- devices used for heating air in supply ventilation systems, air conditioning systems, air heating as well as in dryers.
According to the type of coolant, heaters can be fire, water, steam and electric. .
The most widespread at present are water and steam heaters, which are divided into smooth-tube and ribbed ones; the latter, in turn, are divided into lamellar and spiral-wound.
Distinguish between single-pass and multi-pass heaters. In single-pass, the coolant moves through the tubes in one direction, and in multi-pass, it changes the direction of movement several times due to the presence of partitions in the collector covers (Fig. XII.1).
Heaters perform two models: medium (C) and large (B).
The heat consumption for heating the air is determined by the formulas:
 |
where Q"— heat consumption for air heating, kJ/h (kcal/h); Q- the same, W; 0.278 is the conversion factor from kJ/h to W; G- mass amount of heated air, kg / h, equal to Lp [here L- volumetric amount of heated air, m 3 / h; p is the air density (at a temperature tK), kg / m 3]; with- specific heat capacity of air, equal to 1 kJ / (kg-K); t k - air temperature after the heater, ° С; t n— air temperature before the air heater, °C.
For heaters of the first stage of heating, the temperature tn is equal to the temperature of the outside air.
The outdoor air temperature is assumed to be equal to the calculated ventilation temperature (category A climate parameters) when designing general ventilation designed to combat excess moisture, heat and gases, the MPC of which is more than 100 mg / m3. When designing general ventilation designed to combat gases whose MPC is less than 100 mg / m3, as well as when designing supply ventilation to compensate for air removed through local exhausts, process hoods or pneumatic transport systems, the outside air temperature is assumed to be equal to the calculated outside temperature tn for heating design (climate parameters category B).
In a room without heat surpluses, supply air with a temperature equal to the indoor air temperature tВ for this room should be supplied. In the presence of excess heat, the supply air is supplied at a reduced temperature (by 5-8 ° C). Supply air with a temperature below 10°C is not recommended to be supplied to the room even in the presence of significant heat emissions due to the possibility of colds. The exception is the use of special anemostats.
The required surface area for heating heaters Fк m2, is determined by the formula:
where Q— heat consumption for air heating, W (kcal/h); To- heat transfer coefficient of the heater, W / (m 2 -K) [kcal / (h-m 2 - ° C)]; t cf.T. — average temperature coolant, 0 С; t r.v. is the average temperature of the heated air passing through the heater, °C, equal to (t n + t c)/2.
If the coolant is steam, then the average temperature of the coolant tav.T. is equal to the saturation temperature at the corresponding vapor pressure.
For water temperature tav.T. is defined as the arithmetic mean of the hot and return water temperatures:
The safety factor 1.1-1.2 takes into account the heat loss for air cooling in the air ducts.
The heat transfer coefficient of heaters K depends on the type of coolant, the mass velocity of air movement vp through the heater, the geometric dimensions and design features of the heaters, the speed of water movement through the tubes of the heater.
The mass velocity is understood as the mass of air, kg, passing through 1 m2 of the living section of the air heater in 1 s. Mass velocity vp, kg/(cm2), is determined by the formula
According to the area of the open section fЖ and the heating surface FK, the model, brand and number of heaters are selected. After choosing the heaters, the mass air velocity is specified according to the actual area of the open section of the heater fD of this model:
where A, A 1 , n, n 1 and t- coefficients and exponents, depending on the design of the heater
The speed of water movement in the heater tubes ω, m/s, is determined by the formula:
where Q "is the heat consumption for heating air, kJ / h (kcal / h); rp is the density of water, equal to 1000 kg / m3, sv is the specific heat of water, equal to 4.19 kJ / (kg-K); fTP - open area for coolant passage, m2, tg — temperature hot water in the supply line, ° С; t 0 - return water temperature, 0С.
The heat transfer of heaters is affected by the scheme of tying them with pipelines. With a parallel scheme for connecting pipelines, only part of the coolant passes through a separate heater, and with a sequential scheme, the entire flow of the coolant passes through each heater.
