Cavitation

Cavitation is one of major problems facing pump and inducer designers. Cavitation occurs in liquids when the local pressure is reduced to the vapor pressure of the liquid. Water, one of the most common working fluids, has a high propensity to cavitate under common operating conditions. Pumps and inducers are often damaged by the collapsing micro-bubbles that are formed by the cavitation process and efficiency can deteriorate severely.

ANSYS CFX software includes a state-of-the-art cavitation model based on a volume-of-fluid (VOF) approach [1]. This robust model allows the prediction of the cavitation size and location, very accurately. The animation, below, shows the evolution of the vapor pocket and the corresponding head drop curve at design point for the SHF pump.

Figure 1: Cavitation evolution and corresponding drop curve for the SHP centrifugal pump.

Usually, an inducer is placed upstream of a pump to resist the cavitation. For this purpose, an inducer has a small number of blades (generally 3 blades) with long chords and a very high stagger angle. This particular design makes simulating the flow field through this type of rotating machinery very challenging even without cavitation modeling. Using the VOF model, the flow field has been predicted at design and off design regimes under cavitating conditions [2].

Design Conditions
The shape of the cavitation pockets was compared to the experimental visualizations at design flow rate following the drop curve shown in figure 2-1. Figure 2-2 illustrates the general development of the cavitation in the inducer corresponding to the four operating points in figure 2-1. For the computed figures, the hub is colored in green, the blade in red and the cavitation zone in blue. This zone corresponds to a 10% of vapor. As shown in this figure, the vapor first appears near the shroud at the leading edge of the suction side of the blade (point 1). While the NPSH (cavitation number) decreases, the cavity remains attached to the suction side of the blade but growing towards the blade to blade channel and down towards the hub (point 2). At point 3, the vapor pocket passes to the pressure side of the blade inducing a performance breakdown. At point 4, a sudden drop of the performances appears. This is due to the blockage phenomenon generated by the presence of the pocket in the blade to blade channel from the tip to the hub and also in both side of the blade.

	Location Experiments CFX -Prediction Point 1 Point 2 Point 3 Point 4
Figure 2-1: Head drop curve at design conditions	Figure 2-2: Cavitation evolution vs cavitation number at design conditions

Off design conditions
Figure 3-1 presents the most representative images. One can thus identify:
a) In partial flow rate, backflow vortex cavitation returning upstream of the inducer (figure 3-a).
b) At the design flow rate, the cavitation attached to the blade (figure 3-b).
c) For high flow rate, stable cavities developed on both sides of the blade, characterizing the blockage phenomenon. The development of cavitation is almost identical in the three passages of the inducer (figure 3-c).
For these three types of cavitation, the predicted incipient cavitation number corresponding to 3% of the head drop compares very well the experiments (figure 3-2).

Cavitation type Experiments CFX-Predictions Backflow vortex cavitation Figure 3-a Blade Attached cavitation Figure 3-b Blockage phenomenon Figure 3-c
Figure 3-1: Cavitation type	Figure 3-2: Flow coefficient vs the incipient cavitation number corresponding to 3% of head drop

Transient cavitation
The cavitation model was also tested for transient cavitation conditions [2] and [3]. The turbulence model was modified to induce the unsteadiness of the transient flow. Figure 4 shows an animation of the vapor clouds detaching and collapsing. This model will be applied to predict the transient cavitation in pumps and inducers.

Figure 4: Animation of volume fraction of vapor at different times from ref. [2]

[MPG - 3.3 MB]

References
[1] Bakir, F., Rey, R., Gerber, A. G., Belamri, T. and Hutchinson, B., “ Numerical and experimental investigations of the cavitating behaviour of an inducer”, International Journal of Rotating Machinery, 10:15-25,2004.
[2] Zwart, P., Belamri, T. and Gerber, A. G., “ A Two-Phase Model for Predicting Cavitation Dynamics”, ICMF 2004, Yokohama, Japan, May 30-June 3, 2004. Paper No.152.
[3] Bouer, W., Iben, U. and Vob, M., “ Simulating cavitating flow in injection system”, CFX-USER Conference 2004, Germany.

Brochure: Robust Cavitation Modeling

Photographs courtesy of Lemfi.