Influence Of Buoyancy Forces Research Articles

УЧЕТ СИЛ ПЛАВУЧЕСТИ ПРИ МОДЕЛИРОВАНИИ ТЕЧЕНИЯ СВЕРХКРИТИЧЕСКОЙ ВОДЫ В ВЕРТИКАЛЬНЫХ ТРУБАХ

The results of computer simulation of supercritical water flow in tubes are presented. The regularities of the influence of buoyancy forces on the flow characteristics are investigated.

A parameter model for dredge plume sediment source terms

The presented model allows for fast simulations of the near-field behaviour of overflow dredging plumes. Overflow dredging plumes occur when dredging vessels employ a dropshaft release system to discharge the excess sea water, which is pumped into the trailing suction hopper dredger (TSHD) along with the dredged sediments. The fine sediment fraction in the loaded water-sediment mixture does not fully settle before it reaches the overflow shaft. By consequence, the released water contains a fine sediment fraction of time-varying concentration. The sediment grain size is in the range of clays, silt and fine sand; the sediment concentration varies roughly between 10 and 200 g/l in most cases, peaking at even higher value with short duration. In order to assess the environmental impact of the increased turbidity caused by this release, plume dispersion predictions are often carried out. These predictions are usually executed with a large-scale model covering a complete coastal zone, bay, or estuary. A source term of fine sediments is implemented in the hydrodynamic model to simulate the fine sediment dispersion. The large-scale model mesh resolution and governing equations, however, do not allow to simulate the near-field plume behaviour in the vicinity of the ship hull and propellers. Moreover, in the near-field, these plumes are under influence of buoyancy forces and air bubbles. The initial distribution of sediments is therefore unknown and has to be based on crude assumptions at present. The initial (vertical) distribution of the sediment source is indeed of great influence on the final far-field plume dispersion results. In order to study this near-field behaviour, a highly-detailed computationally fluid dynamics (CFD) model was developed. This model contains a realistic geometry of a dredging vessel, buoyancy effects, air bubbles and propeller action, and was validated earlier by comparing with field measurements. A CFD model requires significant simulation times, which is not available in all situations. For example, to allow correct representation of overflow plume dispersion in a real-time forecasting model, a fast assessment of the near-field behaviour is needed. For this reason, a semi-analytical parameter model has been developed that reproduces the near-field sediment dispersion obtained with the CFD model in a relatively accurate way. In this paper, this so-called grey-box model is presented.

Investigation of mechanisms of supercritical CO2 trapping in deep saline reservoirs using surrogate fluids at ambient laboratory conditions

Geological storage of carbon dioxide relies on the effectiveness of immobilizing CO2 in the pore space of deep geological formations through a number of trapping mechanisms that include capillary, dissolution, and mineral trapping. Improved fundamental understanding of these processes is expected to contribute toward better conceptual models, improved numerical models, more accurate assessment of storage capacities, and optimized placement strategies. However, studying these processes at a fundamental level is not feasible in field settings because fully characterizing the geologic variability at all relevant scales and making observations on the spatial and temporal distribution of the migration and trapping of supercritical CO2 (scCO2) is not practical. The specific goal of this study is to develop and implement an experimental method in intermediate scale test tanks under ambient laboratory conditions to make observations and generate data to improve the understanding of capillary trapping affected by fluid and formation properties. Since it is challenging to visualize multiphase flow processes occurring at high pressure conditions at the meter scale, a testing method was developed based on the use of surrogate test fluids to replace the scCO2 and formation saline water. To set a foundation for extrapolating experimental results to the field, we chose a set of dimensionless groups that define the relative contributions of buoyancy, viscous, and capillary forces to the displacement behavior of immiscible fluids. The experiments were designed with the goal of understanding and accurately quantifying the immobilization of the scCO2 analog in a homogeneous formation confined by a slightly dipping structural barrier. A set of three displacement experiments through unconsolidated sands with variable permeability was conducted in a quasi-two-dimensional flow cell to gain insight into the influence of buoyancy forces on the propagation of the displacing phase. This work takes advantage of laboratory experiments at the intermediate scale to investigate gravitational and hysteresis effects on entrapment of scCO2 currents in brine-saturated reservoirs. Understanding these phenomena at a fundamental level represents a critical step to improve injection strategies and to enhance capillary trapping mechanisms.

