Numerical Simulation of Fluid Flow Caused by Buoyancy Forces During Vacuum Arc Remelting Process

The metallurgical structure and composition of ingots which depend critically on the fluid motion within the molten pool during the vacuum arc remelting(VAR)process have important effect on the subsequent mechanical processes like forging,rolling and welding.In order to determine the fluid motion of molten pool,a 2D finite element model is established using ANSYS10.0 software,combined with the turbulent fluid flow and heat transfer.The fluid motion caused by thermo buoyancy forces is investigated at different VAR processes in the present study.The results indicate that the fluid flows symmetrically along the axis of the molten pool and clockwisely along the circle at the right pool’s profile.It is also shown that the maximum velocity increases with increasing melting rate and a direct proportional relationship exists.

Turbulence Statistics of Thermo-Buoyancy Supercritical Fuel Flow in a Regenerative Cooling Channel

Active regenerative cooling with supercritical hydrocarbon fuel is considered as the most promising thermal protection method.The existence of buoyancy force would lead to strongly anisotropic flow and thermal transport characteristics.It is closely related to the cooling performance of the engine.To elucidate the mechanisms of turbulent transport,the large eddy simulation(LES) was performed to assess turbulence statistics within different turbulence scales.The results indicated that the buoyancy and inertial force together dominated the change of turbulent structure.Moreover,the spatial thermal buoyancy effect significantly suppressed the vertical velocity fluctuation.This is due to the laminar motion caused by the buoyancy force,thereby weakening the thermal transport.For the statistics of velocity fluctuation,it was found that the buoyancy force and inertial force greatly weaken the vertical and streamwise velocity fluctuation,respectively.For the statistics of thermal transport,the results pointed out that the near-wall heat transport characteristics need to be paid more attention.The thickness of the temperature mixing boundary layer led to the attenuation of vertical heat flux,which inhibited vertical temperature diffusion and predisposed to extreme conditions of heat transfer deterioration.The results can enhance the academic understanding of the heat transfer mechanism of supercritical fluids,and give guidance for further applications of thermal protection.

Development of Mathematical Model of Slag Flow Field and Temperature Field in ESR System

The heat transfer phenomena and driving forces of slag bath flow in ESR process were analyzed, and the mathematical models in ESR system were reviewed and evaluated. The electromagnetic force is the main driving force for the flow formation in the ESR slag pool, and the temperature difference in the pool creates a convective flow in the system. The shape of the electrode tip has an effect on electromagnetic field distribution in slag pool, thus affects the flow pattern. Finally an improved mathematical model of slag pool flow was proposed.

THE EFFECT OF MAGNETIC FIELDS ON LOW FREQUENCY OSCILLATING NATURAL CONVECTION WITH PRESSURE GRADIENT

The oscillating natural convection in the presence of transverse magnetic field with time depending pressure gradient is studied. The analysis of the problem is carried out by assuming that the fluid is flowing in a parallel plate configuration. The emphasis is on low frequency oscillating convective flows induced by g-jitter associated with micro gravity because of their importance to the space processing materials. A general solution for an oscillating flow in the presence of transverse magnetic field is carried out. Some special cases of the oscillating flow and its response to an applied magnetic field are performed. It was observed that the behavior of oscillating free convective flows depends on frequency, amplitude of the driving buoyancy forces, temperature gradient,magnetic field and the electric conditions of the channel walls. In the absence of magnetic field, buoyancy force plays a predominant role in driving the oscillatory flow pattern, and velocity magnitude is also affected by temperature gradients. To suppress the oscillating flow external magnetic field can be used. It is also found that the reduction of the velocity is inversely proportional to the square of the applied magnetic field with conducting wall but directly proportional to the inverse of the magnetic field with insulating wall. Detailed calculations and computational results are also carried out to depict the real situation.

