pharos university fluid mechanics for electrical students
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Pharos University Fluid Mechanics For Electrical Students. Dr. A. Shibl . Fluid Mechanics. Is the study of the behavior of fluids at rest or in motion Fluids can be either liquids or gases Liquids flow freely and conform to their containers Gases completely fill their containers. - PowerPoint PPT PresentationTRANSCRIPT
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Pharos UniversityFluid Mechanics For Electrical
Students
Dr. A. Shibl
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Fluid Mechanics
• Is the study of the behavior of fluids at rest or in motion
• Fluids can be either liquids or gases• Liquids flow freely and conform to their
containers • Gases completely fill their containers
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Importance Of Fluid Mechanics
• Utilization of Fluid around us; Air, Water… • Prediction of Fluid flow behavior • Sizing or specifying equipment• Estimate the related energy costs• Estimating the system performance under
different conditions
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Example
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Example
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Fluid Properties:Liquid or Gas
• Liquids are:– Incompressible, DV ≠ f(DP)– Viscous (high viscosity)– Viscosity decreases with temperature
• Gases are:– Compressible, DV = f(DP)– Low viscosity– Viscosity increases with temperature
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PRESSURE
• Pressure:– Force exerted on a unit area– P = Force/Area– Pressure acts uniformly in all directions and
perpendicular to the boundaries in the container– Example: Piston Force
Area= p/4*D2
Pressure=Force/AreaUnit: Psi or Pa (SI)
P
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Density & Sp. Volume
• Density (r) : mass per unit volumer = mass/volume kg/m3, g/cm3, lb/ft3
– Density is a fluid property and slightly dependent on temperature
• Specific Volume (n): Inverse of densityn = 1/r m3/kg
• Specific Gravity ( SG): SG=r/rwater
At same Temp.
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Specific Weight
• Specific Weight = Weight/Volumeg = w/V
• Examples– Calculate the weight of a reservoir of oil if it has a
mass of 825 kg– If the volume is 0.917 m3, compute density,
specific weight, specific gravity
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Equations for Fluid Property
• Circular Area: • Weight: w = m*g Newton• Density: r = m/V Kg/m3
• Specific Weight: g = w/V N/m3
• Specific gravity: SG=r/rwater
Area = p/4*D2
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Viscosity• Dynamic Viscosity
m = Shear Stress/Slope of velocity profile
• Kinematic Viscosity cS (centistokes) or m2/Sec.
yvAF//
Slope = v/y
vn
y
F
cP (centipoise) or Pa-sec
rm
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Newtonian and Non-Newtonian Fluids
• Two types of fluids: Newtonian and Non-Newtonian:
• Newtonian: – Ex.: Water, Oil, Gasoline
DD
D
D
vy
yvyv
AF //
DD
vyf
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Non-Newtonian Fluids
• Time-independent Fluids– Pseudoplastic (Blood Plasma, syrups, inks)– Dilatant (Starch in water)– Bingham (catsup, mustard, toothpaste)
• Time-dependent Fluids– Electrorheological (behavior changes due to
electric field, particles are present)– Magnetorheological (iron powders in fluid)
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Viscosity MeasurementFalling Ball Viscometer
• Viscosity is determined by noting the amount of time a ball takes to travel between two lines
W
Fb Fd
( ) 2
18s f D
V
g gm
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Viscosity MeasurementSaybolt Universal Viscometer
• Measurement is not based on definition of viscosity
• Results are relative, so a standard sample is used for calibration
• Fast and easy
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Saybolt Viscosity– Saybolt Equations:
n (cS) = 0.226t - 195/t, t< 100 SUSn (cS) = 0.220t – 135/t, t> 100 SUSt, amount of time (seconds, SUS, Saybolt Universal
Seconds) it takes for 60 cm3 to flow through orifice (Saybolt viscometer)
– Example:• An oil has a viscosity of 230 SUS at 150° F. Compute
the viscosity in cS and cP. Specific gravity is 0.9.
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Approximate Viscosities of Common Materials(At Room Temperature: 70°F)
Material Viscosity in CentipoiseWater 1 cps
Milk 3 cps SAE 10 Motor Oil 85-140 cps
SAE 20 Motor Oil 140-420 cps
SAE 30 Motor Oil 420-650 cps SAE 40 Motor Oil 650-900 cps
Castrol Oil 1,000 cps Karo Syrup 5,000 cps
Honey 10,000 cps Chocolate 25,000 cps
Ketchup 50,000 cps
Mustard 70,000 cps Sour Cream 100,000 cps
Peanut Butter 250,000 cps
http://www.liquidcontrol.com/etoolbox/viscosity.aspx
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Viscosity ChartTemp. ° F
Temp. C
rm
http://www.klassenhydraulics.com/Reference/viscositychart.htm
m Force
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Hydraulics Fluids for Fluid Power Systems
• Fluid Power– Pneumatics: air-type systems– Hydraulics: liquid-type systems
• Hydraulic Fluids:– Petroleum oils– Water-glycol fluids– High water based fluids (HWBF)– Silicone fluids– Synthetic oils
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Characteristics of Hydraulic Fluids
• Adequate viscosity• Lubricating capability• Cleanliness• Chemical stability• Non-corrosiveness• Ability to resist growth of bacteria• Ecologically acceptable• Low compressibility
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Hydraulic Fluids
• HWBF– Fire resistant– ~40% oil in water
• Water-glycol fluids– Fire resistant– 35 to 50% water
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Hydraulic Fluids
• Petroleum Oils– SAE 10 W, SAE 20-20W (W means rated at
maximum viscosity and cold temperatures)– Engine oils– Additives are required to avoid growth of bacteria
• Silicone Fluids– For high temperature applications
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Pressure
• Pressure:–Absolute = Gage + Atmospheric*–psia = psig + 14.7 psia–*14.7 psia at sea level
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Pressure Scale
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Units of Pressure
• 1 bar = 105 Pa = 0.1 MPa = 100 kPa• 1 atm = 101,325 Pa = 101.325 kPa • 1 atm = 1.012325 bars• 1 mm Hg = 0.13333 kPa• 1 atm = 14.696 psi
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Pressure and Elevation
• Change in pressure in homogeneous liquid at rest due to a change in elevation
DP = ghWhere,DP = change in pressure, kPag = specific weight, N/m3
h = change in elevation, m
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Pressure-Elevation Relationship
• Valid for homogeneous fluids at rest (static)
Free Surface Free Surface
P1
P2
P1 > P2
P2 = Patm + rgh
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Static Fluids: Same elevation and same fluid → same pressure
P2 = Patm + rgh
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Manometers
• Used to measure pressure• DP = gh
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Example: Manometer
• Calculate pressure (psig) or kPa (gage) at Point A. Open end is at atmospheric pressure.
A
Hg: SG = 13.54
Water
0.4 m
0.15 m
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Pressure Measurement Devices
Highly sensitive inclined manometers for systems demanding precise measurement of low pressures
Manometers
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Pressure Measurement DevicesGages
Transducer:
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Barometer and Atmospheric Pressure
Patm = rgh
Patm = 14.psi, 1 atm