Open Area Fraction Research Articles

Experimental Study of Trailing-Edge Bluntness Noise Reduction by Porous Plates

The acoustic and aerodynamic fields of blunt porous plates are examined experimentally in an effort to mitigate trailing-edge bluntness noise. The plates are characterized by a single dimensionless porosity parameter identified in previous works that controls the influence of porosity on the sound field. Hot-wire anemometry interrogates the velocity field to connect turbulence details of specific regions to flow noise directivity and beamforming source maps. Porous plates are demonstrated to reduce the bluntness-induced noise by up to 17 dB and progressively suppress broadband low-frequency noise as the value of the porosity parameter increases. However, an increase in this parameter also increases the high-frequency noise created by the pores themselves. The same highly perforated plate characterized by a large value of the porosity parameter reduces the bluntness-induced vortex shedding that is present in the wake of the impermeable plate. Lastly, pore shape and positional alignment are shown to have a complex effect on the acoustic field. Among the porosity designs considered, plates with circular pores are most effective for low-frequency noise reductions but generate high-frequency noise. No meaningful difference is found between the acoustic spectra from plates of the same open-area fraction with pores aligned along or staggered about the flow direction.

CFD-PB studies on effect of internals on specific interfacial area in a pulsed sieve plate columns

A CFD model coupled with population balance equations is developed to systematically study the effects of column internals on specific interfacial area generated in a pulsed sieve plate column (PSPC). The model has been validated against experimental data on holdup and Sauter mean drop diameter in laboratory scale PSPCs of different geometries. The column internals comprised of sieve plates with varying hole diameter, fractional open area and interplate spacing. Typically the standard sieve plate cartridge is characterised by sieve plates (3.2 mm diameter sieve holes with an fractional open area of 23%) placed 50 mm apart. A systematic response surface methodology has been adopted to determine the relative importance of these parameters on specific interfacial area. 3 factor 3 level Box Behnken design has been adopted. Specific interfacial area decreases with an increase in sieve hole diameter, interplate spacing and open area. Sieve hole diameter is found to be the most significant parameter affecting specific interfacial area. Important hydrodynamic variables like flow velocities in continuous and dispersed phases and how column internals affect them are also discussed.

Droplet penetration through an inclined mesh

Droplets with different Weber numbers We impacting meshes at various inclination angles α were investigated using high-speed photography. It was found that the droplet mesh penetration can be completely suppressed by inclining the mesh. Phase diagrams in the (We, α)-plane determining the expected type of penetration have been determined experimentally for meshes of various structures. It was shown that the Weber numbers for transition between no-penetration and incomplete penetration as well as for transition between incomplete penetration and complete penetration increase monotonically with α. A simple model for predicting transition thresholds is proposed and is validated by comparisons with experimental results. It is shown that both the inclination angle and the mesh open area fraction determine the type of penetration.

Effect of Thickness and Outlet Area Fraction of Macroporous Gas Diffusion Layers on Oxygen Transport Resistance in Water Injection Simulations

Enhanced water removal through the gas diffusion layer (GDL) is important for the design of high-performance proton exchange fuel cells. In this work, the effects of GDL thickness and open area fraction at the GDL/flow field interface are examined under water invasion for a carbon-paper GDL (similar to Toray TGP-H series). Both uncompressed and inhomogeneously compressed samples are considered. Transport in heterogeneous, macroporous GDLs is modeled by means of a hybrid 3D discrete/continuum formulation based on a subdivision of the porous medium into control volumes due to the lack of a well-defined separation between pore and layer scales. Capillary-dominated transport of liquid water is simulated with an invasion percolation algorithm, while oxygen diffusion is simulated with a continuum formulation. Model predictions are validated with previous numerical and experimental data. It is shown that the combination of thin GDLs (mathrm {thickness} sim 100; mu mathrm {m}) and high GDL/flow field open area fractions can facilitate water removal/oxygen supply from/to the catalyst layer and can provide a more uniform oxygen distribution over large cell active areas. In agreement with previous work, porous flow fields with pore sizes comparable to the GDL thickness are good candidates to meet the above requirements, while improving water removal from the flow field (higher gas-phase velocity than conventional millimeter-sized channels) and ensuring a more uniform assembly compression.

The Collection Efficiency of a Large Area PMT Based on the Coated MCPs

Abstract The electron collection efficiency (CE) of the photomultiplier tube based on microchannel plates (MCP-PMT) is limited by the MCP open area fraction. Coating MCP with a high secondary yield material is supposed to be an effective approach to improve CE. Both typical and coated MCP-PMTs are developed. A relative measurement method is proposed to characterize the collection efficiency performance. Results show that the PMT based on the coated MCPs has a significant improvement on CE, a good gain uniformity and a high precise energy resolution.

