Study On Temperature Field Of Ultra Low Temperature Frozen And Reinfocing For Shield Launching

The liquid nitrogen freezing has the characteristics of high freezing speed,low temperature and high strength.It is used in the construction projects of super short construction period such as underground engineering.However,the liquid nitrogen which consume a lot of energy is expensive,and there is no scientific method for calculating the three-dimensional temperature field of liquid nitrogen freezing.In view of the above problems,the freezing of liquid nitrogen was deeply studied,a scientific and reasonable finite element calculation model of liquid nitrogen temperature field was established by comparing with the measured data of engineering.In order to provide reasonable suggestions for the design and construction of liquid nitrogen freezing,and to save energy and reduce construction cost as much as possible,the development law of liquid nitrogen temperature field was studied through finite element calculation.Based on the background of the shield project in Fuzhou metro,the three-dimensional temperature field of liquid nitrogen freezing on the left side of the shield has be simulated by ABAQUS finite element software.The simulation results were compared with the engineering measurement for the reverse heat transfer analysis of liquid nitrogen convection,and the theory of liquid nitrogen convection heat transfer was perfected.In order to further study the influence of different simulation methods on temperature field,the finite element simulation of brine convection and brine heat conduction was carried out base on the right line of brine freezing.The two finite element simulation methods are compared and analyzed.Finally,the comparison between liquid nitrogen freezing and brine freezing on left and right lines was carried out,and the differences between two different freezing methods in freezing area,freezing time and freezing effect were explored.The main conclusions of this paper are as follows:(1)The calculation results of the finite element three-dimensional liquid nitrogen freezing temperature field show that the freezing of liquid nitrogen produces a huge temperature difference in the vertical direction,the lowest temperature is at the depth of 18.3-22.3m,and the lowest temperature is lower than-160℃.The maximum temperature of the frozen wall is near the surface,about-30℃,and the maximum temperature difference is about-130℃ in the vertical direction.Because the thermal conductivity of the underground continuous wall is relatively low,the lowest temperature region in the temperature field occurs between the second row of freezing pipes and the continuous wall.(2)By comparing the measured data with the engineering data,it is proved that the temperature drop trend of the liquid nitrogen temperature field is similar to the measured results.In general,the simulation error is in the acceptable range,and the simulation results are good.The inverse heat transfer analysis of the convective heat transfer system shows that the convective heat transfer coefficient of liquid nitrogen is a function which is affected by the vertical freezing depth and the freezing time.The calculated results show that the simulation results after the correction of the convection heat transfer coefficient are closer to the measured ones than the original ones.(3)Brine freezing convection heat transfer and heat conduction simulation results show that the simulation method of convection heat transfer in vertical freezing simulation of 3D temperature field has its unique advantages.It is proved that the the temperature difference in the vertical direction is caused by the difference of the brine in the freezing pipe.On the horizontal plane,the results of the two simulation methods are very similar.It shows that ifthe effect of velocity is ignored,the result of convective heat transfer is the same as that of heat conduction.The main difference between convective heat transfer and heat conduction is that convection heat transfer takes account of the influence of fluid flow on energy transfer.(4)The comparison between liquid nitrogen freezing and brine freezing temperature field shows that the shape and influence range of three dimensional temperature field of brine freezing and liquid nitrogen freezing are approximately equal.The lowest temperature area occurs in the soil between the two rows of frozen pipes and the diaphragm wall,but the freezing temperature of liquid nitrogen is lowest than-130℃,while the freezing temperature of brine is no more than-30℃.The temperature of liquid nitrogen and frozen soil is much lower than that of brine frozen soil.On the vertical surface,there is a great temperature difference in the freezing temperature field of liquid nitrogen,the freezing range is also changed with the depth,and the freezing temperature field of the brine is evenly distributed,and there is a certain temperature difference in the vertical direction of the freezing period,but with the increase of freezing time,the temperature difference decreases gradually.This paper accurately simulates the freezing temperature field of liquid nitrogen,analyzes the law of its development,and puts forward a number of corresponding suggestions for the design and construction of 3D liquid nitrogen temperature field in time and space,and provides a theoretical basis for the design and construction of liquid nitrogen freezing.