Kinematic Viscosity

9 Key excerpts on "Kinematic Viscosity"

• 

  No longer available |Learn more

  Liquid Physics and Liquid Chemistry (Concepts and Applications)

  • (Author)
  • 2014(Publication Date)
  • Learning Press

    (Publisher)

  Viscosity index is a measure for the change of Kinematic Viscosity with temperature. It is used to characterise lubricating oil in the automotive industry. At one time the petroleum industry relied on measuring Kinematic Viscosity by means of the Saybolt viscometer, and expressing Kinematic Viscosity in units of Saybolt Universal Seconds (SUS). Other abbreviations such as SSU ( Saybolt Seconds Universal ) or SUV ( Saybolt Universal Viscosity ) are sometimes used. Kinematic Viscosity in centistoke can be converted from SUS according to the arithmetic and the reference table provided in ASTM D 2161. ________________________ WORLD TECHNOLOGIES ________________________ Molecular origins Pitch has a viscosity approximately 230 billion (2.3 × 10 11 ) times that of water The viscosity of a system is determined by how molecules constituting the system interact. There are no simple but correct expressions for the viscosity of a fluid. The simplest exact expressions are the Green–Kubo relations for the linear shear viscosity or the Transient Time Correlation Function expressions derived by Evans and Morriss in 1985. Although these expressions are each exact in order to calculate the viscosity of a dense fluid, using these relations requires the use of molecular dynamics computer simulations. ________________________ WORLD TECHNOLOGIES ________________________ Gases Viscosity in gases arises principally from the molecular diffusion that transports momentum between layers of flow. The kinetic theory of gases allows accurate prediction of the behavior of gaseous viscosity. Within the regime where the theory is applicable: • Viscosity is independent of pressure and • Viscosity increases as temperature increases. James Clerk Maxwell published a famous paper in 1866 using the kinetic theory of gases to study gaseous viscosity. To understand why the viscosity is independent of pressure consider two adjacent boundary layers (A and B) moving with respect to each other.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  On-Line Analysis Instrument

  Instrument Technology

  • E. B. Jones(Author)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  T h e oil industry has for m a n y years been concerned with the Kinematic Viscosity of liquids. For these reasons the British Standards Institution now specifies kinematic viscosities. Occasionally, particularly in the study of colloids, the term 'fluidity' is used. This is the reciprocal of the viscosity in poises. Laboratory methods of determining viscosity are largely based u p o n equation 7.1 or u p o n Poiseuille's law for the volume Q discharged in a unit time from a tube of length / a n d radius a, i.e. β = τ τ / ν / 8 η / (7.2) where Ρ is the pressure difference between the ends of the tube and η the d y n a m i c viscosity. T h i s equation also forms the basis of a continuous on-line viscometer in which Q is constant a n d Ρ is measured. T h i s relationship holds 263 MEASUREMENT OF VISCOSITY 264 M E A S U R E M E N T OF V I S C O S I T Y only if the flow is streamlined or laminar, i.e. if the Reynolds n u m b e r * is less than the limiting value. Viscosity m e a s u r e m e n t may also be based u p o n the m e a s u r e m e n t of the drag upon a stationary cylinder when placed in a concentric rotating cylinder containing the liquid to be tested, or upon the d r a g on a rotating cylinder w h e n placed in a stationary concentric cylinder. In m o d e r n industrial instruments used for continuous m e a s u r e m e n t s on the processes, other methods of measuring the force required to produce relative motion in a liquid are also used. Ν on-Newtonian liquids Newton's equation (7.1) is based upon the assumption that the force required to maintain a difference of velocity between two planes in a liquid is proportional to the velocity gradient in the liquid between the planes.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Mechanical Engineers' Handbook, Volume 1

  Materials and Engineering Mechanics

  • Myer Kutz(Author)
  • 2015(Publication Date)
  • Wiley

    (Publisher)

  is the fluid's density. Although the dynamic and kinematic viscosities are clearly related properties, they are dimensionally dissimilar, and it is critically important to always distinguish between them. More is said on the distinction between dynamic and Kinematic Viscosity in the following section on common viscosity units.

