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Why Water Wheel Opportunity Produces Cavitation Phenomenon

May 29, 2020

As a kind of hydraulic energy conversion equipment, the internal flow of water turbine is relatively complicated, which causes cavitation phenomenon in different parts of the turbine and the formation of cavitation in various conditions. Generally speaking, cavitation often occurs in the low-pressure area around the surface of the fluid or in the direction of sudden change and cavitation is a relatively wide range extending from the maximum cavitation point upstream and downstream. Generally speaking, cavitation and cavitation are divided into the following four cases.

1. Airfoil cavitation and cavitation

Airfoil cavitation and cavitation are caused by the pressure drop caused by the water flowing around the blades. The pressure on the back of the blade is often negative, and the pressure distribution is as shown below. When the pressure in the low-pressure area on the back drops below the ambient vaporization pressure, cavitation and cavitation occur. This cavitation and cavitation are closely related to the geometry of the blade airfoil section, so it is called airfoil cavitation and cavitation. Airfoil cavitation and cavitation are the main forms of cavitation and cavitation for counter-turbine turbines. Airfoil cavitation and cavitation are related to operating conditions. When the turbine is in non-optimal conditions, it will induce or aggravate airfoil cavitation and cavitation.


2. Gap cavitation and cavitation

Gap cavitation and cavitation are a form of cavitation and cavitation that occurs when water flow through a narrow channel or gap causes local velocity to rise and the pressure drops to a certain level, as shown in the following figure. Gap cavitation and cavitation mainly occur at the gap between the upper and lower labyrinth rings of the Francis turbine runner, the gap between the outer edge of the blade of the axial flow paddle turbine and the runner chamber, the gap between the blade root and the hub, and the end surface of the guide vane Gap.

3. Local cavitation and cavitation

Local cavitation and cavitation are mainly caused by sudden changes in local flow patterns caused by casting and processing defects that form surface irregularities, sand holes, pores, etc. For example, the local cavitation and cavitation of the paddle turbine generally occurs behind the uneven steps or local dimples connected to the runner chamber; the local cavitation and cavitation may also occur on the blade fixing screws and sealing screws At this point, it is caused by the recess or protrusion of the screw. The cavitation and cavitation damage after the discharge hole of the crown of the runner of the Francis turbine is also a kind of local cavitation and cavitation.

4. Cavitation and cavitation

Cavitation cavitation and cavitation are a unique type of vortex cavitation for counter-turbine turbines, especially counter-turbine turbines. When the counter-turbine is running under normal operating conditions, the outlet of the runner always has a certain circumferential partial velocity, which causes the water flow to rotate in the draft tube, forming a vacuum vortex belt. When the negative pressure in the center of the vortex belt is less than the vaporization pressure, the water flow will produce cavitation, and the rotating vortex belt generally periodically collides with the draft tube wall, causing cavitation and cavitation in the draft tube wall. Cavitation cavitation and cavitation.

Cavitation cavitation and cavitation are generally related to operating conditions. At larger loads, the shape of the vortex zone in the draft tube is cylindrical, as shown in the figure below, it is almost coaxial with the center line of the draft tube, and the diameter is relatively small and stable. Especially under the optimal working conditions, the vortex zone can even disappear. However, under low load, the cavity vortex belt is thick and spiral, and it is also rotating. This eccentric spiral vortex belt is extremely unstable in space and will cause strong cavity cavitation and cavitation.


These are the four cases of cavitation and cavitation. In Francis turbines, airfoil cavitation and cavitation are the main occurrences, while rotary turbines are mainly gap cavitation and cavitation. Among these four types of cavitation and cavitation, airfoil cavitation and cavitation are the most common and serious cavitation and cavitation phenomena. Therefore, everyone should pay attention to this in the usual inspection, repair and maintenance.