Axial Flow Direction Research Articles

Interactions between alluvial fans and axial rivers in Yukon, Canada and Alaska, USA

Abstract In contrast with the archetypal definition of an alluvial fan, this study shows that fans interacting with axial rivers in Yukon and Alaska commonly exhibit asymmetrical morphology in planform. Hypothesis tests relating to the geomorphological characteristics of these alluvial fans were conducted on a dataset of 63 fluvial-dominated fans. A significant relationship existed between fan asymmetry and the direction of axial river flow, which was attributed to two factors supported by examples: (1) axial rivers have a propensity to trim the toes on the up-valley sides of fans; and (2) axial river channels are deflected across the broad valley floors, which allows the profiles on the down-valley sides of fans to be longer than on the up-valley sides. However, an asymmetrical planform morphology does not lead to a significant bias in the spatial distribution of surface streams towards the up-valley sides of fans, which typically have shorter profiles from apex to boundary. If the asymmetry in fan morphology is preserved in the sedimentary record, then the interpretation of fan deposits that developed in broad valleys and that interacted with axial rivers would be improved by understanding this modern analogue.

Large-eddy simulation of nitrogen injection at trans- and supercritical conditions

Large-eddy simulations (LESs) of cryogenic nitrogen injection into a warm environment at supercritical pressure are performed and real-gas thermodynamics models and subgrid-scale (SGS) turbulence models are evaluated. The comparison of different SGS models — the Smagorinsky model, the Vreman model, and the adaptive local deconvolution method — shows that the representation of turbulence on the resolved scales has a notable effect on the location of jet break-up, whereas the particular modeling of unresolved scales is less important for the overall mean flow field evolution. More important are the models for the fluid’s thermodynamic state. The injected fluid is either in a supercritical or in a transcritical state and undergoes a pseudo-boiling process during mixing. Such flows typically exhibit strong density gradients that delay the instability growth and can lead to a redistribution of turbulence kinetic energy from the radial to the axial flow direction. We evaluate novel volume-translation methods on the basis of the cubic Peng-Robinson equation of state in the framework of LES. At small extra computational cost, their application considerably improves the simulation results compared to the standard formulation. Furthermore, we found that the choice of inflow temperature is crucial for the reproduction of the experimental results and that heat addition within the injector can affect the mean flow field in comparison to results with an adiabatic injector.

Implementation of Liquid Crystal Thermography to Determine Wall Temperatures and Heat Transfer Coefficients in a Tube-in-tube Heat Exchanger

Liquid crystal thermography was used in a water-operated concentric tube-in-tube heat exchanger to determine local annular heat transfer coefficients at the inlet region. An annular diameter ratio of 0.54 was considered with the inlet and outlet orientated perpendicularly to the axial flow direction. Both heated and cooled cases were considered at annular Reynolds numbers ranging from 1,000 to 13,800. Wall temperature distributions were directly measured by means of a coating of thermo-chromic liquid crystals. Local heat transfer coefficients at the inlet were higher than those predicted by most correlations, but good agreement was obtained with some literature.

Implementation of chemical reaction equilibrium by Gibbs and Helmholtz energies in tubular reactor models: Application to the steam–methane reforming process

In the tubular reactor models the mass and heat source terms due to chemical reactions in the species mass balances and temperature equation are conventionally determined from kinetic rate expressions. In many cases a kinetic rate model is not available but the chemical equilibrium conversion can be determined from reaction equilibrium calculations minimizing the Gibbs or Helmholtz free energies. Although a process is believed to behave physically like a tubular reactor, previous feasibility and design studies have typically disregarded fluid flow and mass- and heat transfer limitations and performed a classical chemical equilibrium calculation. For non-adiabatic cases, valuable information on the heat transfer flux limitations of the chemical process at the wall (often provided by a specified axial heating/cooling media temperature profile) is lost when simplifying a model representing the physical tubular reactor process behavior by considering a classical thermodynamic system having uniform state properties. For this reason, in the present study, a new type of tubular reactor model, which we name the differential Gibbs (or Helmholtz) reactor model, is presented to improve on the conventional feasibility and design model. In the differential Gibbs (or Helmholtz) reactor model, the chemical conversion and the reaction heat are determined assuming chemical reaction equilibrium conditions along the axial flow direction by minimizing the Gibbs (or Helmholtz) free energy. The new model is verified through comparison with the conventional differential tubular model using the fast reaction kinetics of the steam–methane reforming process and neglected mass diffusion limitation of the catalyst.

