Study On The Dynamics Of The External Flowfield During Venting

Explosion relief venting is one of the methods applied widely in industries to avoid internal explosion. Namely, it can vent the high pressure unburnt and burnt gas mixture or dust to the outside space (generally the ambient air) through a device of low pressure resistance opening, when the explosion is in development and the overpressure exceeds a certain safe threshold(i.e. failure pressure). Because of the complexity and uncertainty in venting process, the inner overpressure could decrease, while the outer overpressure increases rapidly under some suitable conditions, and even external secondary explosions can occur. This gives potential explosion damage to neighboring structure or equipment or personnel in the vicinity. Therefore, it is necessary to investigate the dynamics of the vented external flow field and the mechanism of the secondary explosions during venting.In this paper, with a cylindrical vented vessel connected to a duct, the experiments on the venting to air under different venting conditions (failure pressure, vent area, equivalent ratio of the methane-air gas mixture and the location of ignition source) were performed and the pressure-time profiles of the external flow field were obtained. Moreover, some visualization tests of the external flow field were implemented by using YA-16 high-speed shadowgraph imaging system, and the clear sequential shadowgraph images were also obtained in tests. In addition, the venting process under the typical venting condition was simulated numerically, by using the colocated grids SIMPLE algorithm based on the k turbulent model and 'Eddy dissipation model' turbulent combustion model. Furthermore, the computational shadowgraph images were plotted based on the numerical results and relevant computational flow imaging (CFI) methods.From the experimental and numerical results and computational shadowgraph images, the dynamic characteristics of the vented external flow field, namely,the jet flame, turbulence, vortex , flammable cloud and the structure of the pressure waves in the external flow field during venting, etc., were discussed systematically in detail. In the venting conditions covered in this paper, the leading shock wave generated due to the initial discontinuity of the pressure difference of the external and internal flow field, was firstly coming out of the vent. Then, the combustible gas rushed out and mixed with the oxygen of the air to form combustible cloud. The leading shock wave decayed to sound wave with its front expanding. The vented gas mixture jetted out under expansion for its higher pressure relative to ambient pressure, and the low pressure area near the exit was formed due to the rarefaction waves of the Prandtl-Meyer flow. And the high pressure area (also called 'Suspended shock wave'.) near the axis farther to the exit was also generated due to the convergence of those rarefaction waves reflected from theboundaries of the jet. The occurrence of the combustible cloud, low pressure area of the rarefaction waves and high pressure area of suspended shock were the basic characteristics in the external flow field before the flame spouted out. When the flame penetrated from the vessel(low density) into the duct(high density), it was accelerated due to instabilities occurring on its front and interaction of the shear layer of the duct wall, and its velocity near the exit was up to the maximum and then the jet flame rushed out. Under suitable conditions, for example suitable overpressure intensity and density in the high pressure area, and the area covered by the combustible in the external flow field, etc., the overpressure in the external flow field increased rapidly and even the secondary explosion occurred due to the violent combustion of the external combustible gas ignited by the jet flame. The characteristics of the flame, vortex, leading shock wave and secondary explosion generated later, were showed obviously from the clear sequential experimental shadowgraph images and computational shadowgraph images .In addition, the influenc...