What Is a Compressor Blade?

Compressor: A component of a gas turbine engine that uses high-speed rotating blades to work the air to increase air pressure. The front end of the impeller blade of the compressor is curved and called the guide wheel, and its role is to introduce the gas into the working impeller without impact to reduce the impact loss of the air flow. The compressor impeller of a small supercharger generally integrates the guide wheel and the working impeller. The impeller outlet of the compressor has a diffuser, so that the kinetic energy obtained by the gas in the impeller is converted into pressure as much as possible. The diffuser is divided into two types: vane type and slot type. The compressor housing has air inlets and outlets. The inlet is generally arranged axially, and the flow channel is slightly tapered to reduce the intake resistance. The outlet is generally designed as a volute that gradually expands along the circumference of the flow channel, so that the high-speed airflow continues to expand there and improve the overall efficiency of the supercharger. The compressor is driven by a turbine. Its main performance parameters are: speed, flow, air flow, boost ratio and efficiency.

Chinese name: Compressor
Foreign name: compressor

Drive: turbine
Effect: Pressurize

Compressor Machine Introduction

The ratio of the total air pressure at the outlet of the compressor to the total air pressure at the inlet is called the compressor boosting ratio. When the boosting ratio is the same, the ratio of the theoretically required compression work to the actual mechanical work is called compressor efficiency. Compressors can be divided into two types: centrifugal and axial flow. Hybrid compressors that have two types of features are called compressors. According to the relative speed of the airflow flowing into the rotor blades of the compressor, the compressor can be divided into three forms of subsonic speed, trans-subsonic speed and supersonic speed. ^[1]

Compressor mechanism composition

The centrifugal compressor is composed of a wind guide wheel, an impeller, and a diffuser (Figure 1). Air enters the compressor from the inlet, passes through

Compressor The wind guide, which rotates with the impeller, is guided into the impeller. Under the action of the high-speed rotating impeller, the air is thrown from the center of the impeller to the outer edge of the impeller by centrifugal force, and the pressure gradually increases. After the air flowing out of the impeller enters the diffuser, the speed decreases, the pressure increases again, and finally the air exits the compressor through the outlet pipe.

The centrifugal compressor has an air flow rate of several kilograms to tens of kilograms per second. The supersonic centrifugal compressor has a boost ratio of about 4.5, and the supersonic centrifugal compressor can reach 8 to 10 with an efficiency of about 0.78.

Axial-flow compressor air in the axial-flow compressor mainly flows in the axial direction. It consists of a rotor (also known as a working wheel, colored part in Figure 2) and a stator (also called a rectifier, colored part in Figure 2). A row of rotor blades and a row of stator blades form a single stage. The single-stage supercharging ratio is very small. In order to obtain a higher supercharging ratio, a multi-stage structure as shown in the figure is generally used. After the air is pressurized step by step in the compressor, the density and temperature are also increased step by step.

The air flow rate of the axial compressor is several kilograms per second to 200 kilograms per second. The single-stage supercharging ratio is generally about 1.1 to 2.0, and the efficiency is about 0.85 to 0.88. The multi-stage axial flow compressor has a boost ratio of more than 25. Axial flow compressors have a small area, high boost ratio and high efficiency, and have been widely used in gas turbine engines.

Compressor performance characteristics

The compressors are designed according to the given intake conditions, speed, boost ratio and air flow, but their working conditions (temperature, pressure, speed and air flow in the working environment) actually change. Performance under various operating conditions is called compressor characteristics. At a certain speed, when the compression ratio of the compressor is increased to a certain value, the compressor will enter an unstable working state, which is prone to surge, causing the entire system to generate low-frequency and large-amplitude axial pulsations of the airflow. Even the phenomenon of instantaneous backflow occurs. Compressor surge may cause blade breakage, structural damage, overheating of the combustion chamber, and engine shutdown. To avoid surges:

Adjust the installation angle of certain stages of rectifier blades according to the rotation speed, so that the incoming airflow has a proper angle of attack to avoid the airflow separating and causing surge.

The multi-stage compressor is divided into two rotors with different speeds, which are driven by high and low pressure turbines. Some engines have a three-rotor structure.

The multi-stage axial-flow compressor deflates air from the middle stage to increase the air flow of the front stages, to avoid the angle of attack of the air flow being too large, causing separation and surge.

