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Funded by FCH JU (Grant agreement No. 256823) 1 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 1.

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Presentation on theme: "Funded by FCH JU (Grant agreement No. 256823) 1 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 1."— Presentation transcript:

1 Funded by FCH JU (Grant agreement No. 256823) 1 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 1

2 Funded by FCH JU (Grant agreement No. 256823) 2 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 2  Definitions  Deflagrations  Deflagration to Detonation Transition  Detonations  Prevention and Mitigation  Conclusions

3 Funded by FCH JU (Grant agreement No. 256823) 3 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 3  Deflagration  Transition from Deflagration to Detonation (DDT)  Detonation PhenomenonFlame Speed, m/sPressure kPabar Deflagration< 340  800  8 DDT> 700 Detonation  2000  1,600  16

4 Funded by FCH JU (Grant agreement No. 256823) 4 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 4 Effect of other species on the combustion limits:  The flammable range decreases with increasing concentration of CO 2  The detonability range also decreases with increasing concentration of CO 2

5 Funded by FCH JU (Grant agreement No. 256823) 5 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 5 The flammable range is extended significantly as the temperature is elevated from a typical ambient temperature of 20 °C

6 Funded by FCH JU (Grant agreement No. 256823) 6 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 6 LFLUFLPressureLimiting Oxygen Concentration [bar][% v/v] 14.378.521.54.5 103.972.427.65.8 205.874.125.95.4 It is not clear cut that an elevated pressure leads to either a narrowing or extension of the flammable range. One factor that will influence the measurement of the flammability limits is the way it is measured, that is to say the actual measurement technique and the experimental set-up

7 Funded by FCH JU (Grant agreement No. 256823) 7 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 7  The burning velocity is strongly dependent on the hydrogen concentration  Notice significant scatter in the experimental data  Experimental method greatly influences the velocity measurements  (Maximum) Laminar burning velocity for H 2 is almost an order of magnitude higher than for many hydrocarbons, for example CH 4 (  0.4 m s -1 )

8 Funded by FCH JU (Grant agreement No. 256823) 8 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 8  Ignition of hydrogen-air mixtures at or very near LFL  4.0 % v/v —ignition is not possible (?)  5.0 % v/v —a flame kernel is created, but the flame cannot be sustained  5.5 % v/v —a flame kernel is created and the flame can be sustained, but it is highly buoyant

9 Funded by FCH JU (Grant agreement No. 256823) 9 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 9  We will assume  Stoichiometric hydrogen-air mixture at 20 °C and 1 bar  Quiescent environment  No solid walls or obstacles in the immediate vicinity  The flame grows spherically and the flame surface has a smooth appearance  At some late time, the flame surface becomes “wrinkled” due to  Thermal-diffusive effects  Hydrodynamic instabilities

10 Funded by FCH JU (Grant agreement No. 256823) 10 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 10

11 Funded by FCH JU (Grant agreement No. 256823) 11 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 11  Why is this important?  The burning rate is proportional to the flame surface area  Wrinkling increases the surface area, which in turn leads to an increase in the burning rate  Transition from laminar to turbulent combustion may occur when a threshold has been exceeded, leading to  Increase in burning rate  Increase in pressure where S l and S T are the laminar and turbulent burning velocity respectively, and A l and A T are the flame surfaces of the laminar and turbulent flame

12 Funded by FCH JU (Grant agreement No. 256823) 12 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 12  Fires can be  Non-premixed, for example jet fires  Premixed, for example hobs on a domestic cooker  Explosions are always premixed combustion events  The term Deflagration is commonly used to refer to explosions (especially by lay persons) but does also apply to fires  We will refer to explosions as deflagration in this presentation Fires is covered in another lecture and will therefore not be considered further here

13 Funded by FCH JU (Grant agreement No. 256823) 13 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 13  To reiterate:  Lower Flammability Limit (LFL), below which no combustion can occur due to lack of oxidiser; LFL is 4 % v/v for hydrogen in air  Upper Flammability Limit (UFL), above which no combustion can occur due to lack of fuel; UFL is 75 % v/v for hydrogen in air  However, the flammable range and the energy required to ignite a mixture are dependent on a number of factors:  Temperature of the mixture o Higher temperature extends the flammable range  Pressure of the mixture o Higher pressure narrows/extends the flammable range  Stoichiometry of the mixture o The lowest ignition energy required is for a mixture which nearly stoichiometric

14 Funded by FCH JU (Grant agreement No. 256823) 14 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 14  Anatomy of a deflagration  Pressure wave travelling at the speed of sound (  340 m s -1 in air at 20 °C and 101 kPa)  Flame front, where the reaction takes place, travelling initially at the laminar burning velocity  The expanding hot products push the flammable gas mixture ahead of it Products Unburnt flammable mixture Reaction zone Pressure wave

15 Funded by FCH JU (Grant agreement No. 256823) 15 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 15  Transition from Deflagration to Detonation (DDT) can occur in  Highly confined regions  Highly congested regions  Flame speed well in excess of the speed of sound in the unburnt mixture at the onset of DDT  Different physical mechanism responsible for the high speed flame than for a deflagration  Generates a large pressure spike at DDT, twice the Chapman-Jouguet pressure (somewhere in the region of 3.0 MPa)

16 Funded by FCH JU (Grant agreement No. 256823) 16 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 16  To reiterate:  Lower Detonation Limit (LDL), below which no detonation can occur due to lack of oxidiser; LDL is 11 % v/v for hydrogen in air  Upper Detonation Limit (UDL), above which no detonation can occur due to lack of fuel; UDL is 59 % v/v for hydrogen in air  Detonation cell size  Measure of the sensitivity of the mixture or chemical length scale  Depends on o The physical scale of the geometry Detonability range increases with the size of the flammable cloud for the same mixture stoichiometry o The stoichiometry of the mixture  Not all detonations are self-sustainable, that is to say may revert back to a deflagration