Local non-similarity solution to impact of chemical reaction on MHD mixed convection heat and mass transfer flow over porous wedge in the presence of suction /injection

Combined heat and mass transfer on free, forced, and mixed convection flow along a porous wedge with magnetic effect in the presence of chemical reaction is investigated. The flow field characteristics are analyzed by the Runge-Kutta-Gill scheme with the shooting method as well as the local non-similarity method up to the third level of truncation, which are used to reduce the governing partial differential equations into nine ordinary differential equations. The governing boundary layer equations are converted to a dimensionless form by Falkner-Skan transformations. Because of the effect of suction/injection on the wall of the wedge with buoyancy force and variable wall temperature, the flow field is locally non-similar. Numerical calculations up to the third order level of truncation are carried out as a special case for different values of dimensionless parameters. Effects of the magnetic field strength in the presence of chemical reaction with variable wall temperature and concentration on the dimensionless velocity, temperature and concentration profiles are shown graphically.

Viscoelastic modeling of the diffusion of polymeric pollutants injected into a pipe flow

This study focuses on the transient analysis of nonlinear dispersion of a polymeric pollutant ejected by an external source into a laminar pipe flow of a Newtonian liquid under axi-symmetric conditions.The influence of density variation with pollutant concentration is approximated according to the Boussinesq approximation and the nonlinear governing equations of momentum,pollutant concentration are obtained together with and Oldroyd-B constitutive model for the polymer stress.The problem is solved numerically using a semi-implicit finite difference method.Solutions are presented in graphical form for various parameter values and given in terms of fluid velocity,pollutant concentration,polymer stress components,skin friction and wall mass transfer rate.The model can be a useful tool in understanding the dynamics of industrial pollution situations arising from improper discharge of hydrocarbon pollutants into,say,water bodies.The model can also be quite useful for available necessary early warning methods for detecting or predicting the scale of pollution and hence help mitigate related damage downstream by earlier instituting relevant decontamination measures.

Analysis of the buoyancy-induced heat transferin a rotating cavity

In order to get a better knowledge of the heat transfer in compressor cavities of aero-engines,the simplified rotating cavity with two-plane discs,a shaft and a cylindrical rim has been investigated numerically and compared with the available measurements.The numerical results in agreement with the available experiments show large-scale instabilities.The disk local Nusselt numbers show mainly radial rising distributions for the heated disks with radial rising temperature profiles.In the present work,at the Reynolds number of 20 000,the disk local Nusselt numbers are the correlations of the local Grashof number to the power of 1.89-2.6,and the value of the power is increased as the rotational Reynolds number goes up.At the rotational Reynolds number of 800 000,the local Nusselt numbers are the correlations of the local Grashof number to the power of 0.68-2.6,and the value of the power is decreased as Reynolds number goes up.The area-averaged disk Nusselt number is the correlation of the Reynolds number to the power of 0.479 and the rotational Grashof number to the power of 0.12.

Impact of nanoparticle shape on thermo-solutal buoyancy induced lid-driven-cavity with inclined magnetic-field

The aim of this paper is to numerically investigate the influence of nanoparticles shape on heat and mass transport phenomena in a moving lid cavity under the combined effect of thermo-solutal buoyancy force and magnetic force.The governing equations are transformed into velocity-vorticity form of equations and solved using Galerkin's weighted residual finite-element-technique.The analysis has been carried out with parameters like buoyancy ratio(−5≤N≤5),magnetic field inclination angle(0°≤ϕ≤90°)with four shapes of Al_(2)O_(3) nanoparticle like bricks,blades,platelets and cylinders.The results revealed that the shape factor on Nusselt number is significant depending upon the inclined magnetic field and buoyancy ratio whereas on mass transfer the shape effect is negligible.The diffusion mode of transport process is stronger than the convection mode at higher inclination angle of magnetic field.Based on a given value of N andϕ,blade and cylinder shows the best performance in Nusselt and Sherwood number respectively except the platelet shape that shows maximum frictional loss in terms of wall shear stress.

