Chemical kinetics lecture notes ppt

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Part 3: Chemical Kinetics, HCCI & SI Combustion Reciprocating Internal Combustion Engines Prof. Rolf D. Reitz Engine Research Center University of Wisconsin-Madison 2014 Princeton-CEFRC Summer School on Combustion Course Length: 15 hrs (Mon.- Fri., June 23 – 27, 2014) Copyright ©2014 by Rolf D. Reitz. This material is not to be sold, reproduced or distributed without prior written permission of the owner, Rolf D. Reitz. 1 CEFRC2-3, 2014 Part 3: Chemical Kinetics, HCCI & SI Combustion Short course outline: Engine fundamentals and performance metrics, computer modeling supported by in-depth understanding of fundamental engine processes and detailed experiments in engine design optimization. Day 1 (Engine fundamentals) Part 1: IC Engine Review, 0, 1 and 3-D modeling Part 2: Turbochargers, Engine Performance Metrics Day 2 (Combustion Modeling) Part 3: Chemical Kinetics, HCCI & SI Combustion Part 4: Heat transfer, NOx and Soot Emissions Day 3 (Spray Modeling) Part 5: Atomization, Drop Breakup/Coalescence Part 6: Drop Drag/Wall Impinge/Vaporization/Sprays Day 4 (Engine Optimization) Part 7: Diesel combustion and SI knock modeling Part 8: Optimization and Low Temperature Combustion Day 5 (Applications and the Future) Part 9: Fuels, After-treatment and Controls Part 10: Vehicle Applications, Future of IC Engines 2 CEFRC2-3, 2014 Part 3: Chemical Kinetics, HCCI & SI Combustion http://www.erc.wisc.edu/combustion.php Modes of engine combustion HCCI uses a hybrid combustion strategy. Premixed fuel and air is inducted, but instead of igniting with a spark as in a SI engine, the high temperature from compression causes the mixture to spontaneously react, like in a diesel engine. Ignition occurs at slightly different times at different locations in the chamber. One feature of HCCI combustion is how quickly the fuel is consumed. 3 CEFRC2-3, 2014 Part 3: Chemical Kinetics, HCCI & SI Combustion Curtis, 2014 Chemistry: importance of fuels 4 CEFRC2-3, 2014 Part 3: Chemical Kinetics, HCCI & SI Combustion Daw, 2013 New advanced combustion regimes HCCI 5 CEFRC2-3, 2014 Part 3: Chemical Kinetics, HCCI & SI Combustion Basic combustion concepts – Spark Ignition (SI) How can SI engines operate with engine speeds from 100 to 20,000 rev/min? 2 T Kinetic energy, kV piston burned Integral length scale l L I piston x Turbulence Kinetic energy dissipation rate, 3 e V /L piston piston S T fuel/air 2 Diffusivity, D k /e V L piston piston Because turbulent flame speed, S , scales with rpm T Characteristic Time Combustion (CTC) model Reitz & Bracco, 1983; Abraham, 1985 Species conversion rate (Y, species mass fraction, local equilibrium solution) i t k/e L / V ; c piston piston dY i V Mallard-Le Chatelier propagating wave speed: SD  piston T dt Glassman, 1996 6 CEFRC2-3, 2014 Part 3: Chemical Kinetics, HCCI & SI Combustion Halstead, 1977 Basic combustion concepts – Diesel (CI) Shell Ignition Model Q Ignition  R RH +O 2R 2 Delay  B R R + P + Heat  R R + B  R R + Q Af04  R + Q R + B  B 2R  R termination  2R termination Switch to Characteristic Time Combustion model Turbulence generated by fuel injection t k/e L / V nozzle nozzle c Kong, 1992 7 CEFRC2-3, 2014 Part 3: Chemical Kinetics, HCCI & SI Combustion Turbulent mixing Hot products with Cold reactants Spark-ignition t k/e burned unburned L / V piston piston High turbulence S T - faster combustion Diesel Injected fuel with entrained air air fuel t k/e L / V air nozzle nozzle Delayed ignition (PCCI) - better mixing S =0 T Matalon, 2011 8 CEFRC2-3, 2014 S /S T L Part 3: Chemical Kinetics, HCCI & SI Combustion Summary of combustion regimes • Gasoline engine