A new approach for modeling of streamlined die operations based on a non-quadratic stream function

Talbert and Avitzure introduced the implementation of a quadratic stream function for derivation of velocity field governing through streamlined die extrusion. In the present study, a non-quadratic stream function is introduced to model various streamlined die processes. In the modified formulation an exponent was designated to the stream function in order to obtain a generalized model. The quadratic stream function approach was restricted to processes with axial flow direction. In contrast, the modified model is capable to study the common forming processes i.e. simple and bimetal extrusion and drawing. The actual velocity field was derived from the generalized field by upper bound technique. The plastic flow was studied through distorted bands and elements that obtained by Eulerian description of motion. Subsequently, the modeling of extrusion and drawing proceed by the calculation of the effective plastic strain distribution. Streamlined die has been designed and proposed based on the fact that it requires less external power than the conventional operations and that deforming material encounters smoother plastic flow. The results revealed the length of die highly influences the deformation mode by affecting the shear component of strain. The capability of the modified stream function to capture various metal forming operations with streamlined die was investigated.

Numerical simulation of a freeze–thaw testing procedure for borehole heat exchanger grouts

The amount of research conducted on geothermal energy as a source for heating and cooling demands of buildings, as well as for electrical energy production, has increased substantially in the past decades. The simulation of freezing and thawing is important for geothermal applications involving ground coupled heat pumps. One area of research is the development of grout cements for borehole heat exchangers (BHE). In many cases, BHEs are operated at temperatures below 0° C due to manifold reasons. Hence, the simulation of freezing and thawing cycles (FTC) is important for such geothermal applications, especially in cold regions. Recently, a testing device for measuring and quantifying the influence of FTC stresses on the mechanical integrity and hydraulic properties of BHE grouts was developed (Anbergen, published in 2014). The testing procedure simulates the downhole in situ conditions as confining radial earth pressure, freezing, and thawing directions from the inside to the outside and under saturated conditions. The hydraulic conductivity can be measured in axial flow direction. Thus, statements regarding the susceptibility of grouts against cyclic freezing and thawing stresses can be made. These results differ substantially from earlier findings, as in situ boundary conditions were not simulated sufficiently. For the verification of the procedure’s thermal process, temperature logs were recorded using thermocouples and thermography imaging. The thermal process was simulated using the finite element method (FEM) groundwater, heat, and mass modeling software FEFLOW. FEFLOW is a common software solution for thermohydraulic coupled groundwater applications with mass transport, as well as geothermal applications. However, up until now, the program could not yet simulate phase changes between solid and liquid phases. To enable the program for such simulations, a plug-in was developed. To do this, a C++ code was written and coupled to the simulation routine of the FEM software. The code is based on a modification of the material parameters of fluid and the incorporation of the latent heat effects in the fluid heat capacity. A linear and an exponential approach for the latent heat release were implemented and benchmarked. The code was verified using different analytical solutions and other FEM codes. Finally, the experimental results of the test procedure could be successfully computed using the new plug-in. Thus, it is now possible to compute phase changes with FEFLOW for geothermal applications as well as other applications like permafrost research.

Calculation of temperature field in gas flow with internal heat source

Gas flow sequentially moving through three zones (input z<0, internal heat release 0≤z≤l, and output z>1) of a cylindrical channel was considered. Analytical solutions taking into account the influence of heat source limitation in the axial direction and intensity of air flow in this direction on thermal balance were obtained.

