M. Lopez de Bertodano

Phase distribution in bubbly two-phase flow in vertical ducts

Abstract The lateral phase distribution in bubbly flows in vertical ducts was analyzed using a three-dimensional two-fluid model. The constitutive relations of the model are based on analytic and experimental information on the behavior of a single bubble, and on the assumption of linear superposition of shear-induced and bubble-induced turbulence. The experiments chosen to test the model include available data for pipes and new data obtained in an isosceles triangular duct. While most of the data could be reproduced satisfactorily by the model, some could not. This is attributed to certain physical mechanisms that are still not well understood and therefore were not included in the constitutive relations.

Development of a k-ε Model for Bubbly Two-Phase Flow

An extension of the k-[epsilon] model for bubbly two-phase flow is proposed and tested against experimental data. The basic assumption made is that the shear-induced turbulence and bubble-induced turbulence may be linearly superposed. This assumption results in a model with two time constants that matches both homogeneous two-phase turbulence data (Lance and Bataille, 1991) and pipe data (Serizawa, 1986). The coefficients of the single-phase k-[epsilon] model have not been modified and only one additional coefficient is required: the virtual volume coefficient of the bubbles, which may be determined from first principles. This model not only agrees with the data trends, but it also predicts the turbulence suppression which has been measured for high Reynolds number bubbly air/water flows in pipes.

Phase distribution in complex geometry conduits

Abstract Bubbly air/water two-phase flow data have been taken as an isosceles triangle using hot film probes. It was found that a 3-D two-fluid model was able to predict these data and those taken previously in circular conduits. It appears that mechanically-based CFD predictions of bubbly two-phase flows is possible for many cases of practical concern.

The Prediction of Two-Phase Turbulence and Phase Distribution Phenomena Using a Reynolds Stress Model

The void fraction distribution for turbulent bubbly air/water upflows and downflows in a pipe was analyzed using a three-dimensional two-fluid model. A τ-e turbulence model was used for the continuous (liquid) phase. The τ-e transport equations yield all components of the Reynolds stress tensor for the liquid phase momentum equations

Experiments for entrainment rate of droplets in the annular regime

Abstract Two-fluid model predictions of film dryout in annular flow are limited by the uncertainties in the entrainment rate of droplets from the liquid film. The main cause of these uncertainties is the lack of separate effects experimental data in the range of the operating conditions in industrial applications. Air–water and Freon-113 entrainment rate data have been obtained in 10 mm tubes using the double film extraction technique. These experiments have been scaled to approach high-pressure steam–water flow conditions. The effects of surface tension and density ratio, missing from most previous data sets, have been systematically tested. The entrainment rate mechanism is scaled by the Kelvin–Helmholtz instability at the film surface. Based on this analysis and the new data, a new correlation is proposed that is valid for low-viscosity fluids in small ducts in the ripple-annular regime.

Scaled entrainment measurements in ripple-annular flow in a small tube

High pressure steam–water annular flow was simulated with scaled air–water and Freon-113 experiments. A unique Freon-113 loop was built to obtain low surface tension and high vapor density data on entrainment fraction. The results were compared with two correlations available in the open literature. The Ishii and Mishima correlation was capable to collapse the data for air–water, Freon-113 and steam–water experiments satisfactorily. However the correlation needs to be adjusted for high Weber numbers of the gas phase. The data was limited to the ripple-annular regime because the film extraction technique is not reliable when the film becomes frothy.

The Modeling of Lift and Dispersion Forces in Two-Fluid Model Simulations of a Bubbly Jet

Two-fluid model simulations of a bubbly vertical jet are presented. The purpose of these simulations is to assess the modeling of lift and turbulent dispersion forces in a free shear flow. The turbulent dispersion models used herein are based on the application of a kinetic transport equation, similar to Boltzmanns equation, to obtain the turbulent diffusion force for the dispersed phase. They have already been constituted and validated for the case of particles in homogeneous turbulence and jets and for microscopic bubbles in grid generated turbulence and mixing layers

Entrainment rate of droplets in the ripple-annular regime for small vertical ducts

Two-fluid model predictions of film dryout in annular flow are limited by the uncertainties in the constitutive relations for the entrainment rate of droplets from the liquid film. The main cause of these uncertainties is the lack of separate effects experimental data in the range of the operating conditions in nuclear power reactors. Air/water and Freon-113 entrainment rate data have been obtained in 10 mm tubes using the film extraction technique. These experiments have been scaled to approach high pressure steam-water flow conditions. The effects of surface tension and density ratio, missing from most previous data sets, have been systematically tested. The entrainment rate mechanism is assumed to be a Kelvin-Helmholtz instability. Based on this analysis and two previous correlations, a new correlation is proposed that is valid for low viscosity fluids in small ducts in the ripple annular regime.

Turbulent bubbly two-phase flow data in a triangular duct

Abstract Multidimensional bubbly two-phase flow data was obtained in an isosceles triangular duct having D h = 40 mm and L / D = 73. The data include local measurements of phase distribution, velocity and Reynolds stresses at 10,000 Re 1 bubble = 1250. The measurements were performed with single sensor and X-sensor cylindrical hot film probes of 0.025 mm diameter. These small probes have good bubble penetration characteristics so they are capable of the simultaneous measurement of the liquid phase instantaneous velocity and the phase indicator function. These data are useful for the validation of multidimensional two-fluid CFD models.

DCH dispersal and entrainment experiment in a scaled annular cavity

Abstract The objective of this experiment was to measure the amount of corium dispersal and the droplet size distribution during high pressure melt ejection from a CE reactor. The melt and the steam flowed to the containment through a narrow annular cavity. The experiment was carried out on a 1 20 th scaled model of the cavity and the containment. The scaling was based on dimensionless numbers obtained from a two-phase flow model of the dispersal and entrainment mechanisms in the cavity. Furthermore, the model shows that the flow in the cavity was choked, so high levels of dispersal and entrainment were possible. The experiment consisted of air—water, air—helium, air—woods metal and helium—woods metal tests; the main result being that the level of dispersal was very high in all cases. The woods metal data supported a separated flow model in the cavity, implying that the gas choked velocity was very high and the droplets very small. In contrast, the measured drop sizes for the water tests were much larger than the separated flow model predictions. This discrepancy could not be resolved because the entrainment mechanism is not properly understood at the present time.