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Fluids as constructed. Liquids and liquid state of matter. The structures of liquids and amorphous bodies are similar.

Liquids are called substances that are in a liquid state of aggregation under normal conditions. According to external signs, this state is characterized by the presence of a constant volume for a given portion of liquid, fluidity, the ability to gradually evaporate. The proper form of a liquid is a ball (drop), which forms a liquid under the action of surface tension. This is possible in the absence of gravity. Drops are formed in the free fall of a liquid, and in space spaceship, in conditions of weightlessness, a significant volume of liquid can take the shape of a ball. In a calm state, the liquid spreads over the surface or fills the volume of any vessel. Among inorganic substances, liquids include water, bromine, mercury, and a few stable anhydrous acids (sulfuric, hydrofluoric, etc.). There are a lot of liquids among organic compounds: hydrocarbons, alcohols, acids, etc. Almost all homologous series of organic compounds contain liquids. When cooled, gases pass into a liquid state, and when heated - metals, stable salts, metal oxides.

Liquids can be classified by the nature of their constituent particles into atomic (liquefied noble gases), molecular (most common liquids), metallic (molten metals), ionic (molten salts, metal oxides). In addition to individual substances, mixtures of liquids and solutions of a wide variety of substances in liquids are in a liquid state. The greatest practical value among liquids is water, which is determined by its unique role as a biological solvent. In chemistry and applied fields, liquids, along with gases, are most important as a medium for carrying out all kinds of transformation processes of substances. Liquids are also used to transfer heat through pipes, in hydraulic devices - as a working fluid, as a lubricant for moving machine parts.

In the liquid state of matter, the particles are located at distances close to the sum of their van der Waals radii. The potential energy of the molecules becomes negative in relation to their energy in the gas. To overcome it during the transition to the gaseous state, the molecules need kinetic energy, which is approximately equal to the potential energy. Therefore, the substance is in a liquid state in a temperature range in which the average kinetic energy is approximately equal to the potential energy of interaction or below it, but does not fall to zero.

where e - base of natural logarithms; R - universal gas constant; AN isp - molar heat of vaporization of a liquid; L - constant depending on the properties of the liquid.

Analysis of the equation shows that the vapor pressure of a liquid rises rapidly with increasing temperature, since the temperature is in the denominator of a negative exponent. Equation (7.13) is quite accurately fulfilled provided that the temperature is significantly lower than the critical temperature of the vapor of a given substance.

Upon reaching the temperature at which the vapor pressure of the liquid becomes equal to the atmospheric pressure, the liquid boils. This assumes that there is air above the surface of the liquid. If the liquid is enclosed in a closed vessel, for example, in a cylinder, with a piston producing a pressure equal to atmospheric pressure (101.3 kPa), then when the liquid is heated to the boiling point, vapor above the liquid has not yet

Among the molecules of both gas and liquid, there are both faster and slower molecules relative to the average speed of their movement. Fast molecules overcome attraction and pass into the gas phase in the presence of free volume. During evaporation, due to the loss of faster molecules, the liquid cools. Above the surface of a liquid in a closed volume, a certain vapor pressure is established, which depends on the nature of the liquid and on the temperature. The dependence is expressed by an exponential equation. When the boiling point is exceeded, steam will appear, i.e. gas phase, and the piston will begin to rise as heat is supplied and the volume of steam increases (Fig. 7.4).

Rice. 7.4.

Liquids boiling below the boiling point of water are commonly referred to as volatile. From an open container, they quickly evaporate. At a boiling point of 20-22 ° C, the substance actually turns out to be borderline between a volatile liquid and an easily liquefied gas. Examples of such substances are acetaldehyde CH 3 CHO (bale = 21 ° C) and hydrogen fluoride HF (bale = 19.4 ° C).

Practically important physical characteristics of liquids, in addition to boiling point, are freezing point, color, density, viscosity index, refractive index. For homogeneous media such as liquids, the refractive index is easily measured and serves to identify the liquid. Some constants of liquids are given in table. 7.3.

The balance between the liquid, solid and gaseous phases of a given substance is depicted as status diagrams. In fig. 7.5 shows a diagram of the state of water. The state diagram is a graph showing the dependence of the saturated vapor pressure on temperature for liquid water and ice (curves OA and OV) and the dependence of the melting point of water on pressure (curve OS). The presence of a slight vapor pressure over the ice (curve OV) means that ice can evaporate (sublimate) if the vapor pressure of water in the air is less than the equilibrium pressure above the ice. Dotted line continuing the curve OA to the left of the point O, corresponds to the vapor pressure over supercooled water. This pressure exceeds the vapor pressure over ice at the same temperature. Therefore, supercooled water is unstable and can spontaneously turn into ice. Sometimes, in cold weather, the phenomenon of rain is observed, the drops of which turn into ice when they hit a hard surface. An ice crust forms on the surface. It should be noted that other liquids can also be in an unstable supercooled state.

Some practically important liquids

Name		Density p, g / cm 3 (20 ° С)	Refractive index, u (20 ° C,

Hydrogen fluoride
Sulphuric acid	h 2 so 4


Formic
Acetic acid


Glycerol


Tstrachloride carbon
Chloroform
Nitrobenzene	c g ii 5 no 2

Rice. 75.

The curves divide the diagram into three fields - water, ice and steam. Each point on the diagram represents a specific state of the system. The points inside the fields correspond to the existence of water in only one of the three phases. For example, at 60 ° C and a pressure of 50 k11a, water exists only in a liquid state. Points lying on curves OA, OV and OS, correspond to the equilibrium between the two phases. For example, at temperatures and pressures along the curve OA water and steam are in equilibrium. The intersection point O of the three curves with coordinates 0.61 kPa and 0.01 ° C corresponds to equilibrium between the three phases of water - ice, liquid water and its vapor. This is the so-called triple point of water... The indicated temperature is 0.01 ° C higher than the normal freezing point of 0 ° C water, relative to a pressure of 101.3 kPa. From this it follows that with an increase in external pressure, the freezing point of water decreases. Let's give one more point: at a pressure of 615 atm (6.23-10 4 kPa), the freezing point of water drops to -5 ° C.

