Laminar air flow in the operating room. Laminar mode of fluid movement
Description:
Operating rooms are one of the most critical links in the structure of a hospital building in terms of the importance of the surgical process, as well as providing the special microclimate conditions necessary for its successful implementation and completion. Here, the source of the release of bacterial particles is mainly medical personnel, who are able to generate particles and release microorganisms when moving around the room.
Hospital operating rooms
Air flow control
Over the past decades, in our country and abroad, there has been an increase in purulent-inflammatory diseases caused by infections, which, according to the definition of the World Health Organization (WHO), are commonly called nosocomial infections (HAIs). An analysis of diseases caused by nosocomial infections shows that their frequency and duration are directly dependent on the state of the air environment in hospital premises. To ensure the required microclimate parameters in operating rooms (and industrial clean rooms), unidirectional flow air distributors are used. The results of monitoring the air environment and analyzing the movement of air flows showed that the operation of such distributors provides the required microclimate parameters, but often worsens the bacteriological purity of the air. To protect the critical area, it is necessary that the air flow leaving the device maintains straightness and does not lose the shape of its boundaries, that is, the flow should not expand or contract over the protected area where the surgical
Operating rooms are one of the most critical links in the structure of a hospital building in terms of the importance of the surgical process, as well as providing the special microclimate conditions necessary for its successful implementation and completion. Here, the source of the release of bacterial particles is mainly medical personnel, who are able to generate particles and release microorganisms when moving around the room. The intensity of particles entering indoor air depends on the degree of mobility of people, temperature and air speed in the room. Nosocomial infections tend to move around the operating room with air currents, and there is always a risk of its penetration into the unprotected wound cavity of the patient being operated on. From observations it is obvious that improperly organized operation of ventilation systems leads to intensive accumulation of infection to levels exceeding permissible levels.
For several decades, specialists from different countries have been developing system solutions to ensure air conditions in operating rooms. The air flow supplied to the room must not only assimilate various harmful substances (heat, humidity, odors, harmful substances) and maintain the required microclimate parameters, but also ensure the protection of strictly established areas from infections entering them, that is, the necessary cleanliness of indoor air. The area where invasive interventions are carried out (penetration into the human body) can be called the operating zone or “critical”. The standard defines such an area as an “operating sanitary protection zone” and means by it the space where the operating table, auxiliary tables for instruments and materials, equipment, as well as medical personnel in sterile clothing are located. There is the concept of a “technological core”, which refers to the area where production processes are carried out under sterile conditions, which in meaning can be correlated with the operating area.
To prevent the penetration of bacterial contaminants into the most critical areas, screening methods have become widely used through the use of displacement air flow. Various designs of laminar air flow air distributors were created, and the term “laminar” was later changed to “unidirectional” flow. Currently, you can find a variety of names for air distribution devices in clean rooms, such as “laminar”, “laminar ceiling”, “operating ceiling”, “ operating system clean air”, etc., which does not change their essence. The air distributor is built into the ceiling structure above the protection zone of the room and can be of different sizes depending on the air flow. The recommended optimal area of such a ceiling should be at least 9 m2 in order to completely cover the operating area with tables, equipment and personnel. The displacing air flow at low speeds comes from top to bottom, like a curtain, cutting off both the aseptic field of the surgical intervention zone and the zone of transfer of sterile material from the environment. Air is removed from the lower and upper zones of the room simultaneously. HEPA filters (class H according to) are built into the ceiling structure, through which the supply air passes. Filters trap but do not disinfect living particles.
Currently, much attention is being paid all over the world to the issues of air disinfection in hospitals and other institutions where there are sources of bacterial contamination. The documents voiced requirements for the need to disinfect operating room air with a particle inactivation efficiency of at least 95%, as well as air ducts and climate system equipment. Bacterial particles released by surgical personnel continuously enter the room air and accumulate in it. To ensure that the concentration of particles in indoor air does not reach maximum permissible levels, air control is necessary. Such monitoring must be carried out after installation of climate control systems, maintenance or repair, that is, in the operating mode of a clean room.
The use of unidirectional flow air distributors with built-in ceiling-type ultra-fine filters in operating rooms has become common among designers. Air flows of large volumes go down the room at low speeds, cutting off the protected area from the environment. However, many professionals are unaware that these solutions are not sufficient to maintain adequate levels of air disinfection during surgical procedures.