Dispersed-phase Volume Fraction and Flow Regimes in Oscillatory Liquid-Liquid Two-Phase Flow in Annuli: Comparison of Sieve-Plate and Baffle-Plate Internals

ABSTRACT Liquid-liquid two-phase flows are central to solvent extraction processes. In a columnar solvent extraction contactor, the turn-down ratio can be increased by imposing oscillatory flow over steady flow. This study compares dispersed phase volume fraction and flows regimes for oscillatory liquid-liquid two-phase flow in annuli having sieve-plate and baffle-plate internals. Such flows in annuli are important for specific solvent extraction applications requiring a high surface-to-volume ratio in the contactor geometry. For identical conditions, the volume fraction of the dispersed phase is found to be more in the annulus having baffle plates compared to the annulus having sieve plates. For both types of internals, an increase in dispersed phase velocity or continuous phase velocity, or oscillation velocity leads to enhancement in the volume fraction of the dispersed phase. The dispersed phase volume fraction increases when the spacing between sieve plates or baffle plates is reduced or when the fractional open area is reduced. Retention of the dispersed phase is more in the annulus having a smaller equivalent diameter. The annulus having baffle plates shows a faster transition from mixer-settler to dispersion regime or dispersion regime to the quasi-emulsion regime. Higher dispersed phase volume fraction in the annulus having baffle-plate internals suggests that it is more suitable for mass transfer than the annulus having sieve-plate internals. However, faster regime transitions in the annulus having baffle plates indicate that it is more prone to flooding than the annulus having sieve plates. The correlations that can be used to estimate the volume fraction of the dispersed phase are proposed. These correlations will be useful for design calculations.

A lubricant-infused slip surface for drag reduction

Seaweed and fish have slippery outer surfaces because of the secretion of a layer of mucus. The hydrodynamics over a three-dimensional lubricant-infused slip surface that mimics the mucus layers of seaweed and fish was numerically explored. The morphological features of the lubricant-infused surface were designed to mimic such biological mucus storage systems. The lubricant was assumed to fill the cavity and to be supplemented without limit from the bottom surface of the cavity. The slip motion at the interface between the lubricant and water was simulated by using the volume of fluid method. Simulations were performed for two cavity open area fractions, 40% and 60%, and for three lid thicknesses, 0.01D, 0.03D, and 0.06D, where D is the width of the cavity (D = 400 μm). The simulation was conducted by employing realistic material properties. The contact angle of the lubricant in deionized water was directly measured (θeq = 25.9°). This slippery lubricant layer contributes to drag reduction by lessening the velocity gradient of the surrounding fluid. The hydrodynamics of the slip surface was examined by scrutinizing the effects of varying the open area and the lid thickness on the slip velocity and length, the dispersion area, and the lubricant consumption. The maximum slip velocity and length were obtained in the center of the contact interface, which forms a paraboloid. The effects of varying the cavity open area fraction on the maximum slip velocity and length are significant. The lid thickness affects both the lubricant dispersion pattern and the height to which the lubricant builds up. The lubricant consumption for a cavity open area fraction of 60% is larger than that for 40%. The cavity with an open area fraction of 60% and a lid thickness of 0.06D provides the best drag reduction of the cavities we simulated.

Acoustic scattering by cascades with complex boundary conditions: compliance, porosity and impedance

We present a solution for the scattered field caused by an incident wave interacting with an infinite cascade of blades with complex boundary conditions. This extends previous studies by allowing the blades to be compliant, porous or satisfying a generalised impedance condition. Beginning with the convected wave equation, we employ Fourier transforms to obtain an integral equation amenable to the Wiener--Hopf method. This Wiener--Hopf system is solved using a method that avoids the factorisation of matrix functions. The Fourier transform is inverted to obtain an expression for the acoustic potential function that is valid throughout the entire domain. We observe that the principal effect of complex boundary conditions is to perturb the zeros of the Wiener--Hopf kernel, which correspond to the duct modes in the inter-blade region. We focus efforts on understanding the role of porosity, and present a range of results on sound transmission and generation. The behaviour of the duct modes is discussed in detail in order to explain the physical mechanisms behind the associated noise reductions. In particular, we show that cut-on duct modes do not exist for arbitrary porosity coefficients. Conversely, the acoustic modes are unchanged by modifications to the boundary conditions. Consequently, we observe that even modest values of porosity can result in reductions in the sound power level of $5$ dB for the first mode and $20$ dB for the second mode. The solution is essentially analytic (the only numerical requirements are matrix inversion and root finding) and is therefore extremely rapid to compute.