By the ability to mix with each other, liquids differ sharply from gases. In liquids, in contrast to gases, intermolecular interactions play an important role. Therefore, only such liquids are mixed with each other in any ratios that are sufficiently close in terms of the energy of intermolecular interaction. For example, between water molecules not only Waiderwaals forces act, but also hydrogen bonds are formed. Therefore, various liquids are mixed with water, the molecules of which can also give hydrogen bonds with water: hydrogen fluoride, many oxygen-containing acids, lower members of the homologous series of alcohols, acetone, etc. Liquids that do not form hydrogen bonds or prevent the formation of such bonds between water molecules, do not mix with water, but they can in one way or another, i.e. limited, dissolve. So, alcohols with radicals consisting of four or more carbon atoms are limitedly soluble in water, since radicals, being between water molecules, interfere with the formation of hydrogen bonds and are pushed out of the water volume.

The internal structure of liquids is characterized by both the relatively free mutual movement of molecules and the appearance of a structure that brings the liquid closer to the solid state. It was said above that X-rays are scattered on ordered atoms in crystals. The scattering intensity maxima appear at certain angles of incidence of the initial beam on the plane, formed by atoms inside the crystal. X-ray scattering also occurs in liquids. At a small angle of incidence, corresponding to scattering by closely spaced atoms, a maximum arises, indicating the presence of order in the immediate environment of the atom. However, as the angle of incidence increases, the maxima rapidly decay, which indicates the absence of a regular arrangement for distant atoms. Thus, we can say about liquids that they contain short-range order, Without long-range order.

The structuring of liquids is discovered by studying various physical properties. It is known, for example, that when cooled to 4 ° C, water becomes denser, and upon further cooling, it begins to expand again. This is due to the formation of a more openwork structure corresponding to the direction of hydrogen bonds between molecules. After freezing, these bonds finally stabilize, which follows from the decrease in the ice density.

Liquids are intermediate between gaseous and solid substances. At temperatures close to boiling points, the properties of liquids approach those of gases; at temperatures close to the melting points, the properties of liquids approach the properties of solids. If solid substances are characterized by strict ordering of particles, spreading over distances of up to hundreds of thousands of interatomic or intermolecular radii, then in a liquid substance there are usually no more than a few tens of ordered particles - this is explained by the fact that order between particles in different places of the liquid substance also quickly arises , as it is again "washed out" by the thermal vibration of the particles. At the same time, the total packing density of particles of a liquid substance differs little from a solid substance - therefore, their density is close to the density of solids, and the compressibility is very low. For example, in order to reduce the volume occupied by liquid water by 1%, a pressure of ~ 200 atm is required, while for the same decrease in the volume of gases, a pressure of about 0.01 atm is required. Consequently, the compressibility of liquids is approximately 200: 0.01 = 20,000 times less than the compressibility of gases.

It was noted above that liquids have a certain volume of their own and take the form of a vessel in which they are located; these properties are much closer to the properties of a solid than a gaseous substance. The close proximity of the liquid state to the solid is also confirmed by the data on the standard enthalpies of evaporation ∆Н ° ev and the standard enthalpies of melting ∆Н ° pl. Standard enthalpy of vaporization called the amount of heat required to convert 1 mol of liquid into vapor at 1 atm (101.3 kPa). The same amount of heat is released during the condensation of 1 mol of vapor into a liquid at 1 atm. The amount of heat spent on converting 1 mol of a solid into a liquid at 1 atm is called standard enthalpy of melting(the same amount of heat is released during the "freezing" ("solidification") of 1 mol of liquid at 1 atm). It is known that ∆H ° pl is much less than the corresponding values of ∆H ° ep, which is easy to understand, since the transition from the solid state to the liquid is accompanied by less disturbance of intermolecular attraction than the transition from the liquid to the gaseous state.

A number of other important properties of liquids more closely resemble those of gases. So, like gases, liquids can flow - this property is called fluidity. Resistance to flow is determined by viscosity. The fluidity and viscosity are influenced by the forces of attraction between liquid molecules, their relative molecular weight, and a number of other factors. The viscosity of liquids is ~ 100 times that of gases. Just like gases, liquids are able to diffuse, albeit much more slowly, since liquid particles are packed much more densely than gas particles.

One of the most important properties of a liquid is its surface tension(this property is not inherent in either gases or solids). Intermolecular forces act evenly on a molecule in a liquid from all sides. However, on the surface of the liquid, the balance of these forces is disturbed, and as a result, the "surface" molecules are under the influence of a certain resultant force directed into the liquid. For this reason, the surface of the liquid is in a state of tension. Surface tension Is the minimum force that restrains the movement of liquid particles into the depth of the liquid and thereby keeps the surface of the liquid from contracting. It is the surface tension that explains the "drop-like" shape of freely falling liquid particles.

The main property of a liquid that distinguishes it from other aggregate states is the ability to change its shape indefinitely under the action of tangential mechanical stresses, even arbitrarily small, while practically maintaining the volume. A substance in a liquid state exists in a certain temperature range, below which it passes into a solid state (crystallization or transformation into a solid-state amorphous state - glass), above - into a gaseous state (evaporation occurs). The boundaries of this interval depend on the pressure.

3.1 Physical properties of liquids:

ü Fluidity(The main property. Unlike plastic solids, a liquid does not have a yield point: it is enough to apply an arbitrarily small external force for the liquid to flow.

ü Preservation of volume. One of the characteristic properties of a liquid is that it has a certain volume (under constant external conditions). Liquid is extremely difficult to compress mechanically because, unlike gas, there is very little free space between molecules. Liquids typically expand (expand) when heated and shrink (contract) when cooled.

ü Viscosity. In addition, liquids (like gases) are viscous. It is defined as the ability to resist the movement of one of the parts relative to the other - that is, as internal friction. When adjacent layers of liquid move relative to each other, collision of molecules inevitably occurs in addition to that which is due to thermal motion. The liquid in the vessel, set in motion and left to itself, will gradually stop, but its temperature will rise.

