Full-Range Drift-Flux Correlation for Upward Cocurrent Two-Phase Flows in Vertical Pipes

In nuclear thermal-hydraulic analysis, the void fraction prediction for upward two-phase flows in vertical pipes is essential. The two-fluid model is used as a platform for one-dimensional (One-D) nuclear thermal-hydraulic system analysis codes since it can treat the kinematic and thermal nonequilibrium between phases through interfacial transfer terms. Precise modeling of the area-averaged interfacial drag force in the interfacial momentum transfer term is essential in predicting void fractions accurately. The drift-flux model, which treats the gas-liquid mixture as a pseudo-single fluid yet allows slip between liquid and gas, is widely used in predicting the area-averaged interfacial drag force in two-fluid model-based codes. In the drift-flux model, the distribution parameter and drift velocity are two critical parameters in formulating the area-averaged interfacial drag force. In other applications of the drift-flux model, the one-D drift-flux model is utilized as a simple algebraic tool to predict a one-D void fraction directly from boundary conditions, such as superficial gas and liquid velocities. This study analytically developed a full-range drift-flux correlation for the distribution parameter and drift velocity, which is applicable to a void fraction from 0 to 1 for upward two-phase flows in vertical pipes. First, the critical area-averaged void fraction at the onset of the transition to separated two-phase flows was estimated by considering the similar distributions of void fraction and mixture volumetric flux. Then, the constitutive equations of distribution parameter and drift velocity (or drift-flux correlation) for upward cocurrent two-phase flows, including pure dispersed two-phase flows, transition two-phase flows, and separated two-phase flows, were developed. To validate the new correlation, 419 experimentally obtained void fractions for upward two-phase flows in vertical pipes were collected from nine sources. The comparison between the experimental results and the void fractions calculated by the newly developed full-range drift-flux correlation indicated that the correlation achieved superior prediction performance to that of the existing drift-flux correlations, and it achieved the mean relative deviation and mean absolute relative deviation of - 0.686% and 6.18%, respectively. The validated range of the proposed correlation is: 0.6 cm ≤ D <= 6.7 cm, 0.0338 m/s ≤ <j_g> ≤ 159 m/s, and 0.0226 m/s ≤ <j_f> ≤ 8.46 m/s. © 2025 Takashi Hibiki et al. International Journal of Energy Research published by John Wiley & Sons Ltd.