Turbulence and Phase Distribution in Bubbly Pipe Flow Under Microgravity Condition

Abstract

The role of the turbulence in the void fraction distribution in bubbly pipe flow under microgravity condition is evaluated on the basis of numerical simulations using a Eulerian-Eulerian two-fluid model. In microgravity, the average relative velocity is weak and the void fraction distribution is mainly governed by the turbulence. The simulations show that the turbulent contributions of the added mass force play an important role in the phase distribution phenomenon. It is clearly proved that the turbulence acts on the bubbles distribution not only by the pressure term but also by the turbulent correlations obtained by averaging the added mass force.

A Mechanism on Liquid-Phase Turbulence Modulation by Microbubbles

“…However, the added mass force has a very weak influence on the liquidphase turbulence intensity, which is in good agreement with conclusions of [17,18].When the buoyant force ( B ) is neglected in the computation, the mean slip velocity between microbubbles and liquid is absent, which leads to the absence of the drag and lift force. Under the circumstance (i.e., for Case E), microbubbles have almost no influence on the liquid-phase turbulence intensity (namely, the curves overlap for Cases A and E in Figure 2(a)) as pointed out by Chahed et al [19]. The previous phenomenon further shows that the added mass force, if only being considered, cannot cause any changes in the liquid turbulence intensity.…”

Toward a Multiphase Local Approach in the Modeling of Flotation and Mass Transferin Gas-Liquid Contacting Systems

“…We have to specify the contributions of the average and fluctuating flow fields to this force. Numerical simulations of upward pipe bubbly flow in micro-gravity and in normal gravity conditions show clearly the role of the turbulence and of the interfacial forces on the void fraction distribution, (Chahed et al, 2002). These numerical simulations are compared to the experimental data of Kamp.…”

“…The determination of these scales allows to describe correctly the different effects of the bubbles agitation on the liquid turbulence structure. If from a theoretical point of view, second order is an adequate level for turbulence closure in bubbly flows, the implementation of such turbulence models in two-fluid models clearly improves the predetermination of the turbulence structure in different bubbly flow configurations, (Chahed et al, 2002(Chahed et al, , 2003. Nevertheless, from a practical point of view, second order modeling is still difficult to use and turbulence models based on turbulent viscosity concept, particularly two-equation models, remain widely used in industrial applications.…”

“…In the common formulation of the momentum interfacial exchange only the contributions due the average velocities fields of the liquid and the gas phases is considered while the turbulent contributions of the interfacial force are ignored or eventually expressed via a supplementary dispersion term proportional to the void fraction gradient (Lance & Lopez de Bertonado, 1992). In their model Chahed et al (2002) proposed to take into account beside the average contributions of the interfacial transfer, the turbulent correlations issued from the added mass force and they proved that these turbulent correlations are important in the phase distribution phenomenon, in particular in gas-liquid flow under micro-gravity condition. According to their formulation, the interfacial momentum transfer (4) second order closure of the turbulence developed for bubbly flows, (Chahed & Masbernat, 2003) …”

“…In order to attain this objective, second order turbulence modeling is required and in several recent two-fluid models, the turbulence closures are effectively modeled using second order closure modeling Chahed et al, 2003;Zhou, 2001;Lopez de Bertodano et al, 1990). On the other hand, the turbulence structure of the liquid phase was pointed to have an important role in the phase distribution (Drew & Lahey, 1982); more recently it has been proved that the turbulent contributions of momentum interfacial transfer are important in the phase distribution phenomena, (Chahed et al, 2002). Thus one of the previous questions in the elaboration of efficient prediction tools is the accurate predetermination of the turbulence stress tensors in both liquid and gas phases.…”