Mean Void Research Articles

Biologically induced calcium oxalate mineralization on 15th century lime mortar, Murud Sea fort, India

The historic lime mortar of Murud Seafort, Western India has endured the strong oceanic current in contrast to the basaltic stone blocks used in construction that have extensively weathered. The reason for the survival of the historic lime mortar was predominantely researched. The mortar samples were therefore characterized for their composition and microstructure, using granulometric analysis, thin section analysis, X-ray Fluorescence (XRF), X-ray powder diffraction (XRD), Fourier transform infrared (FTIR), Nuclear Magnetic Resonance (NMR), and Scanning Electron Microscopy with Energy Dispersive X-Ray Analysis (SEM-EDX). The analytical data confirmed the presence of water-insoluble calcium oxalate being formed at the surface crust, superficially on the mortar due to bacterial biomineralization. This biomineralization was responsible for its survival in saline conditions. The mercury intrusion porosimetry (MIP) revealed reduced porosity of the outer mortar surface by 9–14% besides a considerable reduction in mean pore diameter and void ratio to that of the same mortar’s inner core. The bacterial cell count and microbial identification of bacterial isolates method based on 16 S rRNA sequencing identified Bacillus spp. to be mainly responsible for biomineralization. The bacterial action has contributed to the hardening of the superficial material on the outer mortar that saved the historic mortar from the striking sea waves. The granulometric analysis shows the mixing of fine to coarse sized sand grains of basaltic origin sourced from the nearby river channel. With the aid of NMR analysis, it was concluded that calcium phosphate was not as evident as the calcium oxalate on the mortar surface in any of the samples analyzed.

Influence of Interphase Forces on Onset of Fully Developed Subcooled Flow Boiling at High Pressure

Accurate prediction of the onset point of fully developed boiling (FDB) in subcooled flow boiling is crucial for the design of industrial equipment. In this work, Eulerian−Eulerian two fluid model incorporated with the Rensselaer Polytechnic Institute (RPI) wall boiling model was used to investigate high-pressure subcooled boiling flow, with particular attention to the model’s capability of objectively predicting the onset of FDB. After using the experimental data to verify the model, the present work covered a working range of P=6.81−15.5MPa, G=966−2123 kg/m2s, q=750−1500 kW/m2. Results show that the onset of FDB coincided with the inflection point on the axial distribution of boundary void fraction. Lift force had no obvious effect on axial distribution of mean void fraction but had a significant influence on radially local void fraction and determination of the onset of FDB. The turbulent dispersion force, wall lubrication force had a great influence on both axial and radial distribution of void fraction, directly affecting the onset of FDB. Performance of interphase force models in boiling flow was found to be different from that in adiabatic flow, and the underlying mechanism was further revealed. At last, the most feasible interphase force models for predicting onset of FDB were recommended.

Experimental study on the competing effect of ceramic pellets on premixed methane-air flame propagation in a duct

It is urgent to explore effective suppression methods for gas fires and explosions to ensure the safe utilizations of combustible gases in industrial processes. In this work, experiments are performed to study the effect of spherical ceramic pellets on premixed methane-air flame propagation in a closed duct. High-speed schlieren photography and pressure transducers are used to record the flame propagation and pressure transient, respectively. Behaviors of the flame propagating through a section of the duct filled with ceramic pellets in mixtures at different equivalence ratios are scrutinized. Three different diameters of pellets are considered in the experiments. The result shows that the flame can be quenched in the case with a smaller pellet diameter (3 mm) for a wide range of equivalence ratios from fuel-lean to fuel-rich mixture. For larger pellet diameter (5 or 10 mm), flame extinction occurs in fuel-rich mixtures (e.g. Φ = 1.1, 1.2). For the cases of flame surviving through the pellets bed, the pellets show a significant influence on the flame structure and behavior. The flame propagation depends on the porosity and the mean void diameter of the porous media in the pellets bed. Small void diameter is beneficial to flame quenching, while large porosity can accelerate the flame propagation. The pressure dynamics evolution is closely related to the interaction of flame with the pellets, and it depends on whether the flame quenches in the pellets bed. Overall, d = 3 mm ceramic pellets display the best suppression effect on flame propagation and pressure buildup in this study. The results of this study are of great significance to guide the safety design of spherical suppression materials in engineering applications for process safety researchers and engineers.

