Axial Pressure Profiles Research Articles

Taylor–Couette flow and heat transfer in an elliptical enclosure with a rotating inner cylinder

We numerically investigate Taylor–Couette flows within a system consisting of an elliptical outer cylinder and a rotating inner circular cylinder, with particular emphasis on the behavior of Taylor cells. The three-dimensional unsteady Navier–Stokes equations are solved under the assumption of axial periodicity. Also, a scalar transport equation is solved for the heat transfer. Our methodology employs a Fourier-spectral meshless discretization technique, which interpolates variables at scattered points using polyharmonic splines and appended polynomials. A pressure-projection algorithm achieves the time advancement of the flow equations. We present findings for an elliptical enclosure with an aspect ratio of two, examining a range of Reynolds numbers (Re) from subcritical to 300. Our analysis includes streamlines, axial velocity contours, pressure, vorticity, and temperature profiles. The results indicate that the flow remains steady up to Re≈300 before transitioning to an unsteady state at Re≈350.

Flow analysis of screw extrusion in three-dimensional concrete printing

Recent advances in three-dimensional concrete printing necessitated the detailed understanding of the operation and performance of screw extruders. This paper shows that the volumetric output rate can be approximated using the rotating barrel and stationary screw assumption (drag flow equation), which is used routinely in polymer melt extrusion calculations. Verification is provided by comparisons to experimental results available in the literature and to computer flow simulations for fluids with yield stress. Significant insight is obtained using fully three-dimensional simulations. This includes particle pathlines, which form “a helix within a helix,” axial pressure profiles, and the effect of yield stress, which is relatively small on the output rate but large on torque and power. The computer simulation also predicts unyielded flow zones in the extruder channel at low screw rotation speeds.

New 2.5‐Dtwin screw extruder model for the feed section of a co‐rotating twin screw extruder with comparisons to data

AbstractMany polymers are processed and compounded in co‐rotating, fully intermeshing twin screw (TS) extruders. The typical compounding process consists of multiple unit operations including feed introduction, conveying solids away from the feed zone, transitioning from the conveying zone to the kneading block (KB) melting zone, melting, a downstream feed zone, a downstream mixing zone, a devolatilization region, and a pressure generating discharge section. This paper will focus on the first of these unit operations, the conveying of pellets in a partially filled screw leading to compaction and pressure generation in the full screw channel prior to the KB melting zone. Mechanisms for pellet conveying, for heating the polymer pellets, and for developing pressure at the end of the solids conveying screw elements are first presented for low‐density polyethylene (LDPE). The predictions of the model are also compared with a classical set of experiments on high‐density polyethylene (HDPE). The model is then tested using published characteristics of a polyamide‐type polymer. This paper focuses on the physics and engineering concepts that are inherent in the feed section of the TS extruder where the pressure is calculated using the friction of the polymer pellets. The effects of throughput,Q, at a constant rotation speed,N, are examined. Low and highQ/Nratios have significantly different axial pressure profiles.

Effect of bed material size on gas-solid flow characteristics in a CFB at low solid recirculation rates

The gas–solid flow characteristics greatly influence the heat transfer, combustion and pollutant formation processes in a circulating fluidized bed (CFB) boiler. The effects of particle size on the gas–solid flow characteristics, including flow structure, axial pressure and bed density profiles were studied in a two-dimensional CFB cold apparatus under low solid circulation rates (Gs’s). At the same time, an optical fiber probe was used to assess the particle size effect on the formation of particle clusters and their properties under different operating conditions. Results revealed that when the averaged size of bed materials decreased from ∼200 μm to ∼100 μm, at the same operating condition, the bottom dense bed became remarkably higher, and the upper dilute zone had stronger pressure fluctuation, more even axial and horizontal particle distributions, and higher solid density. Gs was more sensitive to the variation of the superficial gas velocity than the bed inventory. In addition, the standard deviation and power spectrum density of instantaneous pressure decreased in the dense bed and increased in the dilute zone. The particle velocity was slightly lower in the lower section of the riser but noticeably greater in the upper riser section. In addition, the clusters formed more frequently and lasted significantly longer. The study showed that flow characteristics in the bed qualitatively changed from that of Type B particles to that Type A particles in the Geldart’s classification, even at a small Gs, and previous studies with Type B particles could very likely not apply in the CFB boilers operating with fine bed materials.

