Experimental Study On The Explosive Boiling Overpressure And Phase Transformation Characteristics Of Superheated Liquid

Liquefied gas is widely used in modern industrial production and life,but at the same time,it also brings lots of risks.Among them,the boiling liquid expand vapor explosion(BLEVE)is a kind of accidents with more serious consequences.When the storage tank of liquefied gas is hit by objects or attacked by fire,due to the rupture of the tank and the rapid decompression,the internal medium is instantly superheated and begin to be nucleated and boiled.In this process,the liquid medium with the gas forms the two-phase flow which is rapidly expanded.The two-phase flow causes a second impact on the tank,and the pressure in the tank rebound,consequently,an explosion accident induced.Once such an accident occurs,it will pose a huge threat to the safety of life and property.Therefore,it is of great significance for accidents prevention to study the mechanism of disaster formation,and the key process is the superheating and explosive boiling of the medium in the tank.In this paper,experimental studies are conducted on the overpressure and phase transition characteristics of the superheated liquid boiling process caused by rapid depressurization in the tank,focusing on the pressure rebound response mechanism and the influencing factors of pressure characteristic parameters during the explosive boiling process.The main work and conclusions are as follows:(1)A small visual experiment platform of explosive boiling is designed and built independently.The experimental storage tank is a vertical rectangular vessel.The experiment uses water as the medium and heats it by means of electric heating.The opening of the vessel is artificially controlled to simulate the rupture process of the tank.Glass windows are set on the experimental vessel to facilitate the observation of the two-phase flow form inside the vessel during the explosive boiling process.The high-frequency dynamic pressure sensor and thermocouples are used to monitor the pressure and temperature response.At the same time,the dynamic evolution process is recorded by a high-speed camera.(2)Through the analysis of the results of the pressure response in the vessel and the dynamic evolution of the two-phase flow in the experimental process,the heterogeneous nucleation boiling characteristics of the medium and the influence of the two-phase flow expansion jet on the pressure response in the explosive boiling process were explored.It was found that the expansion of the mist-like two-phase flow is the direct cause of the pressure rebound at the top of the vessel.The pressure at the top of the vessel presented two rebound stages,and the first rebound peak was high with a short pressure rise duration.In comparison,the second rebound peak was relatively lower with a longer pressure rise duration,and affected by the intermittent expansion of the two-phase flow,the pressure rising rate shows a fluctuating trend.(3)Based on the study of typical experimental phenomena,both discharge area and medium filling rate were changed,and the influences of them on the boiling process of superheated liquid were analyzed.It was found that the medium filling rate affected the liquid energy,while the pressure reduction rate,which finally influenced the superheat rate of liquid,is affected by the gas phase volume and the discharge area.Under the coupling of two mechanisms,as the discharge area increases,the filling rate that produces the maximum pressure peak of the first pressure rebound stage increases in turn.The average pressure rising rate of the second pressure rebound stage can be approximately fitted as an exponential function of the discharge area ratio.With the increase of the filling rate,the index decreases.(4)In addition,through the theoretical calculation of the pure gas phase pressure reduction stage in the discharge process and the comparison of the experimental results with the images of the two-phase flow,the laws of pressure reduction under different discharge conditions were analyzed.The results showed that the inflection point between the experimental pressure and the adiabatic expansion pressure-reducing curve decreases linearly with the increase of the discharge area.