Axial Pressure Profiles Research Articles

Numerical Simulation on Interfacial Characteristics in Supersonic Steam–water Injector Using Particle Model Method

Steam–water injectors have been widely applied in various industrial fields because of their compact and passive features. Despite its straightforward mechanical design, the internal two-phase condensing flow phenomena are extremely complicated. In present study, a numerical model has been developed to simulate steam–water interfacial characteristics in the injectors based on Eulerian–Eulerian multiphase model in ANSYS CFX software. A particle model is available for the interphase transfer between steam and water, in which a thermal phase change model was inserted into the model as a CFX Expression Language (CEL) to calculate interphase heat and mass transfer. The developed model is validated against a test case under a typical operating condition. The numerical results are consistent with experimental data both in terms of axial pressure and temperature profiles, which preliminarily demonstrates the feasibility and accuracy of particle model on simulation of gas–liquid interfacial characteristics in the mixing chamber of injector. Based on the dynamic equilibrium of steam supply and its condensation, interfacial characteristics including the variation of steam plume penetration length and steam–water interface have been discussed under different operating conditions. The numerical results show that steam plume expands with steam inlet mass flow rate and water inlet temperature increasing, while it contracts with the increase of water inlet mass flow rate and backpressure. Besides this, the condensation shock position moves upstream with the backpressure increasing.

Spatially Sampled Pressure Profile Measurements in Externally Pressurized Gas Journal Bearings

Externally pressurized gas journal bearings (EPGJBs) are widely adopted to support high-speed rotors. This article presents novel experimental benchmark test data that enable EPGJB model validation. Axial and circumferential pressure profile measurements are presented for a 40-mm-diameter annular restrictor EPGJB operating at speeds up to 25 krpm. The corresponding rotor position, air mass flow rate, and load capacity are also measured. The effects of supply pressure, rotational speed, and load on the measured variables are presented. The measurements are in good agreement with numerical model computations. The cases in which the model deviates from the measured data are discussed in terms of compressible flow theory.

Comparison of direct flow simulation in a random packed bed of Raschig rings with a macroscale momentum-sink approach

The purpose of the work is to investigate two numerical models of gas flow in a random packed bed of Raschig rings – one based on direct numerical simulation of flow at the level of individual particles and the other simplified approach in which the bed is treated as a homogeneous domain with prescribed pressure drop calculated from Darcy law. The latter model is usually preferred when only average flow characteristics are important in a given application and is expected to provide reasonably accurate results at the macroscopic level. The results show that when the flow enters the bed in a highly non-uniform manner (here – as a jet), the simplified model is not able to reproduce velocity field even averaged over regions of a considerable size. However, the axial average pressure profile is predicted with a good accuracy.

A Network-Theory-Based Comparative Study of Melt-Conveying Models in Single-Screw Extrusion: A. Isothermal Flow.

In many extrusion processes, the metering section is the rate-controlling part of the screw. In this functional zone, the polymer melt is pressurized and readied to be pumped through the die. We have recently proposed a set of heuristic models for predicting the flow behavior of power-law fluids in two- and three-dimensional metering channels. These novel theories remove the need for numerical simulations and can be implemented easily in practice. Here we present a comparative study designed to validate these new methods against experimental data. Extensive experiments were performed on a well-instrumented laboratory single-screw extruder, using various materials, screw designs, and processing conditions. A network-theory-based simulation routine was written in MATLAB to replicate the flow in the metering zones in silico. The predictions of the three-dimensional heuristic melt-conveying model for the axial pressure profile along the screw are in excellent agreement with the experimental extrusion data. To demonstrate the usefulness of the novel melt-flow theories, we additionally compared the models to a modified Newtonian pumping model known from the literature.

