Air stream boundary. Calculation scheme and classification of jets

Let a round cylinder, capable of freely rotating on its axis, be introduced into a stream of water or into the region of the boundary of the air flow. In a certain interval of immersion, in contrast to the mentioned Coapde effect, the cylinder is pushed out of the flow and at the same time rotates in the opposite direction to the expected one - against the “mill wheel”! This effect is observed only under the condition of two-way flow around the cylinder. If the cylinder is recessed so little that it flows around only one side, it rotates “correctly.” But the magnitude of this threshold depth is very small. As deepening continues, the direction of rotation becomes “counter”, then a maximum speed is reached, its drop and, finally, a complete stop when the cylinder is completely immersed in the flow.

If we are talking about a thin jet, comparable in thickness to the diameter of the cylinder, then during anomalous rotation the jet strongly deviates from the cylinder, which can be buried far beyond the geometric axis of the undisturbed jet. However, at some moment the jet jumps over to the other side of the cylinder, and it begins to rotate in the opposite direction, so the phenomenon is hysteretic in nature. As it turned out, the effect is observed not only for a cylinder, but also for a ball and at the boundary of flat and axisymmetric jets, both water and air.

The phenomenon under consideration, due to the combination of rotation and buoyant force, superficially resembles the Magnus effect, but has a completely different nature. The Magnus effect is that a forcedly rotating cylinder or ball experiences, from the side of the oncoming flow, the action of a transverse force associated with forced circulation. If the flow is uniform, then at zero rotation speed there is no transverse force. The effects of anomalous rotation and force interaction considered here arise spontaneously, under the action of a mechanism caused by flow inhomogeneity. In this case, the force also acts on a stationary streamlined body. The angular velocity of rotation of the free cylinder turns out to be exactly proportional to the speed of the incoming flow. This allows us to consider the flow approximately inviscid, but with some circulation, to determine which it is necessary to generalize the Zhukovsky-Chaplygin postulate about the finiteness of the velocity on the sharp edge of the wing to the case of a smooth contour. This generalization assumes that the generated circulation minimizes the maximum velocity on the contour of the streamlined body. This minimax principle makes it possible to qualitatively and partly quantitatively correctly predict the direction and magnitude of circulation in different conditions flow around

LAMINAR AND TURBULENT AIR FLOW

STEADY AIR FLOW

Steady air flow is a flow of air in which the flow speed at any point, as well as the main parameters (pressure, temperature and density) do not change over time. That is, if at certain intervals we measure the speed and other parameters of the air at the same point and in all measurements the parameter values are the same, then this air flow is steady. If the measured quantities change, then the flow is unsteady. In aerodynamics, only steady air flow is considered. The basic concept of aerodynamics is the concept of an elementary stream of air.

Elementary trickle- this is a mentally isolated flow (a small closed circuit in the form of a tube), through lateral surface where air cannot flow either in or out.

Laminar is an air flow in which streams of air move in one direction and are parallel to each other. When the speed increases to a certain value, the streams of air flow, in addition to translational speed, also acquire rapidly changing speeds perpendicular to the direction of translational movement. A flow is formed which is called turbulent, i.e., disorderly.

Boundary layer- this is a layer in which the air speed changes from zero to a value close to the local air flow speed.

When an air flow flows around a body (Fig. 5), air particles do not slide over the surface of the body, but are slowed down, and the air speed at the surface of the body becomes 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, the profile of the body, the state of its surface and the position of the body in the air flow. 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.

Let's consider an air particle A (Fig. 5), which is located between streams of air with velocities U 1 and U 2. Due to the difference in these velocities applied to opposite points of the particle, it rotates, and the closer this particle is to the surface of the body, the more it rotates ( where the speed difference is greatest). 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 speed and the air speed of the boundary layer.

Behind the body, the boundary layer turns into a cocurrent jet, which blurs out and disappears as it moves away from the body. The turbulence in the wake falls on the tail of the aircraft and reduces its efficiency and causes shaking (buffeting phenomenon).

The boundary layer is divided into laminar and turbulent (Fig. 6). In a steady laminar flow of the boundary layer, only internal friction forces due to the viscosity of the air appear, so the air resistance in the laminar layer is low.