Flow Patterns of Oil-Gas and Pressure Gradients in Near-Horizontal Flow Pipeline: Experimental Investigation Using Differential Pressure Transducers

The current investigation aimed to identify pressure gradients and to study the fully developed flow patterns of oil-gas as a blend in a pipe of internal diameter 50 mm and 6 m length with different orientations of 0, 30, and 45-degree. The study was performed at constant values of liquid superficial velocities 0.052, 0.157, 0.262, 0.314, 0.419, and 0.524 m/s, and inlet superficial velocities of gas were ranged from 0.05 to 4.7 m/s at atmospheric pressure. Two pressure transducers located up and downstream were used to measure pressure drops inside the tested pipe. Flow patterns were derived by using the correlation between pressure gradients and time series, the Probability Density Function of differential pressures, pressure gradients with gas superficial velocities, and total pressure losses with mean void fractions. The flow patterns of oil-gas were observed as a uniform stratified flow in the pipe on a 0-degree orientation at various superficial velocities. Stratified, wavy, and slug flow patterns were observed at 30-degree orientation, whereas, bubbly, slug, and churn flow patterns were observed in the pipe of 45-degree orientation. The experiment also showed that pressure drop gradients decreased with increased void fractions, gas superficial velocities, and degree rotations of the flow lines. Finally, the validation of using pressure transducers as a technique for estimating the flow patterns of two-phase flow showed acceptable results with some kind of patterns.

Evolution of granular materials under isochoric cyclic simple shearing.

By means of 3D particle dynamics simulations, we analyze the microstructure of granular materials subjected to isochoric (constant volume) cyclic shearing, which drives the system towards a liquefaction state characterized by loops of jamming-unjamming transition with periodic loss of strength and irreversible accumulation of shear strain. We first show that the macroscopic response obtained by these simulations agrees well with the most salient features of the well-known cyclic behavior of granular materials both before and after liquefaction. Then we investigate the evolution of particle connectivity, force transmission, and anisotropies of contact and force networks. The onset of liquefaction is marked by partial collapse of the force-bearing network with rapid drop of the coordination number and nonrattler fraction of particles, and significant broadening of the contact force probability density function, which begins in the preliquefaction period. We find that the jamming transition in each cycle occurs for a critical value of the coordination number that can be interpreted as the percolation threshold of the contact network and appears to be independent of the initial mean stress, void ratio, and cyclic shear amplitude. We show that upon unjamming in each cycle an isotropic loss of contacts occurs and is followed by the development of high contact anisotropy and a large proportion of particles with only two or three contacts. The higher mobility of the particles also involves a lower degree of frustration of particle rotations and thus lower friction mobilization and tangential force anisotropy. These findings are relevant to both undrained cyclic deformations of saturated soils and rheology of dense non-Brownian suspensions where volume change is coupled with pore liquid drainage conditions.

Effect of Drag Models on the Numerical Simulations of Bubbling and Turbulent Fluidized Beds

AbstractGranular option enabled two‐fluid model coupled with different homogeneous and heterogeneous drag correlations, i.e., Gidaspow, Syamlal and O'Brien, Beetstra, Brucato, Gibilaro, energy minimization multiscale method (EMMS), and Tenneti, are employed for the prediction of bubbling and turbulent fluidized‐bed characteristics. Numerically predicted results are validated against the literature data in terms of bubble shape and diameter, axial and radial variation of time‐averaged void fraction, pressure drop, and bed expansion. The Gidaspow, Syamlal and O'Brien drag model‐predicted mean void fraction and bubble shapes are close to the experiments in bubbling and turbulent fluidized beds with both the 2D and 3D geometries. All drag models overpredicted the pressure drop at low superficial gas velocities in a turbulent fluidized bed.

