Air stream boundary. Main characteristics of turbulent free jets

Laminar is an air flow in which the streams of air move in the same direction and are parallel to each other. When the speed increases to a certain value, the air stream trickles, in addition to the translational speed, also acquire rapidly changing speeds perpendicular to the direction of translational motion. A flow is formed, which is called turbulent, that is, chaotic.

boundary layer

The boundary layer is the layer in which the air velocity varies from zero to a value close to the local air velocity.

When an air flow flows around a body (Fig. 5), air particles do not slide over the surface of the body, but are decelerated, and the air velocity near the surface of the body becomes equal to zero. When moving away from the surface of the body, the air speed increases from zero to the speed of the air flow.

The thickness of the boundary layer is measured in millimeters and depends on the viscosity and pressure of the air, on the profile of the body, the state of its surface and the position of the body in the air stream. The thickness of the boundary layer gradually increases from the leading to the trailing edge. In the boundary layer, the nature of the movement of air particles differs from the nature of the movement outside it.

Consider an air particle A (Fig. 6), which is located between air streams with velocities U1 and U2, due to the difference in these velocities applied to opposite points of the particle, it rotates and the more, the closer this particle is to the surface of the body (where the difference the highest speed). When moving away from the surface of the body, the rotational motion of the particle slows down and becomes equal to zero due to the equality of the air flow velocity and the air velocity of the boundary layer.

Behind the body, the boundary layer passes into a wake, which blurs and disappears as it moves away from the body. The turbulence in the wake hits the tail of the aircraft and reduces its efficiency, causing shaking (Buffing phenomenon).

The boundary layer is divided into laminar and turbulent (Fig. 7). With a steady laminar flow of the boundary layer, only internal friction forces appear due to the viscosity of the air, so the air resistance in the laminar layer is small.

Rice. 5

Rice. 6 Air flow around a body - flow deceleration in the boundary layer

Rice. 7

In a turbulent boundary layer, there is a continuous movement of air streams in all directions, which requires more energy to maintain a random vortex motion and, as a result, a greater resistance of the air flow to the moving body is created.

The coefficient Cf is used to determine the nature of the boundary layer. A body of a certain configuration has its own coefficient. So, for example, for a flat plate, the drag coefficient of the laminar boundary layer is:

for turbulent layer

where Re is the Reynolds number, which expresses the ratio of inertial forces to frictional forces and determines the ratio of two components - profile resistance (shape resistance) and frictional resistance. The Reynolds number Re is determined by the formula:

where V is the air flow velocity,

I - character of body size,

kinetic coefficient of viscosity of air friction forces.

When an air flow flows around a body at a certain point, the boundary layer changes from laminar to turbulent. This point is called the transition point. Its location on the surface of the body profile depends on the viscosity and pressure of the air, the speed of the air streams, the shape of the body and its position in the air flow, and also on the surface roughness. When creating wing profiles, designers tend to take this point as far as possible from the leading edge of the profile, thereby reducing the friction drag. For this purpose, special laminated profiles are used, which increase the smoothness of the wing surface and a number of other measures.

With an increase in the speed of the air flow or an increase in the angle of the body relative to the air flow to a certain value, at some point, the boundary layer is separated from the surface, while the pressure behind this point sharply decreases.

As a result of the fact that the pressure at the trailing edge of the body is greater than behind the separation point, there is a reverse flow of air from the zone of higher pressure to the zone of lower pressure to the separation point, which entails separation of the air flow from the body surface (Fig. 8).

A laminar boundary layer separates more easily from the body surface than a turbulent one.

Continuity equation for an air stream jet

The equation of the continuity of the jet of air flow (the constancy of air flow) is an equation of aerodynamics, which follows from the basic laws of physics - the conservation of mass and inertia - and establishes the relationship between the density, speed and cross-sectional area of the jet of air flow.

Rice. eight

Rice. nine

When considering it, the condition is accepted that the studied air does not have the property of compressibility (Fig. 9).

In a jet of variable cross section, a second volume of air flows through section I for a certain period of time, this volume is equal to the product of the air flow velocity and cross section F.

The second mass air flow m is equal to the product of the second air flow and the air flow density p of the jet. According to the law of conservation of energy, the mass of the air flow of the stream m1 flowing through section I (F1) is equal to the mass m2 of this flow flowing through section II (F2), provided that the air flow is steady:

m1=m2=const, (1.7)

m1F1V1=m2F2V2=const. (1.8)

This expression is called the equation of the continuity of the jet of the air stream of the stream.

F1V1=F2V2= const. (1.9)

So, it can be seen from the formula that the same volume of air passes through different sections of the stream in a certain unit of time (second), but at different speeds.

We write equation (1.9) in the following form:

It can be seen from the formula that the air flow velocity of the jet is inversely proportional to the cross-sectional area of the jet and vice versa.

Thus, the equation of the continuity of the jet of air flow establishes the relationship between the cross section of the jet and the speed, provided that the air flow of the jet is steady.

Static pressure and velocity head Bernoulli equation

air plane aerodynamics

The aircraft, which is in a stationary or moving air flow relative to it, experiences pressure from the latter, in the first case (when the air flow is stationary) it is static pressure and in the second case (when the air flow is moving) it is dynamic pressure, it is often called speed pressure. The static pressure in a stream is similar to the pressure of a liquid at rest (water, gas). For example: water in a pipe, it can be at rest or in motion, in both cases the walls of the pipe are under pressure from the water. In the case of water movement, the pressure will be somewhat less, since a velocity pressure has appeared.

According to the law of conservation of energy, the energy of an air stream in various sections of an air stream is the sum of the kinetic energy of the stream, the potential energy of the pressure forces, the internal energy of the stream and the energy of the body position. This amount is a constant value:

Ekin+Ep+Evn+En=const (1.10)

Kinetic energy (Ekin) - the ability of a moving air stream to do work. She is equal

where m is the mass of air, kgf s2m; V-speed of air flow, m/s. If instead of the mass m we substitute the mass density of air p, then we get the formula for determining the velocity head q (in kgf / m2)

Potential energy Ep - the ability of the air flow to do work under the influence of static pressure forces. It is equal to (in kgf-m)

where Р - air pressure, kgf/m2; F is the cross-sectional area of the air flow filament, m2; S is the path traveled by 1 kg of air through a given section, m; the product SF is called the specific volume and is denoted by v, substituting the value of the specific volume of air into formula (1.13), we obtain

The internal energy Evn is the ability of a gas to do work when its temperature changes:

where Cv is the heat capacity of air at a constant volume, cal / kg-deg; T-temperature on the Kelvin scale, K; A is the thermal equivalent of mechanical work (cal-kg-m).

