Compressor: A component in a gas turbine engine that uses high-speed rotating blades to work on air to increase air pressure.The front part of the compressor impeller blade is curved, which is called the guide wheel. Its function is to guide the gas into the working impeller without impact, so as to reduce the flow impact loss.The compressor impeller of a small supercharger generally integrates the guide wheel with the working impeller.There is a diffuser at the impeller outlet of the compressor to convert the kinetic energy obtained by the gas in the impeller into pressure as much as possible.Diffusers are divided into vane type and slot type.The casing of compressor has inlet and outlet of air flow.The inlet is generally arranged axially, and the flow channel is slightly shrunk to reduce the inlet resistance.The outlet is generally designed as a volute shaped flow channel that gradually expands along the circumference, where the high-speed airflow continues to expand and improve the overall efficiency of the supercharger.The compressor is driven by a turbine, and its main performance parameters include: speed, flow, air flow, boost ratio, efficiency, etc.
The ratio of total pressure of compressor outlet air to total pressure of inlet air is called compressor boost ratio. When the boost ratio is the same, the ratio of theoretically required compression work to actually consumed mechanical work is called compressor efficiency.Compressors can be divided into centrifugal type and axial flow type, and the hybrid compressor with both characteristics is called hybrid compressor.According to the relative velocity of the airflow flowing into the compressor rotor blade, the compressor can be divided into three types: subsonic, transsubsonic and supersonic.[1]
Organization composition
Announce
edit
compressor
Centrifugal compressor is composed of air guide wheel, impeller, diffuser, etc. (Figure 1).Air fromAir inletEnter the compressor and enter the impeller through the guide of the air guide wheel rotating with the impeller.Under the action of high-speed rotating impeller, the air is thrown to the outer edge of the impeller by centrifugal force from the center of the impeller, and the pressure gradually increases. After the air from the impeller enters the diffuser, the speed decreases, and the pressure increases again. Finally, it flows out of the compressor from the outlet pipe.
The air flow of centrifugal compressor is several kilograms to tens of kilograms per second.The pressure ratio of the subsonic centrifugal compressor is about 4.5, and the supersonic centrifugal compressor can reach 8-10, and the efficiency is about 0.78.
Axial flow compressor Air flows mainly along the axial direction in the axial flow compressor.It consists of rotor (also called working wheel, colored part in Figure 2) and stator (also called rectifier, colorless part in Figure 2).The first stage is composed of a row of rotor blades and a row of stator blades. The pressurization ratio of a single stage is very small. In order to obtain a higher pressurization ratio, the multi-stage structure shown in the figure is generally used.After the air is gradually pressurized in the compressor, the density and temperature also increase step by step.
The air flow of axial flow compressor ranges from several kilograms per second to two hundred kilograms per second, the single-stage pressurization ratio is generally about 1.1~2.0, and the efficiency is about 0.85~0.88.The pressurization ratio of multistage axial flow compressor can reach more than 25.Axial flow compressors have been widely used in gas turbine engines due to their small area, high turbocharging ratio and high efficiency.
Performance characteristics
Announce
edit
Compressors are designed according to the given inlet air conditions, speed, boost ratio and air flow, but their working conditions (temperature, pressure, speed and air flow of the working environment) are actually changing. The performance of compressors under various working conditions is called compressor characteristics.At a certain timespeedWhen the pressure ratio of the compressor increases to a certain value, the compressor will enter into an unstable working state, which is easy to occursurgeThe whole system will produce axial pulsation of air flow with low frequency and large amplitude, and even the phenomenon of instantaneous air flow reversal will occur.Compressor surge may cause blade fracture, structural damagecombustion chamberUltra mild engine shutdown.The following measures can be taken to avoid surge:
① Adjust the installation angle of several stages of rectifying blades according to the speed, so that the incoming air flow has an appropriate angle of attack to avoid surge caused by air flow separation.
② The multistage compressor is divided into two rotors with different speeds, which are driven by high and low pressure turbines respectively.Some engines use a 3-rotor structure.
③ The multistage axial flow compressor discharges air from the middle stage to increase the air flow at the front stages, so as to avoid separation and surge caused by excessive angle of attack of the air flow.
④ The multi-stage axial flow compressor makes grooves on the casing of the first stage compressor to make the air flow at the tip of the first stage working wheel blade return through the channel on the casing to reduce the angle of attack of the air flow. This method is called casing treatment.