Relevance of downstream support structure design for oleophilic and oleophobic oil mist filter operating performance

Coalescence filters in applications with high separation efficiency requirements, such as compressed air preparation, usually consist of several layers of nonwoven glass fiber media wrapped around a cylindrical metal support grid. The design of this grid has a crucial effect on gas flow inside the filter and therefore on droplet deposition and liquid transport inside the medium. Experimental studies were carried out with flat filter elements in combination with seven different downstream support grids to investigate the impact of the grids on operating behavior of the filters in oil mist filtration. The grids differed in open area fraction, perforation size and distance and were used in combination with either oleophilic or oleophobic filter media. It was found that the choice of the grid affects liquid transport mechanisms in the media, especially the film formation on the filter rear side of oleophilic filters, thus affecting total pressure loss in quasi-steady state, while impact was less significant for oleophobic media. Separation efficiency was not significantly affected by the choice of grid, while entrainment depended on film coverage of the filter rear side and open area fraction of the grid.

Soft tooling process chain for the manufacturing of micro-functional features on molds used for molding of paper bottles

Paper-based packaging products, due to their excellent sustainable properties are preferred over the glass, plastic, and metal-based packaging solutions. The paper bottle molding process is very new in this category, and the tooling aspect is not well explored. Existing tooling solutions offer certain limitations in the molding process such as clogging of channels, low open-area fraction, low fatigue life, and longer cycle times. With the utilization of additive manufacturing techniques, the tooling can not only be made more economical and produced in shorter time duration but also more efficient. VAT photopolymerization based soft tooling process chain is highlighted and discussed in this work. Design of micro-features and their fabrication is carried out using UV light mask projection method. A characterization approach for precision manufacturing of micro-features using X-ray Computed Tomography is also discussed. Validation of the soft tooling process chain and its effectiveness against the conventional tools is presented and discussed in this work. It is shown that the proposed soft tooling process chain for the manufacturing of molds for paper bottle production is a better performing and cheaper alternative to existing solutions implemented in industry.

Elastic buckling of cellular columns under axial compression

Abstract A calculation of the elastic buckling load for cellular columns with multiple circular openings is required to determine their axial load capacity. Due to the complexity of column geometry and shear effects, the available methods to analyze the elastic buckling load are limited. This study proposes a method of calculation the elastic buckling load about the major axis of a pin-ended cellular column. The analysis is based on simplification of the column geometry and uses an effective length of shear force transfer across openings in the columns. The shear force in the column causes additional deformations of the web-post and the Tee section of cellular columns. The buckling load is derived by using differential equations of total curvature of the buckling curve, which is the sum of curvatures due to moment and shear forces. The proposed buckling load estimate is validated by comparisons with finite element analysis. A parametric study of the column geometry effects on shear, such as the section ratios, the opening ratios, the spacing ratios and the slendernesses, was also conducted. The shear effects clearly increases with open area fraction in the web, and with the section ratios. It was found that the spacing ratio affects the buckling load more than the opening ratio. Overall, the shear effects degrade the buckling load by less than 20% when the slenderness exceeds 50 compared to the Euler buckling load.

Equilibrium configurations of drops or bubbles in an eccentric annulus

The purpose of this paper is to find the zero-gravity equilibrium configurations of liquid drops or bubbles that have sufficient volume to form large-aspect-ratio bridging segments or occluding slugs in the eccentric annulus between two cylinders. In zero gravity, the static problem depends on the contact angle of the fluid segment on the solid support, and two geometric parameters: the radius ratio and the dimensionless distance between the cylinder centres. For both non-wetting and wetting liquids, we find regions of geometric parameter space where only occluding configurations occur, a bistable region where either configuration can occur, and a region where only the non-occluding bridging configuration can occur. For the non-occluding cases, we applied a large-aspect-ratio free-energy minimization approach to predict the cross-sectional shape of the liquid, and a finite element method was used to compute the interface shape of the occluding cases. A Surface Evolver model was used to simulate the three-dimensional shape of both occluding and non-occluding configurations. The simulation results support the theoretical predictions well. The fractional open area of the conduit was determined for both highly wetting and highly non-wetting minority phases. Optimization of the geometric parameters for a given wetting condition could facilitate the segregation and transport of two fluid phases in applications involving large aspect ratios and small pressure driving forces.