It can be seen from the equation that the internal energy of the air flow is directly proportional to its temperature.

Position energy En is the ability of air to do work when the position of the center of gravity of a given air mass changes when it rises to a certain height and is equal to

where h is the change in height, m.

In view of the scanty small values of the separation of the centers of gravity of the air masses along the height in the trickle of the air flow, this energy is neglected in aerodynamics.

Considering all types of energy in relation to certain conditions, it is possible to formulate Bernoulli's law, which establishes a relationship between the static pressure in a trickle of the air flow and the velocity pressure.

Consider a pipe (Fig. 10) of variable diameter (1, 2, 3) in which an air flow moves. Manometers are used to measure the pressure in the sections under consideration. Analyzing the readings of pressure gauges, we can conclude that the lowest dynamic pressure is shown by a pressure gauge of section 3-3. This means that when the pipe narrows, the speed of the air flow increases and the pressure drops.

Rice. ten

The reason for the pressure drop is that the air flow does not produce any work (no friction) and therefore the total energy of the air flow remains constant. If we consider the temperature, density and volume of the air flow in different sections to be constant (T1=T2=T3; р1=р2=р3, V1=V2=V3), then the internal energy can be ignored.

This means that in this case, the transition of the kinetic energy of the air flow into potential energy and vice versa is possible.

When the speed of the air flow increases, then the velocity head increases and, accordingly, the kinetic energy of this air flow.

We substitute the values from formulas (1.11), (1.12), (1.13), (1.14), (1.15) into formula (1.10), taking into account that we neglect the internal energy and position energy, transforming equation (1.10), we obtain

This equation for any section of a trickle of air is written as follows:

This type of equation is the simplest mathematical Bernoulli equation and shows that the sum of the static and dynamic pressures for any section of a steady air flow stream is a constant value. Compressibility is not taken into account in this case. Appropriate corrections are made when compressibility is taken into account.

For clarity of Bernoulli's law, you can conduct an experiment. Take two sheets of paper, holding them parallel to each other at a short distance, blow into the gap between them.

Rice. eleven

The leaves are getting closer. The reason for their convergence is that on the outer side of the sheets the pressure is atmospheric, and in the gap between them, due to the presence of a high-speed air pressure, the pressure decreased and became less than atmospheric. Under the influence of the pressure difference, the sheets of paper bend inward.

wind tunnels

An experimental setup for studying the phenomena and processes that accompany the flow of gas around bodies is called a wind tunnel. The principle of operation of wind tunnels is based on the principle of Galileo's relativity: instead of the motion of a body in a stationary medium, a gas flow around a stationary body is studied. In wind tunnels, the aerodynamic forces acting on the aircraft and the moments are experimentally determined, the pressure and temperature distributions over its surface are studied, the flow pattern around the body is observed, aeroelasticity is studied etc.

Depending on the range of Mach numbers M, wind tunnels are divided into subsonic (M=0.15-0.7), transonic (M=0.7-13), supersonic (M=1.3-5) and hypersonic (M= 5-25), according to the principle of operation - into compressor rooms (continuous operation), in which the air flow is created by a special compressor, and balloon ones with increased pressure, according to the layout of the circuit - into closed and open ones.

Compressor pipes have high efficiency, they are easy to use, but require the creation of unique compressors with high gas flow rates and high power. Balloon wind tunnels are less economical than compressor wind tunnels, since part of the energy is lost when gas is throttled. In addition, the duration of operation of balloon wind tunnels is limited by the gas supply in the cylinders and ranges from tens of seconds to several minutes for various wind tunnels.

The wide distribution of balloon wind tunnels is due to the fact that they are simpler in design and the compressor power required to fill the balloons is relatively small. In wind tunnels with a closed loop, a significant part of the kinetic energy remaining in the gas flow after its passage through the working area is used, which increases the efficiency of the wind tunnel. In this case, however, it is necessary to increase the overall dimensions of the installation.

In subsonic wind tunnels, the aerodynamic characteristics of subsonic helicopters, as well as the characteristics of supersonic aircraft in takeoff and landing modes, are studied. In addition, they are used to study the flow around cars and other ground vehicles, buildings, monuments, bridges, and other objects. Figure shows a diagram of a closed-loop subsonic wind tunnel.

Rice. 12

1 - honeycomb 2 - grids 3 - prechamber 4 - confuser 5 - flow direction 6 - working part with model 7 - diffuser, 8 - knee with rotary blades, 9 - compressor 10 - air cooler

Rice. thirteen

1 - honeycomb 2 - screens 3 - prechamber 4 confuser 5 perforated working part with model 6 ejector 7 diffuser 8 elbow with guide vanes 9 air outlet 10 - air supply from cylinders

Rice. fourteen

1 - compressed air cylinder 2 - pipeline 3 - control throttle 4 - leveling grids 5 - honeycomb 6 - deturbulent grids 7 - prechamber 8 - confuser 9 - supersonic nozzle 10 - working part with model 11 - supersonic diffuser 12 - subsonic diffuser 13 - release into the atmosphere

Rice. fifteen

1 - cylinder with high pressure 2 - pipeline 3 - control throttle 4 - heater 5 - prechamber with honeycomb and grids 6 - hypersonic axisymmetric nozzle 7 - working part with model 8 - hypersonic axisymmetric diffuser 9 - air cooler 10 - flow direction 11 - air supply into ejectors 12 - ejectors 13 - shutters 14 - vacuum vessel 15 - subsonic diffuser

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air jet

Introduction

The theory of jet flows of gas (air) is used in devices of ventilation systems, air showers, air curtains, when calculating the supply or suction of air masses through ventilation grills, burners, etc.

