Viscosity coefficient is a key parameter of a working fluid or gas. In physical terms, viscosity can be defined as the internal friction caused by the movement of particles that make up the mass of a liquid (gaseous) medium, or, more simply, the resistance to movement.
What is viscosity
The simplest empirical experiment for determining viscosity: the same amount of water and oil are poured onto a smooth inclined surface at the same time. Water drains faster than oil. She is more fluid. A moving oil is prevented from draining quickly by the higher friction between its molecules (internal resistance - viscosity). Thus, the viscosity of a liquid is inversely proportional to its fluidity.
Viscosity ratio: formula
In a simplified form, the process of movement of a viscous fluid in a pipeline can be considered in the form of flat parallel layers A and B with the same surface area S, the distance between which is h.
These two layers (A and B) move at different speeds (V and V+ΔV). Layer A, which has the highest speed (V+ΔV), involves layer B, which moves at a lower speed (V). At the same time, layer B tends to slow down the speed of layer A. The physical meaning of the viscosity coefficient is that the friction of the molecules, which are the resistance of the flow layers, forms a force that Isaac Newton described by the following formula:
F=µ × S × (ΔV/h)
Here:
- ΔV is the difference in the velocities of the fluid flow layers;
- h – distance between layers of fluid flow;
- S – surface area of the fluid flow layer;
- Μ (mu) - a coefficient depending on the property of the liquid, called the absolute dynamic viscosity.
In SI units, the formula looks like this:
µ=(F × h) / (S × ΔV)=[Pa × s] (Pascal × second)
Here F is the force of gravity (weight) of the unit volume of the working fluid.
Viscosity value
In most cases, the dynamic viscosity coefficient is measured in centipoise (cP) in accordance with the CGS system of units (centimeter, gram, second). In practice, viscosity is related to the ratio of the mass of a liquid to its volume, that is, to the density of the liquid:
ρ=m / V
Here:
- ρ – liquid density;
- m – mass of fluid;
- V is the volume of liquid.
The relationship between dynamic viscosity (Μ) and density (ρ) is called kinematic viscosity ν (ν – in Greek –nude):
ν=Μ / ρ=[m2/s]
By the way, the methods for determining the viscosity coefficient are different. For example, kinematic viscosity is still measured in accordance with the CGS system in centistokes (cSt) and in fractional units - stokes (St):
- 1St=10-4 m2/s=1 cm2/s;
- 1sSt=10-6 m2/s=1 mm2/s.
Determining the viscosity of water
The viscosity of water is determined by measuring the time it takes for fluid to flow through a calibrated capillary tube. This device is calibrated with a standard fluid of known viscosity. To determine the kinematic viscosity, measured in mm2/s, the fluid flow time, measured in seconds, is multiplied by a constant.
The unit of comparison is the viscosity of distilled water, the value of which is almost constant even when the temperature changes. The viscosity coefficient is the ratio of the time in seconds it takes a fixed volume of distilled water to flow out of a calibrated orifice to that of the fluid being tested.
Viscometers
Viscosity is measured in degrees Engler (°E), Saybolt Universal Seconds ("SUS") or degrees Redwood (°RJ) depending on the type of viscometer used. The three types of viscometers differ only in the amount of fluid flowing out.
Viscometer measuring viscosity in the European unit degree Engler (°E), calculated200cm3 outflowing liquid medium. A viscometer measuring viscosity in Saybolt Universal Seconds ("SUS" or "SSU" used in the USA) contains 60 cm3 of the test fluid. In England, where degrees Redwood (°RJ) are used, the viscometer measures the viscosity of 50 cm3 liquid. For example, if 200 cm3 of a certain oil flows ten times slower than the same volume of water, then the Engler viscosity is 10°E.
Because temperature is a key factor in changing the viscosity coefficient, measurements are usually taken first at a constant temperature of 20°C, and then at higher values. The result is thus expressed by adding the appropriate temperature, for example: 10°E/50°C or 2.8°E/90°C. The viscosity of a liquid at 20°C is higher than its viscosity at higher temperatures. Hydraulic oils have the following viscosities at their respective temperatures:
190 cSt at 20°C=45.4 cSt at 50°C=11.3 cSt at 100°C.
Translate values
Determination of the viscosity coefficient occurs in different systems (American, English, GHS), and therefore it is often necessary to transfer data from one dimensional system to another. To convert fluid viscosity values expressed in degrees Engler to centistokes (mm2/s), use the following empirical formula:
ν(cSt)=7.6 × °E × (1-1/°E3)
For example:
- 2°E=7.6 × 2 × (1-1/23)=15.2 × (0.875)=13.3 cSt;
- 9°E=7,6 × 9 × (1-1/93)=68.4 × (0.9986)=68.3 cSt.
To quickly determine the standard viscosity of hydraulic oil, the formula can be simplified as follows:
ν(cSt)=7.6 × °E(mm2/s)
Having a kinematic viscosity ν in mm2/s or cSt, you can convert it to a dynamic viscosity coefficient Μ using the following relationship:
M=ν × ρ
Example. Summarizing the various conversion formulas for degrees Engler (°E), centistokes (cSt) and centipoise (cP), suppose that a hydraulic oil with a density of ρ=910 kg/m3 has a kinematic viscosity of 12° E, which in units of cSt is:
ν=7.6 × 12 × (1-1/123)=91.2 × (0.99)=90.3 mm2/s.
Because 1cSt=10-6m2/s and 1cP=10-3N×s/m2, then the dynamic viscosity will be:
M=ν × ρ=90.3 × 10-6 910=0.082 N×s/m2=82 cP.
