Development of a vortex risk index for use with single-phase computational fluid dynamics in the simulation of intake structures

Abstract

A new method is developed for assessing the risk of undesirable vortex formation at pump intakes. The method addresses industry demand for a vortex classification system inferred from CFD modelling results. A new vortex risk index is the key result of this research; the numerical value of the index is equivalent to the vortex classification in ANSI/HI-9.8 (2012) standard for interpretation of results of physical modelling test results. The main difference between the present and preceding research is that instead of validating a CFD model that simulates a vortex with some level of accuracy, the focus is on: (i) finding metrics that distinguish acceptable from unacceptable vortices in single-phase CFD models; and (ii) proposing a generalised model setup for intake CFD modelling across different industrial scales. This research was conducted in four parts: • 42 laboratory experiments were conducted to produce all 6 vortex types identified in ANSI/HI-9.8 (2012); • a new two-phase CFD model of air-core vortices was developed, however the computational demands and time to obtain an unsteady solution, render it impractical for current engineering practice, • 42 single-phase CFD models were run to simulate the experimental tests; and • laboratory results, two sets of external prototype data, and CFD results were processed and a new vortex risk index (To) produced for CFD modelling results. The main research results were: • from the physical experiments, a new scaled submergence Froude number (FSS) was produced indicating that when FSS > 0.13 an air-core vortex will form, but no major vortex expected when FSS < 0.084. • Since any remedial measures not involving a change in submergence do not produce a change in FSS, a parameter capable of detecting the change is required, and from single-phase CFD modelling results, a new vortex risk index (To) is developed and shown to correlate strongly with vortex strength and type. Three different risk zones for the formation of vortices are defined based on To, to assess the vortices observed in CFD modelling. If To <1.27 there is a low risk of vortex formation, but if To >3.45 then the risk of vortex formation is high, and anti-vortex measures need to be implemented.