Analysis And Numerical Simulation Of Vaporization Performance Of The LNG Cascade Vaporizer

Liquid natural gas,a kind of clean energy with high calorific value,has received much attention from public society in recent years.Research on equipment of LNG vaporization has also become hotspot in research field.The LNG cascade vaporizer is a kind of heat pipe heat exchanger which realizes the cascade heat transfer among seawater,propane and LNG in the same shell through reasonable arrangement of the evaporation section and the condensing section of the heat pipes.At present,the research on the heat pipe heat exchangers for cryogenic liquid vaporization is rarely reported.In this paper,the heat pipe heat exchanger is applied to LNG vaporization and the coupled heat transfer characteristics of LNG,propane and seawater are studied by combining theoretical analysis and numerical simulation.The thermal resistance model is developed through thermal performance analysis of the LNG cascade vaporizer and the flow field distribution,temperature distribution and pressure distribution of the fluid in the LNG cascade vaporizer under different structural parameters are studied by numerical simulation in this paper.The theoretical results calculated by the thermal resistance model are compared with the numerical simulation results to verify the reasonability of the simulation method.It shows that the thermal resistance model is feasible and the simulation results are reliable.Aiming to obtain the optimum structure parameters of the heat pipes in each cavity of the LNG cascade vaporizer,numerical simulation methods are used to analyze the effects of the heat pipe arrangement and horizontal tube spacing on the outlet temperature,mean convective heat transfer coefficient and pressure drop of the fluid in the LNG cavity,the propane cavity as well as the sea water cavity.The result shows that the comprehensive heat transfer performance of LNG cavity is the best when the heat pipes are in aligned arrangement with 40 mm horizontal tube space;The comprehensive heat transfer performance of propane cavity is the best when the heat pipes are in staggered arrangement with 40 mm horizontal tube space;The comprehensive heat transfer performance of seawater cavity is the best when the heat pipes are in aligned arrangement with 40 mm horizontal tube space and having no vertical baffle plates.In order to study the effects of different operating parameters on the heat transfer performance of LNG cascade vaporizer with real size and under actual working conditions,different mass flow rates of cold and hot liquid as well as the inlet temperature of hot liquid are analyzed using thermal resistance model.The result shows that the heat transfer ability of LNG side can be increased by increasing the mass flow rate of LNG side,and when the mass flow rate of LNG side is 2.8 kg/s,the outlet temperature is 281.6 K,meeting the vaporization requirement as well as raising the heat transfer capacity.When the mass flow rate of seawater is 80 kg/s and 160 kg/s,the outlet temperature of NG is 278 K and 292.2 K respectively and the temperature increase is far less than the mass flow rate increase which shows that the mass flow rate of seawater affects hardly the outlet temperature of LNG side;Meanwhile,the increase of mass flow rate will greatly increase the flow resistance and the pressure drop of the sea side under the mass flow rate of 160 kg/s is about 3.6 times that of the sea side under the mass flow rate of 80 kg/s.The heat transfer performance of the LNG cascade vaporizer is improved with the increase of inlet temperature of hot liquid in seawater cavity.