Effect of cementation on the small-strain stiffness of granite residual soil

It is widely acknowledged that residual and sedimentary soils differ considerably because of how they formed. However, despite extensive advances regarding sedimentary soil, little is known about the small-strain stiffness of residual soil. This paper concerns granite residual soil from Xiamen in China, for which a typical profile is established by in situ investigations. How the stiffness varies with the soil current state, including effective mean stress, void ratio and strain, is established via systematic resonant-column tests on high-quality undisturbed and remolded specimens. Highlighted are the unique properties of the studied soil and how they affect the soil stiffness. It is found that cementation among soil particles plays a critical role in the soil stiffness. Although some existing methods can confirm the existence of cementation in the studied soil, they cannot quantify it. Therefore, a new parameter Rn is proposed to quantify the degree of cementation, with lower Rn corresponding to a more cemented structure. According to the proposed Rn, the studied residual soil in Xiamen has a different degree of cementation from that of some well-studied weathered materials, thereby necessitating separate characterization of this soil. This study improves the understanding of residual soil stiffness at small strain, especially the key effect of cementation in natural soil.

Experimental study of submerged hydraulic jumps generated downstream of rectangular plunging jets

This experimental study analyzes submerged hydraulic jumps generated in a 1.05-m wide stilling basin that receives overflow rectangular impinging jets, with the vertical distance between the sharp-crested weir and the bottom of the plunge pool being 2.20 m. Five submerged hydraulic jumps downstream of non-developed nappe jets were analyzed, with vertical falling distances to break-up length ratios between 0.76 and 0.78. The impingement velocity range was from 6.12 to 6.33 m/s, while the water depth / impingement jet thickness ratios were between 8.45 and 15.00. The velocity field and air entrapped were measured in several cross sections, with the farthest sections being located 1.0 m from the stagnation point. The entrapped air (void fraction and phase change detection) behavior measured with a phase-detection probe was classified as a function of the distance to the stagnation point / impingement jet thickness ratio and/or the mean void fraction of the vertical profiles. The maximum void fractions were observed near the free surface. However, the higher phase change detection frequencies (up to 130-140 Hz) were obtained near the bottom for local void fractions smaller than 0.15-0.20. The non-dimensional velocity distribution seems to be similar to previous published studies, extending their validity range. Furthermore, the free hydraulic jumps formula has been adapted to estimate the air rate in the shear stress layer and the growth of the characteristic length in the plunge pool.

Modelling stress-induced anisotropy in multi-phase granular soils

In this paper, we outline a constitutive model capable of describing anisotropy and many other features of the behaviour of multiphase granular soils, together with the computational framework that enables its numerical implementation. The constitutive model is formulated within the framework of bounding surface plasticity. It can simulate monotonic and cyclic loading for a wide range of stress and saturation states, it includes enhanced descriptions of wetting and drying processes, of anisotropy and of changing compressibility. These features are captured using a single set of parameters by using a combination of isotropic and kinematic hardening. The model is formulated based on the concept of effective stress for unsaturated states that guarantees smooth transitions between unsaturated and fully saturated states. Furthermore, we present unified formulations for saturated and unsaturated states in which the isotropic hardening law and the critical state line are described in a bi-logarithmic space defined by the logarithms of the mean effective stress and void ratio. Moreover, the constitutive model is coupled with a soil water characteristic model that allows consideration of the hysteretic nature of the saturation degree changes upon wetting/drying reversals. The paper describes the numerical implementation, which includes several smoothing techniques to enhance the constitutive model’s performance in numerical modelling during transitions between kinematic hardening and isotropic hardening and drying/wetting reversals. The numerical implementation also includes automatic error control and sub-stepping techniques, suitable for explicit integration algorithms, that give users additional control over the accuracy and speed of the analysis. Lastly, several examples are provided to demonstrate the range of application of the computational framework.

A comparative study of quasi-stable sheet cavities at different stages based on fast synchrotron x-ray imaging

The cavitating flows in a small convergent–divergent channel are studied experimentally using a fast synchrotron x-ray imaging method that enables simultaneous acquisition of the velocity and void fraction fields as well as the two-phase morphology inside the opaque cavitation areas. According to the x-ray measurement results, the quasi-stable sheet cavitation is classified into three representative stages based on the status of the re-entrant flow: (i) at the early stage, the incipient cavity is short and no reverse flow is observed, (ii) at the intermediate stage, an intermittent re-entrant flow is identified underneath the sheet cavity, and (iii) at the developed stage, the re-entrant flow becomes continuous and it can penetrate the entire sheet cavity. The comparison of the three typical sheet cavities illustrates that the occurrence of the re-entrant flow does not result in the transition of sheet-to-cloud cavitation, but it influences the distribution of the mean void fraction and the spectrum of the void fraction variation. The development of cavitation also influences the turbulent velocity fluctuations. On one hand, cavitation alters mean velocity field, which affects velocity fluctuations due to the positive correlation between the velocity gradient and turbulence intensity. One the other hand, the presence of vapor/liquid mixture is observed to greatly suppress velocity fluctuations. The collapse of small vapor structures does not cause an evident augmentation of streamwise velocity fluctuations but increases the shear stress significantly.

