Choosing engine oil is a serious task for every car enthusiast. And the main parameter by which selection should be made is the viscosity of the oil. Oil viscosity characterizes the degree of thickness of the motor fluid and its ability to maintain its properties under temperature changes.
Let's try to figure out in what units viscosity should be measured, what functions it performs and why it plays a huge role in the operation of the entire motor system.
The operation of an internal combustion engine involves the continuous interaction of its structural elements. Let's imagine for a second that the engine is running dry. What will happen to him? First, the frictional force will increase the temperature inside the device. Secondly, deformation and wear of parts will occur. And finally, all this will lead to a complete stop of the internal combustion engine and the impossibility of its further use. Properly selected motor oil performs the following functions:
- protects the motor from overheating,
- prevents rapid wear of mechanisms,
- prevents the formation of corrosion,
- removes soot, soot and fuel combustion products outside the engine system,
- helps to increase the resource of the power unit.
Thus, normal functioning of the motor department without lubricating fluid is impossible.
Important! Pour into the engine vehicle You only need oil whose viscosity meets the requirements of car manufacturers. In this case, the efficiency will be maximum, and the wear of working units will be minimal. You should not trust the opinions of sales consultants, friends and car service specialists if they differ from the instructions for the car. After all, only the manufacturer can know for sure what to fill the engine with.
Oil viscosity index
The concept of oil viscosity implies the ability of a liquid to be viscous. It is determined using the viscosity index. The oil viscosity index is a value that shows the degree of viscosity of the oil fluid during temperature changes. Lubricants with a high degree of viscosity have the following properties:
- When the engine is cold started, the protective film has strong fluidity, which ensures quick and uniform distribution lubricants over the entire working surface;
- heating the engine causes an increase in the viscosity of the film. This property allows you to maintain a protective film on the surfaces of moving parts.
Those. oils with a high viscosity index easily adapt to temperature overloads, while a low viscosity index of a motor oil indicates less ability. Such substances have a more liquid state and form a thin protective film on the parts. In conditions of negative temperatures, motor fluid with a low viscosity index will make it difficult to start the power unit, and at high temperatures it will not be able to prevent high friction forces.
The viscosity index is calculated according to GOST 25371-82. You can calculate it using online services on the Internet.
Kinematic and dynamic viscosity
The degree of ductility of a motor material is determined by two indicators - kinematic and dynamic viscosity.
Engine oil
Kinematic viscosity of an oil is an indicator that reflects its fluidity at normal (+40 degrees Celsius) and high (+100 degrees Celsius) temperatures. The method for measuring this value is based on the use of a capillary viscometer. The device measures the time required for the oil fluid to flow out at given temperatures. Kinematic viscosity is measured in mm 2 /s.
The dynamic viscosity of the oil is also calculated empirically. It shows the resistance force of the oil fluid that occurs during the movement of two layers of oil, spaced 1 centimeter apart and moving at a speed of 1 cm/s. The units of measurement for this quantity are Pascal seconds.
The determination of oil viscosity must take place under different temperature conditions, because the liquid is not stable and changes its properties at low and high temperatures.
A table of motor oil viscosity by temperature is presented below.
Explanation of the engine oil designation
As noted earlier, viscosity is the main parameter of a protective fluid, characterizing its ability to ensure vehicle performance in various climatic conditions.
According to the international SAE classification system, motor lubricants can be of three types: winter, summer and all-season.
Oil intended for winter use is marked with a number and the letter W, for example, 5W, 10W, 15W. The first symbol of the marking indicates the range of negative operating temperatures. Letter W – from English word“Winter” - winter - informs the buyer about the possibility of using the lubricant in harsh low-temperature conditions. It has greater fluidity than its summer counterpart to ensure easy starting at low temperatures. The liquid film instantly envelops the cold elements and facilitates their scrolling.
The limit of negative temperatures at which the oil remains operational is as follows: for 0W - (-40) degrees Celsius, for 5W - (-35) degrees, for 10W - (-25) degrees, for 15W - (-35) degrees.
Summer liquid has a high viscosity, which allows the film to “stick” more firmly to the working elements. At too high temperatures, this oil spreads evenly over the working surface of the parts and protects them from severe wear. This oil is designated by numbers, for example, 20,30,40, etc. This figure characterizes the high-temperature limit in which the liquid retains its properties.