17 Funded by FCH JU (Grant agreement No. 256823) 17 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 17 Products Unburnt flammable mixture  Anatomy of a detonation wave  Shock wave travelling at high speed (  2,000 m s -1 ) and compresses and heats the unburnt flammable mixture  The reaction zone is following closely behind the shock wave  The pre-heating of the fuel-air mixture leads to high velocity and pressure

18 Funded by FCH JU (Grant agreement No. 256823) 18 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 18  The severity of an explosion depends on a number of factors:  Mixture composition—concentration of hydrogen, additives (H 2 O(g), CO 2, …)  Mixture uniformity  Degree of confinement—for example walls and ceiling  Degree of congestion—obstacles such as pipes or vessels  Level of turbulence in the flammable cloud Combustion in a premixed gas cloud Increased pressure Expansion Turbulence enhances the combustion Flow interaction with obstacles Turbulence generation

19 Funded by FCH JU (Grant agreement No. 256823) 19 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 19  Open atmosphere  Essentially unconfined and uncongested  Conventional wisdom suggests that the over-pressure would be low o P open  10 kPa  Very high pressures were generated in the Buncefield incident (December 2005)  Closed vessel  Relatively slow flame  Can still generate high over-pressures  Uniform pressure in the vessel  Connected vessels  Pressure piling, pre-compression of the flammable mixture in the receiving vessel can lead to much higher over-pressures

20 Funded by FCH JU (Grant agreement No. 256823) 20 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 20  Tunnels  Flame directed down the tunnel  Increased turbulence due to : o Surface roughness o Obstacles, such as cars, lorries and coaches o Ventilation

21 Funded by FCH JU (Grant agreement No. 256823) 21 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 21

22 Funded by FCH JU (Grant agreement No. 256823) 22 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 22  Prevention  Passive o Flow restriction o Avoid unnecessary confinement o Natural ventilation  Active o Detection and isolation o Excess flow valve o Mechanical ventilation  Mitigation  Passive o Explosion venting o Separation distance o No ignition sources  Active o Emergency response o Detection o Power shut-down

23 Funded by FCH JU (Grant agreement No. 256823) 23 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 23  Flow restriction to reduce the amount of hydrogen released  Isolation of the system to minimise the amount of hydrogen released  Ventilation  More than one vent is needed and the locations of the vents are important—remember that hydrogen is a very buoyant gas  Natural ventilation usually does not require any human activation  Mechanical ventilation might be more effective, but may require activation either by a human or by a detection system  Detection can be difficult for low concentrations (at or below LFL) and the reliability and drift over time of the sensors must be taken into account

24 Funded by FCH JU (Grant agreement No. 256823) 24 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 24  The vents opens at a set pressure  Combustion takes place in the enclosure  Unburnt mixture is pushed out of the enclosure  External deflagration  Combustion continues in the enclosure  Ejected mixture combusts outside the enclosure

25 Funded by FCH JU (Grant agreement No. 256823) 25 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 25  P1: opening pressure of the vent  P2: external pressure peak  P3: pressure at the peak combustion rate in the enclosure  P4: oscillatory pressure peak

26 Funded by FCH JU (Grant agreement No. 256823) 26 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 26  Severity of a vented deflagration depends on  The ignition location o The closer the ignition location is to the vent the lower the pressure o Very low or no external pressure generation if the ignition location is right next to the vent  Size of enclosure o The most destructive external deflagration occurs when the enclosure is quite small  The vent areas must be adequate  Simulations have indicated that even a non-combusting high- pressure release from a could compromise the structural integrity of an enclosure

27 Funded by FCH JU (Grant agreement No. 256823) 27 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 27  Deflagrations generate overpressures less than about 800 kPa and the flame speed is generally less than the speed of sound in air  Transition from Deflagration to Detonation (DDT) is brought on by turbulence and generates a pressure spike which can be in excess of 3.0 MPa  Detonation can be initiated by a DDT or by direct initiation, which requires large ignition energy (typically by a high-explosive charge), and generates pressure around 1.6 MPa and travels at around 2,000 m s -1  Not all detonations are self-sustaining  Prevention is better than mitigation

28 Funded by FCH JU (Grant agreement No. 256823) 28 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 28 Supplementary Information

29 Funded by FCH JU (Grant agreement No. 256823) 29 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 29 FuelMinimum Ignition (Initiation) Energy Deflagration, mJDetonation, mJ Hydrogen0.0171.0·10 7 Methane0.252.3·10 11 Propane0.282.5·10 9 Ethyne0.0071.29·10 9

30 Funded by FCH JU (Grant agreement No. 256823) 30 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 30 Detonation cell size as a function of the hydrogen concentration Energy required for direct initiation of a spherical detonation

31 Funded by FCH JU (Grant agreement No. 256823) 31 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 31  Damköhler number : Ratio of chemical reaction time scale to characteristic flow (or integral) time scale  Karlovitz number : Ratio between chemical reaction time scale and the Kolmogorov time scale  Lewis number : Ratio between thermal conduction and molecular diffusion  Peclet number : Ratio between heat convection and heat conduction  Prandtl number. Ratio between momentum diffusion and heat diffusion  Reynolds number : Ratio between inertial force and viscous force  Schmidt number : Ratio between momentum diffusion and molecular diffusion  Zel’dovich number: Measure of the temperature sensitivity of the reaction rate

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