The fact is that there are quite a lot of designs of air distribution devices, each of which has its own area of application. Operating room cleanrooms within their “clean” class are divided into classes according to the degree of cleanliness, depending on their purpose. For example, general surgical operating rooms, cardiac surgery or orthopedic operating rooms, etc. Each specific case has its own requirements for ensuring cleanliness.
The first examples of the use of air distributors for clean rooms appeared in the mid-1950s. Since then, it has become traditional to distribute air in clean production rooms through a perforated ceiling when low concentrations of particles or microorganisms are required. The air flow moves through the entire volume of the room in one direction at a uniform speed, usually 0.3–0.5 m/s. The air is supplied through a bank of high-efficiency air filters located on the ceiling of the cleanroom. The air supply is organized on the principle of an air piston moving downward through the entire room, removing contaminants. Air is removed through the floor. This type of air movement contributes to the removal of aerosol contaminants, the sources of which are personnel and processes. This arrangement of ventilation is aimed at ensuring clean air in the room, but requires large air flows and is therefore uneconomical. For cleanrooms of class 1000 or ISO class 6 (ISO classification), the air exchange rate can range from 70 to 160 times per hour.
Subsequently, more rational modular devices of much smaller sizes with low costs appeared, making it possible to select an air supply device based on the size of the protected area and the required air exchange rates of the room, depending on the purpose of the room.
Analysis of the operation of laminar air distributors
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.
However, when such air distributors operate in an operating room, the situation changes significantly. To maintain acceptable levels of bacteriological purity of air in operating rooms, calculated air exchange values usually average 25 times per hour or even less, that is, they are not comparable with the values for production premises. To maintain stable air flow between the operating room and adjacent rooms, excess pressure is usually maintained in it. Air is removed through exhaust devices symmetrically installed in the walls of the lower zone of the room. To distribute smaller volumes of air, as a rule, small-area laminar flow devices are used, which are installed only above the critical area of the room in the form of an island in the middle of the room, instead of using the entire ceiling.
Observations show that such laminar devices will not always provide unidirectional flow. Since there is almost always a difference between the temperature in the supply stream and the ambient air temperature (5-7 ° C), the cooler air leaving the supply device descends much faster than an isothermal unidirectional flow. This is a common occurrence for ceiling diffusers used in public buildings. There is a misconception that laminar floors provide stable, unidirectional airflow regardless of location or method of application. In fact, in real conditions, the speed of low temperature vertical laminar flow will increase as it approaches the floor. The larger the volume of supply air and the lower its temperature relative to the room air, the greater the acceleration of its flow. The table shows that the use of a laminar system with an area of 3 m 2 with a temperature difference of 9 ° C gives a threefold increase in air speed already at a distance of 1.8 m from the beginning of the path. The air speed at the outlet of the supply device is 0.15 m/s, and at the level of the operating table it reaches 0.46 m/s. This value exceeds the acceptable level. It has long been proven by many studies that with excessive inflow flow rates it is impossible to maintain its “unidirectionality”. Analysis of air control in operating rooms, carried out, in particular, by Salvati (1982) and Lewis (Lewis, 1993), showed that in some cases the use of laminar flow units with high air velocities leads to an increase in the level of air contamination in the area of the surgical incision with subsequent risk of infection.
Dependence of air flow speed on area laminar panel and supply air temperature |
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T - difference between the temperature of the supply and ambient air |
When the flow moves, at the initial point the air flow lines will be parallel, then the boundaries of the flow will change, narrowing towards the floor, and it will no longer be able to protect the area determined by the dimensions of the laminar flow unit. At air speeds of 0.46 m/s, the flow will capture low-moving air from the room. Since bacterial particles are constantly released in the room, infected particles will be mixed into the air flow coming from the supply unit, since the sources of their release are constantly operating in the room. This is facilitated by air recirculation resulting from pressurized air in the room. To maintain the cleanliness of operating rooms, according to the standards, it is necessary to ensure an imbalance of air due to the excess of the inflow over the exhaust by 10%. Excess air moves to adjacent less clean rooms. In modern conditions, hermetic sliding doors are often used in operating rooms; excess air has nowhere to go; it circulates throughout the room and is taken back into the supply unit using fans built into it for further cleaning in filters and secondary supply to the room. The circulating air collects all contaminated particles from the air in the room and, moving near the supply flow, can pollute it. Due to the violation of the boundaries of the flow, air from the surrounding space is mixed into it and pathogenic particles penetrate into the sterile zone, which is considered protected.
