Preliminary calculation of the nozzle heating surface.
Q in \u003d V in * (i in // - i in /) * τ \u003d 232231.443 * (2160-111.3) * 0.7 \u003d 333.04 * 10 6 kJ / cycle.
Mean logarithmic temperature difference per cycle.
Velocity of combustion products (smoke) =2.1 m/s. Then the air speed under normal conditions:
6.538 m/s
Average air and smoke temperatures for the period.
935 o C
680 o C
The average temperature of the top of the nozzle in the smoke and air periods
Average tip temperature per cycle
The average temperature of the bottom of the nozzle in the smoke and air periods:
Average nozzle bottom temperature per cycle
We determine the value of the heat transfer coefficients for the top and bottom of the nozzle. For the nozzle of the accepted type at a value of 2240
The actual smoke speed is determined by the formula W d \u003d W to * (1 + βt d). The actual air velocity at temperature t in and air pressure p in \u003d 0.355 MN / m 2 (absolute) is determined by the formula
Where 0.1013-MN / m 2 - pressure under normal conditions.
The value of the kinematic viscosity ν and the coefficient of thermal conductivity λ for combustion products are selected from the tables. At the same time, we take into account that the value of λ depends very little on pressure, and at a pressure of 0.355 MN/m 2, the values of λ at a pressure of 0.1013 MN/m 2 can be used. The kinematic viscosity of gases is inversely proportional to pressure; we divide this value of ν at a pressure of 0.1013 MN / m 2 by the ratio.
Effective beam length for block nozzle
= 0.0284 m
For this nozzle m 2 / m 3; ν \u003d 0.7 m 3 / m 3; m 2 / m 2.
Calculations are summarized in table 3.1
Table 3.1 - Determination of heat transfer coefficients for the top and bottom of the nozzle.
| Name, value and units of measurements | Calculation formula | Advance paynemt | Refined calculation | ||||
| top | bottom | top | Bottom | ||||
| smoke | air | smoke | air | air | air | ||
| Average air and smoke temperatures for the period 0 C | According to the text | 1277,5 | 592,5 | 1026,7 | 355,56 | ||
| Thermal conductivity coefficient of combustion products and air l 10 2 W / (mgrad) | According to the text | 13,405 | 8,101 | 7,444 | 5,15 | 8,18 | 5,19 |
| Kinematic viscosity of combustion products and air g 10 6 m 2 / s | Appendix | 236,5 | 52,6 | 92,079 | 18,12 | 53,19 | 18,28 |
| Determining channel diameter d, m | 0,031 | 0,031 | 0,031 | 0,031 | 0,031 | 0,031 | |
| Actual smoke and air velocity W m/s | According to the text | 11,927 | 8,768 | 6,65 | 4,257 | 8,712 | 4,213 |
| Re | |||||||
| Nu | According to the text | 12,425 | 32,334 | 16,576 | 42,549 | 31,88 | 41,91 |
| Convection heat transfer coefficient a to W / m 2 * deg | 53,73 | 84,5 | 39,804 | 70,69 | 84,15 | 70,226 | |
| 0,027 | - | 0,045 | - | - | - | ||
| 1,005 | - | 1,055 | - | - | - | ||
| Radiant heat transfer coefficient a p W / m 2 * deg | 13,56 | - | 5,042 | - | - | - | |
| a W / m 2 * deg | 67,29 | 84,5 | 44,846 | 70,69 | 84,15 | 70,226 |
The heat capacity and thermal conductivity of brick l nozzles are calculated by the formulas:
C, kJ / (kg * deg) l , W / (m deg)
Dinas 0.875+38.5*10 -5 *t 1.58+38.4*10 -5 t
Fireclay 0.869 + 41.9 * 10 -5 * t 1.04 + 15.1 * 10 -5 t
The equivalent half-thickness of a brick is determined by the formula
mm
Table 3.2 - Physical quantities of the material and the coefficient of heat accumulation for the upper and lower half of the regenerative nozzle
| Name of sizes | Calculation formula | Advance paynemt | Refined calculation | |||
| top | bottom | top | Bottom | |||
| dinas | fireclay | dinas | fireclay | |||
| Average temperature, 0 C | According to the text | 1143,75 | 471,25 | 1152,1 | 474,03 | |
