Volume Fractions Of Cavities Research Articles

Investigation of Cavitation Flow and Entropy Production Characteristics in a Dual-Rotor Turbine Flowmeter

Flow meters are extensively utilized in fields such as chemical engineering, petroleum, and aerospace, and are an indispensable component of modern industry. This paper examines the metrological properties of a dual-rotor turbine flow meter within its measurable flow range through experimental approaches and investigates the cavitation flow dynamics within the flow meter using numerical methods. First, the flow characteristics curve of the dual-rotor turbine flow meter was established experimentally, and the accuracy of numerical simulation results was validated. Secondly, the transient characteristics of the cavitation cavity were revealed using the Z-G-B cavitation model and dynamic mesh technology. Finally, entropy production theory was applied to investigate the energy losses caused by cavitation, analyzing the contributions of different types of energy losses during the cavitation process. Flow calibration experiments and numerical simulations reveal an increase in the meter coefficient of the dual-rotor turbine flow meter in high-flow cavitation zones, indicating that the displayed flow rate is slightly higher during cavitation compared to non-cavitating flows. Transient cavitation flow undergoes three stages: attachment, development, and collapse. At 323 K, the volume fractions of upstream and downstream cavities increase by 38.9% and 48.3%, respectively, with the cavitation cycle duration being 1.21 times that at 298 K. At 343 K, these increases are 75.3% and 239.2%, with the cycle duration being 2.63 times that at 298 K. Among the various sources of loss, the contribution from losses due to pulsating velocity gradients is the most significant, with maximum proportions of 81.95%, 85.1%, and 87.11% at 298 K, 323 K, and 343 K, respectively.

Pore-scale investigation on effects of void cavity distribution on melting of composite phase change materials

Composite Phase Change Materials (PCMs) incorporated with metal foams are promising candidates for thermal management for space exploration. However, void cavities are generated as a result of the volume change of PCM during the melting process, introducing a resistance to heat transfer due to the low thermal conductivity of the void. In this work, the effects of void cavity distribution on the conduction-dominated melting of composite PCMs under microgravity conditions are studied by a two-dimensional pore-scale lattice Boltzmann method, in which a microstructural description of the metal foam is experimentally characterized with the help of X-ray micro-Computed Tomography. Two typical distribution patterns of void cavities are analsed and computed performance is compared (1) a near-wall void cavity, and (2) randomly distributed void cavities. The evolutions of temperature distributions and melting interfaces are compared, and the average liquid fraction and energy stored per width are deduced to describe the energy storage performance. Moreover, the influence of the volume fraction of void cavities is investigated by comparing temperature distributions and energy storage performances of composite PCMs with four different volume fractions of void cavities (0%, 3.7%, 7.6%, 15.2%). After introducing void cavities, the energy stored per width is reduced by 5.7%, 12.3% and 20.2% for randomly distributed void cavities when volume fraction of void cavities is 3.7%, 7.6%, and 15.2%, respectively, and reduced by 42.2%, 64.1% and 79.7% for the near-wall void cavity, respectively. This work initiates the study of the effects of void cavity distribution on composite PCMs, which will stimulate work on structural optimization of thermal management systems.

Comparative study of radiation defects in ion irradiated bulk and thin-foil tungsten

In this study, we employ transmission electron microscope (TEM) to analyze radiation defects in helium (He) and krypton (Kr) ions implanted bulk and thin-foil tungsten. For bulk tungsten, subgrains are formed near the surface region under both He+ and Kr+ irradiation. Dislocation loops are observed beyond ion implanted range. These observations are related to self-interstitial atoms (SIAs) diffusion and clustering. Ordered bubbles are formed in He+ implantation, while no cavities are detected in Kr+ irradiation. In thin-foil tungsten, line up of dislocation loops is found mainly aligns along {101} and {112} slip planes. Both 1/2<111> and 〈100〉 dislocation loops are identified. Compared to He+ irradiation, more 〈100〉 loops are detected in Kr+ irradiation due to higher energy collision cascade. Nanocavities are detected in irradiated thin-foil tungsten besides the formation of high density of interstitial loops. The number density of dislocation loops and the volume fractions of cavities are higher in He+ irradiation than in Kr+ irradiation. The differences in nature of radiation defects is attributed to the higher recombination rate of vacancies and interstitials in bulk sample, the significant surface sink effect in thin-foil irradiation and the chemical and physical effect of implanted ions.

