Application Of Flow Simulation In Gas Turbines

 

1. Introduction to Gas Turbines:

The gas turbine is a prime mover which develops power by burning of fuel. The gas turbines are axial flow machines which convert the heat from combusted fuel into the trust power or shaft power. The hot gases undergo momentum changes when they flow through the passages formed by the stationery and rotating blades.

A jet of hot gases and air mixture is made to flow over rings of moving blades mounted on a shaft and in doing so the velocity of the jet decreases. Its kinetic energy is absorbed by the rings of blades imparting rotary motion to the shaft. A larger part of the power developed by the turbine rotor is consumed for driving a compressor which supplies air under pressure to a combustion chamber, while the remaining power is utilized for doing the external work.

The basic principle on which a gas turbine works is similar to internal combustion reciprocating engine. In both the cases, air is made to enter the prime mover which compresses, air and it is heated by the combustion process and thus raising the pressure still further expanded and finally the expanded products are discharged through the exhaust.

2. Types of Gas Turbines:

(1) Constant Volume or Explosion Type:

The combustion of fuel in this case takes place at constant volume.

The air drawn from the atmosphere is compressed in the compressor to a pressure of 15 N/cm2 to 35 N/cm2 and forced into the combustion chamber through the valves. The oil which is stored in the fuel tank is pumped into the combustion chamber through the injector by means of a fuel injection pump.

Afterwards the injection is stopped and the oil is ignited in presence of air by the spark plug. The ignition takes place at constant volume resulting in an explosion and during this the pressure suddenly rises to about 10 N/cm2 to 145 N/cm2.

(2) Constant Pressure or Continuous Combustion Type Turbine:

It is called continuously combustion type gas turbine. It works on the Joule cycle or Braytron cycle. Fig. 15-4 shows a plant employing this type of turbine. Atmospheric air is drawn and compressed by the compressor which is usually driven by the turbine rotor shaft to a pressure of 15 N/cm2 to 40 N/cm2.

The high temperature and pressure air flows through the annular space between the walls of combustion and mixing chamber. This hot air then meets and mixes with the hot gases from the burner. Thus it is seen that a quantity of air quite in excess of that required for the combustion of oil in the combustion chamber is supplied by the compressor.

3. Types of Compressors Used in Gas Turbines:

1. Centrifugal Compressors:

The principal feature of a centrifugal compressor is illustrated in fig. 15-18. The rotating member known as the impeller consists of a large number of blades and is mounted on the compressor shaft inside the stationary casing. As the impeller rotates, the pressure in the region falls and hence the air enters through the eye and flows radially outwards through the impeller blades as shown in fig. 15-

2. Axial Compressor:

In this type of compressor which is now more commonly used, the air flows in an axial direction right from the intake to the delivery. The working principle is illustrated in fig. 15-19. The stator which encloses the rotor both of which are provided with rings of blades.

As the air enters in the direction shown, it flows through the alternately arranged stator and rotor blade rings. While flowing through each pair of blade rings formed up of one rotor blade ring and one stator blade ring the air gets compressed successively. The air is finally delivered in the direction as shown.

3. Vane Type:

It consists of drum on which a number of vanes are mounted in such a manner that they can slide inside or outside against the spring force. They all the time remain in contact with the inner surface of the supercharger body. The space between the body and the drum decreases from the inlet to the outlet. The medium trapped between any two vanes at inlet goes on decreasing in volume and increasing in pressure as it reaches the outlet.

4. Root Air Blower:

The compressor consists of two rotors of epicycloid shape. Each rotor is fixed to a shaft by a key. The two shafts are connected together by means of gears of equal size. The two rotors revolve at the same speed. It works just like the gear pump where the mixture at the outlet side is at a high pressure.

These compressor are simpler in construction and requires less maintenance. It has comparatively longer life. It works even at lower speed. The centrifugal compressor has poor working characteristics at lower speeds. Vane type compressor has problems of wear of vane tips.

3. Fuels Used in Gas Turbines:

The gas turbine can use fuels in the combustion chamber of three different varieties:

(1) Liquid fuels

(2) Gaseous fuels

(3) Solid fuels.

5. Applications of Gas Turbines:

The gas turbines have wide applications:

(1) Supercharging:

The gas turbines are used for supercharging. A small gas turbine run by the hot exhaust gases which drives the compressor for aviation gasoline engines and for heavy duty diesel engines.

(2) Turbojet and Turbo Propeller Engines:

Every turbojet and turbo propeller engine has a gas turbine. The turbine supplies power only to drive the air compressor in the turbojet engines, while in turbo propelled engines they may drive the propeller in addition to the compressor. The expansion of gases may take place in only one turbine or in a set of low pressure and high pressure turbine. The temperatures at which such turbines are to be operated range from 800°C to 1000°C.

(3) Marine Field:

The gas turbine can also be used in the marine field. These are used for propulsion of ships or power generation on the ship.

(4) Railway:

The gas turbines can be also be used for rail propulsion.

(5) Road Transport:

The gas turbines are used for heavy duty armored vehicles to cruise at high speeds.

(6) Electric Power Generation:

The gas turbines are very popular for electric power generation because of the ability of starting and brought upto full load quickly and less cost in installation and maintenance. As compared to steam power station, the gas turbine power station requires much less water for a particular efficiency.

(7) Industry:

Gas turbines are also employed for industrial purposes, e.g., blast of air for blast furnaces in steel industry, oil and other chemical industries.