The resistance of heaters to the passage of air p, Pa, is expressed by the following formula:
where B and z are the coefficient and exponent, which depend on the design of the heater.
The resistance of the heaters located in series is equal to:
where m is the number of successively located heaters. The calculation ends with a check of the heat output (heat transfer) of the heaters according to the formula
where QK - heat transfer of heaters, W (kcal / h); QK - the same, kJ/h, 3.6 - conversion factor W to kJ/h FK - heating surface area of heaters, m2, taken as a result of calculation of heaters of this type; K - heat transfer coefficient of heaters, W/(m2-K) [kcal/(h-m2-°C)]; tav.v - the average temperature of the heated air passing through the heater, °C; tav. T is the average temperature of the coolant, °С.
When selecting heaters, the margin for the estimated heating surface area is taken in the range of 15 - 20%, for the resistance to air passage - 10% and for the resistance to water movement - 20%.
2005-08-16In a number of cases, it is possible to significantly reduce capital and operating costs by providing autonomous heating of premises with warm air based on the use of heat generators running on gas or liquid fuel. In such units, it is not water that is heated, but air - fresh supply, recirculation or mixed. This method is especially effective for providing autonomous heating of industrial premises, exhibition pavilions, workshops, garages, service stations, car washes, film studios, warehouses, public buildings, gyms, supermarkets, greenhouses, greenhouses, livestock complexes, poultry farms, etc.
Advantages of air heating
There are many advantages of the air heating method over the traditional water heating method in large rooms, we list only the main ones:
- Profitability. Heat is produced directly in the heated room and is almost entirely consumed for its intended purpose. Thanks to direct combustion of fuel without an intermediate heat carrier, a high thermal efficiency of the entire heating system is achieved: 90-94% for recuperative heaters and almost 100% for direct heating systems. The use of programmable thermostats provides the possibility of additional savings from 5 to 25% of thermal energy due to the "standby mode" function - automatically maintaining the temperature in the room work time at the level of + 5-7 ° С.
- The ability to "turn on" the supply ventilation. It's no secret that today, in most enterprises, the supply ventilation does not work properly, which significantly worsens the working conditions of people and affects labor productivity. Heat generators or direct heating systems warm up the air by ∆t up to 90°C - this is quite enough to “make” the supply ventilation work even in the conditions of the Far North. Thus, air heating implies not only economic efficiency, but also an improvement environmental situation and working conditions.
- Little inertia. Units of air heating systems enter the operating mode in a matter of minutes, and due to the high air turnover, the room is completely warmed up in just a few hours. This makes it possible to quickly and flexibly maneuver when heat needs change.
- The absence of an intermediate heat carrier makes it possible to abandon the construction and maintenance of a water heating system that is inefficient for large premises, a boiler house, heating mains and a water treatment plant. Losses in heating mains and their repair are excluded, which makes it possible to drastically reduce operating costs. In winter, there is no risk of defrosting the heaters and the heating system in the event of a prolonged shutdown of the system. Cooling even to a deep "minus" does not lead to defrosting of the system.
- A high degree of automation allows you to generate exactly the amount of heat that is needed. In combination with the high reliability of gas equipment, this significantly increases the safety of the heating system, and a minimum of maintenance personnel is sufficient for its operation.
- Small costs. The method of heating large rooms with the help of heat generators is one of the cheapest and most quickly implemented. The capital costs of building or refurbishing an air system are generally much lower than those of hot water or radiant heating. The payback period for capital expenditures usually does not exceed one or two heating seasons.
Depending on the tasks to be solved, heaters of various types can be used in air heating systems. In this article, we will consider only units that operate without the use of an intermediate heat carrier - recuperative air heaters (with a heat exchanger and removal of combustion products to the outside) and direct air heating systems (gas mixing air heaters).