Ventilation (from Latin ventilatio - ventilation) is the process of removing exhaust air from a room and replacing it with outside air. In necessary cases, this is carried out: air conditioning, filtration, heating or cooling, humidification or dehumidification, ionization, etc. Ventilation provides sanitary and hygienic conditions (temperature, relative humidity, air velocity and air purity) of the indoor air, favorable for human health and well-being, meeting the requirements of sanitary standards, technological processes, building structures, storage technologies, etc.

Also, this term in technology often refers to systems of equipment, devices and instruments for these purposes.

There are two main types of building ventilation: displacement ventilation and mixing ventilation.

Displacement ventilation is predominantly used to ventilate large industrial spaces because it can effectively remove excess heat if properly sized. Air is supplied to the lower level of the room and flows into the working area at low speed. This air must be somewhat colder than the room air for the displacement principle to work. This method provides excellent air quality, but is less suitable for use in offices and other small spaces because the directional air terminal takes up quite a lot of space and it is often difficult to avoid drafts in the work area.

Agitation ventilation is the preferred method of air distribution in situations where so-called comfort ventilation is required. The basis of this method is that the supply air enters the work area already mixed with room air. The calculation of the ventilation system must be made in such a way that the air circulating in the working area is sufficiently comfortable. In other words, the air speed should not be too high and the temperature inside the room should be more or less uniform.

The air jet entering the room entrains and mixes large volumes of ambient air. As a result, the volume of the air jet increases, while its speed decreases the more it penetrates into the room. The mixing of ambient air into the air stream is called ejection.

Rice. 1. Ejection

The air movements caused by the air jet soon thoroughly mix all the air in the room. Airborne contaminants are not only dispersed, but evenly distributed. The temperature in different parts of the room also equalizes.

When calculating mixing ventilation, the most important point is to ensure that the air velocity in the working area is not too high, otherwise a feeling of draft is created.

Rationale

An air shower is a device in the local supply ventilation system that provides a concentrated flow of air that creates a direct impact on a person in the area of the direct impact of this flow on a person.

The air shower is used in fixed workplaces or recreational areas. They are especially effective in industrial premises (rice), where workers are exposed to high temperatures. Installations for air showers happen stationary and mobile.

Air curtain (thermal curtain, air-thermal curtain) - creates an invisible barrier to air flow.

Curtains can be with electric, water, steam, gas heating, as well as without heating.

For installation:

· curtains of vertical installation;

· curtains of horizontal installation;

· concealed installation curtains (built into/behind a false ceiling, doorway).

By type of heating:

Curtains with heating (curtains with heating are usually called air-thermal or thermal curtains, since the screening of the doorway is carried out with heated air);

Curtains without heating (curtains without heating are usually called ("cold flow").

The design of the thermal curtain includes:

· an electric heater or a water heater, as well as large industrial air curtains can be equipped with a steam or gas heater (in case the curtain is heated, there is no such heater in the curtain without heating);

fans

air filter (for models with water heating).

Ventilation grilles are structures that today are widely used in the construction industry for interior and exterior decoration of premises and buildings, laying communication systems. They perform the functions of an air distribution device in ventilation systems of various types. Today, these structures are used in the installation and commissioning of supply and exhaust ventilation.

Modern models of gratings can be used not only for air distribution, but also for its supply or removal. It all depends on the type of ventilation system. Such designs can often be found in private houses, administrative and commercial buildings, office premises. That is, their use is advisable in those rooms where there is a need to create and maintain optimal temperature and humidity indicators.

Scientific theory of air jets

A jet of gas is said to be flooded if it propagates in a medium with the same physical properties as its own. When studying the movement of air in ventilation systems, there are various cases of the propagation of submerged jets. But when considering these cases, the free jet scheme is used as the initial one. A free jet is a jet propagating in an infinite medium. (A jet not limited by solid walls is called a free jet.) In this case, the jet can flow into a stationary medium, as well as into an air stream.

In this case, there are:

· A string jet, a jet flowing into a stream whose velocity direction coincides with the direction of the jet.

· A jet in a drifting flow, if the flow velocity is directed at an angle to the axis of the jet.

· A jet in a counter flow, when the vectors of the longitudinal velocity of the jet and the velocity of the flow are directed towards each other.

According to the type of energy spent on the formation of a jet, there are:

Supply (mechanical) jets created by a fan, compressor, ejector, etc.

· Convective jets formed as a result of heating or cooling of air near hot or cold surfaces of various bodies.

Jets are also distinguished by the shape of the initial section:

· If the cross section is round, then the jet is called asymmetric.

If the section has the form of an infinitely long strip of constant height, then it is called plane-parallel or flat.

The jet and ambient temperatures may be the same or different.

In accordance with this, isothermal and non-isothermal jets are distinguished. On fig. 3 shows an air jet that is formed when air is forced into the room through a hole in the wall. The result is a free air stream. If the temperature of the air in the jet is the same as in the room, it is called a free isothermal jet.

According to the degree of influence of the surrounding space on the nature of the movement of the jet, there are:

jets are free;

semi-limited or flat, moving along the space-limiting plane;

limited (restricted), flowing into the space of finite dimensions, commensurate with the initial dimensions of the jet.

Depending on the jet expiration mode, there can be:

laminar (a flow in which a liquid or gas moves in layers without mixing and pulsations);

turbulent (the form of a liquid or gas flow, in which their elements make disorderly, unsteady movements along complex trajectories, which leads to intense mixing between layers of a moving liquid or gas).

Turbulent jets are observed in ventilation systems. One more definition: if there are rotational velocity components in the initial section, then such a jet is called swirling.

More. In turbulent motion, along with axial motion, there is also transverse motion of particles. In this case, the particles fall outside the jet and transfer their momentum to the masses of motionless air adjacent to the jet, entrain (eject) these masses, giving them a certain speed.

In place of the particles that left the jet, particles from the surrounding air enter it, which slow down the boundary layers of the jet. As a result of this exchange of impulses between the jet and still air, an increase in the mass of the jet and a decrease in velocity at its boundaries appear.