Comparative analysis of hydrogen/air combustion CFD-modeling for 3D and 2D computational domain of micro-cylindrical combustor

A computational study of a micro-cylindrical premixed hydrogen-air combustor using RANS is presented. The influence of the geometric dimensionality of the computational domain is studied by performing numerical simulations with 2D planar, 2D axisymmetric and 3D meshes. A commercial software package with a particular choice of several physical models (regarding turbulence, combustion, radiation) is used for this analysis. A mathematical model describing the hydrogen/air combustion in a cylindrical combustor is developed. The reaction scheme of combustion process with 9 species and 19 steps was simulated by usage Eddy Dissipation Concept (EDC) model. The obtained results from presented CFD model were compared with experimental and numerical data of other publications. The combustion characteristics of developed numerical model such as wall and fluid temperature, hydrogen mass fraction, velocity and pressure were investigated for different geometry types. It is established that the 2D approach to solving combustion problems leads to significant deviations of the obtained results from real ones and can be used only for a preliminary evaluation of the characteristics of the combustion process. Based on the developed model, it is established that the difference between the combustion products temperature at the combustor outlet for the three- and two-dimensional (2D-planar and 2D-axis) geometries is more than 25%, and for the range of the flame formation the temperatures differ by several times. The contours of pressure have minimal visual differences for the all hydrogen burning characteristics that are investigated in this work. The axial pressure profiles for all investigated geometries (2D-axis, 2D-planar and 3D) are almost similar in terms of trend and value.

Impact of catalyst density distribution on the fluid dynamics of an ebullated bed operating at high gas holdup conditions

Experiments were conducted to investigate the impact of particle density distribution on ebullated bed phase holdups and local fluidization behaviour when operating under high gas holdup conditions. Fresh and spent heavy oil hydroprocessing catalyst having relatively narrower and wider density distributions were compared. A 0.5wt.% aqueous ethanol solution was used to obtain relatively high gas holdups as observed in many industrial reactors containing liquid mixtures with surface-active compounds. Axial pressure profiles were used to assess the degree of segregation on liquid-solid and gas-liquid solid fluidized beds. While marginal axial holdup variation occurred when operating the liquid-solid fluidized bed, introduction of gas significantly impacted the fluidized bed dynamics by rendering the bed-freeboard interface diffuse at low superficial liquid velocity as relatively large bubbles were formed. This was observed visually and experimentally based on the pressure profile curvature. At elevated liquid flow rates, the bed interface became more stable due to smaller bubbles being formed because of the greater shear stress at the gas-liquid distributor. Solid holdup was the most affected by the particle density distribution, where bed expansion/contraction was dependent of the liquid flow rate due to varying particle-bubble dynamics. Such information provided guidance on potential factors that can lead to the loss of bed-freeboard interface in the operation of heavy oil hydroprocessors such as the LC-FinerSM.

Modelling and simulation of hydrodynamics in double loop circulating fluidizedbed reactor for chemical looping combustion process

A multiphase CFD model has been developed and implemented in an in-house code for a coupled double loop circulation fluidized bed (DLCFB) reactor which can be utilized for the chemical looping combustion process. The air reactor and the fuel reactor were operated in fast fluidization regime and simulated separately, the connection between the two reactors is realized through specific inlet and outlet boundary conditions. This work represents a first attempt to model and simulate the novel DLCFB system. The model predictions of the axial pressure profiles are in good agreement with the experimental data reported in the literatures. In addition, typical core-annulus structure of radial solid volume fraction distribution can be well predicted in both reactors. These indicate the capability of the model for predicting the cold flow performance of the DLCFB system. Furthermore, the effects of superficial gas velocity, total solid inventory on the flow characteristics have been examined. The results show that an increase of the gas velocity could enhance the solid exchanges between the two reactors. The additional solids were accumulated in the bottom of the reactors when the total solid inventory was increased.

CFD Modeling of Three-phase Flotation Column Incorporating a Population Balance Model

A Computational Fluid Dynamics (CFD) model was developed and validated against published experimental data to understand the effect of slurry concentration on bubbles’ coalescence process in flotation cell. The existing flow field was modelled for two- phase (gas-liquid) and three-phase (gas-liquid-solids). The simulations were carried out using the commercial flow simulation software AVL-FIRE 2009.2 for Jg= 0.94-5.01 cm s-1 Cs= 0-200 kg m-3, respectively. Modelling calculations have been conducted using Eulerian–Eulerian multiphase model with k-ɛ turbulence model for the liquid phase. In the present study, the population balance equation for bubble break-up and bubble coalescence rate and the interfacial exchange of mass and momentum as well as bubble-particles attachment and detachment have been included in the CFD code by writing subroutines in FORTRAN. Comparisons of the predicted results against measured gas holdups and axial pressure profile show reasonable agreement without using any fitting parameter. With the increase of slurry concentration, the gas holdup in a flotation column decreased. These results are in line with those of previous studies. The results also show that the addition of solid particles reduced the size of bubbles in a flotation column.

