Numerical Analysis Of Local Flow And Heat Transfer Of Supercritical LNG In Different Airfoil Shapes And Structures

The miniaturization and compactness of liquefied natural gas(LNG)gasification systems are considered as promising energy systems that can adapt to narrow working Spaces and harsh sea conditions.The Printed circuit heat exchanger(PCHE)is a new heat exchanger with high efficiency,compact structure and non-shaking effect.It is considered to be a better choice for LNG carriers and LNG floating terminal cryoexchanger.In recent years,liquefied natural gas receiving stations have been widely established in China’s coastal areas,and LNG Floating storage and regasification unit(FSRU)has also developed rapidly.As an important equipment in floating storage and regasification unit,LNG carburizer is used to heat LNG from-162℃ to normal temperature under supercritical pressure,which is a pseudophase transition process.The key to study the heat transfer performance of PCHE is the complex convective heat transfer process in the micro-channel.In this thesis,the local flow and heat transfer of supercritical LNG across the pseudophase transition in three different airfoil channels with different structures and shapes have been investigated numerically.It is a process with a large temperature span to heat supercritical LNG from-162℃ to normal temperature,so it is important to determine various physical parameters of supercritical LNG,including critical pressure,critical temperature,density,viscosity and thermal conductivity,etc.According to the physical property value of supercritical LNG,the physical property polynomial function of supercritical fluid is fitted,and by fitting the physical property curve,the subregion study is carried out,including the liquid-like region,the pseudo-critical region and the gas-like region.In this thesis,the local flow and heat transfer characteristics of three different airfoil flow channels with different structures and shapes are studied by means of numerical simulation technology.The secondary flow generated by the shape and structure of the airfoil has an important effect on the performance of local flow and heat transfer.Especially in the liquid-like region,the three types of airfoil fins not only generate mixed disturbance of the velocity vector in the y direction,but also generate secondary flow vortices in the z direction,which can more effectively interfere with the boundary layer and enhance the heat transfer performance.With the increase of temperature,the density and viscosity of supercritical LNG in the pseudo-critical region and gas-like region decrease,heat transfer changes,and secondary flow vortexes are formed above or below the tail of the three airfoil fins.Studies show that under selected conditions,Fin-2 has the best hydraulic performance,especially in the gas-like region,where its f-factor is less than 0.02.This is because the curvature of the leading edge of Fin-2 is changed,the impact surface is reduced,which is conducive to the rapid discharge of local fluid,thus reducing the flow resistance,and secondary flow vortex is generated at the tail of the airfoil.Fin-3 has the best heat transfer performance,and its j factor is 5-10%higher than other airfoil fins.This is because the tail shape of the airfoil is concave,and the secondary flow vortex is located in the concave area,which disturbs the boundary layer and thus improves the heat transfer performance.The influence of the structural parameters of the airfoil fin on the heat transfer performance and resistance is mutually restricted,that is to say,when the structural parameters are changed,the heat transfer will be enhanced at the same time,the resistance will be increased,and the heat transfer will be reduced at the same time.In order to obtain the optimal airfoil fin-runner structure,heat transfer performance and pressure drop are comprehensively considered and heat transfer coefficient(h),pressure drop(ΔPL)and performance evaluation criterion(PEC)are taken as triple objectives,and four factors are considered for each objective.It includes the horizontal distance of airfoil fins(La),the vertical distance of airfoil fins(Lb),the interleaving distance of airfoil fins(Lc)and the shape of airfoil fins.The genetic algorithm was used to solve the multi-objective optimization problem,and the four influencing factors were analyzed,and the runner structure with good heat transfer performance and low-pressure drop was obtained.An equilibrium interval is obtained on the Pareto frontier,where the optimal values of heat transfer coefficient(h),pressure drop(ΔPL)and performance evaluation criterion(PEC)are 194.96(W/m2·K),4262.21(Pa/m)and 0.95,respectively.The design parameters were the horizontal distance La=1～3mm,the vertical distance Lb=2～6mm,the interleaving distance Lc=2mm,and the fin shape was Fin-3.When heat transfer performance is dominant,the third type of airfoil is adopted,and the horizontal distance is 2 mm,the vertical distance is 4 mm,and the staggered distance is 2 mm,which will obtain higher heat transfer performance.