Numerical Study On The Effect Of Buoyancy On The Flow And Heat Transfer Of Supercritical LNG Across The Pseudophase Transition Under Different Structural Channels And Working Conditions

Natural gas（NG）is a high-quality clean and efficient energy source.In order to facilitate transportation and storage,natural gas is usually liquefied into Liquefied Natural Gas（LNG）,which is converted into gas by regasification before being used in industries and households.In the process of regasification,supercritical LNG involves different pseudophase such as liquid-like,pseudo-critical and gas-like,and the fluid is affected by buoyancy,the influence of buoyancy on heat transfer and fluid flow needs to be considered.Additionally,thermophysical properties of supercritical LNG under different pseudo-phases have different effects on heat transfer performance,and the buoyancy effect will make the flow and heat transfer more complicated.Therefore,it is of great significance to study the influence of buoyancy on local flow and heat transfer characteristics of supercritical LNG in liquid-like region,pseudo-critical region,and gas-like region.First of all,in this thesis,the effect of buoyancy on the local flow and heat transfer performance of supercritical LNG under different channel diameters and operating pressures are studied.It was found that the diameter has a major impact on the buoyancy.In the channel with a diameter of 1.5mm,the effect of buoyancy on the heat transfer performance can be neglected,and the buoyancy factor is only 1/6 of that in the channel with a diameter of 3mm.On the other hand,in a channel diameter of 2,2.5,and 3mm,the effect of buoyancy on the heat transfer performance is concentrated in the liquid-like and pseudo-critical regions.At this time,heat transfer enhancement and heat transfer deterioration occur simultaneously,but heat transfer deterioration plays a dominant role.In the gas-like region,the effect of buoyancy decreases and the local convective heat transfer coefficient increases.With the increase of pressure,the influence of buoyancy on flow and heat transfer decreases,and the deterioration of heat transfer weakens.This is attributed to the fact that the fluid temperature near the flat wall of the semi-circular channel is close to the pseudo-critical temperature,and the constant pressure specific heat is higher,thus improving the heat transfer performance.At the same time the turbulent kinetic energy near the channel wall increases,the heat transfer performance is enhanced.The local convective heat transfer coefficient is 1.85 times that of 6MPa,but the heat transfer deterioration is still dominant.In the gas-like region,the effect of buoyancy is negligible.Based on the above studies,this research investigated the effect of buoyancy on the flow and heat transfer performance of supercritical LNG in channels with different cross-sectional shapes,while keeping the hydraulic diameter of the channels constant and changing the cross-sectional shapes.The results indicate that the cross-sectional shape has little effect on the buoyancy factor,while the fluid velocity stratification is limited by the cross-sectional shape.In the liquid-like region,pseudo-critical region and gas-like region,different positions and numbers of vortices are formed.In addition,the temperature field of the channel is also different.In the gas-like region,the effect of buoyancy can be ignored,thus,the temperature inhomogeneity is weakened,and the aggregation of high-temperature fluids is also alleviated.Similar to the above research,heat transfer deterioration and heat transfer enhancement occur simultaneously,and heat transfer deterioration plays a dominant role.The differences in turbulent kinetic energy distribution and specific heat capacity at the bottom of channels among different cross-sectional channels lead to difference of heat transfer performance under different pseudophases.Finally,the comprehensive performance evaluation j/f^1/3 factor is used to evaluate the comprehensive performance of the channels.In the liquid-like region,the comprehensive performance of semi-circular,trapezoidal,and elliptical channels is poor;in the pseudo-critical region,the comprehensive performance of trapezoidal and elliptical channels is poor;in the gas-like region,the comprehensive performance of triangular,trapezoidal,and elliptical channels is poor.At last,in order to suppress the heat transfer deterioration,this study investigated the influence of channel structures on buoyancy effects and heat transfer deterioration in supercritical LNG by using numerical simulation methods.The straight channel with a diameter of 3mm and a combined channel consisted of converging channel,straight channel,and diverging channel are used as the research objects in this study.It is found that the change of the channel diameter affects the range and peak value of the buoyancy effect.At the same time,it was found that the smaller the diameter of the straight channel,the inhibition of heat transfer deterioration is better,but it will increase the fluid flow rate and cause an increase in pressure drop.The j/f^1/3factor is used for comprehensive performance evaluation,and the study found that the diameter of the straight channel has the best comprehensive performance in the liquid-like region;in the pseudo-critical region the optimal structure is difficult to determine;in the gas-like region,Combined channel 1 has the best comprehensive performance.Combined channel 3 is found to be the worst among all structures.