DEM Study on the Instability Behaviour of Granular Materials

This paper presents the discrete element method (DEM) simulations on the instability behaviour of granular materials during constant shear drained condition (CSD). CSD condition was implemented by decreasing mean effective stress on an assembly of particles under strain controlled loading. In this study, the instability condition was predicted at the particle scale level using particle second order work increment (Nicot et al. in Int J Solids Struct 49(10):1252–1258, 2012). The DEM contact parameters have been calibrated to capture the macroscopic responses and the instability behaviour consistently with the laboratory experiments. The effect of different range of initial states at the beginning of CSD condition such as different initial mean effective stress ( $$p_{0}^{^{\prime}}$$ ), void ratio ( $$e_{0}$$ ) and deviatoric stress ( $$q$$ ) on the instability behaviour were analysed. In addition, the micromechanical parameters such as coordination number, anisotropic coefficients (geometric, mechanical) have been extracted to assist in characterising the instability behaviour during CSD condition. The initial stress state of the soil (i.e. at the onset of CSD) has shown a significant influence on the evolution of anisotropic coefficients. An increase in geometric anisotropy and a decrease in mechanical anisotropy with time was observed after the instability.

Investigation of fluidized bed behaviour using electrical capacitance tomography

AbstractThe temporal and cross‐sectional distributions of particles in a 127 mm diameter fluidized bed have been obtained using a new generation, high‐speed electrical capacitance tomography. Two planes of eight electrodes were used and mounted at 160 and 660 mm from the gas distributor which was a 3 mm thick porous plastic plate (maximum pore size of 50‐70 μm). 3 mm diameter, nearly‐spherical polyethylene granules made up the bed. Experiments at sampling frequencies of 200‐2000 cross‐sections per second and gas superficial velocities from just below the minimum fluidization to 83% above minimum fluidization velocities were used. The time series of the cross‐sectional average void fractions have been examined both directly and in amplitude and frequency space. The last two used probability density functions and power spectral densities. The information gathered shows that the fluidized bed was operating in the slugging mode, which is not surprising given the size of the particles. It has been found that an increase in the excess gas velocity above the minimum fluidization velocity resulted in an increase in the mean void fraction, an increase in the length and velocity of the slug bubbles as well as the bed height, and a slight decrease in the slug frequency. The results are presented in a level of detail suitable for comparison with later numerical simulation.

Circadian Patterns in Postvoid Residual and Voided Percentage Among Older Women with Urinary Incontinence.

Background: Women with urinary incontinence incur an increased risk of elevated postvoid residual (PVR) volume and impaired voiding efficiency (i.e., voided percentage (Void%)), but the clinical significance of these parameters remains poorly described. Further characterization of PVR and voiding efficiency may thus be useful in refining the evaluation and management of urinary incontinence. This study aims to explore possible circadian variations in PVR and Void% in older women with stress (SUI), urge (UUI) and mixed urinary incontinence (MUI). Methods: A single center prospective study which enrolled a convenience sample of 90 older women who consulted a tertiary referral hospital for urinary incontinence. Participants underwent an extensive medical interview and were hospitalized to complete a 24-h frequency-volume chart (FVC) with PVR measurement after each void (FVCPVR). Results: FVCPVR analysis demonstrated no differences in mean PVR and Void% between patients with SUI, UUI and MUI. Likewise, no daytime or nighttime differences were observed in mean PVR or Void% within or between groups. Conclusions: No evidence of circadian variation in PVR or Void% was observed in older women with SUI, UUI or MUI.