The decelerated particles of the jet, together with the entrained particles of the surrounding air, form a turbulent boundary layer, the thickness of which continuously increases with distance from the outlet. coming into contact with the stationary medium from the outside (?? = 0), and from the inside - with the core of constant velocity (?? = ?? 0), the boundary layer acquires a variable velocity profile. Fig.4.

The core of constant velocity, as it moves away from the outlet and thickens the boundary layer, narrows until it completely disappears. After that, the boundary layer already fills the entire jet cross section, including the flow axis.

Therefore, further blurring of the jet is accompanied by an increase in its width and, in this case, the velocity on the axis decreases.

The section of the jet, in which the erosion of the core of constant velocity is completed and on the axis of which both halves of the boundary layer merge, is called the transition section. The section of the jet located between the outlet and the transition section, in which the velocity on the axis remains unchanged and equal to the initial velocity?? 0 is called initial. The section following the transition section, in which the velocity on the axis gradually decreases and decays, is called the main section. The boundaries of the jet, both outer and cores of constant velocity, are rectilinear. The point O of intersection of the outer boundaries of the jet is called the pole of the jet.

The static pressure at different points of the jet varies insignificantly and is approximately equal to the pressure of the surrounding space, i.e. the free jet can be considered isobaric.

The main parameters of a turbulent jet are the axial velocity??, diameter D for circular sections, and width?? for flat jets, air consumption?? and average speed.

From the theoretical and experimental studies of Genrikh Naumovich Abramovich, it follows that the main parameters of the jet depend on the turbulence coefficient a, which characterizes the intensity of mixing and depends on the design of the nozzle from which the jet flows. (Genrikh Naumovich Abramovich (1911 - 1995) - Soviet scientist in the field of theoretical and applied gas dynamics).

The greater the turbulence coefficient a, the more intense the mixing and the greater the angle of one-sided expansion of the jet.

Table of turbulence coefficient a and jet expansion angle 2?? for some types of nozzles.

Definition. A jet is a form of flow in which a liquid (gas) flows in an environment filled with a liquid (gas) with physical parameters that differ from it: speed, temperature, composition, etc. Jet flows are diverse - from a rocket engine jet to a jet stream in the atmosphere . An air jet is an air stream formed when it exits an air duct into a large volume space that does not have solid boundaries.

Distribution and form. The air jet consists of several zones with different flow regimes and air velocities. The area of greatest practical interest is the main site. The speed at the center (speed around the central axis) is inversely proportional to the distance from the diffuser or valve, i.e. the farther from the diffuser, the lower the air speed. The air jet is fully developed in the main area, and the prevailing conditions here will have a decisive influence on the flow pattern in the room as a whole.

Main section of the air jet, tilt speed. The shape of the air jet depends on the shape of the diffuser or the outlet of the air distributor. Round or rectangular orifices create a compact cone-shaped air jet. In order for the air jet to be absolutely flat, the orifice must be more than twenty times as wide as its height, or as wide as the room. Air fan jets are obtained by passing through perfectly round orifices, where air can spread in any direction, as in supply diffusers.

Rice. 5. Various types of air jets

ventilation curtain air ejection

speed profile. The air velocity in each part of the jet can be calculated mathematically. To calculate the velocity at a certain distance from the diffuser/valve outlet, it is necessary to know the air velocity at the outlet of the diffuser/valve, its shape and the type of air jet it produces. In the same way, it is possible to consider how the velocities vary in each jet profile.

Using these calculations, velocity curves can be drawn for the entire jet. This makes it possible to identify areas that have the same speed. These areas are called isovels (lines of constant velocity). By making sure that the isovel corresponding to 0.2 m/s is outside the work area, you can be sure that the air velocity will not exceed this level directly in the work area.

Rice. 6. Various air jet isovels

Diffuser coefficient. The diffuser coefficient is a constant value that depends on the shape of the diffuser or valve. The factor can be theoretically calculated using the following factors: the momentum dispersion and contraction of the air jet at the point where it enters the room, and the degree of turbulence created by the diffuser or valve.

In practice, the coefficient is determined for each type of diffuser or damper by measuring the air velocity at at least eight points located at different distances from the diffuser/valve and at least 30 cm apart. These values are then plotted on a logarithmic plot which shows the measured values for the main air jet section, which in turn gives a value for the constant.

The diffuser coefficient makes it possible to calculate the speed of the air jet and to predict the distribution and path of the air jet. This factor is different from the K factor, which is used to enter the correct value for the volume of air leaving the supply air terminal or iris damper. The K factor is described on page 390.

Layering effect. If the air distributor is installed close enough to a flat surface (usually a ceiling), the outgoing air jet is deflected towards it and tends to flow directly over the surface. This effect occurs due to the formation of rarefaction between the jet and the surface, and since there is no possibility of air admixture from the surface, the jet deviates towards it. This phenomenon is called the spreading effect.

Rice. 7. Covering effect

Practical experiments have shown that the distance between the upper edge of the diffuser or damper and the ceiling must not exceed 30 cm in order for a flooring effect to occur. The spreading effect can be used to increase the path of the cold air jet along the ceiling before it enters the work area. The diffuser factor will be slightly higher when a layering effect occurs than when free airflow occurs. It is also important to know how the diffuser or valve is attached when using the diffuser factor to make various calculations.

Non-isothermal air jet. The distribution becomes more difficult when the supply air is warmer or colder than the indoor air. The thermal energy resulting from the difference in air density at different temperatures causes the colder air to move downward (the jet sinks) and the warmer air to move up (the jet to float).

This means that two different forces are acting on the cold jet near the ceiling: the flooring effect, which tries to press it against the ceiling, and thermal energy, which tends to bring it down to the floor.

At a certain distance from the outlet of the diffuser or valve, thermal energy will dominate and the air jet will eventually deviate from the ceiling.

Jet deflection and break-off point can be calculated using formulas based on temperature differentials, diffuser or valve outlet type, airflow rate, etc.

Rice. 8. Air jet separation point (Xm) and deflection (Y)

Important criteria when calculating ventilation. It is important to choose and place the air distributor correctly. It is also important that the temperature and air velocity in the working area are acceptable.

Distance x 0 from pole to outlet:

round jet - x 0 = ;

· flat jet - x 0 = . Where?? 0 - hole diameter or nozzle; ?? 0 - half the height of the flat nozzle.

