Particles In Laminar Flow Research Articles

Modeling of the sedimentation of marine particles in laminar flow. Application to the continental margin

In the present paper we propose an initial stage of modeling of the sedimentation of marine particles in the water-column, excluding the turbulence and the surficial water layer from 0 to 100 m. This marine sedimentation depends on several parameters: the mass concentration of particles, their density and diameters, the water temperature and salinity, the currents, etc… These parameters vary with depth and time. The model that we have developped related the flux to the different parameters; it allows to determine which among them have a prevailing v influence, and to calculate some parameters whose direct measurement is difficult, provided the other parameters are well known. In order to compare the general tendences foreseen by the model and those revealed by experiment, we use experimental data obtained in the southwestern margin of the Gulf of Lions in the Northwestern Mediterranean in the framework of the ECOMARGE programme (France-JGOFS). This comparison shows that our model gives a satisfactory description of those tendences. The calculations revealed the prevailing influence of the current speed and its inclination on the particle flux in the water column. It should be noted that even an extremely slight inclination of the current modifies that flux considerably. This parameter is seldom taken into consideration and in itself it can explain the sedimentation of fine particles which are located optically in the form of nepheloids. These particles, dominanthly mineral, are found in abundance in the traps, at least in the context of continental margins. It is sure that the use of a new generation of current meters and optical instruments will cause an evolution of our hypotheses on marine sedimentation.

Thermophoresis of aerosol particles in laminar flow over inclined plates

We analyze numerically the thermophoretic deposition of small particles due to the impingement of a laminar slot jet on an inclined plate. The governing boundary layer type equations are solved by an implicit finite difference technique using grid adaptation for the cold, hot and adiabatic plate conditions. The particle concentration at the cold plate is found to be almost independent of the location along the plate, and of the angle of inclination of the plate as long as the plate temperature (less than the free stream temperature) and thermophoretic coefficient are constant. Adjacent to the hot plate is a critical layer the thickness of which depends upon both the location along the plate and the angle of inclination. Whether this layer is free or full of particles depends upon the product of Prandtl number and thermophoretic coefficient being less than or greater than unity. Results for the adiabatic plate are very similar to those for the hot plate except that the particle concentration gradient is zero at the plate. The dissipation of mechanical energy also has a significant effect on the particle concentration.

Magnetic filtration of particles in laminar flow through a bed of spheres

We present a theory for the capture of magnetic particles by a bed of spheres in HGMS. The laminar approximation to fluid flow is used in conjunction with the free surface condition (Happel's model) to obtain the fluid velocity profile near a representative collector sphere in the assemblage. An effective medium treatment is used to model the magnetic field, thus taking into account the effects of neighboring spheres. results are reported for the capture of both paramagnetic and diamagnetic particulates by a filter operating either in the longitudinal or transverse design mode.

Effect of diffusion boundary layers on the initial uptake of O 2 by red cells. Theory versus experiment

We have applied theories of mass transfer, in laminar and turbulent flow, to red cells in the stopped-flow apparatus in order to estimate the effect of extracellular diffusion boundary layers on the initial rate of O 2 uptake. We compared the theoretical predictions with the results of our previous stopped-flow experiments with suspensions of red cells to which bovine serum albumin (BSA) had been added to decrease O 2 solubility and diffusivity ( Huxley and Kutchai, 1981). Models of red cells in laminar flow ( Friedlander, 1957, 1961; Harriott, 1962) predict a significant retardation of the rate of O 2 entry into red cells by diffusion boundary layers. Mixing in the stopped-flow apparatus occurs by convective and turbulent mechanisms in addition to simple molecular diffusion. The more complex theory of mass transfer to particles in turbulent flow ( Levich, 1962) shows, because the red cells are highly entrained in the flow, that mixing is not complete on the time scale of initial O 2 uptake. The O 2 permeability of the diffusion boundary layer, predicted by both laminar and turbulent flow theories, approximates the experimentally obtained value. The theories of mass transfer to particles in laminar flow predict the dependence of the rate of O 2 uptake on red cell size observed by other investigators. This suggests that these experimental results may be primarily due to the effect of diffusion boundary layers. Our studies are consistent with the interpretation that red cells in rapid mixing devices may be mixed incompletely with the suspending fluid; thus O 2 transfer is limited by molecular diffusion in the immediate vicinity of the erythrocyte. We conclude that the rates of respiratory gas exchange by red cells in flowing systems may be partly limited by diffusion boundary layers.

Thermophoretic Deposition of Particles in Laminar Flow in a Concentric Annulus

The thermophoretic deposition efficiency of particles in laminar flow in a concentric annulus is considered. A simple, but formally exact expression for the cumulative deposition efficiency is derived (for PrK= 1). This formula is used to calculate the deposition efficiency, and to demonstrate that 100% collection may be easily realized. The effect of the annulus size and the ratio of inner to outer tube wall temperature on the collection efficiency is explored to some extent.

A variational calculus approach to the suspension flow problem using the minimal force hypothesis

Analytic expressions for the velocity profile and distribution of neutrally buoyant particles in laminar flow were obtained as functions of the radial distance. A modified Einstein viscosity model and the hypothesis that the total force on all the particles flowing in the tube is a minimum were used. The methods of the variational calculus were used in the mathematical development. A velocity profile differing only slightly from the parabolic form of that for Hagan-Poiseuille flow was obtained. For particle distribution the equations developed predict a maximum concentration along the center-line for some flows and a maximum concentration in a ring some distance from the center line in other flows. Both of these concentration profiles have been observed experimentally. Quantitative predictions from the equations derived must await further experimental work to permit calculation of the parameters included in the equations. † Present address: Washington Suburban Sanitary Commission, Laurel, MD 20810, and Howard University, Washington, DC 20059, U.S.A. ‡ Present address: Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803, U.S.A.

Study on the Tubular Pinch Effect in a Pipe Flow : I. Lateral migration of a single particle in laminar poiseuille flow

In laminar flow of suspensions through a circular tube, particles migrate into a concentric annular region with the radius about 0.6 of the tube radius. This phenomenon is well-known as the tubular pinch effect. To clarify the mechanism, the influences of relative particle-fluid velocity, Reynolds number and particle diameter on radial particle displacement (lateral migration) were investigated by measuring the motion of a suspended spherical particle in laminar flow through a circular tube. Consequently, it was found that the motions of particles could be classified by the particle Reynolds number. And, especially, the relation between the particle Reynolds number and the equilibrium radial position of particles and the existence of critical particle Reynolds number for shifting from unilateral migration to bilateral migration were clarified.

Behaviour of deformable particles in laminar flow

Behaviour of deformable particles in laminar flow

Lift forces on suspended sediment particles in laminar flow; experiments and sedimentological interpretation

ABSTRACT A sphere carried by laminar flow past a planar boundary experiences a force away from the boundary. The sedimentological implications of this effect, both for transportation of coarse sediment in strong turbu'ent flows and for deposition of fine sediments from oceanic bottom currents, were explored by estimating the lift force on small, nearly neutrally buoyant spheres suspended in laminar flow in a laboratory flume and evaluating how closely this models the lift force in the natural flows. The results suggest that this lift-force effect is not important in natural sedimentary processes.