High mobility promotes intensive detachment of dead skin particles from unprotected areas of the skin of medical personnel and their entry directly into the surgical incision. On the other hand, it should be noted that the development of infectious diseases in the postoperative period is caused by the hypothermic state of the patient, which intensifies when exposed to flows of cold air of increased mobility.
Thus, a laminar flow air diffuser, traditionally used and effective in a cleanroom environment, may be detrimental to operations in a conventional operating room.
This conversation is valid for laminar flow devices, which have an average area of about 3 m 2 - optimal for protecting the operating area. According to American requirements, the air flow velocity at the outlet of laminar panels should not exceed 0.15 m/s, that is, 14 l/s of air should flow into the room from 1 ft 2 (0.09 m 2) of panel area. In our case, this will be 466 l / s (1677.6 m 3 / h) or approximately 17 times / h. According to the standard value of air exchange in operating rooms, it should be 20 times per hour, 25 times per hour, so 17 times per hour fully meets the requirements. It turns out that the value of 20 times per hour corresponds to a room with a volume of 64 m 3.
According to today's standards, the area of a standard operating room (general surgery) should be at least 36 m2. And the requirements for operating rooms for more complex operations (cardiological, orthopedic, etc.) are much higher, and often the volume of such an operating room can exceed 135–150 m 3 . The air distribution system for these cases will require a significantly larger area and air capacity.
In the case of organizing air flow in larger operating rooms, the problem arises of maintaining laminarity of flow from the exit plane to the level of the operating table. Air flow behavior studies have been conducted in several operating rooms. Laminar flow panels were installed in different rooms, which were divided by area into two groups: 1.5–3 m 2 and more than 3 m 3, and experimental air conditioning units were installed that made it possible to change the temperature of the supply air. Repeated measurements of the flow rate of incoming air were carried out at various flow rates and temperature changes, the results of which can be seen in the table.
Criteria for room cleanliness
Correct decisions regarding the organization of air distribution in operating rooms: choosing the rational size of supply panels, ensuring the standard flow rate and temperature of supply air - do not guarantee absolute disinfection of the air in the room. The issue of air disinfection in operating rooms was acutely raised more than 30 years ago, when various anti-epidemiological measures were proposed. And now the goal of the requirements of modern regulatory documents for the design and operation of hospitals is air disinfection, where HVAC systems are presented as the main way to prevent the spread and accumulation of infections.
For example, the standard considers disinfection to be the main goal of its requirements, noting: “a properly designed HVAC system minimizes the airborne transmission of viruses, bacteria, fungal spores and other biological contaminants,” and HVAC systems play a major role in the control of infections and other harmful factors. The requirement for operating room air conditioning systems is highlighted: “the air supply system must be designed to minimize the introduction of bacteria into sterile areas along with the air, while also maintaining the maximum level of cleanliness in the rest of the operating room.”
However, regulatory documents do not contain direct requirements for determining and monitoring the effectiveness of disinfection for various ventilation methods, and designers often have to engage in search activities, which takes a lot of time and distracts from the main work.
In our country there is quite a lot of different regulatory literature on the design of HVAC systems for hospital buildings, and requirements for air disinfection are voiced everywhere, which, for many objective reasons, are practically difficult for designers to implement. This requires not only knowledge of modern disinfection equipment and the correct use of it, but, most importantly, further timely epidemiological monitoring of the indoor air environment, which gives an idea of the quality of operation of HVAC systems, but, unfortunately, is not always carried out. If the cleanliness of clean industrial premises is assessed by the presence of particles (for example, dust particles), then the indicator of air cleanliness in clean rooms of medical buildings is live bacterial or colony-forming particles, the permissible levels of which are given in. To maintain these levels, the air environment should be regularly monitored for microbiological indicators, for which it is necessary to be able to count them. The methodology for collecting and counting microorganisms to assess air purity has not yet been presented in any of the regulatory documents. It is important that the counting of microbial particles should be carried out in the operating room, that is, during the operation. But for this, the design and installation of the air distribution system must be ready. The level of disinfection or the efficiency of the system cannot be determined before it starts operating in the operating room; this can only be done under conditions of at least several operating processes. This poses great difficulties for engineers, since research, although necessary, is contrary to the hospital’s anti-epidemic discipline.