The length of the initial section x n of the jet:

round - x n \u003d;

flat - x n = .

Axial speed?? in the main section at a distance x from the jet pole:

round - ?? = ;

flat - ?? = .

Air consumption?? in the main section at a distance x from the jet pole:

round - ?? = 4.36?? 0();

flat (per unit width nozzle) - ?? = 1.2?? 0 .

The diameter of the round jet in the main section at a distance x from the pole of the jet:

Average speed in the main section of the jet:

round - ?? = ;

flat - ?? = .

Flat jet height:

4,8?? 0 ().

Correct air velocity in the working area. Most air terminal devices are listed in the catalog with a specification called throw length. The jet length is understood as the distance from the inlet of the diffuser or valve to the air jet section, in which the velocity of the flow core decreases to a certain value, usually up to 0.2 m/s. The jet length is indicated and measured in meters.

Rice. 9. The concept of "Jet length"

The first thing to consider when designing air distribution systems is how to avoid too high airflow velocities in the work area. But, as a rule, the reflected or reverse current of this jet enters the working zone: see Fig. 10.

Rice. 10. Reverse airflow with wall mounted diffuser

The reverse air flow rate is approximately 70% of the speed of the main air jet at the wall. This means that a diffuser or damper mounted on the back wall delivering a jet of air with a final velocity of 0.2 m/s will cause an air velocity in the return flow of 0.14 m/s. That corresponds to comfortable ventilation in the working area, the air velocity in which should not exceed 0.15 m/s.

The throw length for the diffuser or valve described above is the same as the length of the room, and in this example is an excellent choice. The acceptable throw length for a wall-mounted diffuser is between 70% and 100% of the room length.

Penetrating power of the air stream. The shape of the room can have a significant impact on the configuration of the flow. When the cross section of the air flow is more than 40% of the cross section of the room, the ejection of room air into the flow will stop. As a result, the air jet will begin to mix its own air. At the same time, an increase in the speed of the supplied air will not solve the problem, since the penetrating ability will remain the same, only the speed of the air stream and the ambient air in the room will increase.

In that part of the room where the main air flow does not reach, other air flows, secondary vortices, will begin to appear. However, if the length of the room is less than three times its height, it can be assumed that the air jet will penetrate to the end of the room.

Rice. 11. Secondary vortices are formed at the farthest end of the room, where the air stream does not reach

Flow around obstacles. The air jet in the presence of obstacles on the ceiling in the form of ceilings, lamps, etc., if they are located too close to the diffuser, may deviate and fall into the working area. Therefore, it is necessary to know what distance should be (A on the graph) between the air supply device and the obstacles for the free movement of the air stream.

Rice. 12. Minimum distance to obstacle

Installation of several air distributors. If one ceiling diffuser is intended to serve the entire room, it should be placed as close to the center of the ceiling as possible, and the total area should not exceed the dimensions shown in fig. 12.

Rice. 12. Small room ventilated by a single ceiling diffuser

If the room is large, it is necessary to divide it into several zones, and place a diffuser in each zone.

Rice. 13. Large room ventilated by multiple ceiling diffusers

A room ventilated by several wall diffusers is also divided into several zones. The number of zones depends on the distance between the diffusers, sufficient to prevent interference with each other. If two air streams are mixed, one air stream with a longer jet length is obtained.

Rice. 14. Large room ventilated by multiple wall diffusers

Warm air supply. The horizontally supplied warm air from the ceiling diffuser heats well rooms with a ceiling height of up to 3.5 meters, raising the room temperature by 10-15°C.

Rice. 15. Horizontal air supply ceiling diffuser

However, in very high rooms, the supply air must be directed vertically downwards if it is also used for space heating. If the temperature difference is not more than 10°C, then the air jet should drop to about 1 m from the floor so that the temperature in the working area becomes comfortable.

Rice. 16. Vertical air supply ceiling diffuser

Cold air supply. If the air supplied along the ceiling is colder than the air in the room, it is important that the air velocity is high enough to ensure that it adheres to the ceiling. If its speed is too low, there is a risk that the thermal energy may direct the air jet down to the floor too soon.

At a certain distance from the diffuser supplying air, the air jet will in any case separate from the ceiling and deviate downwards. This deflection will happen faster for an air jet that is below room temperature, and therefore the jet length will be shorter in this case.

Rice. 17. The difference between the length of isothermal and non-isothermal jets

The air jet must travel at least 60% of the depth of the room before it leaves the ceiling. The maximum air velocity in the working area will thus be almost the same as with isothermal air supply.

When the supply air temperature is lower than room temperature, the air in the room will be cooled to some extent. The acceptable level of cooling (known as the maximum cooling effect) depends on the requirements for air velocity in the work area, on the distance from the diffuser at which the air jet separates from the ceiling, and also on the type of diffuser and its location.

In general, a greater degree of cooling is achieved by using a ceiling diffuser rather than a wall diffuser. This is because the ceiling diffuser diffuses air in all directions and therefore takes less time to mix with the ambient air and to equalize the temperature.

The right choice of air diffuser. The diffusers can be mounted either on the ceiling or on the wall. They are often equipped with nozzles or are perforated to facilitate the mixing of ambient air into the air stream.

Nozzle diffusers are the most flexible devices because they allow individual adjustment of each nozzle. They are ideal for supply air temperatures well below room temperature, especially if they are installed on the ceiling. The distribution pattern can be changed by turning the nozzles in different directions.

Perforated diffusers have a positive effect where the temperature of the air jet is significantly lower than the ambient air temperature. They are not as flexible as nozzle diffusers, but by shielding the supply airflow in different directions, the distribution pattern can be changed.

Wall gratings have a long jet length. They have limited ability to change the distribution pattern and are not well suited for supply air temperatures well below ambient temperature.

Conclusion

So, the air jet is the main element of the operation of ventilation equipment. In this work, the types of ventilation and their equipment, the forms of air jets and their varieties were considered. Particular attention was paid to the use of air jets. Here, in conclusion, you can expand them.