Important! What do the numbers mean? The numbers of the summer parameter in no way mean maximum temperature, at which the vehicle can operate. They are conditional and have nothing to do with the degree scale.
Oil with a viscosity of 30 functions normally at temperatures environment up to +30 degrees Celsius, 40 – up to +45 degrees, 50 – up to +50 degrees.
It is easy to recognize universal oil: its marking includes two numbers and the letter W between them, for example, 5w30. Its use implies any climatic conditions, be it harsh winter or hot summer. In both cases, the oil will adapt to changes and maintain the functionality of the entire engine system.
By the way, the climatic range of universal oil is determined simply. For example, for 5W30 it varies from minus 35 to +30 degrees Celsius.
All-season oils are convenient to use, which is why they are found on the shelves of car dealerships more often than summer and winter options.
To give you a better idea of what motor oil viscosity is appropriate in your area, below is a table showing the operating temperature range for each type of lubricant.
Average oil performance ranges
Having figured out what the numbers in oil viscosity mean, let’s move on to the next standard. The classification of motor oil by viscosity also affects the API standard. Depending on the type of engine, the API designation begins with the letter S or C. S means gasoline engines, C means diesel engines. The second letter of the classification indicates the quality class of the motor oil. And the further this letter is from the beginning of the alphabet, the better quality protective liquid.
For gasoline engine systems, the following designations exist:
- SC – year of manufacture before 1964
- SD – year of manufacture from 1964 to 1968.
- SE – year of manufacture from 1969 to 1972.
- SF – year of manufacture from 1973 to 1988.
- SG – year of manufacture from 1989 to 1994.
- SH – year of manufacture from 1995 to 1996.
- SJ – year of manufacture from 1997 to 2000.
- SL – year of production from 2001 to 2003.
- SM – year of manufacture after 2004
- SN – cars equipped modern system neutralization of exhaust gases.
For diesel:
- CB – year of manufacture before 1961
- CC – year of manufacture before 1983
- CD – year of release before 1990
- CE – year of manufacture before 1990 (turbocharged engine).
- CF – year of manufacture since 1990, (turbocharged engine).
- CG-4 – year of manufacture since 1994, (turbocharged engine).
- CH-4 – year of production: 1998
- CI-4 – modern cars (turbocharged engine).
- CI-4 plus is a much higher class.
What is good for one engine, is in danger of repair for another.
Engine oil
Many car owners are sure that it is worth choosing more viscous oils, because they are the key to long-lasting engine operation. This is a serious misconception. Yes, experts pour oil with a high degree of viscosity under the hoods of racing cars to achieve maximum service life of the power unit. But ordinary passenger cars are equipped with a different system, which will simply choke if the protective film is too thick.
What oil viscosity is permissible to use in the engine of a particular machine is described in any operating manual.
After all, before the launch of mass sales of models, automakers conducted a large number of tests, taking into account possible driving modes and operation technical means in various climatic conditions. By analyzing the behavior of the motor and its ability to maintain stable operation under certain conditions, engineers established acceptable parameters for motor lubrication. Deviation from them can provoke a decrease in the power of the propulsion system, its overheating, an increase in fuel consumption and much more.
Engine oil in the engine
Why is viscosity grade so important in the operation of mechanisms? Imagine for a moment the inside of the engine: there is a gap between the cylinders and the piston, the size of which should allow possible expansion of parts due to high temperature changes. But for maximum efficiency, this gap must have a minimum value, preventing exhaust gases generated during combustion of the fuel mixture from entering the engine system. To ensure that the piston body does not heat up from contact with the cylinders, motor lubricant is used.
The oil viscosity level must ensure the performance of each element of the propulsion system. Manufacturers of power units must achieve an optimal ratio of the minimum gap between the rubbing parts and the oil film, preventing premature wear of the elements and increasing the operating life of the engine. Agree, it is safer to trust official representatives of a car brand, knowing how this knowledge was obtained, than to trust “experienced” motorists who rely on intuition.
What happens when the engine starts?
If your “iron friend” stood in the cold all night, then the next morning the viscosity of the oil poured into it will be several times higher than the calculated operating value. Accordingly, the thickness of the protective film will exceed the gaps between the elements. When a cold engine starts, its power drops and the temperature inside it rises. Thus, the engine warms up.