  The remainder of this chapter begins by discussing the units by which viscosity is measured. Then the distinction between the larger field of rheology and its subfield viscometry is made in the context of differentiating between the so-called Newtonian and non-Newtonian fluids. After that the chapter provides a brief theoretical and mathematical overview of viscosity. Finally, the majority of the chapter provides detailed and practical information on methods for measuring viscosity.

  2 Common Units of Viscosity

  There are several systems of units used with viscosity; many of them are archaic and/or closely tied to one specific viscosity measuring technique (e.g., the Saybolt cup and the Saybolt universal second, and the Krebs unit) or one particular industry (e.g., SAE oil grade and the automotive industry). It is impossible to capture all of these systems in one document, but an attempt is made below to define and relate the most common and standard units associated with viscosity measurement.

  2.1 Absolute Viscosity, μ

  In terms of the SI (Le Système Internationale d'Unités) system of fundamental units, the derived units for absolute viscosity, , are , which is equivalent to (Pascal-seconds). This grouping of units has not received a name of its own. In the closely related cgs (centimeter, gram, and second) system of units, the derived unit of or is called a Poise (after Poiseuille). More commonly a centipoise, of a Poise is used. In the fps (foot, pound, and second) system of units, the units of absolute viscosity are , which is called the reyn (after Osbourne Reynolds). Refer to Table 1

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Physics of Continuous Matter

  Exotic and Everyday Phenomena in the Macroscopic World

  • B. Lautrup(Author)
  • 2011(Publication Date)
  • CRC Press

    (Publisher)

  Over the years the estimate has been refined by means of the kinetic theory of gases, leading to about half the above value [Loeb 1961]. In practice one uses the resulting expression to determine the otherwise rather ill-defined molecular diameter from the measured viscosity (see the margin table). Temperature dependence of viscosity The viscosity of any material depends on temperature. Common experience from the kitchen and industry tells us that most liquids become “thinner” when heated, indicating that the viscosity falls with temperature. Gases on the other hand become more viscous at higher temperatures, simply because the molecules move faster and thus transport more momentum across a surface per unit of time. For an ideal gas it follows from Equation (1.11) on page 8 that the expression is a combinations of constants, so that the viscosity becomes v mol p T . Thus, if the viscosity is 0 at temperature T 0 , it may be estimated as D 0 s T T 0 (15.3) at temperature T . Notice that the viscosity is independent of the pressure. Empirically, the viscosity grows slightly faster with temperature because of molecular attraction. Kinematic Viscosity The viscosity estimate (15.2) seems to point to another measure of viscosity, called the kine-matic viscosity 3 , D : (15.4) Since the estimate, 2 = , does not depend on the unit of mass, this parameter is measured in purely kinematic units 4 of m 2 s 1 (see Table 15.1). In an ideal gas we have / p=T , so that the Kinematic Viscosity will depend on both temperature and pressure, / T 3=2 =p . For isentropic processes in ideal gases it always decreases with increasing temperature (see Problem 15.1). 3 The conflicting use of for both the Kinematic Viscosity and Poisson’s ratio is pervasive in the literature. 4 In the older cgs system, the corresponding unit was called stokes D cm 2 s 1 D 10 4 m 2 s 1 .

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  The Coen & Hamworthy Combustion Handbook

  Fundamentals for Power, Marine & Industrial Applications

  • Stephen Londerville, Charles E. Baukal Jr., Stephen Londerville, Charles E. Baukal Jr.(Authors)
  • 2013(Publication Date)
  • CRC Press

    (Publisher)