ü Free surface formation and surface tension Due to the conservation of volume, the liquid is able to form a free surface. Such a surface is the interface between the phases of a given substance: on one side there is a liquid phase, on the other - a gaseous phase (vapor). If the liquid and gaseous phases of the same substance come into contact, forces arise that tend to reduce the area of the interface - surface tension forces ... The interface behaves like an elastic membrane that tends to contract.

ü Evaporation and condensation

ü Boiling

ü Wetting- a surface phenomenon that occurs when a liquid contacts a solid surface in the presence of steam, that is, at the interfaces of three phases.

ü Miscibility- the ability of liquids to dissolve in each other. An example of miscible liquids: water and ethyl alcohol, an example of immiscible: water and liquid oil.

ü Diffusion. When there are two mixed liquids in the vessel, the molecules, as a result of thermal movement, begin to gradually pass through the interface, and thus the liquids are gradually mixed. This phenomenon is called diffusion (it also occurs in substances in other states of aggregation).

ü Overheating and hypothermia. The liquid can be heated above its boiling point so that boiling does not occur. This requires uniform heating, without significant temperature fluctuations within the volume and without mechanical influences such as vibration. If you throw something into a superheated liquid, it instantly boils. Superheated water can be easily obtained in a microwave oven. Supercooling is the cooling of a liquid below the freezing point without turning into a solid state of aggregation.

1. The liquid state of a substance and its properties.

2.1 Bernoulli's law.

2.2 Pascal's law.

2.3 Laminar flow of liquids.

2.4 Poisel's law.

2.5 Turbulent flow of liquids.

3.1 Measurement of liquid viscosity.

3.2 Measurement of volume and flow rate of liquid

1. The liquid state of a substance and its properties.

It was noted above that liquids have a certain volume of their own and take the form of a vessel in which they are located; these properties are much closer to the properties of a solid than a gaseous substance. The close proximity of the liquid state to the solid is also confirmed by the data on the standard enthalpies of evaporation ∆Н ° ev and the standard enthalpies of melting ∆Н ° pl. The standard enthalpy of vaporization is the amount of heat required to convert 1 mol of liquid to vapor at 1 atm (101.3 kPa). The same amount of heat is released during the condensation of 1 mol of vapor into a liquid at 1 atm. The amount of heat spent on converting 1 mol of a solid into a liquid at 1 atm is called the standard enthalpy of melting (the same amount of heat is released during the "freezing" ("solidification") of 1 mol of liquid at 1 atm). It is known that ∆H ° pl is much less than the corresponding values of ∆H ° ep, which is easy to understand, since the transition from the solid state to the liquid is accompanied by less disturbance of intermolecular attraction than the transition from the liquid to the gaseous state.

One of the most important properties of a liquid is its surface tension (this property is not inherent in either gases or solids). Intermolecular forces act evenly on a molecule in a liquid from all sides. However, on the surface of the liquid, the balance of these forces is disturbed, and as a result, the "surface" molecules are under the influence of a certain resultant force directed into the liquid. For this reason, the surface of the liquid is in a state of tension. Surface tension is the minimum force that restrains the movement of liquid particles into the depth of the liquid and thereby keeps the surface of the liquid from contracting. It is the surface tension that explains the "drop-like" shape of freely falling liquid particles.

Due to the conservation of volume, the liquid is able to form a free surface. Such a surface is the interface between the phases of a given substance: on one side there is a liquid phase, on the other - gaseous (vapor), and possibly other gases, for example, air. If the liquid and gaseous phases of the same substance come into contact, forces arise that tend to reduce the area of the interface - surface tension forces. The interface behaves like an elastic membrane that tends to contract.

Surface tension can be explained by the attraction between liquid molecules. Each molecule attracts other molecules, seeks to "surround" itself with them, and therefore, to leave the surface. Accordingly, the surface tends to shrink. Therefore, when boiling, soap bubbles and bubbles tend to take a spherical shape: for a given volume, a ball has a minimum surface. If only surface tension forces act on the liquid, it will necessarily take a spherical shape - for example, water droplets in zero gravity.

Small objects with a density higher than the density of the liquid are able to "float" on the surface of the liquid, since the force of gravity is less than the force that prevents an increase in surface area.

Wetting is a surface phenomenon that occurs when a liquid contacts a solid surface in the presence of steam, that is, at the interfaces of three phases. Wetting characterizes the "adhesion" of a liquid to the surface and spreading over it (or, conversely, repulsion and non-spreading). There are three cases: non-wetting, limited wetting and complete wetting.

Miscibility is the ability of liquids to dissolve in each other. An example of miscible liquids: water and ethyl alcohol, an example of immiscible: water and liquid oil.

When there are two mixed liquids in the vessel, the molecules, as a result of thermal movement, begin to gradually pass through the interface, and thus the liquids are gradually mixed. This phenomenon is called diffusion (it also occurs in substances in other states of aggregation).

The liquid can be heated above its boiling point so that boiling does not occur. This requires uniform heating, without significant temperature fluctuations within the volume and without mechanical influences such as vibration. If you throw something into a superheated liquid, it instantly boils. Superheated water is easy to get in the microwave.

Subcooling - cooling of a liquid below the freezing point without turning into a solid state of aggregation. As with overheating, hypothermia requires the absence of vibration and significant temperature changes.

If we displace a section of the liquid surface from the equilibrium position, then under the action of restoring forces, the surface begins to move back to the equilibrium position. This movement, however, does not stop, but turns into an oscillatory movement near the equilibrium position and spreads to other areas. This is how waves appear on the surface of the liquid.

If the restoring force is mainly gravity, then such waves are called gravitational waves. Gravitational waves on the water can be seen everywhere.

If the restoring force is mainly the surface tension force, then such waves are called capillary. If these forces are comparable, such waves are called capillary-gravitational waves. Waves on the surface of a liquid are attenuated by viscosity and other factors.

