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Gas Power Cycles Week 11 Gas Power Cycles ME 300 Thermodynamics II 1 Today’s Outline • Gas turbine engines • Brayton cycle • Analysis • Example ME 300 Thermodynamics II 2 Gas Turbine Engine • Produces shaft power by GE H series power generation gas turbine. This 480-megawatt unit has a rated thermal expanding high enthalpy gas efficiency of 60% in combined cycle through a turbine configurations. • A compressor and a combustor produce the enthalpy increasing pressure and temperature, resp. • The spinning turbine rotates a shaft which drives the compressor • Remaining enthalpy can be used to drive a generator or can be expanded in a nozzle producing kinetic energy e.g. thrust ME 300 Thermodynamics II 3 Enthalpy Generation and Conversion Turbine Fuel converts enthalpy Generator to shaft power for Compressor Combustor electricity increases increases pressure Temperature Air (pv) (u) Nozzle converts enthalpy into kinetic energy ME 300 Thermodynamics II 4 Gas Turbine Engine Components http://www.stanford.edu/group/ctr/ResBriefs/temp05/schluter2.pdf www.aem.umn.edu/research/Images/pw_GasTurbine.gif ME 300 Thermodynamics II 5 Combustor Simulations ME 300 Thermodynamics II 6 Schematic • Fresh air enters compressors ~constant pressure • Air compressed to high pressure in compressor e.g. 23:1 • High pressure air is mixed with injected fuel spray in combustor raising temperatures • High enthalpy product gases expand in turbine producing shaft work to run compressor or Open cycle generator ME 300 Thermodynamics II 7 Air Standard Brayton Cycle • Model as closed cycle with ASA • Replace combustion by heat addition at constant pressure • Replace intake/exhaust with heat rejection at constant pressure • Brayton cycle ME 300 Thermodynamics II 8 Brayton Cycle • Four internally reversible processes: – 1-2; Isentropic compression – 2-3: P=constant heat addition – 3-4: Isentropic expansion – 4-1: P=constant heat rejection ME 300 Thermodynamics II 9 Steady Flow Analysis ME 300 Thermodynamics II 10 Thermal Efficiency ME 300 Thermodynamics II 11 Example • A gas turbine power plant T, K operating on an ideal Brayton cycle has a 1300 pressure ratio of 8. The q gas temperature is 300 K P=constant w at the compressor inlet and 1300 K at the turbine rp=8 inlet. Utilizing the ASA, w determine (a) gas q temperature at exits of 300 P=constant compressor and turbine, (b) the back work ratio, s and (c) thermal efficiency. ME 300 Thermodynamics II 12 Example ME 300 Thermodynamics II 13 Example ME 300 Thermodynamics II 14 Example ME 300 Thermodynamics II 15 Optimization ME 300 Thermodynamics II 16 Comments • Maximum temperature in cycle limited by turbine blades maximum temperature ME 300 Thermodynamics II 17 Comments (cont.) • Role of Air in Gas Turbines ME 300 Thermodynamics II 18 Comments (cont.) • Thermal efficiency depends on maximum temperature at turbine inlet ME 300 Thermodynamics II 19 Applications • Aircraft propulsion • Electric power generation ME 300 Thermodynamics II 20 Back Work Ratio ME 300 Thermodynamics II 21 Deviations from Ideal ME 300 Thermodynamics II 22 Example • Assuming a compressor efficiency of 80% and a turbine efficiency of 85%, determine (a) back- work ratio, (b) thermal efficiency, (c) turbine exit temperature of previous gas turbine example. ME 300 Thermodynamics II 23 Example ME 300 Thermodynamics II 24 Example ME 300 Thermodynamics II 25 Summary • Ideal cycle for modern gas turbine engines is Brayton cycle • Four internally reversible processes – can you name them? • Pressure ratio is key parameter • Deviations handled with component isentropic efficiencies ME 300 Thermodynamics II 26 Outline • Performance improvement – Regeneration – Reheat – Intercooling • Examples ME 300 Thermodynamics II 27 Brayton Cycle with Regeneration ME 300 Thermodynamics II 28 Regeneration ME 300 Thermodynamics II 29 Example • Determine thermal efficiency of previous gas turbine engine if a regenerator having an effectiveness of 80% is installed. ME 300 Thermodynamics II 30 Example ME 300 Thermodynamics II 31 Example ME 300 Thermodynamics II 32 Intercooling ME 300 Thermodynamics II 33 Reheating ME 300 Thermodynamics II 34 Intercooling, Reheating, and Regeneration ME 300 Thermodynamics II 35 Ts Diagram ME 300 Thermodynamics II 36 More Stages? ME 300 Thermodynamics II 37 Example • An ideal gas turbine cycle with two stages of compression and two stages of expansion has an overall pressure ratio of 8. Air enters each stage of compressor at 300 K and each stage of turbine at 1300 K. Determine the back work ratio and thermal efficiency assuming (a) no regenerators, (b) ideal regenerator with 100% effectiveness. Compare results with previous example. ME 300 Thermodynamics II 38 Example ME 300 Thermodynamics II 39 Example ME 300 Thermodynamics II 40 Example ME 300 Thermodynamics II 41 Summary • Exhaust temperature of turbine is often higher than temperature of air leaving compressor • Regenerator transfer heat from hot turbine exhaust to high pressure air leaving compressor • Multistage compression with intercooling, regeneration, and multistage expansion with reheating increase thermal efficiency ME 300 Thermodynamics II 42.
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