08.04.3. HERMODYNAMICS CHAPER 9 Gas power cycles Lecturer Axel GRONIEWSKY, PhD 5 th of February08 Most power-producing devices operate on cycles. Complexity of actual cycles are high idealizationsare required (ideal cycle Heat engines: converting thermal energy to work; performance is expressed in terms of the thermal efficiency Wnet wnet or Qin Common idealizations & simplifications No friction is involved no pressure drop in pipes or heat exchangers Expansions and compressions are reversible and quasi-static processes Pipes and equipment are externaly adiabatic unless otherwise is stated Change of kinetic and potential energies are neglected (except fornozzels, diffusers Lecture BME 3 th January 08 Lecture BME 3 th January 08 Property diagrams on p-v and -s, the area enclosed by theprocess curves of a cycle represents the network produced during the cycle heat-addition(q in on-s:process proceeds in the direction of increasing entropy heat-rejection (q out on-s:process proceeds in the direction of decreasing entropy isentropic process(adiabatic& reversible on -s: process proceeds at constant entropy. Any modification that increases the ratio of these two areas will also increase the thermal efficiency of the cycle. Carnot cycle: a reversible cycle, therefore themostefficient cycle operating between two specified temperature limits. L η th hermal efficiency increases with an increase in the average temperature at which heat is supplied to thesystem( H or with adecrease in the average temperature at which heat is rejected ( L from the system. H limit: maximum the components can withstand L limit: of the cooling medium utilized in the cycle H Lecture BME 3 th January 08 3 Lecture BME 3 th January 08 4 Internal combustion engines: working fluid does not undergo a complete thermodynamic cycle (as exhaust gases simplifications and idealizations are required Air-standard assumptions: working fluid is air, behaves as an ideal gas, continuously circulating in a closed loop air has constant specific heats at room temperature (5 C cold-air-standard assumptions processes of the cycle are reversible. combustion process is replaced by a heat-addition process exhaust process is replaced by a heat-rejection process that restoresthe working fluid to its initial state. Lecture BME 3 th January 08 5 RECIPROCAING ENGINES Reciprocating engines: spark-ignition (SI engines or compression-ignition (CI engines; usedinautomobiles, trucks, light aircraft, ships, electric power generators op/botom deadcenter (DC/BDC: positionof the piston when it forms the smallest/largest volume in the cylinder Stroke: distance between DC and BDC Bore: diameter of the piston Intake/Exhaustvalve: valvewhichallow the fuel-air mixture/combustion products to be drawn into/expelled from the cylinder Clearance volume: minimum volume when piston is at DC Displacement volume: volume between DC and BDC Compressionratio (r: r max min BDC DC Mean effective pressure, kpa (MEP: Wnet p Apiston LStroke p displacement or p Wnet ( max min MEP MEP MEP Lecture BME 3 th January 08 6
08.04.3. Four-strokeinternal combustionengines Otto cycle: Isentropic compression(- Constant-volume heat addition(-3 Isentropic expansion(3-4 Constant-volume heat rejection(4- Otto cycle executed in a closed system ( + ( win wout u (kj/kg Heattransfertoand fromthesystem u3 u cv( 3 u4 u cv( 4 w ( 4 net 4 3 ( 3 Isentropic processes 3 4 4 3 hermal Efficiency max, Otto ; r r min Lecture BME 3 th January 08 7 Lecture BME 3 th January 08 8 η th,otto η th,otto (r, r 8 ifris veryhigh air-fuelmix > autoignition prematureignition engineknock it can cause engine damage gasoline blends have goodantiknock characteristics r octane rating: measure of engine knock resistance ofa fuel former gasolin blend: tetraethyl lead (hazardous to health and pollute the environment Workingfluid in actual engines contains larger moleculessuchasco monatomic gas.667 air.4 CO.3 η th,otto, actual 5 30 Cycle Compression Ignition Combustion Otto Air + Fuel Spark Isohoric device sparkplung, carburetor Diesel Air Compression Isobaric fuel injector Lecture BME 3 th January 08 9 Lecture BME 3 th January 08 0 DC op Dead Center BDC Bottom Dead Center IO Intake alve Opens EC Exhaust alve Closes IC Intake alve Closes I Ignition EO Exhaust alve Opens Lecture BME 3 th January 08 Diesel cycle: Isentropic compression(- Constant-pressure heat addition(-3 Isentropic expansion(3-4 Constant-pressure heat rejection(4- Diesel cycle executed in a closed system ( + ( win wout u (kj/kg Heattransfertoand fromthesystem w,3 u3 u p( v3 v + ( u3 u h3 h cp( 3 q u u c out 4 v( 4 w ( 4 net 4 ( 3 ( 3 Cutoffration(r c r c ; r 3 max min hermal Efficiency > rc, Diesel r ( rc Lecture BME 3 th January 08
