Mechanism analysis and relief measure verification of trapped-oil phenomenon in Roots pumps

To address the problems of pressure pulsation, power fluctuation and flow instability caused by the trapped-oil phenomenon when Roots pumps transport high-viscosity hydraulic oil, it is necessary to clarify the trapped-oil mechanism and compare the differences in trapped-oil characteristics induced by different rotor profiles, and propose targeted mitigation measures, thus providing theoretical support for the structural optimization and working condition adaptation of Roots pumps. Firstly, according to whether there was a circular arc transition at the top and root of the rotor profile, it was divided into full working type (such as circular arc profile) and non-full working type (such as involute profile). At the same time, the rotor profile was constructed through parametric modeling, the key structural parameters were defined, and a unified mathematical equation was established. Then, the three-dimensional geometric model of the Roots pump was generated using Siemens NX software, and the CFD (computational fluid dynamics) simulation model was built by Pumplinx software to analyze the operating characteristics of the pump with hydraulic oil as the medium. Finally, square relief grooves were machined at the rotor root, and the differences in trapped-oil characteristics between the pumps with and without relief grooves were compared. The simulation results showed that significant trapped-oil phenomenon occurred in both types of Roots pumps: the maximum pressure increase of the pumps with involute and circular arc profiles was 113.3% and 68.7%, respectively, and no cavitation occurred. After opening the relief grooves, the instantaneous pressure fluctuation amplitude of the pump with involute profile was reduced by 29.5%, and the maximum pressure increase was decreased to 50.4%, whereas the average output flow rate was reduced by 1.2%. In summary, the trapped-oil hazard of non-full working rotor profiles was more serious, and the relief grooves at the rotor root could effectively suppress the trapped-oil phenomenon, but it would lead to a slight reduction in the output flow rate. The research results provide a theoretical basis and technical references for the engineering application of Roots pumps in the high-viscosity liquid transportation field.