Unsteady flow of nanofluids past a vertical flat plate with leading edge accretion or ablation

Unsteady flow of a nanofluid past a semi-infinite vertical flat plate is theoretically investigated. Unsteadiness is caused by steady rates of accretion or ablation at leading edge. Two types of nanofluids, namely, Cu-water and Ag-water are considered, with Prandtl number Pr = 6.2. Governing partial differential equations are transformed into ordinary differential equations, using a similarity transformation. The resulting ordinary differential equations are solved numerically. Results are compared with those of regular fluid (ϕ = 0) with Pr = 0.7 and comparison shows good agreement with previous results. Effects of nanoparticle volume fraction on flow and heat transfer characteristics, influence of buoyancy force and unsteady parameter, are presented and discussed in detail. It is found that effects of the volume fraction on skin friction and wall temperature gradient are more significantly influential in Ag-water solution than in Cu-water solution.

Thermocapillary Convection and Buoyant-Thermocapillary Convection in the Annular Pools of Silicon Melt and Silicone Oil

The thermocapillary convection and buoyant-thermocapillary convection in the annular pool of silicon melt (Pr=0.011) and silicone oil (Pr=6.7) with depth d=10 mm differentially heated at the outer wall and cooled at the inner wall are investigated by 2-D numerical simulation. The numerical results exhibit that the thermocapillary flow is enhanced by buoyancy force for silicon melt while it is weakened for silicone oil. Linear stability analysis indicates that the buoyancy force destabilizes the thermocapillary convection, which is different from that for silicone oil. The detailed reason of different influence of buoyancy force on the thermocapillary flow with different Pr numbers is explained according to present numerical results.

Influence of Buoyancy Force on Thermocapillary Convection Instability in the Differentially Heated Annular Pools of Silicon Melt

The influence of buoyancy force on the thermocapillary convection instability in the annular pools (Ri = 20 mm, Ro = 40 mm, and depth d ranging from 1 to 10 mm) of silicon melt (Pr = 0.011), differentially heated at the outer wall and cooled at the inner wall, is investigated numerically. The critical Marangoni numbers (Mac) for the incipience of oscillatory flow are determined by linear stability analysis (LSA) under both microgravity and normal gravity conditions. The results indicate that the buoyancy force destabilizes the thermocapillary convection under different liquid layer depths from 3 to 10 mm. With increasing the layer depth, the critical Ma number, critical azimuthal wave number and critical phase velocity decrease. Some of 3-D simulation results are compared with those of LSA. 3-D results are found consistent with the LSA results except for a case of D = 0.05 where 3-D simulation gives a stationary 3-D flow under a large Ma.

Theoretical and Numerical Study of Heat Transfer Deterioration in High Performance Light Water Reactor

A numerical investigation of the heat transfer deterioration (HTD) phenomena is performed using the low-Re k-ωturbulence model. Steady-state Reynolds-averaged Navier-Stokes equations are solved together with equations for the transport of enthalpy and turbulence. Equations are solved for the supercritical water flow at different pressures, using water properties from the standard IAPWS (International Association for the Properties of Water and Steam) tables. All cases are extensively validated against experimental data. The influence of buoyancy on the HTD is demonstrated for different mass flow rates in the heated pipes. Numerical results prove that the RANS low-Re turbulence modeling approach is fully capable of simulating the heat transfer in pipes with the water flow at supercritical pressures. A study of buoyancy influence shows that for the low-mass flow rates of coolant, the influence of buoyancy forces on the heat transfer in heated pipes is significant. For the high flow rates, buoyancy influence could be neglected and there are clearly other mechanisms causing the decrease in heat transfer at high coolant flow rates.

