Visualization of dynamic behavior and condensation heat transfer characteristics of liquid film in pipes of different lengths under acoustic field excitation

As a non-contact active heat transfer enhancement method, audible acoustic field has shown great potential in the field of condensation heat transfer. This experiment focuses on analyzing the influence of different frequencies (100–2000 Hz) of audible acoustic field on the morphology of condensation liquid film in condensation pipes of different lengths (400 mm, 800 mm, 1200 mm) through visualization experiments, and establishes the correlation between liquid film response characteristics and condensation heat transfer. The experimental results show that the dynamic response of the condensed liquid film has strict frequency selectivity. Under acoustic excitation, the condensed liquid film can form a spatially ordered multi-stage water curtain structure, with the number of water curtain segments equal to the order of the resonant mode. At non resonant frequencies, the liquid film remains continuous and smooth without visible disturbance. The length of the pipeline affects the spatial response mode of the liquid film by changing the resonance frequency, but the optimal excitation frequency is determined by the impedance between the sound source, pipeline, and liquid film. Under the three pipeline lengths investigated in this experiment, the optimal excitation frequency is 400 Hz.The sound field reduces the thermal resistance on the condensation side by promoting the thinning of the condensation thickness, shortening the heat conduction path, surface waves breaking the thermal boundary layer, and local efficient heat transfer zones due to local fragmentation. When the lengths of the condensing pipes were 400 mm, 800 mm, and 1200 mm, the condensing efficiency increased by 340.3%, 158.3%, and 93.6%, respectively. The strengthening effect decreased with the increase of the condensing pipe length. This study reveals the regulation law of sound field frequency on liquid film morphology and heat transfer efficiency, providing theoretical basis and experimental support for frequency selection and pipeline length selection of sound field enhanced condensation heat transfer systems.