The principle of operation of gas turbine plants

Fig.1. Scheme of a gas turbine unit with a single-shaft gas turbine engine of a simple cycle

Clean air is supplied to the compressor (1) of the gas turbine power unit. Under high pressure, air from the compressor is sent to the combustion chamber (2), where the main fuel, gas, is also supplied. The mixture ignites. When a gas-air mixture is burned, energy is generated in the form of a stream of hot gases. This flow rushes at high speed to the turbine wheel (3) and rotates it. Rotational kinetic energy through the turbine shaft drives the compressor and electric generator (4). From the terminals of the power generator, the generated electricity, usually through a transformer, is sent to the power grid, to energy consumers.

Gas turbines are described by the Brayton thermodynamic cycle. The Brayton/Joule cycle is a thermodynamic cycle that describes the working processes of gas turbine, turbojet and ramjet internal combustion engines, as well as gas turbine external combustion engines with a closed loop of a gaseous (single-phase) working fluid.

The cycle is named after American engineer George Brighton, who invented the reciprocating internal combustion engine that operated on this cycle.

Sometimes this cycle is also called the Joule cycle - in honor of the English physicist James Joule, who established the mechanical equivalent of heat.

Fig.2. P,V diagram Brighton cycle

The ideal Brayton cycle consists of the processes:

• 1-2 Isentropic compression.
• 2-3 Isobaric heat input.
• 3-4 Isentropic expansion.
• 4-1 Isobaric heat removal.

Taking into account the differences between real adiabatic processes of expansion and contraction from isentropic ones, a real Brayton cycle is constructed (1-2p-3-4p-1 on the T-S diagram) (Fig. 3)

Fig.3. T-S chart Brighton cycle
Ideal (1-2-3-4-1)
Real (1-2p-3-4p-1)

The thermal efficiency of an ideal Brayton cycle is usually expressed by the formula:

• where P = p2 / p1 - the degree of pressure increase in the process of isentropic compression (1-2);
• k - adiabatic index (for air equal to 1.4)

It should be especially noted that this generally accepted way of calculating the cycle efficiency obscures the essence of the ongoing process. The limiting efficiency of the thermodynamic cycle is calculated through the temperature ratio using the Carnot formula:

• where T1 is the refrigerator temperature;
• T2 - heater temperature.

Exactly the same temperature ratio can be expressed in terms of the pressure ratios used in the cycle and the adiabatic index:

Thus, the efficiency of the Brayton cycle depends on the initial and final temperatures of the cycle in exactly the same way as the efficiency of the Carnot cycle. With an infinitesimal heating of the working fluid along the line (2-3), the process can be considered isothermal and completely equivalent to the Carnot cycle. The amount of heating of the working fluid T3 in the isobaric process determines the amount of work related to the amount of the working fluid used in the cycle, but in no way affects the thermal efficiency of the cycle. However, when practical implementation heating cycle is usually carried out to the highest possible values ​​limited by the heat resistance of the materials used in order to minimize the size of the mechanisms that compress and expand the working fluid.

In practice, friction and turbulence cause:

• Non-adiabatic compression: for a given total pressure ratio, the compressor discharge temperature is higher than ideal.
• Non-adiabatic expansion: although the turbine temperature drops to the level necessary for operation, the compressor is not affected, the pressure ratio is higher, as a result, the expansion is not enough to provide useful work.
• Pressure losses in the air intake, combustion chamber and outlet: as a result, the expansion is not sufficient to provide useful work.

As with all cyclic heat engines, the higher the combustion temperature, the higher the efficiency. The limiting factor is the ability of the steel, nickel, ceramic or other materials that make up the engine to withstand heat and pressure. Much of the engineering work is focused on removing heat from parts of the turbine. Most turbines also try to recover heat from exhaust gases that are otherwise wasted.

Recuperators are heat exchangers that transfer heat from exhaust gases to compressed air before combustion. In a combined cycle, heat is transferred to the steam turbine systems. And in combined heat and power (CHP), waste heat is used to produce hot water.

Mechanically, gas turbines can be considerably simpler than reciprocating internal combustion engines. Simple turbines may have one moving part: shaft/compressor/turbine/alternate rotor assembly (see image below), not including the fuel system.

Fig.4. This machine has a single stage radial compressor,
turbine, recuperator, and air bearings.

More complex turbines (those used in modern jet engines) may have multiple shafts (coils), hundreds of turbine blades, moving stator blades, and an extensive system of complex piping, combustion chambers, and heat exchangers.

As a general rule, the smaller the motor, the higher the speed of the shaft(s) required to maintain the maximum linear speed of the blades.

The maximum speed of the turbine blades determines the maximum pressure that can be reached, resulting in maximum power, regardless of engine size. The jet engine rotates at about 10,000 rpm and the micro-turbine at about 100,000 rpm.

A gas turbine is an engine in which, in the process of continuous operation, the main organ of the device (the rotor) converts (in other cases, steam or water) into mechanical work. In this case, the jet of the working substance acts on the blades fixed around the circumference of the rotor, setting them in motion. In the direction of the gas flow, turbines are divided into axial (gas moves parallel to the axis of the turbine) or radial (perpendicular movement relative to the same axis). There are both single and multi-stage mechanisms.

A gas turbine can act on the blades in two ways. Firstly, it is an active process, when gas is supplied to the working area at high speeds. In this case, the gas flow tends to move in a straight line, and the curved blade part standing in its way deflects it, turning itself. Secondly, it is a reactive type process, when the gas supply rate is low, but high pressures are used. type in its pure form is almost never found, because in their turbines it is present which acts on the blades along with the reaction force.

Where is the gas turbine used today? The principle of operation of the device allows it to be used for drives of electric current generators, compressors, etc. Turbines of this type are widely used in transport (ship gas turbine installations). Compared to steam counterparts, they have a relatively small weight and dimensions, they do not require the arrangement of a boiler room, a condensing unit.

The gas turbine is ready for operation quite quickly after start-up, develops full power in about 10 minutes, is easy to maintain, requires a small amount of water for cooling. Unlike internal combustion engines, it does not have inertial effects from the crank mechanism. one and a half times shorter than diesel engines and more than twice as light. The devices have the ability to run on low quality fuel. The above qualities make it possible to consider engines of this kind of particular interest for ships on and hydrofoils.

The gas turbine as the main component of the engine has a number of significant disadvantages. Among them, they note high noise, less than diesel engines, efficiency, short life at high temperatures (if the gas medium used has a temperature of about 1100 ° C, then the turbine can be used on average up to 750 hours).

The efficiency of a gas turbine depends on the system in which it is used. For example, devices used in the power industry with an initial temperature of gases above 1300 degrees Celsius, from air in the compressor no more than 23 and no less than 17, have a coefficient of about 38.5% during autonomous operations. Such turbines are not very widespread and are mainly used to cover load peaks in electrical systems. Today, about 15 gas turbines with a capacity of up to 30 MW operate at a number of thermal power plants in Russia. At multi-stage plants, a much higher efficiency index (about 0.93) is achieved due to the high efficiency of structural elements.

