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Unformatted text preview: Advanced fundamental topics Ignition Basic concepts Mathematical theory Dynamics of ignition Effects of the state of the combustible mixture Effects of the characteristics of the ignition source Effects of the flow environment More detailed information:
http://ronney.usc.edu/Lecture2/AME514F06/AME514.ignition.review.pdf AME 514  Fall 2006  Lecture 2 1 Basic concepts Experiments (Lewis & von Elbe, 1961) show that a minimum energy (Emin) (not just minimum T or volume) required to ignite a flame Emin lowest near stoichiometric (typ. 0.2 mJ) but minimum shifts to richer mixtures for higher HCs (why? Stay tuned...) Prediction of Emin relevant to energy conversion and fire safety applications
Minimum ignition energy (mJ) AME 514  Fall 2006  Lecture 2 2 Basic concepts Emin related to need to create flame kernel with dimension ( ) large enough that chemical reaction ( ) can exceed conductive loss rate ( / 2), thus > ( / )1/2 ~ /( )1/2 ~ /SL ~ Emin ~ energy contained in volume of gas with T Tad and radius 4 /SL k (Tad  T ) 4p 3 4p 3 E min d r C p (Tad  T ) 0.3 d r C p (Tad  T ) 34a 2 3 3 3 SL
Successful ignition
Initial profile Initial profile Unsuccessful ignition EMPERATURE Later TEMPERATURE
Still later Later Still later SL DISTANCE DISTANCE AME 514  Fall 2006  Lecture 2 3 Predictions of simple Emin formula Since ~ P1, Emin ~ P2 if SL is independent of P Emin 100,000 times larger in a Hediluted than SF6diluted mixture with same SL, same P (due to and differences) Stoichiometric CH4air @ 1 atm: predicted Emin 0.010 mJ 30x times lower than experiment (due to chemical kinetics, heat losses, shock losses ...) ... but need something more (Lewis number effects): 10% H2air (SL 10 cm/sec): predicted Emin 0.3 mJ = 2.5 times higher than experiments Lean CH4air (SL 5 cm/sec): Emin 5 mJ compared to 5000mJ for lean C3H8air with same SL  but prediction is same for both AME 514  Fall 2006  Lecture 2 4 Predictions of simple Emin formula Emin ~ 3 hard to measure, but quenching distance ( q) (min. tube diameter through which flame can propagate) should be ~ since Pelim = SL,lim q/ ~ q/ 40 constant, thus should have Emin ~ q3P 2 10 Correlation soso Hydrogen (lean) Slope = 0.739
10
1 0 Hydrogen (rich) Methane (lean) Methane (rich) Ethane (lean) Ethane (rich) Propane (lean) Propane (rich) Best fit to all data 10 Minimum ignition energy (mJ) 10 10 10
1 2 Slope = 1
3 10 6 10 5 10 4 10 3 10 2 3 10 1 10
3 0 Pressure * (quenching distance) (atm cm ) AME 514  Fall 2006  Lecture 2 5 More rigorous approach Assumptions: 1D spherical; ideal gases; adiabatic (except for ignition source Q(r,t)); 1 limiting reactant (e.g. very lean or rich); 1step overall reaction; D, , CP, etc. constant; low Mach #; no body forces Governing equations for mass, energy & species conservations (y = limiting reactant mass fraction; QR = its heating value) 1 T = r T = constant + 2 (r 2r v ) = 0 t r r T 1 k 2 T 2 C p + r C p 2 ( r vT ) = 2 r + r QR yZ exp(  E ) + Q(r,t) T t r r r r r y 1 r D 2 y 2 + r v 2 (r y ) = 2 r yZ exp(  E ) r T t r r r r r AME 514  Fall 2006  Lecture 2 6 More rigorous approach Nondimensionalize (note Tad = T + YQR/CP) T ;t Tad T ;Y Tad tAe ;R y ;Le y b Ze b r ;U a v a Ze b ;b E Tad k Q(r,t) ;W r Cp D r C p T Ze b 2 Y Y 1 1 q 1 1 +U 2 R 2Y ) = R Yexp( b (q  1)) ( 2 t R R Le e R R R leads to, for mass, energy and species conservation 2 1 (1/q ) 1 + 2 R = U 0 t R R q 2 q 1 q 1 2 1 +U 2 R + (R,t )q (1 e)Yexp( b (q  1)) + W (R q ) = 2 R R e R R R
