Research On Characteristics Of Large Diameter Buried Heat Pipeline Without Compensation Under Multi Field Coupling

The directly buried laying is an underground laying method developed in recent years.With the continuous expansion of the heating area in China,the pressure and diameter of the directly buried heating pipeline are also increasing.The pressure of some buried pipe network in China reaches 2.5MPa,and the diameter of the pipe is as high as DN 1400mm.At present,the stress calculation method of direct buried pipe network is mainly based on the technical specification for the urban direct buried heating pipeline(CJJ/T 81-98).However,the regulation is specifically applicable to the stress calculation of the buried pipe which is less than or equal to DN500mm.The application scope of the calculation method of the pipe stress in the regulation is limited.And the calculating methods for the stress of the angular,adjustable,three-port are not included in the regulation.With the development of the technology of direct burial laying,the technical specification for the urban directly buried heating pipeline in 2013(CJJ/T 81-2013)has supplemented the technical parameters related to the large diameter pipe.But the experimental verification is not carried out,and the actual operating parameters are not known in the actual operation,and the experimental platform for the large diameter and direct buried heating pipeline is lacking.Designers are required to adopt a more reasonable and comprehensive force design method.In this paper,the classical theory of the soil pressure and the thin shell were used to simplify the large diameter directly buried pipeline into thin-walled shells for analysis and study.The pressure of pipe bearing soil model and back force model of pipe bottom soil were established,which were more suitable for theoretical and practical force.A mathematical model for calculating the coupling of hydraulic,thermal and flow fields in direct buried pipeline was established.Then,the "L" pipe network was taken as an example,and the comprehensive load conditions of the pipe system such as the soil pressure,internal pressure,the heat transfer and flow of the medium in the pipe were deeply considered and compared under the different laying methods(direct burying laying and trench laying).Based on the theory of elastoplastic mechanics,theoretical mechanics,material mechanics,thermodynamics,fluid mechanics and heat transfer,a complete physical model and mathematical model for the medium flow and heat transfer process in the pipe network system were established,and the corresponding definite conditions were determined to figure out the different work conditions.Besides,combined with the modern computer simulation technology,the interaction between pipe and soil,heat and mass heat and mass flow in tube and heat expansion between tube wall were also considered.The coupling relationship of the flow field,temperature field and stress field under different conditions is studied,and the influence rules between various factors were analyzed.At the same time,a comprehensive experimental platform for the stress analysis of a large diameter direct buried heating pipeline was set up,and the thermal process of the large diameter directly buried heating pipeline was tested,and the mechanical parameters and thermal parameters of the thermal process of the directly buried heating pipeline were obtained,which could verify the accuracy of the numerical model.The paper provides theoretical and technical support for the design and construction of the large diameter heating network,and which has important scientific significance and application value.