A traditional modern gas turbine plant (GTP) is a combination of an air compressor, a combustion chamber and a gas turbine, as well as auxiliary systems that ensure its operation. The combination of a gas turbine and an electric generator is called a gas turbine unit.

It is necessary to emphasize one important difference between GTU and PTU. The composition of the PTU does not include a boiler, more precisely, the boiler is considered as a separate source of heat; With this consideration, the boiler is a “black box”: feed water enters it with a temperature of $t_(p.v)$, and steam comes out with parameters $p_0$, $t_0$. A steam turbine plant cannot operate without a boiler as a physical object. In a gas turbine, the combustion chamber is its integral element. In this sense, GTU is self-sufficient.

Gas turbine plants are extremely diverse, perhaps even more than steam turbines. Below we will consider the most promising and most used gas turbines of a simple cycle in the power industry.

circuit diagram such a gas turbine is shown in the figure. Air from the atmosphere enters the inlet of an air compressor, which is a rotary turbomachine with a flow path consisting of rotating and fixed gratings. Compressor pressure ratio p b to the pressure in front of him p a is called the compression ratio of an air compressor and is usually denoted as p to (p to = pb/p a). The compressor rotor is driven by a gas turbine. The compressed air flow is fed into one, two or more combustion chambers. In this case, in most cases, the air flow coming from the compressor is divided into two streams. The first flow is sent to the burners, where fuel (gas or liquid fuel) is also supplied. When fuel is burned, high-temperature combustion products are formed. The relatively cold air of the second flow is mixed with them in order to obtain gases (they are usually called working gases) with a temperature acceptable for parts of a gas turbine.

Working gases with pressure r s (r s < p b due to the hydraulic resistance of the combustion chamber) are fed into the flow path of the gas turbine, the principle of operation of which is no different from the principle of operation of the steam turbine (the only difference is that the gas turbine runs on fuel combustion products, and not on steam). In a gas turbine, the working gases expand to almost atmospheric pressure. p d, enter the outlet diffuser 14, and from it - either immediately into the chimney, or previously into any heat exchanger that uses the heat of the gas turbine exhaust gases.

Due to the expansion of gases in the gas turbine, the latter generates power. A very significant part of it (about half) is spent on the compressor drive, and the rest - on the electric generator drive. This is the net power of the gas turbine, which is indicated when it is marked.

To depict gas turbine diagrams, symbols similar to those used for PTU are used.

There can be no simpler gas turbine, since it contains a minimum of necessary components that provide sequential processes of compression, heating and expansion of the working fluid: one compressor, one or more combustion chambers operating in same conditions, and one gas turbine. Along with simple cycle gas turbines, there are complex cycle gas turbines that may contain several compressors, turbines and combustion chambers. In particular, GT-100-750, built in the USSR in the 70s, belong to this type of gas turbine.

It is made double. High pressure compressor on one shaft KVD and the high-pressure turbine driving it TVD; this shaft has a variable speed. The low pressure turbine is located on the second shaft TND, driving the low pressure compressor KND and electric generator EG; therefore, this shaft has a constant rotational speed of 50 s -1 . Air in the amount of 447 kg / s enters from the atmosphere into KND and is compressed in it to a pressure of approximately 430 kPa (4.3 atm) and then fed into the air cooler IN, where it is cooled with water from 176 to 35 °C. This reduces the work required to compress the air in the high pressure compressor. KVD(compression ratio p k = 6.3). From there, air enters the high pressure combustion chamber. KSVD and combustion products with a temperature of 750 ° C are sent to TVD. From TVD gases containing a significant amount of oxygen enter the low-pressure combustion chamber KSND, in which additional fuel is burned, and from it - into TND. Exhaust gases with a temperature of 390 ° C exit either into the chimney or into a heat exchanger to use the heat of the exhaust gases.

GTU is not very economical due to the high temperature of the flue gases. The complication of the circuit makes it possible to increase its efficiency, but at the same time it requires an increase in capital investments and complicates operation.

The figure shows the GTU V94.3 from Siemens. Atmospheric air from the complex air-cleaning device (KVOU) enters the mine 4 , and from it - to the flow part 16 air compressor. Air is compressed in the compressor. The compression ratio in typical compressors is p k = 13-17, and thus the pressure in the gas turbine tract does not exceed 1.3-1.7 MPa (13-17 atm). This is another major difference between a gas turbine and a steam turbine, in which the steam pressure is 10-15 times greater than the gas pressure in the gas turbine. Small pressure working environment determines the small thickness of the walls of the buildings and the ease of their heating. This is what makes the gas turbine very maneuverable, i.e. capable of quick starts and stops. If it takes from 1 hour to several hours to start a steam turbine, depending on its initial temperature state, then the gas turbine can be put into operation in 10-15 minutes.

When compressed in a compressor, the air heats up. This heating can be estimated by a simple approximate relation:

$$T_a/T_b = \pi_k^(0.25)$$

wherein T b and T a- absolute air temperatures behind and before the compressor. If, for example, T a= 300 K, i.e. the ambient temperature is 27 ° C, and p k \u003d 16, then T b= 600 K and, consequently, the air is heated by

$$\Delta t = (600-273)-(300-273) = 300°C.$$

Thus, the air temperature behind the compressor is 300-350 °C. The air between the walls of the flame tube and the body of the combustion chamber moves to the burner, to which the fuel gas is supplied. Since the fuel must enter the combustion chamber, where the pressure is 1.3-1.7 MPa, the gas pressure must be high. To be able to control its flow into the combustion chamber, the gas pressure is approximately twice as high as the pressure in the chamber. If there is such pressure in the supply gas pipeline, then the gas is supplied to the combustion chamber directly from the gas distribution point (GDP). If the gas pressure is insufficient, then a booster gas compressor is installed between the hydraulic fracturing and the chamber.

The fuel gas consumption is only about 1-1.5% of the air flow from the compressor, so the creation of a highly economical booster gas compressor presents certain technical difficulties.

Inside the flame tube 10 high temperature combustion products are formed. After mixing secondary air at the outlet of the combustion chamber, it decreases somewhat, but nevertheless reaches 1350-1400 °C in typical modern gas turbines.

Hot gases from the combustion chamber enter the flow path 7 gas turbine. In it, gases expand to almost atmospheric pressure, since the space behind the gas turbine communicates either with a chimney or with a heat exchanger, the hydraulic resistance of which is small.