A Composite Model for Melting, Pressure and Fill Factor Profiles in a Metered Fed Closely Intermeshing Counter-rotating Twin Screw Extruder

Abstract A composite model of solids conveying, melting and melt flow in a closely intermeshing counter-rotating twin-screw extruder of modular design has been developed. This is based on combining melt conveying models with new melting and solids conveying models. Computations are made for axial fill factor, pressure, temperature and melting profiles. The results are compared with experiment.

An Analytical Method to Guide mm-Sized Ultrasonic Transducer Design by Demarcating Regimes of Beam-Spreading and Attenuation

In this paper, we present a novel approach that aids in the sizing of micron to mm-sized ultrasonic transducers for enhanced resolution and denser integration. This seeks to optimize penetration-depth which is an important metric of a transducer’s efficacy in different acoustic media. It is generally accepted that in conventional transducers used in clinical imaging, penetration depth is greater for lower driving frequencies which suffer lessened attenuation. However, in acoustic characterization studies, the drops in axial pressure by beam-spreading and attenuation haven’t been individually quantified and their relative effects are poorly understood. We, therefore, undertake a closer investigation of factors that shape the axial pressure profile of a generalized transducer element in a weakly attenuating medium like water and in a highly attenuating medium like tissue. We have found that it is critical to quantify the relative contributions of beam-spreading and attenuation in a medium. We introduce a new parameter called ‘transition distance’ (dTrans) to graphically demarcate the regimes in beam-spreading and attenuation dominate to explain how a low frequency (2 MHz) or a high frequency (5 MHz) gives greater penetration depth in different media.

Sub-millimetre precision of drug delivery in the brain from ultrasound-triggered nanodroplets

Drug-loaded nanoscale cavitation agents, called nanodroplets, are an attractive solution to enhance and localize drug delivery, offering increased stability and prolonged half-life in circulation compared to microbubbles. However, the spatial precision with which drug can be released and delivered into brain tissue from such agents has not been directly mapped. Decafluorobutane lipid-shell droplets (206 +/− 6 nm) were loaded with a fluorescent blood-brain barrier (BBB)-penetrating dye (Nile Blue) and vaporized with ultrasound (1.66 MHz, 10 ms pulse length, 1 Hz pulse repetition frequency), generating transient echogenic microbubbles and delivering the encapsulated dye. The distribution and intensity of released fluorophore was mapped in a tissue-mimicking phantom, and in the brain of rats (Sprague Dawley, N = 4, n = 16). The release and distribution of dye was found to be pressure-dependent (0.2–3.5 MPa) and to occur only above the vaporization threshold of the nanodroplets (1.5 +/− 0.25 MPa in vitro, 2.4 +/− 0.05 MPa in vivo). Dye delivery was achieved with sub-millimetre spatial precision, covering an area of 0.4 to 1.5 mm in diameter, determined by the sonication pressure. The distribution and intensity of dye released at depth in the brain followed the axial pressure profile of the ultrasound beam. Nile Blue (354 Da, LogP 2.7) was compared to Nile Red (318 Da, LogP 3.8) and Quantum Dots (CdSe/ZnS, 5 nm diameter) to visualize the role of molecule size and lipophilicity in crossing the intact BBB following triggered release. Acoustic emissions were shown to predict the successful delivery of the BBB-penetrating dye and the extent of the distribution, demonstrating the theranostic capabilities of nanoscale droplets to precisely localize drug delivery in the brain.

