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
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
Properties involving mass & weight
Properties involving mass & weight
Physical PropertiesPhysical Properties
Properties involving mass & weight
Properties involving mass & weight
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
Properties involving mass & weight
Properties involving mass & weight
Properties involving mass & weight
Properties involving mass & weight
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
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
Fluid classification by response to shear stress
Fluid classification by response to shear stress
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
Surface Tension and Capillary Rise
Surface Tension and Capillary Rise
Surface Tension and Capillary Rise
h
d
FF0 WFF yy
04
2
dhd
dthetah
)cos(4
04
cos2
dhd
Surface Tension and Capillary Rise
Surface Tension and Capillary Rise
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