When gases expand in a gas turbine, power is generated on its shaft. This power is partially used to drive the air compressor, and its excess is used to drive the rotor 1 generator. One of characteristic features GTP is that the compressor requires about half the power developed by the gas turbine. For example, in a gas turbine unit with a capacity of 180 MW (this is the net power) being created in Russia, the compressor capacity is 196 MW. This is one of the fundamental differences between a gas turbine and a steam turbine: in the latter, the power used to compress the feed water even up to a pressure of 23.5 MPa (240 atm) is only a few percent of the steam turbine power. This is due to the fact that water is a low compressible liquid, and air requires a lot of energy to compress.

In the first, rather rough approximation, the gas temperature behind the turbine can be estimated from a simple relationship similar to:

$$T_c/T_d = \pi_k^(0.25).$$

Therefore, if $\pi_k = 16$, and the temperature in front of the turbine T s\u003d 1400 ° С \u003d 1673 K, then the temperature behind it is approximately, K:

$$T_d=T_c/\pi_k^(0.25) = 1673/16^(0.25) = 836.$$

Thus, the gas temperature downstream of the gas turbine is quite high, and a significant amount of heat obtained from fuel combustion literally goes into the chimney. Therefore, during autonomous operation of a gas turbine, its efficiency is low: for typical gas turbines, it is 35-36%, i.e. significantly less than the efficiency of vocational schools. The matter, however, changes drastically when a heat exchanger is installed on the "tail" of the gas turbine unit (a network heater or a waste heat boiler for a combined cycle).

A diffuser is installed behind the gas turbine - a smoothly expanding channel, during the flow in which the velocity pressure of gases is partially converted into pressure. This makes it possible to have a pressure behind the gas turbine that is less than atmospheric pressure, which increases the efficiency of 1 kg of gases in the turbine and, consequently, increases its power.

Air compressor device. As already mentioned, an air compressor is a turbomachine, to the shaft of which power is supplied from a gas turbine; this power is transferred to the air flowing through the flow path of the compressor, as a result of which the air pressure rises up to the pressure in the combustion chamber.

The figure shows a gas turbine rotor placed in thrust bearings; in the foreground, the compressor rotor and stator elements are clearly visible.

From mine 4 air enters the channels formed by the rotary vanes 2 non-rotating inlet guide vane (VNA). The main task of the VNA is to inform the flow moving in the axial (or radial-axial) direction of rotational motion. VNA channels do not fundamentally differ from the nozzle channels of a steam turbine: they are confusing (tapering), and the flow in them accelerates, simultaneously acquiring a circumferential velocity component.

In modern gas turbines, the inlet guide vane is made rotatable. The need for a rotary VNA is caused by the desire to prevent a decrease in efficiency when the GTU load is reduced. The point is that the shafts of the compressor and the electric generator have the same rotational speed, equal to the frequency of the network. Therefore, if VNA is not used, then the amount of air supplied by the compressor to the combustion chamber is constant and does not depend on the turbine load. And you can change the power of the gas turbine only by changing the fuel flow into the combustion chamber. Therefore, with a decrease in fuel consumption and a constant amount of air supplied by the compressor, the temperature of the working gases decreases both before and after the gas turbine. This leads to a very significant reduction in the efficiency of the gas turbine. Rotation of the blades with a decrease in load around the axis 1 by 25 - 30° allows to narrow the flow sections of the VNA channels and reduce the air flow into the combustion chamber, maintaining a constant ratio between the air and fuel consumption. The installation of the inlet guide vane makes it possible to maintain the gas temperature in front of the gas turbine and behind it constant in the power range of approximately 100-80%.

The figure shows the VNA blade drive. A rotary lever is attached to the axes of each blade 2 , which through the lever 4 associated with a swivel ring 1 . If necessary, change the air flow ring 1 rotates with the help of rods and an electric motor with a gearbox; while turning all the levers at the same time 2 and, accordingly, the VNA blades 5 .

The air swirling with the help of VNA enters the 1st stage of the air compressor, which consists of two gratings: rotating and stationary. Both gratings, in contrast to turbine gratings, have expanding (diffuser) channels, i.e. inlet air passage area F 1 less than F 2 at the exit.

When air moves in such a channel, its speed decreases ( w 2 < w 1), and the pressure increases ( R 2 > R one). Unfortunately, to make the diffuser grill economical, i.e. so that the flow rate w 1 to the maximum degree would be converted into pressure, and not into heat, only possible with a small degree of compression R 2 /R 1 (usually 1.2 - 1.3), which leads to a large number of compressor stages (14 - 16 with a compressor compression ratio p k \u003d 13 - 16).

The figure shows the air flow in the compressor stage. From the input (fixed) rotary nozzle apparatus, the air exits at a speed c 1 (see the upper speed triangle), having the necessary circumferential twist (a 1< 90°). Если расположенная за ВНА вращающаяся (рабочая) решетка имеет скорость u 1 , then the relative speed of entering it w 1 will be equal to the difference of vectors c 1 and u 1 , and this difference will be greater than c 1 i.e. w 1 > c one . When moving in the channel, the air speed decreases to the value w 2 and it comes out at an angle b 2 determined by the inclination of the profiles. However, due to the rotation and supply of energy to the air from the rotor blades, its speed With 2 in absolute motion will be greater than c one . The blades of the fixed grid are installed so that the air inlet into the channel is shock-free. Since the channels of this grating are expanding, the velocity in it decreases to the value c" 1 , and the pressure increases from R 1 to R 2. The grid is designed so that c" 1 = c 1, a a "1 = a 1. Therefore, in the second stage and subsequent stages, the compression process will proceed in a similar way. In this case, the height of their gratings will decrease in accordance with the increased air density due to compression.

Sometimes the guide vanes of the first few stages of the compressor are made rotary in the same way as the VNA vanes. This makes it possible to expand the power range of the gas turbine, in which the temperature of the gases in front of the gas turbine and behind it remains unchanged. Accordingly, the economy also increases. The use of several rotary guide vanes allows you to work economically in the range of 100 - 50% of the power.

The last stage of the compressor is arranged in the same way as the previous ones, with the only difference that the task of the last guide vane 1 is not only to increase the pressure, but also to ensure the axial exit of the air flow. Air enters the annular outlet diffuser 23 where the pressure rises to its maximum value. With this pressure, air enters the combustion zone 9 .

Air is taken from the air compressor housing to cool the elements of the gas turbine. To do this, annular chambers are made in its body, communicating with the space behind the corresponding stage. The air from the chambers is removed by pipelines.

In addition, the compressor has so-called anti-surge valves and bypass pipes. 6 , bypassing air from the intermediate stages of the compressor into the outlet diffuser of the gas turbine when it is started and stopped. This eliminates the unstable operation of the compressor at low air flow rates (this phenomenon is called surge), which is expressed in intense vibration of the entire machine.

