Viscosity knowledge

The liquid flowing parallel to each other to move as many of the liquid layer, layers at different speeds, the formation of velocity gradient (dv / dx), which is the flow of basic features.

Since the existence of velocity gradient, the slower flow of the liquid layer block the flow of the liquid layer rapidly, thus. Drag the liquid in motion. In order to maintain a liquid layer velocity gradient movement, must be applied by a liquid layer opposite the anti-resistance force.

Liquid layer in the unit area exerted such a force, called shear stress τ (N/m2).

Shear rate (D) D = d v / d x (S-1)

Shear stress and shear rate to characterize the rheological properties of the two basic parameters

Newton's model in Figure 4-1 to define the fluid viscosity. However, the two different planes parallel to the fluid, has the same area "A", apart from the "dx", and a different flow rate "V1" and "V2" to the same direction of flow, Newton assumed the power to maintain this flow rate is proportional to the different fluid the relative speed or velocity gradient, namely:

τ = ηdv / dx = ηD (Newton formula) η and materials related to the nature of which we call "viscosity."

Viscosity Definition: The area of 1m2 of the board of two immersed liquid, the two plate distance of 1 meter plus 1N if the shear stress, so that the relative velocity between the two plates is 1m / s, then the viscosity of the liquid To 1Pa.s.

Newtonian fluid: the fluid line with Newton formula. Viscosity and temperature only, independent of shear rate and, τ and D is proportional to the relationship.

Non-Newtonian fluid: does not meet the Newton formula τ / D = f (D), to ηa that some (τ / D) the viscosity, said the apparent viscosity.

Determination of viscosity are: dynamic viscosity, kinematic viscosity and viscosity conditions of three measurement methods.

(1) dynamic viscosity: ηt is two liquid layers separated by 1 cm, the area of each is 1 (square centimeter) the relative speed of 1 cm / s arising from the resistance of g / in m seconds. 1 g / cm s = 1 Park General: dynamic viscosity of the industrial units to use the park said.

(2) kinematic viscosity: temperature t ℃, the kinematic viscosity by the symbol γ said that in the International System of Units, the kinematic viscosity unit of Sri Lanka, that is, per square meter (m2 / s), commonly used in determining the actual measurement Sri Lanka, (cst) determined that the unit of Sri Lanka per square millimeter (ie 1cst = 1mm2 / s). Widely used to determine the kinematic viscosity of jet fuel, diesel, lubricants and other liquid petroleum products, dark oil products, after use of lubricants, crude oil viscosity, kinematic viscosity measured by upstream method

(3) conditions Viscosity: refers to the use of different specific viscosity measured by the total units in terms of viscosity, viscosity of countries usually have the following three conditions:

① a viscosity called Sige Le (Engler) viscosity. Is a certain amount of samples, at specified temperature (eg: 50 ℃, 80 ℃, 100 ℃), the total outflow from the viscosity of the sample time required for 200 ml distilled water at 20 ℃ with the same volume of flow required Time (sec) ratio. Temperature t ��, the viscosity of the symbol Et, said viscosity of the conditions the unit is degrees.

② viscosity of race, that Saybolt (sagbolt) viscosity. Is a certain amount of samples, at specified temperature (eg 100 �� F, F210 �� F or 122 �� F, etc.) out from the competition's 200 ml viscometer number of seconds required to "second" unit. Saybolt Viscosity Saybolt Universal Viscosity is divided and the game's heavy oil viscosity (or the game's Furrow (Furol) viscosity) two.

③ viscosity of the Red Ray-Ude (Redwood) viscosity. Is a certain amount of samples, at specified temperature, flow meters from the mine's number of seconds required for 50 ml to "seconds" for the unit. Lei Lei's is divided into a viscosity of 1 (Rt said) and Ray's 2 (with RAt said) two.

Determination of viscosity of the three conditions, commonly used in Europe and the United States, China addition Engler Viscometer using a dark oil and residue oil, the other two viscometer rarely used. Viscosity of the three conditions of representation and units vary, but the relationship between them can be converted through the chart. At the same time a viscosity and kinematic viscosity can also be translated, so much more convenient and flexible.

There are many ways the determination of viscosity, such as the transfer drum method, falling ball method, damping vibration method, cup-type viscometer, capillary method and so on. For low viscosity fluids such as water, ethanol, carbon tetrachloride, etc., commonly used measurement of capillary viscometer; while the viscosity of the fluid, such as castor oil, transformer oil, engine oil, glycerin and other transparent (or translucent) liquid, often falling ball method; the viscosity of 0.1 ~ 100Pa? s range of liquid transfer tube method can also be measured.

Laboratory determination of the principles of viscosity are generally large and by the Stokes formula derived by the formula on Poiseuille viscosity of the expression, obtained viscosity coefficient.

Viscosity depends on the nature and temperature of the liquid, the temperature increases, the viscosity will decrease rapidly. Therefore, to determine viscosity, must accurately control the temperature change makes sense. Determination of viscosity parameters, forecast production process for process control, transportation, and product in the operational use, an important guiding value in the printing, pharmaceutical, oil, automotive and other industries is significant.

In 1845, British mathematician, physicist Stokes (GG Stokes, 1819-1903) and France Navier (CLMH Navier), who derive the viscous fluid mechanics, respectively, the basic equations, namely, Navier - Stokes equations, laying the foundation of traditional fluid mechanics.

1851, Stokes derived solid sphere in a viscous medium, slow and the resistance movement suffered the formula derived in a constant force (gravity) under the action, resistance and flow rate, viscosity coefficient is proportional to namely the resistance of the Storrs formula.

Navier - Stokes equation is the most intricate mathematics in a class of nonlinear equations to find its exact solution is very difficult. Even today, some exact solutions of only 70, only about particular solution is more than one hundred out of solution, is the most complex, has not yet been fully resolved one of the world-class mathematical problem.