Thermal Efficiency is the ratio of the amount of thermal (heat) energy converted to useful work or thrust to the amount of thermal energy contained in the fuel consumed /burned. Today’s gas turbine engines have a thermal efficiency of around 45 to 50 %. Reciprocating engines has a thermal efficiency of about 30%.
There are three primary factors which determine the thermal efficiency of a gas turbine engine; They are:
- Turbine inlet temperatures.
- The higher an engine can increase the temperature of the incoming air, the more thrust that the engine will be able to produce. This is dependent on how much fuel is burned inside the engine. The more fuel we burn, the higher the temperature of the combustion gases, and the more thrust that will be produced. There is , however, a limit to the amount of fuel that we can burn and the amount we can increase the temperature of the air and that limit is the highest temperature the turbine can withstand without damage.
- The higher T.I.T temperatures that the turbine can withstand means that less cooling air is needed. Less cooling air needed means that the compressor can supply less air to the burner which means the compressor can be scaled down. The turbine would now need to extract a corresponding lower amount of energy to turn that compressor. More energy will now be left over to enhance thrust and to increase the thermal efficiency.
- Compression ratio.
- Engines with higher compression ratios can deliver more air to the burner. More air to the burner means we can burn more fuel. Burning more fuel means we can increase the temperature of the gas flow more and that translates into more thrust.
- The greater the compression ratio for a given mass airflow and thrust the lower the engine fuel consumption to produce that thrust. Hence, efficiency goes up. Basically, what we are saying here is that the more energy added to the air by the compressor, the less fuel energy we have to add to get a given amount of thrust.
- The most energy efficient condition occurs when compressor temperatures are low. This is desirable because it allows for a greater temperature rise in the burner, which is accomplished by allowing us to burn more fuel and which results in greater expansion of the combustion gases and more thrust.
- Compressor & Turbine efficiency.
- To better understand Compressor & Turbine efficiency, thinks of eating your favorite nuts like almonds or walnuts. Now, think of eating those same nuts with some of your teeth missing. The efficiency of your chomping will definitely deteriorate.An analogous loss of compressor & turbine efficiency will result from their getting old, damaged or contaminated. The compressor is particularly susceptible to loss of efficiency due to contaminants / dirt adhering to the blades. Compressor section cleaning, using a fluid wash or grit blasting, are common ways to remove these contaminants and restore compressor efficiency. The turbine section is not prone to contaminant build- up due to the high temperatures at which they operate. Turbine section efficiency is most affected by blade erosion, old age or physical damage.
- Loss of compressor and especially turbine efficiency means that the engine will need to burn more fuel in order to produce a given thrust output. This will result in the engine having a higher exhaust gas temperature, a lower RPM and a higher fuel flow at all engine power settings.