The creation of highly economical air compressors is an extremely complex task, which, unlike turbines, cannot be solved only by calculation and design. Since the compressor power is approximately equal to the power of the gas turbine, a deterioration in the efficiency of the compressor by 1% leads to a decrease in the efficiency of the entire gas turbine by 2-2.5%. Therefore, the creation of a good compressor is one of the key problems in the creation of gas turbines. Usually compressors are created by modeling (scaling) using a model compressor created by long experimental refinement.

Gas turbine combustion chambers are very diverse. Above is a gas turbine with two external chambers. The figure shows a GTU type 13E with a capacity of 140 MW from ABB with one remote combustion chamber, the device of which is similar to the device of the chamber shown in the figure. The air from the compressor from the annular diffuser enters the space between the chamber body and the flame tube and is then used for gas combustion and for cooling the flame tube.

The main disadvantage of remote combustion chambers is their large dimensions, which are clearly visible from the figure. To the right of the chamber is a gas turbine, to the left - a compressor. Three holes are visible from above in the body for accommodating anti-surge valves and then - the VNA drive. In modern gas turbines, built-in combustion chambers are mainly used: annular and tubular-annular.

The figure shows an integrated annular combustion chamber. The annular space for combustion is formed by the internal 17 and outdoor 11 fiery pipes. From the inside, the pipes are lined with special inserts 13 and 16 having a thermal barrier coating on the side facing the flame; on the opposite side, the inserts are ribbed, which improves their cooling by air entering through the annular gaps between the inserts inside the flame tube. Thus, the temperature of the flame tube is 750-800 °C in the combustion zone. The frontal microflare burner device of the chamber consists of several hundred burners 10 , to which gas is supplied from four collectors 5 -8 . Turning off the collectors in turn, you can change the power of the gas turbine.

The burner device is shown in the figure. From the collector, gas enters through drilling in the stem 3 to the inner cavity of the shoulder blades 6 swirler. The latter is a hollow radial straight blades that cause the air coming from the combustion chamber to twist and rotate around the axis of the rod. This rotating air vortex receives natural gas from the inner cavity of the swirler blades. 6 through small holes 7 . In this case, a homogeneous fuel-air mixture is formed, which emerges in the form of a swirling jet from the zone 5 . An annular rotating vortex ensures stable combustion of the gas.

The figure shows a tubular-annular combustion chamber GTE-180. Into the annular space 24 between the outlet of the air compressor and the inlet of the gas turbine using perforated cones 3 place 12 flame tubes 10 . The flame tube contains numerous holes with a diameter of 1 mm, arranged in annular rows at a distance of 6 mm between them; distance between rows of holes 23 mm. Through these openings, "cold" air enters from the outside, providing convective-film cooling and the temperature of the flame tube is not higher than 850 °C. A thermal barrier coating 0.4 mm thick is applied to the inner surface of the flame tube.

On the front plate 8 flame tube, a burner device is installed, consisting of a central pilot burner 6 igniting fuel at start-up using a candle 5 , and five main modules, one of which is shown in the figure. The module allows you to burn gas and diesel fuel. Gas through fitting 1 after filter 6 enters the annular fuel gas manifold 5 , and from it into cavities containing small holes (diameter 0.7 mm, step 8 mm). Through these holes, the gas enters the annular space. There are six tangential grooves in the walls of the module 9 , through which the main amount of air supplied for combustion from the air compressor enters. In the tangential slots, the air is twisted and, thus, inside the cavity 8 a rotating vortex is formed, moving towards the outlet of the burner. To the periphery of the vortex through the holes 3 gas enters, mixes with air, and the resulting homogeneous mixture exits the burner, where it ignites and burns. The combustion products enter the nozzle apparatus of the 1st stage of the gas turbine.

The gas turbine is the most complex element of the gas turbine, which is primarily due to the very high temperature of the working gases flowing through its flow path: the gas temperature in front of the turbine of 1350 ° C is currently considered “standard”, and leading companies, primarily General Electric, work on mastering the initial temperature of 1500 °C. Recall that the "standard" initial temperature for steam turbines is 540 °C, and in the future - a temperature of 600-620 °C.

The desire to increase the initial temperature is associated, first of all, with the gain in efficiency that it gives. This is clearly seen from the figure summarizing the achieved level of gas turbine construction: an increase in the initial temperature from 1100 to 1450 °C gives an increase in absolute efficiency from 32 to 40%, i.e. results in fuel savings of 25%. Of course, part of this savings is associated not only with an increase in temperature, but also with the improvement of other elements of the gas turbine, and the initial temperature is still the determining factor.

To ensure long-term operation of a gas turbine, a combination of two means is used. The first means is the use of heat-resistant materials for the most loaded parts that can resist the action of high mechanical loads and temperatures (primarily for nozzle and rotor blades). If steels (i.e. iron-based alloys) with a chromium content of 12-13% are used for steam turbine blades and some other elements, then nickel-based alloys (nimonic) are used for gas turbine blades, which are capable of under real mechanical loads and the required service life to withstand temperatures of 800-850 °C. Therefore, together with the first, a second means is used - cooling the hottest parts.

Most modern gas turbines are cooled using bleed air from various stages of an air compressor. Gas turbines are already in operation, which use water vapor for cooling, which is a better cooling agent than air. Cooling air after heating in the cooled part is discharged into the flow path of the gas turbine. Such a cooling system is called open. There are closed cooling systems in which the coolant heated in the part is sent to the refrigerator and then returned again to cool the part. Such a system is not only very complicated, but also requires the utilization of heat taken from the refrigerator.

The gas turbine cooling system is the most a complex system in GTU, which determines its service life. It ensures not only maintaining an acceptable level of working and nozzle blades, but also body elements, disks carrying working blades, locking bearing seals where oil circulates, etc. This system is extremely branched and organized so that each cooled element receives cooling air of the parameters and in the amount necessary to maintain its optimum temperature. Excessive cooling of parts is just as harmful as insufficient, since it leads to increased costs of cooling air, which requires turbine power to compress in the compressor. In addition, increased air consumption for cooling leads to a decrease in the temperature of the gases behind the turbine, which has a very significant effect on the operation of the equipment installed behind the gas turbine (for example, a steam turbine unit operating as part of a steam turbine). Finally, the cooling system must ensure not only the required temperature level of the parts, but also the uniformity of their heating, which excludes the appearance of dangerous thermal stresses, the cyclic action of which leads to the appearance of cracks.

The figure shows an example of a typical gas turbine cooling circuit. The values ​​of gas temperatures are given in rectangular frames. In front of the nozzle apparatus of the 1st stage 1 it reaches 1350 °C. Behind him, i.e. in front of the working grate of the 1st stage, it is 1130 °C. Even in front of the working blade of the last stage, it is at the level of 600 °C. Gases of this temperature wash the nozzle and working blades, and if they were not cooled, then their temperature would be equal to the temperature of the gases and their service life would be limited to several hours.

To cool the elements of a gas turbine, air is used that is taken from the compressor in that stage where its pressure is slightly higher than the pressure of the working gases in that zone of the gas turbine into which air is supplied. For example, for cooling the nozzle vanes of the 1st stage, cooling air in the amount of 4.5% of the air flow at the compressor inlet is taken from the compressor outlet diffuser, and for cooling the nozzle vanes of the last stage and the adjoining section of the housing - from the 5th stage of the compressor. Sometimes, to cool the hottest elements of a gas turbine, the air taken from the compressor outlet diffuser is first sent to an air cooler, where it is cooled (usually with water) to 180–200 °C and then sent for cooling. In this case, less air is required for cooling, but at the same time, the cost of an air cooler appears, the gas turbine becomes more complicated, and part of the heat removed by the cooling water is lost.

A gas turbine usually has 3-4 stages, i.e. 6-8 rims of gratings, and most often the blades of all rims are cooled, except for the working blades of the last stage. Air for cooling the nozzle vanes is supplied inside through their ends and discharged through numerous (600-700 holes with a diameter of 0.5-0.6 mm) holes located in the corresponding areas of the profile. Cooling air is supplied to the working blades through holes made in the ends of the shank.

In order to understand how cooled blades are arranged, it is necessary to at least in general terms consider the technology of their manufacture. Due to the exceptional difficulty of machining nickel alloys, investment casting is mainly used to produce blades. To implement it, first, casting cores are made from ceramic-based materials using a special technology of molding and heat treatment. The casting core is an exact copy of the cavity inside the future blade, into which cooling air will flow and flow in the required direction. The casting core is placed in a mold, the internal cavity of which fully corresponds to the blade to be obtained. The resulting free space between the rod and the wall of the mold is filled with a heated low-melting mass (for example, plastic), which solidifies. The rod, together with the hardening mass enveloping it, repeating the external shape of the blade, is an investment model. It is placed in a mold, to which the nimonic melt is fed. The latter melts the plastic, takes its place, and as a result, a cast blade appears with an internal cavity filled with a rod. The rod is removed by etching with special chemical solutions. The obtained nozzle vanes practically do not require additional machining (except for the manufacture of numerous holes for the exit of cooling air). Working cast blades require processing of the shank with a special abrasive tool.

The technology described briefly is borrowed from aeronautical technology, where the temperatures achieved are much higher than in stationary steam turbines. The difficulty of mastering these technologies is associated with much larger blade sizes for stationary gas turbines, which grow in proportion to the gas flow rate, i.e. GTU power.

The use of so-called single-crystal blades, which are made from a single crystal, seems very promising. This is due to the fact that the presence of grain boundaries during a long stay at a high temperature leads to a deterioration in the properties of the metal.

The gas turbine rotor is a unique prefabricated structure. Before assembling individual discs 5 compressor and disk 7 gas turbine are bladed and balanced, end parts are manufactured 1 and 8 , spacer 11 and center pin 6 . Each of the discs has two annular collars, on which hirts (named after the inventor - Hirth) are made - strictly radial teeth of a triangular profile. Adjacent pieces have exactly the same collars with exactly the same hirts. With good manufacturing quality of the hirt connection, absolute centering of adjacent disks is ensured (this ensures the radiality of hirts) and assembly repeatability after rotor disassembly.

The rotor is assembled on a special stand, which is an elevator with an annular platform for assembly personnel, inside which assembly is carried out. First, the end part of the rotor is assembled on the thread 1 and tie rod 6 . The rod is placed vertically inside the annular platform and the disk of the 1st stage of the compressor is lowered on top of it with the help of a crane. The centering of the disk and the end part is carried out by hirts. Moving upwards on a special elevator, the installation staff disc by disc [first of the compressor, then the spacer, and then the turbine and the right end 8 ] collects the entire rotor. A nut is screwed onto the right end 9 , and a hydraulic device is installed on the remaining part of the threaded part of the tie rod, squeezing the discs and pulling the tie rod. After drawing the rod, the nut 9 is screwed up to the stop, and the hydraulic device is removed. The stretched rod securely tightens the discs together and turns the rotor into a single rigid structure. The assembled rotor is removed from the assembly stand, and it is ready for installation in the gas turbine.

The main advantage of the gas turbine is its compactness. Indeed, first of all, there is no steam boiler in the gas turbine - a structure that reaches a great height and requires a separate room for installation. This circumstance is connected, first of all, with the high pressure in the combustion chamber (1.2-2 MPa); in the boiler, combustion occurs at atmospheric pressure and, accordingly, the volume of hot gases formed is 12-20 times larger. Further, in a gas turbine, the process of gas expansion takes place in a gas turbine consisting of only 3-5 stages, while a steam turbine having the same power consists of 3-4 cylinders containing 25-30 stages. Even taking into account both the combustion chamber and the air compressor, a 150 MW gas turbine has a length of 8-12 m, and the length of a steam turbine of the same power with a three-cylinder design is 1.5 times longer. At the same time, for the steam turbine, in addition to the boiler, it is necessary to provide for the installation of a condenser with circulation and condensate pumps, a regeneration system of 7-9 heaters, feed turbopumps (from one to three), and a deaerator. As a result, the gas turbine unit can be installed on a concrete base at the zero level of the machine hall, and the STU requires a frame foundation 9-16 m high with the steam turbine placed on the upper foundation slab and auxiliary equipment in the condensation room.

The compactness of the gas turbine allows it to be assembled at the turbine plant, delivered to the engine room by rail or road for installation on a simple foundation. So, in particular, gas turbines with built-in combustion chambers are transported. When transporting gas turbines with remote chambers, the latter are transported separately, but are easily and quickly attached to the compressor-gas turbine module using flanges. The steam turbine is supplied with numerous assemblies and parts, the installation of both itself and numerous auxiliary equipment and connections between them takes several times more time than a gas turbine.

GTU does not require cooling water. As a result, the gas turbine does not have a condenser and a process water supply system with pumping unit and cooling towers (with circulating water supply). As a result, all this leads to the fact that the cost of 1 kW of installed capacity of a gas turbine power plant is much less. At the same time, the cost of the GTU itself (compressor + combustion chamber + gas turbine), due to its complexity, turns out to be 3-4 times more than the cost of a steam turbine of the same power.

An important advantage of a gas turbine is its high maneuverability, determined by a low pressure level (compared to the pressure in a steam turbine) and, consequently, easy heating and cooling without dangerous thermal stresses and deformations.

However, gas turbines also have significant drawbacks, of which, first of all, it should be noted that they are less economical than those of a steam power plant. The average efficiency of sufficiently good gas turbines is 37-38%, and for steam turbine power units - 42-43%. The ceiling for powerful power gas turbines, as it is currently seen, is an efficiency of 41-42% (and maybe even higher, given the large reserves for increasing the initial temperature). The lower efficiency of the gas turbine is associated with the high temperature of the exhaust gases.

