Effects of Clearance and Operating Conditions on Tip Leakage Vortex-Induced Energy Loss in an Axial-Flow Pump Using Entropy Production Method

Tip leakage flow (TLF) is a typical flow phenomenon in the internal flow of axial-flow pumps that has a serious impact on their safety and stability. In this study, numerical simulations are performed to investigate the influence of various tip clearances and operating conditions on the characteristics of the tip leakage vortex (TLV) and energy loss of a prototype of a vertical axial-flow pump. First, based on entropy production theory, the TLV-induced energy loss is quantitatively studied. The entropy production rate caused by turbulence dissipation (EPTD), which is caused by pulsating velocity, contributes the most to the total energy loss. The EPTD at the impeller is principally distributed on the leading edge of the blade due to the influence of the tip clearance. Then, the spatial shape and trajectory of the core of the TLV are discussed, and their correlations with pressure and vorticity are investigated to reveal the spatial distribution characteristics and formation mechanism of TLVs. With increasing tip clearance, the trajectory of the vortex core extends radially outward, and the low-pressure area near the blade tip is consistent with the trajectory of the core of the TLV, which accompanies high vorticity. Fundamentally, pressure gradients and flow separation at the leading edge are the root causes of the TLVs. Lastly, the spatial evolution of TLVs under different calculation schemes is discussed by utilizing the vorticity transport equation, demonstrating that the Coriolis force (CORF) is the main factor that affects the location of a TLV, whereas the vorticity stretching term (VST) has a greater influence on the vorticity variation rate of the TLV than the CORF and plays a predominant role in the spatial development of the TLF.