Gas turbine working with intake, compressor, combustur & Gears

In order to understand the purpose of compressor, intake and combustur there is need to understand the working of turbine in simple words. The air or gas comes from intake and compressed with the help of compressor through adiabatic process as no heat enter or leave the system and then it enters in combustion chamber where it get mixed with fuel and burn with constant pressure after that it is expand to drive the shaft. These mechanism can be divided into four stages 

From stage 1 to 2 the gas is compressed adiabatically as no heat is evolve or absorb. The pressure of gas increases to required limit so, it can enter in the combustion phase. This trend can easily be seen form above figure form 1 to 2. In the next stage from 2 to 3, the compressed air enters in the combustion phase where it is sprayed with fuel and initially ignited with ignitor. In the second stage, the pressure remains constant and heat is entered in the system through combustion process. In the third stage form 3 to 4, the burning air is allowed to expand adiabatically to run the shaft of engine and to extract energy from the system. In this stage pressure drops but no heat enters or leaves the system. To close the cycle the hot air is passed through heat exchange in stage 4 from 4 to 1 where pressure remains constant but heat is drive away from the system

1.1 Compressor:
The compressor is used to compress the air coming from intake and feed it to the combustion chamber. The compressor is negative work mechanism as it consume energy from the system to compress air. The process is adiabatic as no heat energy enter or leave the system at this point. The designing of compressor play a vital role in the working and efficiency of Gas Turbine Engine. There are different types of compressor used depend upon the size, required pressure of gas/air and cost parameters.

The basic types of compressors are centrifugal compressors and axial compressors
  • The centrifugal compressor uses moving impeller to compress air. The impeller rotates and drive air in the direction away from the center of impeller resulting in increase of pressure. To obtain higher pressure it is used in multiple stages.
  • The axial compressor use reciprocating piston to increase pressure. This is useful where high pressure is required in single stage.

1.2 Combustor:
The compressed gas enters in the combustion chamber where it get mixed with fuel and burn. The combustion chamber process occur at constant pressure. The fuel is sprayed on the compressed air by a fuel atomizer and burning is initiated by ignitor. In the basic design, the combustion chamber is divided into two parts, one is the primary portion in which maximum combustion take place and other is secondary portion in which more gas is mixed to cool the mechanism.

1.3 Intake:  
The intake is usually a duct to intake smooth air with minimum turbulence. The length of duct is kept minimum to reduce drag and weight. The requirement is that air must enter in straight line to accommodate maximum air without disturbance and also to reduce corrosion and erosion in the engine, the intake dust must provide clean and undisturbed air.
Purposes of gearboxes used on Gas turbine engine.
In order to drive the accessories like pumps, hydraulic etc, some energy is taken from the main shaft connecting the compressor and turbine through internal gearbox that drives the external gear box and all accessories are connected to external gear box.

2 Internal Gear:
Usually the internal gearbox is complex due to smaller space available near the compressor and turbine. The purpose of this gearbox is to take power from the main shaft and utilize that energy to run different accessories like fuel pump etc. the internal gearbox is normally place between the compressor outlet and combustion chamber but in case of Gas turbines with centrifugal pumps, it can be placed above the compressor. One more limitation of this gearbox is that it is normally placed at high heat, so thermal expansion is considered during designing. It is mostly connected with the main shaft through bevel gear or stub gear in which it is driven with the help of splines and stub gear drive the radial shaft through bevel gears. One more technique is use of narrow shaft parallel to main shaft driven with the help of spur gears and drive the main gearbox with the help of bevel gears.

2.1 Gear train drive:
If there is enough space then internal gear box is connected to external gear box via gear train drive and external gear box get power through simple procedure but now a day it is difficult to find such mechanism due to limitation of space.
2.2 Intermediate gear box:
When the direct aligning of internal gear box with external gear box is not possible then intermediate gear box is installed to connect both internal and external gear boxes as shown in below picture.

2.3 External Gear Box:
The external gear box is connected to all the accessories like pumps, controllers etc. it get power from the main engine shaft through radial gear train or intermediate gear box and with that power run all small parts and controls of gas turbine engine.

3.0 Description of accessories Gear box
In the start of gas turbine engines, the accessories were small and as it was the start of designing so, mostly the accessories were operated through direct interaction with main shaft of engine. The limitation with this practice was that the location of accessory must be such that it can be connected with main shaft through radial shaft. With the passages of time, the designing of gas turbine engine improved and get complex, so, to enhance efficiency and user controls, more accessories like pumps, pneumatic, hydraulics and user controls circuits etc. added in the mechanism furthermore space was limited and all accessories cannot be installed near and radial to main shaft. To make this system run properly, set of gear are installed to distribute and provide power to all accessories.

The coupling of accessories is such that all accessories are directly connected to external gear box and external gear box is driven by internal gear box which is connected with the main rotating shaft of engine.

In some cases, internal gear box is misaligned with the external gear box so an intermediate gear box is required to connect both. The intermediate gear box is driven with the internal gear box and it drives the external gear box.


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