Predicting effective moduli of a Z-reinforced core with cavities using a novel theoretical approach and a micromechanics finite element method

In this paper, a Z-reinforced sandwich structure with cavities is introduced as a promising structural form for modern underwater acoustic stealth structures. To theoretically predict effective moduli of its core layer, called Z-reinforced core with cavities, a novel approach is proposed by applying a multi-level homogenization model in which the composite core is equivalent to an orthotropic material. Via Mori–Tanaka formula, analytic effective moduli of the complex composite core are predicted based on two assumptions: (a) the cavity is assumed as an isotropic material with a weak modulus; (b) the Eshelby tensor in the second-level homogenization is assumed to be equal to the one in the first-level. Meanwhile, a representative volume element (RVE) for the Z-reinforced core with cavities is built and numerical effective moduli are calculated by the micromechanics finite element method (FEM). Several numerical examples are carried out to verify the competencies of the proposed approaches. The obtained results agree well with each other and also with available experimental data. Effects of volume fractions of cavities and reinforcements on effective moduli of the composite core are also discussed. Furthermore, some bending deformation and free vibration analyses for the Z-reinforced sandwich structure with cavities are conducted.

Prediction of material behavior during biaxial stretching of superplastic 5083 aluminum alloy

In order to predict the flow behavior of a superplastic Al-Mg alloy sheet under near biaxial tension mode, finite element simulation was performed using commercial FE software (ABAQUS). To capture the time-dependent plastic behavior of the material, three constitutive models, i.e., the Voce model, a power law, and a newly introduced variable m value viscoplastic (VmV) model, were implemented in FE simulations. The models’ parameters were assessed from the three uniaxial stress-strain curves ranging from 10−3 to 10−1 s−1. To validate the simulation results, Nakazima hemispherical dome testing was performed under isothermal conditions using a constant strain rate of ~ 0.01 s−1 at 450 °C. After the tests, the thickness of the deformed parts was measured and the volume fractions of cavities at different locations were assessed using X-ray micro-tomography, and the impact of the strain path on the rate of cavitation was discussed. Based on the obtained results, when the material behavior was modeled using the VmV model, the accuracy of the prediction was about 2 and 5 times better than the ones from power law and Voce model, respectively. It was also observed that the volume fraction of the cavities exponentially increases with equivalent plastic strain and it depends on the strain path history.

A viscoplastic model based on a variable strain rate sensitivity index for superplastic sheet metals

This paper presents a new phenomenological model for describing the main features of the viscoplastic behavior of superplastic sheet metals, namely, strain hardening, softening, and damage. The proposed model is based on a variable strain rate sensitivity index (m-value) measured from uniaxial tensile tests at different strain rates under constant temperature. In this study, the uniaxial tensile tests were carried out at three strain rates (i.e., 10−3, 10−2, and 10−1 s−1) on a superplastic grade AA5083 aluminum sheet alloy. In addition, the volume fractions of cavities at different plastic strain levels were assessed using X-ray microtomography. The performance of the model was investigated by comparing its predictions with the experimental data. In addition, the model was validated with two sets of reference data for AA5083 aluminum alloy and AZ31 magnesium alloy. In particular, it was observed that the new model could predict the flow behavior of these metals more successfully compared with two reference models; nevertheless, it requires minimal experimentation and calculation efforts.

Microscopic damage mechanism of SA508 Gr3 steel in ultra-high temperature creep

The lower head of reactor pressure vessel (RPV) will endure a great temperature gradient above the phase transition temperature, and the creep and fracture will be the primary failure mode for the RPV material in such a situation. The interrupted creep tests were performed on a typical RPV material, SA508 Gr3 steel, at 800 °C. The microstructure of different creep stages was examined by scanning electron microscopy and transmission electron microscopy. The results showed that the microscopic damage is mainly induced by creep cavities and coarse second-phase particles. Furthermore, the volume fractions of creep cavities and coarse second-phase particles show a linear relationship with the extended creep time. The second-phase particles are determined to be MoC in the second creep stage and Mo2C in the third creep stage, according to the results of selected-area electron diffraction pattern. Combined with energy-dispersive spectrum analysis, the segregation of precipitates caused by the migration of atoms is finally unveiled, which leads to the coarsening of the particles.

