15th European Conference on Turbomachinery Fluid dynamics & Thermodynamics

Paper ID:

ETC2023-168

Main Topic:

Hydraulics Machine

https://doi.org/10.29008/ETC2023-168

Authors

Ted Ørjan Seim Gundersen  - University of Bergen, Norway; OneSubsea, A Schlumberger Company
Victor Moënne-Loccoz - IFP Energies nouvelles
Marine Dupoiron - IFP Energies nouvelles
Erik Andre Torbergsen - OneSubsea, A Schlumberger Company
Boris Balakin - Western Norway University of Applied Sciences
Bjørn Johan Arntzen - University of Bergen, Norway
Alex Christian Hoffmann - University of Bergen, Norway

Abstract

Rotodynamic pumps with open (unshrouded) helico-axial impellers are designed to pump mixtures of gas and liquid, for example when boosting an incoming well stream of oil and gas. Secondary flows can have a significant impact on the pump internal flow field, especially the tip vortex, arising from the leakage of fluid across the impeller tip. In a multistage setup, rotor-stator interaction adds further complexity and unsteadiness to the flow. Experimental data is essential to improve our understanding and characterize the internal flow in these pumps. Flow visualization by high-speed camera recordings can give an initial, qualitative understanding and serve to validate and improve numerical modeling techniques. Here we use high-speed camera recordings to describe the characteristics of the unsteady flow in a multistage helico-axial pump over range of relative flow rates from 135 to 45 % of the best efficiency point. A test rig with a transparent pump casing, comprising three low-specific speed stages has been commissioned in flow loop which allows a variety of pump inlet conditions with water and nitrogen as process fluids. About 2 % volume fraction nitrogen was injected at the inlet and used as tracers in the flow. Relative to previous studies found in the literature, the present work focuses on the flow field observed at very low and low gas fractions and brings novel insight regarding its dynamics and evolvement as the relative flow rate is varied. Recordings at 5000 frames per second of the second stage at a pump speed of 1500 and 2000 rpm reveal secondary flows increasing in size and intensity as the flow coefficient is reduced. At high relative flow rates, the flow through the impeller channels is relatively uniform and the tip leakage flow cannot be seen to affect the main flow. There is however a region of recirculating flow in the diffuser, apparently arising from hub corner separation. The dynamics of the recirculation zone depends on the passing of the downstream impeller blades. As the flow coefficient is reduced towards the best efficiency point and lower, the tip leakage flow becomes significant. This has two main consequences in terms of secondary flows. First, a tip vortex is identified behind the leading edge, and it remains visible at the impeller inlet until the next blade interferes. Remaining structures of the vortex travel downstream through the impeller channel. Secondly, below a certain flow coefficient, the tip leakage becomes strong enough so that it reaches into the upstream diffuser and significantly alters its flow. The impeller inlet conditions are therefore also affected as the incoming flow is deflected towards the hub. CFD simulations have been performed for comparison with flow visualization results. Unsteady RANS simulations reproduce the characteristic secondary flows and structures identified by high-speed video recordings. In the downstream part of the impeller channel however, the remaining structures of the tip vortex are quickly dissipated and disappear due to the nature of the RANS modeling approach. Scale-resolving simulations are required to replicate a wider spectrum of scales through the pump.



ETC2023-168




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