08.04.3. η th,diesel η th,diesel (r, r c, requirement: air > autoignition no engine knock rcan be very high less refined fuels(less expensive SIRLING AND ERICSSON CYCLE Engineswithisothermalheat-addition( H and rejection( L processes η th,otto >η th,diesel ifr Otto r Diesel r c η th,ottoη th,diesel r Diesel -4 η th,diesel, actual 35 40 Dual cycle(rinkler, Seiliger or Sabathe cycle combination of the Otto and the Diesel cycles -: Isentropic compression -3: Addition of heat at constant volume. 3-4: Addition of heat at constant pressure. 4-5: Isentropic expansion. 5-: Rejection of heat at constant volume. Lecture BME 3 th January 08 3 Lecture BME 3 th January 08 4 SIRLING AND ERICSSON CYCLE Stirling cycle external combustion engine -: Isothermalheat-addition ( H -3: Isochoricregeneration(internal heat transfer from working fluid to regenerator 3-4: Isothermalheat-rejection( L 4-: Isochoricregeneration(internal heat transfer from regenerator to working fluid Ericsson cycle external combustionengine -: Isothermalheat-addition ( H -3: Isobaricregeneration(internal heat transfer from working fluid to regenerator 3-4: Isothermalheat-rejection( L 4-: Isobaricregeneration(internal heat transfer from regenerator to working fluid L, Stirling, Ericsson, Carnot H Lecture BME 3 th January 08 5 Bryton Cycle gas turbine -: Isentropic compression (in a compressor -3: Constant-pressure heat addition 3-4: Isentropic expansion(in a turbine 4-: Constant-pressure heat rejection Lecture BME 3 th January 08 6 Bryton cycle steady-flow devices η th,brayton η th,brayton (r p, Energy balance for steady-flow processes ( + ( win wout h (kj/kg Heat transfer to and from the workingfluid h3 h cp( 3 h4 h cp( 4 w ( 4 ( 4 net q cp out cp( 3 ( 3 Isentropic processes p p 3 3 p p4 4 Pressureratio (r p p rp p hermal Efficiency η th, Bryton r p Lecture BME 3 th January 08 7 highr p highη th highr p high H temperaturelimit: 3 < bladescanwithstand r Brayton -6 Other parameters air-fuelratio: AFRm air /m fuel 0.5 air-fuelequvivalenceratio: λafr/afr stoichiometric λ Brayton 3.5 (λ>richmixture; λ<leanmixture Back workratio: BWRW comp /W turb Lecture BME 3 th January 08 8 3
08.04.3. Development of G Increasing the turbine inlet temperature( 3 500 C coating bladeswith ceramic layers cooling bladesabove 850 C Increasing the efficiencies of cycle components Adding modifications to the basic cycle Intercooling regeneration(recuperation Reheating Actual Gas-urbine Cycles of compressors ws hs h ηc wact ha h of turbines wact h3 h4a η w h h s 3 4s Lecture BME 3 th January 08 9 q q Development of G Regeneration: high-pressure air leaving the compressoris heated by heat from the hot exhaust gases in a counter-flowheat exchanger(regenerator or recuperator Larger pressure drop higher effectiveness(ε<0.85 lowerfuel consumption higher price h h reg, act 5 h h h h reg,max 5' 4, reg ( r p 3 qreg, act h5 h 5 ε q h h reg,max 4 4 Lecture BME 3 th January 08 0 Brayton cycle with intercooling, reheating and regeneration multistage compression with intercooling(to reduce specific volume multistage expansionwith reheating(to increase specific volume Wsteady flow dp ( p Jet-propulsion cycle: aircraft gas turbine, operating on an open cycle(lightand compact propeller-driven engine: slightly accelerating a large mass offluid(power jet or turbojet engine:greatly accelerating a small mass of fluid(thrust turboprop engine:combinationof thetwo turbine comp+ aux equ net0 p turb,out >p enviro Gasesenter nozzlewithhigh pressure expandto p ambient v increases thrust is provided hrust(propulsiveforce: F Propulsivepower: W (kw Propulsiveeffect: propulsivepower( p devided by energy input rate(qin o maximize turbine work: p 4 6 8 p and p p p p p p 3 7 9 Lecture BME 3 th January 08 Lecture BME 3 th January 08 urbofan(fanjet engine: large fan driven by the turbine forces a considerableamount of air through a duct (cowl surrounding the engine. Fan exhaust leaves the duct at a higher velocity,enhancing the total thrust of the engine significantly. ypically core flow exit velocity is in the vicinity of Mach and bypass duct velocity is around Mach 0.3. Bypassratio mass flow rate of the bypass stream devidedbythe mass flow rate ofthe core(0: Highspeadexhaustgases are mixed with the lower-speed air, causingreduction in noise urbofan: Pratt& Whitney PW4084 turbofan(boeing 777 aircraft hrust: 373.65 kn Length: 4.87 m Diameter:.84 m Weight: 6800 kg Lecture BME 3 th January 08 3 Lecture BME 3 th January 08 4 4
08.04.3. Propjetengine: gasturbine that drives an aircraft propeller bypassratio ~ (00: (propellersare more efficient than jet engines limited tolow-speed and low-altitude operation their efficiency decreases at highspeeds and altitudes Ramjetengine: uses the engine's forward motion to compress incoming air without a compressor. Cannotproduce thrust at zero airspeed, requires an assisted take-off to accelerate it to a speed where it begins to produce thrust. Ramjets work most efficiently at supersonic speeds around Mach 3. Scramjet engine: type of Ramjet engine in which air flows through at supersonic speeds Lecture BME 3 th January 08 5 Lecture BME 3 th January 08 6 hank you for your attention! Lecture BME 3 th January 08 7 5