Small Scale Experimental Study On Boiling Liquid Expanding Vapor Explosions Mechanism

The development of the modern technical industry makes the people's life convenient; however, at the same time lots of dangerous substances that may happen to burn and explode exist around people broadly. BLEVE, the abbreviation of "boiling liquid expanding vapor explosion", is an extremely harmful physical explosion, and it has the special happening mechanism and the consequences are always totally disasters. Although people have already made some achievements on this subject, some microcosmic mechanisms of the accidents are not very clear and there are lots of doubtful points.In this paper, a study has been made of the conditions that could lead to a BLEVE. A small scale experiment device was built to simulate the occurrence of suddenly initiated release through a top orifice that happened in the high-pressure container containing over heat liquefaction gas or over heat water. As there is some danger in using LPG in the experiments, water was used as the test fluid. High speed camera was used to record the microcosmic evolvement process of the BLEVE and the locomotion characteristic of the two-phase flow after the rupture disc broken. High frequency pressure sensor and other testing ways were used to record the changing process of the super-pressure, and its changing principle in the process of BLEVE was analyzed. At the same time, the factors that influence the super-pressure in the container such as different initial liquid level, the initial liquid temperature and the acreage of the fissure were also researched in the experiment.Two super-pressures peaks in the process of BLEVE accident were observed, and the super-pressure values were 2-3 times as high as the initial pressure in the container before the BLEVE happening. After the rupture disc's opening, the liquid surface boiled firstly and gasified. The explosive boiling which happened on the liquid surface induced the occurrence of the two-phase layer, and the enormous impulse of the two-phase layer to the container's space was the reason for the first pressure peak.After the container broken, the bubbles in the liquid started to move upwards post caused by the decrease of the pressure upside. With the swelling of the two-phase layer, bubbles originally existed in the liquid, grew in size and ruptured. The whole liquid turned into a churn-turbulent bubbly two-phase flow and eventually impacted the vessel top wall. The second pressure peak is possibly due to a dynamic impact of 'liquid hammer' and is maintained by bubbles collapse or something like cavitation at the surface of inner wall of the head-space that occurs with the ejection of two-phase flow. In this phase, the direct contact of the comparatively low-temperature two-phase flow and the high-temperature metal wall could make the entirely break of the container possibly under the coupling effect of the heat and strength.Comparing with the first pressure peak, the longer duration and the thermal-hydraulic interact during the second over-pressure phase would cause severer damage, though its peak value maybe less than the first pressure rising.Liquid heights, opening size and the degree of liquid superheating all had different influences on the duration of depressurization time and the magnitude of measured over-pressure in the BLEVE.The over-stress relied on the superheating degree, the greater the degree of liquid superheat the stronger the over-pressure rising; smaller orifice areas delaied and reduced the magnitude of the first over-pressure but increased the ejection time and thus the second over-pressure phase increased sufficiently was allowed; at the fill level of between 60%-80%, the impact pressure appeared more violent than with other fills and the ejection time increased approximately linear with the liquid heights rising.At the condition of small fissure, the superheat liquid didn't boil explosively for the lower depressurization rate. The violently swell two-phase layer didn't appear. The growth of bubbles was slow. In the liquid of the container the bubble groups with bigger volume appeared. After the bubble groups broken, the pressure inner the container also rise with 0.1 bar.Install the wave insides the container, in the condition of small fissure, it could observe that the growth and movement of bubbles were restrained and the swelling of the two-phase flow was decreased.