Subduction tectonics vs.plume tectonics——Discussion on driving forces for plate motion

Plate tectonics describes the horizontal motions of lithospheric plates,the Earths outer shell,and interactions among them across the Earths surface.Since the establishment of the theory of plate tectonics about half a century ago,considerable debates have remained regarding the driving forces for plate motion.The early"Bottom up"view,i.e.,the convecting mantledriven mechanism,states that mantle plumes originating from the core-mantle boundary act at the base of plates,accelerating continental breakup and driving plate motion.Toward the present,however,the"Top down"idea is more widely accepted,according to which the negative buoyancy of oceanic plates is the dominant driving force for plate motion,and the subducting slabs control surface tectonics and mantle convection.In this regard,plate tectonics is also known as subduction tectonics."Top down"tectonics has received wide supports from numerous geological and geophysical observations.On the other hand,recent studies indicate that the acceleration/deceleration of individual plates over the million-year timescale may reflect the effects of mantle plumes.It is also suggested that surface uplift and subsidence within stable cratonic areas are correlated with plumerelated magmatic activities over the hundred-million-year timescale.On the global scale,the cyclical supercontinent assembly and breakup seem to be coupled with superplume activities during the past two billion years.These correlations over various spatial and temporal scales indicate the close relationship and intensive interactions between plate tectonics and plume tectonics throughout the history of the Earth and the considerable influence of plumes on plate motion.Indeed,we can acquire a comprehensive understanding of the driving forces for plate motion and operation mechanism of the Earth's dynamic system only through joint analyses and integrated studies on plate tectonics and plume tectonics.

Dissipative flow features of Carreau nanofluid with thermal radiation inside plane wall channel:Jeffery-Hamel analysis

The current article communicates a numerical investigation on laminar flow of dissipative generalized Newtonian Carreau nanofluid flowing through vertical conduit with converging and diverging plane walls.Thermal and concentration characteristics due to enthalpy change,activation energy,and non-linear thermal radiation have been examined in the presence of buoyancy forces.The channel walls for both temperature and volumetric fraction are assumed to be isothermal.The instability mechanism of nanofluids is reported using a two-phase nanofluid model,which works reasonably well for nanoparticle concentrations below a certain threshold.A Jeffery-Hamel(J-H)flow model is developed by assuming an incompressible purely radial flow of Carreau nanofluids with heat and mass transportation.Using the suitable non-dimensional variables,the resulting nonlinear partial differential equations are turned into a system of ordinary differential equations.The modified governing equations are then numerically solved using the built-in boundary value problem solver bvp4c,on the template form of commercial software MATLAB.The impacts of material,geometrical and thermophysical parameters governing the J-H problem are discussed and illustrated.Results indicate that higher buoyance forces incline the velocity profiles in converging enclosure,while a slight reduction is perceived in opposing forces.A significant decrease of wall heat transmission is reflected for larger values of activation energy and radiation parameter.For endorsing this communication,a comparison analysis is established with existing research and noticed a remarkable agreement.Practically,the flow inside converging and diverging channels are deployed in nuclear reactors that use plate-type nuclear energies,high heat-flux condensed heat exchangers,high-performance micro-electronic cooling systems,jets,rockets nozzles,and jet propulsion inlet.

VISCOSITY EFFECTS ON THE BEHAVIOR OF A RISING BUBBLE

In the incompressible fluid flow regime, without taking consideration of surface tension effects, the viscosity effects on the behavior of an initially spherical buoyancy-driven bubble rising in an infinite and initially stationary liquid are investigated numerically by the Volume Of Fluid （VOF） method. The ratio of the gas density to the liquid density is taken as 0.001, and the gas viscosity to the liquid viscosity is 0.01, which is close to the case of an air bubble rising in water. It is found by numerical experiments that there exist two critical Reynolds numbers Re1 and Re2 , which are in between 30 and 50 and in between 10 and 20, respectively. As Re 〉 Re1 the bubble will have the transition to toroidal form, and the toroidal bubble will break down into two toroidal bubbles. In this case viscosity will damp the development of the liquid jet and delay the formation of the toroidal bubble. As Re〈Re1 the transition will not happen. As Re2 〈 Re 〈 Re1, the bubble will split from its rim into a toroidal bubble and a spherical cap-like bubble, and as Re〈Re2 the splitting will not occur and the bubble can finally reach a stationary shape. With the decrease of the Reynolds number, the stationary shape changes from spherical-cap bubble with skirt to dimpled peach-like bubble. Before the bubble reaches its stationary shape the vortex structure in the flow field varies with time. The vortex structure corresponding to bubble stationary shape varies with the Reynolds number. It is also found that there exists another critical Reynolds number Re^＊ which is in between Re1 and Re2 , and as Re 〈 Re^＊, after the bubble rises in an accelerating manner for a moment, it will rise with an almost constant speed, and the speed increases with increasing Reynolds number. As Re 〉 Re^＊, it will not rise with a constant speed. The mechanism of the above phenomena has been analyzed theoretically and numerically.

