Optimization Of Double Row Hole Liquid Nitrogen Brine Freezing Scheme

Combined freezing of liquid nitrogen and brine with double row holes is a method for soil freezing reinforcement at the starting and receiving stages of shield.The freezing scheme comprehensively selects the advantages of liquid nitrogen freezing and brine freezing.Firstly.the liquid nitrogen freezing pipe arranged by double row holes is used to freeze and consolidate the soil,and the freezing wall with the required thickness and average temperature can be formed in a short time,which shortens the construction period.When the liquid nitrogen freezing makes the freezing wall meet the design requirements,the liquid nitrogen freezing is stopped and the traditional saline freezing cycle is carried out to maintain the frozen wall in the soil,so as to ensure that the frozen wall is always in the design requirements during the brine maintenance period,which saves the huge cost of continuing to use liquid nitrogen.However,due to the novelty of this kind of freezing scheme,there is a lack of research in scheme design and the development law of temperature field.Therefore,this thesis carried out a feasibility study on the joint freezing of liquid nitrogen and brine with double row holes,analyzed the development law of its temperature field,and clarified the maintenance ability of brine on the freezing effect of liquid nitrogen in the later stage.The research objectives of this thesis are also clear for two points: the research on the temperature field law of the joint freezing of liquid nitrogen and brine with double rows of holes and the design and optimization of the joint freezing program.First of all,numerical simulation is used to study the temperature field law of soil freezing combined with double row of liquid nitrogen brine under typical parameters.Then,the temperature distribution is compared with the pure brine freezing under the same parameters,to show the maintenance effect of the brine freezing in the combined freezing of liquid nitrogen and brine on the early freezing effect of liquid nitrogen.Then,the different levels of the components of the freezing program were compared to determine the level of the factors required by the best program,including freezing tube diameter,liquid nitrogen tube and brine pipe row,brine temperature and liquid nitrogen to brine defreezing time.And through the orthogonal experiment to obtain the main and secondary relationship of each factor on the final freezing maintenance effect of the joint freezing,and select the priority factors of freezing program selection.Finally,by means of physical experiment,the numerical simulation results under typical parameters are compared with the physical experiment results,and the two are compared to verify the test results.The main conclusions of this thesis are as follows:(1)In the combined freezing of double row holes with liquid nitrogen and brine.the soil mass within the thickness range of the designed freezing wall drops rapidly when the liquid nitrogen is frozen.When the freezing cycle of liquid nitrogen is stopped,the temperature of the soil mass will rise obviously and briefly.When the temperature of the soil mass rises close to the temperature of the brine,the temperature recovery rate slows down significantly and does not change at a certain temperature level.During the whole process,the thickness of the freezing wall and the average temperature within the design range have been within the design requirements.(2)The final stable state of the freezing wall under brine freezing is not as good as that under joint freezing,indicating that the brine in joint freezing plays a role of freezing maintenance.(3)It is better to take 144 mm as the diameter of freezing tube in practical engineering: The temperature of brine has a great influence on the average temperature of freezing wall.The lower the temperature of brine,the better the quality of freezing wall: The best freezing effect is achieved when the brine freezing tube spacing is 1.2m,while the liquid nitrogen tube spacing is 1.5m.It is better to control the freezing interval of liquid nitrogen to brine at about 2d,which can speed up the formation of freezing wall in the maintenance stage and reduce the waste of freezing effect of liquid nitrogen.This thesis gives a detailed introduction to the temperature field rule and program optimization of the joint freezing of liquid nitrogen and brine with double row holes,which can provide a reference for future projects using the freezing program.There are 43 figures,17 tables and 96 references in this thesis.