newtonian and non newtonian behaviour of fluids
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This document discusses Newtonian and non-Newtonian fluid behavior. It defines Newtonian fluids as those that obey Newton's viscosity law, where viscosity is constant regardless of shear rate. Examples given are water, benzene, and ethanol. Non-Newtonian fluids' viscosity varies with shear rate and are more complex mixtures like slurries, pastes, and polymer solutions. The document categorizes non-Newtonian behavior as time-independent or time-dependent, and describes examples like shear thinning, shear thickening, thixotropy and rheopectic behavior.
newtonian and non newtonian behaviour of fluids
- 1. Newtonian and non-newtonian behavior of fluids Group Members Waheed Ahmed (2k11-ChE-09) Adnan Rafi (2k11-ChE-16) Ahmed Haroon (2k11-ChE-23) Shahzad Ali Zahid (2k11-ChE-49)
- 2. Fluids which obey the Newton's law of viscosity are called as Newtonian fluids Newton's law of viscosity is given by = shear stress = viscosity of fluid dv/dy = shear rate, rate of strain or velocity gradient Newtonian fluids dy dv
- 3. Examples of Newtonian fluids All gases and most liquids which have simpler molecular formula and low molecular weight such as Water Benzene ethyl alcohol CCl4 Hexane and most solutions of simple molecules are Newtonian fluids
- 4. Non-Newtonian fluids Fluids which do not obey the Newton's law of viscosity are called as non-Newtonian fluids η is the apparent viscosity and is not constant for non-Newtonian fluids dy dv
- 5. Apparent Viscosity If the viscosity is influenced by the shear rate, it is important to specify that the values are different from the constant ones of an ideally viscous fluid. The values obtained are 'apparent viscosity' or 'apparent shear viscosity' values
- 6. Examples of Non-Newtonian fluids Generally non-Newtonian fluids are complex mixtures such as slurries Pastes Gels polymer solutions etc
- 8. Time-Independent Fluid Behavior • Fluids for which the rate of shear at any point is determined only by the value of the shear stress at that point at that instant; these fluids are variously known as “time independent”, “purely viscous. these fluids may be further subdivided into three different types: • Shear-thinning or pseudoplastics • Visco-plastics • Shear-thickening or dilatant
- 9. Time-Independent Fluid Behavior A shear-thinning or pseudoplastic substance is characterized by an apparent viscosity that decreases with increasing shear rate the rate of decrease of the apparent viscosity is not the same for each fluid 1. Shear thinning or pseudoplastic fluids
- 10. The Power-Law or Ostwald-De Waele Model The relationship between shear stress and shear rate for pseudoplastic fluids can often be approximated by a straight line over a limited range of shear rate, and hence this part of the flow curve can be described by the power-law expression τyx =m(˙γyx)n Where n and m, known as the power-law index and the fluid consistency coefficient,for a Newtonian fluid=1 and for a pseudoplastic substance n<1
- 11. flow curves for different types of non- Newtonian fluids
- 12. Pseudoplastics Flow of pseudoplastics is consistent with the random coil model of polymer solutions and melts. At low stress, flow occurs by random coils moving past each other w/o coil deformation. At moderate stress, the coils are deformed and slip past each other more easily. At high stress, the coils are distorted as much as possible and offer low resistance to flow.
- 13. Pseudoplastic (Shear thinning) Foods Shear stress Shear rate Examples: •Banana puree • Orange juice concentrate • Oyster sauce
- 14. Shear Thinning Behavior Shear thinning behavior is often a result of: Orientation of non-spherical particles in the direction of flow. An example of this phenomenon is the pumping of fiber slurries Orientation of polymer chains in the direction of flow and breaking of polymer chains during flow. An example is polymer melt extrusion Deformation of spherical droplets to elliptical droplets in an emulsion. An industrial application where this phenomenon can occur is in the production of low fat margarine Breaking of particle aggregates in suspensions. An example would be stirring paint
- 15. yxB B yx 0 for B yx 0 0yx for B yx 0 Often the two model parameters t0 B and mB are treated as curve fitting constants, even when there is no true yield stress 2. Viscoplastic Fluid Behavior Viscoplastic fluids behave as if they have a yield stress (t0). Until t0 is exceeded they do not appear to flow. A Bingham plastic fluid has a constant plastic viscosity 3. Shear-thickening or Dilatant Fluid Behavior Eq. (*) is applicable with n>1. Viscosity increases with shear stress. Dilatant: shear thickening fluids that contain suspended solids. Solids can become close packed under shear
- 16. Dilatant (Shear thickening) Foods Shear stress Shear rate Examples: • Liquid Chocolate • 40% Corn starch solution
- 19. The apparent viscosity of a fluid changes with time as the fluid is continuously sheared Thixotropic Rheopectic Time-dependent Fluid Behavior
- 20. Thixotropic If the apparent viscosity decreases with time e.g. paints, cream, aqueous iron oxide gels, some drilling mud's Result of a break down in the microstructure of the material as shearing continues This happens when the sheer is exceeded of a limit It leads to non-linear stress-strain behavior
- 21. Thixotropy in a Cement Paste
- 22. Typical experimental data showing thixotropic behavior in red mud suspension
- 23. Rheopectic Is the rare property of some non-newtonian fluids if the apparent viscosity increases with time The longer the fluid undergoes shearing force, the higher its viscosity Examples are gypsum pastes and printer inks Also termed as negative thixotropy
- 24. Rheopectic behavior in a saturated polyester
- 25. Qualitative shear stress–shear rate behavior for thixotropic and rheopectic materials
- 27. Non - newtonian Time independent Time dependent A EC D F GB _ _ Rheological curves of Time - Independent and Time – Dependent Liquids ++
- 28. Visco-elastic Fluid Behavior A visco-elastic fluid displays both elastic and viscous properties. A true visco-elastic fluid gives time dependent behavior Examples steel or aluminum Quartz
- 29. Some phenomena in viscoelastic materials are If the stress is held constant, the strain increases with time If the strain is held constant, the stress decreases with time During rolling, frictional resistance occurs
- 30. Types of viscoelasticity Linear viscoelasticity o It usually happens when the deformations are Small Nonlinear viscoelasticity o Usually happens when the deformations are Large
- 31. Measuring viscoelasticity broadband viscoelastic spectroscopy (BVS) resonant ultrasound spectroscopy (RUS)
- 32. Thanks