VAPOUR PRESSURE AND CAVITATION

We were discussing the basic definition and significance of Kinematic viscosity, Dynamic viscosity, various properties of fluid, type of fluids, Newton’s law of viscosity, Compressibility and bulk modulus and also capillarity, capillary rise and capillary depression in our previous posts. 

We will discuss here the basic concept of vapour pressure and cavitation with the help of this post.

Let us first discuss the concept of vapour pressure

Let us consider a vessel which is contained with liquid as displayed here in following figure. Let us think that we have taken water here in the vessel as liquid at atmospheric pressure and 20⁰C temperature.

Now we are providing the heat energy to the vessel with the help of heat burner. As we are heating the vessel, water will be heated and temperature of water will be increased. 

Evaporation process of water molecules will be started at temperature of 100⁰C as water is stored inside the vessel at atmospheric pressure. Water molecules will be evaporated i.e. water molecules will be converted in to vapour molecules and these vapour molecules will be accumulated within the space between free surface of water and top of vessel. 

These accumulated vapours will apply a pressure over the surface of the water. Pressure applied by the accumulated vapours over the free surface of liquid will be termed as vapour pressure of the liquid. 

Therefore, vapour pressure of liquid could be defined as the pressure at which liquid will be converted in to vapours. 

Now let us consider the same liquid i.e. water contained in the vessel at same condition i.e. at atmospheric pressure and 20⁰C temperature. Let us think that if we reduce the pressure above the surface of water by anyway, boiling point of liquid i.e. water will also be reduced. Therefore water may boil at a temperature of 20⁰C, if we are reducing the pressure above the surface of water. 

Therefore a given liquid may boil even at a normal temperature, if we are reducing the pressure above the surface of liquid below or equivalent to the vapour pressure of liquid at that normal temperature.

Let us discuss now the concept of cavitation

Let us consider that we have one pipe line and water is flowing through this pipe line. As we know that, vaporization of liquid will be started if pressure at any point in flowing liquid becomes equal to or less than the vapour pressure of flowing liquid.

Let us consider one low pressure region in pipe line through which water is flowing. Water will be converted in to vapour in this low pressure region and these vapour bubbles will be carried by this flowing liquid. 

These vapour bubbles will be collapsed and will develop a tremendous rise in pressure, once they will reach in high pressure region. Due to tremendous rise in pressure, material from the wall of pipe line will be eroded and there will be formation of cavities over the surface of pipeline. Such a phenomenon will be termed as cavitation. 

Cavitation could be defined as the phenomenon of formation of vapour bubbles in flowing liquid in a region where the pressure of liquid falls below or equivalent to the vapour pressure of flowing liquid and sudden collapsing of these vapour bubbles in a high pressure region. 

Sudden collapsing of these vapour bubbles will develop tremendous rise in pressure and the metallic surface, over which liquid is flowing, will be subjected with this tremendous rise in pressure.  

Therefore metallic surface, over which liquid is flowing, will be subjected with the pitting action or in simple there will be developed small-small holes over the metallic surface. 

We will now discuss the basic concept of Pascal's Law, in the category of fluid mechanics, in our next post.

Reference:

Fluid mechanics, By R. K. Bansal

Image Courtesy: Google

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