MEEN 3240 MEEN 3240 Laboratory Exercise #6 Liquid Viscosity Lab #6 Objectives: T

Question

MEEN 3240 MEEN 3240 Laboratory Exercise #6 Liquid Viscosity Lab #6 Objectives: The measurement of viscosity by two different methods the Zahn Cups and the rotational viscometer will allow students to intuitively understand the effects of viscosity on fluid flow and will gain the ability to assess the effectiveness of different viscosity measurement techniques. In addition to concepts such as fluid viscosity and drag force, concepts addressed in this lab include Stokes' Law and the Reynolds number.

Explanation / Answer

Liquid Viscosity is nothing but a fluid friction. Like friction between moving solids. Viscosity transforms kinetic energy of motion into heat energy.

Viscosity is an important fluid property when analyzing liquid behavior and fluid motion near solid boundaries. More technically we can say that Viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress. The shear resistance in a fluid is caused by inter molecular friction exerted when layers of fluid attempt to slide by one another.

Viscosity is a property of the fluid which opposes the relative motion between the two surfaces of the fluid in a fluid that are moving at different velocities.

There are two type of liquid viscosity - 1. Dynamic Viscosity 2. Kinematic Viscosity

1. DYNAMIC OR ABSOLUTE VISCOSITY - coefficient of absolute viscosity, is a measure of internal resistance. Dynamic (absolute) viscosity is the tangential force per unit area required to move one horizontal plane with respect to an other plane, at an unit velocity, when maintaining an unit distance apart in the fluid.

The magnitude F of this force is found to be proportional to the speed u and the area A of each plate, and inversely proportional to their separation y:

The proportionality factor in this formula is the viscosity (specifically, the dynamic viscosity) of the fluid, with units of Pa-s i.e Pascal second

In the SI system the dynamic viscosity units are N s/m2 Pa s or kg/(m s) - where 1 Pa s = 1 N s/m2 = 1 kg/(m s)

2. KINEMATIC VISCOSITY - Kinematic viscosity is the ratio of absolute (or dynamic) viscosity to density. It can be obtained by dividing the absolute viscosity of a fluid with the fluid mass density.

In the SI-system the theoretical unit of kinematic viscosity is m2/s - or the commonly used Stoke (St) where

1 St (Stokes) = 10-4m2/s = 1 cm2/s

Stoke comes from the CGS (Centimetre Gram Second) unit system.

Since the Stoke is a large unit it is often divided by 100 into the smaller unit centiStoke (cSt) - where

1 St = 100 cSt

1 cSt (centiStoke) = 10-6 m2/s = 1 mm2/s

1 m2/s = 106 centiStokes.

Viscosity and Reference Temperature - The viscosity of a fluid is highly temperature dependent - and for dynamic or kinematic viscosity to be meaningful the reference temperature must be quoted. In ISO 8217 the reference temperature for a residual fluid is 1000C. For a distillate fluid the reference temperature is 400C.

for a liquid - the kinematic viscosity decreases with higher temperature

for a gas - the kinematic viscosity increases with higher temperature

Measuring Viscosity - Three types of devices are used to measure viscosity

1.Capillary tube viscometer

2.Saybolt viscometer

3.Rotating viscometer

In coating industries, viscosity may be measured with a cup in which the efflux time is measured. There are several sorts of cup – such as the Zahn cup.

Related Questions

Navigate

• Browse (All)
• Browse M
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.