Important! During warming up, you should not give it increased load. A lubricant that is too thick will impede the movement of the main mechanisms and lead to a reduction in the life of the vehicle.
Engine oil viscosity at operating temperatures
After the engine has warmed up, the cooling system is activated. One engine cycle looks like this:
- Pressing the gas pedal increases the engine speed and increases the load on it, as a result of which the friction force of the parts increases (since the too astringent liquid has not yet had time to get into the gaps between parts),
- the oil temperature rises,
- the degree of its viscosity decreases (fluidity increases),
- the thickness of the oil layer decreases (leaks into the gaps between parts),
- friction force decreases,
- The oil film temperature is reduced (partially with the help of the cooling system).
Any motor system works on this principle.
Viscosity of motor oils at a temperature of – 20 degrees
The dependence of oil viscosity on operating temperature is obvious. Just as it is obvious that high level motor protection should not decrease during the entire period of operation. The slightest deviation from the norm can lead to the disappearance of the motor film, which in turn will negatively affect the “defenseless” part.
Each internal combustion engine, although it has a similar design, has unique set consumer properties: power, efficiency, environmental friendliness and torque. These differences are explained by the difference in engine clearances and operating temperatures.
In order to select the oil for a vehicle as accurately as possible, international classifications of motor fluids have been developed.
The classification provided for by the SAE standard informs car owners about the average operating temperature range. API, ACEA, etc. classifications give a clearer idea of the possibility of using lubricant in certain vehicles.
Consequences of filling high viscosity oil
There are times when car owners do not know how to determine the required viscosity of engine oil for their car, and fill in the one recommended by sellers. What happens if the ductility is higher than required?
If oil with high viscosity “splashes” in a well-heated engine, then there is no danger for the engine (at normal speeds). In this case, the temperature inside the unit will simply increase, which will lead to a decrease in the viscosity of the lubricant. Those. the situation will return to normal. But! Regular repetition of this pattern will significantly reduce engine life.
If you suddenly “give on the gas”, causing an increase in speed, the degree of viscosity of the liquid will not correspond to the temperature. This will cause the maximum permissible temperature in the engine compartment to be exceeded. Overheating will cause an increase in friction force and a decrease in the wear resistance of parts. By the way, the oil itself will also lose its properties in a fairly short period of time.
You will not be able to immediately find out that the oil viscosity is not suitable for the vehicle.
The first “symptoms” will appear only after 100-150 thousand kilometers. And the main indicator will be an increase in the gaps between parts. However, even experienced specialists will not be able to definitely connect increased viscosity and a rapid decrease in engine life. It is for this reason that official auto repair shops often neglect the requirements of vehicle manufacturers. In addition, it is profitable for them to repair power units of cars whose warranty period has already expired. That is why choosing the degree of oil viscosity is a difficult task for every car enthusiast.
Viscosity too low: is it dangerous?
Engine oil
Low viscosity can destroy gasoline and diesel engines. This fact is explained by the fact that at increased operating temperatures and loads on the motor, the fluidity of the enveloping film increases, as a result of which the already liquid protection simply “exposes” the parts. Result: increased friction force, increased fuel consumption, deformation of mechanisms. It is impossible to operate a car for a long time with a low-viscosity liquid filled in - it will jam almost immediately.
Some modern engine models require the use of so-called “energy-saving” oils with reduced viscosity. But they can only be used if there are special approvals from car manufacturers: ACEA A1, B1 and ACEA A5, B5.
Oil density stabilizers
Due to constant temperature overloads, the viscosity of the oil gradually begins to decrease. And special stabilizers can help restore it. They can be used in engines of any type whose wear has reached an average or high level.
Stabilizers allow:
Stabilizers
- increase the viscosity of the protective film,
- reduce the amount of soot and deposits on engine cylinders,
- reduce emissions harmful substances in atmosphere,
- restore the protective oil layer,
- achieve “silence” in engine operation,
- prevent oxidation processes inside the motor housing.
The use of stabilizers allows not only to increase the period between oil changes, but also to restore lost beneficial features protective layer.
Types of special lubricants used in production
Spindle machine lubricant has low-viscosity properties. The use of such protection is rational on motors that have a light load and operate at high speeds. Most often, such a lubricant is used in textile production.
