Experimental study on disturbance wave characteristics of the bottom annular liquid film in a horizontal narrow rectangular channel

The disturbance wave characteristics of annular flow in narrow rectangular channels are crucial for the accurate prediction of dry-out type critical heat flux in nuclear reactor fuel assembly systems. To investigate the wave evolution mechanisms and associated characteristics of annular liquid films in a horizontal narrow rectangular channel, the adiabatic air-water flow experiments were carried out using a test section with dimensions of 30mm×1mm×1000mm. An electrical-based liquid film sensor matrix for narrow rectangular geometries was developed to enable high-frequency acquisition of liquid film thickness time-series data on the bottom wide wall. The mean amplitude and frequency of the disturbance waves on bottom wide wall of the horizontal narrow rectangular channel were quantitatively extracted by a threshold identification method. Experimental findings reveal that the mean disturbance wave amplitude exhibits a pronounced upward trend as the superficial liquid velocity rises. Meanwhile, the disturbance wave frequency demonstrates a positive correlation with superficial liquid velocity and increases moderately with superficial gas velocity, without exhibiting a strong linear dependence on gas velocity in the present narrow rectangular channel geometry. Based on these observations, predictive correlations for both amplitude and frequency of disturbance waves within horizontal narrow rectangular ducts were formulated and subsequently validated against both the present experimental dataset and independently published data from existing literature. The validation outcomes confirm that prediction deviations for both models remain bounded within ±25%, demonstrating satisfactory accuracy and practical applicability in engineering contexts. The present study advances the mechanistic understanding of interfacial fluctuation dynamics and disturbance wave development in annular flows confined within rectangular geometries, while simultaneously furnishing a theoretical foundation for assessing dry-out risks and guiding the safety design of compact nuclear reactor fuel assemblies.