Fabrication process for 200 nm-pitch polished freestanding ultrahigh aspect ratio gratings

A fully integrated fabrication process has been developed to fabricate freestanding, ultrahigh aspect ratio silicon gratings with potassium hydroxide (KOH)-polished sidewalls. The gratings are being developed for wavelength-dispersive, soft x-ray spectroscopy on future space telescopes. For this application, the grating needs to have a large open-area fraction and smooth sidewalls (roughness < 1 nm) to maximize efficiency. The prototype gratings fabricated with the process presented here have been tested on a synchrotron beamline and have demonstrated an absolute diffraction efficiency greater than 30% for 2 nm-wavelength x-rays in blazed orders. This efficiency is greater than twice the efficiency of previously fabricated gratings. The fabrication process utilizes silicon-on-insulator wafers where the grating and a cross support are etched in the device layer, and an additional structural support is etched in the handle layer. The device layer and handle layer are both etched via deep reactive-ion etching using a Bosch process. The buried SiO2 layer stops both etches and is removed at the end of the process to create a freestanding structure. The gratings have a pitch of 200 nm, a depth of 4 μm, and the bars are polished via KOH. The polishing process reduces both the roughness and the grating-bar thickness. The finished gratings span an area of approximately 10 by 30 mm, supported by 1 mm-wide hexagons in the handle layer.

Simulation of the electron collection efficiency of a PMT based on the MCP coated with high secondary yield material

Owning to the serious loss of photoelectrons striking at the input electrode of traditional microchannel plate (MCP), photoelectron collection efficiency (CE) of photomultiplier tubes based on MCP (MCP-PMTs) fluctuates around the MCP open area fraction and cannot make a breakthrough. Depositing a thin film of high secondary electron yield material on the MCP is proposed as an effective approach to improve the CE. The available simulation and experimental data to validate it, however, is sparse. In our work, a three-dimensional small area MCP model is developed in CST Studio Suite to evaluate the collection efficiencies of PMTs based on the traditional MCP and the coated one, respectively. Results predict that CE of the PMT based on the coated MCP has a significant increase and a better uniformity, which is expected to reach 100%.

Optimization of the electron collection efficiency of a large area MCP-PMT for the JUNO experiment

A novel large-area (20-inch) photomultiplier tube based on microchannel plate (MCP-PMTs) is proposed for the Jiangmen Underground Neutrino Observatory (JUNO) experiment. Its photoelectron collection efficiency Ce is limited by the MCP open area fraction (Aopen). This efficiency is studied as a function of the angular (θ), energy (E) distributions of electrons in the input charge cloud and the potential difference (U) between the PMT photocathode and the MCP input surface, considering secondary electron emission from the MCP input electrode. In CST Studio Suite, Finite Integral Technique and Monte Carlo method are combined to investigate the dependence of Ce on θ, E and U. Results predict that Ce can exceed Aopen, and are applied to optimize the structure and operational parameters of the 20-inch MCP-PMT prototype. Ce of the optimized MCP-PMT is expected to reach 81.2%. Finally, the reduction of the penetration depth of the MCP input electrode layer and the deposition of a high secondary electron yield material on the MCP are proposed to further optimize Ce.

Calculation of teeter bed height of teetered bed separator based on jet theory

The teeter bed zone is the critical component of the teetered bed separator (TBS), and the calculation of its height plays an important role in optimizing structure of TBS and improving separation performance of coarse slime. This paper simulates the velocity profile in TBS using jet theory and discusses the relationships between velocity profile and axial height as well as fractional open area. The separation of 2–0.3mm coals using TBS with different bed lengths was also performed. The simulation results show that flowing behavior in the teeter bed zone through the distributor could be simplified as single round hole jet in con-current ambient; the fractional open area and axial height of TBS are two important influencing factors, and the even velocity distribution could be easily achieved with greater axial length and larger fractional open area. The height of the teeter bed zone could be calculated using Htb=exp1.0164−φ0.2089×d0, showing that the teeter bed height increases with an increase in hole diameter and a decrease in fractional open area. The experimental results show that good performance with Ep value of 0.117 is achieved using the TBS with bed length of 1000mm, due to the even velocity distribution with minimum flow disturbance resulted from the use of larger bed length.