Recuperative air heaters
In units of this type, fuel mixed with the required amount of air is supplied by the burner to the combustion chamber. The resulting combustion products pass through a two- or three-way heat exchanger. The heat obtained during the combustion of the fuel is transferred to the heated air through the walls of the heat exchanger, and the flue gases are discharged through the chimney to the outside (Fig. 1) - that is why they are called "indirect heating" heat generators.
Recuperative air heaters can be used not only directly for heating, but also as part of a supply ventilation system, as well as for process air heating. The rated thermal power of such systems is from 3 kW to 2 MW. The heated air is supplied to the room through a built-in or remote blower, which makes it possible to use the units both for direct air heating with its delivery through louvered grilles, and with air ducts.
Washing the combustion chamber and the heat exchanger, the air is heated and sent either directly to the heated room through the louvered air distribution grilles located in the upper part, or distributed through the air duct system. An automated block burner is located on the front part of the heat generator (Fig. 2).
The heat exchangers of modern air heaters, as a rule, are made of stainless steel (the furnace is made of heat-resistant steel) and serve from 5 to 25 years, after which they can be repaired or replaced. The efficiency of modern models reaches 90-96%. The main advantage of recuperative air heaters is their versatility.
They can run on natural or liquefied gas, diesel fuel, oil, fuel oil or waste oil - you just have to change the burner. It is possible to work with fresh air, with an admixture of internal and in full recirculation mode. Such a system allows some liberties, for example, to change the flow of heated air, to redistribute the heated air flows into different branches of the air ducts “on the go” using special valves.
In summer, recuperative air heaters can operate in ventilation mode. The units are mounted both in a vertical and horizontal position, on the floor, wall, or built into a sectional ventilation chamber as a heater section.
Recuperative air heaters can even be used for heating rooms of a high comfort category, if the unit itself is moved outside the zone of direct service.
Main disadvantages:
- The large and complex heat exchanger increases the cost and weight of the system compared to mixing type air heaters;
- They need a chimney and a condensate drain.
Direct air heating systems
Modern technologies have made it possible to achieve such purity of combustion natural gas that it became possible not to divert combustion products "into the pipe", but to use them for direct air heating in supply ventilation systems. The gas supplied to combustion completely burns out in the stream of heated air and, mixing with it, gives it all the heat.
This principle is implemented in a number of similar ramp burner designs in the USA, England, France and Russia and has been successfully used since the 1960s at many enterprises in Russia and abroad. Based on the principle of ultra-clean combustion of natural gas directly in the heated air flow, gas mixing air heaters of the STV type (STARVEINE - “star wind”) are produced with a rated thermal output from 150 kW to 21 MW.
The technology of combustion organization itself, as well as a high degree of dilution of combustion products, make it possible to obtain pure warm air in accordance with all applicable standards, practically free of harmful impurities (no more than 30% of MPC). STV air heaters (Fig. 3) consist of a modular burner unit located inside the housing (air duct section), a DUNGS gas line (Germany) and an automation system.
The housing is usually equipped with a hermetic door for ease of maintenance. The burner block, depending on the required thermal power, is assembled from the required number of burner sections of different configurations. The automation of the heaters provides a smooth automatic start according to the cyclogram, control of the parameters of safe operation and the possibility of smooth regulation of the heat output (1:4), which makes it possible to automatically maintain the required air temperature in the heated room.
Application of gas mixing air heaters
Their main purpose is direct heating of fresh supply air supplied to production facilities to compensate for exhaust ventilation and thus improve the working conditions of people.
For premises with a high air exchange rate, it becomes expedient to combine the supply ventilation system and the heating system - in this regard, direct heating systems have no competitors in terms of price / quality ratio. Gas mixing air heaters are designed for:
- autonomous air heating of rooms for various purposes with a large air exchange (K great.5);
- air heating in air-thermal curtains of a cut-off type, it is possible to combine it with heating and supply ventilation systems;
- pre-heating systems for car engines in unheated parking lots;
- thawing and defrosting of wagons, tanks, cars, bulk materials, heating and drying products before painting or other types of processing;
- direct heating of atmospheric air or drying agent in various process heating and drying installations, for example, drying of grain, grass, paper, textiles, wood; applications in painting and drying booths after painting, etc.
