Pharos University Fluid Mechanics For Electrical Students

1. Pharos UniversityFluid Mechanics For Electrical Students Dr. A. Shibl

2. 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

3. 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

4. Example

5. Example

6. 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

7. 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 P Pressure=Force/Area Unit: Psi or Pa (SI)

8. Density & Sp. Volume • Density (r) : mass per unit volume • r = mass/volume kg/m3, g/cm3, lb/ft3 • Density is a fluid property and slightly dependent on temperature • Specific Volume (n): Inverse of density • n = 1/r m3/kg • Specific Gravity ( SG): • SG=r/rwaterAt same Temp.

9. Specific Weight • Specific Weight = Weight/Volume g = 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

10. 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

11. Viscosity • Dynamic Viscosity • = Shear Stress/Slope of velocity profile • Kinematic ViscositycS (centistokes) or m2/Sec. cP (centipoise) or Pa-sec n v F Slope = v/y y

12. Newtonian and Non-Newtonian Fluids • Two types of fluids: Newtonian and Non-Newtonian: • Newtonian: • Ex.: Water, Oil, Gasoline

13. 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)

14. Viscosity MeasurementFalling Ball Viscometer • Viscosity is determined by noting the amount of time a ball takes to travel between two lines W Fb Fd

15. 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

16. Saybolt Viscosity • Saybolt Equations: • (cS) = 0.226t - 195/t, t< 100 SUS • (cS) = 0.220t – 135/t, t> 100 SUS t, 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.

17. http://www.liquidcontrol.com/etoolbox/viscosity.aspx

18. Viscosity Chart m Force Temp. ° F  Temp. C http://www.klassenhydraulics.com/Reference/viscositychart.htm

19. 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

20. Characteristics of Hydraulic Fluids • Adequate viscosity • Lubricating capability • Cleanliness • Chemical stability • Non-corrosiveness • Ability to resist growth of bacteria • Ecologically acceptable • Low compressibility

21. Hydraulic Fluids • HWBF • Fire resistant • ~40% oil in water • Water-glycol fluids • Fire resistant • 35 to 50% water

22. 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

23. Pressure • Pressure: • Absolute = Gage + Atmospheric* • psia = psig + 14.7 psia • *14.7 psia at sea level

24. Pressure Scale

25. 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

26. Pressure and Elevation • Change in pressure in homogeneous liquid at rest due to a change in elevation DP = gh Where, DP = change in pressure, kPa • = specific weight, N/m3 h = change in elevation, m

27. Pressure-Elevation Relationship • Valid for homogeneous fluids at rest (static) P2 = Patm + rgh Free Surface Free Surface P2 P1 P1 > P2

28. Static Fluids: Same elevation and same fluid → same pressure P2 = Patm + rgh

29. Manometers • Used to measure pressure • DP = gh

30. Example: Manometer • Calculate pressure (psig) or kPa (gage) at Point A. Open end is at atmospheric pressure. A 0.15 m Water 0.4 m Hg: SG = 13.54

31. Pressure Measurement Devices Manometers Highly sensitive inclined manometers for systems demanding precise measurement of low pressures

32. Pressure Measurement Devices Gages Transducer:

33. Barometer and Atmospheric Pressure Patm = rgh Patm = 14.psi, 1 atm