Air curtain
To ensure the required air conditions in the operating room, it is important to properly organize the joint work of air inflow and removal. By rationally positioning supply and exhaust devices in the operating room, the nature of air flow can be improved.
In operating rooms, it is impossible to use both the entire ceiling area for air distribution and the floor area for air removal. Floor hoods are unhygienic because they get dirty quickly and are difficult to clean. Bulky, complex and expensive systems have never found their application in small operating rooms. For these reasons, the most rational is the “island” arrangement of laminar panels above the critical area with the installation of exhaust openings in the lower part of the walls. This makes it possible to simulate air flows similar to an industrial clean room in a cheaper and less cumbersome way. A method that has proven successful is the use of air curtains operating on the principle of a protective barrier. The air curtain combines well with the flow of supply air in the form of a narrow “shell” of air at a higher speed, specially organized around the perimeter of the ceiling. The air curtain continuously works for exhaust and prevents the entry of contaminated ambient air into the laminar flow.
To understand the operation of an air curtain, you should imagine an operating room with an exhaust hood arranged on all four sides of the room. The supply air coming from the “laminar island” located in the center of the ceiling will only fall down, expanding towards the sides of the walls as it descends. This solution reduces recirculation zones, the size of stagnant areas in which pathogenic microorganisms collect, and also prevents mixing of the laminar flow with the room air, reduces its acceleration and stabilizes the speed, as a result of which the downward flow covers (locks) the entire sterile area. This helps remove biological contaminants from the protected area and isolate it from the environment.
In Fig. Figure 1 shows a standard air curtain design with slots around the perimeter of the room. When organizing exhaust along the perimeter of the laminar flow, it stretches, it expands and fills the entire zone inside the curtain, as a result of which the “narrowing” effect is prevented and the required speed of the laminar flow is stabilized.
From Fig. Figure 3 shows the values of the actual (measured) speed that occurs with a properly designed air curtain, which clearly demonstrate the interaction of the laminar flow with the air curtain, and the laminar flow moves uniformly. The air curtain eliminates the need to install a bulky exhaust system around the entire perimeter of the room, instead of installing a traditional hood in the walls, as is customary in operating rooms. The air curtain protects the area directly around the surgical personnel and table, preventing contaminated particles from returning to the primary air stream.
After designing an air curtain, the question arises as to what level of disinfection can be achieved during its operation. A poorly designed air curtain will be no more effective than a traditional laminar flow system. A design mistake may be high air speed, since such a curtain will “pull” the laminar flow too quickly, that is, even before it reaches the operating floor. Flow behavior may not be controlled and there may be a risk of contaminated particles leaking into the operating area from floor level. Likewise, an air curtain with a low suction speed cannot effectively block laminar flow and may be drawn into it. In this case, the air condition of the room will be the same as when using only a laminar air supply device. When designing, it is important to correctly determine the speed range and select the appropriate system. This directly affects the calculation of disinfection characteristics.
Despite the obvious advantages of air curtains, they should not be used blindly. The sterile airflow created by air curtains during surgery is not always required. The need to ensure the level of air disinfection should be decided together with technologists, whose role in this case should be surgeons involved in specific operations.
Conclusion
Vertical laminar flow can behave unpredictably depending on its operating conditions. Laminar flow panels used in clean production areas generally cannot provide the required level of disinfection in operating rooms. Air curtain systems help correct the movement pattern of vertical laminar flows. Air curtains are the optimal solution to the problem of bacteriological control of the air environment in operating rooms, especially during long surgical operations and patients with a compromised immune system, for whom airborne infections pose a particular risk.
The article was prepared by A. P. Borisoglebskaya using materials from the ASHRAE journal.
The movement of fluid observed at low speeds, in which individual streams of fluid move parallel to each other and the flow axis, is called laminar fluid movement.
Laminar motion mode in experiments
![](https://i1.wp.com/nektonnasos.ru/articles/laminarnyj_rezhim/pic11.jpg)
A very clear idea of the laminar regime of fluid movement can be obtained from Reynolds' experiment. Detailed description .