Laser drilling of micro-hole arrays in tantalum

X ray collimator optics for space application require an array of high aspect ratio holes of 60:1 with a minimal tantalum (Ta) thickness of ≥2 mm and a very high open area fraction (hole versus wall fraction) of 70% to achieve high collimator efficiency. Each collimator with a drilled area of 110 mm × 70 mm contains several million holes and need a fast drilling process. Laser percussion drilling was performed using an IR pulsed disk laser in a 1 and 2 mm thick Ta plate. A tightly spaced hexagonal closed packed pattern was used to maximize open area fraction with hole-to-hole spacing as small as 80 μm. However, this resulted in a high concentration of debris and a thick recast layer on the remaining walls between the holes. Different process gases were investigated to minimize debris formation and reduce the recast layer thickness. Ramping of pulse energy during the drill cycle was investigated to minimize the adhesion between the substrate and recast layer. Chemical etching was used to remove the debris and recast from the top surface and the inside of the laser-drilled holes. Hole cross sections showed that a high aspect ratio was achieved with a hole diameter of about Ø50 μm in 2 mm thick Ta. To achieve the shortest drilling time of 200 ms per hole, the process parameters were optimized and a hybrid nozzle, with both horizontal and vertical gas flow, was developed and implemented.

Influence of compartment geometry on the residence time of single drops in Kühni extraction columns

In order to use the potential of drop-population balance simulations for the design of Kühni pilot-plant columns, a quantitative understanding of the effect of compartment geometry on drop residence-time distribution is necessary since this is crucial for the accuracy of the simulation. This work presents an experimental method based on single-drop experiments. With this method, the effect of compartment geometry and impeller speed on the drops' residence-time distribution can be quantified systematically. Drop residence times are evaluated not only for the whole compartment but also for different compartment zones, namely the lower zone between bottom stator and impeller, the impeller zone, and the upper zone between impeller and upper stator. It is also shown that by reducing only the height of the lower compartment region, the compartment height can be reduced by 20% without a significant decrease in drop residence time.Based on the experimental findings a stochastic model comprising three sub-models, one for each individual zone (lower toroidal vortex, impeller region, upper toroidal vortex) is developed. In this model, drop movement is described by superimposing drop sedimentation velocity onto the axial velocity of the toroidal vortex, which is a function of the drop's radial position. The model uses stochastic methods to account for stator impact or drops being back-mixed between the individual compartment zones. With this model, it is possible to quantitatively describe not only the mean residence times of drops but also the residence-time distribution for different drop diameters, impeller speeds and open-area fractions of the stator.

Laminar forced convection heat transfer to a single layer of ordered and disordered spheres

We study laminar forced convection heat transfer to single layer arrays of equidistantly and non-equidistantly spaced spheres. We report average Nusselt numbers as a function of geometry and flow conditions, for open frontal area fractions between 0.04 and 0.95, Prandtl numbers between 0.7 and 10, and Reynolds numbers (based on sphere diameter and the free stream velocity) between 0.1 and 100. For equidistantly spaced arrays of spheres we propose a general analytical expression for the average Nusselt number as a function of the Reynolds number, Prandtl number and the open frontal area fraction, as well as asymptotic scaling rules for small and large Reynolds. For all studied Prandtl numbers, equidistant arrays exhibit decreasing average Nusselt numbers for decreasing open frontal area fractions at low Reynolds numbers. For high Reynolds numbers, the Nusselt number approaches that of a single spheres in cross-flow, independent of the open frontal area fraction. For equal open frontal area fractions, the Nusselt number in non-equidistant arrays is lower than in equidistant arrays for intermediate Reynolds numbers. For very low and high Reynolds numbers, non-uniformity does not influence heat transfer.

Laminar‐forced convection mass transfer to ordered and disordered single layer arrays of spheres

Laminar forced convection mass transfer to single layers of equidistantly and nonequidistantly spaced spheres perpendicular to the flow direction is studied. Average Sherwood numbers are reported as a function of geometric configurations and flow conditions, for open frontal area fractions between 0.25 and 0.95, Schmidt numbers between 0.7 and 10, and Reynolds numbers (based on the sphere diameter and the free stream velocity) between 0.1 and 100. For equidistantly spaced arrays of spheres, a general analytical expression is proposed for the average Sherwood number as a function of the Reynolds number, Schmidt number and the open frontal area fraction, as well as asymptotic scaling rules for small and large Reynolds. For all studied Schmidt numbers, equidistant arrays exhibit decreasing average Sherwood numbers for decreasing open frontal area fractions at low Reynolds numbers. For high Reynolds numbers, the Sherwood number approaches that of a single sphere, independent of the open frontal area fraction. For equal open frontal area fractions, the Sherwood number in nonequidistant arrays is lower than in equidistant arrays for intermediate Reynolds numbers. For very low and high Reynolds numbers, nonuniformity does not influence mass transfer. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1400–1408, 2013