Effects of reducing the reactor diameter on the dense gas–solid fluidization of very heavy particles: 3D numerical simulations

In this study, 3D numerical simulations using an Eulerian n-fluid approach of a gas–solid fluidized bed composed of very dense particles of tungsten (19,300kgm−3) were carried out to examine the behavior of this suspension, especially the effects of the reduction of the fluidization column diameter on the fluidization quality. Tungsten was selected as a surrogate material of U(Mo) (Uranium molybdene) which is of interest for new nuclear fuels with limited enrichment. Comparisons between experiments and computations for the axial pressure profile of a 5cm diameter column demonstrate the capability of the mathematical models of the NEPTUNE_CFD code to simulate the fluidization of this powder located outside the classification of Geldart. The numerical results show that the mobility into the bed of these very dense particles is very low. The reduction of the fluidization column diameter from 5cm to 2cm does not have significant effect on the local solid circulation but strongly decreases the axial and radial mixing of the particles due to wall-particles friction effects. These results confirm and allow to better understand the wall effects experimentally evidenced.

Rheological Properties of Collagen Matter Predicted Using an Extrusion Rheometer

AbstractThis article reports on measurements of the rheological properties of slightly compressible collagen liquid (from 6.6 to 8.0% mass fraction of native bovine collagen in water) using a capillary rheometer. The extrusion rheometer is equipped with an annular slit between a circular tube and a central pin. The sample of collagen to be tested is pushed by a piston from a container to the annular measuring section. A manually adjusted hydraulic drive enables the piston velocity and the corresponding shear rate to be varied continuously within the range from 50 to 3000 s−1. Strain gages installed in the outer cylinder monitor the axial pressure profile. It is therefore possible to identify not only the basic nonelastic shear flow characteristics (Herschel Bulkley and Power law model parameters), but also the first normal difference evaluated from the exit pressure. The results of experiments carried out at room temperature show rather broad variability of the parameters.Practical ApplicationsThe rheological properties of native collagenous materials are of primary importance in the design of extrusion processes, for example, extrusion of sausage casings (a description of the flow behavior not only inside the screw extruders, but also while the extruded tubes are being blown) and similar technologies (injection molding, film blowing, rolling, and so forth). Knowledge of the rheological properties of native collagenous vascular grafts is important for an indirect assessment of irradiation effects and the effects of additives on a collagenous matrix (crosslinking). Native collagenous materials are rather “awkward” from the point of view of rheometry. For example, rotational rheometers are limited to a linear oscillation regime (due to discharge of the samples and shear banding), and the application of capillary rheometers is complicated by the inhomogeneity of native collagen (presence of bubbles). The proposed rheometer takes into account the compressibility of the tested samples, and enables an evaluation not only of the shear properties but also of the elastic properties within the range of deformation rates corresponding to the production lines.

