Diffusion Width Research Articles

Influence of bubble size, diffuser width, and flow rate on the integral behavior of bubble plumes

AbstractA large‐eddy simulation based Eulerian‐Lagrangian model is employed to quantify the impact of bubble size, diffuser diameter, and gas flow rate on integral properties of bubble plumes, such as the plume's width, centerline velocity, and mass flux. Calculated quantities are compared with experimental data and integral model predictions. Furthermore, the LES data were used to assess the behavior of the entrainment coefficient, the momentum amplification factor, and the bubble‐to‐momentum spread ratio. It is found that bubble plumes with constant bubble size and smaller diameter behave in accordance with integral plume models. Plumes comprising larger and non‐uniform bubble sizes appear to deviate from past observations and model predictions. In multi‐diameter bubble plumes, a bubble self‐organisation takes place, i.e., small bubbles cluster in the center of the plume whilst large bubbles are found at the periphery of the plume. Multi‐diameter bubble plumes also feature a greater entrainment rate than single‐size bubble plumes, as well as a higher spread ratio and lower turbulent momentum rate. Once the plume is fully established, the size of the diffuser does not appear to affect integral properties of bubble plumes. However, plume development is affected by the diffuser width, as larger release areas lead to a delayed asymptotic behavior of the plume and consequently to a lower entrainment and higher spread ratio. Finally, the effect of the gas flow rate on the integral plume is studied and is deemed very relevant with regards to most integral plume properties and coefficients. This effect is already fairly well described by integral plume models.

The Kelvin-Helmholtz instability in National Ignition Facility hohlraums as a source of gold-gas mixing

Highly resolved radiation-hydrodynamics FCI2 simulations have been performed to model laser experiments on the National Ignition Facility. In these experiments, cylindrical gas-filled hohlraums with gold walls are driven by a 20 ns laser pulse. For the first time, simulations show the appearance of Kelvin-Helmholtz (KH) vortices at the interface between the expanding wall material and the gas fill. In this paper, we determine the mechanisms which generate this instability: the increase of the gas pressure around the expanding gold plasma leads to the aggregation of an over-dense gold layer simultaneously with shear flows. At the surface of this layer, all the conditions are met for a KH instability to grow. Later on, as the interface decelerates, the Rayleigh-Taylor instability also comes into play. A potential scenario for the generation of a mixing zone at the gold-gas interface due to the KH instability is presented. Our estimates of the Reynolds number and the plasma diffusion width at the interface support the possibility of such a mix. The key role of the first nanosecond of the laser pulse in the instability occurrence is also underlined.

Homogenization of viscoplastic constitutive laws within a phase field approach

The present work provides an original consistent framework for combining different constitutive laws, possibly nonlinear, within diffuse interface models (phase field) by homogenization rather than by mixing the material parameters, such as to offer a greater flexibility. The framework relies on the choice of relevant thermodynamic potentials such that the homogenization schemes are natural choices in the thermodynamic formulation. Thus, it justifies previously proposed classic schemes (Ammar et al., 2009b) and gives clues to explore new schemes (Durga et al., 2013; Mosler et al., 2014) by defining relevant potentials. The proposed framework is illustrated by addressing two issues. First, Reuss and Voigt homogenization schemes are shown to deliver the same kinetics of diffusion-controlled transformations in the pure elastic case in the limit of vanishing diffuse interface width. Second, using the Voigt homogenization scheme, we demonstrate that the influence of viscoplasticity, either isotropic or crystalline, on diffusion-controlled transformations is non-trivial and cannot be inferred from simple qualitative arguments. Depending on the competition between the time scales of diffusion and viscoplasticity, the growth kinetics may exhibit intermediate behaviors between purely chemical and elastic cases. Moreover, it is shown how viscoplasticity can change the morphological stability of growing circular precipitates.