Gas viscosity factor
It is determined by the composition (chemical, mechanical) of the gas, the effect of temperature, pressure, and is used in gas-dynamic calculations related to the movement of gas. In practice, the viscosity of gases is taken into account when designing gas field developments, where the coefficient changes are calculated depending on changes in the gas composition (especially important for gas condensate fields), temperature and pressure.
Calculate the viscosity of air. The processes will be similar tothe two streams discussed above. Suppose two gas streams U1 and U2 move in parallel, but at different speeds. Convection (mutual penetration) of molecules will occur between the layers. As a result, the momentum of the faster-moving air stream will decrease, and the initially moving slower one will accelerate.
The viscosity coefficient of air, according to Newton's law, is expressed by the following formula:
F=-h × (dU/dZ) × S
Here:
- dU/dZ is the velocity gradient;
- S – force impact area;
- Coefficient h - dynamic viscosity.
Viscosity index
Viscosity index (VI) is a parameter that correlates changes in viscosity and temperature. A correlation is a statistical relationship, in this case two quantities, in which a change in temperature accompanies a systematic change in viscosity. The higher the viscosity index, the smaller the change between the two values, that is, the viscosity of the working fluid is more stable with temperature changes.
Oil viscosity
The bases of modern oils have a viscosity index below 95-100 units. Therefore, in the hydraulic systems of machines and equipment, sufficiently stable working fluids can be used, which limit the wide change in viscosity under conditions of critical temperatures.
"Favorable" viscosity coefficient can be maintained by introducing into the oil special additives (polymers) obtained during the distillation of oil. They increase the viscosity index of oils foraccount of limiting the change of this characteristic in the allowable interval. In practice, with the introduction of the required amount of additives, the low viscosity index of the base oil can be increased to 100-105 units. However, the mixture obtained in this way deteriorates its properties at high pressure and heat load, thereby reducing the effectiveness of the additive.
In the power circuits of powerful hydraulic systems, working fluids with a viscosity index of 100 units should be used. Working fluids with additives that increase the viscosity index are used in hydraulic control circuits and other systems operating in the low / medium pressure range, in a limited temperature range, with small leaks and in batch operation. With increasing pressure, viscosity also increases, but this process occurs at pressures above 30.0 MPa (300 bar). In practice, this factor is often neglected.
Measurement and indexing
In accordance with international ISO standards, the viscosity coefficient of water (and other liquid media) is expressed in centistokes: cSt (mm2/s). Viscosity measurements of process oils should be carried out at temperatures of 0°C, 40°C and 100°C. In any case, in the oil grade code, the viscosity must be indicated by a figure at a temperature of 40 ° C. In GOST, the viscosity value is given at 50°C. The grades most commonly used in engineering hydraulics range from ISO VG 22 to ISO VG 68.
Hydraulic oils VG 22, VG 32, VG 46, VG 68, VG 100 at 40°C have viscosity values corresponding to their marking: 22, 32, 46, 68 and 100 cSt. Optim althe kinematic viscosity of the working fluid in hydraulic systems ranges from 16 to 36 cSt.
The American Society of Automotive Engineers (SAE) has established viscosity ranges at specific temperatures and assigned them the appropriate codes. The number following the W is the absolute dynamic viscosity Μ at 0°F (-17.7°C) and the kinematic viscosity ν was determined at 212°F (100°C). This indexation applies to all-season oils used in the automotive industry (transmission, motor, etc.).
Effect of viscosity on hydraulics
Determination of the coefficient of viscosity of a liquid is not only of scientific and educational interest, but also carries an important practical value. In hydraulic systems, working fluids not only transfer energy from the pump to hydraulic motors, but also lubricate all parts of the components and remove the generated heat from the friction pairs. The viscosity of the working fluid that is not appropriate for the operating mode can seriously impair the efficiency of all hydraulics.
High viscosity of the working fluid (oil of very high density) leads to the following negative phenomena:
- Increased resistance to hydraulic fluid flow causes an excessive pressure drop in the hydraulic system.
- Deceleration of control speed and mechanical movements of actuators.
- Development of cavitation in the pump.
- Zero or too low air release from hydraulic tank oil.
- Noticeableloss of power (decrease in efficiency) of hydraulics due to high energy costs to overcome the internal friction of the fluid.
- Increased machine prime mover torque caused by increased pump load.
- Rise in hydraulic fluid temperature due to increased friction.
Thus, the physical meaning of the viscosity coefficient lies in its influence (positive or negative) on the components and mechanisms of vehicles, machines and equipment.
Loss of hydraulic power
Low viscosity of the working fluid (oil of low density) leads to the following negative phenomena:
- Decrease in volumetric efficiency of pumps as a result of increasing internal leakage.
- Increase in internal leaks in the hydraulic components of the entire hydraulic system - pumps, valves, hydraulic distributors, hydraulic motors.
- Increased wear of pumping units and jamming of pumps due to insufficient viscosity of the working fluid necessary to provide lubrication of rubbing parts.
Compressibility
Any liquid compresses under pressure. With regard to oils and coolants used in mechanical engineering hydraulics, it has been empirically established that the compression process is inversely proportional to the mass of the liquid per volume. The compression ratio is higher for mineral oils, significantly lower for water, and much lower for synthetic fluids.
In simple low pressure hydraulic systems, the compressibility of the fluid has negligible effect on the reduction of the initial volume. But in powerful machines with high hydraulicpressure and large hydraulic cylinders, this process manifests itself noticeably. For hydraulic mineral oils at a pressure of 10.0 MPa (100 bar), the volume decreases by 0.7%. At the same time, the change in compression volume is slightly affected by the kinematic viscosity and the type of oil.
Conclusion
Determination of the viscosity coefficient allows you to predict the operation of equipment and mechanisms under various conditions, taking into account changes in the composition of a liquid or gas, pressure, temperature. Also, the control of these indicators is relevant in the oil and gas sector, utilities, and other industries.
