Development of a Local Pressure Drop Model for Two-Phase Natural Circulation Systems

This paper presents a comprehensive review of two-phase local pressure drop models in nuclear thermal-hydraulic systems, with a focus on pressure loss estimation arising from sudden flow area changes in two-phase flows. The abrupt area change models implemented in TRAC, TRACE, RELAP, and SPACE are examined, particularly for abrupt area expansions, contractions, and thin plate orifices. These models incorporate the concept of irrecoverable pressure losses (form losses) in the momentum conservation equations. In addition to the geometric considerations, the influence of the Reynolds number on form loss coefficients is comprehensively reviewed.An extended local pressure drop model is proposed to incorporate abrupt area changes, two-phase flow characteristics, and Reynolds number dependencies. The integration of two-phase multipliers and Reynolds number–based correlations is explored to enhance pressure loss predictions under complex flow conditions, which is relevant for modern reactor designs, as low pressure, low-flow velocity, and natural circulation conditions are considered highly important, especially in small modular reactors.To assess the influence of the local pressure drop models on pressure loss behavior and system performance, the models are implemented in SPACE and examined within the Bettis two-phase natural circulation loop and a conceptual reactor system. The simulation results demonstrated a notable deviation in the pressure drop predictions, and consequently, the mass flow estimations, depending on whether two-phase multipliers are applied. The proposed local pressure drop model exhibited improved consistency with the experimental data, particularly under conditions where the two-phase effects are significant. These findings emphasize the importance of incorporating abrupt geometry changes, two-phase dynamics, and Reynolds number effects for reliable thermal-hydraulic system analysis.