Accommodation
Mixing heaters can be built into the air ducts of supply ventilation systems and thermal curtains, into the air ducts of drying plants - both in horizontal and vertical sections. Can be mounted on the floor or platform, under the ceiling or on the wall. As a rule, they are placed in supply and ventilation chambers, but they can also be installed directly in a heated room (according to the category).
With additional equipment, the corresponding elements can serve rooms of categories A and B. Recirculation of indoor air through mixing air heaters is undesirable - a significant decrease in the oxygen level in the room is possible.
Strengths direct heating systems
Simplicity and reliability, low cost and efficiency, the ability to heat up to high temperatures, a high degree of automation, smooth regulation, do not need a chimney. Direct heating is the most economical way - the efficiency of the system is 99.96%. The level of specific capital costs for a heating system based on a direct heating unit combined with forced ventilation is the lowest with the highest degree of automation.
Air heaters of all types are equipped with a safety and control automation system that provides smooth start, maintaining the heating mode and shutting down in case of emergencies. In order to save energy, it is possible to equip air heaters with automatic control taking into account external and internal temperature control, functions of daily and weekly heating programming modes.
It is also possible to include the parameters of a heating system, consisting of many heating units, into a centralized control and dispatching system. In this case, the operator-dispatcher will have operational information about the operation and status of the heating units, clearly displayed on the computer monitor, as well as control their operation mode directly from the remote control center.
Mobile heat generators and heat guns
Designed for temporary use - at construction sites, for heating during off-season periods, technological heating. Mobile heat generators and heat guns run on propane (liquefied bottled gas), diesel fuel or kerosene. Can be both direct heating, and with removal of products of combustion.
Types of autonomous air heating systems
For autonomous heat supply of various premises, various types of air heating systems are used - with centralized heat distribution and decentralized; systems operating entirely on the inflow fresh air, or with full/partial recirculation of internal air.
In decentralized air heating systems, heating and air circulation in the room are carried out by autonomous heat generators located in various sections or work areas - on the floor, wall and under the roof. The air from the heaters is supplied directly to the working area of the room. Sometimes, for better distribution of heat flows, heat generators are equipped with small (local) air duct systems.
For units in this design, the minimum power of the fan motor is typical, so decentralized systems are more economical in terms of power consumption. It is also possible to use air-thermal curtains as part of an air heating system or supply ventilation.
The possibility of local regulation and use of heat generators as needed - by zones, at different times - makes it possible to significantly reduce fuel costs. However, the capital cost of implementing this method is somewhat higher. In systems with centralized heat distribution, air-heating units are used; The warm air produced by them enters the working areas through the duct system.
The units, as a rule, are built into existing ventilation chambers, but it is possible to place them directly in a heated room - on the floor or on the site.
Application and placement, selection of equipment
Each of the types of the above heating units has its undeniable advantages. And there is no ready-made recipe in which case which of them is more appropriate - it depends on many factors: the amount of air exchange in relation to the amount of heat loss, the category of the room, the availability of free space for placing equipment, and financial possibilities. Let's try to form the most general principles appropriate selection of equipment.
1. Heating systems for rooms with little air exchange (air exchange ≤ great,5-1)
The total heat output of the heat generators in this case is assumed to be almost equal to the amount of heat required to compensate for the heat loss of the room, the ventilation is relatively small, so it is advisable to use a heating system based on heat generators of indirect heating with full or partial recirculation of the indoor air of the room.
Ventilation in such rooms can be natural or mixed with outdoor air to recirculate. In the second case, the power of the heaters is increased by an amount sufficient to heat the fresh supply air. Such a heating system can be local, with floor or wall heat generators.