  8.2.4.1 Absolute Viscosity Under careful observation of fluid flow in contact with a solid surface, a moving fluid will see no net motion at the surface. This phenomenon is known as the “no-slip condition.” As seen in Figure 8.1, the velocity profile shows that the flow velocity is dependent on the dis-tance from the surface. The closer the fluid is to the solid surface, the lower the velocity will be and vice versa. With this understanding, we can relate the shear stress encountered by the fluid to the viscosity and volumetric flow rate in the form of τ μ = dV dy (8.4) where τ is the shear†stress†in†N/m †or†lb ft / f 2 2 μ is the absolute dynamic viscosity†in†N s/m †or lb s/ft † f ( ) ⋅ ⋅ 2 2 dV/dy is the velocity†gradient A more common unit used to express viscosity is the poise or centipoises (10 −2 poise). A poise is equivalent to 1 p Dyne s cm g cm s = ⋅ = ⋅ 1 1 2 nobreakspace nobreakspace 1 0 1 0 1 0 1 2 p Pa s N s m kg m s = -= ⋅ = ⋅ . . † . † 1 0 0020885 2 p lb s ft f = ⋅ . † † 8.2.4.2 Kinematic Viscosity More often than not, viscosity is expressed as a ratio of absolute viscosity to density of the fluid, in the form of ν = μ ρ (8.5) where ν is the Kinematic Viscosity in m 2 /s or ft 2 /s. The reason that Kinematic Viscosity is tabulated is due to the frequency in which the term appears in fluid calculations. Kinematic Viscosity is most useful when analyzing systems involving liquids in which the density is unlikely to change significantly with ambient conditions and temperature. In the case of gases, compression, atmospheric pressure change due to elevation, and fluid temperature changes will affect the density, and thus the Kinematic Viscosity. For these reasons, absolute viscosity should be utilized for any gas analysis. More commonly used units for Kinematic Viscosity, depending on application, are the stoke, centistokes, and Saybolt universal seconds (SSU).

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Engineering Tribology

  • John Williams(Author)
  • 2005(Publication Date)
  • Cambridge University Press

    (Publisher)

  In the c.g.s. system the corresponding unit (cm 2 s~ 2 ) is called the stoke. As with the poise this is really too large for practical use and numerical values of Kinematic Viscosity are usually quoted in centistokes (cS); thus 1 cS = 1 x 10~ 2 stoke = 1 mm 2 s~ l . One method of determining viscosity is to measure the time taken for a Lubricants and lubricant properties 19 given volume of fluid to flow out of a container through a narrow capillary tube and various commercial instruments using this principle have been developed. Since most of these depend on gravity-driven flow, they measure the kinematic rather than the absolute viscosity. Figure 1.6 shows the essen-tial features of the Ostwald viscometer which is widely used for this sort of determination. The two glass bulbs, one higher than the other, are connected by a glass capillary whose size is chosen to make inertia and turbulence effects neglig-ibly small. The time of flow for a certain volume of fluid is measured and this is then multiplied by a calibration factor to give the Kinematic Viscosity. As viscosity is very temperature dependent the apparatus must be immersed in a constant-temperature bath to give accurate results. Most tribology analyses require the value of the absolute rather than the Kinematic Viscosity so that if data from viscometers of this type are to be used we also need to know the density of the fluid. Most lubricating oils have densities in the range 800-1000kg m~ 3 and the chart of Fig. 1.7 can be used to make the conversion from cS to cP. The viscosity of commercial lubricants is still sometimes quoted in terms of Saybolt, or Redwood seconds, or Engler degrees. These figures refer to the times that particular volumes of the fluid take to flow through the short length of capillary tubing that is incorporated in each of the corresponding instruments.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Introduction to Chemical Engineering Fluid Mechanics

  • William M. Deen(Author)
  • 2016(Publication Date)
  • Cambridge University Press

    (Publisher)

  Part I Use of experimental data 1 Properties, dimensions, and scales 1.1 INTRODUCTION Fluids, including both gases and liquids, are materials that deform continuously when subjected to shearing forces. If a flowing fluid contains a dye or other tracer, the labeled region tends to change shape from instant to instant. The viscosity of a fluid is a reflection of its resistance to such deformations. This chapter begins with definitions of viscosity and three other material properties that are important in fluid mechanics, namely density, Kinematic Viscosity, and surface tension. Representative values of each are presented, and some of the differences between Newtonian and non-Newtonian fluids are described. Physical quantities have dimensions , such as mass (M), length (L), and time (T). The concepts of mass, length, and time are more fundamental than the units in a particular system of measurement, such as the kilogram (kg), meter (m), and second (s) that under-lie the SI system. To have general validity, a physical relationship must be independent of the observer, including the observer’s choice of units. That can be achieved by making each variable or parameter in an equation dimensionless , which means that the physical quantities are grouped in such a way that their dimensions cancel. Dimensionless param-eters are ratios, and often the numerator and denominator are each the scale of a variable. A scale is the maximum value of something, such as a velocity or force. Thus, the numer-ical value of such a group reveals how two things compare, and thereby offers insight into what is important in the process or phenomenon under consideration and what might be negligible. Several dimensionless groups that arise in fluid mechanics are discussed. Dimensional analysis, the last major topic of the chapter, provides a systematic way to identify the dimensionless groups that are involved in something of interest.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Elements of Friction Theory and Nanotribology

