mechanical properties Brinell hardness and ultimate resilience $\text{UR =}\text{UTS}{}^2\text{2E}$. (where UTS is the ultimate tensile strength and E is the elastic modulus), were examined. Only moderate success was achieved in correlations between “erosion resistance” MDPR_max^?1 and IP and these mechanical properties. However, a good correlation was found between MDPR_max and IP, pertinent to both facilities, of the form MDPR_max^?1 = aIPⁿ, where n is near unity (0.94). The cavitation intensity, as measured by MDPR_max, was found to be 10–20 times greater in the vibratory system, depending on horn amplitude and material. This ratio varies between 5 and 30 if individual materials are considered separately, being greatest for 1018 carbon steel and least for 316 stainless steel. This indicates the important differences in form between these cavitating regimes and the imprecision of material comparisons made in both regimes.     

Effect of nozzle geometry for swirl type twin-fluid water mist nozzle on the spray characteristic

Experimental investigations on the atomization characteristics of twin-fluid water mist nozzle were conducted using particle image velocimetry (PIV) system and particle motion analysis system (PMAS). The twin-fluid water mist nozzles with swirlers designed two types of swirl angles such as 0°, 90° and three different size nozzle hole diameters such as 0.5mm, 1mm, 1.5mm were employed. The experiments were carried out by the injection pressure of water and air divided into 1bar, 2bar respectively. The droplet size of the spray was measured using PMAS. The velocity and turbulence intensity were measured using PIV. The velocity, turbulence intensity and SMD distributions of the sprays were measured along the centerline and radial direction. As the experimental results, swirl angle controlled to droplet sizes. It was found that SMD distribution decreases with the increase of swirl angle. The developed twin-fluid water mist nozzle was satisfied to the criteria of NFPA 750, Class 1. It was proven that the developed nozzle under low pressures could be applied to fire protection system.     

Effect of liquid metal composition and hydrodynamic parameters on cavitation erosion

Cavitation erosion testing machine for low-temperature melting alloy liquid was developed by using a vibratory apparatus. The erosion tests of SUS304 were carried out in three kinds of lead–bismuth and deionized water. We defined a relative temperature as the percentage between freezing and boiling points. At relative temperature at 14 °C, the erosion rate is 10–12 times in various lead–bismuth alloys, and 2–5 times in sodium, as compared with that in deionized water. When SUS304 was exposed to a cavitation in PbBi, the surface was work hardened 20% harder compared with original surface. In deionized water, SUS304 was work hardened by 5%. Therefore, we can conclude that larger collapse pressure can be estimated to act on the specimen surface in lead–bismuth, as compared with that in water.We discussed the effect of hydrodynamic properties on cavitation erosion in a flowing system. It is considered that the erosion rate in sodium is in proportion to 1st to 6th power of flow velocity similarly to that in mercury. The incipient cavitation number is approximately unity irrespective of test liquids. Furthermore, the relation between MDER and cavitation number is expressed as power low of function with an exponent of 2.5.     

Numerical study of hydrofoil geometry effect on cavitating flow

This paper describes the influence of geometrical parameters on the hydrofoil performance in non-cavitating and cavitating flows. The two-phase incompressible Navier-Stokes solver is used to compute the hydrofoil performance in two operating conditions. The new hydrofoil useful for performance improvement is obtained through the optimization design with RSM and the application of nose-drooping geometry. In the optimization design with the response surface model between hydrofoil performance and hydrofoil geometry, it is observed that the design concept of maximum lift-to-drag ratio is appropriate for the improvement of hydrofoil performance. The application of nose-drooping design concept results in the increase of hydrofoil performance in non-cavitating and cavitating flows.     

Effects of applied stress on cavitation erosion

Cavitation erosion under static applied stress and/or alternating stress was studied using steel specimens which were set in close proximity to an oscillating horn in ion-exchanged water. For increasing static applied tensile or compressive stress, weight loss and its rate do not vary in a monotonic fashion but first decrease, then increase through a peak, and then decrease again. Tensile stress except for given stress regimes, and compressive stress at all stress levels, decreases erosion damage compared with zero-stress values. Under alternating stress, the weight loss rate varies with trends similar to those under static applied stress. However, the weight loss rate is larger than for the same static stress, so that the erosion damage is more affected by alternating stress than by static stress. The behaviors under applied stress are discussed through the effect of stress on the erosion particles.     

