Transient Flow Patterns Research Articles

Role of Marangoni convection in Si-Czochralski melts, part I: 3D predictions without crystal

Three-dimensional numerical simulations of flow field and heat transfer in a Czochralski crucible without crystal were carried out under pulling conditions. The study concentrated on the influence of the Marangoni convection in a silicon melt adopting a block-structured finite-volume code. The existing numerical code was extended to implement the surface tension boundary condition in order to investigate the influence of the Marangoni convection on the flow instability mechanism. The transient flow patterns reveal a significant role of Marangoni convection on the fluid mechanical instability at the free surface of the melt. A strong radial flow due to the presence of a surface tension effect is seen to increase the intensity of the buoyant plumes and enhance the net inward radial motion of the plumes. At later stages, the Coriolis force due to large crucible rotation suppresses the radial buoyant convection while substantial radial flow is observed in the presence of the Marangoni effect.

Surface effects on transient three-dimensional flows around rotating spheres at moderate Reynolds numbers

Transient wake flow patterns and dynamic forces acting on a rotating spherical particle with non-uniform surface blowing are studied numerically for Reynolds numbers up to 300 and dimensionless angular velocities up to Ω=1. This range of Reynolds numbers includes the three distinct wake regimes i.e., the steady axisymmetric, the steady non-symmetrical and the unsteady with vortex shedding. The Navier–Stokes equations for an incompressible viscous flow are solved by a finite volume method in a three-dimensional, time accurate manner. An interesting feature associated with particle rotation and surface blowing is that they can affect the near wake structure in such a way that unsteady three-dimensional wake flow with vortex shedding develops at lower Reynolds numbers as compared to flow over a solid sphere in the absence of these effects and thus, vortex shedding occurs even at Re=200. Global properties, such as the lift and drag coefficients, and the Strouhal number are also significantly affected. It is shown that the present data for the average lift and drag coefficients correlate well with: C L /(1+Ω) 3.6=0.11 C D (1+20V S ) 0.2/(1+Ω) Re/1000=24(1+Re 2/3/6)/Re where V S is the average surface blowing velocity normalized by the free stream velocity.

Detached Eddy Simulation of Flow around A-Airfoil

In this study the flow around the Aerospatiale A-airfoil at maximum lift (α = 13.3°) and Re = 2 × 106 is investigated by RANS and DES to quantify the influence of transient flow patterns on the quality of the flow prediction. This flow features a highly unsteady pressure-induced trailing-edge separation. The fairly high Reynolds number makes LES rather unattractive from a practical point of view as the numerical costs scale with ≈ Re1.95 [9] and lead to an unreasonable effort to carry out these simulations. Like LES, DES is designed to capture the unsteady flow features and consequently is supposed to give results superior to RANS. The RANS solution captures the separation, but is notable to predict transient vortex shedding. The application of DES to a 2D domain leads to results comparable to those obtained by 2D-RANS. Only the 3D-DES is capable of predicting both the unsteady flow features and the development of the boundary layer on the airfoil. As expected, the spanwise resolution has a strong impact on the vortex shedding and on the size of the separation bubble. Attention must be paid to both accommodating the full extent of the flow structures and providing the adequate spatial resolution in order to accomplish satisfactory results – that compare favourably with LES – at only a fraction of computational cost.

Energy recovery effectiveness in an enthalpy wheel used in air conditioning systems

SYNOPSIS Maintaining good Indoor Air Quality inside air-conditioned spaces demands more fresh air for ventilation. This in turn increases the heat load on the evaporator coil. For minimising this additional load, a heat recovery unit called an enthalpy wheel is introduced to exchange sensible and latent heat between the fresh air and the return exhaust air. This paper aims to present a simulation of the flow and heat transfer characteristics of the enthalpy wheel. A typical case of hot and humid ambient conditions is considered for analysis. As the wheel is under quasi steady state, transient flow patterns are included to predict the performance of the enthalpy wheel. The amount of energy transferred, during the counter current flows of fresh and return air, is deduced from the heat and mass transfer analysis. It has been found that the effectiveness of heat exchange is nearly 77%. This analysis would be very useful in optimising the design parameters for increasing the heat transferred by the enthalpy wheel.

