Session: 8.3.2 - Pumping Machinery II

Paper Number: 168513

168513 - Experimental and Computational Investigation of Balance Hole Flow Coefficient for Predicting Axial Thrust in an Unshrouded Centrifugal Impeller 

Abstract: 

In centrifugal impellers, axial thrust imbalance can be compensated by incorporating balancing holes that create a recirculation flow from the back to the front of the impeller. These balancing holes increase the leakage flow to the back of the impeller, thereby reducing the pressure distribution at the back and adjusting the axial thrust. In rocket turbopumps, although balance holes cause flow disturbances by interacting with the main suction side flow, they provide benefits such as allowing greater clearance at the balance piston inlet and facilitating rapid pressure reduction in the balance piston chamber during transient shutdown conditions. The design of balance holes is therefore critical to axial thrust control in turbopumps.
However, balance holes involve complex internal flow, and their flow characteristics depend on the pressure distribution on both the front and back sides of the impeller. Particularly, estimating the internal recirculating flow through the balance holes is essential to validate predictions of the flow coefficient of balance holes. However, despite their importance, experimental verification of the effects of balance holes is limited. Most previous studies have focused on simplified models, such as the holes in rotating disks, rather than on actual turbopump impeller systems. Experimental and analytical investigations into the effects of balance holes on their internal flow and the influence on axial thrust have been insufficiently investigated.
In this study, we investigated the internal recirculating flow characteristics and axial thrust behavior in a model impeller with an axial thrust balancing system, simulating a turbopump impeller. The experiments were conducted using a closed-loop water pump test rig. An unshrouded impeller was controlled at specific axial positions using an active magnetic bearing system, and pressure distributions and axial thrust variations were measured under different leakage flow conditions. Additionally, a cobra probe was used to measure velocity at the impeller outlet to quantify the recirculation flow through the balance holes. To further analyze the detailed internal flow affected by balance holes, CFD simulations were conducted to compare with experimental results.
The results indicated that increasing the leakage flow along the back of the impeller resulted in a decrease in the balance hole flow. The experimental results qualitatively agreed with the CFD results for all balance hole configurations. This was attributed to increased pressure loss at the orifice passing the leakage flow, which lowered the pressure distribution at the back side of the impeller and thus reduced the pressure difference across the balance holes. In addition, increasing the diameter or number of balance holes resulted in a more significant pressure reduction on the impeller back side, suggesting that modifying balance hole geometry is effective in adjusting axial thrust.
Furthermore, the flow coefficient of the balance holes was derived from the differential pressure at the inlet and outlet of the balance holes in the CFD results. The coefficient remained nearly constant regardless of the hole diameter or number and was organized based on the velocity ratio inside the balance holes. We confirmed that the flow coefficient of balance holes can be uniquely predicted irrespective of variations in leakage flow or rotational speed. These findings contribute to improving the design of balance holes for more effective axial thrust management in turbopumps.

Presenting Author: Kento Sakai Waseda University

Presenting Author Biography: