Experimental Study On Thermal Stratification Of Liquefied Gas

Liquefied gas tanks would be exposed to fire by accidents during transport or storage process. External heat invasion might lead to tank failure, resulting in tank explosions and other serious accidents. Research on thermal response process of liquefied gas tanks to fire invasion is significance on the analysis of the accident mechanism, proposing preventive measures and improving the device structure.In this paper a set of vertical cylindrical experimental device and a set of cube visible experimental device are established. Heating chip is used to heat the tank to simulate external flame. By recording the internal temperature of the tank, the experiments test the course of thermal stratification. Through the experiments conducted under different heating conditions, the creation condition of thermal stratification is found. The creation and development of the bubbles generating on the internal face of the tank are observed. Combined with the data, this paper proposes a theory that how the movement of bubbles affects the disappearance of the thermal stratification. By changing filling rate, heat flux and the start temperature, different experiments are performed to find out how the different conditions affect the results. The conclusions of this paper are as follows:(1) When the whole sidewall is heated, the temperature rising rate of gas region is higher than the liquid region and it lead to thermal stratification between the two regions. Liquid near the liquid region wall is heated up and the density decreases. By the effect of the buoyancy, liquid near the wall flows up to the surface and the temperature of the surface liquid is higher than the core liquid, which leads to thermal stratification in liquid region. Energy gathers in the liquid surface gradually and it makes liquid in this region overheated, then bubbles generate from the sidewall where contacts the liquid surface. The movement of the bubbles leads to the rollover of the liquid and the disappearance of the stratification. With the bubbles developing downward, the disappearance of the stratification develops downward. When just the liquid region sidewall is heated, bubbles generate from the whole sidewall where contacts the liquid. The movement of the bubbles makes the liquid form a circulation, which leads to the temperature of the liquid uniform and no thermal stratification appears.(2) When tanks are exposed to fire invasion, the lower the filling rate, the temperature and pressure will rise faster, and the thermal stratification will develop faster and disappear later. The higher the heat flux, the pressure and the pressure will rise faster, and the thermal stratification will develop faster. The higher the initial temperature, the time for stratification to disappear will be shorter. The pressure in tanks remains the same with the saturated vapor pressure that corresponds to the temperature of the liquid surface. If the pressure is higher than the saturated vapor pressure, it will be difficult for the thermal stratification to disappear.The innovation point is that this paper proposes a theory that how the generation and development of the bubbles affect the disappearance of the thermal stratification, which is that the movement of the bubbles leads to the rollover of the liquid and it makes the thermal stratification disappeared.