Simulation of the submerged energy nozzle-mold water model system using laser-optical and computational fluid dynamics methods

The present work describes quantitative digital particle image velocimetry measurements of a full-scale water model of a thin slab mold. Different casting speeds and two submerged entry nozzles with one and two outlet ports have been investigated. The flow pattern of the single-port nozzle shows a counterclockwise-rotating double vortex that is nearly steady-state but leads to high stationary surface waves. The flow jets out of the two-port nozzle oscillate and produce a transient flow pattern with low wave amplitudes. The amplitudes for the one-port nozzle show a linear variation with the volumetric flow rate. The experimental results lead to a good interpretation of the flow phenomena and are used to validate steady-state numerical simulations with the commercial program, CFX, on the basis of the Reynolds equations. To describe anisotropic turbulence effects, the Reynolds stress model (RSM) is used for the flat single-port nozzle and the standard k-ɛ model for the mold flow. The calculated mean velocities and wave amplitudes, predicted from pressure distribution at the water surface, are generally in the consensus of the experimental data.

TRANSIENT THERMAL MIXING PERFORMANCE AND FLOW PATTERN IN A ROOM WITH THROUGH-FLOW

TRANSIENT THERMAL MIXING PERFORMANCE AND FLOW PATTERN IN A ROOM WITH THROUGH-FLOW

Cyclic micron-size particle inhalation and deposition in a triple bifurcation lung airway model

Laminar oscillatory inhalation flow as well as micron-particle transport and wall deposition in a representative triple bifurcation airway model have been simulated using a commercial finite-volume code with user-enhanced programs. The computer model has been validated with experimental particle deposition data and deposition patterns for double bifurcations. The transient air flow, particle transport, and wall deposition patterns were analyzed and summarized in terms of particle deposition efficiencies (DEs) and surface density maps. Particle deposition may increase under cyclic flow conditions, i.e., DE-values are typically larger for cyclic flow than for steady flow at the mean flow rate of a given inhalation pulse. The maximal relative difference between these two DE-values may be as high as 50% for 0.02⩽ St mean⩽0.12 during normal inspiration (Q in =30 l/ min) . However, matching inlet Reynolds and Stokes numbers are proposed, which generate under quasi-steady flow conditions very similar DE-values and deposition patterns as in equivalent pulsatile flow. The relative differences between DE-values for cyclic and matching steady cases are less than 5%. The quantitative results of this work are of interest to researchers either conducting health risk assessment studies for inhaled particulate pollutants or analyzing drug aerosol deposition at desired lung target sites.

Numerical simulation of transient temperature and velocity profiles in a horizontal can during sterilization using computational fluid dynamics

In this work, sterilization of a canned liquid food (carrot–orange soup) in a metal can lying horizontally and heated at 121°C from all sides is simulated for a 3-D geometry. Transient temperature, flow pattern, and shapes of the slowest heating zone (SHZ) during natural convection heating of canned liquid foods are predicted. The partial differential equations describing the conservation of mass, momentum and energy conservation are solved numerically using a commercial computational fluid dynamics (CFD) software (PHOENICS), which is based on a finite-volume method of analysis. The simulation shows the influence of natural convection on the liquid-flow pattern and on the movement of the SHZ. The action of natural convection forces the SHZ to migrate towards the bottom of the can as expected. The SHZ eventually stays in a region that is about 20–25% of the can height from the bottom. The secondary flow formation and its effect on the shape of the SHZ are evident. The results of this work are compared with those for vertical can. It shows faster heating in the vertical can, which is expected due to the enhancement of natural convection caused by its longer height.

Thermally polarized (1)H NMR microimaging studies of liquid and gas flow in monolithic catalysts

The feasibility of gas flow imaging in moderately high magnetic fields employing thermally polarized gases at atmospheric pressures is demonstrated experimentally. Two-dimensional spatial maps of flow velocity distributions for acetylene, propane, and butane flowing along the transport channels of shaped monolithic alumina catalysts were obtained at 7 T by (1)H NMR, with true in-plane resolution of 400 &mgr;m and reasonable detection times. The resolution is shown to be limited by the echo attenuation due to rapid molecular diffusion in the imaging gradients of magnetic field. All gas flow images exhibit flow patterns that are not fully developed, in agreement with the range of Reynolds numbers (190-570) and the length of the sample used in gas flow experiments. The flow maps reveal the highly nonuniform spatial distribution of shear rates within the monolith channels of square cross-section, the kind of information essential for evaluation and improvement of the efficiency of mass transfer in shaped catalysts. The water flow images were obtained at lower Re numbers for comparison. These images demonstrate the transformation of a transient flow pattern observed closer to the inflow edge of a monolith into a fully developed one further downstream. Copyright 2000 Academic Press.