  • Enrico Gnecco, Ernst Meyer(Authors)
  • 2015(Publication Date)
  • Cambridge University Press

    (Publisher)

  243 244 Drag in a viscous fluid Table 22.1 Typical values of dynamic viscosity (in mPa·s) Fluid η Air 0.017 Water 0.9 Ethanol 1.2 Mercury 1.5 Glycerol 1.2 × 10 3 which simply states that the mass of the fluid is conserved. Furthermore, appro- priate boundary conditions must be satisfied. A common assumption is the no-slip condition, according to which the velocity v is zero on the solid walls limiting the fluid. Although the no-slip condition is an excellent approximation for most engineering applications, it can be violated in micromechanical devices with very smooth surfaces (see Appendix C). In this chapter we will also assume that the fluid is Newtonian, i.e. that η (at a fixed temperature) does not depend on the rate of change of the velocity at which a fluid layer flows over an adjacent one 2 (the so-called shear rate ˙ γ ). This hypothesis holds well for liquids like water, benzene and light oils, whereas more complex fluids present a non-Newtonian behavior. 3 Using the Navier–Stokes equation it can be proven that the kinetic energy per unit volume of the fluid is dissipated at a rate d E kin dt = − η 2  ∂v i ∂ x j + ∂v j ∂ x i  2 (22.2) and the components of the friction force on the unit area of a bounding wall are τ fric,i = −η  ∂v i ∂ x j + ∂v j ∂ x i − 2 3 δ i j ∂v k ∂ x k  n i , (22.3) where n is a unit vector directed normally into the solid surface. Viscosity of slurry A slurry is a mixture of liquid and fine solid particles which retains fluid prop- erties. If the ratio c between the volume occupied by the particles and the total volume of the suspension is extremely low, the viscosity differs by the amount η = 5cη/2 from the viscosity η of the original fluid. This result, which is strictly 2 We assume that the fluid flows in parallel layers without lateral mixing (laminar flow).This is always the case if the velocity is low enough.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Instrumental Methods for Quality Assurance in Foods

  • Fung(Author)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  9 Viscosity Measurements of Foods Malcolm C. Bourneand M. A. Rao

  Cornell UniversityGeneva, New York

  I.     INTRODUCTION

  Viscosity data must be expressed in fundamental units that are obtainable with well-designed viscometers. Viscosity units that are specific to a particular brand viscometer, such as the time to flow out of a specific cup, will not be applicable universally.

  II.   DEFINITIONS AND UNITS OF MEASUREMENT The fundamental parameters used in viscosity measurement are the following:

  Shear rate is the velocity gradient established in a fluid as a result of an applied shear stress. It is expressed in units of reciprocal seconds (s−1 ) and is denoted by the symbol

  γ ˙

  (gamma dot).

  Shear stress is the stress component applied tangential to the plane on which the force acts. It is a force vector that possesses both magnitude and direction. It is expressed in units of force per unit area. The nomenclature committee of the Society of Rheology recommends that the symbol σ (lower case sigma) be used to denote shear stress. However, at one time, the symbol τ (tau) was used to denote shear stress and many scientists have not yet switched to using σ.

  Viscosity, or correctly dynamic viscosity, is the internal friction of a fluid or its tendency to resist flow. It is denoted by the symbol η (eta) and is defined by the equation:

  η   =

  shear   stress

  shear   rate

  =

  σ

  γ ˙

  The old system of units of measurement (metric) and the new system (SI) for viscosity are given in Table 1 .

  Apparent viscosity is the viscosity of a non-Newtonian liquid expressed as though it were a Newtonian liquid. It is a coefficient calculated from empirical data. Using the above equation, the shear rate employed in the calculation must be specified when the magnitude of apparent viscosity is discussed. It is denoted by the symbol ηa

  Sign up to read

  Learn more about book