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Brayton cycle

Brayton cycle. Uses Auxiliary power generation Stand-alone power generation Naval propulsion Jet engine. Gas Turbine. Advantages High power:weight ratio Compact One-direction motion – vibration Fewer moving parts Better reliability Variety of fuels Low emissions. Disadvantages

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Brayton cycle

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  1. Brayton cycle • Uses • Auxiliary power generation • Stand-alone power generation • Naval propulsion • Jet engine

  2. Gas Turbine • Advantages • High power:weight ratio • Compact • One-direction motion – vibration • Fewer moving parts • Better reliability • Variety of fuels • Low emissions • Disadvantages • Higher cost

  3. Brayton cycle • Working fluid – air • Ideal gas • Specific heats steady • High temperature reservoir • Open or closed model • Steady pressure heat exchange

  4. Cycle Analysis • Net work • Heat in • Thermal efficiency • Back work ratio

  5. Brayton cycle • 1→2 Isentropic compression • 2→3 steady pressure heat addition • 3→4 isentropic expansion • 4→1 steady pressure heat rejection

  6. Brayton cycle • Work in & work out • Heat in & heat out • Thermal efficiency • Pressure ratio • Back work ratio

  7. Brayton Cycle • Approaches • Variable specific heats: Table: h, pr • Steady specific heats: h = CpT • P,v,T relationships: T2 = T1(P2/P1)(k-1)/k

  8. Compressor • Essential to compress large volumes of air for efficiency of cycle • Centrifugal • Axial: more common; rotor and stator blades

  9. Example • A simple Brayton cycle has a rp = 12, a compressor inlet at 300K, and a turbine inlet at 1000K. Determine the mass flow of air needed when the net power output is 70MW. Specific heats are constant.

  10. Example • An ideal air-standard Brayton cycle has air entering the compressor at 100kPa,300K, & 5m3/s. The compressor ratio is 10; the turbine inlet is at 1400K. • Find power generated, bwr, and thermal efficiency.

  11. Brayton cycle • Irreversibilities: isentropic efficiency • Gas turbine power plant operating at steady state receives air at 100kpa & 300K. Air is compressed to 500kPa and reaches a maximum cycle temperature of 920K. The isentropic efficiencies of the compressor and turbine are both at 83%. • Find the thermal efficiency and bwr of the cycle.

  12. Brayton cycle • Regenerator • Effectiveness

  13. Regeneration • Capital costs • Pressure losses

  14. Brayton cycle • Ideal • ηth =45.6% • With regenerator • ηth =57%

  15. Practice problem • A gas turbine power plant operates on a Brayton Cycle between 100 & 1200kPa. Air enters the compressor at 30oC, 150 m3/min and leaves the turbine at 500oC. Isentropic efficiencies of 82% and 88% apply to the compressor & turbine respectively. Assume specific heats are variable. • Find: net power output, bwr, thermal efficiency; (659kW, 0.63, 31.9%)

  16. Practice Problem • A gas turbine has a regenerator. Air enters the compressor at 100kPa & 20oC. Compressor ratio is 8. Maximum cycle temperature is 800oC. The cold air stream leaves the regenerator 10oC cooler than the hot air stream entering the regenerator. The engine produces 150kW. • Find: heat added & heat rejected from the cycle; 303kW & 153kW.

  17. Assignment • Chapter 9: sections 9.5 through 9.10

  18. Brayton cycle • Isentropic compression power • Isothermal power • Intercooler

  19. Brayton cycle • Reheat

  20. Brayton cycle • Ideal • ηth =45.6% • With irreversibilities • ηth =24.9% • With regenerator * ηth =56.8%

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