Even in ancient times, people first set sail, and the wind carried their boats through the water or sleighs through the ice and snow. However, since then, the air currents have found so much work that it is worth mentioning it separately. The ships sail to this day. They float on rivers, lakes and even oceans. The undoubted advantages of this method of transportation are cleanliness and silence (gasoline stains do not remain on the water and the engine does not make noise), and you do not have to buy gasoline. Athletes, on the other hand, sail not only on boats, but even just on boards.

Other athletes use air currents for free flight.

Air is also used for completely earthly work. In the old days, the wind turned the wings of the windmill. Now, in place of the millstones, an electricity generator has been installed, which converts wind energy into electrical energy - a wind power plant has turned out.

We talked only about natural air currents - winds. But you can also create wind artificially. The simplest thing is to blow.

Wind occurs when there is a difference in atmospheric pressure: in one place the pressure is higher, in another it is lower, the air begins to move from the side of high pressure to the side of low. This means that if we pump out air from somewhere (we create low pressure), then air will immediately rush there from all sides. If, on the contrary, we create high pressure somewhere, air will rush out from there. Now let's leave the air only one way to freedom - through a narrow tube. A very strong wind will begin to blow in the tube. When you have to deflate an air mattress, pay attention to how much air is blown out through the valve!

Such artificial winds are used, for example, in pneumatic mail (air mail).

Now let's take a pipe and create a reduced air pressure at one end. The air from outside will immediately rush into the pipe, capturing all light objects along the way. We got a vacuum cleaner.

The same vacuum cleaner principle is used when loading flour. It is not poured, but simply sucked out of the car to the warehouse and back. By the way, flour is also ground with the help of wind, because the grains are quite light.

The use of air jet in the mining industry. The ventilation jet, after passing through all mine workings, can carry a significant amount of low-potential thermal energy, which is released into the atmosphere after ventilation of mining operations. The use of the energy potential of the ventilation jet of mines, depending on the ventilation scheme, the natural temperature of the rocks and the remoteness of the mining enterprise from the industrial infrastructure, can have different indicators of economic efficiency and environmental impact.

And here is another example of using an air jet. A plasma torch is a modern metal cutting device (although it was invented in the 20th century), it uses air (or any plasma-forming gas) in its work. Air (Air) or other plasma-forming gas (mixture of gases), having passed through the channel inside the electrode assembly and the swirling mechanism, forms a vortex flow swirling along the longitudinal axis of the plasma torch electrode and exiting through the nozzle channel geometrically coaxial with it.

References

1. E.S. Laptev. Fundamentals of hydraulics and aerodynamics. Almaty, 2016.

2. N.N. Belyaev, P.B. Mashikhina. The use of air jets to intensify the evaporation process.

3. Article "Air shell of the earth" Ispolzovanije_vetra.html.

4. Article "Application of air flow swirlers to improve the efficiency of wind turbines". http://vikidalka.ru/2-196929.html.

5. Article "Air currents". http://ru-ecology.info/term/19749/.

6. Article “Combines of the future. Air jet use. http://svistun.info/zemledelie/211.

7. Staroverov I.G. Handbook of the designer of industrial, residential and public buildings and structures. Air heating with concentrated air supply with parallel direction of air jets. Air heating with concentrated air supply with a fan direction of air jets.

8. Article "Theory of air jets". Vecotech. http://vecotech.com.ua/podbor-e-montazh-dimohodov/666.html.

9. Article "Internal structure and principle of operation of the plasma torch of air-plasma metal cutting installations." http://www.spektrplus.ru/d_plazm.htm.

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Formation of an air jet in the process of overcoming violations of sound pronunciation

The main purpose of the respiratory apparatus is the implementation of gas exchange, that is, the delivery of oxygen to the tissues of the body and the removal of carbon dioxide from them. And this exchange takes place due to the periodic renewal of air in the lungs, which occurs during the alternation of the respiratory phases - inhalation and exhalation.

There are three main types of breathing:

Clavicular

Rib (thoracic)

Diaphragmatic (abdominal)

With clavicular breathing, the shoulder girdle and upper ribs rise, and mainly the upper part of the chest expands.

With a costal (thoracic) chest, the chest expands forward and to the sides.

In diaphragmatic breathing, the diaphragm descends and increases mainly the lower part of the chest; the abdominal wall protrudes.

Pure types of breathing are not actually observed. In any type of breathing, the diaphragm is active to a greater or lesser extent. Therefore, in practice, one can only speak of predominantly key, predominantly abdominal, or clavicular breathing.

Types of breathing depend on gender, age, profession.

Thus, in women, the chest type of breathing is more often observed, in men - the abdominal type, in manual workers, the abdominal type of breathing prevails, in persons engaged in clerical and generally sedentary work, the chest type.

Children usually have a mixed type of breathing, i.e., the middle one between the abdominal and chest.

With deep or full breathing, three types of breathing are combined - clavicular, thoracic, abdominal.

Within 1 minute there are 16-20 complete respiratory movements (inhalations and exhalations).

The duration of inhalation is almost equal to the duration of exhalation (the ratio of inhalation time to exhalation time is approximately 1: 1.25).

This is the physiological respiration necessary for life.

But in order for a child to start speaking, he must master a special type of breathing - speech-breathing.

This term refers to the ability of a person in the process of speaking to take a deep enough breath in a timely manner and rationally expend air during exhalation. Example: (our Tanya).

Speech breathing is the basis of sounding speech, the source of the formation of sounds, voices. It provides normal voice formation, helps to correctly observe pauses, maintain smoothness of speech, change the volume, use speech melody.

The development of speech breathing in a child begins already at the age of 6 months, the respiratory system is being prepared for the implementation of voice reactions, and is completed by the age of 10.

The formation of speech breathing involves, among other things, the production of an air stream. The development of an air jet is considered one of the necessary and significant conditions for setting sounds. Work on the education of the air stream begins at the preparatory stage of the formation of the correct sound pronunciation, along with the development of phonemic hearing and articulatory motor skills. (tab. 1)

The system of speech therapy work at the preparatory stage for the formation of an air jet is based on the development of the following main oppositions in a child with dyslalia (Table 2).