Turbine lubrication. Its main feature is to protect all working mechanisms from oxidation and premature wear. The optimal viscosity of turbine oil allows it to be used in turbocompressor drives, gas, steam and hydraulic turbines.
VMGZ or all-season hydraulic thickened oil. This liquid is ideal for equipment used in the regions of Siberia, the Far North and Far East. This oil is intended for internal combustion engines equipped with hydraulic drives. VMGZ is not divided into summer and winter oils, because its use only implies low-temperature climates.
The raw materials for hydraulic oil are low-viscosity components containing a mineral base. In order for the oil to reach the desired consistency, special additives are added to it.
The viscosity of hydraulic oil is shown in the table below.
OilRite is another lubricant used for the preservation and treatment of mechanisms. It has a waterproof graphite base and retains its properties in the temperature range from minus 20 degrees Celsius to plus 70 degrees Celsius.
conclusions
A clear answer to the question: “What is the best viscosity of motor oil?” no and cannot be. The thing is that the required degree of ductility for each mechanism - be it a weaving loom or a racing car engine - is different, and it cannot be determined “at random”. The required parameters of lubricating fluids are calculated by manufacturers empirically, so when choosing a fluid for your vehicle, you are primarily guided by the instructions of the developer. And after that, you can refer to the table of motor oil viscosity by temperature.
Viscosity- this is the property of a liquid to resist shearing forces. Viscosity is a property inherent in both droplet liquids and gases, which manifests itself only when moving, cannot be detected at rest, and manifests itself in the form of internal friction when adjacent particles of the liquid move. Viscosity characterizes the degree of fluidity of a liquid and the mobility of its particles. The viscosity of liquids explains the resistance and loss of pressure that occurs when they move through pipes, channels and other channels, as well as when foreign bodies move in them.
Isaac Newton was actively involved in the study of the properties of internal friction of a liquid, laying the foundations for the doctrine of viscosity. Newton suggested (later confirmed by experiment) that the resistance forces arising during such sliding of layers are proportional to the area of contact of the layers and the sliding speed. As a result, I. Newton obtained a dependence characterizing the relationship between viscosity and the phenomenon of internal friction, which was called the law of the same name.
Let the liquid flow along a flat wall in parallel layers. Each layer will move at its own speed, and the speed of the layers will increase as they move away from the wall.
Let us consider two layers of liquid moving at a distance Δy from each other. Since there is a frictional force between the layers and due to mutual braking, the different layers have different speeds, and layer A moves with speed v, and layer B moves with speed (v+Δv). The value Δv is the absolute shift of layer A over layer B, and the value Δv/Δy is the relative shift, or velocity gradient. Then, during movement, a tangential stress τ (tau) arises, which characterizes friction per unit area (internal friction stress).
Internal friction stress has a physical meaning depending on:
Where F tr- internal friction force, N; S- contact area of surfaces, m2.
Then, according to Newton’s law, the relationship between stress and relative shift will be:
those. internal friction stress proportional to the velocity gradient.
Proportionality factor µ (mu) is called dynamic viscosity coefficient. From the formula it is clear that the dynamic coefficient of viscosity is numerically equal to the internal friction stress in the case when the relative speed of two planes A and B, spaced from each other at a distance of 1 m, is equal to 1 m/s.
The dimension of the dynamic viscosity coefficient follows from the formula. Since the voltage τ is the force per unit area, then its dimension is equal to:
Speed gradient dimension:
Hence the dimension of the dynamic viscosity coefficient:
Thus, the unit of measurement of dynamic viscosity in the SI system of units is taken to be:
IN physical system The unit of dynamic viscosity is the poise, denoted by “ P»:
The dynamic viscosity of droplet liquids, the molecules of which are located very close to each other, decreases with increasing temperature due to an increase in the speed of Brownian motion, which weakens the holding bonds, that is, the adhesion force.
Dependence of coefficient μ on temperature is generally expressed by the formula:
where is the value at t= 0°C; A And b- experimental coefficients depending on the physicochemical properties (type) of the liquid; t- liquid temperature in °C.
In gases, the forces of attraction between molecules manifest themselves only under strong compression, and in normal conditions gas molecules are in a state of chaotic thermal motion and friction of gas layers against each other occurs only due to the collision of molecules. As the temperature increases, the speed of the molecules increases, the number of their collisions increases, and the viscosity increases.
For fresh water, Poiseuille obtained the formula:
For air, Millikan's formula is known:
In hydraulics, to characterize the viscous properties of gases and vapors, sometimes instead of the dynamic one, another viscosity coefficient is used, denoted by the letter η (eta) and related to the dynamic coefficient by the equation
where g is the acceleration of gravity, m/s 2 .
Obviously, this viscosity coefficient η has the dimension:
In this case, the unit of measurement η in the technical system of units is
In hydraulics and manufacturing, the so-called kinematic viscosity coefficient ν(nu), defined as the ratio of dynamic viscosity to density:
Dimension of kinematic viscosity coefficient:
In the SI system, the unit adopted for ν is .
Unit of measurement of the coefficient ν in a physical system serves as Stokes, denoted by “ St»:
For example, the kinematic coefficient of viscosity of water is equal to
The reciprocal of dynamic viscosity is called fluidity.
Viscosity for all droplet liquids decreases with increasing temperature. To obtain accurate hydraulic calculations, it is recommended to have a graph (or table) of the dependence of viscosity on temperature, based on special determinations in the laboratory. You should be very careful when handling various kinds nomograms and formulas used to determine the viscosity of a mixture of two or more different petroleum products.
A graph characterizing the dependence of changes in liquid viscosity on temperature is called viscogram(Fig. 1.3).
Fig.1.3. Viscogram
To determine the viscosity of a liquid at any arbitrary temperature T the Reynolds-Filonov formula is used with sufficient accuracy:
Where ν - viscosity at a known temperature T , u- coefficient of viscogram steepness, which characterizes the angle of inclination of the tangent viscogram to the abscissa axis (Fig. 1.4) and is determined by the formula:
Fig. 1.4 Determination of the viscogram slope coefficient
Thus, it is possible to characterize any liquid and determine its viscosity at any temperature, knowing the coordinates of two arbitrary points on the viscogram. It is worth noting that for droplet liquids the viscogram coefficient is positive, however, there are liquids whose viscosity changes little with temperature changes; for gaseous liquids, the viscogram coefficient is negative. There are liquids whose viscosity depends little on temperature; they are complex chemical compounds and are used as workers in hydraulic machines, such as viscous couplings.
There are liquids for which I. Newton's law does not apply. Unlike ordinary Newtonian fluids, these fluids are called non-Newtonian, or abnormal.
Values of kinematic viscosity ν of water and air
The viscosity of different types of liquid of the same name, for example, oil, depending on chemical composition and molecular structure can have different meanings.
For viscous oils, average values u= 0.05 + 0.1 per 1°C.
The viscosity of liquids, as experiments show, also depends on pressure. As pressure increases, it usually increases. The exception is water, for which at temperatures up to 32 ° C the viscosity decreases with increasing pressure. At pressures encountered in practice (up to 20 MPa), the change in the viscosity of liquids is very small and is not taken into account in conventional hydraulic calculations.
VISCOSITY, the property of a liquid (or gas) to resist flow.
Viscosity is also considered as one of the transfer phenomena that determines the dissipation of energy during deformation of the medium. The viscosity of solids has a number of features and is usually considered separately (see Internal friction).
During laminar movement of a liquid between two plane-parallel plates, one of which is stationary and the other moves with speed ν, the molecular layer immediately adjacent to the lower plate remains stationary, and the layer adjacent to the upper plate will move at maximum speed (Fig.) . The flow of a liquid is characterized by a velocity gradient γ? = dv/dz, indicating the rate of change in velocity from layer to layer in the direction perpendicular to the movement of the liquid. If the speed changes linearly, then γ?= v/d, where d is the distance between the plates. The quantity γ is also called the shear rate.
According to the fundamental law of viscous flow established by I. Newton (published in 1687), the shear stress τ = F/S causing fluid flow is proportional to the gradient of the flow velocity: τ = ηγ?. The proportionality coefficient η is called the coefficient of dynamic viscosity, or simply viscosity. It characterizes the fluid's resistance to flow. Viscosity can also be thought of as a measure of the energy dissipated in the form of heat as a fluid flows. Energy dissipation occurs due to the transfer of momentum. The values of the viscosity coefficient and the power W dissipated per unit volume due to viscosity are related by the relation: W = ηγ? 2.
The relation established by Newton is valid only in the case when η does not depend on the shear rate. Media in which this condition is satisfied are called Newtonian (see Newtonian fluid).
The SI unit of dynamic viscosity is Pa s [in CGS it is poise (dyne s/cm2): 1 poise = 0.1 Pa s]. The value φ= 1/η, the reciprocal of viscosity, is called fluidity. Also often considered is the kinematic viscosity ν = η/ρ (where ρ is the density of the substance), measured in m 2 / s (SI) and Stokes (GHS). The viscosity of liquids and gases is measured using viscometers (see Viscometry).
The viscosity of ideal gases is determined by the relation: η = (1/3)mn??, where m is the mass of the molecule, n is the number of molecules per unit volume, ? - average speed of molecules, ? is the free path of the molecule.
The viscosity of gases increases when heated, while the viscosity of liquids, on the contrary, decreases. This is due to the different molecular mechanisms of viscosity in these systems. There are two mechanisms of momentum transfer: kinetic (not involving collisions between molecules) and collisional. The first is predominant in rarefied gas, the second - in dense gas and liquid.
In gases, the distances between molecules are significantly greater than the radius of action of molecular forces, therefore the viscosity of gases is a consequence of the chaotic (thermal) movement of molecules, as a result of which molecules move from layer to layer, slowing down the flow. Since the average speed of molecules? increases with increasing temperature, the viscosity of gases increases when heated.
The viscosity of liquids, where the distance between molecules is much smaller than in gases, is primarily due to intermolecular interactions that limit the mobility of molecules. As the temperature increases, the mutual movement of molecules becomes easier, intermolecular interactions weaken and, consequently, the internal friction of the liquid decreases.
The viscosity of a liquid is determined by the size and shape of the molecules, their relative position and the strength of intermolecular interactions. Viscosity depends on the chemical structure of the liquid molecules. Yes, viscosity organic matter increases with the introduction of polar groups and rings into the molecule. In homologous series (saturated hydrocarbons, alcohols, organic acids etc.) the viscosity of compounds increases with increasing molecular weight.
The viscosity of solutions depends on their concentration and can be either greater or less than the viscosity of a pure solvent. The viscosity of extremely dilute suspensions depends linearly on the volume fraction φ of suspended particles: η = η 0 (1 + αφ) (Einstein formula), where η 0 is the viscosity of the dispersion medium. The coefficient α depends on the shape of the particles; in particular, for spherical particles α = 2.5. A similar dependence of viscosity on volume fraction is observed in solutions of globular proteins.
Viscosity can vary within wide limits. The following are the viscosity values of some liquids and gases at a temperature of 20°C (in 10 -3 Pa s): gases - hydrogen 0.0088, nitrogen 0.0175, oxygen 0.0202; liquids - water 1.002, ethanol 1.200, mercury 1.554, nitrobenzene 2.030, glycerol 1.485.
Liquid helium has the lowest viscosity. At a temperature of 2.172 K it goes into a superfluid state, in which the viscosity is zero (see Superfluidity). The viscosity of gases is hundreds of times less than the viscosity of ordinary liquids. The viscosity of molten metals is close in order of magnitude to the viscosity of ordinary liquids.
Polymer solutions and melts have high viscosity. The viscosity of even dilute polymer solutions is significantly higher than the viscosity of low molecular weight compounds. This is due to the fact that the sizes of polymer macromolecules are so large that different sections of the same macromolecule end up in layers moving at different speeds, which causes additional resistance to flow. The viscosity of more concentrated polymer solutions becomes even higher due to the entanglement of macromolecules with each other. One of the methods for estimating the molecular weight of polymers is based on measuring the viscosity of solutions.
The presence in polymer solutions of spatial structures formed by the adhesion of macromolecules leads to the appearance of so-called structural viscosity, which (unlike the viscosity of Newtonian liquids) depends on the shear stress (or speed) (see Rheology). When a structured fluid flows, work external forces is spent not only on overcoming internal friction, but also on destroying the structure.
Lit.: Landau L. D., Akhiezer A. I., Lifshits E. M. Kurs general physics. Mechanics and Molecular physics. 2nd ed. M., 1969; Filippova O. E., Khokhlov A. R. Viscosity of dilute polymer solutions. M., 2002; Schramm G. Fundamentals of practical rheology and rheometry. M., 2003.