Longitudinal pulsed detonation instability in a rotating detonation combustor

The peculiar phenomenon of longitudinal pulsed detonation (LPD) in a rotating detonation combustor (RDC) is studied using hydrogen–air mixtures, by utilizing: (i) two air injection schemes having different inlet areas, and (ii) a convergent nozzle assembly with different spacers that affixes to the RDC exit. By varying the air injection pressure ratio and the backpressure, the regime of occurrence and the mechanism of this pulsed detonation instability are investigated. Immense evidence to suggest that LPD is caused by a peculiar detonation initiation mechanism enabled by a reflected shock wave from the RDC exit is discovered through an ensemble axial pressure profile analysis. Distance–time plots show that a single cycle of the pulsed detonation has two components: a fast-moving axial forward decaying detonation wave (with 75% of the ideal detonation speed) and a slower reflected detached shock wave (with 30% of the ideal speed). When the weak reflected wave comes in contact with the fresh reactants at the RDC headwall, another strong axial detonation is produced, thereby continuing the cycle without external ignition. LPD is also found to have three diverse facets, namely, inception, sustenance and operating frequency. For similar backpressures, the two air injection schemes have completely different operating regimes, leading to the inference that while backpressure is necessary for the onset of the pulsed detonation instability, by virtue of enabling reflected shock waves from the exit, lower air injection pressure ratio dictates the sustenance of the instability. A narrow band of injection pressure ratios, between 1.4 and 1.85, under back-pressurized RDC operation has high proclivity to produce sustained periodic longitudinal pulsed detonations in the combustor. Above this range, stable rotating detonation is preferred, and below this range, the operation is distinguished by the mixed presence of both rotating and pulsed detonations for a given test point, finally breaking down into chaotic instability for lower pressure ratios. The frequency of the pulsed detonation operation is found to depend on the initial combustor pressure and equivalence ratio, with higher frequency observed with an increase in backpressure and equivalence ratio.

Axial stress localization facilitates pressure propagation in gelled pipes

Paraffin wax-oil gels are unique rheological fluids which undergo shear degradation starting at a deformation (shear strain level) of approximately 1%. Flow commencement in pipelines filled with wax-oil gels is a complex hydrodynamic process involving propagation of acoustic, diffusive, and rheological degradation pressure wave fronts. Dynamic simulation informed by qualified rheological relations provides useful insight into the physical nature of these flow processes. Eulerian simulations are presented which emulate known physical phenomena and essential characteristics of wax-gel flow dynamics. A constitutive rheological equation set accounts for deformation-driven reduction in yield stress and viscosity terms. No explicit time-dependent rheological parameters are utilized in the equations. Rheological yielding alters the nature of the dominant pressure wave from inherently diffusive towards self-sharpening. Axial stress localization effectively sequentializes the gel breakage process, quantified by reduced length of the pressure wave-front zone. Ultimately, axial stress localization allows flow in longer pipe segments, albeit with a concomitant time delay. Viscous behavior and yielding degradation behavior are shown to account for upward and downward concavity in transient axial pressure profiles, respectively. Overall, a unique synergy between gel compressibility and gel degradation is revealed. Deformation-coupled interaction between compressibility and degradation allows pressure propagation and subsequent sustained flow through a gel material which is otherwise immobile in the incompressible case.

Modeling Submicron Particles Collection in Laminar Forced Convection Gas Flow by a Rectangular Venturi Scrubber

Venturi scrubbers are usually used for large particles cleaning in turbulent gaseous flow. In this work, submicron particles scrubbing in laminar forced convection dusty air flow in a rectangular venturi scrubber have been numerically simulated. Hydrodynamics effects and scrubbing process are investigated in detail. Results are presented as flow velocity, axial pressure, streamlines pattern, particles and droplets mass fraction profile, and collect efficiency. They show that venturi scrubbers can be efficient for submicron particles scrubbing. In fact, a better collect efficiency is obtained at high particles-droplets residence time, high ratio droplets concentration/particles concentration, low venturi diameter ratioand low Reynolds numbers. There is a critical Reynolds number value for which the collect efficiency becomes very low and tends to be constant.

Pressure and apparent voidage profiles for riser with an abrupt exit (T-shape) in a CFB riser operating above fast fluidization regimes

The influence of abrupt exit (T-shaped) configuration on the riser axial pressure profile in a large-scale circulating fluidized bed (CFB) is examined. A new analysis was developed to predict the axial voidage along the length of influence in the exit region with T-shape geometry. The exit region was characterized using non-dimensional analysis of the continuum equations (balances of masses and momenta) that described multiphase flows. In addition to deceleration length due to abrupt exit, the boundary condition for the solid fraction at the top of the riser and the fully developed regions, were measured using an industrial scale circulating fluidized bed (CFB) of 0.3m diameter and 15m tall. The operating factors affecting the flow development in the exit region were determined for three materials of various sizes and densities in core annular and dilute regimes of the riser. Performance data were taken from statistically designed experiments over a wide range of Fr (0.5–39), Re (8–600), Ar (29–3600), load ratio (0.2–28), riser to particle diameter ratio (375–5000), and gas to solid density ratio (138–1381). A series of correlations were developed to predict the voidage at the exit of the riser and length of influence due to the exit geometry. The correlations are based on gas and solid properties, operating conditions, and riser geometry.

