Abstract
Rocket pumps mainly use the Balance Piston (BP) system as an axial thrust self-balancing system for their large thrust. Although the BP system is statically stable, this system becomes unstable under some dynamic conditions. Therefore, some theoretical research has been previously conducted and identified its cause as fluid compressibility. However, their results used theoretical analysis, and very few examples examined the effect of fluid compressibility on three-dimensional unsteady fluid forces.
In this study, we performed a compressible CFD analysis in a liquid hydrogen turbopump with unshrouded impellers and compared the results with those of an incompressible analysis. Through the comparison, we investigated the influence of the fluid compressibility of liquid hydrogen on the pump’s axial characteristics.
According to the compressible CFD results, we found a significant decrease in fluid density in tip clearance due to the vortex from tip leakage flow occurring local temperature increase. In the BP system, because of the pressure loss and temperature rise associated with passage through the orifices, fluid density changed approximately 10% in the downstream of the BP chamber compared to the impeller inlet.
The unsteady-state calculation simulating forced axial vibration revealed that a lower damping coefficient, higher stiffness coefficient, and lower damping ratio were calculated in the compressible analysis than in the incompressible analysis. These findings were because No. 1 orifice loss of the BP system associated with forced vibration changed fluid density and static pressure in the compressible analysis, which resulted in changing the mass flow response into the BP system.