The multi-stage axial-flow compressor is slotted in the casing of the first-stage compressor, so that the airflow at the tip of the blade of the first-stage working wheel passes through the groove on the casing to reduce the angle of attack of the airflow. This method is called box processing.

Blade vibration Compressor blades often cause cracks or even fractures due to vibration. Vibration is divided into two types: one is the blade vibration that occurs under the action of a periodic external force, which is called forced vibration. The periodic external force comes from the mutual interference between the working wheel blades and the rectifier blades, and the rotating stall of the working wheel blades. The other type is formed by the vibration of the blade itself and the interference with the adjacent blade itself, which is called blade self-excited vibration or blade flutter. To avoid blade flutter, two adjacent blades on the work wheel can be made with different thicknesses to change their natural frequencies.

Working principle of compressor

Turbojet engines work according to a "working cycle". It draws air from the atmosphere, compresses and heats it

Main components of axial compressor After this process, the energy and momentum-derived air is expelled from the propulsion nozzle at a speed of up to 2000 feet per second (610 meters per second) or about 1400 mph (2253 km / hour). When the high-speed jet stream is ejected from the engine, the compressor and turbine are driven to continue to rotate at the same time, maintaining the "working cycle". The mechanical layout of the turbine engine is relatively simple, because it contains only two main rotating parts, namely the compressor and the turbine, and one or several combustion chambers. However, not all aspects of this engine have this simplicity because thermal and aerodynamic issues are more complex. These problems are caused by the high operating temperature of the combustion chamber and turbine, the ever-changing air flow through the compressor and turbine blades, and the design work of the exhaust system that exhausts the gas and forms a propulsion jet stream.

Compressor Surge Principle

Compressor surge is a low-frequency, high-amplitude oscillation that occurs along the axis of the compressor. This low-frequency, high-amplitude airflow oscillation is a great source of excitation force. It will cause strong mechanical vibration of the engine parts and overheating of the hot end, and cause severe damage to the parts in a short time. Therefore, the compressor is not allowed to work in the surge zone under any state.

Compressor surge

The sound of the engine changes from a whistle to a low level; the vibration of the engine increases; the total pressure and flow of the compressor outlet fluctuate greatly; the speed is unstable, the thrust suddenly drops and there is a large fluctuation; the exhaust temperature of the engine rises Causes over-temperature; in severe cases, firing occurs, the air flow is interrupted and the flameout stops. Therefore, once the above phenomenon occurs, measures must be taken immediately to bring the compressor out of the surge working state.

The root cause of the compressor

Because the angle of attack is too large, the airflow is separated at the blade back and this airflow separation is extended to the entire cascade channel.

Compressor use

1. Air source for pneumatic equipment 2. Inflate (tire) 3. Spray paint and spray 4. Inflate (sterilized and filtered) in aerobic fermentation tank 5. Air source for plasma cutting 6. Air source for pneumatic tools.

Compressor related classification

Centrifugal compressor

Consists of four parts: air intake system, impeller, diffuser, and air collecting pipe

The air is sucked in the center (inlet) of the impeller, and the centrifugal force causes the air to enter the diffuser channel from the radial direction at high speed.

Rotor and diffuser blades come in various shapes and are selected according to pressure-speed characteristics

Advantages: simple structure, reliable work, stable performance

Disadvantages: low efficiency, large windward area

After the 1950s, except for small turboshafts and turboprop engines, centrifugal compressors were no longer used

Cooperate with axial compressor as the final stage of compressor

The centrifugal compressor boost ratio in the study can reach more than 12

The minimum flow rate of a centrifugal compressor is limited by surge conditions, and the maximum flow rate is limited by blocking conditions

Variable speed, inlet throttling, outlet throttling, and adjustable inlet guide vanes can be used to adjust to expand the range of operating conditions

Obstruction: The airflow is contracted by the action of the blades and the curvature of the streamline,

A local supersonic zone is formed near the entrance, and the supersonic speed is extended to the whole

The phenomenon that the gas flow reaches the maximum at the throat section and cannot be increased any more

Compressor axial flow

Compressors with gas flowing near the axis, also known as axial flow blowers; moving blade acceleration

Axial compressor The fluid, the stationary vane acts as a diffuser, converting speed into pressure rise. Reverse process similar to a reaction turbine

Axial flow compressors are widely used in gas turbine installations, blast furnace blasts, air separation, natural gas liquefaction, heavy oil catalysis, etc.

