Flow pattern map and time-frequency spectrum characteristics of nitrogen-water two-phase flow in small vertical upward noncircular channels

We experimentally investigate the vertical upward gas-liquid two-phase flow in a 2 × 0.81 mm small rectangular channel and an equilateral triangle channel with a 2 mm side length. The two channels have same hydraulic diameters (Dh = 1.15 mm). We first present an experimental flow pattern map with nitrogen and water superficial velocities ranging from 0.08 m/s to 11.82 m/s and 0.12 m/s to 1.52 m/s, respectively. We also compare the flow pattern transition criteria between the triangle and rectangular cross sections by using the same hydraulic radius. We employ the influence rule that small passage geometry limits the flow pattern transition criteria. Thereafter, we comparatively analyze the transition boundaries of experiment flow patterns with other results in literature and classical prediction models. Results show that the cross-sectional shapes and experimental conditions of the experimental pipeline significantly affect the changes in the flow regime. Given the differential pressure signal of the two-phase flow, we propose two effective quadric time-frequency representations, namely, the adaptive optimal kernel time-frequency representation (AOK TFR) and data reduction sub-frequency band wavelet (DA SFBW) to investigate the complex behavior of vertical upward gas-liquid flows. We extract the positive power spectral density of the singular value-frequency entropy, singular value-frequency entropy, damping ratio, and vibration mode to characterize the evolution of flow patterns. The results suggest that the AOK TFR method can reveal flow dynamic details, whereas the DA SFBW based method is a powerful tool for characterizing the dynamical characteristics of different vertical upward gas-liquid flow patterns.