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Fluid Mechanics

Turbines and compressors are vital in fluid mechanics, converting energy between fluids and mechanical systems. From gas turbines in jet engines to hydraulic turbines in dams, these devices play crucial roles in power generation and industrial processes.

Thermodynamics is key to understanding turbine and compressor performance. Efficiency metrics like isentropic efficiency help engineers optimize these machines, balancing power output with energy losses. Design considerations vary based on application, from high-temperature materials in gas turbines to moisture control in steam turbines.

Types and Applications of Turbines and Compressors

Types of turbines and compressors

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  • Turbines convert fluid energy into mechanical energy
    • Gas turbines convert thermal energy from high-temperature, high-pressure gases into mechanical energy used in power generation (power plants), aircraft propulsion (jet engines), and industrial applications (oil and gas)
    • Steam turbines convert thermal energy from high-pressure steam into mechanical energy used in power generation (coal and nuclear power plants) and industrial processes (chemical manufacturing)
    • Hydraulic turbines convert kinetic energy from flowing water into mechanical energy used in hydroelectric power generation (dams and run-of-river systems)
  • Compressors increase the pressure of a fluid by converting mechanical energy into fluid energy
    • Centrifugal compressors increase fluid pressure by converting kinetic energy into pressure energy used in gas processing (natural gas), refrigeration (air conditioners), and air conditioning systems (buildings)
    • Axial compressors increase fluid pressure by progressively compressing it along the axis of rotation used in gas turbines (power generation) and jet engines (aircraft propulsion)
    • Positive displacement compressors increase fluid pressure by reducing its volume in a confined space used in refrigeration (refrigerators), air conditioning (heat pumps), and compressed air systems (pneumatic tools)

Thermodynamics and Performance Characteristics

Thermodynamics of turbines and compressors

  • Turbines involve an expansion process that converts high-pressure, high-temperature fluid into low-pressure, low-temperature fluid while extracting work
    • Isentropic expansion process is an ideal process where the entropy remains constant
    • Actual expansion process involves irreversibilities such as friction and heat transfer resulting in a lower efficiency compared to the isentropic process
  • Compressors involve a compression process that converts low-pressure, low-temperature fluid into high-pressure, high-temperature fluid while requiring work input
    • Isentropic compression process is an ideal process where the entropy remains constant
    • Actual compression process involves irreversibilities such as friction and heat transfer resulting in a higher work input requirement compared to the isentropic process

Performance metrics for turbines and compressors

  • Power output quantifies the rate of work produced or consumed
    • Turbines: W˙=m˙(h1h2)\dot{W} = \dot{m}(h_1 - h_2), where W˙\dot{W} is power output, m˙\dot{m} is mass flow rate, and h1h_1 and h2h_2 are inlet and outlet specific enthalpies
    • Compressors: W˙=m˙(h2h1)\dot{W} = \dot{m}(h_2 - h_1), where W˙\dot{W} is power input, m˙\dot{m} is mass flow rate, and h1h_1 and h2h_2 are inlet and outlet specific enthalpies
  • Isentropic efficiency compares the actual performance to the ideal isentropic process
    • Turbines: ηt=h1h2h1h2s\eta_t = \frac{h_1 - h_2}{h_1 - h_{2s}}, where h2sh_{2s} is the specific enthalpy at the outlet for an isentropic process
    • Compressors: ηc=h2sh1h2h1\eta_c = \frac{h_{2s} - h_1}{h_2 - h_1}, where h2sh_{2s} is the specific enthalpy at the outlet for an isentropic process
  • Pressure ratio, defined as the ratio of outlet pressure to inlet pressure, affects the power output, efficiency, and size of turbines and compressors

Design Considerations and Operating Principles

Design of gas and steam turbines

  • Gas turbines operate on the Brayton cycle, which consists of:
    1. Isentropic compression in the compressor
    2. Constant-pressure heat addition in the combustion chamber
    3. Isentropic expansion in the turbine
    4. Constant-pressure heat rejection in the exhaust
  • Gas turbine components include:
    • Compressor increases the pressure of the incoming air
    • Combustion chamber mixes the compressed air with fuel and ignites the mixture
    • Turbine extracts work from the high-temperature, high-pressure gases
  • Gas turbine design considerations involve materials selection for high-temperature operation, cooling systems for turbine blades, and emission control technologies (catalytic converters)
  • Steam turbines operate on the Rankine cycle, which consists of:
    1. Isentropic compression in the pump
    2. Constant-pressure heat addition in the boiler
    3. Isentropic expansion in the turbine
    4. Constant-pressure heat rejection in the condenser
  • Steam turbine components include:
    • Boiler generates high-pressure steam by heating water
    • Turbine extracts work from the high-pressure steam
    • Condenser condenses the low-pressure steam back into water
    • Pump increases the pressure of the water and returns it to the boiler
  • Steam turbine design considerations involve steam quality and moisture separation, blade erosion prevention, and thermal efficiency optimization through regenerative heating (feedwater heaters) and reheating stages (intermediate pressure turbines)
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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.