Laminar air flow. Laminar and turbulent air flow

Over the past ten years, both abroad and in our country, the number of purulent-inflammatory diseases due to infections that have acquired the name “nosocomial infections” (HAI) has increased, as defined by the World Health Organization (WHO). Based on the analysis of diseases caused by nosocomial infections, we can say that their duration and frequency directly depend on the state of the air environment of hospital premises. In order to ensure the required microclimate parameters in operating rooms (and industrial clean rooms), unidirectional flow air distributors are used. As shown by the results of environmental monitoring and analysis of air flows, the operation of such distributors can provide the required microclimate parameters, but negatively affects the bacteriological composition of the air. To achieve the required degree of protection of the critical zone, it is necessary that the air flow that leaves the device does not lose the shape of its boundaries and maintains straightness of movement, in other words, the air flow should not narrow or expand over the zone selected for protection in which the surgical table is located.

In the structure of a hospital building, operating rooms require the greatest responsibility due to the importance of the surgical process and the provision of the necessary microclimate conditions for this process to be successfully carried out and completed. The main source of release of various bacterial particles is the medical personnel themselves, who generate particles and release microorganisms while moving around the room. The intensity of the appearance of new particles in the air space of a room depends on the temperature, the degree of mobility of people, and the speed of air movement. The nosocomial infection, as a rule, moves around the operating room with air currents, and the probability of its penetration into the vulnerable wound cavity of the patient being operated on never decreases. As observations have shown, improper organization of ventilation systems usually leads to such a rapid accumulation of infection in the room that its level may exceed the permissible norm.

For several decades now, foreign experts have been trying to develop system solutions to ensure the necessary air conditions in operating rooms. The air flow that enters the room must not only maintain microclimate parameters, assimilate harmful factors (heat, smell, humidity, harmful substances), but also maintain the protection of selected areas from the possibility of infection, and therefore ensure the required cleanliness of operating room air . The area in which invasive operations are performed (penetration into the human body) is called the "critical" or operating zone. The standard defines such a zone as an “operating sanitary protection zone”; this concept means the space in which the operating table, equipment, tables for instruments, and medical personnel are located. There is such a thing as a “technological core”. It refers to the area in which production processes are carried out under sterile conditions; this area can be meaningfully correlated with the operating room.

In order to prevent the penetration of bacterial contamination into the most critical areas, screening methods based on the use of air flow displacement have become widespread. For this purpose, laminar air flow air distributors of various designs have been developed. Later, "laminar" became known as "unidirectional" flow. Today you can find a variety of names for air distribution devices for clean rooms, for example, “laminar ceiling”, “laminar”, “clean air operating system”, “operating ceiling” and others, but this does not change their essence. The air distributor is built into the ceiling structure above the protected area of ​​the room. It can be of different sizes, it depends on the air flow. The optimal area of ​​such a ceiling should not be less than 9 m2, so that it can completely cover the area with tables, staff and equipment. The displacing air flow in small portions slowly flows from top to bottom, thus separating the aseptic field of the surgical exposure zone, the zone where sterile material is transferred from the environmental zone. Air is removed from the lower and upper zones of the protected room simultaneously. HEPA filters (class H according to) are built into the ceiling, which allow air flow through them. Filters only trap living particles without disinfecting them.

Recently, at the global level, attention has increased to the issues of disinfecting the air environment of hospital premises and other institutions in which sources of bacterial contaminants are present. The documents set out the requirements that it is necessary to disinfect the air in operating rooms with a particle deactivation efficiency of 95% or higher. Climate system equipment and air ducts are also subject to disinfection. Bacteria and particles released by surgical personnel continuously enter the room air and accumulate there. In order to prevent the concentration of harmful substances in the room from reaching the maximum permissible level, it is necessary to constantly monitor the air environment. This control is mandatory after installation of the climate system, repair or maintenance, that is, while the clean room is in use.

It has already become commonplace for designers to use ultra-fine unidirectional flow air distributors with built-in ceiling-type filters in operating rooms.

Air flows with large volumes slowly move down the room, thus separating the protected area from the surrounding air. However, many specialists do not worry that these solutions alone will not be enough to maintain the required level of air disinfection during surgical operations.

A large number of design options for air distribution devices have been proposed, each of them has its own application in a specific area. Special operating rooms within their class are divided into subclasses depending on their purpose according to the degree of cleanliness. For example, cardiac surgery, general, orthopedic operating rooms, etc. Each class has its own requirements for ensuring cleanliness.