Application of Wray–Agarwal Turbulence Model for Numerical Simulation of Gas-Solid Flows in Circulating Fluidized Bed Risers

Abstract In recent decades, increasing attention has been paid on accurate modeling of circulating fluidized bed (CFB) risers to provide valuable guidance to design, optimization, and operation of reactors. Turbulence model plays an important role in the accurate prediction of complex gas-solid flows. Recently developed Wray–Agarwal (WA) model is a one-equation turbulence model with the advantages of high computational efficiency and competitive accuracy with two-equation models. In this paper for the first time, the Eulerian–Eulerian approach coupled with different turbulence models including WA model, standard κ–ɛ model, and shear stress transport (SST) κ–ω model is employed to simulate two-phase flows of gas phase and solid phase in two CFB risers, in order to assess accuracy and efficiency of WA model compared to other well-known two-equation models. Predicted gas-solid flow dynamic characteristics including the gas-solid volume fraction distributions in radial and axial directions, pressure profiles, and solid mass flux distributions are compared with data obtained from an experiment in detail. The results demonstrate that the WA model is very promising for accurate and efficient simulation of gas-solid multiphase flows.

IFAR liner benchmark challenge #1 – DLR impedance eduction of uniform and axially segmented liners and comparison with NASA results

This paper presents the contribution from the German Aerospace Center (DLR) to the first liner benchmark challenge under the framework of the International Forum for Aviation Research (IFAR). Therefore, two sets of acoustically damping wall treatments, called ‘liner samples’, have been produced by additive manufacturing based on the design data provided by NASA coordinating this benchmark. These liner samples have been integrated and acoustically characterized in the liner flow test facility DUCT-R at DLR Berlin as well as in the liner flow test facility GFIT at NASA Langley. Besides the dissipation coefficients and the axial pressure profiles, the liner wall impedance was educed by first determining the axial wave numbers and then applying a straightforward method based on the one-dimensional Convected Helmholtz Equation. Finally, the comparison of the liner impedance values to the NASA results show a fairly good agreement.

The effects of partial preheating on pressure propagation and Flow-Restart phenomena in a clogged pipeline with a weakly compressible gel

The present article aims to decipher the effect of preheating a segment of the pipe on the pressure propagation mechanisms and flow restart operation in a gelled pipeline. During the restart operation, shear-thinning thixotropic rheology governs the gel properties, where the viscosity is a function of shear strain and thermal history. A finite volume method is employed to solve the governing equations for a weakly compressible gel. Rheology of the initial gel incorporates the effect of temperature distribution from the preheating stage. Flow restart in the thixotropic gel involves three different timescales: namely, the compressive diffusion timescale (acoustic wave propagation timescale), the viscous timescale, and the gel degradation timescale. In an isothermal case, the flow restart occurs at the gel degradation timescale, whereas flow restart in the preheated gel takes place at the viscous timescale. Time evolution of nonlinear axial pressure profile and residual viscosity provides a theoretical understanding of the gel degradation process in the thermal diffusion-controlled and advection-controlled flow regimes, defined in terms of Peclet number (Pe). The results indicate that the gel degradation process is affected by thermal-induced melting in addition to shear melting. Partial preheating tends to dislodge the gel into plugs, and this is more significant at low Pe and high pristine gel strength. The local Nusselt number (Nu) at the heated portion of the wall during flow restart suggests that the wall-heat transfer is prominent when the pressure wavefront has not reached the outlet.

Experimental Investigation of Inner Nozzle Surface Modifications on the Flow Regimes in Steam/Water Flow

Mixing of two phase flows can be enhanced by modifying the inner surface of gas/vapor injection chamber. In the present study, we perform experimental PIV investigation focusing the planar region across the supersonic steam jet injection into the water in a transparent rectangular duct 4 ft high. Axial pressure profile of the rectangular channel is obtained to highlight the impact of inner surface modifications of supersonic nozzle on the steam-water interfacial hydrodynamics for inlet pressure ranging between 1.5 and 3.0 bars. Results show elongation of the vortical structures with increase in pressure. The profile of the spatial scalar fluctuations intensity decreases with peaks at certain locations indicating formation of shocks within the steam jet in the upstream region. Turbulence induced mixing is observed as the interfacial structure breaks down causing local minima. However, due to the inner surface modifications the local minima shifted from the nozzle’s exit up to 1.7% lengthwise than the smooth inner surface. The intensity of high pressure locations are found to be vary between 4 and 5% along the flow channel. However, due to the inner surface modifications, the lengths of the high pressure crests reduced by 3.4–5.2%.