Lagrangian mixing in decaying stably stratified turbulence

Direct numerical simulations are used to study mixing and dispersion in decaying stably stratified turbulence from a Lagrangian perspective. The change in density of fluid particles owing to small-scale mixing is extracted from the simulations to provide insight into the mixing process. These changes are driven by temporally and spatially intermittent events that are strongly suppressed as the stratification increases and overturning motions disappear. This occurs for times Nt > 2n, i.e. after one buoyancy period, where N is the buoyancy frequency. The role of small-scale mixing processes in the density (or buoyancy) flux is analysed. After an initial transient, we find that diapycnal displacements due to mixing dominate the dispersion of fluid particles, even in weak stratification. The relationship between the diapycnal diffusivity and vertical dispersion coefficients is found to be strongly dependent on stratification. Models for the mixing following fluid particles are investigated. The time scale for the density changes due to small-scale mixing is shown to be approximately independent of N and instead remains linked to the energy decay time scale which is relatively insensitive to stratification. There are large changes in the structure of these flows as they evolve under the influence of buoyancy forces. We investigate these changes and their relationship to mixing. We find that strong mixing events are closely linked to the presence of overturning regions in the flow, and that they occur close to (but not within) these regions. The results reported here have implications for the development of improved models of diffusion in stably stratified turbulence.

Instability of buoyant‐thermocapillary flows in non‐cylindrical liquid bridges

AbstractThe flow driven by buoyancy and thermocapillarity in a liquid bridge between coaxial differentially heated disks held by surface tension forces is investigated. We numerically determine the stability of axisymmetric flows by a linear stability analysis. In the present approach, the influence of buoyancy forces on the flow stability is taken into account through static meniscus deformations as well as through internal buoyancy. To elucidate the physical instability mechanisms the energy balance given by Reynolds‐Orr‐like equations is considered term by term.

Influence of buoyancy forces on Marangoni flow instabilities in liquid bridges

The influence of buoyancy forces on oscillatory Marangoni flow in liquid bridges of different aspect ratio is investigated by three‐dimensional, time‐dependent numerical solutions and by laboratory experiments using a microscale apparatus and a thermographic visualisation system. Liquid bridges heated from above and from below are investigated. The numerical and experimental results show that for each aspect ratio and for both the heating conditions the onset of the Marangoni oscillatory flow is characterized by the appearance of a standing wave regime; after a certain time, a second transition to a travelling wave regime occurs. The three‐dimensional flow organization at the onset of instability is different according to whether the bridge is heated from above or from below. When the liquid bridge is heated from below, the critical Marangoni number is larger, the critical wave number (m) is smaller and the standing wave regime is more stable, compared with the case of the bridge heated from above. For the critical azimuthal wave number, two correlation laws are found as a function of the geometrical aspect ratio A.

EXPERIMENTAL STUDY OF CONVECTIVE HEAT TRANSFER INSTABILITIES IN TRANSIENT RESPONSE ALONG A COUNTERCURRENT HEAT EXCHANGER

This article deals with experimental investigations to reveal the effect of combined natural and forced convection in a horizontal tubular countercurrent heat exchanger in both transient and steady states. In transient response, the temperature profiles of hot fluid in a cross section of the inner tube show the influence of buoyancy forces and radial conduction. These profiles are distorted in time and lead to the appearance of oscillating temperature along the inner tube of the heat exchanger. The transient Richardson dimensionless number is calculated to show these effects. Also, a spectral analysis using the discrete Fourier transform has been performed to characterize these temperature instabilities. Using this technique, the influence of flow rates of hot and cold fluids is investigated.

The influence of the wall temperature on the development of heat transfer and secondary flow in a coiled heat exchanger

In the present study the development of mixed convective flow is studied in a helically coiled heat exchanger with an axially varying wall temperature for Re = 500, Pr = 5 and δ = 1 14 and compared to the constant wall temperature boundary condition. In the method used the parabolized equations are solved using a finite difference discretization scheme. The influence of buoyancy forces is analyzed on heat transfer and secondary flow. For all Grashof numbers studied it appears that both heat transfer, quantified by the Nusselt number, and secondary flow, quantified by the relative kinetic energy, exhibit a wavy behaviour in axial direction. For higher Grashof numbers, however, this phenomenon diminishes for the case with an axially varying wall temperature due to stabilizing stratification effects.

