THE COMBUSTION PROCESS IN GAS TURBINES

 The combustion process is of critical importance to the gas turbine cycle because in this process only heat energy, which is later converted into work by the turbine, is released from the fuel.  Therefore, any losses incurred in the combustion process have a direct effect on the thermal efficiency of the cycle.

Despite the importance of the combustion process, normally it receives lesser attention than the expansion, compression, and heat-transfer processes.  Owing to its extreme complication, this process is not completely understood, and no rigorous analysis has been made.  An empirical or trial-and-error approach is therefore adopted instead.  Furthermore, it is not too difficult to achieve a combustion efficiency of 98 % with a static pressure loss of less than 5%.  This, of course, raises the question of the efficacy of attempts at further improvement.  However, there is a real need for reducing the size of combustors, increasing their life and reliability, and providing a direct approach to design planning that is both economical of time and materials.  This last need is especially evident in view of the diverse design procedures for combustors in current use.              

The primary purpose of the combustor is to provide space for the chemical reaction of fuel and air, the air being supplied by the compressor and the products of combustion being delivered to the turbine.

In carrying out this purpose, the combustor must fulfill the following important requirements.

• The combustor must ensure the quick start up of the plant.
• The combustion must take place at higher efficiency, because any losses incurred in the combustion process will have a direct effect on the thermal efficiency of the cycle.
• Pressure losses/drop should be as low as possible.
• The hydraulic resistance must be as low as feasible.
• The geometry and arrangement of the combustor must provide for stability of combustion over a wide range of air fuel ratio, which varies from 40:1 to 200:1.
• Ignition must be reliable and accomplished with ease over a wide range of atmospheric conditions.
• High degree of mixing (Thorough mixing of air with fuel and as well as with combustion products) must be provided for effective complete combustion and uniform temperature distribution in the combustion gases supplied to the turbine.
• Carbon deposits should not be formed under any conditions to enhance the life of the combustion chamber.
• Combustion gases going to the turbine should be cooled adequately so as not to affect the blades.
• For longer service life, the combustor must be cooled properly especially in the hottest portions.
• The combustor must be simple in design, safe in operation, and easy and inexpensive in manufacture. 
• Combustors of mobile and transportable gas-turbine plants must have a low- mass and small dimensions as for as possible. 

The Combustion Process :

The combustion process is divided into three zones to understand it easily. These three zones are:

• Primary zone,
• Secondary zone,
• Tertiary zone.

In the primary zone about 15 to 20% of air is introduced around the jet of fuel. When this comparatively rich mixture burns, it provides the high temperature necessary to sustain the ignition. In the secondary zone, about 30% of air is introduced to complete the combustion. This air must be introduced carefully at the right points, because if too much cold air is injected, the flame may be chilled locally, thereby reducing the reaction rate in that neighborhood. Finally, in the tertiary zone the remaining air is mixed with these gases to cool them upto the temperature suitable for the blade material. Sufficient turbulence must be created so that hot and cold gases are thoroughly mixed to give the required uniform temperature distribution at the outlet of the combustion chambers. In actual combustion chamber, there is no line of demarcation between the zones but it is helpful to make these distinctions when analyzing the process.

The zonal method of introducing the air will not by itself give a self-piloting flame in an air stream which is moving an order of magnitude faster than the flame speed in a burning mixture. The second essential feature is therefore a re-circulating flow pattern, which directs some of the burning mixture in the primary zone back on to the incoming fuel and air. One way of achieving this is shown in figure 4.1(a), which is a typical British practice. This is also called as downstream type of combustion chamber where the fuel is injected in the same direction as the air stream.

Here the primary air is introduced through twisted radial vanes so that the resulted vortex motion may induce a region of low pressure along the axis of the chamber. This vortex motion is sometimes enhanced by injecting the secondary air through short tangential chutes in the flame tube, instead of through plain holes in figure. The net result is that the burning gases tend to flow towards the region of low pressure, and some portion of them is swept round towards the jet of fuel as indicated by arrows.

Many other solutions are possible for the stable flame. One American practice is to dispense with swirl vanes and achieve the recirculation by a careful positioning of the holes in the flame tube downstream of a hemispherical baffle as shown in above image . The below image shows a possible solution using upstream injection, which gives a good mixing of the fuel and primary air. It is difficult to avoid overheating of the fuel injector, however, an upstream injection is employed more for after burners (or reheat) in the jet pipe of aircraft engines than in main combustion system. After burners, operate only for short periods of thrust boosting. 

The above image illustrates a vaporizer system wherein the fuel is injected at low pressure into walking stick shaped tubes placed in the primary zone. A rich mixture of fuel vapour and air issues from the vaporizer tubes in the upstream direction to mix with the remaining air passing through holes in a baffle around the fuel supply pipes.  Most chambers are designed to have air inlet velocity of not more than 80 m/sec. at full load.

Pressure Loss:

There is some pressure drop in the combustion chamber whether the combustion is direct or indirect. The factors contributing towards this loss are:

• Heating process.
• Skin friction and turbulence.                                                                                                            

Pressure loss due to heating process only, is known as fundamental loss or hot loss. Due to friction and turbulence, also loss takes place and that is known as cold loss, which contributes more. 

Combustor Parts:

The below image shows the essential elements of a combustor. It consists of the following main elements.

1. Outer casing.
2.  A perforated combustion liner.
3. Cross Fire tube (Cross flame tube)
4.  Fuel injection systems (fuel nozzles)
5. Initial ignition system such as spark plugs.
6. Ultra violet flame detectors
7. Transition pieces