Application of Network Analysis to Flow Systems with Alternating Wave Channels: Part B. (Superimposed Drag-Pressure Flows in Extrusion).

Due to progress in the development of screw designs over recent decades, numerous high-performance screws have become commercially available in single-screw extrusion. While some of these advanced designs have been studied intensively, others have received comparatively less attention. We developed and validated a semi-numerical network-theory-based modeling approach to predicting flows of shear-thinning polymer melts in wave-dispersion screws. In the first part (Part A), we systematically reduced the complexity of the flow analysis by omitting the influence of the screw rotation on the conveying behavior of the wave zone. In this part (Part B), we extended the original theory by considering the drag flow imposed by the screw. Two- and three-dimensional melt-conveying models were combined to predict locally the conveying characteristics of the wave channels in a discretized flow network. Extensive experiments were performed on a laboratory single-screw extruder, using various barrel designs and wave-dispersion screws. The predictions of our semi-numerical modeling approach for the axial pressure profile along the wave-dispersion zone accurately reproduce the experimental data. Removing the need for time-consuming numerical simulations, this modeling approach enables fast analyses of the conveying behavior of wave-dispersion zones, thereby offering a useful tool for design and optimization studies and process troubleshooting.

Online optical monitoring of polymer melting in a twin‐screw extruder

AbstractAn experimental setup containing a sliding online optical device is used to monitor in real‐time the melting process of a commercial polypropylene in a corotating intermeshing twin‐screw extruder. Turbidity and birefringence are measured at several axial locations upstream and along the first restrictive zone of the screw, where melting develops. The experiments are performed using different set barrel temperatures, extruder feed rates, and screw speeds, to generate distinct flow histories and, accordingly, changes in the onset and rate of melting of the polymer. The local flow conditions are characterized in terms of residence time distribution and data equivalent to axial pressure profiles. Turbidity and birefringence are sensitive to changes in the operating conditions providing a coherent description of melting. The onset of melting seems to take place in partially filled conveying elements, and then melting develops quickly as the latter become fully filled, and is completed well before flow through the kneading block.

Microscopic Pressure Tensor in Cylindrical Geometry: Pressure of Water in a Carbon Nanotube.

The microscopic pressure tensor plays an important role in understanding the mechanical stability, transport, and high-pressure phenomena of confined phases. The lack of an exact formulation to account for the long-range Coulombic contribution to the local pressure tensor in cylindrical geometries prevents the characterization of molecular fluids confined in cylindrical pores. To address this problem, we first derive the local cylindrical pressure tensor for Lennard-Jones fluids based on the Harasima (H) definition, which is expected to be compatible with the Ewald summation method. The test of the H-definition pressure equations in a homogeneous system shows that the radial and azimuthal pressure have unphysical radial dependence near the origin, while the axial pressure gives physically meaningful values. We propose an alternative contour definition that is more appropriate for cylindrical geometry and show that it leads to physically realistic results for all three pressure tensor components. With this definition, the radial and azimuthal pressures are of Irving-Kirkwood (IK) type, and the axial pressure is of Harasima type. Because of the practical interest in the axial pressure, we develop a Harasima/Ewald (H/E) method for calculating the long-range Coulombic contribution to the local axial pressure for rigid molecules. As an application, the axial pressure profile of water inside and outside a (20, 20) single-wall carbon nanotube is determined. The H/E method is compared to the IK method, which assumes a spherically truncated Coulombic potential. Detailed analysis of the pressure profile by both methods shows that the water confined in the nanotube is in a stretched state overall in the axial direction.