Air distributors for clean rooms were first used in the mid-50s of the last century. Since then, the distribution of air in industrial premises has become traditional in cases where it is necessary to ensure reduced concentrations of microorganisms or particles, all this is done through a perforated ceiling. The air flow moves in one direction through the entire volume of the room, while the speed remains uniform - approximately 0.3 - 0.5 m/s. The air is supplied through a group of high efficiency air filters located on the ceiling of the clean room. The air flow is supplied according to the principle of an air piston, which rapidly moves down through the entire room, removing harmful substances and contaminants. Air is removed through the floor. This air movement can remove aerosol contaminants originating from processes and personnel. The organization of such ventilation is aimed at ensuring the necessary cleanliness of the air in the operating room. Its disadvantage is that it requires a large air flow, which is not economical. For cleanrooms of class ISO 6 (according to ISO classification) or class 1000, an air exchange rate of 70-160 times per hour is allowed. Later, more efficient modular-type devices came to replace them, having smaller sizes and low costs, which allows you to choose an air supply device based on the size of the protection zone and the required air exchange rates in the room, depending on its purpose.

Operation of laminar air diffusers

Laminar flow devices are designed for use in clean production rooms for distributing large volumes of air. Implementation requires specially designed ceilings, room pressure regulation and floor hoods. If these conditions are met, laminar flow distributors will certainly create the necessary unidirectional flow with parallel flow lines. Due to the high air exchange rate, conditions close to isothermal are maintained in the supply air flow. Designed for air distribution with extensive air exchanges, ceilings provide low starting flow rates due to their large area. Control of changes in air pressure in the room and the result of the operation of exhaust devices ensure the minimum size of air recirculation zones; the principle of “one pass and one exit” works here. Suspended particles fall to the floor and are removed, making recycling virtually impossible.

However, in an operating room, such air heaters work somewhat differently. In order not to exceed the permissible levels of bacteriological purity of the air in operating rooms, according to calculations, air exchange values ​​are about 25 times per hour, and sometimes even less. In other words, these values ​​are not comparable to those calculated for industrial premises. To maintain stable air flow between the operating room and adjacent rooms, positive pressure is maintained in the operating room. The air is removed through exhaust devices that are installed symmetrically in the walls of the lower zone. To distribute smaller volumes of air, laminar flow devices of a smaller area are used; they are installed directly above the critical area of ​​​​the room as an island in the middle of the room, rather than occupying the entire ceiling.

Based on observations, such laminar air distributors will not always be able to provide unidirectional flow. Since a difference of 5-7 °C between the temperature in the supply air stream and the ambient air temperature is inevitable, the cooler air leaving the supply device will fall much faster than a unidirectional isothermal flow. This is a common occurrence for ceiling diffusers installed in public spaces. The opinion that laminar floors provide a unidirectional, stable air flow in any case, regardless of where and how they are used, is erroneous. Indeed, in real conditions, the speed of a vertical low-temperature laminar flow will increase as it descends towards the floor.

With an increase in the volume of supply air and a decrease in its temperature relative to the room air, the acceleration of its flow increases. As shown in the table, thanks to the use of a laminar system with an area of ​​3 m 2 and a temperature difference of 9 ° C, the air speed at a distance of 1.8 m from the outlet increases three times. At the exit from the laminar device, the air speed is 0.15 m/s, and in the area of ​​the operating table - 0.46 m/s, which exceeds the permissible level. Many studies have long proven that with an increased speed of the inflow flow, its “unidirectionality” is not maintained.

An analysis of air control in operating rooms by Lewis (1993) and Salvati (1982) found that in some cases the use of laminar flow units with high air velocities increases the level of airborne contamination in the area of ​​the surgical incision, which can lead to to its infection.

The dependence of the change in air flow speed on the supply air temperature and the size of the laminar panel area is shown in the table. When air moves from the starting point, the flow lines will run parallel, then the boundaries of the flow will change, narrowing towards the floor will occur, and, therefore, it will no longer be able to protect the area determined by the dimensions of the laminar flow unit. Having a speed of 0.46 m/s, the air flow will capture the low-moving air of the room. And since bacteria are continuously entering the room, contaminated particles will enter the air flow coming out of the supply unit. This is facilitated by air recirculation, which occurs due to air pressure in the room.