CFD investigation of effect of helical wire-wrap parameters on the thermal hydraulic performance of 217 fuel pin bundle

In fast reactors, the fuel pins are wound with helical wire-wrap spacer to provide support for the fuel pins and to provide space for sodium coolant to flow through the bundle. Due to the helical wire-wrap spacer, the coolant not only flows in axial direction but also in a transverse direction. This transverse flow provides better mixing of coolant among the sub channels and due to this, the heat transfer coefficient of the coolant increases. But, the frictional resistance to flow also increases. The effect of helical wire wrap parameters on the flow and temperature distributions of sodium in heat generating 217 pin fuel bundle and the variation of friction factor and Nusselt number as a function of helical wire parameters have been predicted. Toward this, the statistically averaged 3-dimensional conservation equations of mass, momentum and energy are solved along with k–ε turbulence model by varying the helical pitch and diameter of the spacer wire using customized Computational Fluid Dynamics (CFD) code CFDEXPERT. The geometric details of the fuel pin bundle and heat flux are similar to that of the Indian Prototype Fast Breeder Reactor (PFBR) that is currently in an advanced stage of construction.Based on detailed parametric study, it is found that the transverse flow induced by wire-wrap increases if the helical pitch is reduced. As a consequence of this, the friction factor and the Nusselt number increase with reduction in helical pitch. When the wire diameter is reduced, the pin bundle becomes tighter with high resistance for transverse flow. As a result of the reduced cross flow, the friction factor as well as Nusselt number decrease when wire diameter is reduced. Based on the computational data, suitable correlations have been derived for estimation of Nusselt number in 217 pin fuel bundle for a range of practical interest in fast reactor core design. It is also seen that the clad temperature decreases with decrease in helical pitch and increase in wire diameter due to increase in the transverse flow and associated enhancement in heat transfer coefficient. It is seen that the sodium temperature difference between the central sub-channels in the various rows of pin bundle and the peripheral sub-channels at the same hexagonal face is lower for shorter helical pitch and larger helical wire diameter which is attributed to the enhanced heat exchange due to higher cross flow in these cases.

Performance of two fluid model based numeric tool with pressure and enthalpy as independent variables for simulation of two phase flow in axial and radial direction

Abstract Several numeric tools have been developed and reported in literature for simulation of two phase flow conditions. Most of the tools make use of the classical homogeneous equilibrium model of two phase flow which is reported to perform better under high pressure conditions with relatively small temperature gradients. System codes such as RELAP make use of two fluid model of two phase flow. Such system codes model the flow of two phases in one dimension in the axial direction. The provision of modeling flow in the direction perpendicular to the axial flow direction exists with the approximation of not considering the flux terms in the momentum equations in the perpendicular direction. This approach is reported to work well for situations with predominant single directional flows. To analyze certain situations with predominant flow in axial as well as radial directions a numeric tool (FLOW) has been developed. The approach employs the same momentum equation for axial as well as radial direction incorporating the flux terms in both directions. The pressure and the enthalpy of the fluid are used as independent variable in the formulation of the tool. The results are found to be in good agreement with 3D CFD simulations.

Effect of Thread Helix Angle on the Axial Metal Flow of Cross Wedge Rolling Thread Shaft

When rolling thread shaft in cross wedge rolling, forming quality of thread is affected by many process parameters such as the thread helix angle, which is a special and complex parameter. However, there is few study of the effect of the thread helix angle on thread shaft forming. In this paper how thread helix angle affects axial metal flow was analyzed from the perspective of rolling force and the forming process of thread shaft was simulated with different helix angles by the deform-3D software. Through the measurement of the point coordinates, the effect of the helix angle on the axial metal flow of rolled pieces was analyzed. Finally, the conclusion was obtained that helix angle causes the asymmetry of axial metal flow with small difference values for asymmetry and the axial metal flow of rolled pieces increases as the helix angle increases. The rolling thread segment can be ensured in the middle of rolled pieces by moving the blank a suitable distance in the reverse direction of the axial flow direction.

