Research on ultrasonic crystal removal equipment and crystal removal efficiency for tunnel drainage system pipelines

Blockage in tunnel drainage pipelines caused by crystal deposition can induce lining deterioration such as cracking, spalling, and leakage, highlighting the need for effective crystal removal in tunnel maintenance. This study develops and validates an ultrasonic crystal removal device for corrugated drainage pipes through combined numerical simulation and indoor experiments. COMSOL Multiphysics was used to evaluate the sound-pressure field under straight and oblique installations at 20, 28, 40, and 52 kHz, indicating that 40 kHz provides the most favorable acoustic conditions in the present geometry and that the straight installation yields stronger and more uniform excitation than the oblique installation. Based on these findings, the fabricated device was tested at 40 kHz and 50 W in a CaCl₂-NaHCO₃ recirculating crystallization-removal experiment, and removal efficiency was quantified using key mass-based indices, specifically the crystal removal rate and the remaining crystal mass. The results show a clear two-stage removal process, with most mass reduction occurring in the first 30 days and a slower stage thereafter as residual deposits become denser and more strongly adhered. Removal performance exhibits strong spatial dependence and decreases with distance from the transducer. In the straight installation, the closest sections achieved removal rates of 98% and 97% with residual masses of 4.8 g and 7.8 g, whereas the oblique installation showed pronounced directional asymmetry, with only facing-side sections reaching 95% and 90% and several back-facing or distant sections remaining below 45% with residual masses exceeding 120 g. Overall, the experiments corroborate the simulations and demonstrate that straight installation at 40 kHz enables higher, more stable, and more spatially uniform crystal removal performance, providing guidance for mitigating crystal-induced blockage in tunnel drainage systems.