Core precession: flow structures and energy

SUMMARY Experiments using a precessing liquid-filled oblate spheroid with ellipticity (ab)/a= 1/400 extend and clarify earlier research. They yield flow data useful for estimating flows in the Earth’s liquid core. Observed flows illustrate and confirm a nearly rigid liquid sphere with retrograde drift and lagging a cavity (mantle) axis in precession. The similarities of the observed lag angle with that computed for a rigid sphere, and earlier energy dissipation research both support the use of a rigid sphere analytical model to predict energy dissipation and first-order flow within the core‐mantle boundary (CMB). Second-order boundary layer and interior cylindrical flow structures also are photographed and measured. Interior flows are never turbulent or unstable at near-Earth parameters, although complex and transient flow patterns are observed within the boundary layer. Other mechanisms proposed to explain net heat loss from the Earth and maintenance of the geodynamo typically require acceptance of some critical but unproven premise. Precession and CMB configuration are known with certainty and precision. Analytical diculties have been the obstacle. Experiments illustrate the consequences of precession and ellipticity, provide criteria for validating analytical and numerical models, and may yield direct knowledge of the Earth’s deep interior with careful scaling.

An investigation of deactivation of bacteria in a canned liquid food during sterilization using computational fluid dynamics (CFD)

Thermal processing of a liquid food always results in important biochemical changes such as bacteria deactivation and nutrient concentration changes. To estimate these changes the liquid food needs to be tagged and followed, which is a difficult task for most flow conditions. In this study, the computational fluid dynamics (CFD) code PHOENICS is used to predict temperature distribution and concentration of the live bacteria in a can filled with liquid food. The governing equations for continuity, momentum and energy are solved numerically together with bacteria concentration, using a finite volume method. Arrhenius equation was used to describe bacteria deactivation kinetics, and it was introduced to the existing software package using a FORTRAN code. The diffusion of bacteria was modelled using the modified Brownian diffusion equation. Natural convection that occurs during thermal sterilization of viscous liquid (the aqueous solution of sodium carboxy-methyl cellulose (CMC)) in a cylindrical can heated from all sides, has been studied. Saturated steam at 121°C was used as the heating medium, and the model liquid was assumed to have constant properties except for the viscosity (temperature dependent) and density (Boussinesq approximation). The simulations have provided transient flow pattern, live bacteria concentration and temperature profiles, which highlight the slowest heating zone (SHZ) resulted from different periods of heating. The results show that the action of natural convection forces the SHZ to migrate towards the bottom of the can, and eventually stay in a region that is about 10–15% of the can height from the bottom. The secondary flow formation and its effect on the shape of the SHZ were evident. The simulations also show how the concentration of the live bacteria depends on both temperature distribution and flow pattern. The effect of diffusion on the rate of sterilization has been found to be negligible in the cases simulated in this study.

Direct simulation of natural convection cooling in a vertical circular cylinder

The transient processes of cooling-down and stratifying an initially homogeneous fluid by natural convection in a vertical circular cylinder have been investigated numerically. The transient flow patterns are identified by the visualization of the transient evolving processes in the cylinder. The results show that vigorous flow activities concentrate mainly in the vertical thermal boundary layer along the sidewall and in the horizontal region which is the lower part of the domain where the cold intrusion flow is embedded. The transient flow patterns at the unsteady and quasi-steady stages are analysed, including the activities of the travelling waves in the vertical thermal boundary layer along the sidewall and the cold intrusions in the horizontal region. A scaling analysis is used to characterize the development of the vertical thermal boundary layer on the sidewall and the stratification in the cylinder. It is found that the numerical solutions agree very well with the scaling results.