Phase-field simulation of tilted growth of dendritic arrays during directional solidification

The tilted growth of dendritic arrays during directional solidification of non-axially oriented crystals has been simulated in two-dimensional system by using a quantitative phase-field model. A convergence study with respect to the diffuse interface width has been carried out, showing that the results are reasonably independent of the diffuse interface width for W0/d0⩽11.3. A parametric study is performed to investigate the effects of the primary spacing, the misorientation angle and the pulling velocity on the tip undercooling, the upper limit of primary spacing, and the tilted angle. Results show that the tip undercooling decreases when increasing the primary spacing or decreasing the misorientation angle. It has been found that the upper limit, resulting from tertiary branching, depends on both the solidification condition (the pulling velocity and the thermal gradient) and the preferred crystalline orientation (the misorientation angle) for the tilted growth of dendritic arrays. Previous rotation law, describing the relation between the tilted angle and the Péclet number, is evaluated according to phase-field simulation results. Moreover, the effect of the development of sidebranches on the growth direction selection was discussed. Further studies on the microstructure evolution from the onset of planar instability during directional solidification show that the selection of the steady-state primary spacing is sensitive to the misorientation angle.

Analysis of the gas-dynamic performance of a vaned diffuser with given velocity distribution along the vane's surfaces

Extension of the effective range of vaned diffusers is one of the promising ways to improve the centrifugal compressor's stages which are used in numerous fields of industry. The new method of profiling of the diffuser vanes has been developed using Stratford's results and boundary layer theory by Loytsanskiy. The developed method is based on the solution of the inverse task of gas-dynamic using given velocity distribution along the vane's surface. Comparison of the results of numerical simulations for different diffusers has shown that the performance of the diffuser designed with the resulting velocity distribution are better. Influence of the vane profile, number of the vanes, diffuser outlet diameter and the diffuser width on diffuser characteristics has been investigated. The results of the simulations have been used to formulate recommendations on the design of high-effectiveness vaned diffusers for centrifugal stages of different types.

Comparisons RANS and URANS numerical results with experiments in a vaned diffuser of a centrifugal pump

The paper presents the analysis of the performance and the internal flow behaviour in the vaned diffuser of a radial flow pump using PIV (particles image velocimetry) technique, pressure probe traverses and numerical simulations. PIV measurements have been performed at different hub to shroud planes inside one diffuser channel passage for a given rotational speed and various flow rates. For each operating condition, PIV measurements have been made for different angular positions of the impeller. Probe traverses have also been performed using a 3 holes pressure probe from hub to shroud diffuser width at different radial locations in between the two diffuser geometrical throats. The numerical simulations were realized with the two commercial codes: i-Star CCM+ 8.02.011 (RANS (Reynolds Averaged Navier Stokes) turbulence model, frozen rotor and unsteady calculations), ii-CFX 10.0 (turbulence modelled with DES model (Detached Eddy Simulation) combining RANS with LES (Large Eddy Simulation), unsteady calculations). Comparisons between numerical (fully unsteady calculations) and experimental results are presented and discussed for two flow rates. In this respect, the effects of fluid leakage due to the gap between the rotating and fixed part of the pump model are analysed and discussed.

Effect of Ni solute on grain boundary diffusivity and structure of βSn

The effect of Ni impurity on the self-diffusivity of βSn in the (101) symmetric tilt grain boundary is investigated. Using molecular dynamics simulations over a temperature range of 300–450K. It is shown that Ni solute decreases the grain boundary self-diffusivity of βSn as the amount of solute in grain boundary increases. We also study the solute effect on the diffusive width of grain boundary and observe that low concentrations of Ni solute lead to shrinking of the grain boundary width relative to the pure βSn grain boundary.

Dating of inkpad seals using the oil diffusion (migration) method

Inkpad seals are often placed on important documents, and determining the date when a seal was affixed can be important to assess the authenticity of a document. In this study, four different brands of inkpads and three different types of paper were used to make diachronic (age-changing) samples of the seals. An oil diffusion (migration) method was employed to date the inkpad seals by comparing diffusion (migration) distances of a blending agent (castor-oil or chlorinated paraffin oil) of the same stamped impression measured at different times or by comparing the mean oil diffusion width associated with the purported date and the mean oil diffusion width associated with the suspected date. After testing, curves were created to show the relationship between the diffusion distance of ink and the date of the seals. The experimental results showed that the oil diffusion method was applicable for the dating of inkpad seals under certain conditions.

