The Dynamic Two-Fluid Model OLGA: Theory and Application

Abstract

Dynamic two-fluid models have found a wide range of application in the simulation of two-phase-flow systems, particularly for the analysis of steam/water flow in the core of a nuclear reactor. Until quite recently, however, very few attempts have been made to use such models in the simulation of two-phase oil and gas flow in pipelines. This paper presents a dynamic two-fluid model, OLGA, in detail, stressing the basic equations and the two-fluid models applied. Predictions of steady-state pressure drop, liquid holdup, and flow-regime transitions are compared with data from the SINTEF Two-Phase Flow Laboratory and from the literature. Comparisons with evaluated field data are also presented. IntroductionThe development of the dynamic two-phase-flow model OLGA started as a project for Statoil to simulate slow transients associated with mass transport, rather than the fast pressure transients well known from the nuclear industry. Problems of interest included terrain slugging, pipeline startup and shut-in, variable production rates, and pigging. This implied simulations with time spans ranging from hours to weeks in extreme cases. Thus, the numerical method applied would have to be stable for long timesteps and not restricted by the velocity of sound.A first version of OLGA based on this approach was working in 1983, but the main development was carried out in a joint research program between the Inst. for Energy Technology (!FE) and SINTEF, supported by Conoco Norway, Esso Norge, Mobil Exploration Norway, Norsk Hydro A/S, Petro Canada, Saga Petroleum, Statoil, and Texaco Exploration Norway. In this project, the empirical basis of the model was extended and new applications were introduced. To a large extent, the present model is a product of this project.Two-phase flow traditionally has been modeled by separate empirical correlations for volumetric gas fraction, pressure drop, and flow regimes, although these are physically interrelated. In recent years, however, advanced dynamic nuclear reactor codes like TRAC,l1 RELAP-5,2 and CATHARE3 have been developed and are based on a more unified approach to gas fraction and pressure drop. Flow regimes, however, are still treated by separate flowregime maps as functions of void fraction and mass flow only. In the OLGA approach, flow regimes are treated as an integral part of the two-fluid system.The physical model of OLGA was originally based on smalldiameter data for low-pressure air/water flow. The 1983 data from the SINTEF Two-Phase Flow Laboratory showed that, while the bubble/slug flow regime was described adequately, the stratified/annular regime was not. In vertical annular flow, the predicted pressure drops were up to 50% too high (see Fig. 1). In horizontal flow, the predicted holdups were too high by a factor of two in extreme cases.These discrepancies were explained by the neglect of a droplet field, moving at approximately the gas velocity, in the early model. This regime, denoted stratified-or annular-mist flow, has been incorporated in OLGA 84 and later versio...

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