Stages of an axial compressor = one row of rotor blades + (next) one row of static co-lobes

The rotor blade is fixed on the rotating drum, and the stator blade is fixed on the cylinder

Rotating blades, kinetic energy fluid, pressure rises slightly; static columns, fluid pressure further increases

High pressure ratio device, compressor stages> 20

Imported guide vanes, without pressure rise, do not belong to the first stage of the compressor.

Purpose: to obtain the required flow field distribution when the airflow enters the first stage

The air is continuously compressed by the axial compressor, and the specific volume of air decreases and the density increases. Therefore, the cross-sectional area of the channel of the axial compressor is gradually reduced and converged. The cross-sectional area of the outlet of the compressor is much smaller than the cross-sectional area of the inlet.

Compressor runner vs turbine runner

Cross-sectional area deceleration, boost kinetic energy is converted into boost

Cross-sectional area speed increase, pressure drop kinetic energy increase

Note: relative speed

Compressor booster principle

When the airflow passes through the elementary stage, the rotor blades pressurize the airflow to make the total pressure and temperature of the airflow at the elementary stage outlet higher than the inlet.

Elementary efficiency of the compressor: obtain the same total pressure boost ratio,

Ideal Adiabatic Compression Work / Actual Compression Work

The elementary airflow parameters of the compressor vary greatly along the leaf height because:

The tangent speed u of the element level of the work wheel is not equal along the leaf height, so that the work performed by the work wheel on the airflow is not equal along the leaf height.

In the air rotating flow field behind the work wheel, a radial pressure difference must be generated. The larger the radius, the higher the static pressure, which will cause the gas micelles to generate centripetal acceleration.

Change the shape of the blade (workwheel blade and deflector blade are twisted)

A stall in an axial flow compressor does not occur simultaneously along the entire annulus, but first in a certain part of the blade, and the stall area is not fixed on these blades. The stall zone moves in a direction opposite to the direction of rotation with respect to the working wheel cascade.

Multistage axial flow compressors are prone to surge in the following two situations:

When working at a certain speed, if the outlet back pressure increases to reduce the air flow to a certain level, surge will occur

Surge is prone to occur when the engine deviates from the design working condition and reduces the speed

Design of a multi-stage axial flow compressor with a relatively low boost pressure, the change in the cross-sectional area of the inlet and outlet is small, and surge is not easy to occur

When surge occurs, a strong unstable working phenomenon occurs: the airflow flowing through the compressor generates a strong low-frequency and high-amplitude oscillation along the axis of the compressor. Amplitude pulsation, accompanied by a strong shot

Compressor anti-asthma measures

Bleed from one or several intermediate sections of a multi-stage axial compressor

When the speed of the compressor is lower than a certain value, the vent valve is opened. The purpose is to increase the air flow of the compressors of the previous stages and avoid the separation of the air flow due to the excessive attack angle of the previous stages. The intermediate stage bleed also avoids the excessive inlet velocity of the compressors of the subsequent stages, the angle of attack is too small, or even a negative value, which reduces the boost ratio and efficiency.

Simple and uneconomical (putting the compressed air into the surrounding atmosphere loses the mechanical work of compressing this part of the air)

The first stage uses adjustable inlet guide vanes and stationary vanes. At low speeds, they can be closed. Increase the axial speed of the airflow to prevent stalling, so that it can approach the optimal operating conditions. (Adjustable imported guide vanes and stationary vanes are also available for the last stages)

Adopt biaxial or triaxial structure

The single-stage supercharging ratio is very small from 1.15 to 1.35. In order to obtain a higher supercharging ratio, a multi-stage structure is generally used. After the air is pressurized step by step in the compressor, the density and temperature are also increased step by step.

The main performance parameters of axial flow compressor: pressure, flow, power, efficiency, speed.

The minimum flow is limited by surge conditions, and the maximum flow is limited by blocking conditions. Variable speed, inlet throttling, outlet throttling, and adjustable stationary blades can be used to adjust to expand the range of operating conditions