Evolution of Diffusion-Related Degradation of Polymeric Lubricant Under Laser Heating: A Molecular Dynamics Study

Molecular dynamics simulation coupled with a modified coarse-grained bead-spring model is employed to investigate the diffusion-related degradation of lubricant film under laser heating. The lubricant surface morphology and depletion profiles during laser heating are studied. It is observed that lubricant film degrades due to thermal diffusion and undergoes severe depletion with increasing laser heating duration, resulting in raised ridges around the depleted zone. The diffusion width and depth are evaluated to further explore diffusion-related degradation instability. As expected, the diffusion depth increases rapidly with the heating duration and the laser power, while the width would fluctuate around a constant value after initial rapid rise and following slight reduction. In addition, a Gaussian temperature gradient is formed in the radial direction due to the thermal transfer between the heated and surrounding beads. It is also shown that the laser power plays an important role in the temperature gradient and hence greatly influences the diffusion-related degradation of lubricant film on a solid surface.

Analysis and measurement of the impact of diffuser width on rotating stall in centrifugal compressors

In compression systems, instability has long been an important issue. However, compared to axial machines, relatively little work has been done on the stability of centrifugal machines. Especially, many analytical models of stabilities have been developed to predict and control rotating stall, using compressor characteristic. However, stability models for centrifugal compressors are not scarce. Much research on compressor stability has focused on stalling flow coefficient and rotating stall phenomenon at the stalling flow coefficient. Given this situation, this paper presents a stability analysis of centrifugal compressors to predict rotating stall inception as well as the speed and number of cells. This analysis involves the use of compressor geometries, a steady compressor characteristic, and threedimensional flow analysis in the diffuser. The flow field perturbations at the axial inlet duct, impeller, and radial exit duct are determined via an eigenvalue analysis. The predictions are validated against experimental results from compressors with three different diffuser widths. The model accurately predicts the rotating stall inception flow coefficient. As the compressor characteristic becomes less steep with increasing diffuser width, the stalling flow coefficient increases. Also, experiment validates the model prediction that, depending on the dominant mode of flow perturbation, the number of rotating stall cells can be changed from three to two cells in the tested configurations. Furthermore, the cell speed increases as the flow coefficient decreases for a given number of stall cells. However, when the stall cell number is reduced, the cell speed decreases.

Comparisons between numerical calculations and measurements in vaned diffuser of SHF impeller

The paper presents analysis of the performance and the internal flow behaviour in the vaned diffuser of a radial flow pump using PIV (Particle Image Velocimetry) and pressure probe traverses. PIV measurements have already been performed at middle height inside one diffuser channel passage for a given speed of rotation and various mass flow rates. These results have been already presented in several previous communications. New experiments have been performed using a three-hole pressure probe traverses from hub to shroud diffuser width at different radial locations between the two diffuser geometrical throats. Numerical simulations are also realized with the commercial codes Star CCM+ 7.02.011 and CFX. Frozen rotor and fully unsteady calculations of the whole pump have been performed. Comparisons between numerical results, previous experimental PIV results and new probe traverses one's are presented and discussed for one mass flow rate. In this respect, a first attempt to take into account fluid leakages between the rotating and fixed part of the pump has been checked since it may affect the real flow structure inside the diffuser.

Investigation of the Stage Performance and Flow Fields in a Centrifugal Compressor with a Vaneless Diffuser

The effect of the width of the vaneless diffuser on the stage performance and flow fields of a centrifugal compressor is studied numerically and experimentally. The diffuser width is varied by reducing the diffuser flow area from the shroud side (i.e., pinching the diffuser). Seven different diffuser widths are studied with numerical simulation. In the modeling, the diffuser widthb/b2is varied within the range 1.00 to 0.50. The numerical results are compared with results obtained in previous studies. In addition, two of the diffusers are further investigated with experimental measurement. The main finding of the work is that the pinch reduces losses in the impeller associated with the tip-clearance flow. Furthermore, it is shown that a too large width reduction causes the flow to accelerate excessively, resulting in a highly nonuniform flow field and flow separation near the shroud.

