Gas Power Systems 4

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Gas Power Systems - 4

• The Gas Turbine

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Overview

• Gas Turbines
• The Carnot cycle as an air standard cycle
• The Ericcson Cycle
• The Brayton Cycle
• The standard cycle
• The reheat cycle
• Inter-cooling

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Gas modern gas turbine cycle
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The modern gas turbine cycle

• Long-haul automotive power.
• Large aircraft flight envelopes
• Commercial aircraft
• Altitude (30,000 to 40,000 ft)
• Long range and low specific fuel consumption
  (sfc)
• Moderate speed and thrust

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The modern gas turbine cycle

• Military aircraft
• High altitude ( gt 40,000 ft)
• Moderate range and high specific fuel consumption
  (sfc)
• High speed and large thrust
• Relevant cycles
• The Ericsson and Brayton cycles

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General features of the aircraft gas turbine
engine
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The air-standard turbine cycle

• Open system modeled as a closed system - fixed
  with fixed mass flow.
• Air is the working fluid.
• Ideal gas assumptions are applied.
• Approximate the combustor as the high temperature
  source.
• Internally reversible processes.

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Ideal gas power cycles
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Air-standard Carnot cycle
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Air-standard Carnot cycle
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Limitations of the air-standard Carnot cycle

• Heat addition at constant temperature is
  difficult and costly.
• Work is required because fluid expands.
• Heat addition is limited because a large change
  in volume would imply a low mean pressure in the
  heat addition process.
• Frictional effects might become too great if the
  mean pressure is too low.

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Ideal gas power cycles
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Air-standard Ericsson cycle
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The air-standard Rescission cycle

• Constant pressure heat addition and rejection
• Constant temperature compression and expansion

Qb-c
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The Air Standard Ericcson Cycle
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Thermal EfficiencyEriccson Cycle
Qb-c
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Ideal gas power cycles
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The Brayton cycle
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The Brayton Cycle

• Modern gas turbines operate on an open Brayton
  cycle.
• Ambient air is drawn at the inlet.
• Exhaust gases are released to the ambient
  environment.
• The air standard Brayton cycle is a closed cycle.
• All processes are internally reversible.
• Air is the working fluid and assumed an ideal gas.

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General features of the aircraft gas turbine
engine
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The gas turbine processes

• Isentropic compression to TH
• Constant pressure heat addition at TH
• Isentropic expansion to TC
• Constant pressure heat rejection at TC

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4
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s1 s2
s3 s4
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Thermal efficiency of the ideal Brayton cycle
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Thermal EfficiencyIdeal Brayton Cycle
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Cycle efficiency

• Ideal gas assumptions apply.
• All processes internally reversible
• Compressor and turbine efficiency are each 100.
• Assume fuel added in the combustor is a small
  percent (mass or moles) of total flow, and thus
  air properties provide a good estimate of cycle
  performance.

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End of Gas Power Systems - 4
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Key terms and concepts
Ericcson cycle Brayton cycle Air-standard cycles