Chemical inhibiting of hydrogen-air detonations Sergey M. Frolov

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1 Chemical inhibiting of hydrogen-air detonations Sergey M. Frolov Semenov Institute of Chemical Physics Moscow, Russia

2 Outline Introduction Theoretical studies Classical 1D approach (ZND-model) Detailed reaction mechanism Reaction mechanism for inhibitor Solution algorithm Results of calculations for H air Results of calculations for H air inhibitor Experimental studies Experimental procedure Experimental results for H air Experimental results for H air inhibitor Conclusions Acknowledgments

3 Introduction Ideas on chemical inhibiting of hydrogen air detonations were put forward long ago at testing piston engines operating on hydrogen. As detonation suppression additives, methane, iodine, iodine hydrogen, organic iodeeds, as well as various metalorganic and nitrogen-organic compounds were considered. The effect of detonation suppression additives on the detonation properties of hydrogen air mixtures were explained mostly by the low-temperature reactions of chain termination. The search for effective detonation suppression additives has been revived recently in view of new developments in hydrogen technologies, in particular fuel cells. Recent studies demonstrate that small additives of unsaturated hydrocarbons to hydrogen air mixtures can inhibit detonations due to termination of high-temperature chain-branching oxidation reactions. 1D Zel dovich theory of detonability limits was proved to provide satisfactory uantitative predictions for detonability limits depending on mixture composition, initial temperature and pressure, as well as the percentage of inert diluents in hydrogen air mixture.

4 Objective The objective of this work is to apply the 1D theory of detonability limits to the problem of chemical inhibiting of hydrogen air detonations by small additives of effective gaseous detonation-suppression agents.

5 Theoretical studies

6 Structure of 1D steady-state detonation Complete expansion and cooling plane: D - u = 0 T = T 0 Complete expansion plane: D - u = 0 T > T 0 CJ plane u CJ = acj Shock wave D D u u CJ u u s D 5 x Zone of cooled and expanded detonation products 4 Zone of afterburning expansion, and cooling of detonation products 3 Reaction zone 0 Initial mixture 1

7 Variation of mixture state D - u = 0 0,0 0,1 0, 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 T = 1,1 T 0 35 s CJ s Detonation Hugoniot Integral curve D Complete expansion and cooling plane: D 5 x Zone of cooled and expanded detonation products u Complete expansion plane: D - u = 0 T > T 0 4 Zone of afterburning expansion, and cooling of detonation products CJ plane u CJ = acj u CJ 3 u Shock wave Reaction zone u s 0 D Initial mixture 1 p/p Shock Hugoniot Rayleigh lines CJ point 0 0,0 0,1 0, 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 C CJ 3 Flow turning point 4 1,5 CJ point v / v 1

8 Governing euations U v ; U d( p v dx U d v D v o ( H dx ) U ; ) D. + Chemical kinetics Zeldovich (1940), Rybanin (1966), Frolov (1986) Skin friction Heat flux Reynolds number d w Re Drag coefficient 10 5 < Re < 10 6 C f Re Heat transfer coefficient Nu d Nu C Re ~ h Reynolds analogy C 1 h C f

9 Boundary conditions Complete expansion and cooling plane: D - u = 0 T = T 0 Complete expansion plane: D - u = 0 T > T 0 CJ plane u CJ = acj Shock wave D D u u CJ u u s D 5 x Zone of cooled and expanded detonation products 4 Zone of afterburning expansion, and cooling of detonation products 3 Reaction zone 0 Initial mixture 1 x : 1; T T 1 1 s N i1 H c i i p s s u u s s sus s D, 1 1 ci ci 1 s p 1 1 D N 1 i1 i, H c i 1 D,

10 Validation of hydrogen oxidation mechanism 300 0%Н +air 50 5%Н +air t ind, s T = 100 K t ind, s T = 100 K 0 0,50 0,55 0,60 0,65 0,70 0,75 0,80 0,85 0, / T, 1/K 0 0,60 0,65 0,70 0,75 0,80 0,85 0, / T, 1/K Exp. Fukutani et al. (1999) Temperature range of interest T > 100 K

11 Reaction mechanism: inhibitor C3H6 (propylene) C3H6 H ( M ) C3H7 ( M ) k exp T (l, mole, s), Tsang (1991) C 3H7 O C3H6 HO k (l, mole, s), Warnatz (1984)

12 Solution algorithm: shooting techniue D Complete expansion and cooling plane: D - u = 0 T = T 0 D u Complete expansion plane: D - u = 0 T > T 0 CJ plane u CJ = acj u CJ u Shock wave u s D +C f -C f C f (root) 5 x Zone of cooled and expanded detonation products dv dx 4 Zone of afterburning expansion, and cooling of detonation products P, M,.. 0 P, M,.. 0 CJ 3 Reaction zone 0 Initial mixture 1 P/P 0 CJ point X, mm Detonation velocity D is the problem eigenvalue

13 Detonability limits: hydrogen - air Limiting tube diameter, mm Calc. Direct exper d = Limiting tube diameter, mm T 1 = 48 K T 1 = 98 K H, %(vol.) H, %(vol.)

