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» What is cavitation?

Cavitation is the creation of low-pressure vapour/gas filled cavities in a fluid.

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The creation of cavities becomes more difficult as the pressure increases downstream of the cavitation generator. The initial focus of
Whirlwind International was the application of cavitation in oil and gas wells for the removal of dirt and scale.
Consequently the R&D of Whirlwind International focused on generating cavitation under high-pressure conditions, in
principle up to 500 bar.

Whirlwind International uses fluid driven vibration instruments, also called hydro-vibrators, to produce a flare of cavitation
bubbles. These bubbles violently implode once they leave the direct influence area of the hydrovibrator.
The implosions cause suction and vibration effects that dislodge dirt in the inflow area of a
well or scale deposits on the wall of a well.

Hydrodynamic cavitation
The principle of cavitation is shown in the picture below, where fluid flow from the left hits a static ball
on a stem:


Figure 1: Hydrodynamic cavitation development

The next picture shows the collapse of a single bubble in contact with a wall:


The experiment with the static ball in the above picture is performed at near-atmospheric pressure
conditions, i.e. with a fluid pressure around the ball of ca 1 bar absolute. It is relatively easy to create
cavitation at such conditions.
As can be seen in the right hand picture of figure 1, a cloud or flare of bubbles is generated at high
fluid velocities. With further increase of the fluid flow rate the flare may change in the centre to a single
cavity core; this condition is often referred to as power (super) cavitation.
The cavitation effect and the size of the flare will dramatically decrease with an increase of the fluid
pressure around the ball. At pressures encountered in a deep well the cavitation effect reduces to nil
or insignificant. This holds in principle true for all static barriers, such as nozzles, weirs or narrow
openings.

Technology details
The picture below provides an overview of the working principles of the vibration tools of Whirlwind International:


Figure 3: Working Principles

Fluid is forced through a vibration instrument. Narrowing of the flow path in the vibrator instrument
causes a loose body to vibrate. With increasing flow rate of the fluid, turbulence becomes so strong
that tiny cavitation nuclei are formed. These nuclei grow immediately downstream of the nucleation
zone into a cloud of bubbles in a similar way that is shown in the static experiment of figure 1.
Further away from the moving body the pressure conditions are such that the bubbles can no longer
grow and eventually the bubbles collapse. This creates implosions, strong vortices and shock waves.
The visible effect of the cloud of growing and collapsing bubbles is referred to as the flare (plume - or
in Dutch: PLUIM).

Power Cavitation
Cavitation created by static bodies or barriers in a fluid stream is referred to as passive cavitation.
Power cavitation is a form of cavitation that is stimulated by specific enhancement effects, as
illustrated in the picture below:


Figure 4: Power cavitation

The enhancement effect can be best explained by visualizing that the vibrator tool generates
cyclic full shut off of the flow (so-called ‘stop-flow’). This causes a cyclic water hammer
shockwave that enhances cavitation at moments that the (reflected) high-pressure peak of the
shockwave hits the stop-flow vibrator and weaker cavitation as the low-pressure peak ‘passes by’.
This effect causes cyclic bursts of stronger and weaker cavitation compared to passive
cavitation.
 
The value of Power Cavitation is twofold: it will be possible to generate cavitation at higher
downstream pressure and stronger cavitation at ‘normal’ downstream pressure conditions than is
achieved with passive cavitation. Due to Power Cavitation
new applications are being unlocked, such as enhanced killing power for bacteria and possibly
even viruses (bug killing applications).

Most hydrodynamic tools on the market claiming a cavitation effect are ‘passive’. With passive tools
strong cavitation may be generated at near atmospheric conditions that however dies away very
quickly with increasing downstream pressures. Usually the demonstrations for tools with passive
cavitation are based on effects only noticeable at near atmospheric pressure, thus potentially
misleading a customer for the use of such tools in high-pressure environments as encountered in deep
wells. Our R&D has focused on demonstrating the technology at representative pressure conditions.


Figure 5: System Parameters

 
 
 
Ontwikkeling & Realisatie: Sandr