It is known that sounds are pronounced in the exhalation phase. As a rule, occlusive plosives and occlusive-fricative consonants are pronounced shortly, the air stream is weak. Sonor and slot sounds require a strong long-lasting air jet.

The pronunciation of most sounds of late ontogeny requires a directed air jet.

Directions of speech therapy work at the preparatory stage of the formation of sound pronunciation.

Preparatory stage

Directions of correction 1Formation of phonemic hearing

2. Formation of speech breathing

3. Formation of articulatory motility

Oppositions produced during the formation of an air jet

air jet

(when making hissing sounds) Narrow

(when pronouncing hissing sounds Cold

(when making whistling sounds)

Weak Strong

Scattered Directional

b]Three main directions of the air jet:

1) the air jet is directed directly at the center of the tongue. This is typical for the pronunciation of most sounds; labial (V, V, F, F, posterior lingual (K, K. G, G. X, X), anterior lingual (T, T, D, D, whistling (S, S, Z, Z, C)

2) the air jet is directed upwards in the center of the tongue. This is typical for the pronunciation of hissing (Ш,Ж,Ш,Ч) sounds and vibrants (Р, Р).

3) the air stream is directed along the lateral edges of the tongue. This is typical for the pronunciation of closing-passing (L, L) sounds.

In accordance with the listed directions of the passage of the air stream in the oral cavity, the following exercises are used in speech therapy work:

1. "Blow the snowflakes off the hill." "Punish a naughty tongue." "Groove".

2. "Tricks".

3. "There is a hunter in the swamp"

The development of an air jet can be carried out before articulation gymnastics or simultaneously with articulation gymnastics. Since the cheeks, lips, tongue take an active part in the formation of the air jet.

Articulation exercises performed on exhalation:

"Indians". On the exhale, pronounce "Bl-bl-bl".

"Punish the naughty tongue." On the exhale, pronounce "Pya-pya-pya."

"Machine gun" On the exhale, it is pronounced "T-t-t."

"Motor". On the exhale, pronounce "Rrr".

"Beetle" On the exhale, it is pronounced "F-zh-zh".

In the system of speech therapy work on the education of an air jet, the main directions can be distinguished:

1. Blowing with closed lips.

2. Blowing through lips stretched out with a tube.

4. Blow on the tongue.

Let's take a closer look at each direction.

1. Blowing with closed lips. To strengthen the muscles of the cheeks, the following exercises can be considered preparatory:

* “Inflate two balloons” Inflate the cheeks and hold air in them.

* "Rolling balls" Cheeks are inflated one by one.

* "Thin". Draw in the cheeks with closed lips and with the mouth slightly open.

* "Blowing through lips stretched out with a tube." Tension of the circular muscle of the mouth.

Without puffing out your cheeks, blow through lips that are close together and slightly pushed forward, forming a round “window” in the middle.

Blow off any soft object (cotton ball, paper snowflake, etc.) from the palm raised to the mouth. Blow on a piece of cotton wool tied to a thread. You can blow from the bottom up on the dandelion fluffs, try to keep them in the air longer.

Blow on a sailboat, napkin, leaf, weather vane, etc.

Blow on a pencil lying on the table so that it rolls (on hexagonal)

Blowing out the candle.

Inflating balloons, rubber toys.

Blowing soap bubbles.

Blast using whistles. Hooters, pipes, harmonica.

Racing through the water of paper boats, celluloid toys, for example, inflating "fish". Children are offered to alternately blow on light toys in a basin of water.

Blow strongly into the water until it splashes.

You can stretch the threads horizontally and tie light paper birds, butterflies, dragonflies to the threads hanging vertically on it.

Blow - rolling along the groove of light wooden or celluloid balls.

3. Blow through lips stretched in a smile.

* "Propeller" To form a narrow gap between the lips drawn together in a slight smile. The corners of the mouth are pressed against the teeth. A stream of air directed into this gap, the child cuts through the movements of the index finger from side to side. If the gap is formed correctly and the jet is strong enough, the sound from the air dissected by the finger is clearly audible.

* To form a narrow slit between the lips drawn together in a slight smile. The child is offered to put a wide tip of the tongue between the lips. Blow on the tip of your tongue.

* To form a narrow slit between the lips drawn together in a slight smile. “Slap” the tongue with your lips, exhaling the sounds of py-py-py.

4. Blowing on the tongue.

* In the middle of the tongue along its front edge, “make a path” - put a match with a cut off head and let the breeze blow off the paper leaves.

* Holding the tongue wide behind the upper teeth, you need to blow on its tip. Instructions: "Smile. Show me your teeth. Keep your tongue wide at the top. Do you feel the breeze? Blow like that one more time. Feel how I blow! You can use a mirror so that the child can see the position of his tongue.

* Put a wide tongue on the lower lip. Roll the edges of the tongue so that a groove forms. Easy to blow through the groove.

* "Blow the snowflakes off the hill"

Smile. Show me your teeth. Open your mouth. Hold the tip of your tongue behind your lower teeth.

Raise your tongue up. Blow on your tongue.

In the process of corrective work on the formation of an air jet, it is important to adhere to the following methodological recommendations.

* Exercises are carried out in a well-ventilated area.

* It is better to perform exercises while standing, with a free position of the body in space. The chest is expanded. Follow your posture.

* Attention is drawn to the fact that the child inhales deeply and calmly, through the nose. Exhalation through the mouth should be easy, smooth, without tension.

* Monitor the accuracy of the direction of the air jet.

* Short-term exercises (from 30 seconds to 1.5 minutes). Hyperventilation of the lungs leads to an abundant supply of oxygen to the cerebral cortex, as a result of which dizziness may occur.

* Dosage of quantity and pace of exercises. Intensive blowing is carried out no more than 5 times in 1 session, within a few seconds.

* Do not puff out your cheeks.

* Do not retain exhaled air. You can hold your cheeks with your hands to use tactile control.

* In the initial stages, you can use a mirror to attract visual control.

* Control over the exhaled air stream is carried out with the help of a cotton swab brought to the child's mouth: if the exercise is performed correctly. The cotton will deflect.

* Exercises can be performed under the account.

Rice. 49. Air jet from the end of a round pipe.

