Air enters a gas turbine with two stages of compression and two stages of expans

Question

Air enters a gas turbine with two stages of compression and two stages of expansion at 100 kPa and 17oC. The system uses a regenerator as well as reheating and intercooling. The pressure ratio across each compressor is 4. 300 kJ/kg of heat are added to the air in each combustion chamber. The regenerator increases the temperature of the cold air by 20oC. Determine the thermal efficiency of the system assuming isentropic operations for all compressor and turbine stages and using constant specific heats at room temperature.

Explanation / Answer

Thermal efficiency is a measure of the output energy divided by the input energy in a system. It must be between 0% and 100%. A thermal efficiency of 100% would mean that all energy put into a system comes out, albeit in a different form. Heat engines and refrigerators both have associated thermal efficiencies, though they are trying to accomplish opposite objectives. Real-world thermal efficiencies generally fall significantly below 100% due to a variety of reasons. In a gasoline engine, input energy is stored in the chemical bonds of a hydrocarbon fuel. A hydrocarbon molecule consists entirely of hydrogen and carbon. When these molecules are combined with oxygen, they can chemically react and form carbon monoxide and water; in essence, the hydrocarbon molecule is split up and combined with oxygen atoms. The part of this reaction that is useful to an engine, though, is the heat that is released. Heat released from gasoline combustion is the relevant input energy in thermal efficiency.

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