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Abstract: A pre-swirl system with a multi-chamber structure is crucial to the secondary air system of an aero engine. The airflow within the pre-swirl system (characterized by high-speed rotation and compressible flow) is complicated. During transient processes in aeroengine operation, the pre-swirl system is subjected to upstream fluctuations, which is a less studied aspect. This paper delves into the unsteady flow characteristics within the pre-swirl system. We investigate the influence of different pressure-fluctuation boundary conditions, corresponding to step function, ramp function, and sine function, on the transient response characteristics of the pre-swirl system. The results indicate that the response characteristics are strongly affected by the upstream boundary conditions. An obvious overshoot phenomenon is observed in the actual temperature drop under the step and ramp function conditions. The peak time t_p of the step function is 75% shorter compared to the ramp function. Furthermore, the flow parameters exhibit nonlinear growth during the transient process, emphasizing the need for consideration in future quasi-steady simulations. For the sine function condition, the pressure-fluctuation frequency minimally affects stable values of mass flow rate and actual temperature drop but exerts a substantial influence on the maximum deviation of ∆T_sys. As the frequency increases from 100 Hz to 200 Hz, the ∆T_sys decreases from around ±13 K to ±10 K.