Three-component solids velocity measurements in the middle section of a riser

Coincident three-component solids velocity measurements are performed in a 8.7-m-high cylindrical riser (internal diameter 0.10m) of a pilot-scale cold-flow Circulating Fluidized Bed set-up, using two Laser Doppler Anemometry (LDA) probes. Experiments are performed with superficial gas velocities of 3.5 and 5.3m/s at riser heights from 0.9 to 5.5m and with an average solids volume fraction of 0.0002. At the lower riser heights, the solids flow is observed to be highly disturbed due to the asymmetrical position of both the air and solids inlet. A fully developed parabolic axial solids velocity field is observed about 2m above the solids inlet line. The measured radial and azimuthal particle velocity components are significantly smaller than the axial one. The particle velocity fluctuations in the axial flow direction are larger than in the radial and azimuthal directions, showing that the solids flow is anisotropic. However, both the radial and azimuthal velocity fluctuations are higher than the corresponding mean velocities, as observed when studying the turbulence intensity in the three directions. All fluctuating velocities are decreasing significantly with increasing riser height, i.e. as the flow becomes more developed. No average down-flow of solids is recorded, not even close to the wall, due to the highly diluted flow. The total particle shear stress profiles designate a mean transport of fluctuating momentum along the riser radius due to velocity fluctuations. The particle fluctuation energy calculated based on experimental data indicates that the solids flow is dominated by wall-particle rather than intra-particle collisions.

Stratigraphic Architecture of An Experimental Basin With Interacting Drainages

Abstract The stratigraphic architecture of intracontinental rift basins is defined by a dynamic relationship between depositional processes associated with the basin floor and flanking tributary streams. The resulting depositional belts are sensitive to a variety of factors, including basin geometry, subsidence rate, and sediment discharge. The Experimental EarthScape run in 2006 (XES06) examined the development of fluvial morphology and alluvial architecture as a function of subsidence and sediment flux in an experimental basin based on the form of a simple half graben. Sediments from tributary drainages were introduced into the axial stream through toe cutting and realignment of transverse drainage courses to parallel the prevailing axial-flow direction. Transverse sediment contributions to the axial stream were almost equally apportioned over a wide range of sediment discharges tested in the experiments. Sediment tracers showed a larger contribution of footwall-derived sediment into the axial belt, probably due to more frequent and aggressive toe cutting by axial streams. Changes in the axial–transverse deposit boundary to external forcing (by subsidence and sediment discharge), and relatively rapid intrastage stabilization of the depositional belts, resembles the large-scale self-organization observed in moving boundaries that define the morphology of fluviodeltaic systems. Basin sedimentation was matched to subsidence in order to maintain a constant base level, which made the location and width of the axial belt sensitive to the relative sediment fluxes from the transverse systems, rather than the axis of maximum subsidence. The asymmetrical subsidence pattern and the transverse-fan morphology influenced the preservation of sedimentary sequences. Stage-bounding stratigraphic lacunae were well preserved in the hanging-wall succession, providing a reliable record of basin development.

Research on Characteristics of Spiral Flow Generator in a Horizontal Liquid-Solid Circulation Fluidized Bed

An experimental study is carried out to investigate the structure parameters and pressure drop characteristics of a spiral flow generator in a horizontal liquid-solid circulation fluidized bed. The CCD image measurement and data processing system was used to study the particles distribution at different angles covering 10°, 15°, 20°and 25°normal to the axial flow direction. Increasing the vane angle resulted in lower particles non-uniform results. The vane angle has important influence on generation of the spiral flow in horizontal circulation fluidized bed. The pressure loss with spiral flow generator is higher than that without spiral flow generator. The average increase in pressure losses was about 110% in comparison with that for the pipe without spiral flow generator. The vane angle of 15° represented the minimum pressure drop values.

ANALYSIS OF TWO-DIMENSIONAL FLOW IN DUCTS WITH CIRCULAR SAW GITT FORMULATION IN PRIMITIVE VARIABLES

In this paper the Generalized Integral Transform Technique is employed to produce hybrid solutions for the velocity and pressure fields of a newtonian fluid in two dimensional flow. The problem is formulated by using primitive variables and the necessary mathematical manipulations were used to obtain the Poisson equation for the pressure field. The momentum equations in the axial direction of flow and Poisson are transformed to remove the transversal dependency. The resulting transformed fields are solved with the IMSL numerical subroutine, DBVPFD. The obtained results for the longitudinal velocity profile at the center of the channel are compared with the available data in the open literature for validation and model fitting. Even so, studies are carried out about the convergence of the solution for the velocity profile in the centerline as well as testing different values of the scale factor of axial coordinate for the choice of a factor which can fit perfectly for comparison with available data. Interest practical datas such as: friction factor and mean velocity are obtained along the duct for a entry condition into the parallel flow channel (v = 0).