Three-dimensional modelling of the multiphase hydrodynamics in a separated-gasification chemical looping combustion unit during full-loop operation

Separated-gasification chemical looping combustion (SG-CLC) is a clean utilization technology for fossil fuels, which can separate CO2 during high-performance combustion process with low degradation of oxygen carrier and zero-energy penalty. As an important part of the optimization, modelling and prediction of the multiphase hydrodynamics in the SG-CLC unit were conducted in this work. This unit consists of a gasifier (GR), a reduction reactor (RR) and an air reactor (AR). For thorough understanding of the flow behaviors of three phases (gas, oxygen carrier and sand phases), a three-dimensional computational fluid dynamics (CFD) model for SG-CLC system was first implemented. The main purpose is to predict multiphase hydrodynamics at the fuel side of this SG-CLC system due to its significant effects on combustion efficiency. The axial pressure profile predicted by the CFD model was validated by experimental data to demonstrate its feasibility. Then, based on this model, the hydrodynamics of different phases at fuel side (GR and RR) were investigated under fundamental condition, which indicated the gas-solid flow mechanism in circulation establishment process. Furthermore, effects of the operation condition on the hydrodynamics properties were investigated. The local gas-solid slip velocity in RR was fitted as the function of superficial fluidizing number Nr and axial position, which showed high accuracy for the prediction in this system.

Effects of Riser Diameter on Solids Holdup and Particle Velocity Profiles in Circulating Fluidized Bed Riser Systems

Abstract Tests were performed in a 0.1-m diameter small circulating fluidized bed (SCFB) and 0.3 m diameter cold flow circulating fluidized bed (CFCFB) riser systems located at the National Energy Technology Laboratory (NETL) to study the effects of riser diameter on the riser hydrodynamics. These tests were performed at solids circulation rates of Gs = 20 and 75 kg/m2 s and superficial gas velocities of Ug = 5.8 and 6.5 m/s using high-density polyethylene (HDPE) pellets with a density of 0.863 g/cm3, particle size range of 600–1400 µm (with a Sauter mean diameter of 871 µm, placing them in the Geldart B classification). Comparisons of riser axial pressure and solids fraction profiles, radial particle velocity profiles, and radial profiles of higher statistical moments and select chaos analysis parameters were considered. The results showed that for a given Ug and Gs, the smaller diameter riser exhibited characteristics associated with more dilute solids flow than that observed in the larger diameter riser. Additionally, the larger diameter riser exhibited a downward flow of solids near the wall under all test conditions, whereas the smaller diameter riser data exhibited little or no indications of solids downflow near the wall. These findings suggest that, from an industrial standpoint, a direct scaleup of small-scale tests cannot readily be accomplished as the solids holdup and the solids velocity profiles in small units (those normally tested in the laboratory) are not similar to those of large units and the performance of large units can therefore not be predicted from small-scale tests.

Differences in Pressure Recovery Between Balloon Expandable and Self-expandable Transcatheter Aortic Valves.

Pressure recovery downstream of the aortic valve constitutes an important factor affecting the calculation of pressure gradient (PG) across the valve and therefore the accuracy of the calculated aortic valve area. Some clinical studies hypothesized that stent and valve cusps design contribute to flow acceleration and Doppler-measured valve gradients across the balloon-expandable transcatheter aortic valve. This study aims at elucidating the physical mechanisms behind pressure recovery variations between Edwards SAPIEN 3 and Medtronic Evolut TAVs through the measurements of sensitive and precise axial pressure profiles. A 23mm Edwards SAPIEN3 and a 26mm Medtronic Evolut were deployed in a pulse duplicator. A Millar catheter was used to record 50 cycles of pressure data along the centerline of the valve chamber upstream and downstream of the valve. The peak PG obtained with SAPIEN at vena contracta (VC) is 18.83 ± 0.75mmHg and after recovery, 9.56 ± 0.78mmHg. For Evolut at VC, peak PG is 18.25 ± 0.63mmHg and after recovery, 10.3 ± 0.57mmHg. The differences in peak PG at VC and at the recovery were statistically significant (p < 0.001). With SAPIEN 3 at VC, the mean PG obtained is 10.11 ± 0.63mmHg and after recovery 7.06 ± 0.46mmHg. For Evolut, mean PG at VC is 10.45 ± 0.67mmHg and after recovery 7.99 ± 0.61mmHg. The differences between the mean PG between the two valves was not statistically significant at VC (p = 0.71) but significant post-recovery (p < 0.00001). While gradients at the VC are higher with the SAPIEN 3, the net gradient after pressure recovery is significantly lower compared to Evolut TAV. Efficiency ofpressure recovery significantly depends on valve type due tostent interference with the recovering blood flow.