Understand the importance of molecular organization at polymer–polymer interfaces in excitonic solar cells

To improve the photovoltaic power conversion efficiency of organic solar cells it is necessary to increase the open circuit voltage and short circuit current. Prior work has shown that both these device properties can be improved by using donor–acceptor systems composed of two polymers, which favors excitons dissociation and charge generation, and subsequent charge transport to the electrodes. However, device performance depends strongly on the experimental conditions under which the device was created, because they determine the molecular structure of the interface between the two polymers. Polymer chains can have different conformations relative to the interface, creating different arrangements of the conjugated segments whose disorder degree can affect energy and charge transfer. Thus, understanding this effect is of utmost importance to improve the efficiency of all-polymer excitonic solar cells. In this work, we present the results obtained using a Monte Carlo model that takes into account the arrangement of the polymer strands at polymer–polymer interfaces and considers the main physical processes mediating exciton and charge dynamics. Our results show that the amount of charge extracted from the interface is sensitive to the orientation of polymer strands at interfaces and on the diffusive layer width formed by the mixture of both polymers.

Numerical simulation of cross section electron-beam induced current in thin-film solar-cells for low and high injection conditions

Electron-beam induced current measurements (EBIC) in the cross-section configuration can be used to characterize electronic properties of thin-film solar-cells with a spatial resolution in the submicrometer range. Assuming low injection conditions and complete charge carrier collection in the depletion region, the minority charge-carrier diffusion length and width of the space charge region can be extracted from EBIC data using an analytical expression. In the present work, we evaluate the validity of the assumptions underlying the analytical description by using numerical device simulation to describe EBIC profiles perpendicular to the pn-junction of thin-film solar cells. We find that under low injection conditions, the analytical description provides good results if the minority charge-carrier diffusion length in the absorber layer is significantly larger than the width of the space charge region. On the other hand, the analytical description of the EBIC profiles deviates significantly from the numerical simulation for short diffusion lengths and also for high injection conditions. Experimental EBIC profiles of Cu(In,Ga)Se2 solar cells are evaluated to obtain local minority carrier diffusion-lengths and to illustrate high-injection and low-injection effects in the measurements.

Control of Unstable Flow Using Rough Surfaces

The function of centrifugal blowers/compressors is limited at low-mass flow rates by fluid flow instabilities leading to rotating stall. These instabilities limit the flow range in which they can operate. An experimental investigation was conducted to investigate a model of radial vaneless diffuser at stall as well as stall-free operating conditions. The speed of the blower was kept constant at 2000 RPM, while the mass flow rate was reduced gradually to investigate the steady and unsteady flow characteristics of the diffuser. These measurements were reported for diffuser diameter ratios, Do / Di, of 1.5, 1.75 and 2.0 with diffuser width ratio, b / Di, of 0.055. The rotating stall pattern with one stall cell was dominant over the pattern with two cells which appeared at flow rates lower than the critical. In addition, the instability in the diffuser was delayed to a lower flow coefficient when rough surfaces were attached to one or both walls of the diffuser with the lowest values achieved by attaching the rough surface to the shroud wall. Results show that the roughness has no significant effect on stall cell frequencies.

Experimental study of centrifugal compressor tip clearance and vaneless diffuser flow fields

Three vaneless diffuser designs, varying in diffuser width, for centrifugal compressor were studied experimentally. Along with the diffuser width, the tip clearance was altered. The compressor overall performance and diffuser flow fields were studied for each of the three diffusers at four different tip clearances. For the diffuser flow fields, the total pressures were measured with probes traversed over the diffuser width both at the diffuser inlet and outlet. Along with the total pressures, the static pressures were measured adjacent to the probes. This enabled the axial flow angle and velocity distributions to be studied. The pinches tested improved the stage efficiency mainly by suppressing the secondary flow region present near the shroud at the impeller outlet. This leads to a lower strain rate, resulting in lower losses. The efficiency decrease due to the increased tip clearance was similar with and without pinch present. This indicates that in the diffuser, the main source of the tip clearance associated losses is the tip jet, and the tip clearance vortices mix out already in the impeller.

