An Assessment of the Virtual Mass Force in RELAP5/MOD3.3 for the Bubbly Flow Regime

One-dimensional two-fluid models used for the simulation of large, industrial-scale problems require many simplifying assumptions to make a closed model that is tractable, applicable to a wide variety of flow regimes, and computationally efficient. Of particular interest here is the virtual mass force and the simplified form used in the RELAP5/MOD3.3 model. Comparison of the characteristics of the simplified model with a more complete two-fluid model for bubbly two-phase flow shows a remarkable similarity. Comparison to experimental data is also surprisingly favorable—provided that the flow conditions are determined appropriately. Namely, the characteristic analysis determined that a drift velocity for distorted bubbly flow, rather than for churn-turbulent flow, matches the data more accurately. The study is concluded by implementing a distorted bubbly drift velocity correlation into the RELAP5/MOD3.3 code. A comparison of the void wave speeds with the data confirms the results of the characteristic analysis.