Numerical simulation of natural convection heating of canned food by computational fluid dynamics

Natural convection heating within a can of liquid food during sterilization is simulated by solving the governing equations for continuity, momentum and energy conservation for an axisymmetric case using a commercial Computational Fluid Dynamics (CFD) package ( PHOENICS). Transient flow patterns and temperature profiles within model liquids (sodium carboxy-methyl cellulose (CMC) and water) have been predicted. The model liquids, CMC and water, were assumed to have constant properties except for the viscosity (temperature dependent) and density (Boussinesq approximation). It has been shown that the action of natural convection forces the slowest heating zone (SHZ) to migrate towards the bottom of the can as expected. The shape and the size of the SHZ area are different for CMC and water. The magnitude of the axial velocity was found to be in the range of 10 −5–10 −4 m/s for CMS and of 10 −2–10 −1 m/s for water, these magnitudes of course vary with time and position in the can. The time required for the SHZ to reach the sterilization temperature of 100°C was 1800 s for CMC and only, 150 s for water.

Compaction-driven fluid flow in viscoelastic rock

Compaction driven fluid flow is inherently unstable such that an obstruction to upward fluid flow (i.e. a shock) may induce fluid-filled waves of porosity, propagated by dilational deformation due to an effective pressure gradient within the wave. Viscous porosity waves have attracted attention as a mechanism for melt transport, but are also a mechanism for both the transport and trapping of fluids released by diagenetic and metamorphic reactions. We introduce a mathematical formulation applicable to compaction driven flow for the entire range of rheological behaviors realized in the lithosphere. We then examine three first-order factors that influence the character of fluid flow: (1) thermally activated creep, (2) dependence of bulk viscosity on porosity, and (3) fluid flow in the limit of zero initial connected porosity. For normal geothermal gradients, thermally activated creep stabilizes horizontal waves, a geometry that was thought to be unstable on the basis of constant viscosity models. Implications of this stabilization are that: (1) the vertical length scale for compaction driven flow is generally constrained by the activation energy for viscous deformation rather than the viscous compaction length, and (2) lateral fluid flow in viscous regimes may occur on greater length scales than anticipated from earlier estimates of compaction length scales. In viscous rock, inverted geothermal gradients stabilize vertically elongated waves or vertical channels. Decreasing temperature toward the earth's surface can induce an abrupt transition from viscous to elastic deformation-propagated fluid flow. Below the transition, fluid flow is accomplished by short wavelength, large amplitude waves; above the transition flow is by high velocity, low amplitude surges. The resulting transient flow patterns vary strongly in space and time. Solitary porosity waves may nucleate in viscous, viscoplastic, and viscoelastic rheologies. The amplitude of these waves is effectively unlimited for physically realistic models with dependence of bulk viscosity on porosity. In the limit of zero initial connected porosity, arguably the only model relevant for melt extraction, travelling waves are only possible in a viscoelastic matrix. Such waves are truly self-propagating in that the fluid and the wave phase velocities are identical; thus, if no chemical processes occur during propagation, the waves have the capacity to transmit geochemical signatures indefinitely. In addition to solitary waves, we find that periodic solutions to the compaction equations are common though previously unrecognized. The transition between the solutions depends on the pore volume carried by the wave and the Darcyian velocity of the background fluid flux. Periodic solutions are possible for all velocities, whereas solitary solutions require large volumes and low velocities.

Compaction-driven fluid flow in viscoelastic rock

Compaction driven fluid flow is inherently unstable such that an obstruction to upward fluid flow (i.e. a shock) may induce fluid-filled waves of porosity, propagated by dilational deformation due to an effective pressure gradient within the wave. Viscous porosity waves have attracted attention as a mechanism for melt transport, but are also a mechanism for both the transport and trapping of fluids released by diagenetic and metamorphic reactions. We introduce a mathematical formulation applicable to compaction driven flow for the entire range of rheological behaviors realized in the lithosphere. We then examine three first-order factors that influence the character of fluid flow: (1) thermally activated creep, (2) dependence of bulk viscosity on porosity, and (3) fluid flow in the limit of zero initial connected porosity. For normal geothermal gradients, thermally activated creep stabilizes horizontal waves, a geometry that was thought to be unstable on the basis of constant viscosity models. Implications of this stabilization are that: (1) the vertical length scale for compaction driven flow is generally constrained by the activation energy for viscous deformation rather than the viscous compaction length, and (2) lateral fluid flow in viscous regimes may occur on greater length scales than anticipated from earlier estimates of compaction length scales. In viscous rock, inverted geothermal gradients stabilize vertically elongated waves or vertical channels. Decreasing temperature toward the earth’s surface can induce an abrupt transition from viscous to elastic deformation-propagated fluid flow. Below the transition, fluid flow is accomplished by short wavelength, large amplitude waves; above the transition flow is by high velocity, low amplitude surges. The resulting transient flow patterns vary strongly in space and time. Solitary porosity waves may nucleate in viscous, viscoplastic, and viscoelastic rheologies. The amplitude of these waves is effectively unlimited for physically realistic models with dependence of bulk viscosity on porosity. In the limit of zero initial connected porosity, arguably the only model relevant for melt extraction, travelling waves are only possible in a viscoelastic matrix. Such waves are truly self-propagating in that the fluid and the wave phase velocities are identical; thus, if no chemical processes occur during propagation, the waves have the capacity to transmit geochemical signatures indefinitely. In addition to solitary waves, we find that periodic solutions to the compaction equations are common though previously unrecognized. The transition between the solutions depends on the pore volume carried by the wave and the Darcyian velocity of the background fluid flux. Periodic solutions are possible for all velocities, whereas solitary solutions require large volumes and low velocities. © Elsevier, Paris

