Understanding the effect of temperature on the viscosity of the fluid is very important. As the temperature of the liquid fluid increases its viscosity decreases. In gases its opposite, the viscosity of the gases fluids increases as the temperature of the gas increases.
Importance of Temperature for the Fluids

The viscosity is the property of the fluid that resists the flow of the fluids like liquids and gases. Understanding the effect of temperature on the viscosity of the fluid is very important. In engines the lubricating oil is heated to very high temperatures due to combustion of the fuel, hence it is vital to know whether the lubricating oil, which is a fluid, will be viscous enough to be able to carry out the lubrication of the moving parts of the engine at those high temperatures.

There are chances that at high temperature the fluid may loose its viscosity, which may render it useless when used in high temperature applications like lubrication of the engine. In some cases the fluid may even start evaporating at high temperature. On the other hand, in the refrigerating compressors there is very low temperature on the suction side of the compressor and high temperature on the discharge side of the compressor. Hence the fluid used as the lubricant in the refrigerating compressors should be able to maintain its viscosity as high as well as low temperatures.

Whenever the fluid is used for any application, operating temperature should be given due considerations. The fluid that can sustain those operating temperatures should only be selected for those applications; otherwise the very purpose of using the fluid will be lost.

Effect of Temperature on Liquid and Gas Fluids

Let us see the effect of temperature on liquids and gases fluid:

1) Liquids: As the temperature of the liquid fluid increases its viscosity decreases. In the liquids the cohesive forces between the molecules predominates the molecular momentum transfer between the molecules, mainly because the molecules are closely packed (it is this reason that liquids have lesser volume than gases. The cohesive forces are maximum in solids so the molecules are even more closely packed in them). When the liquid is heated the cohesive forces between the molecules reduce thus the forces of attraction between them reduce, which eventually reduces the viscosity of the liquids.

The liquids used as the lubrication fluid and for number of other applications should be selected properly considering the operating temperatures. At high temperatures the liquids loose viscosity; hence in the engine the fluid used for lubrication should be such that it should be able maintain its viscosity even at the high temperatures. At low temperatures the viscosity of the fluid increases, hence in the refrigerating compressor the fluid selected for the lubrication should be such that it is able to maintain value of viscosity at the lowest and highest temperatures inside the compressor.

For liquids: µ = µo/ (1 + αt + βt2)

Where: µ - Viscosity of the liquid at t degree Celsius n poise

µo – Viscosity of the fluid at 0o Celsius in poise

α, β – are the constants

2) Gases: In gases there is opposite phenomenon. The viscosity of the gases increases as the temperature of the gas increases. The reason behind this is again the movement of the molecules and the forces between them. In the gases the cohesive forces between the molecules is lesser, while molecular momentum transfer is high. As the temperature of the gas is increased the molecular momentum transfer rate increases further which increases the viscosity of the gas.

For gases: µ = µo + αt + βt2

Reference

Book: Fluid Mechanics and Hydraulic Machines by Dr. R. K. Bansal