Interfacial drag force in one-dimensional two-fluid model

In the two-fluid model, the interfacial momentum transfer is the key closure relation for defining the degree of coupling between phases. The two approaches utilized in one-dimensional system analysis codes, namely the "drag coefficient approach" (RELAP5/MOD2 and TRAC-PF1/MOD1) and "drift-velocity approach" (RELAP5/MOD3 and TRACE V5) to describe the interfacial drag force, are discussed in detail. Shortcomings in the current and past forms of interfacial drag are identified. Considering the code drawbacks, two approaches which are conceptually consistent with the two-fluid model are discussed. The first approach is an improved form of the drag coefficient approach and can be easily implemented with the classical two-fluid model without requiring any additional development. The second formulates the most consistent and advanced description of interfacial drag force for the use with the two-fluid model with two-group interfacial area transport equation. The framework of the two-group interfacial drag, which models spherical/distorted bubbles (group-1) and cap/slug/churn-turbulent bubbles (group-2) separately, is presented. Proper description of distribution parameter, drag coefficient, relative velocity, bubble size/shape characterization and interfacial area concentration is discussed, providing a comprehensive road map to advanced closure of the one-dimensional interfacial drag force.