CFD simulation on influence of suspended solid particles on bubbles' coalescence rate in flotation cell

A computational fluid dynamics (CFD) model was used to investigate the influence of solid concentration on bubbles' coalescence rate in flotation cell using Eulerian–Eulerian approach. CFD simulations were performed with AVL-FIRE 2009.2, and the existing flow field was modelled for two-phase (gas–liquid) and three-phase (gas–liquid–solids). The liquid phase was treated as a continuum and the gas phase (bubbles) and solid particles were considered as dispersed phases. The population balance equation for bubble break-up and bubble coalescence rate and the interfacial exchange of mass and momentum as well as bubble–particle attachment and detachment have been included in the CFD code by writing subroutines in FORTRAN. This investigation focused on studying the effect of solid particle on bubble break-up and bubble coalescence rate in the flotation cell at different superficial gas velocity values. The results predict that the presence of solid particles reduced the gas holdup in a flotation column. With the increase of the superficial gas velocity the size of gas bubble that were generated inside the cell decreased, leading to increased gas holdup. The result also shows that the Sauter mean diameter of bubbles decreases with the increase of solid concentration. Reasonably good agreement was obtained between simulation and experimental results for the effect of solid concentration on gas hold-up and axial pressure profile. In the current study, the froth zone was neglected, only the pulp zone was simulated. This is a deficiency of the present model, as the pulp is only one part of the flotation process, and it is physically linked to the froth. However, the model is a step towards gaining a complete view to describing the processes within a flotation cell through inspect the impact of presence of solid particles on the bubble coalescence rate under the different operation conditions.

A composite model for an intermeshing counter‐rotating twin‐screw extruder and its experimental verification

A composite simulation model of solids conveying, melting and melt flow in a closely intermeshing counter‐rotating twin‐screw extruder has been developed. The model is based on combining new melt conveying models with melting and solids conveying models, and the die is included into considerations. A general approach is applied using fully three‐dimensional non‐Newtonian FEM modeling for melt conveying to develop screw pumping characteristics which are implemented into the composite model. Several screw configurations are considered including non‐classical elements. Computations are made for axial fill factor, pressure, temperature, and melting profiles. The results are validated experimentally. POLYM. ENG. SCI., 55:2838–2848, 2015. © 2015 Society of Plastics Engineers

CFD modeling of riser with Group B particles

A series of tests were conducted in a large cold flow CFB unit (18m long riser and 30cm in diameter) with two different Group B powders. The impact of particle nature on the riser hydrodynamics was investigated at similar operating conditions through pressure taps and extraction probes. To study the impact of particle size and sphericity, sand and glass beads were used. Resulting axial pressure profiles are very different in both the acceleration and in the fully developed region. These differences can most probably be explained by the difference in shape between the solids. Higher sphericity seems to generate smaller pressure drop. Through extraction probe measurements, the core-annulus regime was found on the developed zone of the riser. An assessment of a commercial CFD code to predict riser flow was carried out. For glass beads, simulation results agree reasonably well with experimental pressure profiles, but the core annulus structure is underpredicted. However, pressure drop along the riser is strongly underestimated in sand simulations.

Turbulence Model Effects on Cold-Gas Lateral Jet Interaction in a Supersonic Crossflow

Computational fluid dynamic predictions of surface pressures resulting from a sonic lateral jet venting into a supersonic crossflow from a cone–cylinder–flare missile are compared to archival wind-tunnel data. Predictions of axial and azimuthal pressure profiles were found to be very dependent on the turbulence model, with some models performing relatively poorly. Menter’s baseline model gave very good to excellent predictions and was used to perform additional validations of other flow conditions, jet nozzle configurations, and jet pressure ratios: again with excellent agreement. The study found that, even with the observed variations in surface pressure, the aerodynamic forces and moments produced by the lateral jet interaction were much less sensitive to the turbulence model. However, an estimate of the trajectory and strength of the counter-rotating vortex pair showed that, although there was little effect of the turbulence model on the location of the vortex pair, the induced vorticity varied by over 30%. This difference can be large enough to impact the prediction of the resultant forces and moments if there are fins or other appendages in the wake of the counter-rotating vortex pair.

