What are the effects of changed in atmospheric condition on the efficiency of gas turbine?

What the effects are of changed in atmospheric condition on the efficiency of gas turbine?

 

The ideal gas turbine Theory of operation

 

In a practical gas turbine, gases are first accelerated in either a centrifugal or axial compressor. These gases are then slowed using a diverging nozzle known as a diffuser; these processes increase the pressure and temperature of the flow. In an ideal system, this is isentropic. However, in practice, energy is lost to heat, due to friction and turbulence. Gases then pass from the diffuser to a combustion chamber, or similar device, where heat is added. In an ideal system, this occurs at constant pressure (isobaric heat addition). As there is no change in pressure the specific volume of the gases increases. In practical situations this process is usually accompanied by a slight loss in pressure, due to friction. Finally, this larger volume of gases is expanded and accelerated by nozzle guide vanes before energy is extracted by a turbine. In an ideal system, these gases are expanded isentropic and leave the turbine at their original pressure. In practice this process is not isentropic as energy is once again lost to friction and turbulence.So larger thrust is valuable from gas turbine exit gas exhaust

 

Effect of temperature

 

On a cold day the density of the air increases so that the mass of air entering the compressor for a given engine speed is greater, hence the thrust or shaft horse-power, is higher. The denser air does, however, increase the power required to drive the compressor or compressors; thus the engine will require more fuel to maintain the same engine speed or will run at a reduced engine speed if no increase in fuel is available.

 

On a hot day the density of the air decreases, so that reducing the mass of air entering the compressor and, consequently, decreases the thrust of the engine for a given shaft horse-power. Because less power will be required to drive the compressor, the fuel control system reduces the fuel flow to maintain a constant engine rotational speed or turbine entry temperature, as appropriate; however, because of the decrease in air density, the thrust will be lower. At a temperature of45 deg.C., depending on the type of engine, a thrust loss of up to20 per cent may be experienced.

This means that some sort of thrust growth, such as water injection, may be required.

 

The fuel control system controls the fuel flow so that the maximum fuel supply is held practically constant at low air temperature conditions, whereupon the engine speed falls but, because of the increased mass airflow as a result of the increase in air density, the thrust remains the same.

 For example, the combined acceleration and speed control fuel system schedules fuel flow to maintain a constant engine r.p.m., hence thrust increases as air temperature decreases until, at a predetermined compressor delivery pressure, the fuel flow is automatically controlled to maintain a constant compressor delivery pressure and,

                                

Effect of altitude

 

With increasing altitude the ambient air pressure and temperature are reduced. This affects the engine in two interrelated ways:

The fall of pressure reduces the air density and hence the mass airflow into the engine for a given engine speeds. This causes the thrust or shaft horse-power to fall. The fuel control system, as described in, adjusts the fuel pump output to match the reduced mass airflow, so maintaining a constant engine speed.

The fall in air temperature increases the density of the air, so that the mass of air entering the compressor for a given engine speed is greater.

This causes the mass airflow to reduce at a lower rate and so compensates to some extent for the loss of thrust due to the fall in atmospheric pressure. At altitudes above36,089 feet and up to65,617 feet, however, the temperature remains constant, and the thrust or shaft horse-power is affected by pressure only.

 

The performance of the gas turbine engine is dependent on the mass of air enteringthe engine. At a constant speed, the compressor pumps a constant volume of air into the engine with no regard for air mass or density. If the density of the air decreases, the same volume of air will contain less mass, so less power is produced. If air density increases, power output also increases as the air mass flow increases for the same volume of air. Atmospheric conditions affect the performance of the engine since the density of the air will be different under different conditions. On a cold day, the air density is high, so the mass of the air entering the compressor is increased. As a result, higher horsepower is produced. In contrast, on a hot day, or at high altitude,air density is decreased, resulting in a decrease of output shaft power

 

I think that Mr Osama Abdalla shows us all the physics my thanks...i will add some useful information, every engine is equiped with his owen ECU,if you talk about the aircraft engine...what's more important for aircarft engine is always thrust and fuel consumption....every enigne is designed for a specific altitude...so there's an important change in all parameters of flight temperature ,pressure,density those are stat parameters...for all this we had mathematical model for the atmospher that allow to us calculating all the changes in the stat parameters...the ECU ''electronic control unit" will do all this work  such air/fuel ratio for example