spark-ignition with flame propagation: High turbulence for high flame speed  heat losses. Issues: NOx and UHC/CO, knock (CR, fuels), throttling losses  low thermal efficiency TE 25% • Diesel engine with spray (diffusion) combustion: Rich mixtures (soot) & high temperatures (NOx)  higher TE 45% • H/Premixed Charge Compression Ignition – LTC, chemistry controlled (CR): Sensitive to fuel, poor combustion/load control, low NOx-soot  TE 50% spark-ignition diesel H/PCCI 9 CEFRC2-3, 2014 Part 3: Chemical Kinetics, HCCI & SI Combustion Williams, 1988 Premixed volumetric combustion & chemical kinetics Species and energy conservation equations  cs ii  (uD )   ( )    i i i t  Constant volume combustion – Well-Stirred-Reactor (WSR) n r dY W '' ' ii  (  ) (Y,T), i 1,...,n  k,, i k i k s dt  k1 ' ''  k,, i k i nn ss I specific internal energy      YY ii  (Y ,T)       k f,, k b k chemical label WW ii  11 ii    n reactions r n species s ; reactant/product stoichiometric coefficients mass fraction n s  dT 1 e() T dY ii molecular weight (YY ,TT ) ( , )  dt c (Y,T) W dt i1 vi e species energy i e i 10 CEFRC2-3, 2014 Part 3: Chemical Kinetics, HCCI & SI Combustion Law, 2006 Homogeneous charge: no spatial gradients Y i  / 7 i A 1.610 (cm,mol,s), b 1.83, E 11.6(kJ / mol)  t 13 1.4 10 0 2 n s h  T f ,i i    13 1.2 10 tc i1 p 1.5 13 k 1 10  Consider single overall reaction 12 8 10 CH 4 k 1 12 k 6 10 CH OH  CH  H O 4 3 2 12 4 10 0.5 b k AT exp( E / RT) 12 2 10 0 0  d CH CH 4 4 0 500 1000 1500 2000 2500  kCH OH  4 dt W T CH 4 11 CEFRC2-3, 2014 Part 3: Chemical Kinetics, HCCI & SI Combustion Reitz, 1981 0.6 m=0.25 HCCI: Ignition delay FU() 0.5 Y i 0.4  / i m=0.5  t 0.3 0 n s h  T f ,i i Cold boundary 0.2  m=1.0  tc difficulty i1 p 0.1 U 0 Consider single component system 0 0.2 0.4 0.6 0.8 1 U U TT  unburned U  1.0 TT  burned unburned Example: dU mm 1  F(U)  bU (1U) dt Ignition 11 delay : mt b For U  U o mm UU 0 bt U So, time to reach, say, 5U : 0 o 4 Ignition delay: 1 bt  m mbt  Const. F (m,m;(1 m);U) 21 m 5mU 0 U 12 CEFRC2-3, 2014 m+1 m F(U)=U (1-U)Part 3: Chemical Kinetics, HCCI & SI Combustion Warnatz, 2006 Combustion chemistry models – CH (15 spec, 31 react.) 4 CH + 2 O = CO + 2 H O 4 2 2 2 Methyl Reactions Hydrogen-Oxygen Chain 13 CH + O  CH O + H 3 2 1 H + O  OH +O 14 CH +OH  CH O + H +H 2 3 2 2 H + O  OH + H 15 CH +OH  CH O + H 2 3 2 2 c 3 H + OH H O + H 16 CH + H  CH 2 2 3 4 4 H O + O 2 OH 23 CH + H  CH + H 2 3 2 2 Hydroperoxyl Formation and Consumption 28 CH + OH CH + H O 3 2 2 b 5 H + O + M  HO + M Formaldehyde Reactions 2 2 6 HO + H  2 OH 17 CH O + H CHO + H 2 2 2 7 HO + H  H + O 18 CH O + OH CHO + H O 2 2 2 2 2 8 HO + H  H O + O Formyl Reactions 2 2 9 HO + OH  O + H O 19 CHO + H CO + H 2 2 2 2 Conversion of Carbon Monoxide to Carbon Dioxide 20 CHO + OH CO + H O 2 End 10 CO + OH  CO + H 21 CHO + O  CO + HO 2 2 2 Methane Consumption 22 CHO + M CO + H + M Initiation Start 11 CH + H  H + CH 4 2 3 H atom abstraction 12 CH + OH  H O + CH Methylene Reactions 4 2 3 24 CH + O  CO + H 2 2 2 2 Methylidyne Reactions 25 CH + O  CO + OH + H 2 2 27 CH + O  CHO + O 2 26 CH +H  CH + H 2 2 31 CH + OH  CH O + H 2 29 CH + OH  CH O + H 2 2 30 CH + OH  CH + H O 2 2 Conversion to products by sequential fragmentation by H abstraction 13 CEFRC2-3, 2014 H O chemistry 2 2 High temperature Part 3: Chemical Kinetics, HCCI & SI Combustion Lu, 2009 Brakora, 2013 Chemical kinetic mechanisms for engine simulations Requirements for mechanisms for practical engine simulations: • Size can not be too large due to CPU time limitation 100 species • Capable of predicting auto-ignition delay time accurately • Contain proper reactions for pollutant formation precursors Biodiesel surrogates - Significant mechanism reduction is required. Soy biodiesel - Methyl: - palmitate (C16:0) - stearate (C18:0) - oleate (C18:1) - linoleate (C18:2) CH 4 - linolenate (C18:3) C C C 2 4 9 14 CEFRC2-3, 2014 Part 3: Chemical Kinetics, HCCI & SI Combustion Warnatz, 2006 Hydrocarbon kinetics - NTC Second Stage Ignition H O = OH + OH 2 2 Acceleration by 1400 Q•OOH branching 1200 O 2 1000 . 