Experimental hydrodynamic study of gas‐particle dense suspension upward flow for application as new heat transfer and storage fluid

AbstractThis paper focuses on a new concept of Heat Transfer Fluid (HTF) for Concentrating Solar Plants (CSP) applications through fluidized bed. CSP plants with very high concentration (such as solar tower plant technology) offer good efficiencies because of high operating temperatures. CSP efficiency could be greatly increased through more efficient HTF. Molten salts, mineral oils, water and air have some of the following drawbacks: limited range of operating temperatures, corrosiveness, high pressure, low energy storage capacity and toxicity.To replace classical HTF, Dense Particle Suspension (DPS) fluidized with air (approximately 40 % of solid) is proposed. DPS has a volume heat capacity similar to those of liquid HTF, does not need pressurization, is safe, inert and is only limited by the maximal working temperature of the receiver material (1100 K), thus opening new opportunities for high efficiency thermodynamic cycles. This work is the hydrodynamic study of a gas‐solid dense suspension upward flow at ambient temperature, in a vertical 2‐tube bundle of small diameter tubes, which have their bottom immersed in a slightly pressurized fluidized bed (pressure approximately equal to the ratio of the solid weight in a tube over its cross section area). This type of flow is yet implemented in the field of hyper‐dense phase vertical conveying of powders and it is currently under development for solar receivers using dense suspensions of particles as heat transfer and storage medium. This application was patented by Flamant and Hemati in 2010 (France 1058565 (2010) CNRS/INP Toulouse, G. Flamant, H. Hemati; PCT Extension, No. WO 2012/052661 A2), and its development is funded by the European Commission. In this technological breakthrough, the concentrated solar energy is collected, carried and stored directly by the fine particles flowing upward, with a suspension void fraction close to that of a dense fluidized bed. Contrary to circulating fluidized bed “risers”, it offers a good contact area between the wall and the particles.The important hydrodynamic and thermal coupling required a step‐by‐step approach. Ambient flows had to be understood and controlled first. Thus a 2‐pass “cold” mock‐up, each pass composed of two vertical parallel tubes, was built. Pressure drop, solid weight and helium volume fraction measurements demonstrated the ability to handle a regular solid upward flow (imperative here), with solid flow rates from 20 to 130 kg.h−1, with void fractions from 0.57 to 0.63 and with an even distribution of the solid flow rate between the tubes. Moreover, the governing parameters of this flow were established as: the solid feeding flow rate, the fluidization velocity, the solid holdup, the freeboard pressure and the aeration velocity. The secondary air injection, also called “aeration”, is the most important parameter for the stability and the even distribution of the total solid flow rate in the tubes. The 1D modelling of the suspension flow in the tubes was also performed in the flow direction. The flow structure was described using the bubble‐emulsion model formalism, and by adding the solid entrainment by the bubble wake. Predictions of the model are compared with the experimental measurements of driving pressure and axial pressure profile along the tubes.

Riser hydrodynamic study with different Group B powders

Experiments were conducted in a large cold flow CFB unit (18m long riser and 30cm in diameter) with three Group B powders. Sand, glass beads and a metal ore, with similar particle size, were used to study the impact of particle density and sphericity. The impact of particle nature on the riser hydrodynamics was investigated at similar operating conditions. Resulting axial pressure profiles are very different in both the acceleration and the fully developed region. These differences can most probably be explained by the difference in shape between the solids. Higher sphericity seems to generate less pressure drop and smaller values of drag force. The influence of particle shape on acceleration resulting from gas–solid momentum transfer and the impact on particle concentration in the fully developed region are then studied. To analyze the results, a monodimensional momentum balance study was conducted, resulting in a new formulation for the average drag force, based on experimental results.