Influence of the thickness of the liquid layer on the ratio of the dimensions of a convection cell

A theory is developed for a new type of transition — a change in the ratio of the longitudinal and transverse dimensions of a convection cell as the thickness of a liquid layer is varied. A sudden change in the ratio of the cell dimensions takes place because of a change in the predominant mechanism for excitation of convection. The governing influence of buoyancy forces gives way to one of thermocapillary forces, and they in turn give way to the influence of thermoelectric forces for yet thinner layers. As the layer thickness is reduced gradually at a fixed external heating, the ratio of the dimensions will take on the values 0.7, 0.65, and 1, respectively.

Influence of buoyancy forces on the flow of gases through packed beds at elevated pressures

AbstractThe flow of gases through packed‐bed columns at elevated pressures was investigated by displacement experiments with a stepwise change in the tracer concentration. The experiments with different tracers, flow rates, pressures, particle sizes, tube diameters, and flow directions were used to illustrate the effect of buoyancy forces on the apparent fluid mixing in packed beds. The experimental results exhibit a strong influence of the density differences between the tracer and carrier gas on the flow behavior at higher pressures. Axial dispersion significantly increases due to hydrodynamical instabilities when the density increases with the height, and is reduced in the presence of stable density gradients. The effect of density differences on the fluid flow becomes more pronounced with increasing tube diameters. It can be argued that in pressure equipment the usual correlations for mass and heat transfer may not be valid any more and that plug flow in such equipment is hardly possible to achieve.

Developing mixed convection in a coiled heat exchanger

In this paper the development of mixed convection in a helically coiled heat exchanger for Re = 500, Pr = 5 and δ = 1 14 is studied. The influence of buoyancy forces (Gr = ¢O (10 5)) on heat transfer and secondary flow is analyzed. In the method used the parabolized equations are solved using a finite difference discretization. The code is tested on mixed convection flow in a 90° curved tube of which the results are compared to the results obtained with an elliptical code. For the helically coiled tube a constant wall temperature is considered. It appeared that heat transfer is highly influenced by secondary flow induced by centrifugal and buoyancy forces. For low Grashof numbers a splitting phenomenon of the temperature field is observed due to large secondary velocities, resulting in two separated areas of fluid. For high Grashof numbers the fluid in the coiled pipe becomes almost linearly startified which results in small secondary velocities. A wavy behaviour in the Nusselt number is observed for medium Grashof numbers.

On the Entrance Effects and the Influence of Buoyancy Forces on the Fluid Flow In Rtp Reactors

ABSTRACTFlow and heat transfer effects play a critical role in chemical vapour deposition (CVD) reactors. Holographic interferometry offers the possibility to study the flow dynamics of an entire region in real time under process conditions with negligible disturbances of the investigated process. The advantage of the method is the possibility of accurate measurements of flow patterns and temperature profiles in a fast dynamic process under actual experimental conditions. Mixed and forced flow conditions were studied for two gases with distinct different thermophysical properties. The influence of the gas inlet system and buoyancy forces on the resulting fluid flow are highlighted. A special attention was drawn on the visualization of edge effects.