Another disadvantage of gas turbines is the impossibility of using low-grade fuels in them, at least at present. It can work well only on gas or good liquid fuel such as diesel. Steam power units can operate on any fuel, including the poorest quality.

The low initial cost of thermal power plants with gas turbines and at the same time relatively low efficiency and high cost of the fuel used and maneuverability determine the main area for individual use of gas turbines: they should be used in power systems as peak or backup power sources operating several hours a day.

At the same time, the situation changes dramatically when the heat of the gas turbine exhaust gases is used in heating plants or in a combined (steam-and-gas) cycle.

A turbine is an engine in which the potential energy of a compressible fluid is converted into kinetic energy in the blade apparatus, and the latter in the impellers into mechanical work transmitted to a continuously rotating shaft.

Steam turbines by their design represent a heat engine that is constantly in operation. During operation, superheated or saturated water vapor enters the flow path and, due to its expansion, forces the rotor to rotate. Rotation occurs as a result of the steam flow acting on the blade apparatus.

The steam turbine is part of the steam turbine design, which is designed to generate energy. There are also installations that, in addition to electricity, can generate thermal energy - the steam that has passed through the steam blades enters the network water heaters. This type of turbine is called industrial-cogeneration or cogeneration type of turbines. In the first case, steam extraction is provided for industrial purposes in the turbine. Complete with a generator, a steam turbine is a turbine unit.

Steam turbine types

Turbines are divided, depending on the direction in which the steam moves, into radial and axial turbines. The steam flow in radial turbines is directed perpendicular to the axis. Steam turbines can be one-, two- and three-case. The steam turbine is equipped with a variety of technical devices that prevent the ingress of ambient air into the casing. These are a variety of seals, which are supplied with water vapor in a small amount.

A safety regulator is located on the front section of the shaft, designed to turn off the steam supply when the turbine speed increases.

Characteristics of the main parameters of the nominal values

· Turbine rated power- the maximum power that the turbine must develop for a long time at the terminals of the electric generator, with normal values ​​​​of the main parameters or when they change within the limits specified by the industry and state standards. A controlled steam extraction turbine can develop power above its nominal power if this is in accordance with the strength conditions of its parts.

· Turbine economic power- the power at which the turbine operates with the greatest efficiency. Depending on the parameters of live steam and the purpose of the turbine, the rated power can be equal to the economic power or more by 10-25%.

· Nominal temperature of regenerative feed water heating- the temperature of the feed water downstream of the last heater in the direction of the water.

· Rated cooling water temperature- the temperature of the cooling water at the inlet to the condenser.

gas turbine(fr. turbine from lat. turbo swirl, rotation) is a continuous heat engine, in the blade apparatus of which the energy of compressed and heated gas is converted into mechanical work on the shaft. It consists of a rotor (blades fixed on disks) and a stator (guide vanes fixed in the housing).

Gas having a high temperature and pressure enters through the turbine nozzle apparatus into the low pressure area behind the nozzle part, simultaneously expanding and accelerating. Further, the gas flow enters the turbine blades, giving them part of its kinetic energy and imparting torque to the blades. The rotor blades transmit torque through the turbine discs to the shaft. Beneficial features gas turbine: a gas turbine, for example, drives a generator located on the same shaft with it, which is the useful work of a gas turbine.

Gas turbines are used as part of gas turbine engines (used for transport) and gas turbine units (used at thermal power plants as part of stationary GTUs, CCGTs). Gas turbines are described by the Brayton thermodynamic cycle, in which air is first adiabatically compressed, then burned at constant pressure, and then adiabatically expanded back to starting pressure.

Types of gas turbines

- Aircraft and jet engines

- Auxiliary power unit

- Industrial gas turbines for electricity production

- Turboshaft engines

- Radial gas turbines

- Microturbines

Mechanically, gas turbines can be considerably simpler than reciprocating internal combustion engines. Simple turbines may have one moving part: shaft/compressor/turbine/alternate rotor assembly (see image above), not including the fuel system.

More complex turbines (those used in modern jet engines) may have multiple shafts (coils), hundreds of turbine blades, moving stator blades, and an extensive system of complex piping, combustion chambers, and heat exchangers.

As a general rule, the smaller the motor, the higher the speed of the shaft(s) required to maintain the maximum linear speed of the blades. The maximum speed of the turbine blades determines the maximum pressure that can be reached, resulting in maximum power, regardless of engine size. The jet engine rotates at about 10,000 rpm and the micro-turbine at about 100,000 rpm.

A turbine is any rotating device that uses the energy of a moving working fluid (fluid) to produce work. Typical turbine fluids are: wind, water, steam and helium. Windmills and hydroelectric power stations have used turbines for decades to turn electric generators and produce energy for industry and housing. Simple turbines have been known for much longer, the first of them appeared in ancient Greece.

In the history of power generation, however, gas turbines themselves appeared not so long ago. The first practical gas turbine started generating electricity in Neuchatel, Switzerland in 1939. It was developed by the Brown Boveri Company. The first gas turbine to power an aircraft also ran in 1939 in Germany, using a gas turbine designed by Hans P. von Ohain. In England in the 1930s, the invention and design of the gas turbine by Frank Whittle led to the first turbine-powered flight in 1941.

Figure 1. Scheme of an aircraft turbine (a) and a gas turbine for ground use (b)

The term "gas turbine" is easily misleading because for many it means a turbine engine that uses gas as fuel. In fact, a gas turbine (shown schematically in Figure 1) has a compressor that supplies and compresses gas (usually air); the combustion chamber, where the combustion of fuel heats the compressed gas and the turbine itself, which extracts energy from the flow of hot, compressed gases. This energy is enough to power the compressor and remains for useful applications. A gas turbine is an internal combustion engine (ICE) that uses the continuous combustion of fuel to produce useful work. In this, the turbine differs from carburetor or diesel internal combustion engines, where the combustion process is intermittent.

Since the use of gas turbines began in 1939 simultaneously in the power industry and in aviation, different names are used for aviation and land-based gas turbines. Aviation gas turbines are called turbojet or jet engines, and other gas turbines are called gas turbine engines. AT English language there are even more names for these generally similar engines.

Use of gas turbines

In an aircraft turbojet, the energy from the turbine drives a compressor that draws air into the engine. The hot gas leaving the turbine is expelled into the atmosphere through the exhaust nozzle, which creates thrust. On fig. 1a shows a diagram of a turbojet engine.

Figure 2. Schematic representation of an aircraft turbojet engine.

A typical turbojet engine is shown in fig. 2. Such engines create thrust from 45 kgf to 45,000 kgf with a dead weight of 13 kg to 9,000 kg. The smallest engines drive cruise missiles, the largest - huge planes. The gas turbine in fig. 2 is a turbofan engine with a large diameter compressor. Thrust is created both by the air that is sucked in by the compressor and the air that passes through the turbine itself. The engine has big sizes and is capable of generating high thrust at low takeoff speeds, making it most suitable for commercial aircraft. The turbojet engine does not have a fan and creates thrust with air that passes completely through the gas path. Turbojets have small frontal dimensions and produce the most thrust at high speeds, making them most suitable for use in fighter aircraft.

