1. Gas Turbines, THERMAL ENGINEERING - II ME 4003 notes in english

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Subject - THERMAL ENGINEERING - II ME 4003
Branch - Mechanical Engineering
Semester - 4th Semester

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GAS TURBINES

A gas turbine is a type of internal combustion engine that converts fuel energy into mechanical energy through high-speed rotating blades. It operates on the Brayton cycle, which consists of four major thermodynamic processes: compression, combustion, expansion, and exhaust.

Gas turbines are used in various applications, including aircraft propulsion, power generation, and industrial mechanical drives. They offer advantages such as a high power-to-weight ratio, continuous combustion, and simple construction compared to reciprocating internal combustion (I.C.) engines.

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1.1 Introduction and Derivation for Work and Efficiency of Air-Standard Brayton Cycle

Introduction to Brayton Cycle

The Brayton cycle is the thermodynamic cycle used in gas turbines. It involves:

1. Compression (isentropic process) – Air is compressed, increasing pressure and temperature.

2. Heat Addition (constant pressure process) – Fuel is added, and combustion occurs.

3. Expansion (isentropic process) – Hot gases expand through the turbine, producing mechanical work.

4. Heat Rejection (constant pressure process) – Remaining heat is expelled to the surroundings.

This cycle is an open-cycle process when used in aircraft and power plants, meaning fresh air enters the system continuously. However, in some industrial applications, a closed-cycle process is used, where the working fluid is recirculated.

Work and Efficiency Derivation

To analyze the performance of a gas turbine, we derive expressions for work output and efficiency.

Work Done by the Brayton Cycle

Total work output in the cycle is the difference between turbine work and compressor work:

$$W_{net} = W_{turbine} - W_{compressor}$$

Using thermodynamic equations, the work done per unit mass in an ideal gas turbine cycle is given by:

$$W = C_p \left( T_3 - T_4 \right) - C_p \left( T_2 - T_1 \right)$$

where:

• $C_p$ = specific heat at constant pressure

• $T_1, T_2, T_3, T_4$ = temperatures at different points in the cycle

Efficiency of the Brayton Cycle

The thermal efficiency ($\eta$) of an ideal Brayton cycle is given by:

$$\eta = 1 - \frac{1}{r_p^{(\gamma -1)/\gamma}}$$

where:

• $r_p$ = pressure ratio ($P_2/P_1$)

• $\gamma$ = ratio of specific heats ($C_p/C_v$)

This equation shows that higher pressure ratios lead to higher efficiency.

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1.2 Gas Turbine Classification

Gas turbines can be classified based on operating cycle, application, and shaft arrangement.

1. Based on the Operating Cycle

• Open Cycle Gas Turbine

  • Fresh air enters the compressor, passes through the combustion chamber and turbine, and then is expelled.

  • Commonly used in aircraft and power plants.

  • Simple in design but requires continuous air intake.

• Closed Cycle Gas Turbine

  • The working fluid (air or helium) is continuously recirculated.

  • Heat exchangers replace combustion chambers.

  • Used in nuclear and space applications due to better efficiency.

2. Based on Application

• Aero Gas Turbines – Used in jet engines for aircraft propulsion.

• Industrial Gas Turbines – Used for power generation in power plants and mechanical drives.

• Marine Gas Turbines – Used in ships, especially naval vessels.

3. Based on Shaft Arrangement

• Single-Shaft Gas Turbine

  • A single shaft connects the compressor, turbine, and power output.

  • Common in power plants.

• Two-Shaft Gas Turbine

  • Separate shafts for the compressor and turbine.

  • Allows better speed control.

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1.3 Comparison of Gas Turbines with Reciprocating I.C. Engines and Steam Turbines

Gas turbines differ significantly from reciprocating engines and steam turbines in terms of operation, efficiency, and applications.

Comparison with Reciprocating I.C. Engines

Feature	Gas Turbine	Reciprocating I.C. Engine
Operation	Continuous combustion	Intermittent combustion
Efficiency	Lower at small scales	Higher at small scales
Power-to-Weight Ratio	High	Low
Start-up Time	Quick	Slow
Components	Compressor, combustor, turbine	Piston, cylinder, valves

Gas turbines are more suitable for large power output applications, while I.C. engines are better for small-scale operations.

Comparison with Steam Turbines

Feature	Gas Turbine	Steam Turbine
Working Medium	Air + Fuel	Steam
Efficiency	Lower	Higher at high power
Start-up Time	Fast	Slow
Size and Weight	Compact	Large and heavy
Fuel Requirement	Requires high-quality fuel	Can use low-grade fuels

Steam turbines are ideal for large-scale power generation, while gas turbines are preferred for aerospace and quick start applications.

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1.4 Applications and Limitations of Gas Turbines

Applications of Gas Turbines

Gas turbines are used in power generation, transportation, and industrial applications due to their efficiency and high-speed operation.

1. Aircraft Propulsion

   • Jet engines in commercial and military aircraft.

   • High power-to-weight ratio makes them ideal for aviation.

2. Power Generation

   • Used in gas turbine power plants for electricity generation.

   • Can work alone or in combined cycle plants with steam turbines.

3. Industrial Applications

   • Used to drive compressors, pumps, and generators.

   • Common in refineries and chemical plants.

4. Marine Propulsion

   • Used in naval ships and submarines.

   • Lighter and faster than diesel engines.

Limitations of Gas Turbines

Despite their advantages, gas turbines have some limitations:

• Lower Efficiency at Small Scales

  • Efficiency is lower than reciprocating engines when used for small power output.

• High Initial Cost

  • Manufacturing and installation are expensive.

• Fuel Quality Requirement

  • Requires high-quality fuel to avoid turbine damage.

• Complex Cooling System

  • The turbine blades operate at extremely high temperatures, requiring advanced cooling techniques.

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Conclusion

Gas turbines are essential in aviation, power generation, and industrial applications. They operate on the Brayton cycle, offering high power output and efficiency. While they have some limitations, their fast start-up time, lightweight design, and continuous combustion process make them superior to reciprocating engines in many cases.