with boundary conditions (R,0) = e;Y (R,0) = 1;U(R,0) = 0 for all R (Initial condition: T = T , y = y , (R,t ) = e;Y (R,t ) = 1;U(R,t ) = 0 as R Y U = = = 0 at R = 0 and as R R R R
AME 514  Fall 2006  Lecture 2 7 Steady (?!?) solutions If reaction is confined to a thin zone near r = RZ (large )
1 e Rz R + e; Y = 1 z R Le R R < Rz : q = q * = constant; Y = 0 R > Rz : q = Rz = * T* 1 e T T =e+ or T * = T + ad Tad Le Le a b 2eLea Z d b 1 exp *  1 = ;SL = ;d exp Le 2 q b 2 SL This is a flame ball solution  note for Le < > 1, T* > < Tad; for Le = 1, T* = Tad and RZ = Generally unstable R < RZ: shrinks and extinguishes R > RZ: expands and develops into steady flame RZ related to requirement for initiation of steady flame  expect Emin ~ Rz3 ... but stable for a few carefully (or accidentally) chosen mixtures AME 514  Fall 2006  Lecture 2 8 Steady (?!?) solutions How can a spherical flame not propagate???
1.2
C ~ 11/r T* Temperature 1 )
Fuel concentration  T 0.8
f Propagating flame (/ = 1/10) T
Interior filled with combustion products T ~ 1/r ) / (T Normalized temperature 0.6 (T  T 0.4
Reaction zone 0.2 0 0.1 1 10 Radius / Radius of flame 100 Fuel & oxygen diffuse inward Heat & products diffuse outward Space experiments show ~ 1 cm diameter flame balls possible Movie: 500 sec elapsed time QuickTimeand a Video decompressor are needed to see this picture. AME 514  Fall 2006  Lecture 2 9 Lewis number effects Energy requirement very strongly dependent on Lewis number! 1000 100 = 1/7 = 10 (Le = 1)10
3 3 z z 1 0.1 0.01 /R R 0.001 0 0.5 1 1.5 2 Lewis number d b 1 From the relation Rz = exp *  1 Le 2 q From computations by Tromans and Furzeland, 1986 AME 514  Fall 2006  Lecture 2 10 Lewis number effects Ok, so why does min. MIE shift to richer mixtures for higher HCs? Leeffective = effective/Deffective Deff = D of stoichiometrically limiting reactant, thus for lean mixtures Deff = Dfuel; rich mixtures Deff = DO2 Lean mixtures  Leeffective = Lefuel Mostly air, so eff air; also Deff = Dfuel CH4: DCH4 > air since MCH4 < MN2&O2 thus LeCH4 < 1, thus Leeff < 1 Higher HCs: Dfuel < air, thus Leeff > 1  much higher MIE Rich mixtures  Leeffective = LeO2 CH4: CH4 > air since MCH4 < MN2&O2, so adding excess CH4 INCREASES Leeff Higher HCs: fuel < air since Mfuel > MN2&O2, so adding excess fuel DECREASES Leeff Actually adding excess fuel decreases both and D, but Const1 Const 2 Const 3 decreases more =a ~ ;D ~ +
eff mix M mix O2 M mix MO 2 AME 514  Fall 2006  Lecture 2 11 Dynamic analysis RZ is related (but not equal) to an ignition requirement Joulin (1985) analyzed unsteady equations for Le < 1