A new experimental method for characterizing boiling regimes of water in vertical milli-channels

Over the past few decades, high-performance heat-transfer devices have been studied for numerous applications. Boiling flow in such confined spaces induces high-frequency flow instabilities that directly influence the heat-transfer performances. A small-scale water vaporizer with 10 mm-sized channels is experimentally studied over a wide range of vapor quality (0.05–1). The peculiarity of the setup is a transparent window that allows for high speed imaging of the boiling flow up to 5 bars. This work focuses on a post-processing method based on spatio-temporal diagrams. Hydrodynamics of the boiling flow can be observed on the diagrams enabling to understand the boiling mechanisms and the flow regimes inside the channels. Four regimes are observed, namely bubbly, slug, churn and annular flow. Diagrams are also used to estimate the local mean void fraction over time in channels, the local vapor quality, the slip ratio and a flow-regime map. For qualities under 10 %, the bubbly flow is predominant in channel. For higher qualities, if the specific mass flow rate of water is below 37 kg/s/m², only the slug flow and the annular flow are observed, whereas the churn flow is also observed for larger flow rates.

Experimental Study of Turbulence and Entrained Air Characteristics in a Plunging Breaking Solitary Wave

The entrained air and turbulence characteristics under a breaking solitary wave on a 1:20 sloping beach are investigated through laboratory measurement. Free surface elevation is obtained from wave gauge measurements. Wave breaking process is captured in detail by a high-speed camera. The bubble image velocimetry (BIV) is used to measure the velocity and the fiber optic reflectometer (FOR) is used to capture instantaneous void fraction in the aerated region. The mean void fraction and velocities in the aerated region are obtained by ensemble averaging over 22 repetitions. Results show that the maximum mean void fraction is 0.6 in the collapsing cavity region and is 0.35 in the splash up region. The time series of the mean void fraction has good synchronization with the instantaneous images taken by high-speed camera. The maximum horizontal velocity occurs in the splash up region and reaches 1.17C shortly after the plunging jet hits the water surface, with C being the phase speed of the primary wave. The turbulence intensities over the entire aerated region are presented and discussed. The measured data can be used for the calibration and verification of the numerical model for aerated flows simulation under breaking waves in the surf zone.

Computational fluid dynamics modeling of liquid–gas flow patterns and hydraulics in the cross-corrugated channel of a plate heat exchanger

The morphology of liquid–gas flows in corrugated plate heat exchangers is complex due to the intricate channel geometry. To date, only a few experimental visualizations have been performed to study the two-phase flow characteristics in plate heat exchangers. In this paper, we perform pioneering computational fluid dynamics simulations of the adiabatic liquid–gas (water-air) flow in a cross-corrugated channel of a plate heat exchanger. The standard volume-of-fluid technique is used to capture the complex phase-interfaces constructed by the cross-corrugated walls. In order to reduce the computational cost, the computational domain is simplified by a series of assumptions. The bubbly flow, slug flow, churn flow and film flow are modeled by varying the superficial velocities of both phases, and the validity of these patterns is proved by comparison with experimental results. A flow regime map is developed based on the numerical results, and the transitions between the regimes are discussed. The predicted pressure drop shows good agreement with the experimental data. The two-phase multiplier for general prediction of the friction factor in the cross-corrugated channel is calibrated. The mean void fraction in the channel is quantified by the numerical simulation. The void fraction model from Zuber and Findlay is found to be applicable to the cross-corrugated channel, which is further modified for general use for these types of channel flows.

Porous shape memory dental implant by reactive sintering of TiH2-Ni-Urea mixture.

We produced bifurcated bone-like shape memory implant (BL-SMI) with desirable tooth-root fixation capability by compact-sintering of TiH2-Ni-urea mixture. The primary constituents of the porous product were Ni and Ti. We could adjust the pores' shape, size, and interconnectivity for favorite bone ingrowth by using urea as a space holder. Without urea, we obtained an average porosity of 0.30, and a mean void size of 100 μm. With 70 vol % urea, we got 62% interconnected pores of 400 μm average size. Aging allowed us to tune the austenite-martensite transformation temperatures towards the needed body tissue arouse. Differential scanning calorimetry measured the transformation temperatures. Their austenite start, austenite peak, and austenite finish values were As = 4, Ap = 22, and Af = 34 °C, respectively. They retained functional shape recovery and superelastic effect at the body temperature. Mechanical properties, including Young's modulus of the specimens, matched well to maxilla and mandible bone tissue. The measured Young's modulus of the NiTi specimens was as low as 3.5 GPa, which decreased to ∼2.1 GPa with further porosity increase at higher space holder percentages. Superelasticity regime and low Young's modulus of the implant could potentially prevent stress-shielding from the surrounding bone tissues and give rise to secure fixation of the implant into the bone socket. Bending tests showed 0.9 mm recoverable deflection for specimens which assisted immediate self-fixation of the implant into the jaw bone cavity.