Evaluation of interfacial excess contributions in different phase-field models for elastically inhomogeneous systems

In elastically inhomogeneous solid materials, the presence of strains causes changes in both morphology and phase equilibria, thereby changing the mechanical and chemical properties. For any given initial phase- and grain-structure, it is difficult to determine experimentally or analytically these changes in properties. Phase-field models coupled with micro elasticity theory can be used to predict the morphological and chemical evolution of such strained systems, but their accuracy with respect to interfacial excess contributions has not been tested extensively. In this study, we analyse three existing phase-field schemes for coherent two-phase model systems and a Cu6Sn5–Bct-Sn system. We compare the chemical composition and stress state obtained in the simulations with analytical values calculated from Johnson's (Johnson 1987 Metall. Trans. A 18 233–47) model. All schemes reproduce the shift in chemical composition, but not the strains. This deviation is due to excess interfacial energy, stresses, and strains not present in the analytical results, since all three schemes are based on assumptions different from the stress and strain relations at equilibrium. Based on this analysis, we introduce a new scheme which is consistent with the analytical calculations. We validate for the model system that this new scheme quantitatively predicts the morphological and chemical evolution, without any interfacial excess contributions and independent of the diffuse interface width.

Experimental study of centrifugal compressor vaneless diffuser width

Seven different vaneless diffuser designs for a centrifugal compressor, varying only in diffuser width, were studied experimentally. The studied diffuser widths versus impeller exit width were 1.0, 0.903, 0.854, and 0.806. Three of the narrowed diffusers had the width reduced from the hub and shroud divided evenly, and the three others had the width reduced only from the shroud. The total and static pressures, the total temperature and the flow angles at the diffuser inlet and outlet were measured at the design rotational speed with three different mass flows. The impeller and diffuser performance was studied along with the axial distributions of flow angles and velocities in the diffuser. The results revealed that the pinch improved the compressor stage and impeller performance but deteriorated the diffuser performance. The pinch clearly decreased the secondary flow region present near the shroud. The pinch implemented in the shroud is more beneficial than pinch divided between the hub and the shroud. In order to obtain the beneficial effects of pinch, the pinch should be sufficient. However, excessive pinch deteriorates the compressor performance.

Impact Of Diffuser Width On The Performance Of AP1000 Reactor Coolant Pump

For the problem of incapable to determine the width of spherical diffuser, four groups of spherial diffusers with different width were designed by using the speed coefficient method. Numerical investigation of internal flow in the nuclear coolant pumps with these diffusers was made based on ANSYS-CFX software. Aimed at the efficiency, ditribution of pressure and velocity vector on the meridian plane of each volute scheme was analysised emphatically. By comparing the performance curves of the coolant pumps predicted by the numerical simulation, we can draw that the head and efficiency of pump decreased as the increasing of diffuser width and performanced worst at the width of d=1.7R, then the performance no longer changes when diffuser width continue to reduce.

An SCR-Incorporated BJT Device for Robust ESD Protection With High Latchup Immunity in High-Voltage Technology

A silicon-controlled rectifier (SCR)-incorporated BJT with high holding voltage is developed for electrostatic discharge (ESD) protection in a 0.6 μm high-voltage 10 V process. This device consists simply of a floating P+ diffusion incorporated in a parasitic NPN BJT. A robust 6-7 kV ESD threshold and high-latchup-immune holding voltage of 15-18 V can be achieved by layout optimization of the NPN-N+ -collector to floating P+ -diffusion spacing and the floating P+ diffusion width. It can be equivalently regarded as parallel connection of an incorporated PNPN SCR part and an NPN BJT part. The incorporated SCR part is further composed of a parasitic SCR in series with a reverse-biased PN diode formed by the floating P+ region and N-well. The further analysis shows that the floating P+ diffusion is the key part of this SCR-incorporated BJT. The parasitic reverse-biased PN diode sustains most of the high holding voltage. The parasitic NPN BJT plays a major role in ESD current conduction, while the incorporated SCR in series with the reverse-biased PN diode is the secondary conduction path.