The liquid flows out of the tank through a transparent pipe and goes through the tap to the drain. Thus, the liquid flows at a certain small and constant flow rate.
At the entrance to the pipe there is a thin tube through which a colored medium enters the central part of the flow.
When paint enters a flow of liquid moving at low speed, the red paint will move in an even stream. From this experiment we can conclude that the liquid flows in a layered manner, without mixing and vortex formation.
This mode of fluid flow is usually called laminar.
Let us consider the basic laws of the laminar regime with uniform movement in round pipes, limiting ourselves to cases where the pipe axis is horizontal.
In this case, we will consider an already formed flow, i.e. flow in a section, the beginning of which is located from the inlet section of the pipe at a distance that provides the final stable form of velocity distribution over the flow section.
Bearing in mind that the laminar flow regime has a layered (jet) character and occurs without mixing of particles, it should be assumed that in a laminar flow there will only be velocities parallel to the pipe axis, while transverse velocities will be absent.
![](https://i1.wp.com/nektonnasos.ru/articles/laminarnyj_rezhim/pic2.jpg)
One can imagine that in this case the moving liquid seems to be divided into an infinitely large number of infinitely thin cylindrical layers, parallel to the axis of the pipeline and moving one inside the other at different speeds, increasing in the direction from the walls to the axis of the pipe.
In this case, the velocity in the layer directly in contact with the walls due to the adhesion effect is zero and reaches its maximum value in the layer moving along the axis of the pipe.
Laminar flow formula
The accepted motion scheme and the assumptions introduced above make it possible to theoretically establish the law of velocity distribution in the cross section of the flow in laminar mode.
![](https://i0.wp.com/nektonnasos.ru/articles/laminarnyj_rezhim/pic3.jpg)
To do this, we will do the following. Let us denote the internal radius of the pipe by r and choose the origin of coordinates at the center of its cross section O, directing the x axis along the axis of the pipe, and the z axis vertically.
Now let’s select a volume of liquid inside the pipe in the form of a cylinder of a certain radius y and length L and apply Bernoulli’s equation to it. Since due to the horizontal axis of the pipe z1=z2=0, then
![](https://i2.wp.com/nektonnasos.ru/articles/laminarnyj_rezhim/pic4.jpg)
where R is the hydraulic radius of the section of the selected cylindrical volume = y/2
τ – unit friction force = - μ * dυ/dy
Substituting the values of R and τ into the original equation we get
![](https://i0.wp.com/nektonnasos.ru/articles/laminarnyj_rezhim/pic5.jpg)
By specifying different values of the y coordinate, you can calculate the velocities at any point in the section. The maximum speed will obviously be at y=0, i.e. on the axis of the pipe.
![](https://i0.wp.com/nektonnasos.ru/articles/laminarnyj_rezhim/pic6.jpg)
![](https://i1.wp.com/nektonnasos.ru/articles/laminarnyj_rezhim/pic7.jpg)
In order to represent this equation graphically, it is necessary to plot the velocity on a certain scale from some arbitrary straight line AA in the form of segments directed along the fluid flow, and connect the ends of the segments with a smooth curve.
The resulting curve will represent the velocity distribution curve in the cross section of the flow.
![](https://i2.wp.com/nektonnasos.ru/articles/laminarnyj_rezhim/pic8.jpg)
The graph of changes in friction force τ across a cross section looks completely different. Thus, in a laminar mode in a cylindrical pipe, the velocities in the cross section of the flow change according to a parabolic law, and the tangential stresses change according to a linear law.
The results obtained are valid for pipe sections with fully developed laminar flow. In fact, the liquid that enters the pipe must pass a certain section from the inlet section before a parabolic velocity distribution law corresponding to the laminar regime is established in the pipe.
Development of laminar regime in a pipe
The development of a laminar regime in a pipe can be imagined as follows. Let, for example, liquid enter a pipe from a large reservoir, the edges of the inlet hole of which are well rounded.
![](https://i1.wp.com/nektonnasos.ru/articles/laminarnyj_rezhim/pic9.jpg)
In this case, the velocities at all points of the inlet cross section will be almost the same, with the exception of a very thin, so-called wall layer (layer near the walls), in which, due to the adhesion of the liquid to the walls, an almost sudden drop in speed to zero occurs. Therefore, the velocity curve in the inlet section can be represented quite accurately in the form of a straight line segment.
