Bubbly Flow In Vertical Channel Research Articles

Effect of interfacial drag force model on code prediction for upward adiabatic two-phase bubbly flow in vertical channels

Accurate modeling of the interfacial drag force is one of the keys to predicting thermo-fluid parameters using one-dimensional nuclear thermal-hydraulic system analysis code architected through the two-fluid model. The interfacial drag force appears in the interfacial momentum transfer term and governs the velocity slip or the relative velocity between gas and liquid phases. The most straightforward method to model the interfacial drag force is to model the force through the drag law (drag law approach). A drag coefficient and interfacial area concentration should be given to close the interfacial drag force model. Among them, the modeling of the interfacial area concentration has been one of the weakest links in the interfacial drag force modeling due to the lack of reliable data covering a wide test condition including prototypic nuclear reactor conditions and lack of physically sound interfacial area model. To avoid a considerable uncertainty in the prediction of the interfacial area concentration, Andersen and Chu (1982) proposed the interfacial drag force model using the drift-flux parameters (Andersen approach). The Andersen approach is practical for the simulation of a slow transient flow and a steady flow. Major system analysis codes such as USNRC TRACE have adopted the Andersen approach in the interfacial drag force modeling. Some attempts to improve the code performance have been considered using the drag law approach with the interfacial area transport equation. The dynamic modeling of the interfacial area concentration has the potential to improve the prediction accuracy of the interfacial area concentration in a transient flow and developing flow. Due to the importance of the improved interfacial drag force modeling, the implementation and evaluation of the interfacial area transport equation in USNRC TRACE code has been performed by Talley et al. (2011, 2013). The study claimed that the introduction of the interfacial area transport equation into the TRACE code improved the code performance in an adiabatic bubbly flow analysis significantly. The present study assessed the code calculation made by Talley et al. (2011) and identified several issues in the code calculation results. The present study analytically demonstrated that the drag law approach became identical with the Andersen approach for the distorted particle regime (or a major bubble shape regime in bubbly flow) due to the balancing-out of the interfacial area concentration (or bubble size) in the numerator and denominator of the interfacial drag force formulation. The code calculation using TRAC code endorsed the analytical assessment of the insignificant or no merit of the interfacial area transport equation in the code performance of the adiabatic bubbly flow analysis. The present study also pointed out the inconsistency of the code calculation made by Talley et al. (2011).

Heat transfer in turbulent bubbly flow in vertical channels

This paper examines the convective heat transfer in the turbulent bubbly flow between two parallel walls under a constant heat flux condition with a Reynolds number of up to Re=5600 based on the channel width and the average velocity. Direct numerical simulation is used to fully resolve the turbulent flow and the motion of bubbles. The channel is vertical and the flow is directed upward so that nearly spherical bubbles accumulate at the wall but deformable bubbles remain in the middle. The results show that the presence of bubbles enhances the mixing and consequently the heat transfer for both nearly spherical and deformable bubbles, as compared with results for single-phase flows.

Multiscale considerations in direct numerical simulations of multiphase flows

Direct Numerical Simulations of multiphase flows have progressed rapidly over the last decade and it is now possible to simulate, for example, the motion of hundreds of deformable bubbles in turbulent flows. The availability of results from such simulations should help advance the development of new and improved closure relations and models of the average or large-scale flows. We review recent results for bubbly flow in vertical channels, discuss the difference between upflow and downflow and the effect of the bubble deformability and how the resulting insight allowed us to produce a simple description of the large scale flow, for certain flow conditions. We then discuss the need for the development of numerical methods for more complex situations, such as where the flow creates spontaneous thin films and threads, or where additional physical processes take place at a rate that is very different from the fluid flow. Recent work on capturing localized small-scale processes using embedded analytical models, focusing on the mass transfer from bubbles in liquids with low mass diffusivity, suggests one approach. We conclude by discussing immediate needs for progress on the theoretical framework for describing the large-scale motion of multiphase flows and the need for multiscale methods to capture physical processes taking place at diverse length and time scales.

Influence of local void fraction distribution on turbulent structure of upward bubbly flow in vertical channel

Turbulent structure of upward dilute bubbly flow with 1 mm bubbles in a vertical channel is investigated experimentally. Small amount of surfactant is added to water to avoid bubble coalescence and to control local void fraction distribution. Liquid phase velocity is measured using two-dimensional laser Doppler Velocimetry. In 1-Pentanol solution of 20 ppm, bubbles have half-slip surface and migrate strongly toward the channel wall due to the shear-induced lift force which leads to wall-peaked distribution of local void fraction. On the other hand, in Triton X-100 solution of 2 ppm, bubbles become fully-contaminated and do not migrate toward the wall or the channel centre due to near-zero lift force, causing uniform distribution of local void fraction in the wall-normal direction. Once bubbles accumulate near the wall, transport of turbulent energy produced by the wall shear towards the channel centre is blocked. Then turbulence induced by the bubble motion becomes dominant in a wide core region (so-called pseudo turbulence). By contrast, in the case of the uniform distribution of bubbles, a mechanism of a turbulent energy transport which is the same as that of a single-phase turbulence still exists and furthermore the bubble-induced turbulence is added on it.