Upward bubbly flows in a square pipe with a sudden expansion: Bubble dispersion and reattachment length

We investigate the bubble dynamics and subsequent changes in the liquid-phase flow characteristics of upward bubbly flows in a square pipe with a sudden expansion (expansion ratio of 4.0). The experiments are conducted under three liquid-phase Reynolds numbers of 0 (stationary), 420 (laminar) and 6000 (turbulent). The inlet volume void fraction ranges about 0−1.0%, and we use a high-speed two-phase particle image velocimetry and shadowgraphy to measure two phases simultaneously. It is observed that after the expansion, smaller bubbles tend to migrate toward the wall and larger bubbles rise in a core region, which is more encouraged by the steeper velocity gradient with increasing the Reynolds number. This bubble distribution is further analyzed by estimating the interfacial forces acting on rising bubbles. Affected by this bubble distribution, the enhanced turbulence in the inlet flow energizes the separating shear layer, resulting in the reduction of reattachment length behind the edge. Finally, we suggest an empirical relation for a two-phase flow reattachment length in terms of Reynolds number and mean void fraction, which explains the contribution of added dispersed phase on the mixing enhancement in the backstep flows.

Study of a Cavitating Venturi Tube by Lumped Parameters

The hydrodynamic cavitation in a Venturi tube is studied both theoretically and experimentally. A lumped parameter model was developed to describe the accumulation and dissipation of energy in the biphasic flow as a function of the bubble population characteristics (mean volume, standard deviation, and void fraction). Resistance, capacitance, inductance, and frequency lumped-parameters were identified applying the fluid conservation equations (mass and momentum) along with electrical/hydraulic analogies. Experiments with 1,2-propanediol were carried out in a hydraulic circuit composed of valves, a pump, and a Venturi nozzle. The acoustic noise generated (at different cavitation regimes) by the passage of the fluid through the tube was acquired with a piezoelectric sensor. After processing the experimental signals, the system frequency at each operation condition was determined. Plausible estimations of the void fraction were obtained at different experimental frequencies by evaluating a theoretical expression of the frequency lumped-parameter. This semi-empirical technique might be a low-cost alternative when the void fraction of a flow needs to be determined and tomography devices are not available.

On the dynamic modeling and control of the cold-end system in a direct air-cooling generating unit

Direct air-cooling is considered as a promising technique in power generation because of its distinct water-saving advantages, where ambient air is utilized instead of water, to cool down the exhaust steam in the direct air-cooling condenser. However, the relatively poor heat transfer characteristics and thermodynamic properties of air also brings higher backpressure in the direct air-cooling generating unit compared with conventional water-cooling generating unit, as well as the sensitivity of condenser performance to meteorological and ambient conditions. Therefore, the unit backpressure control for the direct air-cooling generating unit has been a challenging task in the recent years. To this end, a control-oriented mechanism dynamic cold-end system model is built for the direct air-cooling generating unit in this paper. The modified moving boundary method is utilized to capture the dynamic process of exhaust steam condensation in the condenser, wherein a time-variant mean void fraction is adopted to enhance model accuracy and robustness. Validation results with operating data have shown the established model can capture the dynamics of direct air-cooling cold-end system with satisfied accuracy. Based on the established model, a model predictive controller with feedforward compensation is proposed to keep unit backpressure steady at the desired set-point while suppressing the time-varying stochastic ambient temperature disturbances by regulating the rotation speed of fan array appropriately. The simulation results have verified the merits and effectiveness of the proposed strategy in achieving both satisfactory backpressure regulation and disturbance rejection performance.