Abstract
In a liquid-hydrogen-fuel rocket engine system, a turbopump conveys fuel from a tank to the engine. A rotor in the pump can move axially to balance fluid force that works on it, and this mechanism is called “balance piston (BP).” In rare cases, however, an unsteady axial vibration of the rotor occurred while its rotational speed was decreasing after the engine had stopped. Many researchers considered the cause of the problem was fluid force that worked on the BP and endeavored to find out what the key factor was, but it was still not unveiled. In this paper, we constructed a dynamic model that computes the fluid force and represents it in the form of added mass, added damping coefficient and added stiffness. By adding them to structural mass, structural damping coefficient and structural stiffness, we obtain total mass, total damping coefficient and total stiffness. If they are negative, unsteady vibration possibly occurs. We conducted simulations with the constructed model under various computational conditions and sought which factor brings about the unsteady vibration. Then, we found that pressure fluctuation at the inlet of the BP and clearance of the BP played significant role in added damping and added stiffness, while pressure fluctuation at the outlet of the BP and circumferential velocity at the inlet of the BP had little effect. Moreover, total damping coefficient became negative when the phase of the pressure fluctuation was around π/2, but total mass and total stiffness kept positive under any conditions. Therefore, we concluded that the main reason of the unsteady vibration of the turbopump is negative damping because of certain pressure fluctuation at the inlet of the BP.