To maintain the cleanliness of operating rooms, according to the standards, it is necessary to ensure air imbalance by increasing the inflow by 10% more than the exhaust. Excess air enters adjacent, untreated rooms. In modern operating rooms, sealed sliding doors are often used, then excess air cannot escape and circulates throughout the room, after which it is taken back into the supply unit using built-in fans, then it is cleaned in filters and re-supplied into the room. The circulating air flow collects all contaminated substances from the air in the room (if it moves near the supply flow, it can pollute it). Since the boundaries of the flow are violated, it is inevitable that air from the room will be mixed into it, and, consequently, the penetration of harmful particles into the protected sterile zone.

Increased air mobility entails intensive exfoliation of dead skin particles from open areas of the skin of medical personnel, after which they enter the surgical incision. However, on the other hand, the development of infectious diseases during the rehabilitation period after surgery is a consequence of the patient’s hypothermic state, which is aggravated when exposed to moving currents of cold air. So, a well-functioning traditional laminar flow air diffuser in a cleanroom can be as beneficial as it can be detrimental during an operation performed in a conventional operating room.

This feature is typical for laminar flow devices with an average area of ​​about 3 m2 - optimal for protecting the operating area. According to American requirements, the air flow rate at the outlet of a laminar flow device should not be higher than 0.15 m/s, that is, 14 l/s of air should enter the room from an area of ​​0.09 m2. In this case, 466 l/s (1677.6 m 3 / h) or about 17 times per hour will flow. Since, according to the standard value of air exchange in operating rooms, it should be 20 times per hour, according to - 25 times per hour, then 17 times per hour fully corresponds to the required standards. It turns out that the value of 20 times per hour is suitable for a room with a volume of 64 m 3.

According to current standards, the area of ​​general surgery (standard operating room) should be at least 36 m 2. However, higher requirements are imposed on operating rooms intended for more complex operations (orthopedic, cardiological, etc.), often the volume of such operating rooms is about 135 - 150 m 3. For such cases, an air distribution system with a larger area and air capacity will be required.

If air flow is provided for larger operating rooms, this creates the problem of maintaining laminar flow from the outlet level to the operating table. Air flow studies were conducted in several operating rooms. In each of them, laminar panels were installed, which can be divided into two groups based on the occupied area: 1.5 - 3 m 2 and more than 3 m 2, and experimental air conditioning installations were built that allow you to change the value of the supply air temperature. During the study, measurements were taken of the speed of the incoming air flow at various air flow rates and temperature changes; these measurements can be seen in the table.

Criteria for the cleanliness of operating rooms

To properly organize the circulation and distribution of air in the room, it is necessary to select a rational size of the supply panels, ensure the standard flow rate and temperature of the supply air. However, these factors do not guarantee absolute air disinfection. For more than 30 years, scientists have been solving the issue of disinfecting operating rooms and proposing various anti-epidemiological measures. Today, the requirements of modern regulatory documents for the operation and design of hospital premises face the goal of air disinfection, where the main way to prevent the accumulation and spread of infections is HVAC systems.

For example, according to the standard, the main purpose of its requirements is disinfection, and it states that “a properly designed HVAC system minimizes the airborne spread of viruses, fungal spores, bacteria and other biological contaminants”, a major role in the control of infections and other harmful factors HVAC system plays. It defines requirements for indoor air conditioning systems, which state that the design of the air supply system should minimize the penetration of bacteria along with the air into clean areas, and maintain the highest possible level of cleanliness in the remainder of the operating room.

However, regulatory documents do not contain direct requirements reflecting the determination and control of the effectiveness of disinfection of premises with various ventilation methods. Therefore, when designing, you have to engage in searches, which take a lot of time and do not allow you to do your main work.

A large amount of regulatory literature has been produced on the design of HVAC systems for operating rooms; it describes requirements for air disinfection that are quite difficult for the designer to meet for a variety of reasons. To do this, it is not enough just to know modern disinfecting equipment and the rules for working with it; you also need to maintain further timely epidemiological monitoring of indoor air, which creates an impression of the quality of operation of HVAC systems. This, unfortunately, is not always observed. If the assessment of the cleanliness of industrial premises is based on the presence of particles (suspended substances), then the indicator of cleanliness in clean hospital premises is represented by live bacterial or colony-forming particles, their permissible levels are given in. In order not to exceed these levels, regular monitoring of indoor air is necessary for microbiological indicators; this requires counting microorganisms. The collection and calculation methodology for assessing the level of air cleanliness was not given in any regulatory document. It is very important that the counting of microorganisms should be carried out in the work area during the operation. But this requires a ready-made design and installation of an air distribution system. The degree of disinfection or the effectiveness of the system cannot be determined before starting work in the operating room; this is established only during at least several operations. Here a number of difficulties arise for engineers, because the necessary research contradicts the observance of anti-epidemic discipline in hospital premises.