In non-aeronautical gas turbines, part of the energy from the turbine is used to drive the compressor. The remaining energy - "useful energy" is removed from the turbine shaft at an energy utilization device such as an electric generator or a ship's propeller.

A typical land based gas turbine is shown in fig. 3. Such installations can generate energy from 0.05 MW to 240 MW. The setup shown in fig. 3 is a gas turbine derived from the aircraft, but lighter. Heavier units are designed specifically for ground use and are called industrial turbines. Although aircraft-derived turbines are increasingly being used as primary power generators, they are still most commonly used as compressors for pumping natural gas, powering ships, and used as supplementary power generators during periods of peak demand. Gas turbine generators can turn on quickly, supplying energy when it is most needed.

Figure 3. The simplest, single-stage, land-based gas turbine. For example, in energy. 1 - compressor, 2 - combustion chamber, 3 - turbine.

The most important advantages of a gas turbine are:

1. It is able to generate a lot of power with a relatively small size and weight.
2. The gas turbine operates in a constant rotation mode, unlike reciprocating engines operating with constantly changing loads. Therefore, turbines last a long time and require relatively little maintenance.
3. Although the gas turbine is started using auxiliary equipment such as electric motors or another gas turbine, starting takes minutes. For comparison, the start-up time of a steam turbine is measured in hours.
4. A gas turbine can use a variety of fuels. Large land-based turbines typically use natural gas, while aviation turbines tend to use light distillates (kerosene). Diesel fuel or specially treated fuel oil can also be used. It is also possible to use combustible gases from the process of pyrolysis, gasification and oil refining, as well as biogas.
5. Typically, gas turbines use atmospheric air as the working fluid. When generating electricity, a gas turbine does not need a coolant (such as water).

In the past, one of the main disadvantages of gas turbines has been low efficiency compared to other ICEs or steam turbines power plants. However, over the past 50 years, improvements in their design have increased thermal efficiency from 18% in 1939 on a Neuchatel gas turbine to the current efficiency of 40% in simple cycle operation and about 55% in combined cycle (more on that below). In the future, the efficiency of gas turbines will increase even more, with efficiency expected to rise to 45-47% in the simple cycle and up to 60% in the combined cycle. These expected efficiencies are substantially higher than other common engines such as steam turbines.

Gas turbine cycles

The sequence diagram shows what happens when air enters, passes through the gas path and exits the gas turbine. Typically, a cyclogram shows the relationship between air volume and system pressure. On fig. 4a shows the Brayton cycle, which shows the change in the properties of a fixed volume of air passing through a gas turbine during its operation. The key areas of this cyclogram are also shown in the schematic representation of the gas turbine in fig. 4b.

Figure 4a. Brayton's cycle diagram P-V coordinates for the working fluid, showing the flows of work (W) and heat (Q).

Figure 4b. Schematic illustration of a gas turbine showing points from the Brayton cycle diagram.

The air is compressed from point 1 to point 2. The pressure of the gas increases while the volume of the gas decreases. The air is then heated at constant pressure from point 2 to point 3. This heat is produced by the fuel being introduced into the combustion chamber and burning continuously.

Hot compressed air from point 3 begins to expand between points 3 and 4. The pressure and temperature in this interval fall, and the volume of gas increases. In the engine in Fig. 4b, this is represented by the flow of gas from point 3 through the turbine to point 4. This produces energy that can then be used. In fig. 1a, the flow is directed from point 3" to point 4 through the outlet nozzle and produces thrust. "Useful work" in Fig. 4a is shown by curve 3'-4. This is the energy capable of driving the drive shaft of a ground turbine or creating thrust for an aircraft engine. Cycle Brighton ends in Fig. 4 with a process in which the volume and temperature of the air decrease as heat is released into the atmosphere.

Figure 5. Closed loop system.

Most gas turbines operate in an open cycle mode. In an open circuit, air is taken from the atmosphere (point 1 in Figs. 4a and 4b) and expelled back into the atmosphere at point 4, so the hot gas is cooled in the atmosphere after it is expelled from the engine. In a gas turbine operating in a closed cycle, the working fluid (liquid or gas) is constantly used to cool the exhaust gases (at point 4) in the heat exchanger (shown schematically in Fig. 5) and is sent to the compressor inlet. Since a closed volume with a limited amount of gas is used, a closed cycle turbine is not an internal combustion engine. In a closed cycle system, combustion cannot be sustained and the conventional combustion chamber is replaced by a secondary heat exchanger that heats the compressed air before it enters the turbine. Heat is provided by an external source, e.g. nuclear reactor, coal fluidized bed furnace or other heat source. It was proposed to use closed-cycle gas turbines in flights to Mars and other long-term space flights.

A gas turbine that is designed and operated according to the Bryson cycle (Figure 4) is called a simple cycle gas turbine. Most gas turbines on aircraft operate on a simple cycle to keep the weight and frontal dimension of the engine as small as possible. However, for land or sea use, it becomes possible to add additional equipment to the simple cycle turbine in order to increase the efficiency and/or power of the engine. Three types of modifications are used: regeneration, intermediate cooling and double heating.

Regeneration provides for the installation of a heat exchanger (recuperator) on the way of exhaust gases (point 4 in Fig. 4b). Compressed air from point 2 in fig. 4b is preheated on the heat exchanger by exhaust gases before entering the combustion chamber (Fig. 6a).

If the regeneration is well implemented, that is, the efficiency of the heat exchanger is high, and the pressure drop in it is small, the efficiency will be greater than with a simple turbine cycle. However, the cost of the regenerator should also be taken into account. The regenerators were used in gas turbine engines in the Abrams M1 tanks - the main battle tank of Operation Desert Storm - and in experimental gas turbine engines of vehicles. Gas turbines with regeneration increase efficiency by 5-6% and their efficiency is even higher when operating under partial load.

Intercooling also involves the use of heat exchangers. An intercooler (intercooler) cools the gas during its compression. For example, if the compressor consists of two modules, high and low pressure, an intercooler should be installed between them to cool the gas flow and reduce the amount of work required to compress in the high pressure compressor (Fig. 6b). The cooling agent can be atmospheric air (so-called air coolers) or water (eg sea water in a ship's turbine). It is easy to show that the power of a gas turbine with a well designed intercooler is increased.

double heating is used in turbines and is a way to increase the power output of a turbine without changing the operation of the compressor or increasing the operating temperature of the turbine. If the gas turbine has two modules, high and low pressure, then a superheater (usually another combustor) is used to reheat the gas flow between the high and low pressure turbines (Fig. 6c). It can increase the output power by 1-3%. Dual heating in aircraft turbines is realized by adding an afterburner at the turbine nozzle. This increases traction, but significantly increases fuel consumption.