Formally speaking, for the equilibrium coexistence of a liquid phase with other phases of the same substance - gaseous or crystalline - strictly defined conditions are required. So, at a given pressure, a strictly defined temperature is needed. Nevertheless, in nature and in technology everywhere, liquid coexists with steam, or also with a solid state of aggregation - for example, water with water vapor and often with ice (if we consider vapor as a separate phase, which is present along with air). This is due to the following reasons.

Non-equilibrium state. It takes time for the liquid to evaporate until the liquid has completely evaporated, it coexists with steam. In nature, water evaporation constantly occurs, as well as the reverse process - condensation.

Closed volume. The liquid in a closed vessel begins to evaporate, but since the volume is limited, the vapor pressure rises, it becomes saturated even before the liquid has completely evaporated, if its amount was large enough. When the state of saturation is reached, the amount of evaporated liquid is equal to the amount of condensed liquid, the system comes to equilibrium. Thus, in a limited volume, the conditions necessary for the equilibrium coexistence of liquid and vapor can be established.

Presence of atmosphere in conditions earth gravity... Affects the liquid Atmosphere pressure(air and steam), while for steam almost only its partial pressure... Therefore, liquid and vapor above its surface correspond to different points on the phase diagram, in the region of existence of the liquid phase and in the region of existence of the gaseous phase, respectively. This does not cancel evaporation, but evaporation takes time during which both phases coexist. Without this condition, liquids would boil and evaporate very quickly.

2.1 Bernoulli's Law - is a consequence of the law of conservation of energy for a stationary flow of an ideal (that is, without internal friction) incompressible fluid:

Density of the liquid,

Flow rate,

The height at which the considered fluid element is located,

The pressure at the point in space where the center of mass of the considered fluid element is located,

Acceleration of gravity.

The constant on the right side is usually called pressure, or full pressure, as well by the Bernoulli integral... The dimension of all the terms is the unit of energy per unit volume of the liquid.

This ratio, deduced by Daniel Bernoulli in 1738, was named after him Bernoulli equation... For horizontal pipe h= 0 and the Bernoulli equation takes the form:

This form of the Bernoulli equation can be obtained by integrating the Euler equation for a stationary one-dimensional fluid flow, at a constant density ρ:

According to Bernoulli's law, the total pressure in a steady flow of liquid remains constant along this flow.

Total pressure consists of the weighted (ρ gh), static (p) and dynamic (ρν 2/2) pressures.

It follows from Bernoulli's law that with a decrease in the flow cross section, due to an increase in speed, that is, dynamic pressure, the static pressure drops. This is the main reason for the Magnus effect. Bernoulli's law is also valid for laminar gas flows. The phenomenon of a decrease in pressure with an increase in the flow rate underlies the operation of various types of flow meters (for example, a Venturi pipe), water and steam jet pumps. And the consistent application of Bernoulli's law led to the emergence of a technical hydromechanical discipline - hydraulics.

Bernoulli's law is valid in its pure form only for liquids whose viscosity is zero, that is, those liquids that do not stick to the pipe surface. In fact, it has been experimentally established that the velocity of a liquid on the surface of a solid is almost always exactly zero (except for the cases of jet separation under some rare conditions).

2.2 Pascal's Law is formulated as follows:

The pressure exerted on a liquid (or gas) in any one place on its border, for example, by a piston, is transmitted unchanged to all points of the liquid (or gas).

The main property of liquids and gases- to transmit pressure without changing in all directions - is the basis of the design of hydraulic and pneumatic devices and machines.

How many times the area of one piston is greater than the area of another, the same number of times the hydraulic machine gives a gain in power.

2.3 Laminar flow(lat. lamina- plate, strip) - a flow in which a liquid or gas moves in layers without mixing and pulsations (that is, random rapid changes in velocity and pressure).

Laminar flow is possible only up to a certain critical value of the Reynolds number, after which it becomes turbulent. The critical value of the Reynolds number depends on the specific type of flow (flow in a round pipe, flow around a ball, etc.). For example, for a flow in a round tube

The Reynolds number is determined by the following relationship:

ρ is the density of the medium, kg / m 3;

v- characteristic speed, m / s;

L- characteristic size, m;

η - dynamic viscosity of the medium, N * s / m 2;

ν is the kinematic viscosity of the medium, m 2 / s ();

Q- volumetric flow rate;

A is the cross-sectional area of the pipe.

The Reynolds number as a criterion for the transition from laminar to turbulent flow and vice versa works relatively well for pressure flows. With the transition to free-flow flows, the transition zone between the laminar and turbulent regimes increases, and the use of the Reynolds number as a criterion is not always legitimate. For example, in reservoirs, the formally calculated values of the Reynolds number are very large, although a laminar flow is observed there.

2.4 Equation or Poiseuille's law- the law that determines the flow rate of a liquid at a steady flow of a viscous incompressible liquid in a thin cylindrical pipe of circular cross-section.

According to the law, the second volumetric flow rate of the liquid is proportional to the pressure drop per unit length of the tube (pressure gradient in the tube) and to the fourth power of the radius (diameter) of the tube:

Q- the flow rate of the liquid in the pipeline;
D- pipeline diameter;
v- fluid velocity along the pipeline;
r- distance from the pipeline axis;
R- radius of the pipeline;
p 1 − p 2 - pressure difference at the inlet and outlet from the pipe;
η is the viscosity of the liquid;
L- pipe length.

Poiseuille's law works only for laminar flow and provided that the length of the tube exceeds the so-called length of the initial section, which is necessary for the development of a laminar flow in the tube.

The Poiseuille flow is characterized by a parabolic velocity distribution along the tube radius. In each cross section of the tube average speed half the maximum speed in this section.