Air curtain method

Properly organized joint work of air supply and removal ensures the required air conditions in the operating room. To improve the nature of air flow in the operating room, it is necessary to ensure a rational relative position of exhaust and supply devices.

Rice. 1. Analysis of the air curtain operation

Using both the entire ceiling area for air distribution and the entire floor for exhaust is not possible. Exhaust units on the floor are unhygienic as they quickly become dirty and difficult to clean. Complex, bulky and expensive systems are not widely used in small operating rooms. Therefore, the most rational is considered to be the “island” placement of laminar panels above the protected area and the installation of exhaust openings in the lower part of the room. This makes it possible to organize air flows similar to clean industrial premises. This method is cheaper and more compact. Air curtains are successfully used to act as a protective barrier. The air curtain is connected to the flow of supply air, forming a narrow “shell” of air at a higher speed, which is specially created along the perimeter of the ceiling. Such a curtain constantly works for exhaust and prevents polluted ambient air from entering the laminar flow.

To better understand how an air curtain works, you can imagine an operating room with a hood installed on all four sides of the room. The air flow, which comes from the “laminar island” located in the center of the ceiling, can only go down, while expanding towards the sides of the walls as it approaches the floor. This solution will reduce recirculation zones and the size of stagnation areas where harmful microorganisms accumulate, prevent room air from mixing with laminar flow, reduce its acceleration, stabilize speed and block the entire sterile zone with downward flow. This helps to isolate the protected area from the surrounding air and allows biological contaminants to be removed from it.

Rice. Figure 2 shows a standard air curtain design with slits around the perimeter of the room. If you organize an exhaust along the perimeter of the laminar flow, it will stretch, the air flow will expand and fill the entire area under the curtain, and as a result, the “narrowing” effect will be prevented and the required speed of the laminar flow will be stabilized.

Rice. 2. Air curtain diagram

In Fig. Figure 3 shows the actual air speed values ​​for a properly designed air curtain. They clearly show the interaction of the air curtain with a laminar flow that moves uniformly. An air curtain allows you to avoid installing a bulky exhaust system along the entire perimeter of the room. Instead, as is customary in operating rooms, a traditional hood is installed in the walls. The air curtain serves to protect the area surrounding the surgical personnel and the table, preventing contaminated particles from returning to the initial air flow.

Rice. 3. Actual velocity profile in the air curtain cross section

What level of disinfection can be achieved using an air curtain? If it is poorly designed, it will not provide any greater effect than a laminar system. You can make a mistake at high air speed, then such a curtain can “pull” the air flow faster than necessary, and it will not have time to reach the operating table. Uncontrolled flow behavior can threaten the penetration of contaminated particles into the protected area from floor level. Also, a curtain with insufficient suction speed will not be able to completely block the air flow and may be drawn into it. In this case, the air mode of the operating room will be the same as when using only a laminar device. During design, the speed range must be correctly identified and the appropriate system selected. The calculation of disinfection characteristics depends on this.

Air curtains have a number of obvious advantages, but they should not be used everywhere, because it is not always necessary to create a sterile flow during surgery. The decision on the level of air disinfection required is made jointly with the surgeons involved in these operations.

Conclusion

Vertical laminar flow does not always behave predictably, which depends on the conditions of its use. Laminar flow panels, which are used in clean production rooms, often do not provide the required level of disinfection in operating rooms. The installation of air curtain systems helps control the movement patterns of vertical laminar air flows. Air curtains help to carry out bacteriological control of the air in operating rooms, especially during long-term surgical interventions and the constant presence of patients with weak immune systems, for whom airborne infections are a huge risk.

The article was prepared by A. P. Borisoglebskaya using materials from the ASHRAE journal.

Literature

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Depending on the method of ventilation, the room is usually called:

a) turbulently ventilated or rooms withnon-unidirectional air flow;

b) rooms with laminar, or unidirectional, air flow.

Note. The professional vocabulary is dominated by the terms

"turbulent air flow", "laminar air flow".

