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Gas Turbine Introduction

Combustion engines convert the chemical energy in the fuel to mechanical work, e.g. in a car, or kinetic energy to produce thrust, e.g. in an airplane. This energy conversion process involves heat as an intermediate energy form, and thus the efficiency is limited by the Carnot theory.
Combustion engines predominate transport (road, water and air) and power generation, due to the reliability, ruggedness, high power density, and low cost. This trend is predicted to continue into the foreseeable futureCombustion engines predominate transport (road, water and air) and power generation, due to the reliability, ruggedness, high power density, and low cost. This trend is predicted to continue into the foreseeable future.
When the combustion occurs outside the system where the working fluid does work, the engine is called external combustion engine. An engine undertaking the Carnot cycle is an external combustion engine. Practical examples: steam engines, Sterling engines. In these engines, the working fluid, e.g. steam, does not participate in the combustion process. Working fluid is the fluid that undergoes the thermodynamic cycles and does the work.
If the combustion occurs inside the system, it is called internal combustion engine. Working fluid in this case burns in the engine and the combustion product subsequently does the work. Practical examples: gas turbines, spark-ignition engines, diesel engines. Internal combustion raises the temperature of the working fluid much faster than external heat addition which relies on heat transfer. Therefore, internal combustion engines usually have higher energy density (power per engine weight) than external combustion engines.

Application of gas turbines
Cut-away view of the Rolls-Royce Trent 900 turbofan engine, used on Airbus A380:



Pros/cons of gas turbines
Advantages 1.Small size and mass for a given power output, i.e. high W/m3 or W/kg 2. Low vibration: parts rotating and not reciprocating 3. Reliable: low dynamic stresses from near constant speed operation 4. Low cost per kW at larger sizes
Disadvantages 1. Thermal efficiency is usually lower than diesel engines – thermal efficiency of gas turbines is directly related to the maximum temperature in the cycle (turbine inlet temperature), which is limited by materials of turbine blades. However, efficiencies higher than diesel engines can be achieved by combining gas turbine cycles with steam turbine cycles. 2. Relatively poor dynamic performance – less suitable for automotive and other ‘stop-go’ applications.

Gas Turbine Cycles
Brayton cycle
• Assumptions: 1. Ideal cycle: reversible, no pressure losses, no heat losses to the surrounding. 2. Air-standard cycles: air is the working fluid with constant specific heat P c ; combustion and turbine-exhaust expansion replaced with heat addition and heat rejection, respectively.