Vortex cavitation

The effect of polymer solutions, ejected at very small flow rates at the tip of a finite span hydrofoil, on the inception and extent of tip vortex cavitation is investigated. The results show that the dessinent cavitation numbers are significantly reduced by polymer additives as compared with those in pure water. Moreover, for operating conditions well below critical, the presence of polymer in the vortex core results in the nearly complete elimination of the vapour cavity. These effects can be interpreted on the basis of a modification of the velocity field due to the viscoelastic properties of the polymer solutions. L.D.A. measurements indicate a large decrease of the tangential velocity component in the transition region between the inner solid body rotation co.e and the outer potential vortex flow. Also, a change of the slope of the velocity within the core is observed. 

Experimental evidence on tip vortex cavitation inhibition by polymer solution ejection at the tip of a finite span hydrofoil has been presented. The phenomenon is shown to be directly related to the modification of the tangential velocity profiles of a non cavitating vortex during polymer solution ejection as compared to pure-water flow. Indeed, integration of... 

Inge, C. and Bark, G. Tip vortex cavitation in water and in dilute polymer solutions. Technical Report TRITA-MEK-83-12, Royal Inst, of Technology, Stockholm, Sweden. (1983). 

Fruman, D.H. Tip vortex cavitation inhibition by polymer additives. Proceedings of the A.S.M.E. Cavitation and Polyphase Flow Forum, New-Orleans. (1984). 

Vortex cavitation occurring on the tips of rotating blades is known as tip vortex cavitation. Cavities form in the cores of vortices in regions of high shear. This type of cavitation is not restricted to rotating blades. It can also occur in the separation zones of bluff bodies. 

Ezure, T., et al., 2013. Effects of fluid viscosity on occurrence behavior of vortex cavitation — vortex strucmres and occurrence condition. In Proceedings of NURETH-15, Pisa, Italy, May 12-17, 2013. 

Fig. 4. Typical design elements foi wet deagglomeiation in low viscosity systems (a) a high, ipm lotoi (shown below its normal position within stator) produces turbulence and cavitation as blades pass each other (b) a rotating disk creates a deep vortex to rapidly refresh the surface, and up- and downtumed teeth at the edge cause impact, turbulence, and sometimes cavitation and (c) the clearance of a high rpm rotor can be reduced as the batch...

Internal Flow. Depending on the atomizer type and operating conditions, the internal fluid flow can involve compHcated phenomena such as flow separation, boundary layer growth, cavitation, turbulence, vortex formation, and two-phase flow. The internal flow regime is often considered one of the most important stages of Hquid a tomiza tion because it determines the initial Hquid disturbances and conditions that affect the subsequent Hquid breakup and droplet dispersion. 

The characteristic time-scales mentioned above take into account some but not all practical considerations. For example, really intense stirring (rpm > 500) in the CSTR is not recommended for in situ studies since a deep vortex ivill be formed in the liquid, gas ivill be entrained, and tivo-phase flow w ill occur in the recycle line. Also, two-phase flow will generally cause cavitation in a mechanical pump (possibly stopping flow) and induce irreproducible spectroscopic measurements. 

Many draw nozzles, especially those in the bottom of vessels, plug because of the presence of vortex breakers. Many designers routinely add complex vortex breakers to prevent cavitation in pumps. But vortex breakers are needed only in nozzles operating with high velocities and low liquid levels. Corrosion products, debris, and products of chemical degradation can more easily foul and restrict nozzles equipped with vortex breakers. 

Bottom liquid outlets. Sufficient residence time must be provided in the bottom of the column to separate any entrained gas from the leaving liquid. Gas in the bottom outlet may also result from vortexing or from forthing caused by liquid dropping from the bottom tray (a waterfall pool effect). Vortex breakers are commonly used, and liquid-drop height is often restricted. Inadequate gas separation may lead to bottom pump cavitation or vapor choking the outlet line. 

Vortex Breaker A desnce located inside a vessel at the outlet connection. Generally consisting of plates welded together to form the shape of a cross. The vortex breaker prevents cavitation in the liquid passing through the outlet connection. 

Cavitation in pumps] pump rpm too fast/suction resistance too high/clogged suction line/suction pressure too low/liquid flowrate higher than design/en-trained gas/no vortex breaker. 

Figure 2 - Backscattered light signature from tip vortex during for 3=9° and U = 4.46 m/s. (a) Cavitation without ejection (b) With polymer ejec-tion q. = 9 10"7 mVs and (c) with polymer ejection q. = 1.48 10"° m /s.

The tangential velocities weremeasured at a station. 20 m downstream from the tip of the wing. This distance was selected because it was expected that the whole fluid has been set in rotation as a result of the roll-un at the tip and that vortex diffusion was still limited [7]. Measurements were conducted, for non cavitating conditions, at a free stream velocity of 5 m/s, two incidence angles, 5 and 10°, and five ejection conditions. 

If this condition is verified, this risk of cavitation in the inlet pipe to the pump is avoided. On the other hand, engineering rules for positioning the suction depth /(depth are provided in order to avoid drawing air in, which may happen if a vortex is generated in the lower basin by the suction. Such rules compare the suction depth / depth to the diameter of the pipe, the dimensions of the basin, and the distance from the suction port to the basin walls. 

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