Blade vibration compressor blades often crack or even break due to vibration.There are two types of vibration: one is blade vibration under periodic external force, called forced vibration.The periodic external force comes from the mutual interference between the working wheel blade and the rectifier blade, the rotating stall of the working wheel blade, etc.The other is caused by the vibration of the blade itself and the mutual interference with the vibration of adjacent blades, which is called self-excited blade vibration or blade flutter.In order to avoid blade flutter, two adjacent blades on the working wheel can adopt different thicknesses to change their natural frequencies.
working principle
Announce
edit
Main components of axial flow compressor
The turbojet engine operates on a "duty cycle".It draws air from the atmosphere, and after the process of compression and heating, the air that gets energy and momentum is discharged from the propulsion nozzle at a speed of up to 2000 feet/second (610 meters/second) or about 1400 miles/hour (2253 kilometers/hour).When the high-speed jet flows out of the engine, it drives the compressor and turbine to continue rotating to maintain the "working cycle".The mechanical layout of turbine engine is relatively simple, because it only contains two main rotating parts, namely compressor and turbine, and one or several combustion chambers.However, not all aspects of this engine have this simplicity, because thermal and aerodynamic problems are relatively complex.These problems are caused by the high operating temperature of the combustion chamber and turbine, the constantly changing airflow through the compressor and turbine blades, and the design work of the exhaust system that discharges the gas and forms the propulsion jet flow.
Surge principle
Announce
edit
Compressor surge is a low frequency and high amplitude oscillation phenomenon of air flow along the compressor axis.This low frequency and high amplitude air flow oscillation is a great source of exciting force, which will lead to strong mechanical vibration of engine parts and overtemperature of hot end, and cause serious damage to engine parts in a very short time. Therefore, the compressor is not allowed to enter the surge area under any conditions.
Surge phenomenon
Announce
edit
The sound of the engine changed from whistle to deep;The vibration of the engine increases;The total pressure and flow at the compressor outlet fluctuate significantly;The speed is unstable, the thrust drops suddenly and fluctuates greatly;The engine exhaust temperature rises, causing overheating;In serious cases, blasting will occur, and the gas flow will be interrupted, resulting in flameout and shutdown.Therefore, once the above phenomenon occurs, measures must be taken immediately to make the compressor exit the surge working state.
Root cause
Announce
edit
As the attack angle is too large, the air flow is separated at the blade back, and this air flow separation seriously extends to the entire cascade channel.
purpose
Announce
edit
1. Air supply for pneumatic equipment 2. Inflate (tires) 3. Paint and spray 4. Inflate (sterilized and filtered) in aerobic fermentation tank 5. Air supply for plasma cutting 6. Air supply for pneumatic tools.
Related classification
Announce
edit
Centrifugal
It is composed of intake system, impeller, diffuser and gas collector
Air is sucked into the center (inlet) of the impeller, and centrifugal force makes the air enter the diffuser channel from radial direction at high speed. In the diffuser, the air flow is decelerated to obtain pressure rise
Blades of rotor and diffuser have various shapes, which are selected according to the requirements of pressure velocity characteristics
Advantages: simple structure, reliable operation and stable performance
Disadvantages: low efficiency, large windward area
After the 1950s, centrifugal compressors were no longer used except for small turboshaft and turboprop engines
Cooperate with axial flow compressor as the last stage of compressor
The turbocharging ratio of the centrifugal compressor under study can reach more than 12
The minimum flow of centrifugal compressor is limited by surge condition, and the maximum flow is limited by blocking condition
Variable speed, inlet throttle, outlet throttle, adjustable inlet guide vane and other methods can be used for adjustment to expand the range of operating conditions
Blocking: the airflow shrinks due to the effect of blades and streamline curvature,
A local supersonic zone is formed near the inlet, and the supersonic zone extends to the whole
The phenomenon that the gas flow reaches the maximum value and cannot be increased when the throat section is
Axial flow type
Axial compressor
The compressor with gas flowing near the axial direction is generally called axial flow blower;The moving blade accelerates the fluid, and the stationary blade acts as a diffuser to convert the speed into pressure rise.Similar to the reverse process of reaction turbine
Axial flow compressors are widely used for medium pressure air supply and other gases in gas turbine devices, blast furnace blowing, air separation, natural gas liquefaction, heavy oil catalysis and other devices
Axial compressor stage=one row of rotor blades+one row of stator blades
The rotor blade is fixed on the drum, and the stator blade is fixed on the cylinder
Moving blade, kinetic energy fluid, pressure slightly increased;Stator train, the pressure of fluid increases further
High pressure ratio device, compressor stage>20
The inlet guide vane has no pressure rise and does not belong to the first stage of compressor.
