Research On The Mechanisms Of Thermal Stratification And Explosive Boiling Of Liquefied Gas

In the process of storing and transporting liquefied gas, tanks are the key equipments and major hazards. When a tank is exposed to fire impingement, the temperature and pressure in the tank will rise and the tank may crack. The blowdown from the crack may lead to a boiling liquid expanding vapor explosion (BLEVE). The studies for thermal response and BLEVE of liquefied gas can supply theories to prevent these accidents. Many researches in this field have been done and some valuable conclusions were obtained. However, the process involves the coupled heat transfer and non-equilibrium mass transfer, which complicate the matter. Therefore, further researches are needed to reveal the mechanisms about the thermal stratification and BLEVE. The major works as following were conducted in the thesis.(1) Several experimental systems were set up to model the thermal response and BLEVE of liquefied gas tanks. The experimental conditions (like heating area, heating power and liquid level, etc.) can be controlled. The temperatures can be recorded and the convection and boiling of the fluid can be observed. The transient pressure responses in blowdown process can be recorded.(2) The thermal stratification tests were conducted. Through analyzing the temperature response of the fluid and the two-phase flow near the inner wall, the mechanisms on the formation and progress of thermal stratification were gained:the climbing warmer boundary flow forms and maintains the stratification and the boiling has strong power to eliminate the stratification. Based on the above mechanisms, the effects of heating conditions and fluid conditions on the thermal stratification were studied. When the vapor and liquid wall are heated together, the vapor zone will transfer heat to liquid zone. The stronger is the heating on the vapor wall, the stratification degree of liquid is bigger and the stratification lasts for a longer time. Through affecting the distribution of heat flux into the fluid, development of stratification layer, rise rate of vapor temperature and the boiling conditions, the physical characters of fluid would impact the thermal stratification.(3) Both depressurization tests and numerical simulation were carried out to study the mechanism of BLEVE. Some laws were founded as flows. The rapid depressurization makes the liquid superheated and explosive boiling. Under the actions of the depressurization and boiling, the pressure response was a "drop-rebound-drop" process. The ability of pressure rebound reaches the peak when the filling rate is about60%. The bigger venting size leads to the bigger pressure drop, pressure rebound, pressure drop rate and pressure rebound rate. The stratification will reduce the total fluid energy and the release rate of the energy, so the pressure rebound was lower under the stratification condition.(4) On basis of the thermal response principles of tanks and the effect of stratification on explosive boiling, a safety device preventing BLEVE was proposed and patented. The risk of BLEVE can be reduced by the device in virtue of its ability reducing the energy in the tank and cooling the vapor wall well.