Fully resolved numerical simulation of reactive mixing in a T-shaped micromixer using parabolized species equations

This paper introduces and exploits a hybrid numerical approach for fully resolved numerical simulations of reactive mixing in T-shaped microreactors and thereby enables a computational analysis of how chemical reactions interact with convective and diffusive transport. The approach exploits the fast redirection of the flow inside the mixing channel, resulting in a flow field with positive axial flow component everywhere after a short entry zone. This allows handling the axial flow direction as a pseudo-time variable, so that the evolution of the concentration profile can be computed consecutively on successive cross sections, following the main axial flow direction. With this approach the finest length scales, given by the Batchelor length scale, can be resolved for such a reactive mixing process inside a T-microreactor at stationary flow conditions. This allows for a detailed analysis of the mixing state as well as important characteristics of the reactive mixing process like yield and selectivity. The concrete numerical simulations yield local diffusion times inside the reactor, reveal the influence of the strength of the secondary flow on the progress of the chemical reaction and show how local selectivities result from the species transport.

The model of laminar pulsatile flow in tubes in rolling motion

The laminar pulsatile flow in tubes in rolling motion is investigated theoretically. The theoretical model of laminar flow in rolling motion is developed and the velocity correlation is also derived. The effect of rolling motion on velocity and frictional resistance factor is analyzed. The rolling motion mainly affects on the laminar flow by the tangential force. The centrifugal force does not affect on the flow. The tangential force affects on the flow in axial direction, its radial effect is very weak and could be omitted. There are two critical rolling points in rolling motion. After the first critical rolling point, the flowing velocity next to the wall reverses. Moreover, the flow rate at the tube cross-section becomes negative after the second critical rolling point. The buoyancy force is only one part of the effects that affects on the average velocity of a natural circulation system in rolling motion. The effect of Womersley number on the velocity is significant, which can not only affect on the average velocity but also on the oscillating period and velocity amplitude. The rolling motion does not affect on the average frictional resistance of laminar pulsatile flow. If the rolling motion is very serious, the flow is at a transitional or turbulent flow state, in this case the effect of rolling motion on the average frictional resistance is considerable.

Computer aided design optimisation of microfluidic flow distributors

This study reports on a quantitative study of the influence of the most important geometrical design parameters for micro-machined flow distributors with uniform cross-section and filled with diamond-shaped pillars having their longest dimension oriented perpendicular to the axial flow direction. It was found that the shape of the bands eluting from the distributor improves with increasing aspect ratio (AR) of the pillars, both in terms of global warp and local axial dispersion. Increasing the AR from 5 to 25 reduces the distributor length needed to bring the maximal transversal velocity difference below 5% from 170 μm to 15 μm when using pillars with axial width of 5 μm. To solve the problem that high AR pillar distributors only have a limited number of exit points, and therefore produce bands with a strong local warp, one can conceive mixed size distributors, wherein a zone filled with several rows of very high AR pillars is followed by one or more zones consisting of pillars with a smaller AR. With such a design, the variance of the eluting bands can be reduced to only 30% of the variance of a single size distributor.

On the Origin of Paleocurrent Complexity Within Deep Marine Channel Levees

Abstract Complex patterns of temporal and spatial variation in paleoflow within individual turbidites have been inferred from studies of both recent and ancient submarine channel–levee sands and sandstones. Previous interpretations of these paleocurrents have invoked “shifting patterns of meandering streamlines” in essentially unconfined flows, and in some cases topographic deflection of flows as they spilled from the channel. However, other flow indicators, such as current ripples, suggest that overbank flow may exhibit more uniform flow directions. Here we re-examine the spatial and temporal evolution of overbank flow on submarine channel levees, using grain fabric data from the Cretaceous Rosario Formation and extant data from the recent Amazon channel. We integrate these observations with our theoretical understanding of the long-term evolution of overbank flow processes to propose a process-based model of evolution of overbank flow across submarine channel levees. Overbank flow over levees is inferred to be dominated by three main processes: (1) gravity forces that attempt to direct flow down the local maximum slope; (2) inherited momentum from the channelized flow, which in many cases is closer to the axial flow direction; (3) development over time of an overbank wedge of fluid that moves broadly parallel to the direction of the main channel. The interaction among these processes varies both in space and time, and collectively accounts for the observed paleoflow variations inferred from deposits. This study may allow better interpretation of depositional sub-environments within levee turbidites, both in core and in outcrop.