Driving Modes I am air

There are two driving modes air: laminar? and turbulent?. Laminar? The mode is characterized by the ordered movement of air particles along parallel trajectories. Mixing in the flow occurs as a result of the interpenetration of molecules. In a turbulent mode, the movement of air particles is chaotic, mixing is caused by the interpenetration of individual volumes of air and therefore occurs much more intensely than in a laminar mode.

With stationary laminar movement, the speed of the air flow at a point is constant in magnitude and direction; during turbulent motion, its magnitude and direction are variable in time.

Turbulence is a consequence of external (carried into the flow) or internal (generated in the flow) disturbances?. Turbulence ventilation flows are usually of internal origin. Its cause is vortex formation when the flow flows around irregularities?walls and objects.

The criterion of foundations? turbulent regime is the Rhea number?Nolds:

R e = uD / h

Where And - average air speed in indoors;

D - hydraulically? room diameter;

D= 4S/P

S - cross-sectional area premises;

R - perimeter of the transverse sections of the room;

v- kinematic?air viscosity coefficient.

Rhea number? Nolds, above which the turbulent movement of the abutment?clearly, is called critical. For premises it is equal to 1000-1500, for smooth pipes - 2300. V premises air movement is usually turbulent; when filtering(in clean rooms)possible as laminar?, and turbulent? mode.

Laminar flow units are used in clean production rooms and serve to distribute large volumes of air, providing for specially designed ceilings, floor hoods and room pressure regulation. Under these conditions, the operation of laminar flow distributors is guaranteed to provide the required unidirectional flow with parallel flow lines. A high air exchange rate helps maintain conditions close to isothermal in the supply air flow. Ceilings designed for air distribution with large air exchanges, due to their large area, provide a low initial air flow velocity. The operation of exhaust devices located at floor level and control of air pressure in the room minimize the size of recirculation flow zones, and the principle of “one pass and one exit” is easily implemented. Suspended particles are pressed against the floor and removed, so there is little risk of them being recirculated.

Lung compliance quantitatively characterizes the extensibility of lung tissue at any time of change in their volume during the inhalation and exhalation phases. Therefore, distensibility is a static characteristic of the elastic properties of lung tissue. However, during breathing, resistance to the movement of the external respiration apparatus arises, which determines its dynamic characteristics, among which the most important is resistance the flow of air as it moves through the airways of the lungs.

The movement of air from the external environment through the respiratory tract to the alveoli and in the opposite direction is influenced by the pressure gradient: in this case, the air moves from an area of ​​high pressure to an area of ​​low pressure. When you inhale, the air pressure in the alveolar space is less than atmospheric pressure, and when you exhale, the opposite is true. Airway resistance air flow depends on the pressure gradient between the oral cavity and the alveolar space.

Airflow through the respiratory tract may be laminar, turbulent and transitional between these types. Air moves in the respiratory tract mainly in a laminar flow, the speed of which is higher in the center of these tubes and lower near their walls. With laminar air flow, its speed linearly depends on the pressure gradient along the airways. At the points of division of the respiratory tract (bifurcation), laminar air flow becomes turbulent. When turbulent flow occurs in the airways, a breathing noise occurs, which can be heard in the lungs with a stethoscope. The resistance to laminar gas flow in a pipe is determined by its diameter. Therefore, according to Poiseuille's law, the resistance of the airways to air flow is proportional to their diameter raised to the fourth power. Since the resistance of the airways is inversely related to their diameter to the fourth power, this indicator most significantly depends on changes in the diameter of the airways caused, for example, by the release of mucus from the mucous membrane or the narrowing of the lumen of the bronchi. The total cross-sectional diameter of the airways increases in the direction from the trachea to the periphery of the lung and becomes largest in the terminal airways, which causes a sharp decrease in the resistance to air flow and its speed in these parts of the lungs. Thus, the linear velocity of the flow of inhaled air in the trachea and main bronchi is approximately 100 cm/s. At the border of the air-conducting and transition zones of the respiratory tract, the linear speed of the air flow is about 1 cm/s; in the respiratory bronchi it decreases to 0.2 cm/s, and in the alveolar ducts and sacs - to 0.02 cm/s. Such a low speed of air flow in the alveolar ducts and sacs causes insignificant resistance moving air and is not accompanied by significant expenditure of energy from muscle contraction.