Depth distribution of disorder and cavities in high dose helium implanted silicon characterized by spectroscopic ellipsometry

Helium was implanted into Cz 〈111〉 p-type silicon wafers at 40 keV for various doses between 1×10 16 He + cm −2 and 1×10 17 He + cm −2. Furnace Anneals were subsequently applied at temperatures ranging from 650 °C up to 1000 °C. The optical models for the as-implanted samples are based on the coupled half-Gaussian model developed by Fried et al. As point defects and cavities coexist in the as-implanted sample, the original model combining two half-Gaussian profiles was modified. Both the distributions of point defects and cavities were described by Gaussian profiles in the new model, each of them using two coupled half-Gaussians. The optical model allows the overlapping of the profiles of defects and cavities. A fitting procedure, called ‘multiple random search’, was applied to minimize the probability of getting in a local minimum. Using this new model, the measured spectra were well fitted, while there was no acceptable fit possible with conventional models. The annealed samples could be well described by cavity profiles only. Different models were investigated including Gaussian profiles or profiles with arbitrary distributions using independent volume fractions of cavities in the sub-layers. The results were crosschecked by transmission electron microscopy micrographs, showing a good agreement.

Comparative study of porosity in 3Y-TZP superplastic ceramics by ultra-small-angle neutron scattering and scanning electron microscopy image analysis

Recent measurements of cavitation in superplastically strained 3Y-TZP ceramics by scanning electron microscopy and ultra small-angle neutron scattering (USANS) led to systematic differences in volume fractions of cavities determined by both methods. We show that the scattering curves measured by double crystal USANS diffractometers can be calculated directly from scanning electron micrographs taken from a single section of the material, without any assumptions of cavity shape or distribution. This technique was used to compare the USANS measurements with electron micrographs and showed that the calculated and measured curves agree well for medium scattering vectors. This result confirms that the electron micrographs represent well the microstructure of small cavities up to about 0.6 μm in radius. On the contrary, the calculations resulted in systematically larger intensities for small scattering vectors. This indicates that large cavities are responsible for the observed differences in volume fraction for the samples with nominal strain above 100%. An explanation by an inhomogeneous distribution of large cavities generated at high strains in the bulk of the tensile specimen and/or by surface artifacts is suggested.

Damage in a viscoplastic material—Part II. Overall behaviour

The overall behaviour of a damaged viscoplastic material under axisymmetric loading is addressed in this paper. A linear viscous matrix containing aligned spheroidal cavities is first considered. Its overall behaviour is estimated by means of a differential scheme. The damage-induced morphological anisotropy, due to the shape of the voids, is then analyzed. It is shown to be significant even at low volume fractions of cavities ( ƒ < 0.05 ), when the latter are prolate or oblate. These results are then used to predict the behaviour of a nonlinear viscous matrix containing aligned spheroidal cavities, by means of a variational principle proposed by Ponte Castañeda (1991). Two types of nonlinear matrix behaviour are considered: a power law corresponding to metal forming at moderate strain rates and a linear law with nonzero intercept suited to high strain rates.The constitutive equations of such nonlinear damaged materials have been used in a companion paper in order to study the influence of matrix compressibility on the growth of a spheroidal cavity during straining.

Cavitational Strain Contribution to Tensile Creep in Vitreous Bonded Ceramics

Analysis of the role of cavitation during uniaxial creep deformation in vitreous bonded ceramics reveals that the cavity volume contributes only to the strain in the direction parallel to the tensile stress regardless of the shape and orientation of cavities. Creep asymmetry results from the fact that cavitation preferentially contributes to axial tensile strain while the strain observed under the same conditions in compression is produced only by volume‐conserving mechanisms. The contribution of cavitational strain in the axial tensile strain is equal to the volume fraction of cavities and proportional to the difference between tensile and compressive strains in the axial direction. The density change method and a newly proposed method based on the difference in the axial strains were used for separating the cavitational from the true tensile strain in self‐reinforced silicon nitride. Both methods consistently revealed more than 90% contribution of cavitation to the total tensile strain. Cavitation is concluded to be the dominant mechanism of tensile creep deformation in vitreous bonded ceramics because the reported volume fractions of cavities during their deformation are usually in the range of 70–90% of tensile strain.

