Experimental investigation of characteristic frequency of intermittent gas-liquid vertical downward flow

The estimation and prediction of the characteristic frequency of intermittent structures present in intermittent vertical downward two-phase flow is of primary importance for the design and operations of systems involving this kind of flow. This paper aims to present, for the first time, a detailed quantitative analysis of the characteristic frequencies of intermittent flows in this flow configuration. The study was conducted through a series of experiments in a 30 mm ID pipe using an air–water mixture. Absolute pressure time series were acquired at three axial locations and analyzed using the Welch method to extract the characteristic frequencies. Visual observations using a high-speed camera enabled the identification and discussion of the distinct structures present in the three flow regimes (cap bubble, slug and churn) of intermittent flow. Based on an analysis of the forces acting during transitions between the different flow regimes, the dimensionless mixture Froude number and the modified Lockhart–Martinelli parameter were employed for mapping the three flow regimes, demonstrating high predictive capabilities. In addition, the analysis of the measured characteristic frequencies showed that they increase during the flow development, reaching a fully developed value at 56.33 L/D . These frequencies are strongly influenced by both the gas and liquid superficial velocities. Interestingly, the measured total pressure drops were found to be directly related to both the characteristic frequencies and the flow regime type. Finally, a new empirical correlation was introduced, outperforming the existing one. • Comprehensive study on characteristic frequencies of intermittent flow in vertical downward pipes. • Three flow regimes identified and analyzed: cap bubble, slug, and churn flows, via high-speed visualization. • New flow regime map proposed using mixture Froude and modified Lockhart-Martinelli numbers. • Characteristic frequency increases during the flow development and is influenced by gas and liquid superficial velocities. • New correlation outperforms existing model, using gas-based Strouhal number and input liquid fraction.