Gas-liquid two-phase flow in horizontal pipes: Time-frequency characteristics of acoustic responses for different flow patterns

Accurate characterization of gas–liquid two-phase flow patterns is important for understanding heat-transfer behavior and ensuring the operational reliability of industrial systems. However, conventional measurement methods may disturb the flow field or depend on pipeline transparency, limiting their application under practical operating conditions. In this study, a horizontal gas–liquid two-phase flow visualization platform integrated with high-frequency acoustic sensors was developed to acquire acoustic signals from different flow patterns. Band-pass filtering and Fourier transform were employed to analyze the acoustic energy and intensity characteristics in the time and frequency domains. The results indicate that the dominant acoustic signals of gas–liquid two-phase flow are concentrated within 18–68 kHz. Stratified flow exhibits smooth signals with low energy, slug flow produces distinct periodic pulses, strong slug flow presents larger and more intense fluctuations, and annular flow generates continuous high-frequency and high-amplitude responses. The average acoustic energy increases from below 0.01 J for stratified flow to above 1.5 J for annular flow. These results demonstrate that the time–frequency characteristics of acoustic responses can effectively characterize different gas–liquid two-phase flow patterns and provide a physical basis for non-invasive flow monitoring in horizontal pipes.