2.5 T urbulent T flow (from Latin turbulentus - violent, disorderly), a form of flow of a liquid or gas, in which case their elements perform disordered, unsteady movements along complex trajectories, which leads to intensive mixing between layers of moving liquid or gas (see Turbulence). The most thoroughly studied T. t. In pipes, channels, boundary layers near solid bodies in a stream of liquid or gas, as well as the so-called. free T. t. - jets, traces of solid bodies moving relative to liquid or gas, and mixing zones between flows of different speed, not separated by c.-l. solid walls. T. t. Differ from the corresponding laminar flows both in their complex internal structure (Fig. 1), and the distribution of the averaged velocity over the flow cross section and integral characteristics - the dependence of the mean over the cross section or max. speed, flow rate, as well as coeff. resistance from the Rey-nolds of the Re number. The profile of the average velocity of T. t. In pipes or channels differs from parabolic. profile of the corresponding laminar flow with a faster increase in velocity at the walls and a smaller curvature towards the center. part of the flow (Fig. 2). With the exception of a thin layer near the wall, the velocity profile is described by a logarithmic law (i.e., the velocity is linearly dependent on the logarithm of the distance to the wall). Resistance coefficient:

Friction stress on the wall,
- the density of the liquid,
is its velocity, averaged over the flow cross section) is related to Re by the ratio

Averaged velocity profile: a - for laminar, 6 - for turbulent flow.

3.1 Measuring the viscosity of a liquid .

Kinematic viscosity is a measure of the flow of a resisting fluid under the influence of gravity. When two fluids of equal volume are placed in identical capillary viscometers and move by gravity, the viscous fluid takes longer to flow through the capillary. If one fluid takes 200 seconds to drain and the other takes 400 seconds, the second fluid is twice as viscous as the first on the kinematic viscosity scale.

Absolute viscosity, sometimes called dynamic or simple viscosity, is the product of the kinematic viscosity and density of a fluid:
Absolute Viscosity = Kinematic Viscosity * Density
The dimension of kinematic viscosity is L 2 / T, where L is length and T is time). The SI unit of kinematic viscosity is 1 cSt (centiStokes) = mm 2 / s. The absolute viscosity is expressed in centipoise (cPoise). The SI unit of absolute viscosity is millipascal-second 1 mPa * s = 1 cP.

An instrument for measuring viscosity is called a viscometer. Viscometers can be classified into three main types:

A... Capillary viscometers measure the flow rate of a fixed volume of liquid through a small orifice at a controlled temperature. Shear rate can be measured from about zero to 106 s -1 by changing the capillary diameter and applied pressure. Types of capillary viscometers and their operating modes:
Glass capillary viscometer (ASTM D 445) - The liquid passes through an opening of the set - diameter under the influence of gravity. The shear rate is less than 10 s -1. The kinematic viscosity of all automotive oils is measured with capillary viscometers.
High Pressure Capillary Viscometer (ASTM D 4624 and D 5481) - A fixed volume of liquid is squeezed out through a diameter glass capillary under the applied gas pressure. The shear rate can be changed up to 106 s -1. This technique is commonly used to simulate the viscosity of engine oils in working main bearings. This viscosity is called viscosity at high temperature and high shear (HTHS) and measured at 150 ° C and 106 s -1. HTHS viscosity is also measured with a tapered bearing simulator, ASTM D 4683 (see below).

B... Rotational viscometers use the torque on a rotating shaft to measure the resistance of a fluid to flow. Rotational viscometers include the Cold Cranking Simulator (CCS), the Mini Rotational Viscometer (MRV), the Brookfield Viscometer, and the Tapered Bearing Simulator (TBS). The shear rate can be changed by changing the dimensions of the rotor, the gap between the rotor and the stator wall, and the speed.
Cold Roll Simulator (ASTM D 5293) - CCS measures apparent viscosity in the range of 500 to 200,000 cP. The shear rate is between 104 and 105 s -1. The normal operating temperature range is 0 to -40 ° C. CCS showed excellent correlation with engine start at low temperatures. The SAE J300 viscosity grading defines the low temperature viscosity efficiency of engine oils within the CCS and MRV limits.

Mini Rotary Viscometer (ASTM D 4684) - The MRV test, which is related to the oil pumpability mechanism, is a measurement at low shear rate. The main feature of the method is the slow cooling rate of the sample. The sample is prepared to have a specific thermal history that includes heating, slow cooling, and impregnation cycles. The MRV measures the apparent residual voltage which, if greater than a threshold value, indicates a potential pump failure problem associated with air ingress. Above a certain viscosity (currently defined as 60,000 cP by SAE J 300), oil can cause pumpability failure through a mechanism called the "limited flow effect". An SAE 10W oil, for example, should have a maximum viscosity of 60,000 cP at -30 ° C without residual stress. This method also measures the apparent viscosity at shear rates from 1 to 50 s -1.
Brookfield viscometer - determines the viscosity in a wide range (from 1 to 105 Poise) at a low shear rate (up to 102 s -1).
ASTM D 2983 is primarily used to determine the low temperature viscosity of automotive gear oils, automatic transmission oils, hydraulic oils and tractor oils. Temperature - Testing ranges from -5 to -40 ° C.
ASTM D 5133, Brookfield Scan Method, measures the Brookfield viscosity of a sample when cooled at a constant rate of 1 ° C / hr. Like the MRV, the ASTM D 5133 method is intended to determine the pumpability of an oil at low temperatures. This test determines the point of structure formation, defined as the temperature at which the sample reaches a viscosity of 30,000 cP. The structure index is also defined as the fastest rate of increase in viscosity from -5 ° C to the lowest test temperature. This method finds use in engine oils and is required by ILSAC GF-2. Tapered Bearing Simulator (ASTM D 4683) - This technique also measures the viscosity of engine oils at high temperatures and high shear rates (see High Pressure Capillary Viscometer). Very high shear rates are obtained due to the extremely small clearance between the rotor and the stator wall.

Viscosity Index (VI) is an empirical number that indicates the degree of change in the viscosity of an oil within a given temperature range. A high VI means a relatively small change in viscosity with temperature, and a low VI means a large change in viscosity with temperature. Most mineral base oils have a VI between 0 and 110, but the VI for multigrage oils often exceeds 110.
The determination of the viscosity index requires the determination of the kinematic viscosity at 40 ° C and 100 ° C. Thereafter, VI is determined from tables according to ASTM D 2270 or ASTM D 39B. Since VI is determined from viscosity at 40 ° C and 100 ° C, it is not related to low temperature or HTHS viscosity. These values are obtained with CCS, MRV, Brookfield low temperature viscometer and high shear viscometers.
SAE has not used IV to classify engine oils since 1967 because the term is technically obsolete. However, API 1509, API 1509, describes a base oil classification system using IV as one of several parameters to provide principles for oil interchangeability and viscosity scale universality.