Rice. 5. Change in air flow speed in the boundary layer

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

Rice. 7. Laminar and turbulent flow

In a turbulent border 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 consequence of this, a greater resistance to the air flow is created to the moving body.

To determine the nature of the boundary layer, the coefficient C f is used. A body of a certain configuration has its own coefficient. So, for example, for a flat plate the resistance coefficient of the laminar boundary layer is equal to:

for a turbulent layer

where R e is the Reynolds number, expressing the ratio of inertial forces to friction forces and determining the ratio of two components - profile resistance (shape resistance) and friction resistance. Reynolds number R e is determined by the formula:

where V is the air flow speed,

I - character body size,

γ is the kinetic coefficient of viscosity of air friction forces.

When an air flow flows around a body, at a certain point the boundary layer transitions 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, as well as the surface roughness. When creating wing profiles, designers strive to place this point as far as possible from the leading edge of the profile, thereby reducing friction drag. For this purpose, special laminated profiles are used to increase the smoothness of the wing surface and a number of other measures.

When the speed of the air flow increases or the angle of position of the body relative to the air flow increases to a certain value, at a certain point the boundary layer is separated from the surface, and the pressure behind this point sharply decreases.

As a result of the fact that at the trailing edge of the body the pressure is greater than behind the separation point, a reverse flow of air occurs from a zone of higher pressure to a zone of lower pressure to the separation point, which entails separation of the air flow from the surface of the body (Fig. 7).

A laminar boundary layer comes off more easily from the surface of a body than a turbulent boundary layer.

In solids, the distances between molecules are very small and the forces of mutual attraction between the molecules are large. Molecules undergo slight vibrational movements.

U gaseous substances the distances between molecules are much greater than the molecules themselves, mutual attraction is very small, molecules move in different directions and at different speeds. The energy of all molecules together is considered as the internal energy of the substance.

Air is considered as a collection of a large number of molecules, as a continuous medium in which individual particles come into contact with each other. Introduction to continuity of the medium allows you to significantly simplify the study of liquids and gases.

In addition, in aerodynamics wide application found principle reversibility of movement. According to this principle, instead of considering the motion of a body in a stationary medium, one can consider the motion of the medium relative to a stationary body.

The speed of the oncoming undisturbed flow in reverse motion is equal to the speed of the body itself in still air.

Aerodynamic forces will be the same both for a body moving in still air and for a stationary body flown by air, if the speed of the body relative to the air is the same.

Reversal of motion is widely used when conducting experiments in wind tunnels, as well as in theoretical studies where the concept is used air flow.

By air flow called the directed movement of chaotically moving particles.

If at any point in space occupied by a flow of liquid or gas, the pressure, density, magnitude and direction of the flow velocity do not change over time, the movement of this flow is called established. If these parameters at a given point in space change over time, then the movement is called unsteady.

There are various methods studying the movement of liquids and gases. One of them is that the movement of individual particles is considered at each point in space at a given moment in time. In this case, the so-called streamlines are examined.

Current line is a line whose tangent at each point coincides with the velocity vector at that point. The set of streamlines is contained in some tubecurrent and forms an elementary a trickle of current . Each selected stream can be represented as flowing in isolation from total mass gas

Dividing the flow into streams gives a visual representation of the complex flow of gas in space. The basic laws of motion—conservation of mass and conservation of energy—can be applied to an individual stream. Using equations expressing these laws, it is possible to carry out a physical analysis of the interaction solid with gas (air).

According to the nature of the flow, the air flow can be laminar and turbulent.

Laminar is an air flow in which streams of air move in one direction and are parallel to each other.

As the speed increases, air particles, in addition to translational speed, acquire rapidly changing speeds perpendicular to the direction of translational motion. A flow is formed which is called turbulent , i.e. disorderly.

Boundary layer

Boundary layer called a thin layer of inhibited gas that forms on the surface of bodies flowing around a flow. The viscosity of the gas in the boundary layer is the main cause of the formation of drag force.

When flowing around a body, gas particles passing very close to its surface will experience strong deceleration. Starting from a certain point near the surface, the flow velocity decreases as it approaches the body and becomes zero at the surface itself. The distribution of velocities in other sections of the surface is similar (Fig. 2.1).

