Temperature and velocity dependent drift-flux model for gas–liquid flow in geothermal systems

Multiphase gas–liquid flow governs mass and energy transport in geothermal reservoirs, wellbores, and surface facilities. Because direct resolution of interfacial fluid dynamics is generally impractical, simplified approaches such as drift-flux formulations are widely used to predict fluid-phase behavior including phase slip and void fraction. However, most commonly used correlations were developed for air–water and oil–gas systems under near-ambient conditions, introducing uncertainty in their applicability to geothermal environments where fluid properties vary strongly with temperature. In this study, the performance of widely used drift-flux correlations was evaluated using 421 measurements obtained from water–N 2 and water–NaCl–N 2 experiments conducted in the Geothermal-Flowloop (GFL) at the University of Iceland under conditions of 20–180 °C and 1–20 barg. Void fraction was determined by gamma densitometry, and the results were compared with predictions from classical drift-flux formulations. At ambient temperatures (20–30 °C), the existing models reproduced the measurements well, and comparison with two independent flow-loop datasets confirmed the consistency of the data. However, all major correlations progressively overpredict void fraction with increasing temperature and total superficial velocity, indicating that important temperature- and flow-dependent effects are not adequately represented. To address these deficiencies, a modified drift-flux model was developed from the Zuber–Findlay framework. Analysis of the experimental data showed that phase slip increases systematically with decreasing liquid viscosity and increasing superficial velocity. Accordingly, the classical drift-velocity expression was reformulated using normalized liquid viscosity and total superficial velocity, and a transition function was additionally introduced to extend applicability across different flow regimes. The resulting formulation substantially improves agreement with experimental observations across the full range of temperatures, flow velocities, and void fractions, reducing systematic void-fraction overestimation, lowering RMSE from 0.20 to 0.05 at 180 °C, and reproducing the observed thermal and flow-rate trends. The results demonstrate that explicit incorporation of temperature-dependent fluid properties and flow-dependent slip behavior is important for accurate multiphase flow modelling in geothermal systems and provide an extended drift-flux framework applicable to high-temperature gas–liquid flows.