Purpose: to obtain the required flow field distribution when the airflow enters the first stage
The air is continuously compressed through the axial compressor, and the specific volume of air decreases and the density increases.Therefore, the channel cross-sectional area of the axial flow compressor is gradually reduced in a convergent form, and the compressor outlet cross-sectional area is much smaller than the inlet cross-sectional area
Compressor passage vs turbine passage
Cross-sectional area↗ Deceleration, pressure increase Kinetic energy is converted into pressure increase
Cross-sectional area↘ Increase in speed and decrease in pressure Increase in kinetic energy
Note: Relative speed
Supercharging principle
When the air flow passes through the elementary stage, the rotor blades work to pressurize the air flow, so that the total pressure and total temperature of the air flow at the outlet of the elementary stage are higher than those at the inlet
Compressor elementary stage efficiency: obtain the same total pressure boost ratio,
Ideal adiabatic compression work/actual compression work
The flow parameters of compressor elementary stage vary greatly along the blade height direction because:
The tangent speed u of the working wheel primitive level is not equal along the blade height, which makes the work done by the air flow of the working wheel set not equal along the blade height.
In the air rotating flow field behind the working wheel, there must be a radial pressure difference. The larger the radius, the higher the static pressure, and the centripetal acceleration of the gas micro cluster
Change the blade shape (the working wheel blade and deflector blade are twisted)
The stall in one stage of an axial flow compressor does not occur at the same time along the entire torus, but first occurs at a certain part of some blades, and the stall area is not fixed on these blades.The stall zone moves in the opposite direction to the rotation direction relative to the working wheel cascade.
Multi stage axial flow compressor is prone to surge under the following two conditions:
When working at a certain speed, if the outlet back pressure increases and the air flow decreases to a certain extent, surge will occur
When the engine deviates from the design working condition and reduces the speed, surge is easy to occur
Multi stage axial flow compressor with low pressurization ratio is designed, and the change of inlet and outlet cross-sectional area is small, so it is not easy to surge
When surge occurs, there is a strong unstable working phenomenon: the airflow flowing through the compressor produces a strong oscillation of low frequency and high amplitude along the axis direction of the compressor, the average pressure at the compressor outlet drops sharply, and the total pressure, flow rate and flow rate at the outlet produce large fluctuations, accompanied by a strong blasting sound
Anti surge measures
① Bleeding from one or more intermediate sections of multistage axial flow compressor
When the compressor speed is lower than a certain value, the vent valve will be opened to increase the air flow of the previous stages of the compressor and avoid airflow separation due to excessive attack angle of the previous stages.The intermediate stage bleeding also avoids the phenomenon that the inlet flow rate of the compressor in the later stages is too large, the attack angle is too small, or even negative, which reduces the boost ratio and efficiency
Simple and uneconomical (put compressed air into the surrounding atmosphere, and lose the mechanical power to compress this air)
② The first stage adopts adjustable inlet guide vanes and stationary vanes. At low speed, they can close to improve the axial speed of air flow, prevent stall, so as to approach the optimal operating condition.(Adjustable inlet guide vane and stationary vane can also be used for the last several stages)
③ Double or three-axis structure
The single-stage turbocharging ratio is very small, 1.15~1.35. In order to obtain a high turbocharging ratio, the multi-stage structure is generally adopted.After the air is pressurized step by step in the compressor, the density and temperature also increase step by step
Main performance parameters of axial compressor: pressure, flow, power, efficiency, speed.
The minimum flow is limited by the surge condition, and the maximum flow is limited by the blocking condition.Variable speed, inlet throttle, outlet throttle and adjustable stationary blade can be used for adjustment to expand the range of operating conditions
Correlation comparison
Centrifugal compressor
Advantages: the compressor has high stage pressure ratio, good operating range, good efficiency within the operating range, easy manufacturing, low cost and light weight
Disadvantages: large cross-sectional area, loss increases with the number of stages up to 2 stages
Axial compressor
Excellent: high peak efficiency, high pressure ratio can be achieved with many stages with low loss, small cross sectional area and large mass flow
Lack: the operating range with good efficiency is narrow, the manufacturing cost is high, the weight is large, and the starting power (possibly) is high