| Bulk density, r kg / m 3 | According to the text | |||||
| Thermal conductivity coefficient l W/(mgrad) | According to the text | 2,019 | 1,111 | 2,022 | 1,111 | |
| Heat capacity С, kJ/(kg*deg) | According to the text | 1,315 | 1,066 | 1,318 | 1,067 | |
| Thermal diffusivity a, m 2 / hour | 0,0027 | 0,0018 | 0,0027 | 0,0018 | ||
| F 0 S | 21,704 | 14,59 | 21,68 | 14,58 | ||
| Heat accumulation coefficient h to |  | 0,942 | 0,916 | 0,942 | 0,916 | |
As is obvious from the table, the value of h to >, i.e. the bricks are used thermally throughout its entire thickness. Accordingly, to the above compiled, we accept the value of the thermal hysteresis coefficient for the top of the nozzle x=2.3, for the bottom x=5.1.
Then the total heat transfer coefficient is calculated by the formula:
for the top of the nozzle
58.025 kJ / (m 2 cycle * deg)
for the bottom of the nozzle
60.454 kJ / (m 2 cycle * deg)
Average for the nozzle as a whole
59.239 kJ / (m 2 cycle * deg)
Nozzle heating surface
22093.13 m2
Nozzle volume
= 579.87 m 3
The area of the horizontal section of the nozzle in the clear
\u003d 9.866 m 2
Remember
- What instrument is used to measure air temperature? What kinds of rotation of the Earth do you know? Why does the day and night cycle occur on Earth?
How does the earth's surface and atmosphere heat up? The sun radiates a huge amount of energy. However, the atmosphere transmits only half of the sun's rays to the earth's surface. Some of them are reflected, some are absorbed by clouds, gases and dust particles (Fig. 83).
Rice. 83. Consumption of solar energy coming to Earth
When the sun's rays pass through, the atmosphere from them almost does not heat up. As the earth's surface heats up, it becomes a heat source itself. It is from it that the atmospheric air is heated. Therefore, the air in the troposphere is warmer near the earth's surface than at altitude. When climbing up, every kilometer, the air temperature drops by 6 "C. High in the mountains, due to the low temperature, the accumulated snow does not melt even in summer. The temperature in the troposphere changes not only with height, but also during certain periods of time: days, years.
Differences in air heating during the day and year. During the day, the sun's rays illuminate earth's surface and warm it up, it heats up the air. At night, the flow of solar energy stops, and the surface, along with the air, gradually cools.
The sun is highest above the horizon at noon. This is the time when the most solar energy comes in. However, the highest temperature is observed after 2-3 hours after noon, since it takes time for heat to transfer from the Earth's surface to the troposphere. The lowest temperature is before sunrise.
The air temperature also changes with the seasons. You already know that the Earth moves around the Sun in an orbit and earth's axis permanently inclined to the plane of the orbit. Because of this, during the year in the same area, the sun's rays fall on the surface in different ways.
When the angle of incidence of the rays is steeper, the surface receives more solar energy, the air temperature rises and summer comes (Fig. 84).
Rice. 84. The fall of the sun's rays on the earth's surface at noon on June 22 and December 22
When the sun's rays are more tilted, the surface heats up slightly. The air temperature at this time drops, and winter comes. Most warm month in the Northern Hemisphere - July, and the coldest - January. In the Southern Hemisphere, on the contrary: the most cold month year - July, and the warmest - January.
From the figure, determine how the angle of incidence of the sun's rays differs on June 22 and December 22 at parallels of 23.5 ° N. sh. and yu. w .; at the parallels of 66.5° N. sh. and yu. sh.
Think about why the warmest and coldest months are not June and December, when the sun's rays have the largest and smallest angles of incidence on the earth's surface.
Rice. 85. Average annual air temperatures of the Earth
Indicators of temperature changes. To identify the general patterns of temperature changes, an indicator of average temperatures is used: average daily, average monthly, average annual (Fig. 85). For example, to calculate the average daily temperature during the day, the temperature is measured several times, these indicators are summed up, and the resulting amount is divided by the number of measurements.
Define:
- average daily temperature according to four measurements per day: -8°C, -4°C, +3°C, +1°C;
- the average annual temperature of Moscow using the table data.
Table 4
Determining the change in temperature, usually note its highest and lowest rates.
The difference between the highest and lowest readings is called the temperature range.
The amplitude can be determined for a day (daily amplitude), month, year. For example, if highest temperature per day is + 20 ° С, and the smallest - + 8 ° С, then the daily amplitude will be 12 ° С (Fig. 86).
Rice. 86. Daily temperature range
Determine how many degrees the annual amplitude in Krasnoyarsk is greater than in St. Petersburg, if the average temperature in July in Krasnoyarsk is +19°С, and in January it is -17°С; in St. Petersburg +18°C and -8°C respectively.
On maps, the distribution of average temperatures is reflected using isotherms.
Isotherms are lines connecting points with the same average temperature air for a certain period of time.
Usually show isotherms of the warmest and coldest months of the year, i.e. July and January.
Questions and tasks
- How is air heated in the atmosphere?
- How does the air temperature change during the day?
- What determines the difference in the heating of the Earth's surface during the year?
Research carried out at the turn of the 1940s-1950s made it possible to develop a number of aerodynamic and technological solutions that ensure the safe overcoming of the sound barrier even by serial aircraft. Then it seemed that the conquest of the sound barrier creates unlimited possibilities for a further increase in flight speed. In just a few years, about 30 types of supersonic aircraft were flown, of which a significant number were put into mass production.
The variety of solutions used has led to the fact that many problems associated with flights at high supersonic speeds have been comprehensively studied and solved. However, new problems were encountered, much more complex than the sound barrier. They are caused by the heating of the structure. aircraft when flying at high speed in dense layers of the atmosphere. This new obstacle was once called the thermal barrier. Unlike the sound barrier, the new barrier cannot be characterized by a constant similar to the speed of sound, since it depends both on flight parameters (speed and altitude) and airframe design (constructive solutions and materials used), and on aircraft equipment (air conditioning, cooling systems, etc.). P.). Thus, the concept of "thermal barrier" includes not only the problem of dangerous heating of the structure, but also issues such as heat transfer, strength properties of materials, design principles, air conditioning, etc.
The heating of the aircraft in flight occurs mainly for two reasons: from the aerodynamic braking of the air flow and from the heat release of the propulsion system. Both of these phenomena constitute the process of interaction between the medium (air, exhaust gases) and a streamlined solid body (aircraft, engine). The second phenomenon is typical for all aircraft, and it is associated with an increase in the temperature of engine structural elements that receive heat from the air compressed in the compressor, as well as from combustion products in the chamber and exhaust pipe. When flying at high speeds, the internal heating of the aircraft also occurs from the air decelerating in the air channel in front of the compressor. When flying at low speeds, the air passing through the engine has a relatively low temperature, as a result of which dangerous heating of the airframe structural elements does not occur. At high flight speeds, the heating of the airframe structure from hot engine elements is limited by additional cooling with low-temperature air. Typically, air is used that is removed from the air intake using a guide separating the boundary layer, as well as air captured from the atmosphere using additional intakes located on the surface of the engine nacelle. In two-circuit engines, air from the external (cold) circuit is also used for cooling.
Thus, the level of the thermal barrier for supersonic aircraft is determined by external aerodynamic heating. The intensity of heating of the surface flowed around by the air flow depends on the flight speed. At low speeds, this heating is so insignificant that the increase in temperature can be ignored. At high speed, the air flow has a high kinetic energy, and therefore the temperature increase can be significant. This also applies to the temperature inside the aircraft, since the high-speed flow, stagnant in the air intake and compressed in the engine compressor, becomes so high that it is unable to remove heat from the hot parts of the engine.
The increase in the temperature of the aircraft skin as a result of aerodynamic heating is caused by the viscosity of the air flowing around the aircraft, as well as its compression on the frontal surfaces. Due to the loss of speed by air particles in the boundary layer as a result of viscous friction, the temperature of the entire streamlined surface of the aircraft increases. As a result of air compression, the temperature rises, however, only locally (mainly the nose of the fuselage, the windshield of the cockpit, and especially the leading edges of the wing and plumage), but more often reaches values that are unsafe for the structure. In this case, in some places there is an almost direct collision of the air flow with the surface and full dynamic braking. In accordance with the principle of conservation of energy, all the kinetic energy of the flow is converted into heat and pressure energy. The corresponding temperature rise is directly proportional to the square of the flow velocity before braking (or, without wind, to the square of the aircraft speed) and inversely proportional to the flight altitude.
Theoretically, if the flow around is steady, the weather is calm and cloudless, and there is no heat transfer through radiation, then heat does not penetrate into the structure, and the skin temperature is close to the so-called adiabatic stagnation temperature. Its dependence on the Mach number (speed and flight altitude) is given in Table. 4.
Under actual conditions, the increase in the temperature of the aircraft skin from aerodynamic heating, i.e., the difference between the stagnation temperature and the ambient temperature, turns out to be somewhat smaller due to heat exchange with the environment (by radiation), neighboring structural elements, etc. In addition, the flow is completely decelerated only at the so-called critical points located on the protruding parts of the aircraft, and the heat influx to the skin also depends on the nature of the boundary layer of air (it is more intense for a turbulent boundary layer). A significant decrease in temperature also occurs when flying through clouds, especially when they contain supercooled water drops and ice crystals. For such flight conditions, it is assumed that the decrease in the skin temperature at the critical point compared to the theoretical stagnation temperature can reach even 20-40%.
Table 4. Dependence of the skin temperature on the Mach number
Nevertheless, the overall heating of the aircraft in flight at supersonic speeds (especially at low altitude) is sometimes so high that an increase in the temperature of individual elements of the airframe and equipment leads either to their destruction, or, at least, to the need to change the flight mode. For example, during studies of the XB-70A aircraft in flights at altitudes of more than 21,000 m at a speed of M = 3, the temperature of the leading edges of the air intake and the leading edges of the wing was 580-605 K, and the rest of the skin was 470-500 K. Consequences of increasing the temperature of aircraft structural elements Such high values can be fully estimated if we take into account the fact that already at temperatures of about 370 K, organic glass, which is widely used for glazing cabins, softens, fuel boils, and ordinary glue loses its strength. At 400 K, the strength of duralumin is significantly reduced, at 500 K, the chemical decomposition of the working fluid in the hydraulic system and the destruction of seals occur, at 800 K, titanium alloys lose the necessary mechanical properties, at temperatures above 900 K, aluminum and magnesium melt, and steel softens. An increase in temperature also leads to the destruction of coatings, of which anodizing and chromium plating can be used up to 570 K, nickel plating up to 650 K, and silver plating up to 720 K.
After the appearance of this new obstacle in increasing the speed of flight, research began to eliminate or mitigate its consequences. Ways to protect the aircraft from the effects of aerodynamic heating are determined by factors that prevent the temperature rise. In addition to the flight altitude and atmospheric conditions, the degree of heating of the aircraft is significantly affected by:
is the coefficient of thermal conductivity of the sheathing material;
- the size of the surface (especially the frontal) of the aircraft; -flight time.
It follows that the simplest ways to reduce the heating of the structure are to increase the flight altitude and limit its duration to a minimum. These methods were used in the first supersonic aircraft (especially experimental ones). Due to the rather high thermal conductivity and heat capacity of the materials used to manufacture heat-stressed aircraft structural elements, from the moment the aircraft reaches high speed to the moment individual structural elements are heated to the design temperature of the critical point, it usually takes quite a long time. big time. In flights lasting several minutes (even at low altitudes), destructive temperatures are not reached. Flight at high altitudes takes place under conditions of low temperature (about 250 K) and low air density. As a result, the amount of heat given off by the flow to the surfaces of the aircraft is small, and the heat exchange takes longer, which greatly alleviates the severity of the problem. A similar result is obtained by limiting the speed of the aircraft at low altitudes. For example, during a flight over the ground at a speed of 1600 km / h, the strength of duralumin decreases by only 2%, and an increase in speed to 2400 km / h leads to a decrease in its strength by up to 75% compared to the initial value.
Rice. 1.14. Temperature distribution in the air duct and in the engine of the Concord aircraft during flight with M = 2.2 (a) and the temperature of the skin of the XB-70A aircraft during flight at a constant speed of 3200 km/h (b).
However, the need to ensure safe operating conditions over the entire range of used speeds and flight altitudes forces designers to look for appropriate technical means. Since the heating of aircraft structural elements causes a decrease in the mechanical properties of materials, the occurrence of thermal stresses on the structure, as well as deterioration in the working conditions of the crew and equipment, such technical means used in current practice can be divided into three groups. They respectively include the use of 1) heat-resistant materials, 2) design solutions that provide the necessary thermal insulation and allowable deformation of parts, and 3) cooling systems for the cockpit and equipment compartments.
In aircraft with a maximum speed of M = 2.0-1-2.2, aluminum alloys (duralumin) are widely used, which are characterized by relatively high strength, low density and retention of strength properties with a slight increase in temperature. Durals are usually supplemented with steel or titanium alloys, from which the parts of the airframe that are subjected to the greatest mechanical or thermal loads are made. Titanium alloys were used already in the first half of the 50s, at first on a very small scale (now details from them can be up to 30% of the weight of the airframe). In experimental aircraft with M ~ 3, it becomes necessary to use heat-resistant steel alloys as the main structural material. Such steels retain good mechanical properties at high temperatures typical for flights at hypersonic speeds, but their disadvantages are high cost and high density. These shortcomings in a certain sense limit the development of high-speed aircraft, so other materials are also being researched.
In the 1970s, the first experiments were made on the use of beryllium in aircraft construction, as well as composite materials based on boron or carbon fibers. These materials still have a high cost, but at the same time they are characterized by low density, high strength and rigidity, as well as significant heat resistance. Examples of specific applications of these materials in the construction of the airframe are given in the descriptions of individual aircraft.
Another factor that significantly affects the performance of a heated aircraft structure is the effect of so-called thermal stresses. They arise as a result of temperature differences between the outer and inner surfaces of the elements, and especially between the skin and the internal structural elements of the aircraft. Surface heating of the airframe leads to deformation of its elements. For example, warping of the wing skin may occur in such a way that it will lead to a change in aerodynamic characteristics. Therefore, many aircraft use brazed (sometimes glued) multilayer skin, which is characterized by high rigidity and good insulating properties, or internal structural elements with appropriate expansion joints are used (for example, in the F-105 aircraft, the spar walls are made of corrugated sheet). Experiments are also known for cooling the wing with fuel (for example, in the X-15 aircraft) flowing under the skin on the way from the tank to the combustion chamber nozzles. However, at high temperatures, the fuel usually undergoes coking, so such experiments can be considered unsuccessful.
Currently, various methods are being investigated, among which is the application of an insulating layer of refractory materials by plasma spraying. Other methods considered promising have not found application. Among other things, it was proposed to use a "protective layer" created by blowing gas onto the skin, "sweating" cooling by supplying a liquid with a high evaporation temperature to the surface through the porous skin, as well as cooling created by melting and entraining part of the skin (ablative materials).
A rather specific and at the same time very important task is to maintain the appropriate temperature in the cockpit and in the equipment compartments (especially electronic), as well as the temperature of the fuel and hydraulic systems. Currently, this problem is being solved by using high-performance air conditioning, cooling and refrigeration systems, effective thermal insulation, the use of hydraulic fluids with a high evaporation temperature, etc.
The problems associated with the thermal barrier must be addressed comprehensively. Any progress in this area removes the barrier to of this type aircraft in the direction of higher flight speed, without excluding it as such. However, the desire for even greater speeds leads to the creation of even more complex structures and equipment requiring the use of higher quality materials. This has a noticeable effect on the weight, purchase price, and the cost of operating and maintaining the aircraft.
From those given in table. 2 of these fighter aircraft shows that in most cases the maximum speed of 2200-2600 km / h was considered rational. Only in some cases is it believed that the speed of the aircraft should exceed M ~ 3. Aircraft capable of developing such speeds include the experimental X-2, XB-70A and T. 188 machines, the reconnaissance SR-71, and the E-266 aircraft.
1* Refrigeration is the forced transfer of heat from a cold source to a high-temperature environment with artificial opposition to the natural direction of heat movement (from a warm body to a cold one when the cooling process takes place). The simplest refrigerator is a household refrigerator.
All life processes on Earth are caused by thermal energy. The main source from which the Earth receives thermal energy is the Sun. It radiates energy in the form of various beams - electromagnetic waves. The radiation of the Sun in the form of electromagnetic waves propagating at a speed of 300,000 km / s is called, which consists of rays of various lengths that carry light and heat to the Earth.
Radiation can be direct or diffuse. If there were no atmosphere, the earth's surface would receive only direct radiation. Therefore, radiation that comes directly from the Sun in the form of direct sunlight and with a cloudless sky is called direct. She carries the largest number warmth and light. But, passing through the atmosphere, the sun's rays are partially scattered, deviate from the direct path as a result of reflection from air molecules, water droplets, dust particles and turn into rays going in all directions. Such radiation is called diffuse. Therefore, it is also light in those places where direct sunlight (direct radiation) does not penetrate (forest canopy, shady side of rocks, mountains, buildings, etc.). Scattered radiation also determines the color of the sky. All solar radiation coming to the earth's surface, i.e. direct and scattered, called the total. The earth's surface, absorbing solar radiation, heats up and itself becomes a source of heat radiation into the atmosphere. It is called terrestrial radiation, or terrestrial radiation, and is largely delayed by the lower layers of the atmosphere. The solar radiation absorbed by the earth's surface is spent on heating water, soil, air, evaporation and radiation into the atmosphere. Earthy, not defining temperature regime troposphere, i.e. the sun's rays passing through everything do not heat it. The largest amount of heat is received and heated to the highest temperatures by the lower layers of the atmosphere, directly adjacent to the heat source - the earth's surface. As you move away from the earth's surface, the heating weakens. That is why in the troposphere, with height, an average of 0.6 ° C decreases for every 100 m of ascent. This general pattern for the troposphere. There are times when the overlying layers of air are warmer than the underlying ones. This phenomenon is called temperature inversion.
The heating of the earth's surface differs significantly not only in height. The amount of total solar radiation directly depends on the angle of incidence of the sun's rays. The closer this value is to 90°, the more solar energy the earth's surface receives.
In turn, the angle of incidence of the sun's rays on a certain point on the earth's surface is determined by its geographical latitude. The strength of direct solar radiation depends on the length of the path that the sun's rays travel through the atmosphere. When the Sun is at its zenith (near the equator), its rays fall vertically on the earth's surface, i.e. overcome the atmosphere in the shortest way (at 90 °) and intensively give up their energy to a small area. As you move away from the equatorial zone to the south or north, the length of the path of the sun's rays increases, i.e. the angle of their incidence on the earth's surface decreases. More and more, the rays begin to slide along the Earth, as it were, and approach the tangent line in the region of the poles. In this case, the same beam of energy is scattered over a larger area, and the amount of reflected energy increases. Thus, where the sun's rays fall on the earth's surface at an angle of 90 °, they are constantly high, and as they move towards the poles, it becomes progressively colder. It is at the poles, where the sun's rays fall at an angle of 180 ° (i.e., tangentially), that there is the least amount of heat.
Such an uneven distribution of heat on the Earth, depending on the latitude of the place, makes it possible to distinguish five thermal zones: one hot, two and two cold.
The conditions for heating water and land by solar radiation are very different. The heat capacity of water is twice that of land. This means that with the same amount of heat, land heats up twice as fast as water, and when it cools, the opposite happens. In addition, water evaporates when heated, which consumes a considerable amount of heat. On land, heat is concentrated only in its upper layer, only a small part of it is transferred to the depth. In water, the rays immediately heat up a significant thickness, which is also facilitated by the vertical mixing of water. As a result, water accumulates heat much more than land, retains it longer and spends it more evenly than land. It heats up slower and cools down slower.
The surface of the land is not uniform. Its heating depends to a large extent on physical properties soils and, ice, exposure (the angle of inclination of land areas in relation to the incident sun's rays) slopes. Features of the underlying surface determine the different nature of the change in air temperatures during the day and year. The lowest air temperatures during the day on land are observed shortly before sunrise (no influx of solar radiation and strong terrestrial radiation at night). The highest - in the afternoon (14-15 hours). During the year in the Northern Hemisphere, the most high temperatures air on land are observed in July, and the lowest - in January. Above the water surface, the daily maximum air temperature is shifted and is observed at 15-16 hours, and the minimum is 2-3 hours after sunrise. The annual maximum (in the Northern Hemisphere) is in August, and the minimum is in February.
Aerodynamic heating heating of bodies moving at high speed in air or other gas. A. n. - the result of the fact that air molecules incident on the body are decelerated near the body. If the flight is made at the supersonic speed of cultures, braking occurs primarily in the shock wave (See shock wave) ,
occurring in front of the body. Further deceleration of air molecules occurs directly at the very surface of the body, in
boundary layer (See boundary layer). When air molecules decelerate, their thermal energy increases, i.e., the temperature of the gas near the surface of the moving body increases, the maximum temperature to which the gas can be heated in the vicinity of the moving body is close to the so-called. braking temperature: T 0 = T n + v 2 /2c p ,
where T n - incoming air temperature, v - body flight speed cp is the specific heat capacity of the gas at constant pressure. So, for example, when flying a supersonic aircraft at three times the speed of sound (about 1 km/sec) the stagnation temperature is about 400°C, and when the spacecraft enters the Earth’s atmosphere with the 1st cosmic velocity (8.1 km/s) the stagnation temperature reaches 8000 °C. If in the first case, during a sufficiently long flight, the temperature of the aircraft skin reaches values close to the stagnation temperature, then in the second case, the surface of the spacecraft will inevitably begin to collapse due to the inability of the materials to withstand such high temperatures. Heat is transferred from regions of a gas with an elevated temperature to a moving body, and aerodynamic heating occurs. There are two forms A. n. - convective and radiation. Convective heating is a consequence of heat transfer from the outer, "hot" part of the boundary layer to the surface of the body. Quantitatively, the convective heat flux is determined from the ratio q k = a(T e -T w), where T e - equilibrium temperature (the limiting temperature to which the surface of the body could be heated if there was no energy removal), T w - actual surface temperature, a- coefficient of convective heat transfer, depending on the speed and altitude of the flight, the shape and size of the body, as well as other factors. The equilibrium temperature is close to the stagnation temperature. Type of coefficient dependence a from the listed parameters is determined by the flow regime in the boundary layer (laminar or turbulent). In the case of turbulent flow, convective heating becomes more intense. This is due to the fact that, in addition to molecular thermal conductivity, turbulent velocity fluctuations in the boundary layer begin to play a significant role in energy transfer. As the flight speed increases, the air temperature behind the shock wave and in the boundary layer increases, resulting in dissociation and ionization.
