Study on a horizontal axial flow pump during runaway process with bidirectional operating conditions

Area of Science:

• Fluid Mechanics
• Turbomachinery

Background:

• Ultra-low head pump stations are susceptible to bidirectional water delivery demands, posing a risk of runaway accidents under both forward and backward operating conditions.
• Understanding the distinct characteristics of these runaway processes is crucial for ensuring operational safety and efficiency in such systems.

Purpose of the Study:

• To analyze and compare the bidirectional runaway processes in a horizontal axial flow pump under forward runaway conditions (FRC) and backward runaway conditions (BRC).
• To investigate the flow dynamics and identify key parameters influencing the runaway behavior in both scenarios.

Main Methods:

• Numerical simulation using the Shear-Stress Transport (SST) k-ω model and the Volume of Fluid (VOF) model to capture water surface dynamics.
• Integration of the torque balance equation to determine real-time rotational speed during simulated bidirectional runaway events.
• Application of the vortex transport equation and swirl number to characterize flow behavior.

Main Results:

• The runaway process was categorized into five distinct stages: drop, braking, rising, convergence, and runaway, based on torque curve analysis.
• During the rising stage, increased pressure difference on blades, driven by faster flow impacting the pressure surface and separation on the suction surface, leads to abnormal torque increase.
• The convergence and runaway stages showed significantly higher pulsation amplitudes of torque and axial force under FRC compared to BRC.

Conclusions:

• The synchronization of vortex rope rotation frequency with the main pressure fluctuation frequency in the impeller under FRC amplifies pulsation amplitudes.
• Under BRC, guide vanes inhibit vortex formation, leading to less pronounced pulsations.
• The findings highlight critical differences in runaway dynamics, essential for designing safer and more reliable ultra-low head pump systems.