Effect of separation distance on cavitation erosion of vibratory and stationary specimens in a vibratory facility

Vibratory cavitation erosion with vibratory and stationary specimens is studied for three materials in tap water at room temperature. The separation distance is varied from 0.127 to 6.096 mm. Test materials were commercially pure lead, soft (1100-O) aluminum and type 316 stainless steel. The double-horn amplitude was 58.4 μm (2.3 × 10^?3in) in a 20 kHz facility. The total duration of all tests was 10 min.The weight loss of both vibratory and stationary specimens of course depends on materials. The weight loss of stationary specimens is best correlated as a function of the reciprocal of the separation distance.     

A comparison of liquid impact erosion and cavitation erosion

A comparison has been made between the response of several metals and alloys to multiple liquid jet impact and their response to ultrasonically induced cavitation. For the single-phase metals and alloys, namely aluminum, iron, Al-1%Cu, Al-1%Mg and Cu-30%Zn, the failure mechanism is the same for the two techniques. However, there are significant differences for Al-4%Cu and Al-9%Mg. The former alloy fails in cavitation by the growth of macroscopic fatigue-like striated pits, whereas under liquid impact the fracture appears more like transcrystalline cleavage. The differences in the Al-9%Mg alloy are even more dramatic: under cavitation the surface becomes covered with macroscopic striated pits, whereas under liquid impact failure is intercrystalline. Although the magnitude and duration of the pressure pulses produced by the two techniques are similar, all the materials studied required three to four orders of magnitude more impacts by avitation than by liquid impact for a given amount of erosion. The greater erosive power of the liquid impact is attributed to the shearing effects of lateral flow of the liquid after impact.     

Effect of laser processing on the cavitation erosion of Cr-Mo-Cu alloy cast iron

The cavitation behaviour of Cr-Mo-Cu alloy cast iron (cylinder liner material of diesel engines) samples processed by a continuous wave CO₂ laser was investigated with a magnetostriction apparatus. Untreated samples manifested the lowest resistance to cavitation erosion. Laser surface-fused and phase transformation hardened samples came next, but the improvement was very small. Samples clad with a Co 50 alloy by means of a laser exhibited excellent cavitation resistance, so much so that no erosion hole was created on the surface of the alloy layer after a 600 min cavitation test. The main mechanism in the cavitation erosion process for each microstructure is described and discussed.     

高压喷嘴的射流仿真研究

针对高压喷嘴射流发散快以及速度稳定性差等问题,将Fluent仿真技术应用到高压喷嘴的流场分析中.根据高压喷嘴结构特点,为喷嘴及外部喷射区域建立了轴对称的几何模型;并根据喷射过程中流体速度的变化情况进行了网格的划分与加密,然后对高压喷嘴的淹没与非淹没射流流场以及不同收缩角时的非淹没射流流场进行了仿真分析;在流体理论的基础上提取了各种情况下的速度分布图,并对其结果进行了对比分析.研究结果表明,收缩角的不同对喷嘴流场的轴线的速度变化影响较大,当收缩角为10°时流场速度的稳定性最好;此外,通过对淹没射流与非淹没射流的流场情况进行比较,得出了非淹没射流对速度的集中更为理想的结论;仿真分析结果也为高压喷嘴结构及喷射形式的选取提供了一定的依据.     