Analysis of the Flow Distribution in a Particle Bed Reactor for Nuclear Thermal Propulsion

Nuclear thermal propulsion (NTP) is regarded as the preferred option for the upcoming crewed interstellar exploration due to its excellent performance compared to the current most advanced chemical propulsion systems. Over the past several decades, many novel concepts have been proposed, among which the particle bed reactor (PBR) is the most efficient, compact, and lightweight method. Its unique features, such as the extremely high power density and the radial flow path of coolant in the fuel region, introduce many challengeable issues to the thermal hydraulic design of PBR, with the flow distribution being representative. In this work, the flow distribution process within the core is analyzed based on the understanding of the axial pressure profile in a dummy PBR. A “flow shift” phenomenon leading to the hot spot in the core is introduced first, and three methods, i.e., decreasing the pressure drop within the hot gas channel, increasing the flow resistance on the cold frit or hot frit, and changing the flow pattern from “Z” to “U”, are proposed to reduce the “flow shift” and the consequent temperature mal-distribution. The pros and cons of using cold frit or hot frit to distribute the coolant are also discussed. Finally, by using three numerical examples, these analyses are demonstrated. The findings here may provide technical support for PBR design.

Effect of the solids inventory and fluidization gas velocity on the hydrodynamics of a circulating fluidized bed

The hydrodynamics behavior of a bench-scale circulating fluidized bed (CFB) system has been analyzed in terms of the axial pressure profile and solids circulation rate. Experimental runs were carried out at room conditions using beds of quartz sand and air as the fluid phase in order to know the effect of the overall solids inventory and the superficial gas velocity used in the riser. Static pressure signals obtained from the gas-particle flow were registered at several positions along the CFB loop and the solids circulation rate was measured by using a diverter valve installed at the standpipe section. A statistical analysis applied on the experimental data show that the solids inventory and the superficial gas velocity in the range here studied lead to significant changes on the hydrodynamics of the circulating fluidized bed system.Keywords: Circulating fluidized bed riser, gas-particle hydrodynamics, static pressure signals, solids circulation rate.

Controlling kinetic and diffusive length-scales during absorptive hydrogen removal in methane dehydroaromatization on MoCx/H-ZSM-5 catalysts

Addition of Zr metal absorbent to MoCx/H-ZSM-5 in the form of staged-bed, stratified-bed, and interpellet physical mixtures effectively scavenges H2 from catalyst proximity, enhancing maximum single-pass benzene + naphthalene yield during methane dehydroaromatization (DHA) reactions to 14–16% compared to 8% in formulations without zirconium. The coupling of spatially-distinct catalytic and absorptive functions is achieved by dispersive/diffusive transport which conveys H2 to staged Zr both co- and counter-current to bulk advection, thereby suppressing axial H2 partial pressure profiles along the catalyst bed and enhancing net aromatization rates. We evince hitherto unreported significance of dispersive hydrogen transport during methane DHA by measurement of Péclet number, Pe = 1.32, in H2 tracer studies with step-change or impulse input to inert catalyst proxies. Kinetic limits to methane pyrolysis are quantified by Damköhler number, Da, for synthesis of benzene, DaB = 0.15, and naphthalene, DaN = 0.03, determined from kinetic studies which rigorously account for reversibility of DHA reactions. Detailed reaction-transport models synthesize interplay of kinetic, diffusive, and convective length-scales captured by Péclet and Damköhler number to predict influence of catalyst-absorbent proximity and process flow-conditions on aromatization rates. Systematic control of catalyst bed-length, L, or linear flow velocity, u, predictably alters Pe and Da to effect improvements in methane conversion with and without Zr metal, corroborating results from simulation of the reaction-transport model.