Combined cycle gas turbine power plant is often abbreviated as CCGT. Combined cycle means a power plant in which a gas turbine and a steam turbine are used together to achieve greater efficiency than when used separately. The gas turbine drives an electric generator. Turbine exhaust gases are used to produce steam in a heat exchanger, this steam drives a steam turbine which also produces electricity. If steam is used for heating, the plant is called a cogeneration power plant. In other words, in Russia the abbreviation CHP (Heat and Power Plant) is commonly used. But at CHP plants, as a rule, not gas turbines work, but ordinary steam turbines. And the used steam is used for heating, so CHP and CHP are not synonymous. On fig. 7 is a simplified diagram of a cogeneration power plant, showing two heat engines installed in series. The top engine is a gas turbine. It transfers energy to the lower engine - the steam turbine. The steam turbine then transfers the heat to the condenser.

Figure 7. Diagram of a combined cycle power plant.

The efficiency of the combined cycle \(\nu_(cc) \) can be represented by a rather simple expression: \(\nu_(cc) = \nu_B + \nu_R - \nu_B \times \nu_R \) In other words, it is the sum of the efficiency of each of the stages minus their work. This equation shows why cogeneration is so efficient. Assume \(\nu_B = 40%\) is a reasonable upper bound for the efficiency of a Brayton cycle gas turbine. A reasonable estimate of the efficiency of a steam turbine operating on the Rankine cycle at the second stage of cogeneration is \(\nu_R = 30% \). Substituting these values ​​into the equation, we get: \(\nu_(cc) = 0.40 + 0.30 - 0.40 \times 0.3 = 0.70 - 0.12 = 0.58 \). That is, the efficiency of such a system will be 58%.

This is the upper bound for the efficiency of a cogeneration power plant. The practical efficiency will be lower due to the inevitable loss of energy between stages. Practically in the cogeneration systems put into operation in recent years, an efficiency of 52-58% has been achieved.

Gas turbine components

The operation of a gas turbine is best broken down into three subsystems: compressor, combustion chamber, and turbine, as shown in Fig. 1. Next, we will briefly review each of these subsystems.

Compressors and turbines

The compressor is connected to the turbine by a common shaft so that the turbine can turn the compressor. A single shaft gas turbine has a single shaft connecting the turbine and compressor. A two-shaft gas turbine (Fig. 6b and 6c) has two conical shafts. The longer one is connected to a low pressure compressor and a low pressure turbine. It rotates inside a shorter hollow shaft that connects the high pressure compressor to the high pressure turbine. The shaft connecting the turbine and high pressure compressor rotates faster than the shaft of the turbine and low pressure compressor. A three-shaft gas turbine has a third shaft connecting the turbine and the medium pressure compressor.

Gas turbines can be centrifugal or axial, or a combination. The centrifugal compressor, in which compressed air exits around the outer perimeter of the machine, is reliable, usually costs less, but is limited to a compression ratio of 6-7 to 1. They were widely used in the past and are still used today in small gas turbines.

In more efficient and productive axial compressors, compressed air exits along the axis of the mechanism. This is the most common type of gas compressor (see figures 2 and 3). Centrifugal compressors consist of a large number of identical sections. Each section contains a rotating wheel with turbine blades and a wheel with fixed blades (stators). The sections are arranged in such a way that the compressed air sequentially passes through each section, giving some of its energy to each of them.

Turbines have a simpler design than a compressor, since it is more difficult to compress the gas flow than to cause it to expand back. Axial turbines like those shown in fig. 2 and 3 have fewer sections than a centrifugal compressor. There are small gas turbines that use centrifugal turbines (with radial gas injection), but axial turbines are the most common.

The design and manufacture of a turbine is difficult because it is required to increase the lifetime of the components in the hot gas stream. The design reliability issue is most critical in the turbine's first stage, where temperatures are highest. Special materials and a sophisticated cooling system are used to make turbine blades that melt at a temperature of 980-1040 degrees Celsius in a gas stream whose temperature reaches 1650 degrees Celsius.

The combustion chamber

A successful combustion chamber design must satisfy many requirements, and its proper design has been a challenge since the days of the Whittle and von Ohin turbines. The relative importance of each of the requirements for the combustion chamber depends on the application of the turbine and, of course, some requirements conflict with each other. When designing a combustion chamber, compromises are inevitable. Most of the design requirements are related to the price, efficiency and environmental friendliness of the engine. Here is a list of basic requirements for a combustion chamber:

1. High fuel combustion efficiency under all operating conditions.
2. Low fuel underburning and carbon monoxide (carbon monoxide) emissions, low nitrogen oxide emissions under heavy load and no visible smoke emissions (minimization of environmental pollution).
3. Small pressure drop when gas passes through the combustion chamber. 3-4% pressure loss is a typical pressure drop.
4. Combustion must be stable in all modes of operation.
5. Combustion must be stable at very low temperatures and low pressure at high altitude (for aircraft engines).
6. Burning should be even, without pulsations or disruptions.
7. The temperature must be stable.
8. Long service life (thousands of hours), especially for industrial turbines.
9. Usability different types fuel. Land turbines typically use natural gas or diesel fuel. For aviation kerosene turbines.
10. The length and diameter of the combustion chamber must match the size of the engine assembly.
11. The total cost of owning a combustion chamber should be kept to a minimum (this includes initial cost, operating and maintenance costs).
12. The combustion chamber for aircraft engines must have a minimum weight.

The combustion chamber consists of at least three main parts: shell, flame tube and fuel injection system. The shell must withstand the operating pressure and may be part of the gas turbine design. The shell closes a relatively thin-walled flame tube in which combustion and the fuel injection system take place.

Compared to other types of engines, such as diesel and reciprocating automobile engines, gas turbines produce the least amount of air pollutants per unit of power. Among gas turbine emissions, unburned fuel, carbon monoxide (carbon monoxide), oxides of nitrogen (NOx) and smoke are of greatest concern. Although the contribution of aircraft turbines to total pollutant emissions is less than 1%, emissions directly into the troposphere doubled between 40 and 60 degrees north latitude, causing a 20% increase in ozone concentrations. In the stratosphere where supersonic aircraft fly, NOx emissions cause ozone depletion. Both effects are detrimental. environment, so reducing nitrogen oxides (NOx) in aircraft engine emissions is what needs to happen in the 21st century.

This is a fairly short article that tries to cover all aspects of turbine applications, from aviation to energy, without relying on formulas. To get better acquainted with the topic, I can recommend the book "Gas Turbine in Railway Transport" http://tapemark.narod.ru/turbo/index.html. If you skip the chapters related to the specifics of the use of turbines on the railway, the book is still very understandable, but much more detailed.