3.2 Measurement of volume and flow rate of liquid.

To measure the flow rate of liquids, flow meters are used based on various principles of operation: flow meters of variable and constant differential pressure, variable level, electromagnetic, ultrasonic, vortex, thermal and turbine.

To measure the amount of a substance, flow meters with integrators or counters are used. The integrator continuously sums up the readings of the device, and the amount of the substance is determined by the difference in its readings for the required period of time.

Measurement of flow and quantity is a complex task, since the physical properties of the measured flows affect the readings of the devices: density, viscosity, phase ratio in the flow, etc. Physical properties the measured flows, in turn, depend on the operating conditions, mainly on temperature and pressure.

If the operating conditions of the flow meter differ from the conditions under which it was calibrated, then the error in the readings of the device may significantly exceed the permissible value. Therefore, for commercially available devices, restrictions on the area of their application are established: according to the properties of the measured flow, maximum temperature and pressure, the content of solid particles or gases in a liquid, etc.

Differential pressure flow meters

The operation of these flow meters is based on the occurrence of a pressure drop across a restricting device in a pipeline when a liquid or gas flow moves through it. With a change in the flow rate Q, the value of this pressure drop Δp also changes.

For some orifice devices as flow rate-to-differential pressure transducers, the transfer coefficient is determined experimentally and its values are summarized in special tables. Such constriction devices are called standard.

The simplest and most common restriction device is the diaphragm. The standard diaphragm is a thin disc with a circular hole in the center. Its transmission coefficient significantly depends on the durability of the diaphragm and especially the leading edge of the hole. Therefore, diaphragms are made of materials that are chemically resistant to the measured medium and resistant to mechanical wear. In addition to the diaphragm, a Venturi nozzle and a Venturi tube are also used as standard orifice devices, which create a lower hydraulic resistance in the pipeline.

The orifice of a variable pressure differential flow meter is a primary transducer in which the flow rate is converted into a differential pressure.

Differential pressure gauges are used as intermediate converters for flow meters with variable differential pressure. Differential pressure gauges are connected to the restriction device by impulse tubes and are installed in the immediate vicinity of it. Therefore, in flowmeters with variable differential pressure, differential pressure gauges are usually used, equipped with an intermediate transducer to transmit the measurement results to the operator's panel (for example, diaphragm differential pressure gauges DM).

As well as when measuring pressure and level, separating vessels and diaphragm seals are used to protect differential pressure gauges from the aggressive effects of the measured medium.

A feature of primary converters of variable pressure drop meters is the quadratic dependence of the pressure drop on the flow rate. In order for the readings of the flow meter to linearly depend on the flow rate, a linearizing transducer is introduced into the measuring circuit of the variable differential pressure flow meters. Such a converter is, for example, a linearization block in an NP-PZ intermediate converter. When the differential pressure gauge is directly connected with a measuring device (for example, KSD), linearization is performed in the device itself using a curve with a quadratic characteristic.

Constant differential pressure flowmeters

Liquid or gas flow rates can also be measured at a constant differential pressure. To maintain a constant pressure drop when changing the flow rate through the orifice, it is necessary to automatically change its flow area. The simplest way is to automatically change the flow area in the flowmeter.

The rotameter is a vertical conical tube containing a float. The measured flow Q, passing through the rotameter from bottom to top, creates a differential pressure before and after the float. This differential pressure, in turn, creates a lift that balances the weight of the float.

If the flow rate through the rotameter changes, then the pressure drop will also change. This will lead to a change in the lift and therefore to an imbalance of the float. The float will begin to mix. And since the tube of the rotameter is conical, then the flow area in the gap between the float and the tube will change, as a result there will be a change in the pressure drop, and, consequently, in the lifting force. When the differential pressure and lift return to their previous values, the float will balance and stop.

Thus, each value of the flow through the rotameter Q corresponds to a certain position of the float. Since for a conical tube the area of the annular gap between it and the float is proportional to the height of its rise, the scale of the rotameter is uniform.

The industry produces rotameters with glass and metal tubes. For rotameters with a glass tube, the scale is marked directly on the surface of the tube. For remote measurement of the position of the float in a metal tube, intermediate linear displacement transducers into a unified electrical or pneumatic signal are used.

In rotameters with an electrical output signal, the plunger of the differential-transformer converter moves along with the float. Rotameters with pneumatic output signal use a magnetic coupling to communicate the position of the float to the transmitter. It consists of two permanent magnets. One - double - moves together with the float, the other, mounted on the lever of the transducer of displacement to compressed air pressure, moves together with the lever after the first magnet.

Rotameters are also available for measuring the flow of highly aggressive media. Rotameters are jacketed for steam heating. They are designed to measure the flow rate of crystallizing media.

Variable level flow meters

It is known from hydraulics that if the liquid flows freely through the hole in the bottom of the tank, then its flow rate Q and the level in the tank H are related. Consequently, the level in the tank can be used to judge the flow rate from it.

The operation of variable level flow meters is based on this principle. Obviously, the tank itself with a hole in the bottom plays the role of a primary converter here. The output signal of such a transducer is the level in the tank. Therefore, any of the considered level gauges can serve as an intermediate transducer of the measuring circuit of a variable level flow meter.

Variable level flowmeters are usually used to measure the flow rate of corrosive and contaminated liquids when they are discharged into tanks under atmospheric pressure.

Electromagnetic flowmeters

The action of electromagnetic flow meters is based on the law of electromagnetic induction, according to which e will be induced in a conductor moving in a magnetic field. d. s, proportional to the speed of movement of the conductor. In electromagnetic flow meters, the role of a conductor is played by an electrically conductive liquid flowing through pipeline 1 and crossing the magnetic field 3 of electromagnet 2. In this case, emf will be induced in the liquid. etc. with. U, proportional to the speed of its movement, i.e., to the flow rate of the liquid.

