Recent Changes - Search:




edit SideBar

Multi-peaks structure of pressure transients and underlying physical phenomena

Vented deflagration pressure dynamics has multi-peak structure. The number of peaks and their magnitude depend on various explosion conditions. Detailed analysis of this peak structure was performed in paper [1]. Vented deflagration of 10% natural gas-air mixture in a vessel of volume V=0.76 m3 with venting area F=V2/3/9.2 and low vent release pressure panel (inertia 3.3 kg/m2) was analysed. Four distinct pressure peaks were registered for the case of vented deflagration in low-strength enclosures (Figure 1).

Fig. 1. Multi-peak structure of vented explosion.

The following deflagration phenomena and pressure peak are identified for vented deflagration with initially closed vent cover. After ignition deflagration develops in closed vessel until moment (a) when vent cover is released. So, pressure peak P1 is associated with vent release. At moment (b), when time derivative of pressure is equal to zero, the volume of gases produced by combustion inside the vessel is equal to volume of gases flowing out of the vessel, i.e. \DeltaVdefl=\DeltaVoutflow. It is known that venting of combustion products is more efficient for reducing explosion pressure compared to flammable mixture as for the same pressure drop at the vent a velocity of flow is proportional to square root of temperature. This explains that moment (c) on pressure transient corresponds to the beginning of burnt gas outflow. After it the combustion products “flow through” flammable mixture, which just escaped the vessel, and external explosion occurs at moment (d) with corresponding peak pressure P2. Helmholz oscillations could be generated after external explosion with characteristic wave length longer than resonator (vessel) size. The Helmholz oscillations can induce Rayleigh-Taylor instability [2]. Increase of mass burning rate due to increase of flame front area implies growth of pressure with time. At moment (e) the contact of flame front with walls of the vessel is sufficient for the heat losses to compensate the heat release by combustion and pressure peak P3 is formed. At final stage of the explosion when combustion proceed mainly close to walls and in corners high-frequency oscillations may be generated. Rayleigh criterion is applied in this case: "If heat be given to the air at the moment of greatest condensation, or taken from it at the moment of greatest rarefaction, the vibration is encouraged" [3]. Oscillations of peak P4 can be eliminated by taking adequate measures, such as lining the interior of the vessel to be protected with damping material.

It is important to note that when the vent cover failure pressure is increased to 7.5 kPa the second and third peaks practically can no longer be seen on the pressure transient. For relatively high vent release pressures, which are characteristic for process equipment, only two pressure peaks (one - vent release, another - mixture burnout) can be observed.

1.Cooper M.G., Fairweather M., Tite J.P., On the mechanisms of pressure generation in vented explosions, Combustion and Flame, 1986, Vol. 65, Issue 1, pp.1-14.
2.Taylor, G.I., The instability of liquid surfaces when accelerated in a direction perpendicular to their planes. I, Proc. Roy. Soc. (London) 1950, A201:192-196. Rayleigh, L., Investigation of the character of the equilibrium of an incompressible heavy fluid of variable density, Scientific Papers, ii, 200-7, Cambridge, England, 1900.
3.Rayleigh, J.W.S., The explanation of certain acoustical phenomena, Nature, 1878, pp.319-321.

<< Amplification of pressure effect due to pre-compression (Mache effect) | Content | Turbulence generated by venting process >>

Edit - History - Print - Recent Changes - Search
Page last modified on December 10, 2008, at 02:26 PM