The Efficacy of Air Injection in Mitigating Silt Erosion on Hydroturbine Blades: A Computational Study

Abstract

The present study proposes a distinctive solution to counter the development of hydroturbine blade erosion under the influence of silt-laden water stream by creating a partial air shield through air injection on the pressure surface of a hydrofoil. The multiphase interactions between incoming quartz particle-water suspension and injected air through a full-span slit positioned near leading edge of NACA 4412 hydrofoil is numerically investigated to identify the erosion wear behavior with and without air injection. To accurately predict the erosion distribution profile in the entrained external boundary layer regime, the Euler–Euler–Lagrange model is adopted along with the K-omega SST turbulence scheme. The initial simulations were carried out at a 20° angle of attack for water speeds ranging from 5 to 20 m/s and silt concentrations ranging from 1000 to 4000 ppm to study the silt erosion, and the results showed that the substantial erosion at the leading edge followed by minimal erosion on the bottom side of the guide vane. Further, the simulations were conducted over hydrofoil at angle of attack (AOA) 10° and 20° subjected to silt stream accelerating at a constant velocity of 5 m/s for varying injection rates ranging from 7.5 to 27.5 m/s, respectively. For realistic evaluation, the grain size samples were averaged experimentally and first three sizes were selected in relative proportions to form silt concentrations of 2500 and 5000 ppm. Moreover, to assess the sensitivity of erosion intensity on the guide vane, the air injection AOA was also varied at 30° and 90°. Results revealed that any incremental change in hydrofoil attack angle and ppm concentration is directly proportional to enhanced erosion rates and overall drag. The erosion rate intensity is found to be maximum on the leading edge which then decreases as the silt particles advances in the stream-wise direction after losing momentum during the first strike. An introduction of air injection then exhibits interesting insights, as the mitigation of particle trajectory is initially scattered in distinct horizontal patches up till a critical injection rate and later developed a vertical line-like distribution which is attributed to the rapid change in adverse pressure gradient. The predicted McLaury erosion rates and particle mass concentration were found to constantly decrease both in distribution locus and intensity at incrementing rates of injection. The aforementioned observations remained unchanged/showcased minimal change at different angles of injection, but the critical injection rate shifted to a higher value on increasing hydrofoil’s AOA. Almost 35% of hydrofoil surface is free from erosion pitting at critical injection rate, and about 70–85% surface region can be protected at higher injection rates as estimated numerically. This showcases effectiveness of air injection in attenuating erosion wear by forming a protective air blanket, and influencing particle trajectories at a small compromise of 6–9% decrease in overall pressure coefficient.