On the contrary, the greatest airway resistance air flow occurs at the level of segmental bronchi due to the presence in their mucous membrane of secretory epithelium and a well-developed smooth muscle layer, i.e., factors that most influence both the diameter of the airways and the resistance to air flow in them. One of the functions of the respiratory muscles is to overcome this resistance.

Studying the properties of liquid and gas flows is very important for industry and utilities. Laminar and turbulent flow affects the speed of transportation of water, oil, and natural gas through pipelines for various purposes and affects other parameters. The science of hydrodynamics deals with these problems.

Classification

In the scientific community, the flow regimes of liquids and gases are divided into two completely different classes:

• laminar (jet);
• turbulent.

A transition stage is also distinguished. By the way, the term “liquid” has a broad meaning: it can be incompressible (this is actually a liquid), compressible (gas), conducting, etc.

Background

Back in 1880, Mendeleev expressed the idea of ​​the existence of two opposite flow regimes. British physicist and engineer Osborne Reynolds studied this issue in more detail, completing his research in 1883. First practically, and then using formulas, he established that at low flow speeds, the movement of liquids takes on a laminar form: layers (particle flows) hardly mix and move along parallel trajectories. However, after overcoming a certain critical value (it is different for different conditions), called the Reynolds number, the fluid flow regimes change: the jet flow becomes chaotic, vortex - that is, turbulent. As it turned out, these parameters are also characteristic of gases to a certain extent.

Practical calculations of the English scientist showed that the behavior of, for example, water strongly depends on the shape and size of the reservoir (pipe, channel, capillary, etc.) through which it flows. Pipes with a circular cross-section (such as are used for the installation of pressure pipelines) have their own Reynolds number - the formula is described as follows: Re = 2300. For flow along an open channel, it is different: Re = 900. At lower values ​​of Re, the flow will be ordered, at higher values ​​- chaotic .

Laminar flow

The difference between laminar flow and turbulent flow is the nature and direction of water (gas) flows. They move in layers, without mixing and without pulsations. In other words, the movement occurs evenly, without random jumps in pressure, direction and speed.

Laminar flow of liquid is formed, for example, in narrow living beings, capillaries of plants and, under comparable conditions, during the flow of very viscous liquids (fuel oil through a pipeline). To clearly see the jet flow, just open the water tap slightly - the water will flow calmly, evenly, without mixing. If the tap is turned off all the way, the pressure in the system will increase and the flow will become chaotic.

Turbulent flow

Unlike laminar flow, in which nearby particles move along almost parallel trajectories, turbulent fluid flow is disordered. If we use the Lagrange approach, then the trajectories of particles can intersect arbitrarily and behave quite unpredictably. The movements of liquids and gases under these conditions are always nonstationary, and the parameters of these nonstationarities can have a very wide range.

How the laminar regime of gas flow turns into turbulent can be traced using the example of a stream of smoke from a burning cigarette in still air. Initially, the particles move almost parallel along trajectories that do not change over time. The smoke seems motionless. Then, in some place, large vortices suddenly appear and move completely chaotically. These vortices break up into smaller ones, those into even smaller ones, and so on. Eventually, the smoke practically mixes with the surrounding air.

Turbulence cycles

The example described above is textbook, and from its observation, scientists have drawn the following conclusions:

1. Laminar and turbulent flow are probabilistic in nature: the transition from one regime to another does not occur in a precisely specified place, but in a rather arbitrary, random place.
2. First, large vortices appear, the size of which is larger than the size of a stream of smoke. The movement becomes unsteady and highly anisotropic. Large flows lose stability and break up into smaller and smaller ones. Thus, a whole hierarchy of vortices arises. The energy of their movement is transferred from large to small, and at the end of this process disappears - energy dissipation occurs at small scales.
3. The turbulent flow regime is random in nature: one or another vortex can end up in a completely arbitrary, unpredictable place.
4. Mixing of smoke with the surrounding air practically does not occur in laminar conditions, but in turbulent conditions it is very intense.
5. Despite the fact that the boundary conditions are stationary, the turbulence itself has a pronounced non-stationary character - all gas-dynamic parameters change over time.

There is another important property of turbulence: it is always three-dimensional. Even if we consider a one-dimensional flow in a pipe or a two-dimensional boundary layer, the movement of turbulent vortices still occurs in the directions of all three coordinate axes.

Reynolds number: formula

The transition from laminarity to turbulence is characterized by the so-called critical Reynolds number:

Re cr = (ρuL/µ) cr,

where ρ is the flow density, u is the characteristic flow speed; L is the characteristic size of the flow, µ is the coefficient cr - flow through a pipe with a circular cross-section.