The output signal of such a primary converter is picked up by two insulated electrodes 4 and 6, installed in the wall of the pipeline. The section of the pipeline on both sides of the electrodes is covered with electrical insulation 7 in order to exclude the shunting of the induced electric power. etc. with. through the liquid and the pipe wall.

The degree of aggressiveness of the measured media for electromagnetic flowmeters is determined by the insulation material of the pipe and the electrodes of the primary converter. For this purpose, flowmeters use rubber, acid-resistant enamel and fluoroplastic. The most resistant to aggressive media is a flowmeter with a PTFE insulating coating and graphitized PTFE electrodes.

During the operation of the flow meters, the zero and the calibration of the device must be checked periodically, at least once a week. For verification, the primary transducer is filled with the measured liquid. After that, the operating mode switch on the front panel of the measuring unit is moved to the "Measure" position and the "Zero" potentiometer is set to the zero mark. When the switch is moved to the "Calibration" position, the arrow of the device should stop at 100%. Otherwise, the arrow is brought to this mark by the "Calibration" potentiometer.

A distinctive feature of electromagnetic flow meters is the absence of additional pressure losses at the site. measurements. This is due to the lack of parts protruding into the pipe. A particularly valuable property of such flowmeters, in contrast to other types of flowmeters, is the ability to measure the flow rate of aggressive, abrasive and viscous liquids and slurries.

Ultrasonic flow meters

The operation of these flowmeters is based on the addition of the velocity of propagation of ultrasound in a liquid and the velocity of the liquid flow itself. The emitter and receiver of ultrasonic pulses of the flow meter are located at the ends of the measuring section of the pipeline. The electronic unit contains a pulse generator and a meter for the time it takes the pulse to travel the distance between the emitter and the receiver.

Before starting operation, the flowmeter is filled with a liquid, the flow rate of which will be measured, and the time it takes for the pulse to travel this distance in a standing medium. When the flow moves, its speed will add up with the speed of ultrasound, which will lead to a decrease in the travel time of the pulse. This time, converted in the block into a unified current signal, will be the less, the greater the flow rate, i.e., the greater its flow rate Q.

Ultrasonic flow meters offer the same benefits as electromagnetic flow meters and can also measure the flow of non-conductive liquids.

Vortex flowmeters

The operation of such flowmeters is based on the phenomenon of the appearance of vortices when the flow meets a non-streamlined body. When the flow meter is in operation, the vortices are detached alternately from opposite sides of the body located across the flow. The vortex separation frequency is directly proportional to the flow velocity, ie, to its volumetric flow rate Q. At the vortex site, the flow velocity increases, the pressure decreases. Therefore, the frequency of vortex formation can be measured, for example, with a pressure gauge, the electrical output of which is fed to a frequency meter.

Thermal flow meters

The heat flow meter consists of a heater 1 and two temperature sensors 2 and 3, which are installed outside the tube 4 with the measured flow. With a constant power of the heater, the amount of heat taken from it by the flow will also be constant. Therefore, with an increase in the flow rate Q, the heating of the flow will decrease, which is determined by the temperature difference measured by the temperature sensors 3 and 2. To measure high flow rates, not the entire flow Q is measured, but only part of it Q1, which is passed through the pipe 4. This pipe shunts the section of the pipeline 5 equipped with a throttle 6. The flow area of the throttle determines the upper limit of the range of measured flow rates: the larger this cross section, the higher flow rates can be measured (with the same heater power).

Turbine flow meters

In such flowmeters, the measured flow drives a turbine rotating in bearings. The speed of rotation of the turbine is proportional to the flow rate, that is, to the flow rate Q. To measure the speed of rotation of the turbine, its housing is made of a non-magnetic material. Outside the housing, a differential-transformer converter is installed, and an edge of a ferromagnetic material is made at one of the turbine blades. When this blade passes by the converter, its inductive resistance changes and the voltage on the secondary windings U out changes with a frequency proportional to the flow rate Q. The measuring instrument of such a flow meter is a frequency meter that measures the frequency of voltage change.

High-speed counters

These meters are similar in design to turbine flow meters. The difference between them lies in the fact that flowmeters measure the speed of rotation of the turbine, and in counters, the number of its revolutions, which is then recalculated for the amount of liquid that has passed through the counter for the time interval of interest, for example, for a month.

Euler's Equation Navier-Stokes Equations Diffusion Equation Hooke's Law

As a rule, a substance in a liquid state has only one modification. (The most important exceptions are quantum liquids and liquid crystals.) Therefore, in most cases, a liquid is not only a state of aggregation, but also a thermodynamic phase (liquid phase).

All liquids are usually divided into pure liquids and mixtures. Some mixtures of liquids are of great importance for life: blood, sea water, etc. Liquids can function as solvents.

Physical properties of liquids

Fluidity

The main property of liquids is fluidity. If an external force is applied to a section of liquid in equilibrium, then a flow of liquid particles arises in the direction in which this force is applied: the liquid flows. Thus, under the action of unbalanced external forces, the liquid does not retain the shape and relative position of the parts, and therefore takes the form of the vessel in which it is located.

Unlike plastic solids, a liquid has no yield point: it is enough to apply an arbitrarily small external force for the liquid to flow.

Volume preservation

One of the characteristic properties of a liquid is that it has a certain volume (under constant external conditions). Liquid is extremely difficult to compress mechanically because, unlike gas, there is very little free space between molecules. The pressure exerted on a liquid enclosed in a vessel is transmitted without change to each point in the volume of this liquid (Pascal's law is also true for gases). This feature, along with very low compressibility, is used in hydraulic machines.

Liquids typically expand (expand) when heated and shrink (contract) when cooled. However, there are exceptions, for example, water is compressed when heated, at normal pressure and temperatures from 0 ° C to about 4 ° C.

Viscosity

In addition, liquids (like gases) are viscous. It is defined as the ability to resist the movement of one part relative to another - that is, as internal friction.

