Ayat Karimi

Advanced Experimental and Numerical Techniques for Cavitation Erosion Prediction

This book provides a comprehensive treatment of the cavitation erosion phenomenon and state-of-the-art research in the field. It is divided into two parts. Part 1 consists of seven chapters, offering a wide range of computational and experimental approaches to cavitation erosion. It includes a general introduction to cavitation and cavitation erosion, a detailed description of facilities and measurement techniques commonly used in cavitation erosion studies, an extensive presentation of various stages of cavitation damage (including incubation and mass loss), and insights into the contribution of computational methods to the analysis of both fluid and material behavior. The proposed approach is based on a detailed description of impact loads generated by collapsing cavitation bubbles and a physical analysis of the material response to these loads. Part 2 is devoted to a selection of nine papers presented at the International Workshop on Advanced Experimental and Numerical Techniques for Cavitation Erosion (Grenoble, France, 1-2 March 2011), representing the forefront of research on cavitation erosion. Innovative numerical and experimental investigations illustrate the most advanced breakthroughs in cavitation erosion research.

Comparison of erosion mechanisms in different types of cavitation

Abstract A new cavitation erosion device producing vortex cavitation has been extensively used. A comparative study between various cavitation erosion situations was carried out to verify the ability of this vortex cavitation generator to produce realistic cavitation erosion with respect to that observed in hydraulic machinery. For this purpose, specimens of indium and α + β brass were subjected to different cavitation erosion situations in a Francis turbine model, a cavitation water tunnel, a vibratory cavitation device and our vortex cavitation generator. The surface deformation and the development of damage in exposed specimens were examined using scanning electron microscopy. In all cases, except for the vibratory cavitation device, the damage starts with the formation of isolated hollows and craters of similar morphologies and sizes, produced by collapse impingements. The accumulation of isolated damage distributed statistically over the specimens results in erosion. Meanwhile, for vibratory cavitation the damage is initially scattered uniformly over the specimen surface and develops progressively. In spite of this, the topographies of severely eroded surfaces in various types of cavitation did not present noticeable differences. However, transmission electron microscopy observations of subsurface microstructures in eroded specimens indicate the same arrangements of dislocations and the appearance of largescale deformation twins. Hardened superficial layers in specimens exposed to flow cavitation are thicker than those in vibratory cavitation, which leads to higher erosion rates.

Ripple formation in solid-liquid erosion

Abstract The formation of ripples on the surface of materials subjected to small-angle impingement erosion has been widely observed. In order to determine the origin and development of surface ripples, ductile (stainless steels and copper-based alloys) and brittle (thermal spray ceramics and cermets) materials were eroded in laboratory and fields tests. Scanning electron microscopy investigation revealed that the predominant wear mechanisms were scratching in metallic alloys, spray particle break-out in ceramics and removal of the binding matrix in cermets. Despite substantial differences in erosion mechanism, all the materials tested formed ripple patterns on their surfaces. It was determined that the ripple size increases with time and can attain a steady state that reflects local fluid flow conditions. Cermets and ceramics display the onset of rippling under more severe erosive conditions than metals. Ripple formation can induce cavitation, particularly on the lee side of ridges. Tests using water free of solid particles also gave rise to ripples, thereby confirming the importance of flow in their formation. Several hypotheses and models are discussed with respect to the results and a model based on the interaction between eddies and surface profile is presented.

Impact Load Measurements in an Erosive Cavitating Flow

Impact load measurements were carried out in a high-speed cavitation loop by means of a conventional pressure sensor flush-mounted in the region of closure of the cavity where maximum damage was observed. The sensor was dynamically calibrated by the ball drop test technique. Pressure pulse amplitudes were measured at different velocities and constant cavitation number and cavity length. It was found that pressure pulse height spectra follow a simple exponential law, which depends upon two parameters interpreted as a reference peak rate and a reference load. By exploring the dependence of both parameters on flow velocity, it was possible to show that the various histograms measured at different velocities can be reduced to a unique non-dimensional one and derive scaling laws, which enable to transpose results from one velocity to another. The measured values of impact loads are compared to similar data in the literature, and the impact load spectra are discussed with respect to pitting test results available from a previous investigation. It is concluded that an uncertainty remains on the measured values of impact loads and that a special effort should be made to compare quantitatively pitting test results and impact load measurements. To evaluate the coherence of both sets of data with each other, it is suggested to introduce two-dimensional histograms of impact loads by considering the size of the impacted area in addition to the measured impact load amplitude. It is conjectured that the combination of impact load measurements and pitting test measurements should allow the determination of such two-dimensional histograms, which are an essential input for analyzing the material response and computing the progression of erosion with exposure time. [DOI: 10.1115/1.4005342]