molecules. The resulting atoms, ions and electrons diffuse into a colder region - to the surface of the body. There is a back reaction (recombination) ,
going with the release of heat. This makes an additional contribution to the convective A. n. Upon reaching the flight speed of about 5000 m/s the temperature behind the shock wave reaches values at which the gas begins to radiate. Due to the radiant transfer of energy from areas with elevated temperature to the surface of the body, radiative heating occurs. In this case, radiation in the visible and ultraviolet regions of the spectrum plays the greatest role. When flying in the Earth's atmosphere at speeds below the first space speed (8.1 km/s) radiative heating is small compared to convective heating. At the second space velocity (11.2 km/s)
their values become close, and at flight speeds of 13-15 km/s and higher, corresponding to the return to Earth after flights to other planets, the main contribution is made by radiative heating.
A particularly important role of A. n. plays when spacecraft return to the Earth's atmosphere (for example, Vostok, Voskhod, Soyuz). To combat A. n. spacecraft are equipped with special thermal protection systems (see Thermal protection). Lit .: Fundamentals of heat transfer in aviation and rocket technology, M., 1960; Dorrens W. Kh., Hypersonic flows of viscous gas, transl. from English, M., 1966; Zeldovich Ya. B., Raizer Yu. P., Physics of shock waves and high-temperature hydrodynamic phenomena, 2nd ed., M., 1966. N. A. Anfimov.
Big Soviet encyclopedia. - M .: Soviet encyclopedia. 1969-1978 .
See what "Aerodynamic heating" is in other dictionaries:
Heating of bodies moving at high speed in air or other gas. A. n. the result of the fact that air molecules incident on the body are decelerated near the body. If the flight is made with supersonic. speed, braking occurs primarily in shock ... ... Physical encyclopedia
Heating of a body moving at high speed in air (gas). Noticeable aerodynamic heating is observed when the body moves at supersonic speed (for example, when the warheads of intercontinental ballistic missiles) EdwART. ... ... Marine Dictionary
aerodynamic heating- Heating of the surface of a body streamlined with gas, moving in a gaseous medium at high speed in the presence of convective, and in the presence of hypersonic speeds and radiative heat exchange with the gaseous medium in the boundary or shock layer. [GOST 26883… … Technical translator's guide
An increase in the temperature of a body moving at high speed in air or other gas. Aerodynamic heating is the result of deceleration of gas molecules near the surface of the body. So, when a spacecraft enters the Earth's atmosphere at a speed of 7.9 km / s ... ... encyclopedic Dictionary
aerodynamic heating- aerodinaminis įšilimas statusas T sritis Energetika apibrėžtis Kūnų, judančių dujose (ore) dideliu greičiu, paviršiaus įšilimas. atitikmenys: angl. aerodynamic heating vok. aerodynamische Aufheizung, f rus. aerodynamic heating, m pranc.… … Aiškinamasis šiluminės ir branduolinės technikos terminų žodynas- an increase in the temperature of a body moving at high speed in air or other gas. A. i. the result of deceleration of gas molecules near the surface of the body. So, at the entrance of the cosmic. apparatus into the Earth's atmosphere at a speed of 7.9 km / s, the rate of air at the surface pa ... Natural science. encyclopedic Dictionary
Aerodynamic heating of the rocket structure- Heating of the surface of the rocket during its movement in dense layers of the atmosphere at high speed. A.n. - the result of the fact that air molecules incident on a rocket are decelerated near its body. In this case, the transfer of kinetic energy occurs ... ... Encyclopedia of the Strategic Missile Forces
Concorde Concorde at the airport ... Wikipedia
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