For example, for a flow with speed u in a pipe, L is used as Osborne Reynolds showed that in this case 2300

A similar result is obtained in the boundary layer on the plate. The distance from the leading edge of the plate is taken as a characteristic size, and then: 3 × 10 5

Concept of speed disturbance

Laminar and turbulent fluid flow, and, accordingly, the critical value of the Reynolds number (Re) depend on a large number of factors: pressure gradient, height of roughness tubercles, intensity of turbulence in the external flow, temperature difference, etc. For convenience, these total factors are also called velocity disturbance , since they have a certain effect on the flow rate. If this disturbance is small, it can be extinguished by viscous forces tending to level the velocity field. With large disturbances, the flow may lose stability and turbulence occurs.

Considering that the physical meaning of the Reynolds number is the ratio of inertial forces and viscous forces, the disturbance of flows falls under the formula:

Re = ρuL/µ = ρu 2 /(µ×(u/L)).

The numerator contains double the velocity pressure, and the denominator contains a quantity of the order of friction stress if the thickness of the boundary layer is taken as L. The high-speed pressure tends to destroy the balance, but this is counteracted. However, it is not clear why (or the velocity pressure) leads to changes only when they are 1000 times greater than the viscous forces.

Calculations and facts

It would probably be more convenient to use the velocity disturbance rather than the absolute flow velocity u as the characteristic velocity in Recr. In this case, the critical Reynolds number will be of the order of 10, that is, when the disturbance of the velocity pressure exceeds the viscous stresses by 5 times, the laminar flow of the fluid becomes turbulent. This definition of Re, according to a number of scientists, well explains the following experimentally confirmed facts.

For an ideally uniform velocity profile on an ideally smooth surface, the traditionally determined number Re cr tends to infinity, that is, the transition to turbulence is actually not observed. But the Reynolds number, determined by the magnitude of the speed disturbance, is less than the critical one, which is equal to 10.

In the presence of artificial turbulators that cause a burst of speed comparable to the main speed, the flow becomes turbulent at much lower values ​​of the Reynolds number than Re cr determined from the absolute value of the speed. This makes it possible to use the value of the coefficient Re cr = 10, where the absolute value of the speed disturbance caused by the above reasons is used as the characteristic speed.

Stability of laminar flow in a pipeline

Laminar and turbulent flow is characteristic of all types of liquids and gases under different conditions. In nature, laminar flows are rare and are characteristic, for example, of narrow underground flows in flat conditions. This issue worries scientists much more in the context of practical applications for transporting water, oil, gas and other technical liquids through pipelines.

The issue of laminar flow stability is closely related to the study of the perturbed motion of the main flow. It has been established that it is exposed to so-called small disturbances. Depending on whether they fade or grow over time, the main flow is considered stable or unstable.

Flow of compressible and incompressible fluids

One of the factors influencing the laminar and turbulent flow of a fluid is its compressibility. This property of a liquid is especially important when studying the stability of unsteady processes with a rapid change in the main flow.

Research shows that laminar flow of incompressible fluid in pipes of cylindrical cross-section is resistant to relatively small axisymmetric and non-axisymmetric disturbances in time and space.

Recently, calculations have been carried out on the influence of axisymmetric disturbances on the stability of the flow in the inlet part of a cylindrical pipe, where the main flow depends on two coordinates. In this case, the coordinate along the pipe axis is considered as a parameter on which the velocity profile along the pipe radius of the main flow depends.

Conclusion

Despite centuries of study, it cannot be said that both laminar and turbulent flow have been thoroughly studied. Experimental studies at the micro level raise new questions that require reasoned computational justification. The nature of the research also has practical benefits: thousands of kilometers of water, oil, gas, and product pipelines have been laid throughout the world. The more technical solutions are implemented to reduce turbulence during transportation, the more effective it will be.

To reduce pollution in high-class clean rooms, special ventilation systems are used in which the air flow moves from top to bottom without turbulence, i.e. laminar. With a laminar air flow, dirt particles from people and equipment are not scattered throughout the room, but are collected in a flow near the floor.