s q(s ) dc (s) ds ( ) ln ( c (s )) + = c (s ) 2 ds s  s 0 2 * 2 (q Q R(s ) Le a t ) q ;c ;s 4p * 2 e 1 Le Rz2 Rz 14pl RzTad (q ) ( , and q are the dimensionless radius, time and heat input) and found at the optimal ignition duration 2 E min e 1 Le 13 14b * r ad C p (Tad  T ) Rz e q Le which has the expected form Emin ~ {energy per unit volume} x {volume of minimal flame kernel} ~ { adCp(Tad  T)} x {Rz3}
AME 514  Fall 2006  Lecture 2 12 Dynamic analysis Joulin (1985) Radius vs. time duration Minimum ignition energy vs. ignition AME 514  Fall 2006  Lecture 2 13 Effect of spark gap & duration Expect "optimal" ignition duration ~ ignition kernel time scale ~ RZ2/ Duration too long  energy wasted after kernel has formed and propagated away  Emin ~ t1 Duration too short  larger shock losses, larger heat losses to electrodes due to high T kernel Expect "optimal" ignition kernel size ~ kernel length scale ~ RZ Size too large  energy wasted in too large volume  Emin ~ R3 Size too small  larger heat losses to electrodes
Sloane & Ronney, 1990
Detailed chemical model Kono et al., 1976 1step chemical model AME 514  Fall 2006  Lecture 2 14 Effect of flow environment Mean flow or random flow (i.e. turbulence) (e.g. inside IC engine or gas turbine) increases stretch, thus Emin Kono et al., 1984 DeSoete, 1984 AME 514  Fall 2006  Lecture 2 15 Effect of ignition source Laser ignition sources higher than sparks despite lower heat losses, less asymmetrical flame kernel  maybe due to higher shock losses with shorter duration laser source? 10 Minimum ignition energy (mJ)
1
ps laser ns laser Lewis & von Elbe Sloane & Ronney Ronney Kingdon & Weinberg 0.1 4 5 6 7 8 9 10 11 12 Mole percent CH4 in air
Lim et al., 1996
AME 514  Fall 2006  Lecture 2 16 References
De Soete, G. G.: 20th Symposium (International) on Combustion, Combustion Institute, 1984, p. 161. DixonLewis, G., Shepard, I. G.: 15th Symposium (International) on Combustion, Combustion Institute, 1974, p. 1483. Frendi, A., Sibulkin, M.: "Dependence of Minimum Ignition Energy on Ignition Parameters," Combust. Sci. Tech. 73, 395413, 1990. Joulin, G.: Combust. Sci. Tech. 43, 99 (1985). Kingdon, R. G., Weinberg, F. J.: 16th Symposium (International) on Combustion, Combustion Institute, 1976, p. 747.9924. Kono, M., Kumagai, S., Sakai, T.: 16th Symposium (International) on Combustion, Combustion Institute, 1976, p. 757. Kono, M., Hatori, K., Iinuma, K.: 20th Symposium (International) on Combustion, Combustion Institute, 1984, p. 133. Lewis, B., von Elbe, G.: Combustion, Flames, and Explosions of Gases, 3rd ed., Academic Press, 1987. Lim, E. H., McIlroy, A., Ronney, P. D., Syage, J. A., in: Transport Phenomena in Combustion (S. H. Chan, Ed.), Taylor and Francis, 1996, pp. 176184. Ronney, P. D., Combust. Flame 62, 120 (1985). Sloane, T. M., Ronney, P. D., "A Comparison of Ignition Phenomena Modelled with Detailed and Simplified Kinetics," Combustion Science and Technology, Vol. 88, pp. 113 (1993). Tromans, P. S., Furzeland, R. M.: 21st Symposium (International) on Combustion, Combustion Institute, 1986, p. 1891. AME 514  Fall 2006  Lecture 2 17 ...
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This note was uploaded on 09/19/2008 for the course AME 514 taught by Professor Ronney during the Fall '06 term at USC.
 Fall '06
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