后锥角对串联空化腔室的空化特性的数值研究

针对串联式空化腔室的空化强度和效率存在的问题,展开了对其影响较大的后锥角β值的数值研究,为今后的空化腔室设计提供理论依据。采用Mixture多相流模型,以及k-?两方程湍流模型,在Fluent软件中仿真模拟后锥角β值连续变化的多种串联空化腔室流体域模型,抽取分析计算结果中的压力,速度及空泡体积分数的云图,进而研究后锥形角β值对空化流场的影响。研究结果表明:在后锥形角β值取15?附近能够获得强度相对较高的空化现象;产生空泡体积的大小、负压力的区域大小以及沿中轴线维持恒定速度的能力三者成正相关关系;后锥角β值直接影响着空化流场的分布情况,决定着最终喷射到工件表面的作用范围。     

Material and velocity effects on cavitation erosion pitting

Cavitation erosion during the incubation period was investigated via pitting tests conducted on three different materials: an Aluminum alloy, a Nickel Aluminum Bronze alloy and a Duplex Stainless Steel. Pitting tests were conducted in a cavitation tunnel in the velocity range 45–90 m/s at a constant cavitation number. The test section was made of a straight nozzle 16 mm in diameter discharged into the radial 2.5 mm space between two flat walls. Cavitation appears in the form of a toroidal cavity attached to the nozzle exit and damage on the samples facing the nozzle is concentrated in a circular ring centered in the cavity closure region. The exposure time was adjusted to avoid pit overlapping. The material surface was examined using a conventional contact profilometer which allowed us to identify the pits, count them, and measure their main characteristics such as depth, surface area, and volume. From these the pitting rate, the coverage rate, and the depth of deformation rate were defined. Pits were classified according to their diameter. For all materials and operating conditions, pitting rate appears to follow an exponential law in relation to the pit diameter. This law depends upon two parameters only, which were identified as the coverage time τ (i.e. the time required for the surface to be covered by erosion pits) and a characteristic pit diameter δ, which corresponds to the pits whose contribution to the coverage process is the highest. Scaling laws for pitting were derived accounting for both material properties and flow velocity, and a procedure to make pitting test results non-dimensional is proposed. The influence of the material on pitting test results was analyzed. It is shown that the damage is not correlated in simple terms with the elastic limit determined from conventional tensile tests and it is conjectured that other parameters such as the strain rate might play a significant role and should be included in the analysis. The effect of flow velocity on both parameters τ and δ was analyzed and a classical power law was found for the influence of the flow velocity on pitting rate for all three materials. Finally, some analysis and discussion is given concerning distributions of pit volume and pit depth.     

Scaling study of cavitation pitting from cavitating jets and ultrasonic horns

Cavitation erosion prediction and characterization of cavitation field strength are of interest to industries suffering from cavitation erosion detrimental effects. One means to evaluate cavitation fields and materials is to examine pitting rates during the incubation period, where the test sample undergoes localized permanent deformations shaped as individual pits. In this study, samples from three metallic materials, an Aluminum alloy (Al 7075), a Nickel Aluminum Bronze (NAB) and a Duplex Stainless Steel (SS A2205) were subjected to a vast range of cavitation intensities generated by cavitating jets at different driving pressures and by an ultrasonic horn. The resulting pitted sample surfaces were examined and characterized with a non-contact 3D optical scanner and the resulting damage computer-analyzed. A statistical analysis of the pit population and its characteristics was then carried out. It was found that the various cavitation field strengths can be correlated to the measured pit distributions and that two characteristic quantities: a characteristic number of pits per unit surface area and unit time, and a characteristic pit diameter or a characteristic pit depth can be attributed to a given “cavitation intensity level”. This characterization concept can be used in the future to study the cavitation intensity of the full scale and to develop methods of full scale predictions based on model scale erosion data.     

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     

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.     

Effect of process parameter on the kerf geometry in abrasive water jet milling

This research work introduces a model to predict the kerf profile in abrasive water jet slot milling in aluminium 7075-T651. The experimentation allowed modelling the maximum cutting depth and the width at the half of maximum depth in terms of four process parameters: pressure, abrasive mass flow rate, stand-off distance, and traverse feed rate. It is shown that the introduction of the maximum depth and the width at the half of maximum depth in a Gaussian function is suitable to describe the kerf profile. The definition of an equivalent traverse feed rate at every instant along the jet trajectory introduces the effect of the jet acceleration in the model. Thus, the model is capable of predicting the kerf profile at constant traverse feed rate and at variable traverse feed rate due to direction changing trajectories. The variations of cutting depth along a slot are also studied in order to avoid the cutting parameters combinations which lead to low repetitiveness conditions and then irregular milling surfaces.