Download
1 / 62
720 likes | 1.28k Vues
In the name of Allah, Most Gracious, Most Compassionate. >> Viscosity. Group Members Aqsan Naveed Ahmad Sattar Umer Zahid Mohsin Mahmood. Aqsam Naveed. >>> Definition. Viscosity is an internal property of a fluid that offers resistance to flow.
E N D
>> Viscosity • Group Members • Aqsan Naveed • Ahmad Sattar • Umer Zahid • Mohsin Mahmood
>>> Definition • Viscosity is an internal property of a fluid that offers resistance to flow. • The resistance of a substance to flow. For example, water has a lower viscosity than molasses and flows more easily. Viscosity is related to the concept of shear force • It can be understood as the effect of different layers of the fluid exerting shearing force on each other, or on other surfaces, as they move against each other.
Types of “Viscosity ” Dynamic (absolute) Viscosity • It is the tangential force per unit area required to move one horizontal plane with respect to the other at unit velocity when maintained a unit distance apart by the fluid. • The shearing stress between the layers of non turbulent fluid moving in straight parallel lines can be defined for a Newtonian fluid
The dynamic or absolute viscosity can be expressed like • τ = μ dc/dy (1) • where • τ = shearing stress • μ = dynamic viscosity • Equation (1) is known as the Newtons Law of Friction.
In the SI system the dynamic viscosity units are N s/m2, Pa s or kg/m s where • 1 Pa s = 1 N s/m2 = 1 kg/m s • The dynamic viscosity is also often expressed in the metric CGS (centimeter-gram-second) system as g/cm.s, dyne.s/cm2 or poise (p) where • 1 poise = dyne s/cm2 = g/cm s = 1/10 Pa s • For practical use the Poise is to large and it's usual divided by 100 into the smaller unit called the centiPoise (cP) where • 1 p = 100 cP • Water at 68.4oF (20.2oC) has an absolute viscosity of one - 1 - centiPoise.
Kinematic Viscosity • is the ratio of absolute or dynamic viscosity to density - a quantity in which no force is involved. Kinematic viscosity can be obtained by dividing the absolute viscosity of a fluid with it's mass density • ν = μ / ρ (2) • where • ν = kinematic viscosity • μ = absolute or dynamic viscosity • ρ = density • In the SI-system the theoretical unit is m2/s or commonly used Stoke (St)where • 1 St = 10-4 m2/s
Since the Stoke is an unpractical large unit, it is usual divided by 100 to give the unit called Centistokes (cSt) where • 1 St = 100 cSt • 1 cSt = 10-6 m2/s • Since the specific gravity of water at 68.4oF (20.2oC) is almost one (1), the kinematic viscosity of water at 68.4oF is for all practical purposes 1.0 cSt.
Viscosity and Reference Temperatures • The viscosity of a fluid is highly temperature dependent and for either dynamic or kinematic viscosity to be meaningful, the reference temperature must be quoted. In ISO 8217 the reference temperature for a residual fluid is 100oC. For a distillate fluid the reference temperature is 40oC. • For a liquid - the kinematic viscosity will decrease with higher temperature • For a gas - the kinematic viscosity will increase with higher temperature
Comparison of temperaturedependent • viscosities • Gas: oxygen • Increase with temperature • Liquid: aniline • Decrease with temperature
Other Commonly used Viscosity Units • Saybolt Universal Seconds (or SUS, SSU) • Saybolt Universal Seconds (or SUS) is used to measure viscosity. The efflux time is Saybolt Universal Seconds (SUS) required for 60 milliliters of a petroleum product to flow through the calibrated orifice of a Saybolt Universal viscometer, under carefully controlled temperature and as prescribed by test method ASTM D 88. This method has largely been replaced by the kinematic viscosity method. Saybolt Universal Seconds is also called the SSU number (Seconds Saybolt Universal) or SSF number (Saybolt Seconds Furol).
Kinematic viscosity versus dynamic or absolute viscosity can be expressed as • ν = 4.63 μ / SG (3) • where • ν = kinematic vicosity (SSU) • μ = dynamic or absolute viscosity (cP)
Degree Engler • Degree Engler is used in Great Britain as a scale to measure kinematic viscosity. Unlike the Saybolt and Redwood scales, the Engler scale is based on comparing a flow of the substance being tested to the flow of another substance - water. Viscosity in Engler degrees is the ratio of the time of a flow of 200 cubic centimetres of the fluid whose viscosity is being measured - to the time of flow of 200 cubic centimeters of water at the same temperature (usually 20oC but sometimes 50oC or 100oC) in a standardized Engler viscosity meter.