Distance R, at which the speed decreases is called the thickness of the boundary layer, and the change in speed along the thickness of the boundary layer is called speed gradient.

Fig.2.1 Change in air flow velocity in the boundary layer

The thickness of the boundary layer is measured in millimeters and depends on the viscosity and pressure of the air, the shape of the body, the state of its surface and the position of the body in the air flow. The thickness of the boundary layer gradually increases from the front of the body to the back.

At the boundary of the boundary layer, the particle velocity becomes equal to the free-stream velocity. Above this limit there is no velocity gradient, so the viscosity of the gas practically does not appear.

Thus, in the boundary layer, particle velocities change from the speed of the external flow at the “border” of the boundary layer to zero on the surface of the body.

Due to the velocity gradient, the nature of the movement of gas particles in the boundary layer differs from their movement in the potential layer. In the boundary layer due to the speed difference U 1 -U 2 the particles begin to rotate (see Fig. 2.2).

The closer the particle is to the surface of the body, the more intense the rotation. The boundary layer is always vortex and is therefore called a surface vortex layer.

Rice. 2.2 Air flow around a body - flow deceleration in the boundary layer

Gas particles from the boundary layer are carried away by the flow into a region located behind the streamlined body, called accompanying jet. The velocities of particles in the accompanying jet are always less than the velocity of the external flow, because particles emerge from the boundary layer already slowed down.

Types of boundary layer flow. At a low free-stream velocity, the gas in the boundary layer flows calmly in the form of separate layers. This boundary layer is called laminar (Fig. 2.3, a). The boundary layer is vortex, but the gas movement is ordered, the layers do not mix, and the particles rotate within the same thin layer.

If energetic mixing of particles in the transverse direction occurs in the boundary layer and the entire boundary layer is randomly vortexed, such a boundary layer is called turbulent (Fig. 2, b).

In a turbulent boundary layer, there is continuous movement of air streams in all directions, which requires more energy. Air flow resistance increases.

With)

Rice. 2.3 Laminar and turbulent flow

A laminar boundary layer is formed at the front part of the streamlined body, which then turns into a turbulent one. This boundary layer is called mixed (Fig. 2.3, c).

In a mixed flow, at a certain point the boundary layer transitions from laminar to turbulent. Its location on the surface of the body depends on the speed of the streams, the shape of the body and its position in the air flow, as well as on the roughness of the surface. The position of the point is determined by the coordinate X s(Fig.2.3,) .

For smooth airfoils, the transition point usually lies at a distance approximately equal to 35% of the chord length.

When creating wing profiles, designers strive to place this point as far as possible from the leading edge, thereby increasing the extent of the laminar part of the boundary layer. For this purpose, special laminated profiles, and also increase the smoothness of the wing surface and a number of other measures.

Boundary layer separation. When flowing around a body with a curved surface, the pressure and velocities at different points on the surface will be different (Fig. 2.4). When the flow moves from point A to point B, a diffuse expansion of the flow occurs.

A B

Rice. 2.4 Boundary layer flow near the separation point

Therefore, the pressure increases and the speed decreases, since the particle velocities at the very surface of the body are very small, under the influence of the pressure difference between points A and B in this area, the gas moves in the opposite direction. At the same time, the external flow continues to move forward.

Because of reverse flow The external gas flow is pushed away from the surface of the body. The boundary layer swells and breaks away from the surface of the body. The point on the surface of the body at which the boundary layer is separated is called separation point .

The separation of the boundary layer leads to the formation of vortices behind the body. The position of the separation point depends on the nature of the flow in the boundary layer. In turbulent flow, the point of flow separation lies much further downstream than in laminar flow. The vortex region behind the body in this case is much smaller. This paradoxical phenomenon is explained by the fact that during turbulent motion more intense transverse mixing of particles occurs.

Boundary layer separation is observed when flowing around curved surfaces, for example, a wing profile at high angles of attack. This phenomenon is very dangerous, because... leads to a sharp decrease in lift, a significant increase in resistance to flow, loss of stability and controllability of the aircraft, and vibrations.

The phenomenon of flow stall depends on the shape and condition of the surface of the body, the nature of the air flow in the boundary layer. Bodies that have an elongated shape with smooth outlines (streamlined) are not subject to flow stall, unlike non-streamlined bodies.