14 1D Detonation structure: 5%(vol.) Н -air u, m/s T, K d = 195 mm d = 8 mm t, s d = 195 mm d = 8 mm t, s T u p, MPa H, %(vol.),8,6 d = 195 mm,4, d = 8 mm,0 1,8 1,6 1,4 1, t, s 0,0 0,15 0,10 d = 195 mm 0,05 d = 8 mm 0, t, s P Н

15 Effect of inhibitor on ignition delay of hydrogen air mixture t ind, s % 0% 0, p 1, MPa T, K % 1% t, s [H], % (vol.) 0,10 0,08 0,06 0,04 0,0 0% 1% 0, t, s! Volume fraction, %(vol.) 0,01 1E-3 1E-4 1E-5 1E-6 1E-7 1% [C 3 H 6 ] [C 3 H 7 ] [HO ] [HO ] 0% 1E t, s

16 Detonability limits: hydrogen air inhibitor (C3H6) Limiting tube diameter, mm % 1% 0% P 0 = 1 atm T 0 = 98 K Narrowing of detonability limits (even in wide tubes!) Increasing the limiting tube diameter Shifting the minimum of U-curve [H ] % (vol.)

17 T, K [H O ], %(wt.) 1D Near-limiting detonation structure: %(vol.) Н air inhibitor (C3H6) 0% 1% ,010 0,009 0,008 0,007 0,006 0,005 0,004 0,003 0,00 0,001 t, s 0, t, s 0% d = 6 mm D = 168 m/s 1% d = 9 mm D = 1748 m/s [H], %(wt.) [HO ], %(wt.) 0,5 0,0 0,15 0,10 0,05 0% 1% 0, ,08 1% 0,07 0,06 0,05 0,04 0,03 0,0 0,01 0% t, s 0, t, s

18 Detonation velocity deficit without (0%) and with (1%) C3H6 1,00 0,98 0,96 0,94 D/D CJ 0,9 0,90 1% Decreasing allowable deficit 0,88 0,86 0% 0, [H ], %(vol.)

19 Experimental studies

20 Detonation tube at Institute of Structural Macrokinetics

21 Schematic of experimental setup Detonation tube Photodiodes Ignition (~3 J) Pressure transducers Data auisition Data acuisition system Overfilling with H-O Initiation section (H-O) Tube length: 15 m Tube diameter: 101 mm Mixtures: 33.8% H + air (+ C4H8) 45.0% H + air (+ C3H6, C4H8) Initial conditions: P 0 = 1 atm, T 0 = 98 K

22 x, m Time distance diagram: 33.8% H + air 4 test runs with registered detonation D = m/s Maximum distance for detonation establishment t, ms Azatyan et al. (007)

23 Time distance diagram: 33.8% H + air + iso-c4h8 x, m Distance to decay ~.0%.% pure H -air %.% a b Shock wave Reaction front t, ms Azatyan et al. (007)

24 Time distance diagram: 33.8% H + air +.5% (iso-c4h8 or C3H6) x, m pure H -air x, m t, ms C3H6 C4H8 a Shock wave Reaction front b Limiting tube diameter, mm % % 1% [H ] % (vol.) 0% iso-butylene vs. propylene: effect of molecular structure iso-butylene: easier and uicker detonation decay (-bond) predicted limiting tube diameter for H air.5% C3H6 is 61 mm high sensitivity of detonation go no go conditions to governing parameters 61 mm t, ms Azatyan et al. (007), Frolov et al. (009)

25 Conclusions Mathematical modeling of chemical inhibiting of hydrogen air detonations has been performed. 1D detonation model with detailed chain-branching reaction mechanism of hydrogen oxidation describes satisfactorily all main effects of chemical inhibitors on the detonation. Calculations indicate that chemical inhibitors narrow the concentration limits of detonations and increase the limiting tube diameter, in which the steadystate detonation propagation is still possible. Despite the inhibitors introduce additional exothermal reactions and additional hydrogen oxidation reactions, their presence results in detonation suppression. Inhibitors lead to the deceleration of the overall reaction process which manifests itself by increasing ignition delay time and decreasing concentrations of atomic hydrogen main carrier of chain-branching reaction. The effect of inhibitors is mainly determined by the specific dependence of the rate of chain-branching reaction on temperature which differs considerably from the regular Arrhenius dependence.

26 Acknowledgments The author would like to thank Prof. Azatyan V.V. for providing the inhibiting mechanism of n-propylene and experimental data, and M.Sc. Medvedev S.N. for performing calculations and data processing. This work was supported by the Russian Foundation for Basic Research (grant ).

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