In a state of balance different phases substances are at rest relative to each other. With their relative motion, braking forces (viscosity) appear, which tend to reduce the relative speed. The mechanism of viscosity can be reduced to the exchange of momentum of the ordered movement of molecules between different layers in gases and liquids. The emergence of viscous friction forces in gases and liquids is referred to as transfer processes. The viscosity of solids has a number of significant features and is considered separately.
DEFINITION
Kinematic viscosity is defined as the ratio of dynamic viscosity () to the density of the substance. It is usually designated by the letter (nu). Then we write the mathematical definition of the kinematic viscosity coefficient as:
where is the gas (liquid) density.
Since in expression (1) the density of the substance is in the denominator, then, for example, rarefied air at a pressure of 7.6 mm Hg. Art. and a temperature of 0 o C has a kinematic viscosity twice that of glycerin.
Kinematic viscosity of air at normal conditions is often considered equal, therefore, when moving in the atmosphere, Stokes’ law is used when the product of the body’s radius (cm) and its speed () does not exceed 0.01.
The kinematic viscosity of water under normal conditions is often considered to be of the order of , therefore, when moving in water, Stokes’ law is applied when the product of the body’s radius (cm) and its speed () does not exceed 0.001.
Kinematic viscosity and Reynolds numbers
Reynolds numbers (Re) are expressed using kinematic viscosity:
where are the linear dimensions of a body moving in matter, and is the speed of movement of the body.
In accordance with expression (2), for a body moving at a constant speed, the number decreases if the kinematic viscosity increases. If the Re number is small, then in the frontal resistance the forces of viscous friction prevail over the forces of inertia. And vice versa, big numbers Reynolds, which are observed at low kinematic viscosities, indicate the priority of inertia forces over friction.
The Reynolds number is small at a given value of kinematic viscosity, when the size of the body and the speed of its movement are small.
Units of measurement of kinematic viscosity coefficient
The basic SI unit for kinematic viscosity is:
Examples of problem solving
EXAMPLE 1
| Exercise | A metal ball (its density is equal to ) is uniformly lowered in a liquid (the density of the liquid is equal to kinematic viscosity). At what maximum possible diameter of the ball will the flow around it remain laminar? Consider that the transition to turbulent flow occurs at Re=0.5. Take the diameter of the ball as the characteristic size. |
| Solution | Let's make a drawing Using Newton's second law, we obtain the expression: where is the Archimedes force and is the force of viscous friction. In projection onto the Y axis, equation (1.1) will take the form: In this case we have: Wherein:
Substituting results (1.3)-(1.5) into (1.2), we have: The Reynolds number is defined in our case as:
|
In industry, scientific activity It is often necessary to calculate the viscosity coefficient of a liquid. Working with conventional or dispersed media in the form of aerosols and gas emulsions requires knowledge of the physical properties of these substances.
What is the viscosity of a liquid?
Newton also laid the foundation for the science of rheology. This branch studies the resistance of a substance during movement, i.e. viscosity.
In liquids and gases, molecules interact continuously. They hit each other, are pushed off, or simply fly by. As a result, the layers of matter seem to interact with each other, imparting speed to each of them. The phenomenon of such interaction between molecules of liquids/gases is called viscosity, or internal friction.
To better examine this process, it is necessary to demonstrate an experiment with two plates, between which there is a liquid medium. If you move the top plate, the layer of liquid “sticking” to it will also begin to move at a certain speed v1. After a short period of time, we notice that the underlying layers of liquid also begin to move along the same trajectory at speeds v2, v3...vn, etc., with v1>v2, v3...vn. The speed of the lowest one remains zero.
Using a gas as an example, it is almost impossible to carry out such an experiment, since the forces of interaction of molecules with each other are very small, and it will not be possible to register this visually. Here we also talk about layers, about the speed of movement of these layers, therefore viscosity also exists in gaseous media.
Newtonian and non-Newtonian media
A Newtonian fluid is a fluid whose viscosity can be calculated using Newton's formula.
Such media include water and solutions. The viscosity coefficient of a liquid in such media may depend on factors such as temperature, pressure or atomic structure of the substance, but the velocity gradient will always remain unchanged.
