R.V.A. Oliemans

A turbulent diffusion model for particle dispersion and deposition in horizontal tube flow

Abstract Particle dispersion and deposition in a horizontal turbulent tube flow have been studied with a Turbulent Diffusion Model. Dispersion and deposition are modelled as the combined process of turbulent diffusion and gravitational settling fluxes. The particle diffusion coefficient is expressed in terms of the fluid diffusivity, taking into account the inertial effect and the crossing trajectories effect. The analytical solution for the particle concentration in a one-dimensional problem between two horizontal plates is found, and is used to calculate the relative deposition between the top and the bottom wall. It is investigated how this relative deposition depends on the particle diameter, the height of the channel and the Froude number. The one-dimensional analytical solution is used to predict the two-dimensional deposition flux in a tube, and it is investigated how this depends on the particle diameter and the Froude number. The expression for the deposition flux contains the characteristic physical parameters of the deposition problem that have not been recognized in earlier work.

Direct numerical simulation analysis of local flow topology in a particle-laden turbulent channel flow

The results of point-particle Eulerian–Lagrangian direct numerical simulation (DNS) calculations of dilute particle-laden turbulent channel flow are used to study the effect of the particles on the local flow topology. It is found that in the viscous sublayer, the flow becomes increasingly more two-dimensional as the two-way coupling effect (due to interaction between particles and fluid flow) increases with increasing particle load. Beyond the viscous sublayer the modifications in flow topology are not strongly related to the preferential concentration of particles in the flow field, which is in contrast to previous channel flow simulations. The effect of particles on the turbulent flow beyond the viscous sublayer is mostly a result of the overall changing near-wall dynamics of the fluid flow.

Results from a two-dimensional turbulent diffusion-model for dispersion and deposition of droplets in horizontal annular dispersed gas/liquid flow

Abstract A previously used one-dimensional Turbulent Diffusion-model is extended to two dimensions. A quasi-stationary annular dispersed gas/liquid flow is simulated by adding the contributions for different particle sizes of a large number of annular line sources from roll wave tops in the upstream direction. One or more local maxima in the droplet concentration are predicted, if droplet sources are present all along the tube wall. A local maximum in the concentration has been found in experiments as well, but it was always solely attributed to the presence of a secondary gas flow. The droplet deposition flux as calculated in the two-dimensional model shows good agreement with a previously derived analytical expression for the droplet deposition flux. Finally we derive how the two-dimensional deposition flux depends on the Stokes- and Froude-numbers. For an arbitrary Froude-number it is predicted droplets up to which Stokes-number are able to deposit at the top of the tube.

EFFECT OF THE PARTICLE-INDUCED TURBULENCE-MODIFICATION ON TWO-EQUATION MODELS FOR PARTICLE-LADEN WALL-BOUNDED TURBULENT FLOWS

In particle-laden flows, the particles can promote large changes in the turbulence characteristics of the fluid. Current two-equation models — in particular, the k – ɛ model — take into account only the “direct interaction” between the particles and the surrounding fluid, and do not consider the “indirect effects”, due to the disruption the particles promote in the turbulence dynamics itself. This disruption can promote large changes in the “standard values” of the “constants” of the k – ɛ model, which are not currently taken into account. In this paper, we study the influence of the particle-turbulence interaction on the k – ɛ model, using point-particle Eulerian-Lagrangian DNS/LES simulations, in a fully-developed turbulent channel flow laden with small heavy spheric particles. We focus on the effect the particles have on the constant C µ , associated with the kinematic eddy-viscosity. The particles promote large non-uniform changes in the value of C µ , which can increase or decrease, depending on the distance to the wall and the Reynolds number. The changes in the value of C µ can be understood in terms of the particle-effects on: (i) the balance between the Reynolds shear-stress and the turbulence kinetic energy, and (ii) the balance between production and dissipation of turbulence kinetic energy. The particles promote a large (very roughly) uniform decrease in the value of the kinematic eddy-viscosity; this suggests that in simple wall-bounded flows it might be possible to take into account the effect of the particles on the kinematic eddy-viscosity using simpler mixing-length models, without the use of a two-equation framework.

Measurements of Film Characteristics Along a Stationary Taylor Bubble Using Laser Induzed Fluorescence

In this study we measure the characteristics of the falling film along a stationary Taylor bubble. A Taylor bubble is a large gas bubble in a vertical tube that spans the tube diameter and rises with a high speed through the liquid. The film characteristics are used to verify the model proposed by Delfos (1996) for the gas flux out of the Taylor bubble, the so-called entrainment flux. We developed a laser induced fluorescence (LIF) technique to measure the instantaneous film thickness.