TRANSITION TO DETONATION IN NON-UNIFORM H2-AIR: CHEMICAL KINETICS OF SHOCK-INDUCED STRONG IGNITION

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1 TRANSITION TO DETONATION IN NON-UNIFORM H2-AIR: CHEMICAL KINETICS OF SHOCK-INDUCED STRONG IGNITION L.R. BOECK*, J. HASSLBERGER AND T. SATTELMAYER INSTITUTE OF THERMODYNAMICS, TU MUNICH Dr.-Ing. Lorenz Böck 1

Explosion hazard assessment as part of nuclear reactor safety engineering Large quantities of H 2 can be released in a severe accident scenario Spatial concentration gradients are

2 Explosion hazard assessment as part of nuclear reactor safety engineering Large quantities of H 2 can be released in a severe accident scenario Spatial concentration gradients are omnipresent J. Stewering, GRS, Expert meeting on containment safety, Cologne, Germany, 2014 Transverse gradients portray a simplification that can be investigated scientifically Dr.-Ing. Lorenz Böck 2

3 Sub-processes of a confined explosion Ignition Flame acceleration Onset of detonation Detonation propagation Deflagration-to-Detonation Transition Images: Ignition: Peter Katzy Onset ofdetonation: Florian Ettner How do transverse concentration gradients influence DDT in H 2 -air mixtures? Dr.-Ing. Lorenz Böck 3

4 Experiment Dr.-Ing. Lorenz Böck 4

5 Experimental setup - overview Dr.-Ing. Lorenz Böck 5

6 Experimental setup - geometry Dr.-Ing. Lorenz Böck 6

7 Generation of transverse concentration gradients (1) Injection (2) Deflection, H 2 layer formation (3) Diffusion Concentration gradient slope is defined by diffusion time t d Dr.-Ing. Lorenz Böck 7

8 Onset of detonation in obstructed configurations Dr.-Ing. Lorenz Böck 8

9 Onset of detonation in obstructed channels - homogeneous Strong ignition (local explosion) after shock reflection initiates onset of detonation Dr.-Ing. Lorenz Böck 9

10 Onset of detonation by shock reflection empirical: v O(a pr ) Shock reflection Local explosion at obstacle surface Diffraction around obstacle Secondary hot-spot at channel wall Formation of detonation from secondary hot-spot Detonation propagation in the macroscopic geometry Strong ignition as the first requirement for onset of detonation Mitigation by expansion fan (Thomas et al. 2002) Secondary hot spot may also occur at the channel center line (Kellenberger and Ciccarelli 2014) Zeldovich mechanism Minimum size of confining geometry for detonation propagation Dr.-Ing. Lorenz Böck 10

11 A 1D/0D model describing gasdynamics and chemical kinetics of the onset of detonation Shock reflection (1D) large obstacle spacing Chemical kinetics in post-reflected-shock gas (constant volume reactor) Criterion for the fast deflagration precursor shock to cause a local explosion and thus potentially the onset of detonation Dr.-Ing. Lorenz Böck 11

12 Explosion limits diagram extended second explosion limit Lee and Hochgreb (1998) Belles (1959) suggested to determine detonability by using the extended second explosion limit as a criterion Dr.-Ing. Lorenz Böck 12

13 Explosion limit determination Shepherd, J.E. (2009) Detonation in gases: Peak in reduced effective activation energy corresponds to the extended second explosion limit: Temperature dependence of the induction reactions, expressed as one-step Arrhenius law Reaction mechanism by Ó Conaire et al. (2004) Cantera, Goodwin et al. (2014), Version 2.1.2, Dr.-Ing. Lorenz Böck 13

14 Strong ignition by shock reflection A critical overpressure of bar needs to be reached to cause strong ignition; minor influence of H 2 concentration Dr.-Ing. Lorenz Böck 14

15 Onset of detonation in obstructed channels gradients 22.5 vol. %, steep gradient (t d = 3 s) 1 Shock reflection 2 3 Local explosion at the upper obstacle surface Detonation initiation by secondary wall reflection Dr.-Ing. Lorenz Böck 15

16 The relation between overpressure and flame speed Critical local flame Mach number ( ) before onset of detonation by shock reflection can occur Good accordance with the empirical criterion v a pr M F,y=0.06m = v a re M F,y=0.06m = v a re,y=0.06m Dr.-Ing. Lorenz Böck 16

17 Summary Chemistry-based criterion for strong ignition as a first step of onset of detonation Strong ignition is observed in obstructed channels as a first crutial step Strong ignition occurs beyond the extended second explosion limit Critical overpressure and flame Mach number ( p 10bar, M F ) Minor influence of H 2 concentration Conclusions for mixtures with transverse concentration gradients Flames in non-uniform mixtures need to reach the same critical flame Mach number, determined locally Critical flame speed is thus higher in gradient mixtures than in a homogeneous mixture at equal average H 2 concentration Outlook Criterion for strong ignition can be applied to more complex shock configurations, e.g. at lower obstacle spacings Dr.-Ing. Lorenz Böck 17

Visit our open-access DDT database www.td.mw.tum.

18 Visit our open-access DDT database Data from flame acceleration, DDT and detonation experiments Conventional and optical data H 2 -air mixtures (homogeneous, concentration gradients, water mist, ) Dr.-Ing. Lorenz Böck 18

Cellular structure of detonation wave in hydrogen-methane-air mixtures

Open Access Journal Journal of Power Technologies 91 (3) (2011) 130 135 journal homepage:papers.itc.pw.edu.pl Cellular structure of detonation wave in hydrogen-methane-air mixtures Rafał Porowski, Andrzej