Example - Converting between Kinematic and Absolute Viscosity for Air • Kinematic viscosity of air at 1 bar (105 Pa, N/m2) and 40oC is 16.97 cSt (16.97 10-6 m2/s). • The density of air estimated with the Ideal Gas Law • ρ = p / R T • where • ρ = density (kg/m3) • p = absolute pressure (Pa, N/m2) • R = individual gas constant (J/kg K) • T = absolute temperature (K) • ρ = (105 N/m2) / ((287 J/kg/K) (273 oC + 33 0C) • = 1.113 kg/m3 • Absolute viscosity can be expressed as • μ = (1.113 kg/m3) (16.97 10-6 m2/s) • = 1.88 10-5 (kg/m s, Ns/m2, P)
Different types of Viscosity Shear-thickening liquids -low viscosity when stirred slowly, but viscosity increases when mixed quickly Shear thinning liquids -high viscosity when mixed slowly, easier to mix quickly Bingham liquids -at low stress behaves like a solid, high stress behaves like a liquid Granular materials -like Bingham, but do not have a well-defined viscosity
Types of Fluid on Viscosity Basis • Newtonian Fluids • Non-Newtonian Fluids
Newtonian Fluids • Fluids for which the shearing stress is linearly related to the rate of shearing strain are designated as Newtonian Fluids. • Newtonian materials are referred to as true liquids since their viscosity or consistency is not affected by shear such as agitation or pumping at a constant temperature. Fortunately most common fluids, both liquids and gases, are Newtonian. Water and oils are examples of Newtonian liquids.
Shear-thinning or Pseudoplastic Liquids • Shear-thinning or pseudoplastic liquids are those whose apparent viscosity decreases with increasing shear rate. Their structure is time-independent.
Pseudoplastic Liquids Grease
Thixotropic Fluids • Thixotropic liquids have a time-dependent structure. The apparent viscosity of a thixotropic liquid decreases with increasing time, at a constant shear rate. • Ketchup and mayonnaise are examples of thixotropic materials. They appear thick or viscous but are possible to pump quite easily.
Dilatant Fluids • Shear Thickening Fluids or Dilatant Fluids increase their viscosity with agitation. Some of these liquids can become almost solid within a pump or pipe line. With agitation, cream becomes butter and Candy compounds, clay slurries and similar heavily filled liquids do the same thing.
Dilatant Fluids Starch in Water
Bingham Plastic Fluids • Bingham Plastic Fluids have a yield value which must be exceeded before it will start to flow like a fluid. From that point the viscosity will decrease with increase of agitation. Toothpaste, mayonnaise and tomato catsup are examples of such products.
Bingham Plastic Fluids Coal Tar
Viscosity and Temperature • Kinematic viscosity of liquids like water, mercury, oils SAE 10 and oil no. 3 - and gases like air, hydrogen and helium are indicated below. Note that • for liquids viscosity decreases with temperature • for gases viscosity increases with temperature
Measurement of viscosity • Measure flow in thepresence of agradient of speed • Poiseuille’s formulafor flow through acylindrical tubesubject to apressure drop P
Types of Viscometers • Redwood Viscometer • Engler Viscometer • Saybolt Viscometer • Ostwald viscometer • Falling-ball viscometer
Redwood • A standard British-type viscometer in which the viscosity is determined by the time, in seconds. • Required for a certain quantity of liquid to pass out through the orifice under given conditions • It is used for determining viscosities of petroleum oils.
Working • The inner cylinder is filled to the marker level and the outer cylinder filled so as to have sufficient height to give good heat transfer. • The first reading is taken by removing the stopper from the orifice. The recorded values were the water and test liquid temperatures and the time to collect say 50cc of the liquid being recorded.
Conti…………….. • The collected 50cc of liquid must be put back into the inner cylinder and the used cylinder inverted to empty completely. • The level of the sample is checked as, if more of the liquid is needed it would be added. The temperature is then raised and the agitator is used again continuously. This is done until the liquid is brought within 1 degree of the water temperature. The same parameters were again measured. This procedure is repeated for 5 samples in total (at least).
Engler Viscometer • An instrument used in the measurement of the degree Engler, a measure of viscosity; the kinematic viscosity ν in stokes for this instrument is obtained from the equation ν = 0.00147t - 3.74/t, where t is the efflux time in seconds.
Saybolt viscometer • It is Used to determine the viscosity of petroleum products at specified temperatures between 70 to 210 °F. • Stainless steel made, the Saybolt viscometer is supplied complete with two interchangeable orifices Furol and Universal, oil bath, electric heater with digital thermoregulator, stirrer, cooling coil, viscosity flask. • Thermometers are not included and must be ordered separately
Ostwald viscometer • Need a small-diameter tube (capillary) • Measure time of flow of a specific volume through the capillary • Constant pressure drop across the capillary • Use Poiseuille’s equation to calculate viscosity • Must be calibrated with a known material
Falling-ball viscometer • Measure the terminal velocity of a ball falling in a fluid • Use Stokes Law for the viscous drag to determine viscosity
Features • Continuous monitoring of viscosity, and temperature • Two 4-20mA outputs for remote recording • Two analog outputs and RS485 communication • Compact, explosion-proof systemt • Automatic self-cleaning • Readout in centipoises, cSt, or SSU • Optional LCD display
