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.