1. Gas Power Cycles
Cengel & Boles, Chapter 8
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2. Analysis of Power Cycles - Basics
Power cycle = Heat engine
Recall thermal efficiency:
Carnot heat engine:
The Carnot cycle has the maximum possible efficiency, but is not a realistic model for a power cycle since it is so impractical
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3. Analysis of Power Cycles - Basics, cont.
More practical models are called ideal cycles - they are internally reversible but typically have external irreversibilities
Ideal cycle assumptions include:
absence of friction
quasi-equilibrium processes
pipes and connections between various components are well-insulated, i.e., heat transfer is negligible
negligible KE and PE effects (except in diffusers and nozzles)
negligible pressure drop in HXers
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4. Gas Power Cycles
Working fluid remains in gaseous phase throughout cycle
Common gas cycles
Otto*: spark-ignition ICE engine, closed system
Diesel*: compression-ignition ICE engine, closed system
Dual: Otto/Diesel combo, closed system
Stirling: ext. combustion, closed system
Ericsson: ext. combustion, control volume
Brayton*: gas turbine engine or power plant, control volume
* covered in this course
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5. Internal Combustion Engine (ICE) terms
Bottom-dead center (BDC) – piston position where volume is maximum
Top-dead center (TDC) – piston position where volume is minimum
Clearance volume – minimum cylinder volume (VTDC = V2)
Compression ratio (r)
Displacement volume
Mean Effective Pressure (MEP)
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6. ICE terms, cont.
Spark-ignition (SI) engine - reciprocating engine where air-fuel combustion is initiated by a spark plug
Compression-ignition (CI) engine - reciprocating engine where air-fuel combustion is initiated by compression
Four-stroke engine - piston executes intake, compression, expansion, and exhaust in four strokes while crankshaft completes two revolutions
Two-stroke engine - piston executes intake, compression, expansion, and exhaust in two strokes while crankshaft completes one revolution
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7. Analysis of Gas Power Cycles
Air-standard assumptions:
working fluid is a fixed mass of air which is modeled as a closed system and behaves as an ideal gas
all processes are internally reversible unless stated otherwise
combustion process is replaced by a heat addition process from an external source
exhaust process is replaced by a heat rejection process that restores air to its initial state
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8. Analysis of Gas Power Cycles, cont.
Constant specific heat approach (aka cold-air standard) - for approximate analysis only
where cv , cp are evaluated at 25°C, 1 atm
Variable specific heat approach - for more accurate analysis
where u and h obtained from Table A-17
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9. Analysis of Gas Power Cycles, cont.
Isentropic compression/expansion
if compression ratio (v1/v2) is known, e.g., in Otto or Diesel cycles, use
(find u2 or h2 from vr2 in Table A-17)
if pressure ratio (P2/P1) is known, e.g., in a Brayton cycle, use
(find u2 or h2 from Pr2 in Table A-17)
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10. Otto Cycle Analysis Thermal efficiency
Heat addition (process 2-3, v = const)
Heat rejection (process 4-1, v = const)
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11. Diesel Cycle Analysis Thermal efficiency
Heat addition (process 2-3, P = const)
Heat rejection (process 4-1, v = const)
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12. Cold-Air Standard Thermal Efficiency
Otto Cycle
Diesel Cycle
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13. The Brayton Cycle Ideal cycle for gas turbine engines and power plants
The air-standard Brayton cycle has a closed-loop configuration, even though most applications are open-loop
Basic components:
Compressor (increases pressure of gas)
Heat exchanger or combustor (const P heat addition)
Turbine (produces power)
Heat exchanger (const P heat rejection)
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14. Air-Standard Brayton Cycle Analysis
Compressor
Combustor (heat addition)
Turbine
Heat Exchanger (heat rejection)
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15. Air-Standard Brayton Cycle Analysis, cont.
Thermal Efficiency
Back Work Ratio
as discussed in Ch. 6, a gas compressor requires much greater work input per unit mass than a pump for a given pressure rise; thus the rbw for a gas power cycle (40-60%) is much greater than that for a vapor power cycle (1-2%)
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16. Air-Standard Brayton Cycle Analysis, cont.
Cold-air standard thermal efficiency
High pressure ratios (rp =P2/P1) yield the highest thermal efficiency, however, moderate pressure ratios often yield a higher power-to-weight ratio
Maximum turbine inlet temperature is around 1700 K, imposed by metallurgical properties
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17. Improving Gas Turbine Cycle Performance
Regeneration - utilizes turbine exhaust gas to preheat air entering the combustor; this reduces heat addition requirement and increases thermal efficiency
Multistage turbine with reheat - similar to vapor power cycles; increases thermal efficiency
Compressor intercooling - gas is cooled between compressor stages; decreases compressor work and bwr, increases thermal efficiency
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18. Gas Turbine Aircraft Propulsion
Gas turbines are ideal for aircraft propulsion due to high power-to-weight ratio
Basic turbojet engine - inlet diffuser, compressor, combustor, turbine, exit nozzle
Turbofan engine - inlet fan brings in additional air which bypasses engine core and increases thrust from nozzle
Turboprop engine - turbine powers a propeller, which provides primary thrust
Ramjet - high-speed air is compressed by ram effect and then heated by combustor; thrust is developed by nozzle w/o need for compressor or turbine
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