Deepwater oil and gas reservoirs are typically characterized by considerable depth, high temperature, and high pressure, which lead to variations in crude oil viscosity across different reservoir types. When multiphase pumps transport media with different viscosity conditions, the internal flow patterns become unstable, readily inducing pressure pulsations and shaft vibrations, thereby compromising the stability of the transportation system. This study systematically investigates the influence of liquid viscosity on pressure pulsations and force characteristics during gas-liquid transport in a multiphase pump. Four liquid media with distinct viscosity conditions, water (1.01 mPa s), light oil (4.42 mPa s), medium oil (12.46 mPa s), and heavy oil (39 mPa s), were used, along with air (0.018 mPa s) as the gas phase. Numerical simulations were conducted in conjunction with signal analysis techniques, including variational mode decomposition and the comprehensive evaluation index (CEI). The results indicate that both the pump head and efficiency decrease with increasing medium viscosity. The pressure pulsation characteristics and axial force behavior inside the pump are jointly governed by rotor-stator interaction and viscosity. As viscosity rises, the peak-to-peak pressure at the impeller outlet increases, reaching 2.1 times that observed under water conditions when heavy oil is used. Under such high-viscosity conditions, macroscale energy becomes dominant, and the broadband pulsations observed with water are eliminated. Furthermore, the average axial force is 8% higher than that under water conditions. With increasing viscosity, the CEI of the pump's radial force declines, reaching its minimum under heavy-oil conditions, where the radial force performance is optimal. These findings contribute to the enhancement of operational stability and the optimal design of multiphase pumps.
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