800 600 400 200 0 555 580 605 630 655 680 705 730 Initial Temperature (K) First Stage Ignition Isomerization steps 15 CEFRC2-3, 2014 Induction Period (s)Part 3: Chemical Kinetics, HCCI & SI Combustion Warnatz, 2006 Alkane fuel oxidation Example: Propane Energy release CO + OH CO2 + H High Smaller R HCO temperature 1200K Chain branching CH , C H ,… 3 2 5 reaction b-scission H + O OH + O 2 Blue flame 1100K Thermal decomposition Fuel: RH R H O OH + OH +OH, H,… 2 2 + n-C H • + O 3 7 2 Negative Internal H atom abstraction Chain propagation temperature 800 QOOH ROO 900K coefficient QO + OH +O 2 Olefin channel OOQOOH Internal H atom Chain branching abstraction Cool flames 800K HOOQ’O + OH HOOQ’OOH •CH CH CH OOH 2 2 2 OQ’O + OH Isomerization 16 CEFRC2-3, 2014 Part 3: Chemical Kinetics, HCCI & SI Combustion Mehl, 2009 HCCI combustion kinetics Typical HCCI Combustion H R• Temperature and Heat Release Rate Fast High Temperature profiles - RH Combustion • • + O 2 OO• H O 2 2 + HO • 2 OOH • O + + •OH + O 2 O + •OH OOH Ethers/ olefins •OO Degenerate - •OH Branching Path O O CAD TDC •OH O + + HOO • Aldehydes/ketones 17 CEFRC2-3, 2014 HRR T, P Part 3: Chemical Kinetics, HCCI & SI Combustion Patel, 2004 Mechanism reduction – identify key reaction steps ERC n-heptane mechanism Burnout stage 6 6 6 1 1. n . n- -C C H H + + O OH H= = C C H H - -2 2+ + H H O O 7 7 16 16 7 7 15 15 2 2 2 2. C . C k ke ett = = C C H H C CO O + + C CH H O O + + O OH H 7 7 12 12 5 5 11 11 2 2 nd 2 st 3 3. H . H O O + + M M = = O OH H + + O OH H + + M M stage 2 2 2 2 1 stage 4 4.. H HO O + + H HO O = = H H O O + + O O 2 2 2 2 2 2 2 2 2 2 5 5.. C CH H + + H HO O = = C CH H + + H H O O 4 4 2 2 3 3 2 2 2 2 2 6 6.. C CO O + + O OH H= = C CO O + + H H 2 2 3,4,5 , 7 , 7 7 7. C . C H H O O + + O O = = C C k ke ett + + O OH H 7 7 15 15 2 2 2 2 7 7 12 12 1,7 Time 18 CEFRC2-3, 2014 Temperature Part 3: Chemical Kinetics, HCCI & SI Combustion Ra, 2008 Reduced mechanisms: match shock tube and RCM data First stage (t1), main ignition (t2=tig) delay 10 Expts: Fieweger, 1997 1 0.1 Cal, tig Exp, tig Predicted ignition delay times Cal, t1 validated against shock tube tests Exp, t1 (data from Fieweger) 0.01 =1.0 and P=40 bar n-heptane/air 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1000/initial temperature 1/K 19 CEFRC2-3, 2014 ignition delay msPart 3: Chemical Kinetics, HCCI & SI Combustion Ra, 2011 Mechanism reduction methodology tetra-decane: ROO+O =R-keto+OH 2 10 Reduction of reaction pathways and species number baseline A = k×A base k=2 - combination of chemical lumping, graphical k=0.5 1 reaction flow analysis and elimination methods Reaction rate optimization - ignition delay curve sensitivity analysis 0.1 Pre-exponential: A = k×A base 0.01 600 700 800 900 1000 1100 1200 1300 1400 Ignition delay sensitivity coefficient initial temperature K (logtt  log ) 10kk 10 12 ignition delay sensitivity ST ( ) 100 25 60 ig gradient sensitivity log t log (k k ) 10 base 10 1 2 50 20 40 Ignition delay gradient sensitivity coefficient 30 15 20 d log t d log t 10kk 10 12 10 10  dT dT 0 ST ( )100 gr 5 log (kk ) -10 10 1 2 -20 0 -30 Positive S : counter-clockwise rotation gr -5 -40 Negative S : clockwise rotation gr 650 750 850 950 1050 1150 1250 1350 1450 initial temperature K 20 CEFRC2-3, 2014 ignition sensitivity % ignition delay ms gradient sensitivity coefficient