CHE215 - Lecture 1

Fluid

A substance that deforms continuously when acted on by shearing stress of any magnitude.

Shearing Stress

• force per unit area

• created whenever a tangential force acts on a surface.

Common solids being acted on by shearing stress

• will initially deform but will not continuously deform or flo

Rheology

Study of Materials

Applied shearing stress is small

Material behave as a solid

Stress exceeds the critical value or large shearing stress

Substance will flow

Fluid molecules

• High degree of motion freedom

• Assume shape

• Occasionally occupy the volume

Fluids may include:

1. Gases

2. Liquids

3. Any form oof matter than behave as a fluid

4. Mixture of two or more of the three

Gases

• Highest degree of molecular motion freedom

• Lightest molecular weight

• Occupy the volume

Liquids

• Have hindered yet enough molecular motion

• Assume shape but not the volume of the container

Macroscopic values

• Average value of the quantity under consideration

• Average behavior of molecules in a given volume

Fluid is treated as continuum

All the fluid characteristics of interest vary continuously throughout the field

Density

• Mass per unit volume

• Used to characterize the mass of a fluid system\

• p = m/v

Specific volume, v

• The volume per unit mass of the fluid

• v = 1/p

Density, temperature

The sensitivity of ___ to ___ makes it a critical physical property in fluid flow.

Specific Weight

• Weight per unit volume of a fluid

• Used to characterize the weight of a fluid system

• γ = ρg

Specific Gravity, SG

• Ratio of density of the fluid to the density of water at a specified temperature

• 4C and 1000kg/m³

• SG = ρ/ρ_{water at 4C}

Viscosity

• The fluid between the two plates moves with velocity u = u(y) that would be found to vary linearly, u = U_y / b

• Velocity is constant since du/dy = U / b

Velocity gradient

• du/dy

• developed in the fluid between the plates

Shear Strain

Rate of angular deformation

Newtonian Fluids

Shear stress is linear to the shear strain

Apparent Viscosity, U_app

Slope of the shear stress vs. rate of strain

Shear Thinning Fluids or Pseudoplastic

• Apparent viscosity decreases with increasing shear rate

• The harder the fluid is sheared, the less viscous it becomes.

Shear thickening fluids or dilatant

• Apparent viscosity increases with increasing shear rate.

• The harder the fluid is sheared, the more viscous it becomes.

Compressibility (e_v)

The measure of the changes in volume and density in response to changes in the normal forces acting on the volume.

Bulk Modulus (E_v)

A measure of how easily the volume of a particular fluid can be changes when there is a change in pressure. A measure of compressibility of a fluid.

Large bulk modulus value

Fluid is relatively incompressible

Vapor Pressure

Pressure exerted by the vapor at the liquid surface when the vapor phase is in equilibrium with the liquid phase.

Boiling point

Temperature at which the vapor pressure is equal to prevailing pressure

Cavitation

• Formation of bubbles in pumps

• Vapor pressure goes down at a certain level while force goes up

Speed of Sound

Disturbances introduced at some point in the fluid propagate at a finite velocity.

Sonication or Ultrasonication

• Speed up dissolution by breaking up intermolecular interaction

• Provide energy for chemical reaction

• Stirring when mechanical stirring is not possible

• Degassing under vacuum

Viscosity in liquids vs. gases

• Liquids: Viscosity decreases with an increase in temperature

• Gases: Viscosity increases with an increase in temperature