Constructions

In general, clean rooms include the following basic elements:

enclosing wall structures (frame, blind and glazed wall panels, doors, windows);

sealed panel and cassette ceilings with built-in raster lamps;

antistatic floors;

Clean-Zone Floor Covering Clean-Zone is supplied in standard rolls, to be professionally installed as a wall-to-wall covering floor, creating a permanent and unavoidable trap for dirt.

air preparation system (supply, exhaust and recirculation ventilation units, air intake devices, air distributors with final filters, air control devices, sensor equipment and automation elements, etc.);

control system for engineering systems of clean rooms;

airlocks;

transfer windows;

filter and fan modules for creating clean zones inside clean rooms.

Electronics industry is one of the largest consumers of cleanrooms in the world. The requirements for the level of cleanliness in this industry are the most stringent. The trend of constant growth of these requirements has led to qualitatively new approaches to creating clean environments. The essence of these approaches is to create isolating technologies, i.e. in the physical separation of a certain volume of clean air from the environment. This separation, usually hermetically sealed, eliminated the influence of one of the most intense sources of pollution - humans. The use of insulating technologies entails the widespread introduction of automation and robotization. The use of clean rooms in microelectronics has its own characteristics: the requirements for the cleanliness of the air environment for aerosol particles come to the fore. Increased demands are also placed on the cleanroom grounding system, especially in terms of ensuring the absence of static electricity. Microelectronics requires the creation of clean rooms of the highest cleanliness classes with the installation of perforated raised floors to improve air flow lines, i.e. increasing the unidirectionality of flow.

Clean production facilities must provide conditions for maximum cleanliness of production; ensure insulation of the internal volume; entrance to clean rooms through a special vestibule (gateway).

The pressure in a clean room should be greater than atmospheric pressure, which helps push dust out of it. In the airlock, personnel clothing is blown to remove dust particles.

In clean rooms, laminar air flows are created, and turbulent flows that are created by rotating and moving parts of equipment are unacceptable. It is necessary to ensure that there are no heated things that contribute to the formation of convection currents.

Typically, a lattice floor and a lattice ceiling are used.

Clean rooms contain a minimum of equipment

Since the production of clean rooms is very expensive, local dust removal zones are used.

One of the effective ways to reduce costs when creating clean room complexes is zoning of the clean room into local areas, which may differ from each other both in the air cleanliness class and in the functional purpose (only product protection, or protection of both the product and the environment).

Thus, inside a clean room with a low cleanliness class, clean zones with a higher cleanliness class than the room where they are located can be created above critical areas of the technological process.

The main purpose of clean zones:

maintaining specified air parameters in the local workspace;

protection of the product from environmental influences.

According to the definition given in GOST R ISO 14644-1-2000, a clean zone is a defined space in which the concentration of airborne particles is controlled, constructed and operated to minimize the entry, release and retention of particles within the area, and allowing other parameters such as temperature, humidity and pressure to be controlled as necessary.

Clean zones can be constructed structurally either as part of the overall cleanroom ventilation system, or as stand-alone products.

The first method is applicable when the location of clean zones is laid down at the design stage of creating a clean room and is not subject to change for the entire period of its operation, as well as when it is necessary to supply supply air to the working space of the clean zone.

The second method involves the possibility of changing the location of clean zones, which provides greater opportunities for changing the technological process and upgrading equipment. In this case, clean zones, designed as independent products, can either be attached to the power structures of the clean room, or be mobile autonomous products that can be moved within the clean room.

Most often, clean production conditions are used with minimal personnel, using semi-automatic machines. Local installations are often used. Recently, cluster installations have begun to be used.

Specifications:

1 Ultimate pressure in a clean, empty and degassed chamber, Pa 1.33x10-3

2 Pressure recovery time 1.33x10-3 Pa, min 30

3 Dimensions of the working chamber, mm Diameter Height 900 1000

4 Number of plasma accelerators with metal cathodes (SPU-M) with plasma flow separation, pcs. 3

5 Number of pulsed plasma accelerators with graphite cathodes (IPU-S) with plasma flow separation, pcs. 4

6 Number of extended ion sources for cleaning and assistance (RIF type), pcs. 1

7 Heating of substrates, 0С 250

8 Technological equipment: Single planetary set, pcs. Double planetary, pcs. 1 1

9 Process gas injection system

10 Process control and management system

11 High-vacuum pumping: two diffusion pumps operating in parallel NVDM-400 with a capacity of 7000 l/s each

12 Forevacuum pumping: AVR-150 forevacuum unit with a capacity of 150 l/s

13 Maximum electrical power consumed by a vacuum installation, kW, no more than 50

14 Area occupied by a vacuum installation, m2 25