On fig. 49 shows the structure of an air jet flowing from the open end of a cylindrical tube. The jet expands as it exits the hole. The measurements show that as we move away from the hole, the velocity in the expanding flow decreases, and the temperature and concentration of gaseous impurities change in cases where the air temperature in the room and the content of the same gases in it differ from the initial ones characterizing the jet. The expansion of the jet, the drop in speed, as well as the change in temperature and concentration of impurities occur due to the fact that the supply jet is drawn into the flow (sucked in) by the surrounding air. The mixing begins at the outer boundaries and gradually penetrates into the depth of the jet. As a result, two sections are formed along the length of the jet - the initial and the main one. In the initial section, where the masses of air from the room have not yet had time to completely mix with the jet, a cone-shaped core is preserved (unshaded part in Fig. 49) with the initial parameters of the flow. In the main section of the jet, the core is already completely washed out.

These features of the jet structure are very important from the point of view of hygiene. If the worker's head enters the initial section of the supply air jet, he will breathe clean air, even if the atmosphere in the room is significantly polluted.

The fact that the concentration of impurities and the temperature not only in the initial, but in the main section of the jet may differ from the corresponding ones in the environment, allows the supply jet as a whole to be used to create a limited zone of cleaner air than in the room and, depending on hygienic requirements warmer (in cold rooms) or colder (in hot shops).

It has been established that the expansion angle of the initial section of the jet depends on the shape of the inlet nozzle. The smallest angle is when air flows out of the open section of the cylindrical pipe. If a hole of a different shape is taken, and also if the hole is provided with a grate or other device that disturbs the flow of the jet, then the expansion angle will increase, and the air flow rates along the jet will decrease faster, since the admixture of ambient air will be more intense. In this case, the initial section, the cleanest region of the jet, will be shortened accordingly. An increase in the angle of expansion of the initial section of the jet is resorted to if it is necessary to increase the area of the zone blown by the jet. The angle of expansion of the main section of the jet is practically independent of the shape of the inlet nozzle and in all cases is approximately equal to 22°.

A characteristic property of the supply jet is its range. The speed in the jet, although decreasing with distance from the inlet, can still be felt at considerable distances. In this case, the decrease in speed is the slower, than (ceteris paribus) the larger the size of the hole.

The range of the inlet jet is a positive feature in cases where the hygienic task requires blowing the body with an air stream at a significant distance from the worker from the inlet. The range is also used when installing an air curtain and in cases where the jet can deflect the flow of polluted air into the zone of action of the exhaust air inlet.

If it is necessary to avoid the sensation of an unpleasant blast, for example, when installing general ventilation, they tend to reduce the range and release air at low speeds in order to obtain permissible mobility (0.2-0.5 m / s) at the workplace. A rapid reduction in the initial velocity and flow dispersion can be achieved by using special designs of air distributors. The temperature of the supply jet affects the propagation conditions of the supply jet. If the temperature of the jet and the environment is the same, the axis of the jet is rectilinear. If the jet air is warmer than the room air, then the jet axis bends upwards, and when the air temperature of the jet is lower than in the room, the jet axis curves downwards.

The stated provisions refer to the so-called free jet flowing into an unlimited space, i.e., practically when propagating far from the enclosures of the room. If the expanding jet touches the surface of a wall, ceiling or floor, then it "sticks" to this surface. The structure of the jet changes in this case - it begins to expand one-sidedly and its range increases.

The basic laws that govern the movement of turbulent free jets are the same as for limited flows. Their motion is described by equations (VI, 19), they are also affected by molecular and turbulent stresses, pulsating velocities. However, the absence of firm boundaries also determines a number of their features.
On fig. 44 shows a diagram of a free jet.

The starting point of a free jet is called the pole of the jet. In practice, however, the initial cross section of the jet always has some dimensions. In this case, the pole of the jet is defined as the point of intersection of the outer boundaries of the jet.
When the air flow exits the initial section AB (see Fig. 44), the jets are separated at its edge, resulting in the formation of an expanding turbulent boundary layer A "AC B B". Between its internal boundaries AS and BS there is a core of constant velocities, within which the longitudinal velocities remain constant (Fig. 45) and equal to the average velocity in the initial section.

Longitudinal velocities in a free jet have a maximum value on its axis, decreasing to zero at the outer boundary. The absolute values of the velocities also decrease with distance from the initial section.
A very important property of free jets is the constancy of pressure in the entire volume of the jet and its equality to the air pressure outside the jet.
The central core of the jet, through each cross section of which the same amount of air passes per unit time, equal to that in the initial section, is called the core of constant mass.
The space between the core of constant mass and the outer boundary of the jet is occupied by the attached masses, which are carried away by the core of constant mass and move in the same direction, constituting an integral part of the free jet. The volume of the attached masses increases in the direction of motion. The added masses play an important role in the mass exchange between the free jet and the environment, since they are the "mediator" of the exchange between the clean air of the core and the polluted air in which the free jet propagates. This exchange occurs as a result of the presence of transverse pulsating velocity components at the outer boundary of the free jet.
Extensive studies of free jets in mine workings were carried out by V. N. Voronin. The range of the free jet, according to V. N. Voronin, is equal to
(VI.39);
where S is the cross-sectional area of the working;
b is the maximum distance from the wall of the working, supplying air (or from the ventilation pipe), to the wall of the working, to which the free jet propagates;
a is the jet structure coefficient equal to 0.06–0.08. Air consumption in an arbitrary section of the main section of the round jet, separated by a distance X from the outlet with radius R0, is equal to
(VI.40)
where (Q0 is the air flow rate in the initial section.
The highest intensity of turbulent pulsations in the main section of the jet, determined by formula (VI.34), is observed at a distance of 0.2–0.5 of the jet radius. The turbulence intensity increases along the jet, while the pulsation frequency decreases. The largest vortices are observed in the axial part of the jet. Characteristic is the constancy of the mixing path in the jet cross section and proportionality to its distance from the mouth. The swirling of the jet significantly increases the mixing path and hence its mixing capacity.
Free jets are of great importance in mine ventilation: they operate in chamber-shaped workings, in the bottomhole spaces of dead-end workings ventilated by blowers, in the spaces between the mounting frames, etc.

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