Migration forces of transcatheter aortic valves in patients with noncalcific aortic insufficiency

Transcatheter aortic valves have been successfully implanted into the calcified leaflets of patients with severe aortic stenosis. However, their stability in patients with noncalcified aortic insufficiency is unknown. Similar to thoracic and abdominal aortic stent grafts, transcatheter aortic valves are subjected to antegrade ejection forces during systole. However, retrograde migration forces into the left ventricle are also generated by the diastolic pressure gradient across the closed valve. It has been suggested that leaflet calcification anchors the prosthesis, and measurements of migration forces should be considered before clinical trials in noncalcified aortic insufficiency. The objective of this study was to use computational fluid dynamics simulations to quantify forces that could potentially dislodge the prosthesis. A computational fluid dynamics model was developed to simulate systolic flow through a geometric mesh of the aortic root and transcatheter aortic valves. Hemodynamic measurements were made at discrete moments during ejection. Unsteady control volume analysis was used for calculations of force on the mesh. Results of the simulation indicate that a total force of 0.602 N acts on the transcatheter aortic valves during systole, 99% of which is in the direction of axial flow. The largest contributor to force was the dynamic pressure gradient through the transcatheter aortic valves. This antegrade force is approximately 10 times smaller than the retrograde force (6.01 N) on the closed valve during diastole. Our model simulated systolic flow through a transcatheter aortic valve and demonstrated migration into the left ventricle to be of greater concern than antegrade ejection.

Computational fluid dynamics simulation of transcatheter aortic valve degeneration☆☆☆

Studied under clinical trials, transcatheter aortic valves (TAV) have demonstrated good short-term feasibility and results in high-risk surgical patients with severe aortic stenosis. However, their long-term safety and durability are unknown. The objective of this study is to evaluate hemodynamic changes within TAV created by bioprosthetic leaflet degeneration. Computational fluid dynamics (CFD) simulations were performed to evaluate the hemodynamics through TAV sclerosis (35% orifice reduction) and stenosis (78% orifice reduction). A three-dimensional surface mesh of the TAV within the aortic root was generated for each simulation. Leaflets were contained within an open, cylindrical body without attachment to the sinus commissures representing the stent. A continuous surface between the annulus and TAV excluded the geometry of the native calcified leaflets and prevented paravalvular leak. Unsteady control volume analysis throughout systole was used to calculate leaflet shear stress and total force on the TAV. Sclerosis increased total force on the TAV by 63% (0.602-0.98 N). Advancement of degeneration from sclerosis to stenosis was accompanied by an 86% increase in total force (1.82 N) but only a 32% increase in peak wall shear stress on the leaflets. Of the total force exerted on the TAV, 99% was in the direction of axial flow. Shear stresses on the TAV were greatest during peak systolic flow with stress concentrations on the tips of the leaflets. In the normal TAV, the aortic root geometry and physiologic flow dominate location and magnitude of shear stress. Following leaflet degeneration, the specific geometry of the stenosis dictates the profile of axial velocity leaving the TAV and shear stress on the leaflets. A dramatic increase in peak leaflet shear stress was observed (115 Pa stenosis vs. 87 Pa sclerosis and 29 Pa normal). CFD simulations in this study provide the first of its kind data quantifying hemodynamics within stenosed TAV. Stenosis leads to significant forces of TAV during systole; however, diastolic forces predominate even with significant stenosis. Substantial changes in peak shear stress occur with TAV degeneration. As the first implanted TAV begin to stenose, the authors recommend watchful examination for device failure.