Flow disruption can occur as a result of violation of aircraft operating rules: reaching critical angles of attack, violation of alignment. With careless maintenance, local flow disruptions occur due to loose fitting of hatch covers, incomplete closing of valves and other reasons. Dangerous vibrations of aircraft parts occur.

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Air jet

Introduction

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

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

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

There are two main methods of ventilation of buildings: displacement ventilation and mixing ventilation.

Displacement ventilation is primarily used to ventilate large industrial spaces as it can effectively remove excess heat if properly sized. Air is supplied to the lower level of the room and flows into the work area at low speed. This air must be slightly cooler 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.

Mixing 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 supplied air enters the work area already mixed with room air. The ventilation system must be calculated in such a way that the air circulating in the work 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 stream entering the room draws into the flow and mixes large volumes of surrounding air. As a result, the volume of the air stream increases, while its speed decreases the further it penetrates into the room. Mixing ambient air into the air flow is called ejection.

Rice. 1. Ejection

The air movements caused by the air stream soon thoroughly mix all the air in the room. Pollutants in the air are not only atomized, but also distributed evenly. The temperature in different parts of the room is also equalized.

When calculating ventilation by mixing, the most important point is to ensure that the air speed in the work area is not too high, otherwise there will be a feeling of draft.

Rationale

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

Air showers are used in fixed workplaces or rest areas. Particularly effective in production premises(fig), where workers are exposed high temperature. Installations for air showers are stationary and mobile.

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

The curtains can be electrically, water, steam, gas heated, or without heating.

For installation:

· curtains of vertical installation;

· curtains of horizontal installation;

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

By heating type:

· heated curtains (heated curtains are usually called air-thermal or thermal curtains, since the doorway is shielded 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 water heater, as well as large industrial thermal curtains can be equipped with a steam or gas heater (if the curtain is heated, the curtain without heating does not have this kind of heater);

· fans;

· air filter(for water heated models).

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

Modern models of grilles 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 homes, administrative and commercial buildings, and office premises. That is, their use is advisable in those rooms where there is a need to create and maintain optimal temperature and humidity levels.

Scientific theory of air jets

A gas stream is called flooded if it propagates in a medium with the same physical properties, which she herself has. When studying the movement of air in ventilation systems, various cases of propagation of flooded jets are encountered. But when considering these cases, the free jet scheme is used as the initial one. A free jet is a jet propagating in an unbounded environment. (A jet not limited by solid walls is called free.) The jet can flow into a stationary medium, as well as into an air stream.

In this case there are:

· String jet, a jet flowing into a stream whose speed 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 flow velocity are directed towards each other.

According to the type of energy spent on the formation of the jet, they are distinguished:

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

· Convective jets formed due to 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 circular, then the jet is called asymmetrical.

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

Jet temperatures and environment may be the same or different.

In accordance with this, a distinction is made between isothermal and non-isothermal jets. In Fig. Figure 3 shows an air stream that is formed when air is forced into the room through a hole in the wall. As a result, a free air stream appears. If the temperature of the air in the stream is the same as in the room, it is called a free isothermal stream.

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

· free jets;

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

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

Depending on the flow mode, the jets can be:

laminar (flow in which liquid or gas moves in layers without mixing or pulsation);

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

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

Read 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 bordering the jet, entraining (ejecting) these masses, giving them a certain speed.

In place of the particles that leave the jet, particles from the surrounding air enter it, which slow down the boundary layers of the jet. As a consequence of this exchange of impulses between the jet and motionless air, an increase in the mass of the jet and a decrease in speed at its boundaries appears.

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

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

Therefore, further erosion of the jet is accompanied by an increase in its width and at the same time the speed 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 meet is called the transition section. A section of the jet located between the outlet and the transition section, in which the speed on the axis remains unchanged and equal initial speed?? 0 is called initial. The section following the transition section, in which the speed on the axis gradually decreases and fades, is called the main section. The boundaries of the jet, both external and the core of constant speed, 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 changes 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 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 unilateral expansion of the jet.

