Fluid Properties

Fluid PropertiesFluid Properties

CVEN 212Spring 2015

Riyadh Al-Raoush, PhD, PE

Fluid PropertiesFluid Properties

System, Extensive & Intensive Properties Mass and Weight Relationships between Pressure and volume

Ideal Gas Law Flow of Heat Bulk Modulus of Elasticity

Viscosity Vapor Pressure Surface Tension

Definition of a FluidDefinition of a Fluid

“a substance that deforms continuously when subjected to a shear stress, no matter how small that shear stress may be” - Streeter, Wylie, Bedford

System,Extensive & Intensive Properties

System,Extensive & Intensive Properties

Extensive properties related to the total

mass of the system represented by upper-

case letters e.g., M: mass ; W:

weight

Intensive properties independent of the

amount of fluid designated by

lowercase letters e.g., p: pressure; :

density

System: a given quantity of matter

Properties involving mass & weight


Specific Weight, weight per unit

volume [force/volume] Appendix b and c

Mass Density, mass per unit volume [mass/volume] Appendix b and c

= g



Physical PropertiesPhysical Properties



Specific Gravity, S ratio of specific weight

of a given fluid to the specific weight of water at a standard reference temperature

[-] independent of units!

Find gasoline, given Sgasoline= 0.75 water= 9810 N/m3

Sgasoline = gasoline /water

gasoline= 7357.5 N/m3





Equation of State for Gases(Ideal Gas Law)

Equation of State for Gases(Ideal Gas Law)

Equation of state for an ideal gas p= RT

R the gas constant, Tables give value of R for various gases

to determine the mass density of the gas = p/RT

P: absolute pressure : mass densityT: absolute temp [K or R]

Properties involving the flow of heat

Properties involving the flow of heat

Specific Heat, c describes the capacity

of a substance to store thermal energy

for gases: cv: specific volume

remains constant cp: pressure held

constant

Specific Internal Energy, u energy that a substance

possesses because of the state of the molecular activity for ideal gas, u is a

function of T only

specific enthalpy, h h=u+p/ function of T only

Bulk Modulus of ElasticityBulk Modulus of Elasticity

Relates the change in volume to a change in pressure

measures the “compressibility” of the fluid

pressure waves

VdVdpEv /

Ev: bulk modulus of elasticity dp: incremental pressure

change V: fluid volume dV: the incremental volume

change

Fluid Deformation between Parallel Plates

Fluid Deformation between Parallel Plates

Side view

Force F causes the top plate to have velocity U.What other parameters control how much force is required to get a desired velocity?

Fy

U

Fluid ViscosityFluid Viscosity

Examples of highly viscous fluids ______________________(Run a Video)

Fundamental mechanisms Gases - transfer of molecular momentum

Viscosity __________ as temperature increases. Viscosity __________ as pressure increases.

Liquids - cohesion and momentum transfer Viscosity ____________ as temperature increases. Relatively independent of pressure (incompressible)

molasses, tar, 20w-50 oil

increases

_______increases

decreases

Role of ViscosityRole of Viscosity

Statics Fluids at rest have no relative motion between

layers of fluid and thus du/dy = 0 Therefore the shear stress is zero and is

independent of the fluid viscosity Flows Fluid viscosity is very important when the fluid

is moving

Shear Stress/ViscosityShear Stress/Viscosity

change in velocity with respect to distance

AF

2mN

yU

yU

dydu

yAUF AU

Fy

2msNdimension of

s1

Tangential force per unit area

Rate of angular deformation

rate of shear

Shear Stress/ViscosityShear Stress/Viscosity

Fluid classification by response to shear stress


Newtonian Ideal Fluid Ideal plastic

NewtonianIdeal Fluid

Ideal plastic

Shear stress

Rat

e of

def

orm

atio

ndydu

dydu



Example: Measure the viscosity of water

Example: Measure the viscosity of water

The inner cylinder is 10 cm in diameter and rotates at 10 rpm. The fluid layer is 2 mm thick and 10 cm high. The power required to turn the inner cylinder is 50x10-6 watts. What is the dynamic viscosity of the fluid?

Inner cylinder

Outer cylinder

Thin layer of water

SolutionSolution

hrPt

322

23-32

6-

s/mN 1.16x10m) (0.1m) (0.05(1.047/s)2

m) (0.002 W)10(50

x

U A thrF

22

Pt

hrP322

Inner cylinder

Outer cylinder

Thin layer of waterr 2rh

Fr

tAUAF

dydu

yu

Dynamic and Kinematic Viscosity

Dynamic and Kinematic Viscosity

Kinematic viscosity is a fluid property obtained by dividing the dynamic viscosity by the fluid density

3mkg

smkg

[m2/s]

Surface TensionSurface Tension

molecules below the surface act on each other through forces that are equal in all directions

molecules near the surface have a greater attraction for each other than they do for molecules below the surface

Surface Tension and Capillary Rise




h

d

FF0 WFF yy

04

2

dhd

dthetah

)cos(4

04

cos2

dhd



Run a Video

Surface TensionSurface Tension

Pressure increase in a spherical droplet

rp 2

pr2

2r

pr2 = 2r

Surface Tension - ExamplesSurface Tension - Examples

Vapor PressureVapor Pressure

Def’n: pressure at which a liquid will boil the vapor pressure of water at 212 F is 14.7

psia (i.e., atmospheric pressure) at 70 F, the vapor pressure is 0.363 psia

Cavitation: “boiling” in flowing liquids; e.g., suction side of a pump

Vapor PressureVapor Pressure

010002000300040005000600070008000

0 10 20 30 40

Temperature (C)

Vapo

r pre

ssur

e (P

a)liquid

What is vapor pressure of water at 100°C? 101 kPa

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Fluid Properties