Prediction model of fluctuation frequencies induced by continuity waves in slug flow

Abstract

[Objective] Slug flow is one of the main gas-liquid two-phase flow regimes. Its fluctuation characteristics not only impact the safe operation of multi-phase pumps and the measurement accuracy of multi-phase flowmeters but also arouse flow-induced vibrations in various pipeline sectors, such as elbows, valves, and places where pipelines are connected in parallel. Therefore, conducting a systematic study on fluctuation frequencies induced by slug flow is of utmost importance for securing the operation safety of these pipelines and associated fittings. [Methods] By leveraging the dynamics model of slug flow and recognizing the inherent continuity wave characteristics of slug flow, a novel continuity wave model was developed. This model employed the Strouhal number to characterize the dimensionless fluctuation frequencies induced by slug flow. Through extensive literature investigations, existing empirical calculation models for fluctuation frequencies induced by slug flow in horizontal pipelines were summarized. Additionally, an experimental study was conducted to examine fluctuation frequencies induced by air-water two-phase slug flow in a 30 m long horizontal pipeline with an inner diameter of 24 mm. Conducting probes were used to measure the fluctuation frequencies within the parameter ranges of 0.23 m/s to 1.85 m/s for superficial liquid velocity and 0.88 m/s to 10.0 m/s for superficial gas velocity. The experimental data and calculation results based on the new model were taken to compare with the prediction results obtained using 12 different empirical formulas proposed by various scholars. [Results] Analysis of the experimental data revealed that the fluctuation frequencies induced by continuity waves in slug flow increased with rising superficial liquid velocities, approximating a linear trend. At low superficial liquid velocities, the fluctuation frequency initially decreased and subsequently increased with the increase of superficial gas velocities. However, the influence of superficial gas velocities on slug frequencies became less noticeable at high superficial liquid velocities. The proposed model had an average relative error of approximately -15%, lower than the mean level observed in the empirical formulas, and the root-mean-square relative error was the lowest, approximately 26%. [Conclusion] Error analysis demonstrates that the prediction results of fluctuation frequencies induced by slug flow obtained from the proposed continuity wave model align more closely with the experimental data compared to those obtained from existing empirical formulas, exhibiting a greater degree of scattering.

Keywords

• two-phase flow
• slug flow
• continuity wave
• flow-induced vibration
• fluctuation frequency