If it is impossible to place the unit in a heated room or when organizing maintenance of several rooms, a centralized type system can be used: heat generators are located in the ventilation chamber (an extension, on the mezzanine, in an adjacent room), and the heat is distributed through the air ducts.
During working hours, heat generators can operate in partial recirculation mode, simultaneously heating the mixed supply air, during non-working hours, some of them can be turned off, and the rest can be switched to an economical standby mode of + 2-5 ° C with full recirculation.
2. Heating systems for rooms with a large air exchange rate, constantly in need of supplying large volumes of fresh air supply (Air exchange great)
In this case, the amount of heat required to heat the supply air may already be several times greater than the amount of heat required to compensate for heat losses. Here, it is most expedient and economical to combine an air heating system with a supply ventilation system. The heating system can be built on the basis of direct air heating installations, or on the basis of the use of recuperative heat generators in a design with a higher degree of heating.
The total heat output of the heaters must be equal to the sum of the heat demand for supply air heating and the heat required to compensate for heat losses. In direct heating systems, 100% of the outdoor air is heated, ensuring the supply of the required volume of supply air.
During working hours, they heat the air from outside to the design temperature of + 16-40 ° C (taking into account overheating to ensure heat loss compensation). In order to save money during non-working hours, you can turn off part of the heaters to reduce the supply air flow, and switch the rest to the standby mode of maintaining +2-5°C.
Recuperative heat generators in standby mode allow for additional savings by switching them to full recirculation mode. The lowest capital costs in organizing centralized heating systems are when using the largest possible heaters. Capital costs for STV gas mixing air heaters can range from 300 to 600 rubles/kW of installed heat output.
3. Combined air heating systems
The best option for rooms with significant air exchange during working hours with a single-shift operation, or an intermittent work cycle - when the difference in the need for supply of fresh air and heat during the day is significant.
In this case, it is advisable to separate the operation of two systems: standby heating and supply ventilation combined with a heating (reheating) system. At the same time, recuperative heat generators are installed in the heated room or in the ventilation chambers to maintain only the standby mode with full recirculation (at the calculated outdoor temperature).
The supply ventilation system, combined with the heating system, provides heating of the required volume of fresh supply air up to + 16-30 ° C and heating of the room to the required operating temperature, and for economy purposes it is switched on only during working hours.
It is built either on the basis of recuperative heat generators (with an increased degree of heating), or on the basis of powerful direct heating systems (which is 2-4 times cheaper). It is possible to combine the forced-air heating system with the existing water heating system (it can remain on duty), the option is also applicable for the staged modernization of the existing heating and ventilation system.
With this method, operating costs will be the lowest. Thus, using air heaters of various types in various combinations, it is possible to solve both problems at the same time - both heating and forced ventilation.
There are a lot of examples of the use of air heating systems and the possibilities of their combination are extremely diverse. In each case, it is necessary to carry out thermal calculations, take into account all the conditions of use and perform several options for selecting equipment, comparing them in terms of feasibility, capital costs and operating costs.
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 production 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 serial 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 aircraft structure 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 the flight parameters (speed and altitude) and the airframe design (design solutions and materials used), and on the 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 generation of the propulsion system. Both of these phenomena constitute the process of interaction between the medium (air, exhaust gases) and the streamlined solid(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 by 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 means of radiation), neighboring structural elements, etc. In addition, complete deceleration of the flow occurs 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 for the manufacture of heat-stressed structural elements of the aircraft, from the moment the aircraft reaches high speed until the moment the 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 in 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, which are 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. At present, this problem is 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 pushes the barrier for this type of aircraft towards higher flight speeds, without excluding it as such. However, the desire for even higher speeds leads to the creation of even more complex structures and equipment that require the use of better materials. This has a noticeable effect on the weight, purchase price, and the cost of operating and maintaining the aircraft.
From the 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.
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., Raiser 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:
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