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

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

Distribution and shape. The air stream consists of several zones with different flow regimes and air movement speeds. The area of greatest practical interest is the main site. The center velocity (velocity around the central axis) is inversely proportional to the distance from the diffuser or valve, i.e. the further from the diffuser, the less speed air. The air flow develops fully in the main area, and the conditions prevailing here will have a decisive influence on the flow regime in the room as a whole.

Main section of the air stream, tilt speed. The shape of the air stream depends on the shape of the diffuser or the passage opening of the air distributor. Round or rectangular passage holes create a compact, conical air stream. In order for the air stream to be completely flat, the passage opening must be more than twenty times wider than its height or as wide as the room. Air fan jets are obtained by passing through perfectly round passage openings, where air can spread in any direction, as in supply diffusers.

Rice. 5. Various types air jets

ventilation curtain air ejection

Speed profile. The air speed in each part of the jet can be calculated mathematically. To calculate the speed at a certain distance from the outlet of the diffuser/valve, it is necessary to know the speed of the air at the outlet of the diffuser/valve, its shape and the type of air stream that it forms. 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 speed). By making sure that the isovel corresponding to 0.2 m/s is located outside the work area, you can be sure that the air speed will not exceed this level directly in the work area.

Rice. 6. Various air stream isovels

Diffuser coefficient. The diffuser coefficient is a constant value that depends on the shape of the diffuser or valve. The coefficient can be calculated theoretically using the following factors: the impulse dispersion and constriction of the air stream at the point where it is introduced into 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 valve by measuring the air speed at a minimum of eight points located at different distances from the diffuser/valve and at least 30 cm from each other. These values are then plotted on a logarithmic scale, which shows the measured values for the main portion of the air stream, which in turn gives the value for the constant.

The diffuser coefficient makes it possible to calculate air stream speeds and predict the distribution and path of the air stream. This factor is different from the K factor, which is used to set the correct volume of air leaving the supply air distributor or iris valve. 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 stream is deflected towards it and tends to flow directly along the surface. This effect occurs due to the formation of a vacuum between the jet and the surface, and since there is no possibility of air mixing from the surface, the jet is deflected in its direction. This phenomenon is called the spreading effect.

Rice. 7. Layering effect

Practical experiments have shown that the distance between the top edge of the diffuser or valve and the ceiling should not exceed 30 cm for a layering effect to occur. The layering effect can be used to increase the path of the cold air stream along the ceiling before introducing it into the work area. The diffuser coefficient will be slightly higher when a spreading effect occurs than when there is a free air flow. It is also important to know how the diffuser or valve is attached when using the diffuser coefficient to make various calculations.

Non-isothermal air jet. Distribution becomes more complex when the supplied air is warmer or cooler than the indoor air. Thermal energy resulting from differences in air density at different temperatures causes the cooler air flow to move downward (the jet sinks) and the warmer air to rush upward (the jet floats).

This means that two different forces act on the cold jet near the ceiling: the layering effect, which tries to press it towards the ceiling, and thermal energy, which tends to lower it to the floor.

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

The jet deflection and lift-off point can be calculated using formulas based on temperature differentials, diffuser or valve outlet type, air flow speed, etc.

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

Important criteria when calculating ventilation. It is important to select and place the air distributor correctly. It is also important that the temperature and air speed in the work 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.

Length of the initial section x n of the jet:

round - x n = ;

flat - x n = .

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

· round - ?? = ;

· flat - ?? = .

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

· round - ?? = 4.36?? 0();

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

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

Average speed in the main section of the jet:

· round - ?? = ;

· flat - ?? = .

Flat jet height:

4,8?? 0 ().

Correct speed air in the work area. For most air distribution devices, the catalog contains a characteristic called jet length. The length of the jet is understood as the distance from the supply opening of the diffuser or valve to the cross section of the air stream, in which the speed of the flow core decreases to a certain value, usually up to 0.2 m/sec. The length of the jet is designated and measured in meters.

Rice. 9. The concept of "Jet length"

The first thing taken into account when calculating air distribution systems is how to avoid too high air flow rates in the work area. But, as a rule, the reflected or reverse current of this jet enters the working area: see Fig. 10.

Rice. 10. Reverse air flow with wall-mounted diffuser

The speed of the reverse air flow is approximately 70% of the speed of the main air flow at the wall. This means that a diffuser or valve installed on a rear wall, supplying a stream 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. This corresponds to comfortable ventilation in the work area, the air speed in which should not exceed 0.15 m/s.