Experimental Investigation of the Secondary Flow in a Rotating Smooth Channel Subjected to Thermal Boundary Conditions

In the current study,thermal boundary conditions are considered in a rotating smooth channel with a square cross-section to investigate the secondary flow and compare it to that of the same channel without heating.The measurement is conducted at three streamwise planes(X=445 mm,525 mm,605 mm).The flow parameters are the Reynolds number(Re=4750,which was based on the average longitudinal or primary velocity U and the hydraulic diameter D of the channel cross-section),the rotation number(Ro=?D/U,where?is the rotational velocity,ranging from 0 to 0.26),and the aspect ratio of the channel cross-section(AR=1,which is calculated by dividing the channel height by the channel width).The leading and trailing walls are heated under a constant heat flux qw=380 W/m^2,and the top and bottom walls are isothermal at room temperature.This work is in a series with our previous work without thermal boundary conditions.Based on the experimental data,we obtained a four-vortex regime.There is a counter-rotating vortex pair near the leading side and the trailing side.Because the leading and trailing walls are heated,the buoyancy force increases the relative vertical position of the vortex pair near the trailing side from 5%to 12.5%of the hydraulic diameter.When moving upstream along the streamwise direction,the upper vortex near the trailing wall becomes weaker,whereas the lower vortex becomes stronger.As the rotational speed increases,the vortex pair near the trailing side is inhibited by the Coriolis force.Under heated thermal boundary conditions,the vortex pair near the trailing side reappears due to the effect of buoyancy force.These results indicate that the buoyancy force has a substantial effect on the secondary flow regime under thermal boundary conditions.

Effect of Actual Cooling Rate of Ladle Stream on Persistent Metallurgical Performance of a Given Tundish

To evaluate the effect of actual cooling rate of liquid steel in the ladle on the metallurgical performances of a tundish, a transient and coupled computational model was developed to reveal the flow fields, temperature fields, residence time distribution of the molten steel and the inclusion removal efficiency in a typical single-strand tundish with given geometry and process parameters. The results showed that, with the decrease of the ladle stream cooling rate, the temperature difference of bulk flow at the outlet of tundish over a normal casting period decreased from 11.3 to 2.6 K, and the dead volume fraction of the tundish decreased from 17.58% to 14. 35%, while the inclusion removal efficiency was increased especially for the inclusions with the diameter less than 50 μm, whose removal ratio could be increased by 20.62%. When the cooling rate was less than 0.3 K · min-1 , however, the variation rates of the three evaluation criterions above declined significantly, which suggested that a critical value existed for the effect of the cooling rate of ladle stream on the tundish performances. The establishment of the critical ladle stream cooling rate should be very important to achieve persistent metallurgical properties of tundish over the whole casting stage, together with the reasonable ladle insulation design.

Evaluation of the use of surrogate Laminaria digitata in eco-hydraulic laboratory experiments

Inert surrogates can avoid husbandry and adaptation problems of live vegetation in laboratories. Surrogates are generally used for experiments on vegetation-hydrodynamics interactions, but it is unclear how well they replicate field conditions. Here, surrogates for the brown macroalgae Laminaria digitata were developed to reproduce its hydraulic roughness. Plant shape, stiffness and buoyancy of L. digitata were evaluated and compared to the properties of inert materials. Different surrogate materials and shapes were exposed to unidirectional flow. It is concluded that buoyancy is an important factor in low flow conditions and a basic shape might be sufficient to model complex shaped plants resulting in the same streamlined shape.