Study On The Development Law Of Superimposed Temperature Field Of Combined Freezing Of Liquid Nitrogen And Brine

The construction conditions of urban underground engineering are complex and unpredictable.The pipe jacking construction process is likely to cause the movement of surface and underground soil.In order to ensure the safety of the tunnel repair construction,artificial freezing method is often used for the tunnel soil during the pipe jacking construction.For reinforcement,this thesis takes the Ningbo pipe jacking section D pipe gallery relocation project as the background,uses the method of combined freezing of liquid nitrogen and brine to relieve the problem of the pipe jacking,and explores the development law of the superimposed temperature field of the combined freezing of liquid nitrogen and brine.First,use Comsol software to assign different cold source loads to the brine freezing tube and the liquid nitrogen freezing tube,combined with the temperature field change characteristics of the liquid nitrogen freezing process,and construct in turn to obtain the simultaneous freezing of both liquid nitrogen and brine,first liquid nitrogen and then brine freezing,and Three-dimensional finite element models under three working conditions: salt water first and liquid nitrogen freezing.Analyze the development process of the freezing curtain under these three conditions,obtain the development cloud map and temperature cloud map of the freezing curtain,select the different paths of five different sections in the superimposed area of liquid nitrogen and brine,and analyze the temperature distribution of the special path.Take the thickness of the frozen wall and the average temperature of the special path.In particular,in order to explore the law of uneven changes in the temperature of the liquid nitrogen pipe wall with the depth of the pipe,under the condition that the liquid nitrogen and brine are frozen at the same time,several special paths along the different depth sections of the liquid nitrogen pipe are selected to analyze the temperature distribution of the special paths,And obtain the freezing wall thickness and average temperature.The weakest path was finally determined by analyzing the freezing wall thickness and average temperature formed on different paths.At the same time,the corresponding path data obtained from the test of various working conditions were compared,and the most suitable freezing plan with the strongest freezing effect and the most economical freezing plan was selected.Then,according to similar criteria,a model test under the condition of simultaneous freezing of liquid nitrogen and brine was designed,and the model of the jacking pipe,ground connecting wall,freezing pipe and test box was constructed according to the condition that the geometric reduction ratio was equal to 10.After that,thermal insulation cotton was laid in the test box and the return pipeline to obtain similar boundary conditions.The test was completed according to the time sequence of the actual project freezing.After the test,the data results obtained were studied in depth,and the temperature measurement of the special path was obtained.Point temperature changes,and analyze the frozen wall thickness and average temperature of the special path.Finally,comparing the results of physical simulation and numerical simulation,it was verified that these two methods obtained similar results.From this,it can be inferred that the model used in the numerical calculation is correct,and the model test sequence is also reasonable.For further development The follow-up more in-depth research work provides a theoretical reference.The paper has 62 figures,5 tables,and 91 references.