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

Penetrating ability of the air stream. The shape of the room can have an impact significant influence to the flow configuration. 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 stream will begin to mix in its own air. In this case, increasing 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 surrounding 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 stream 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 stream, if there are 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 work area. Therefore, it is necessary to know what distance should be (A on the graph) between the device supplying air and 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 an entire room, it should be placed as close to the center of the ceiling as possible, and total area should not exceed the dimensions shown in Fig. 12.

Rice. 12. Small room ventilated by one 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 several ceiling diffusers

The 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 stream with a longer jet length is obtained.

Rice. 14. Large room ventilated by several wall diffusers

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

Rice. 15. Horizontal air supply with ceiling diffuser

However, in very high rooms, the supplied air must be directed vertically downward if it is also used for heating the room. If the temperature difference is no more than 10°C, then the air stream should drop to approximately 1 m from the floor so that the temperature in the work area becomes comfortable.

Rice. 16. Vertical air supply of ceiling diffuser

Cold air supply. If the air supplied along the ceiling is cooler than the air in the room, it is important that the air flow 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 force the air stream down towards the floor too early.

At a certain distance from the diffuser supplying air, the air stream will in any case separate from the ceiling and deflect downward. This deviation will happen faster for an air stream that has a temperature below room temperature, and therefore in this case the length of the stream will be shorter.

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

The air stream must travel at least 60% of the depth of the room before it leaves the ceiling. The maximum air speed in the working area will therefore be almost the same as when supplying isothermal air.

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

All in all, high degree cooling is achieved by using a ceiling diffuser rather than a wall diffuser. This is because a ceiling diffuser spreads air in all directions, so it takes less time to mix with the surrounding air and equalize the temperature.

Choosing the right air distributor. Air distributors can be mounted either on the ceiling or on the wall. They are often equipped with nozzles or perforations, which makes it easier to mix ambient air into the air flow.

Nozzle diffusers are the most flexible devices because they allow each nozzle to be individually configured. They are ideal for supply air that is significantly lower than the room temperature, especially if they are installed on the ceiling. The distribution pattern can be changed by rotating the nozzles in different directions.

Diffusers with perforation give a positive effect where the temperature of the air stream is significantly lower than the ambient temperature. They are not as flexible as nozzle diffusers, but by shielding the supplied airflow in different directions, the distribution pattern can be changed.

Wall grilles have longer length jets. They have limited opportunities to change the distribution pattern and are not very suitable for supply air at a temperature significantly lower than the ambient temperature.

Conclusion

So, the air stream is the main element of the operation of ventilation equipment. In this work, types of ventilation and their equipment, shapes of air jets and their varieties were considered. Special attention emphasis was placed on the use of air jets. Here in conclusion we can expand them.

Back in time immemorial people set a sail for the first time, and the wind carried their boats across the water or sleighs across ice and snow. However, since then the air currents have had so much work to do that it is worth special mention. Sailing ships still operate today. They float along rivers, lakes and even oceans. The undoubted advantages of this method of transportation are cleanliness and silence (there are no gasoline stains on the water and no engine noise), and you don’t have to buy gasoline. Athletes sail not only on boats, but even just on boards.

Other athletes use air currents to fly freely.

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

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

Wind occurs when there is a difference atmospheric pressure: in one place the pressure is higher, in another - 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 (create low pressure), then air will immediately rush there from all sides. If, on the contrary, we create somewhere high blood pressure, the 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 your air mattress, notice how much air flows out of the valve!

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

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

The same vacuum cleaner principle is used when loading flour. It is not poured, but simply sucked from the machine to the warehouse and back. By the way, they also grind flour using the wind, because the grains are quite light.

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

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

Literature used

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 “The air shell of the earth” Ispolzovanije_vetra.html.

4. Article “Use of air flow swirlers to increase the efficiency of wind turbines.” http://vikidalka.ru/2-196929.html.

5. Article “Air flows”. http://ru-ecology.info/term/19749/.

6. Article “Combine harvesters of the future. Using an air jet." http://svistun.info/zemledelie/211.

7. Staroverov I.G. Directory of designers 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 fan direction of air streams.

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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