On the One-Dimensional Modeling of Vertical Upward Bubbly Flow

Abstract

The one-dimensional two-fluid model approach has been traditionally used in thermal-hydraulics codes for the analysis of transients and accidents in water-cooled nuclear power plants. This paper investigates the performance of RELAP5/MOD3 predicting vertical upward bubbly flow at low velocity conditions. For bubbly flow and vertical pipes, this code applies the driftvelocity approach, showing important discrepancies with the experiments compared. Then, we use a classical formulation of the drag coefficient approach to evaluate the performance of both approaches. This is based on the critical Weber criteria and includes several assumptions for the calculation of the interfacial area and bubble size that are evaluated in this work. A more accurate drag coefficient approach is proposed and implemented in RELAP5/MOD3. Instead of using the Weber criteria, the bubble size distribution is directly considered. This allows the calculation of the interfacial area directly from the definition of Sauter mean diameter of a distribution. The results show that only the proposed approach was able to predict all the flow characteristics, in particular the bubble size and interfacial area concentration. Finally, the computational results are analyzed and validated with cross-section area average measurements of void fraction, dispersed phase velocity, bubble size, and interfacial area concentration.

Development and assessment of a one-dimensional CFD solver for boiling flows in bubbly regimes

“…The first model is a drift flux based model, which principle is that both continuous and dispersed phases are solved as an indivisible system (Peña-Monferrer et al, 2018c). In order to calculate the drag coefficient, a balance in the direction of flow is performed considering pressure, buoyancy and the interfacial drag between both phases.…”

“…The depressurization in the system and its influence in the bubbles size is described by an expansion factor f i that is related to the inlet values (Peña-Monferrer et al, 2018c). At each node i the relation states…”

“…Knowing that the bubble size follows a normal distribution, the Sauter mean diameter in this case can be calculated from its popular formulation (Peña-Monferrer et al, 2018c).…”

“…Particularly, the work presented in this section is the extension of the work presented in (Peña-Monferrer et al, 2018c). The cited work analyses the capability of the one-dimensional code RELAP5 to predict the behaviour of a bubbly flow under low velocity conditions and it proposes a new drag model that considers the bubble size distribution to improve the accuracy of the results.…”