Melting heat transfer characteristics of a horizontal ice cylinder immersed in quiescent saline water

Experiments were performed to determine the effect of salinity level on the melting heat transfer characteristics of a horizontal ice cylinder immersed in quiescent saline water. Emphasis was placed on interpreting the heat transfer mechanism which dominates the solid-liquid interface situation. Measurements were carried out for saline water of 0.5–3.5 wt% in salinity, while the ambient temperature ranged from 1.8 to 24.0°C. Flow visualization was employed to investigate the transient flow patterns and corresponding solid-liquid interface locations. It was found that the flow patterns around the ice cylinder were a strong function of the saline water concentration, which then considerably affected the local heat transfer coefficient along the melting ice cylinder.

A numerical study of unsteady free convection from a sphere in a porous medium

Abstract In this paper a transient free convection flow from a sphere which is embedded in a porous medium has been considered. The temperature or heat flux of the sphere is suddenly raised and subsequently maintained at constant values over the surface. The coupled governing equations of the Darcy flow and energy are solved numerically for both small and large values of the Rayleigh number using a classical finite-difference method. Transient and steady-state flow and temperature patterns around the sphere are discussed in detail and a comparison of the overall Nusselt number with some earlier results shows very good agreement.

B234 振動平板による容器内の流れ(二次元CFDとPIVによる検証)

The two-dimensional flow field, induced by a flat plate starting to oscillate in a cylindrical vessel, was analyzed numerically using the SIMPLER method based on a fully implicit equation coupling with the moving-boundary-fitted grid technique. The transient flow pattern was discussed with the change in lift coefficient. The fully-developed flow patterns agreed well with the experimental results based on a PIV (particle imaging velocimetry) measurement, and the feasibility of 2D-CFD code was verified.

Thermal-Hydraulic Instability of Boiling Natural Circulation Loop with a Chimney. 3rd Report. Instability at High System Pressure.

Experiments have been conducted to investigate thermal-hydraulic instabilities at system pressure ranging from 1 to 7.2 MPa in a boiling natural circulation loop with a chimney. Stability maps in reference to the system pressure, the channel inlet subcooling, and heat flux are presented. Two different types of instabilities were observed in the facility at relatively low and high system pressures. Both of the instability mechanisms were clarified by investigation of the transient flow pattern and the response of the driving force of the circulation to momentum energy.

Observation of liquid motion in a precessing oblate spheroid

We summarize experimental results for transient flow patterns in a liquid-filled oblate spheroid, and compare the observed flows with existing geodynamo theories and evidence. Flow patterns are traced from a state near inertial rest through spin-up at a constant cavity spin rate, to forced precession where the cavity spin axis precesses at a constant rate about an inertially fixed direction displaced from the cavity spin axis by a constant half-coning angle. Space- and time-dependent flow with various instabilities are recorded for precession-induced boundary layer and deep interior flow velocities. Slow precession induces cylindrical shear layers and mostly closed meridional convection in the bulk of the core, the same features as found in buoyancy-driven convection models and in a ‘weak Taylor state’ solution for a numerical αω dynamo with small non-zero viscous coupling. Observed precessional flows exhibit significant departures from geostrophic equilibrium.