Effects of Tip Clearance and Surface Roughness on Small-Scale Turbopump Impeller Performance

Abstract

Centimeter-scale turbopump impellers typically used in liquid rocket engines of small launch vehicles suffer from reduced performance due to manufacturing challenges and nonuniform geometric scaling. This thesis aims to characterize the impact of impeller blade tip clearance and surface roughness on the performance of small-scale turbopump impellers by assessing the dominant flow features, quantifying the underlying loss mechanisms, and determining the sensitivity of performance losses to changes in tip clearance and surface roughness. The study identifies the primary flow features governing impeller performance to be blade tip leakage flow and secondary flow. The analysis identified two distinct flow regimes based on tip clearance: above 5% of tip clearance, the losses are predominantly due to blade tip leakage flow, whereas below this threshold, losses are governed by both secondary flow and blade tip leakage flow. For tip clearances above 5% of the blade span, blade tip leakage flow is estimated to contribute more than 80% of total impeller loss. A 1% change in tip clearance is estimated to result in a 0.8% loss in efficiency. The calculations suggest increasing surface roughness reduces the effective tip clearance due to increased viscous effects in the tip gap, but strengthens the secondary flow. This lowers the effective tip clearance that separates the flow regimes. The contribution of blade tip leakage loss to total impeller loss decreases by up to 22% for surface roughness increased from an Rₐ value of 1 µm to 10 µm. The strengthened secondary flow at higher surface roughness increases mixing of the blade tip leakage flow with the blade passage flow, leading to larger regions of blockage. Increasing the surface roughness from an Rₐ value of 1 µm to 10 µm results in a 4% loss in impeller efficiency. This study demonstrates that surface roughness is more impactful on small-scale impeller performance than blade tip clearance, and so manufacturing for smooth surfaces should be prioritized over reducing the blade tip clearance gap.