When adjacent layers of liquid move relative to each other, collisions of molecules in addition to that caused by thermal motion inevitably occur. Forces arise that inhibit the ordered movement. In this case, the kinetic energy of ordered motion is converted into thermal energy - the energy of the chaotic motion of molecules.

The liquid in the vessel, set in motion and left to itself, will gradually stop, but its temperature will rise.

Free surface formation and surface tension

If the liquid and gaseous phases of the same substance come into contact, forces arise that tend to reduce the area of the interface - surface tension forces. The interface behaves like an elastic membrane that tends to contract.

Therefore, when boiling, soap bubbles and bubbles tend to take a spherical shape: for a given volume, a ball has a minimum surface. If only surface tension forces act on the liquid, it will necessarily take a spherical shape - for example, water droplets in zero gravity.

Evaporation and condensation

Diffusion

Overheating and hypothermia

Density Waves

Although liquid is extremely difficult to compress, nevertheless, when pressure changes, its volume and density still change. This does not happen instantly; so, if one section is compressed, then such compression is transmitted to other sections with a delay. This means that elastic waves, more specifically, density waves, are capable of propagating inside the liquid. Together with the density, other physical quantities also change, for example, temperature.

If during the propagation of a wave, the density changes only slightly, such a wave is called a sound wave, or sound.

If the density changes strongly enough, then such a wave is called a shock wave. The shock wave is described by other equations.

Density waves in a liquid are longitudinal, that is, the density changes along the direction of wave propagation. There are no transverse elastic waves in the liquid due to the non-conservation of the shape.

Elastic waves in a liquid decay with time, their energy gradually transforms into thermal energy. The reasons for attenuation are viscosity, "classical absorption", molecular relaxation, and others. In this case, the so-called second, or bulk viscosity, which is internal friction with a change in density, works. The shock wave, as a result of attenuation, after some time passes into a sound wave.

Elastic waves in a liquid are also subject to scattering by inhomogeneities arising from the chaotic thermal motion of molecules.

Waves on the surface

If the restoring force is mainly gravity, then such waves are called gravitational waves (not to be confused with gravity waves). Gravitational waves on water can be seen everywhere.

If the restoring force is mainly the surface tension force, then such waves are called capillary.

If these forces are comparable, such waves are called capillary-gravitational waves.

Waves on the surface of a liquid are attenuated by viscosity and other factors.

Coexistence with other phases

The presence of the atmosphere in the conditions of earth's gravity. The liquid is affected by atmospheric pressure (air and steam), while for steam, almost only its partial pressure should be taken into account. Therefore, liquid and vapor above its surface correspond to different points on the phase diagram, in the region of existence of the liquid phase and in the region of existence of the gaseous phase, respectively. This does not cancel evaporation, but evaporation takes time during which both phases coexist. Without this condition, liquids would boil and evaporate very quickly.

Theory

Mechanics

The study of the motion and mechanical equilibrium of liquids and gases and their interaction with each other and with solids is devoted to the section of mechanics - hydroaeromechanics (often also called hydrodynamics). Hydroaeromechanics is part of the more general branch of mechanics, continuum mechanics.

Fluid mechanics is a branch of fluid mechanics that deals with incompressible fluids. Since the compressibility of liquids is very small, it can be neglected in many cases. Gas dynamics is devoted to the study of compressible liquids and gases.

Hydromechanics is subdivided into hydrostatics, in which the equilibrium of incompressible fluids is studied, and hydrodynamics (in the narrow sense), in which their motion is studied.

The movement of conductive and magnetic fluids is studied in magnetohydrodynamics. Hydraulics are used to solve applied problems.

The basic law of hydrostatics is Pascal's law.

2. Liquids from diatomic molecules consisting of identical atoms (liquid hydrogen, liquid nitrogen). Such molecules have a quadrupole moment.

4. Liquids consisting of polar molecules linked by dipole-dipole interaction (liquid hydrogen bromide).

5. Associated liquids, or liquids with hydrogen bonds (water, glycerin).

6. Liquids consisting of large molecules for which the internal degrees of freedom are essential.

Liquids of the first two groups (sometimes three) are usually called simple. Simple liquids have been studied better than others; of complex liquids, water is the best studied. This classification does not include quantum liquids and liquid crystals, which are special cases and should be considered separately.

Statistical theory

The structure and thermodynamic properties of liquids are most successfully investigated using the Percus-Yevik equation.

If we use the model of solid balls, that is, we consider liquid molecules as balls with a diameter d, then the Percus-Yevik equation can be solved analytically and the equation of state for the liquid can be obtained:

where n- the number of particles per unit volume, - dimensionless density. At low densities, this equation transforms into the equation of state for an ideal gas: ... For extremely high densities,, the equation of state of an incompressible fluid is obtained:.

The model of hard balls does not take into account the attraction between molecules, therefore, there is no sharp transition between liquid and gas when external conditions change.

If more accurate results are needed, then the best description of the structure and properties of a liquid is achieved using perturbation theory. In this case, the model of hard balls is considered to be the zero approximation, and the forces of attraction between the molecules are considered to be a perturbation and give corrections.

Cluster theory

One of the modern theories is "Cluster theory"... It is based on the idea that a liquid is represented as a combination of a solid and a gas. In this case, particles of the solid phase (crystals moving over short distances) are located in a cloud of gas, forming cluster structure... The particle energy corresponds to the Boltzmann distribution, while the average energy of the system remains constant (provided it is isolated). Slow particles collide with clusters and become part of them. Thus, the configuration of the clusters is constantly changing, the system is in a state of dynamic equilibrium. When creating an external influence, the system will behave according to the Le Chatelier principle. Thus, it is easy to explain the phase transformation:

When heated, the system will gradually turn to gas (boiling)
When cooled, the system will gradually turn into a solid (freezing).

Experimental study methods

The structure of liquids is studied using X-ray structural analysis, electron diffraction and neutron diffraction.

Links

Thermodynamic states of matter
Physics: Solid Liquid Gas Plasma