【作者】 王俊；

【导师】 石荣剑； 崔海涛；

【作者基本信息】 中国矿业大学 ， 建筑与土木工程（专业学位）， 2021， 硕士

【摘要】 液氮冻结技术因其冻结温度低、冻结速度快的特点在市政地下工程施工中常被用来加固软弱地层,满足富含水软弱地层中开挖施工的围护要求,特别是在抢险等应急工程中,液氮冻结能在较短时间内完成地层的冻结加固,提升土体强度,快速恢复后续施工,更能体现其超低温冻结的技术优势。液氮冻结主要依靠液氮气化后吸收的潜热来实现冻结过程,所以液氮气化效率直接影响冻结效果,造成冻结管长度方向上液氮冻结效果的不均匀,沿冻结管长度方向上形成的冻结壁厚度和平均温度差别较大,影响冻结加固的效果。在供液管上开孔的基础上,分别在每个开孔位置两端的冻结管内壁上设置环向隔板,形成液氮分段冻结器结构,可以改变分段内液氮的流场分布及液氮气化过程,从而调节冻结管长度方向上的冻结效果,改善冻结壁的不均匀性。本文利用物理模型试验和数值模拟的方法,对液氮分段冻结条件下的温度场演变规律进行研究,获得了液氮冻结过程中温度场的分布特征。首先,基于相似理论设计完成了液氮分段冻结的模型试验,分析了供液管开孔、开孔高度、两端隔板宽度对液氮冻结温度场分布特征的影响,获得了不同参数条件下冻结管内部分布特征和外部土体温度的变化规律。研究结果表明,隔板分段内供液管开孔的结构可以明显改善冻结均匀性,扩大隔板宽度增强冻结温度场效果显著,供液管开孔位置位于隔板与隔板中间或者距离上部隔板较近时对冻结过程温度场的分布特征影响较小。其次,提出用等效传热系数的参数来表征液氮冻结过程的热传递过程,并利用Comsol软件对物理模型试验获得的土体冻结温度场进行了分析,反演了不同条件下液氮冻结的等效传热系数,获得了冻结管长度方向上等效传热系数的分布规律。研究表明,隔板形成分段结构优化了冻结管长度方向上等效传热系数的分布特征,供液管是否开孔会显著影响冻结效果。最后,根据双排布孔液氮冻结工程的典型参数,基于等效传热系数的分布特征分析了液氮冻结过程中温度场的演化过程,研究了土体竖向导热对隔板位置土体冻结过程的影响,对比了供液管开孔对冻土发展速度及形成冻结壁质量的差异性,获得了双排布孔条件下液氮分段冻结温度场的分布特征。研究成果为合理确定液氮分段冻结器的加工参数提供依据,也可以为进一步优化液氮冻结的均匀性提供基础,具有较重要的工程应用价值。文中共有图86幅,表30个,参考文献77篇。更多还原

【Abstract】 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.更多还原