Gas turbines, based on the Brayton cycle, are powerhouses in energy conversion. They compress air, mix it with fuel, ignite the mixture, and use the resulting hot gases to spin turbines, generating power for various applications.

Understanding the Brayton cycle is crucial for grasping gas power systems. It connects to other cycles like Otto and Diesel, but stands out for its continuous combustion process and wide-ranging applications in aviation, power generation, and industry.

Brayton Cycle Components and Processes

Brayton Cycle Overview

• Brayton cycle represents a thermodynamic cycle describing gas turbine engine operation
• Consists of four main processes: compression, combustion, expansion, and exhaust
• Working fluid (usually air) undergoes these processes to generate mechanical power

Compression Process

• Ambient air drawn into the compressor and compressed to high pressure
• Compressor typically an axial or centrifugal type with multiple stages
• Compression increases air pressure and temperature, preparing it for combustion
• Compressor work input required to achieve desired pressure ratio

Combustion Process

• Compressed air enters combustion chamber where fuel is injected
• Fuel-air mixture ignited resulting in high-temperature, high-pressure gas
• Combustion adds heat to the system, increasing the energy of the working fluid
• Combustion temperature limited by material constraints and NOx emission regulations

Expansion and Exhaust Processes

• Hot combustion gases expand through the turbine section, driving turbine blades
• Turbine generates mechanical power to drive the compressor and the load (generator, propeller)
• Expanded gases released into the atmosphere during exhaust process
• Exhaust gases contain significant thermal energy for potential heat recovery or thrust generation

Thermodynamics of Gas Turbine Systems

First and Second Laws of Thermodynamics

• First law (conservation of energy) analyzes energy balance in gas turbine systems
• Change in internal energy equals heat added minus work done by the system
• Second law (entropy) determines maximum theoretical efficiency of gas turbine systems
• Entropy of an isolated system always increases over time

Ideal and Real Brayton Cycles

• Ideal Brayton cycle assumes isentropic compression and expansion (no change in entropy)
• Real gas turbine systems experience irreversibilities (friction, heat transfer losses)
• Actual efficiency lower than ideal efficiency due to irreversibilities and losses
• Isentropic efficiencies of compressor and turbine account for real system losses

Key Thermodynamic Parameters

• Pressure ratio: ratio of compressor discharge pressure to inlet pressure
• Higher pressure ratios generally lead to higher efficiencies
• Specific heat ratio (k) of working fluid affects temperature changes during compression and expansion
• Turbine inlet temperature (TIT) critical for performance and efficiency, limited by material constraints

Brayton Cycle Gas Turbine Performance

Efficiency and Power Output

• Thermal efficiency: ratio of net work output to heat input, indicates energy utilization
• Power output depends on mass flow rate, pressure ratio, and turbine inlet temperature
• Increasing these parameters generally leads to higher power output
• Specific fuel consumption (SFC) measures fuel efficiency (fuel consumed per unit power output)

Component Efficiencies and Losses

• Compressor and turbine efficiencies significantly impact overall cycle performance
• Higher component efficiencies result in improved cycle efficiency and reduced losses
• Pressure losses in ducts, combustion chamber, and heat exchangers affect performance
• Cooling air extraction for turbine blade cooling reduces overall efficiency

Turbine Inlet Temperature (TIT)

• TIT is a critical parameter affecting gas turbine performance and efficiency
• Higher TITs enable higher power output and efficiency
• Advanced materials (superalloys, ceramic matrix composites) and cooling technologies required
• Advancements in TIT have been a major driver of gas turbine performance improvements

Efficiency and Power Output of Brayton Cycles

Ideal Brayton Cycle Efficiency

• Thermal efficiency of ideal Brayton cycle calculated using pressure ratio and specific heat ratio
• Formula: ηth = 1 - (1/rp)^((k-1)/k), where rp is pressure ratio and k is specific heat ratio
• Efficiency increases with higher pressure ratios and lower specific heat ratios
• Ideal efficiency represents the maximum theoretical efficiency achievable

Real Gas Turbine Efficiency

• Actual thermal efficiency lower than ideal efficiency due to irreversibilities and losses
• Isentropic efficiencies of compressor and turbine account for real system losses
• Polytropic efficiency considers the non-ideal behavior of the working fluid during compression and expansion
• Overall efficiency depends on component efficiencies, pressure losses, and cooling air extraction

Power Output Calculation

• Power output determined using mass flow rate, specific work output, and mechanical efficiency
• Specific work output is the difference between turbine work and compressor work per unit mass
• Formula: Pout = ṁ × (wt - wc) × ηm, where ṁ is mass flow rate, wt and wc are specific turbine and compressor work, and ηm is mechanical efficiency
• Heat input required for a given power output calculated based on thermal efficiency and fuel heating value

Applications of Gas Turbine Systems

Aviation Industry

• Gas turbines widely used for aircraft propulsion (turbofan engines for commercial aircraft, turboshaft engines for helicopters)
• Provide high power-to-weight ratios and enable efficient long-distance travel
• Jet engines based on the Brayton cycle, with modifications such as multi-stage compression and expansion

Power Generation Sector

• Gas turbines employed in simple cycle and combined cycle power plants
• Simple cycle plants use gas turbine exhaust directly for power generation
• Combined cycle plants utilize exhaust heat to generate steam for a steam turbine, achieving higher overall efficiencies
• Gas turbines offer flexibility, quick start-up, and low emissions compared to other fossil fuel power plants

Oil and Gas Industry

• Gas turbines used for power generation and mechanical drive in offshore platforms, gas compression stations, and LNG plants
• Provide reliable and efficient power solutions in remote locations
• Gas turbines can operate on a variety of fuels, including natural gas, diesel, and heavy oils

Marine and Industrial Applications

• Gas turbines used for ship propulsion and onboard power generation in the marine industry
• Offer high power density, quick start-up, and low emissions compared to traditional diesel engines
• Industrial applications include cogeneration (combined heat and power), process heating, and mechanical drive
• Gas turbines provide reliable and efficient power solutions for industries such as refineries, chemical plants, and manufacturing facilities