Non-Newtonian liquids are media in which the above-mentioned value can change, which means that Newton’s formula will not apply here. Such substances include all dispersed media (emulsions, aerosols, suspensions). This also includes blood. We'll talk about this in more detail later.
Blood as the internal environment of the body
As you know, 80% of the blood is plasma, which has a liquid aggregate state, and the remaining 20% are erythrocytes, platelets, leukocytes and various inclusions. Human red blood cells have a diameter of 8 nm. When stationary, they form aggregates in the form of coin columns, while significantly increasing the viscosity of the liquid. If the blood flow is active, these “structures” disintegrate, and internal friction is correspondingly reduced.
Medium viscosity coefficients
The interaction of layers of the medium with each other affects the characteristics of the entire liquid or gas system. Viscosity is one example of a physical phenomenon called friction. Thanks to it, the upper and lower layers of the medium gradually equalize the speed of their current, and ultimately it becomes equal to zero. Viscosity can also be characterized as the resistance of one layer of a medium to another.
To describe such phenomena, two qualitative characteristics of internal friction are distinguished:
- dynamic viscosity coefficient (dynamic viscosity of the liquid);
- kinetic coefficient of viscosity (kinetic viscosity).
Both quantities are related by the equation υ = η / ρ, where ρ is the density of the medium, υ is the kinetic viscosity, and η is the dynamic viscosity.
Methods for determining liquid viscosity
Viscometry is the measurement of viscosity. On modern stage In the development of science, the value of liquid viscosity can be found in practical ways in four ways:
1. Capillary method. To carry it out, you need to have two vessels connected by a glass channel of small diameter known length. You also need to know the pressure values in one vessel and in the other. The liquid is placed in a glass channel, and over a certain period of time it flows from one flask to another.
Further calculations are made using the Poiseuille formula to find the value of the fluid viscosity coefficient.
In practice, liquid media can be mixtures heated to 200-300 degrees. An ordinary glass tube under such conditions would simply become deformed or even burst, which is unacceptable. Modern capillary viscometers are made of high-quality and resistant material that can easily withstand such loads.
2. Medical method according to Hesse. To calculate the viscosity of a liquid in this way, it is necessary to have not one, but two identical capillary installations. In one of them a medium is placed in advance known value internal friction, and in the other - the test liquid. Next, two time values are measured and a proportion is made by which they arrive at the desired number.
3. Rotational method. To carry it out, it is necessary to have a structure of two coaxial cylinders. This means that one of them must be inside the other. Liquid is poured into the space between them, and then the inner cylinder is accelerated. This angular velocity is also imparted to the fluid. The difference in torque allows the viscosity of the medium to be calculated.
4. Determination of liquid viscosity by the Stokes method. To conduct this experiment, you must have a Heppler viscometer, which is a cylinder filled with liquid. Before starting the experiment, make two marks on the cylinder and measure the length between them. Then they take a ball of a certain radius R and lower it into the liquid medium. To determine the speed of its fall, find the time it takes the object to move from one mark to another. Knowing the speed of the ball, you can calculate the viscosity of the liquid.
Practical application of viscometers
Determining the viscosity of a liquid is of great practical importance in the oil refining industry. When working with multiphase, dispersed media, it is important to know them physical properties, especially internal friction. Modern viscometers are made of durable materials, and advanced technologies are used in their production. All this together allows you to work with high temperature and pressure without harm to the equipment itself.
Fluid viscosity plays a big role in industry because transportation, processing and production of, for example, oil depend on the internal friction values of the liquid mixture.
What role does viscosity play in medical equipment?
The flow of the gas mixture through the endotracheal tube depends on the internal friction of this gas. A change in the viscosity of the medium here has a different effect on the penetration of air through the apparatus and depends on the composition of the gas mixture.
Introduction medicines, vaccines via a syringe are also a shining example effects of medium viscosity. We are talking about pressure drops at the end of the needle when injecting liquid, although it was initially believed that this physical phenomenon could be neglected. Emergence high pressure at the tip - this is the result of internal friction.
Conclusion
The viscosity of the medium is one of physical quantities, which has great practical application. In the laboratory, industry, medicine - in all these areas, the concept of internal friction appears very often. The operation of the simplest laboratory equipment may depend on the degree of viscosity of the medium used for research. Even the processing industry cannot do without knowledge in the field of physics.