As we move away from the entrance, due to friction at the walls, the layers of liquid adjacent to the boundary layer begin to slow down, the thickness of this layer gradually increases, and the movement in it, on the contrary, slows down.
The central part of the flow (the core of the flow), not yet captured by friction, continues to move as one whole, with approximately the same speed for all layers, and the slowdown of movement in the near-wall layer inevitably causes an increase in the speed in the core.
![](https://i1.wp.com/nektonnasos.ru/articles/laminarnyj_rezhim/pic10.jpg)
Thus, in the middle of the pipe, in the core, the flow velocity increases all the time, and near the walls, in the growing boundary layer, it decreases. This occurs until the boundary layer covers the entire flow cross section and the core is reduced to zero. At this point, the formation of the flow ends, and the velocity curve takes on the parabolic shape usual for the laminar regime.
Transition from laminar to turbulent flow
Under certain conditions, laminar fluid flow can become turbulent. As the speed of the flow increases, the layered structure of the flow begins to collapse, waves and vortices appear, the propagation of which in the flow indicates increasing disturbance.
Gradually, the number of vortices begins to increase, and increases until the stream breaks into many smaller streams mixing with each other.
The chaotic movement of such small streams suggests the beginning of the transition from laminar flow to turbulent. As the speed increases, the laminar flow loses its stability, and any random small disturbances that previously caused only small fluctuations begin to develop rapidly.
Video about laminar flow
In everyday life, the transition from one flow regime to another can be traced using the example of a stream of smoke. At first, the particles move almost parallel along time-invariant trajectories. The smoke is practically motionless. Over time, large vortices suddenly appear in some places and move along chaotic trajectories. These vortices break up into smaller ones, those into even smaller ones, and so on. Eventually, the smoke practically mixes with the surrounding air.
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.
Table of contents of the topic "Breathing. Respiratory system.":1. Breathing. Respiratory system. Functions of the respiratory system.
2. External breathing. Biomechanics of breathing. Breathing process. Biomechanics of inspiration. How do people breathe?
3. Exhale. Biomechanism of exhalation. The process of exhalation. How does exhalation occur?
4. Change in lung volume during inhalation and exhalation. Function of intrapleural pressure. Pleural space. Pneumothorax.
5. Breathing phases. Volume of the lung(s). Breathing rate. Depth of breathing. Pulmonary air volumes. Tidal volume. Reserve, residual volume. Lung capacity.
6. Factors influencing pulmonary volume during the inspiratory phase. Extensibility of the lungs (lung tissue). Hysteresis.
7. Alveoli. Surfactant. Surface tension of the fluid layer in the alveoli. Laplace's law.
9. Flow-volume relationship in the lungs. Pressure in the airways during exhalation.
10. Work of the respiratory muscles during the respiratory cycle. The work of the respiratory muscles during deep breathing.
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.
There are two different forms, two modes of fluid flow: laminar and turbulent flow. The flow is called laminar (layered) if along the flow each selected thin layer slides relative to its neighbors without mixing with them, and turbulent (vortex) if intense vortex formation and mixing of the liquid (gas) occurs along the flow.
Laminar the flow of liquid is observed at low speeds of its movement. In laminar flow, the trajectories of all particles are parallel and their shape follows the boundaries of the flow. In a round pipe, for example, the liquid moves in cylindrical layers, the generatrices of which are parallel to the walls and axis of the pipe. In a rectangular channel of infinite width, the liquid moves in layers parallel to its bottom. At each point in the flow, the speed remains constant in direction. If the speed does not change with time and magnitude, the motion is called steady. For laminar motion in a pipe, the velocity distribution diagram in the cross section has the form of a parabola with a maximum speed on the pipe axis and a zero value at the walls, where an adhering layer of liquid is formed. The outer layer of liquid adjacent to the surface of the pipe in which it flows adheres to it due to molecular adhesion forces and remains motionless. The greater the distance from the subsequent layers to the pipe surface, the greater the speed of subsequent layers, and the layer moving along the pipe axis has the highest speed. The profile of the average speed of a turbulent flow in pipes (Fig. 53) differs from the parabolic profile of the corresponding laminar flow by a more rapid increase in speed v.
