Characterization of slug flows in horizontal piping by signal analysis from a capacitive probe

The slug flow is a common occurrence in gas–liquid piping flows. Usually it is an undesirable flow regime since the existence of long lumps of liquid slug moving at high speed is unfavorable to gas–liquid transportation, so that considerable effort has been devoted to study its hydrodynamic characteristics. In this work, a capacitive probe was used for dynamic measurements in the horizontal air–water slug flows, for several flow rates. The acquired signals were representative of the effective liquid layer thickness near every cross sectional area of the flow, instead of merely the holdup or void fraction in a finite volume of the flow. This was possible because probe had a thin sensing electrode that minimizes the axial length effect on the measurements. Tests were performed in a 34 mm i.d. acrylic pipe, 5 m long; in which slug flows as well as stratified-smooth and stratified-wavy flows were generated. Signal analysis techniques were applied for flow regime identification and toward characterization of these two-phase flows: Power Spectrum Density (PSD) from Fourier Transform and Probability Density Function (PDF) from Statistical Analysis. Therefore, PSD and PDF graphs were taken as signatures of each flow under test and a correlation was calculated for each PSD and PDF set of data, which showed to be a robust parameter for correct flow regime identification.