# VAPOR PRESSURE AND CAVITATION – 1 • Saturation temperature Ts: The temperature at which a pure substance changes phase at a given pressure. • Saturation pressure ps: The pressure at which a pure substance changes phase at a given temperature. • Vapour pressure (pv): The pressure exerted by its vapor in phase equilibrium with its liquid at a given temperature. It is identical to the saturation pressure ps of the liquid (pv = ps). • There is a possibility of the liquid pressure in liquid-flow systems dropping below the vapor pressure at some locations, and the resulting unplanned vaporization.

VAPOR PRESSURE AND CAVITATION – 2

VISCOSITY

Viscosity: A property that represents the internal resistance of a fluid to

motion or the “fluidity”.

Drag force: The force a flowing fluid exerts on a body in the flow

direction. The magnitude of this force depends, in part, on viscosity

A fluid moving relative to

a body exerts a drag

force on the body, partly

because of friction

caused by viscosity.

The behavior of a fluid in

laminar flow between two

parallel plates when the

upper plate moves with a

constant velocity.

Fluids for which the rate of deformation is

proportional to the shear stress are called

Newtonian fluids.

Shear stress

Shear force

 dynamic viscosity

kg/m  s or N  s/m2 or Pa  s

1 poise = 0.1 Pa  s

VISCOSITY Newton’s Viscosity Relation

The rate of deformation (velocity gradient)

of a Newtonian fluid is proportional to

shear stress, and the constant of

proportionality is the viscosity.

Variation of shear stress with the

rate of deformation for

Newtonian and non-Newtonian

fluids (the slope of a curve at a

point is the apparent viscosity of

the fluid at that point).

VISCOSITY Newtonian & Non Newtonian Fluids

Kinematic viscosity,

m2/s or stoke (1 stoke = 1 cm2/s)

The viscosity of liquids decreases and the

viscosity of gases increases with temperature.

VISCOSITY – 2

This equation can be used to calculate the viscosity of a

fluid by measuring torque at a specified angular velocity.

Therefore, two concentric cylinders can be used as a

viscometer, a device that measures viscosity.

L length of the cylinder

number of revolutions per unit time