Figure 9Profiles (diagrams) of laminar and turbulent fluid flows in pipes
The average value of the velocity in the cross section of a round pipe under steady laminar flow is determined by the Hagen-Poiseuille law:
(8)
where p 1 and p 2 are the pressure in two cross sections of the pipe, spaced apart at a distance Δx; r - pipe radius; η - viscosity coefficient.
The Hagen-Poiseuille law can be easily verified. It turns out that for ordinary liquids it is valid only at low flow rates or small pipe sizes. More precisely, the Hagen-Poiseuille law is satisfied only at small values of the Reynolds number:
(9)
where υ is the average speed in the cross section of the pipe; l- characteristic size, in this case - pipe diameter; ν is the coefficient of kinematic viscosity.
The English scientist Osborne Reynolds (1842 - 1912) in 1883 carried out an experiment according to the following scheme: at the entrance to the pipe through which a steady flow of liquid flows, a thin tube was placed so that its opening was on the axis of the tube. Paint was supplied through a tube into the liquid stream. While laminar flow existed, the paint moved approximately along the axis of the pipe in the form of a thin, sharply limited strip. Then, starting from a certain speed value, which Reynolds called critical, wave-like disturbances and individual rapidly decaying vortices arose on the strip. As the speed increased, their number became larger and they began to develop. At a certain speed, the strip broke up into separate vortices, which spread throughout the entire thickness of the liquid flow, causing intense mixing and coloring of the entire liquid. This current was called turbulent .
Starting from a critical speed value, the Hagen-Poiseuille law was also violated. Repeating experiments with pipes of different diameters and with different liquids, Reynolds discovered that the critical speed at which the parallelism of the flow velocity vectors is broken varied depending on the size of the flow and the viscosity of the liquid, but always in such a way that the dimensionless number took on a certain constant value in the region of transition from laminar to turbulent flow.
The English scientist O. Reynolds (1842 - 1912) proved that the nature of the flow depends on a dimensionless quantity called the Reynolds number:
(10)
where ν = η/ρ - kinematic viscosity, ρ - fluid density, υ av - average fluid velocity over the pipe cross-section, l- characteristic linear dimension, for example pipe diameter.
Thus, up to a certain value of the Re number there is a stable laminar flow, and then in a certain range of values of this number the laminar flow ceases to be stable and individual, more or less quickly decaying disturbances arise in the flow. Reynolds called these numbers critical Re cr. As the Reynolds number increases further, the motion becomes turbulent. The region of critical Re values usually lies between 1500-2500. It should be noted that the value of Re cr is influenced by the nature of the entrance to the pipe and the degree of roughness of its walls. With very smooth walls and a particularly smooth entrance into the pipe, the critical value of the Reynolds number could be raised to 20,000, and if the entrance to the pipe has sharp edges, burrs, etc. or the pipe walls are rough, the Re cr value can drop to 800-1000 .
In turbulent flow, fluid particles acquire velocity components perpendicular to the flow, so they can move from one layer to another. The velocity of liquid particles increases rapidly as they move away from the pipe surface, then changes quite slightly. Since liquid particles move from one layer to another, their speeds in different layers differ little. Due to the large velocity gradient at the pipe surface, vortices usually form.
Turbulent flow of liquids is most common in nature and technology. Air flow in. atmosphere, water in seas and rivers, in canals, in pipes is always turbulent. In nature, laminar movement occurs when water filters through the thin pores of fine-grained soils.
The study of turbulent flow and the construction of its theory is extremely complicated. The experimental and mathematical difficulties of these studies have so far been only partially overcome. Therefore, a number of practically important problems (water flow in canals and rivers, the movement of an aircraft of a given profile in the air, etc.) have to be either solved approximately or by testing the corresponding models in special hydrodynamic tubes. To move from the results obtained on the model to the phenomenon in nature, the so-called similarity theory is used. The Reynolds number is one of the main criteria for the similarity of the flow of a viscous fluid. Therefore, its definition is practically very important. In this work, a transition from laminar flow to turbulent flow is observed and several values of the Reynolds number are determined: in the laminar flow region, in the transition region (critical flow) and in turbulent flow.
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