Basic knowledge of viscosity

Dec 17, 2019

Kinematic viscosity is the ratio of the dynamic viscosity of a liquid to the density of the fluid ρ at the same temperature. The unit is (m ^ 2) / s. Represented by a lowercase letter v.
  Note: The unit that has been used is St (斯)
  The relationship between St (s) and (m ^ 2) / s is: 1 (m ^ 2) / s = 10 ^ 4St = 10 ^ 6cSt. (Where "cSt" is pronounced "centis")
  The flowing liquid is regarded as many liquid layers moving in parallel with each other.The speed of each layer is different, forming a speed gradient (dv / dx). This is the basic characteristic of the flow.
  Due to the existence of the velocity gradient, the slower liquid layer retards the flow of the faster liquid layer, so the liquid generates motion resistance. In order to maintain a certain velocity gradient motion of the liquid layer, a reaction opposite to the resistance must be applied to the liquid layer. To force.
  This kind of force exerted on the unit liquid layer area is called the shear stress τ (N / m2).
  Shear rate (D) D = d v / d x (S-1)
  Shear stress and shear rate are two basic parameters that characterize the rheological properties of the system
  Newton uses the pattern in Figure 4-1 to define the viscosity of a fluid. Two different plane but parallel fluids have the same area "A", separated by a distance "dx", and flow in the same direction at different flow rates "V1" and "V2". Newton assumes that the force to maintain this different flow rate is proportional to the fluid's Relative velocity or velocity gradient, ie:
  τ = ηdv / dx = ηD (Newton's formula) where η is related to the material properties, we call it "viscosity".
  Definition of viscosity: two plates with an area of ​​1m2 are immersed in a liquid, and the distance between the two plates is 1 meter.If a 1N shear stress is added to make the relative velocity between the two plates 1m / s, the viscosity of the liquid is 1Pa .s.
  Newtonian fluid: A fluid that conforms to Newton's formula. Viscosity is only related to temperature and has nothing to do with shear rate. Τ and D are proportional.
  Non-Newtonian fluid: Does not conform to Newton's formula τ / D = f (D), and ηa represents the viscosity at a certain (τ / D), which is called apparent viscosity.
  There are three methods for measuring viscosity: dynamic viscosity, kinematic viscosity and conditional viscosity.
  (1) Dynamic viscosity: ηt is the resistance generated when the two liquid layers are 1 cm apart and each area is 1 (square centimeter) when the relative moving speed is 1 cm / sec. The unit is grams / rime • second. 1g / cm • sec = 1poise In general: the industrial viscosity unit is expressed in poise.
  (2) Kinematic viscosity: At the temperature t ° C, the kinematic viscosity is represented by the symbol γ. In the international unit system, the unit of kinematic viscosity is Si, that is, square meters per second (m2 / s), and centiseconds are commonly used in actual measurement. cst) means the unit of centimeter is square millimeter per second (ie 1cst = 1mm2 / s). Kinematic viscosity is widely used to measure the viscosity of liquid petroleum products such as jet fuel oil, diesel oil, lubricating oil, dark petroleum products, used lubricants, crude oil, etc. The kinematic viscosity is measured by the countercurrent method
  (3) Conditional viscosity: refers to the viscosity expressed in conditional units measured with different specific viscometers. There are three types of conditional viscosity commonly used in various countries:
  ① Engle viscosity is also called Engler viscosity. It is a certain amount of sample. At a specified temperature (such as: 50 ° C, 80 ° C, 100 ° C), the time required to flow out a 200 ml sample from an Engelson viscometer is the time required for distilled water to flow out at the same volume at 20 ° C (Seconds) ratio. When the temperature is tº, the Engelson viscosity is represented by the symbol Et, and the unit of the Engelson viscosity is the degree of condition.
  ② Sai's viscosity, that is, Sagbolt viscosity. It is a certain amount of sample, the number of seconds required to flow 200 ml from the Sai's viscometer at a specified temperature (such as 100ºF, F210ºF, 122ºF, etc.), in "seconds" units. Sai's viscosity is divided into Sai's universal viscosity and Sai's heavy oil viscosity (or Furol viscosity).
  ③ The Rayleigh viscosity is the Redwood viscosity. It is a certain amount of sample, the number of seconds required to flow out of 50 ml from the Leinometer at a specified temperature, in seconds. The Rayleigh viscosity is further divided into Rayleigh No. 1 (represented by Rt) and Rayleigh 2 (represented by RAt).
  The above three conditional viscometry methods are commonly used in Europe and the United States. Except for the dark lubricating oil and residual oil, which are measured by Enshi viscometer, the other two viscometers are rarely used. The three conditioned viscosity expression methods and units are different, but the relationship between them can be converted by the chart. At the same time, Engelson viscosity and kinematic viscosity can be converted, which is much more convenient and flexible.
  The nature of internal friction between fluids is called the viscosity of the fluid.
  Dynamic viscosity refers to the ratio of the internal friction force per unit contact area of ​​a fluid to the rate of change of flow velocity perpendicular to the direction of motion.
  Kinematic viscosity refers to the ratio of dynamic viscosity to the density of a fluid at the same temperature and pressure.