The influence of convection during solidification on fragmentation of the mushy zone of a model alloy

Experiments have been conducted to observe fragmentation events in a model alloy (succinonitrile and acetone) solidifying in the presence of forced convection in the superheated melt. Measurements of fragmentation rates have been made, and an attempt was made to relate the results to the controllable parameters of the system. A microscope-video system recorded the mushy zone-melt interface, and the fragmentation process and fragmentation rates could be determined from a frame-by-frame analysis of the video images. Experiments were conducted for varying cooling rates, overall temperature differences, melt flow rates, and for two different concentrations of acetone (1.3 and 6.1 wt pct). Significant dendritic fragmentation occurred for all runs. In addition, the influence of buoyancy forces is clearly evident from particle motion near the mushy zone-melt interface. Fragmentation rates appear to correlate well with the magnitude of particle velocities near the interface, with increasing fragmentation being associated with higher particle velocity magnitude (either in the same or the opposite direction to the mean flow) for the 1.3 wt pct acetone mixture. However, the correlation is quite different for the higher concentration. The relationship between these results and the possible mechanisms for fragmentation are discussed. Although it appears that either constitutional remelting or capillary pinching are likely of importance, hydrodynamic shear forces or some other mechanism as yet undiscovered cannot be completely discounted, although circumstantial evidence suggests that mechanical shearing is inconsistent with observations made both here and already in published literature. The results provide a step in the development of solidification models that incorporate fragmentation processes in the mushy zone as an important mechanism of grain refinement and a potential source of macrosegregation in ingots and large castings.

Large-scale structure in the far field of byoyant jets

The flow structure and entrainment mechanisms in the far field of a round vertical buoyant jet have been studied experimentally by use of an optical technique based on laser-induced fluorescence (LIF). A large number of essentially instantaneous tracer concentration profiles were recorded for each experimental run by combining LIF with linear photodiode array imaging and high-speed digital data acquisition. Analysis of the resulting high-resolution flow images indicates that the far-field region is dominated by the periodic passage of structures spanning the entire radial flow extent. Ambient fluid is entrained by vortical motions and is transported to regions deep into the flow interior. Correlation analysis discloses that the passage frequency of the structures scales with the local mean velocity and flow width. Conditional averaging of the data indicates that the downstream frontal region of the structure is well mixed and at higher concentration level than the back and side regions where ambient fluid is intermittently present. This results in an axial concentration gradient within the structure, analogous to the ramp-like pattern previously observed in heated air jets. In comparison to the momentum-driven flow the ambient fluid presence in the flow interior is greatly increased when body forces are the driving mechanism. This appears to result from the influence of buoyancy forces in the production of turbulent vortices at the integral scale. An important feature of both the momentum-driven and buoyancy-driven flows investigated is the strongly intermittent character of the concentration field. This raises the issue of the appropriateness of gradient-diffusion theories for the description of such flows.

Review of existing experimental findings on forced convection heat transfer to supercritical

A literature survey of existing experimental findings on steady state and transient forced convection heat transfer to supercritical helium is given, and the deterioration of heat transfer, the influences of buoyancy force and correlations of heat transfer are also discussed.

Numerical study of continental collision: influence of buoyancy forces and an initial stiff inclusion

Summary. We study simple models of collision between two continental plates patterned after the geometry of the India-Asia collision, but which may be applicable to more general situations. The collision is modelled by the indentation of a viscoplastic plate (Asia) by a rigid punch (India). Although deformation is three-dimensional in nature, a plate thickness of the order of 100 km permits a 2-D approach for wavelengths longer than a few hundred kilometres. We assume an Airy compensation at the crustmantle boundary throughout the deformation. We can then reduce the problem to a 2-D one within the context of thin plate theory. This model makes it possible to study crustal thickening during collisions. The opposition between buoyancy forces, which tend to flatten the topography, and viscous resistance, appears clearly and we show the existence of a relaxation mechanism associated with the volumetric part of the deformation. The asymptotic behaviour of the model is of the incompressible plane strain type and thus crustal thickening tends to a maximum value. When this limit is reached no more thickening is possible and distensive phenomena can be induced. Buoyancy effects can be described by a nondimensional parameter, the Argand number Ar, which is the ratio between the relaxation time and the characteristic time of the viscous flow. A finite element approximation, based on a material description of the finite deformation, is used to investigate the evolution of the indentation process. The role of pre-existing heterogeneity (stiff inclusion) is also considered. Lateral boundary conditions on the plate are chosen to approximate the boundary interactions that are believed to apply for the Asian plate, i.e.