Water droplet erosion and microstructure of laser-nitrided Ti6A14V

Abstract The ability of laser nitriding to improve water droplet erosion resistance of Ti6Al4V alloy was studied. Using a CO 2 continuous laser, a layer of about 400 μm thickness was nitrided and another layer of 400–500 μm was only heat affected. Electron microscopy observations showed that the microstructure of nitrided layers consists essentially of TiN compounds which are embedded in Ti(α) matrix. Depending on the nitrogen concentrations within the feeding gas, the titanium nitrides exhibited plate-like shape or dendritic morphologies. Below the nitrided layer a thickness of 50–100 μm of samples underwent martensitic structure which in its turn gives rise progressively to bimodal (α + β) base material. Laser nitriding increased microhardness from 370–400 to 650–800kgf mm −2 , and enhanced erosion resistance significantly compared with untreated Ti6A14V and hardened 12% Cr stainless steel. The mechanism of material removal by erosion was changed from work hardening and platelets detachment in untreated samples to brittle fracture by formation of large flakes and spalling in nitrided layers. Advanced stages of erosion are accompanied by the appearance of macrocracks of ten in the nitrided zone, but some of them propagate even into heat-affected area. The annealing at 650 and 700°C of the laser-nitrided samples results in precipitation of β phase rich in vanadium.

First-principles study of the effect of nitrogen vacancies on the decomposition pattern in cubic Ti1−xAlxN1−y

The effect of nitrogen substoichiometry on the isostructural phase stabilities of the cubic Ti1−xAlxN1−y system has been investigated using first-principles calculations. The preferred isostructu ...

Cavitation and Cavitation Erosion

In this chapter, an introduction to cavitation and cavitation erosion is presented. Cavitation involves the development of various types of vapor structures (such as attached cavities, travelling bubbles, vortical cavities, bubble clouds) in liquid flow due to a drop in the local pressure below a critical value usually close to the vapor pressure. These structures generally originate from cavitation nuclei, typically gaseous microbubbles contained in the liquid. The critical pressure of a nucleus is defined as the particular value of the pressure below which no equilibrium is possible. If a nucleus is subject to pressure lower than its critical pressure, it will explosively grow into a macroscopic cavitation bubble. The bubble will collapse when transported by the liquid flow into regions of pressure recovery. If the collapse occurs near a wall, the resulting high amplitude and small duration impulsive loads may cause local damage. Repeated impulsive loads may cause increasing cavitation erosion damage. The response of the material to cavitation impulsive loads is discussed and material properties most relevant to cavitation erosion, such as sensitivity to strain rate, are presented.

A novel synthetic strategy for bioinspired functionally graded nanocomposites employing magnetic field gradients

In order to mimic the complex architecture of many bio-materials and synthesize composites characterized by continuously graded composition and mechanical properties, an innovative synthetic strategy making use of magnetic field gradients and based on the motion of superparamagnetic Fe3O4@SiO2 core–shell nanoparticles is adopted. It is demonstrated that by lowering the viscosity of the system through particle functionalization, and increasing the magnetic force acting on the nanoparticles upon optimization of a simple set-up composed of two permanent magnets in repulsion configuration, the magnephoretic process can be considerably accelerated. Thus, owing to the magnetic responsiveness of the Fe3O4 core and the remarkable mechanical properties of the SiO2 shell, approximately 150 μm thick polymeric films with continuous gradients in composition and characterized by considerable increments in elastic modulus (up to ≈70%) and hardness (up to ≈150%) when going from particle-depleted to particle-enriched regions can be synthesized, even in times as short as 1 hour. The present methods are highly promising for a more efficient magnetic force-based synthesis of inhomogeneous soft materials whose composition is required to be locally tuned to meet the specific mechanical demands arising from non-uniform external loads.

Nanoindentation of Functionally Graded Polymer Nanocomposites: Assessment of the Strengthening Parameters through Experiments and Modeling

NNanoindentation tests were carried out on the surface of polymer nanocomposites exhibiting either graded or homogeneous distributions of Fe3O4@silica core-shell nanoparticles in a photocurable polymeric matrix. The results reveal a complex interplay between graded morphology, indentation depth and calculated modulus and hardness values, which was elucidated through numerical simulations. First, it was experimentally shown how for small (1 µm) indentations, large increases in modulus (up to +40%) and hardness (up to +93%) were obtained for graded composites with respect to their homogeneous counterparts, whereas at a larger indentation depth (20 µm) the modulus and hardness of the graded and homogeneous composites did not substantially differ from each other and from those of the pure polymer. Then, through a Material Point Method approach, experimental nanoindentation tests were successfully simulated, confirming the importance of the indentation depth and of the associated plastic zone as key factors for a more accurate design of graded polymer nanocomposites whose mechanical properties are able to fulfill the requirements encountered during operational life.

Laboratory Testing Methods of Cavitation Erosion

This chapter presents in detail several cavitation erosion testing methods commonly used in the laboratory. The vibratory cavitation apparatus (G32) is described with its two variants, the direct method using a specimen attached to the vibrating tip of the ultrasonic horn and the alternative method using a fixed specimen facing the horn tip. In the cavitating jet apparatus (G134 and its variants), a jet is discharged at high pressure and velocity in a cell whose pressure may be controlled to adjust the cavitation number. This results in a shear type cavitation whose aggressiveness may be enhanced by a proper design of the nozzle shape and piping assembly. A high-speed cavitation tunnel equipped with a radial divergent test section is also presented. This particular test section generates an unsteady cavity attached to the nozzle exit with cavitation erosion damage concentrated in the cavity closure region. Usual testing procedures together with typical erosion patterns and mass loss results obtained in such facilities are also presented.