Structure and growth of hydrogenated microcrystalline silicon: Investigation by transmission electron microscopy and Raman spectroscopy of films grown at different plasma excitation frequencies

Abstract Microcrystalline silicon prepared by plasma-enhanced chemical vapour deposition consists of variable volume fractions of amorphous phase, grain boundaries, cavities and crystalline grains. In this paper the structural properties, which strongly depend on the growth conditions, were investigated in detail by transmission electron microscopy and by Raman spectroscopy. A columnar structure parallel to the growth direction is observed for all conditions investigated. By increasing the plasma excitation frequency the crystalline volume fraction and the grain sizes are enhanced. Simultaneously an increase in the growth rate can be achieved, which is accompanied by an increasing etch rate of amorphous material. In addition, spherical voids were found predominantly in samples prepared at a low plasma excitation frequency. The growth of a porous initial layer containing a high density of ‘crack-line’ voids is observed when high plasma excitation frequencies are applied. These results suggest that the microcrystalline growth is governed by the preferential etching of the amorphous phase. In addition, chemical reactions have to be taken into account to explain the formation of the spherical voids.

An approximate calculation on the elastic constants of a solid containing varying volume fractions of cavities

An approximate calculation for the elastic constants of a solid containing varying volume fractions of cavities is presented. The calculation is based on the mechanisms of deformation of solids containing varying volume fractions of cavities namely straining, bending and stretching. The work concentrates first on the deformation of solids containing a small number of cavities and then secondly on the deformation of the solid containing a large fraction of cavities. The analysis represents the unification of two modes of deformation of solids containing cavities in the dilute case with that of the concentrated case. It is found that K f K = (1 − φ) 2 (1 + ∝φ) + (1 − 2v)[1 −φ 2 e(1 − φ) 2 + (1 − φ e)(1 − φ)] (1 − 2v f G f G = (1 − φ) 2 I + βφ + 3 8 e G φ[φ 2 e(1 − φ) 2 + (1 − φ e)(1 − φ)] , where ∝= 1 2 (1 + v) (1 − 2v) , β = 2 (4 − 5v) 7 − 5v , φ is the volume fraction, K is the bulk modulus of the material, K f is the effective bulk modulus of the material with cavities, G is the shear modulus of the material, G f is the effective modulus of the material with cavities, e is the elastic modulus of the material, ν is the Poisson ratio of the material, ν f Poisson ratio of the cellular material and φ e is the fraction of solid in the struts.

Superplastic deformation characteristics of microduplex Ti–6Al–4V alloy

A study has been made of the superplastic tensile behaviour of a microduplex Ti–6AI–4V alloy in the temperature range 760–940°C. Maximum superplastic elongations were obtained for a temperature of 880°C at which the volume fraction of β–phase was approximately 0·4. At temperatures below about 850°C, the main microstructural changes which occurred involved dynamic grain refinement and cavitation. The cavities, which occurred at α/α and α/β boundaries, interlinked parallel to the tensile axis at high strains. However, the volume fractions of cavities observed, although not quantitatively assessed, were always relatively small and had no significant effect on the fracture behaviour of tensile specimens, all of which pulled down to a fine point or chisel edge at failure. At temperatures greater than 850°C, no cavities were observed except near the fracture surface, and the most important structural change was grain growth. At all the temperatures examined, the structural changes which occurred during superplastic deformation were accompanied by strain hardening.MST/323

Superplastic deformation characteristics of microduplex Ti–6Al–4V alloy

Abstract A study has been made of the superplastic tensile behaviour of a microduplex Ti–6AI–4V alloy in the temperature range 760–940°C. Maximum superplastic elongations were obtained for a temperature of 880°C at which the volume fraction of β–phase was approximately 0·4. At temperatures below about 850°C, the main microstructural changes which occurred involved dynamic grain refinement and cavitation. The cavities, which occurred at α/α and α/β boundaries, interlinked parallel to the tensile axis at high strains. However, the volume fractions of cavities observed, although not quantitatively assessed, were always relatively small and had no significant effect on the fracture behaviour of tensile specimens, all of which pulled down to a fine point or chisel edge at failure. At temperatures greater than 850°C, no cavities were observed except near the fracture surface, and the most important structural change was grain growth. At all the temperatures examined, the structural changes which occurred during...