Research On Distribution Characteristics Of Temperature Field Of Liquid Nitrogen Substage Freezing

Because of its low freezing temperature and fast freezing speed,liquid nitrogen freezing technology is often used in the construction of municipal underground engineering to reinforce the weak ground and meet the requirements of excavation and construction in the soft ground with rich water,especially in emergency projects such as emergency rescue.Among them,liquid nitrogen freezing can complete the freezing and reinforcement of the ground in a relatively short time,increase the strength of the soil,and quickly resume subsequent construction,which can better reflect its technical advantages of ultra-low temperature freezing.The freezing of liquid nitrogen mainly relies on the latent heat absorbed after liquid nitrogenization to achieve the freezing process,so the efficiency of liquid nitrogenization directly affects the freezing effect,resulting in uneven freezing of the liquid nitrogen along the length of the freezing tube,and freezing along the length of the freezing tube The wall thickness and the average temperature are quite different,which affects the effect of freezing and strengthening.On the basis of the openings on the liquid supply pipes,annular partitions are set on the inner walls of the freezing pipes at both ends of each opening position to form a liquid nitrogen segmented freezer structure,which can change the flow field distribution of liquid nitrogen in the segments And the process of liquid nitrogenization,so as to adjust the freezing effect in the length direction of the freezing pipe,and improve the unevenness of the freezing wall.In this thesis,physical model tests and numerical simulations are used to study the evolution of the temperature field under the condition of staged freezing of liquid nitrogen,and the distribution characteristics of the temperature field during the freezing process of liquid nitrogen are obtained.First,based on the similar theory design,the model test of liquid nitrogen segmented freezing was completed,and the influence of the opening of the liquid supply pipe,the height of the opening,and the width of the partitions at both ends on the distribution characteristics of the freezing temperature field of liquid nitrogen was analyzed,and different parameter conditions were obtained.The internal distribution characteristics of the lower freezing pipe and the change law of the external soil temperature.The research results show that the structure of the openings of the liquid supply pipes in the partitions can significantly improve the uniformity of freezing,and the effect of expanding the width of the partitions to enhance the freezing temperature field is significant,and the position of the openings of the liquid supplying pipes is located in the middle or the distance between the partitions and the partitions.The closeness of the upper partition has little effect on the distribution characteristics of the temperature field during freezing.Secondly,it is proposed to use the parameters of heat transfer coefficient to characterize the heat transfer process of the liquid nitrogen freezing process,and use Comsol software to analyze the soil freezing temperature field obtained from the physical model test,and invert the freezing of liquid nitrogen under different conditions.The heat transfer coefficient,the distribution law of the heat transfer coefficient in the length direction of the freezing tube is obtained.Research shows that the partition structure formed by the partition optimizes the distribution characteristics of the convective heat transfer coefficient in the length direction of the freezing pipe,and whether the liquid supply pipe is open or not will significantly affect the freezing effect.Finally,according to the typical parameters of the liquid nitrogen freezing project with double-row holes,the evolution process of the temperature field during the freezing process of liquid nitrogen is analyzed based on the distribution characteristics of the heat transfer coefficient,and the influence of the vertical conduction heat of the soil on the soil freezing at the position of the partition is studied.The influence of the process is compared with the difference between the opening of the supply pipe on the development speed of frozen soil and the quality of the frozen wall formed,and the distribution characteristics of the temperature field of liquid nitrogen segmented freezing under the condition of double-row holes are obtained.The research results provide a basis for rationally determining the processing parameters of the liquid nitrogen segmented freezers,and can also provide a basis for further optimizing the uniformity of liquid nitrogen freezing,which has relatively important engineering application value.There are 86 figures and 30 tables as well as 77 references in the thesis.