synonymKinematic viscosity(kinematic viscosity) generally refers to kinematic viscosity
Kinematic viscosity, English translation:.
The kinematic viscosity is theDynamic viscosityAt the same temperaturefluidThe ratio of density ρ.The unit is (m ^ 2)/s.It is represented by the lowercase letter v.Note: The units used previously are St cSt and m ^ 2/sProgression rateThe relationship is: 1 (m ^ 2)/s=10 ^ 4St=10 ^ 6cSt.(where "cSt" is read as "centis") The flowing liquid is regarded as many liquid layers moving in parallel with each other, and each layer has different velocities, formingvelocity gradient(dv/dx), which is the basic feature of flow.
ApproverCommittee for Examination and Approval of Physical Terms
[See the contents]《Physical Terms (Second Edition)》Science Press, 1996
[Time of Publication] 1996[1]
Calculation formula
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This force applied on unit liquid layer area is called shear stress τ (N/m2)
Shear rate (D) D=d v/d x (S-1)
Shear stress and shear rate are two basic parameters to characterize the rheological properties of the system
Newton defined the viscosity of the fluid in the pattern shown in Figure 4-1.Two parallel fluids with different planes have the same area "A", a distance "dx", and flow in the same direction at different flow rates "V1" and "V2". Newton assumed that the force to maintain this different flow rate is proportional to the relative velocity or velocity gradient of the fluid, namely:
τ=η dv/dx=η D (Newtonian formula), where η is related to material properties, we call it "dynamic viscosity".
Definition of dynamic viscosity: immerse two plates with an area of 1m2 in the liquid, and the distance between the two plates is 1m. If 1N shear stress is added, so that the relative speed between the two plates is 1m/s, the viscosity of the liquid is 1Pa. s.
Kinematic viscosity is the ratio of dynamic viscosity to density.
Newtonian fluid: fluid that conforms to Newton's formula.Viscosity is only related to temperature, not to shear rate, and τ is proportional to D.
Non Newtonian fluid: does not conform to the Newtonian formula τ/D=f (D), and η a represents the viscosity at a certain (τ/D), called apparent viscosity.[2]
Viscosity measurement
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There are three measurement methods for dynamic viscosity, kinematic viscosity and conditional viscosity.
(1) Dynamic viscosity: η t is the resistance generated when the two liquid layers are 1 cm apart and their areas are 1 (square centimeter) respectively and the relative moving speed is 1 cm/s, and the unit is g/li m · s.1 g/cm · s=1 poise General: industrial dynamic viscosity unit is expressed in poise.
(2) Kinematic viscosity: at the temperature t ℃, the kinematic viscosity is represented by the symbol γ. In the International System of Units, the unit of kinematic viscosity is s, that isQuadratic meters per second (mtwo/s)[3]In the actual measurement, it is often used in centistokes, and (cst) means centistokes in square millimeters per second (i.e. 1cst=1mmtwo/s)。Kinematic viscosity is widely used to measure air jetfuel oil、diesel oil, lubricating oil and other liquid petroleum products The viscosity and kinematic viscosity of dark petroleum products, used lubricating oil and crude oil are measured by counter current method
(3) Conditional viscosity: refers to the viscosity measured by different specific viscometers and expressed in conditional units. There are three kinds of conditional viscosity commonly used in various countries:
①Engler viscosityAlso called Engler viscosity.It is the ratio of the time (s) required for a certain amount of sample to flow 200 ml of sample from the Engler viscometer at the specified temperature (such as 50 ℃, 80 ℃, 100 ℃) to the time (s) required for distilled water to flow out the same volume at 20 ℃.When the temperature is t º, the Engler viscosity is expressed by the symbol Et, and the unit of Engler viscosity is condition degree.
② Saybolt viscosity, that is, sagbolt viscosity.It is the number of seconds required for a certain amount of sample to flow 200 ml from the Saybolt viscometer at a specified temperature (such as 100 º F, F210 º F or 122 º F), in seconds.Saybolt viscosity is divided into Saybolt universal viscosity and Saybolt heavy oil viscosity (or Furol viscosity).
③ Redwood viscosity refers to Redwood viscosity.It refers to the number of seconds required for a certain amount of sample to flow out 50 ml from the Raytheon meter at the specified temperature, in "seconds".Rexhlet viscosity is divided into Rexhlet No. 1 (Rt) and Rexhlet No. 2 (RAt).
The above three conditional viscosity measurement methods are commonly used in Europe and the United States. In China, except for measuring dark lubricating oil and residual oil with Engler viscometer, the other two viscometers are rarely used.The three conditional viscosities are expressed in different ways and units, but their relationship can be converted through charts.At the same time, Engler viscosity and kinematic viscosity can also be converted, which is much more convenient and flexible
The property of internal friction between fluids is called fluid viscosity.
Dynamic viscosity refers to the ratio of the internal friction force on the unit contact area of the fluid to the rate of change of the flow velocity perpendicular to the direction of motion.
Kinematic viscosity refers to the ratio of dynamic viscosity and fluid density at the same temperature and pressure.
