Thermal-mechanical Coupling Analysis Of Buried Heating Pipe And Soil Syste

The direct burial laying method has fast construction,low heat loss,small floor space and is not easy to damage,which is the main pipeline form of urban central heating in China.At present,the national industry standard "Technical Regulations for Urban Heating Direct Buried Hot Water Pipelines"(CJJ/T81-2013)only applies to pipelines with a diameter of no more than 1200 mm.However,due to the actual needs of the project,the upper limit of pipe diameter has long exceeded this value and continues to grow,and high pressure and large diameter have become the inevitable development trend of direct buried heating pipelines.Awareness of the thermal/mechanical characteristics of large-diameter pipes is extremely limited.In this paper,the method of indoor experiment combined with numerical simulation is used to study an actual large-diameter direct buried heating project,which aims to reveal the distribution characteristics of temperature field and stress field of pipelines under thermodynamic coupling,and the variation law of thermodynamic coupling field under different working conditions.Firstly,representative samples of pipeline composition materials were collected,temperature-related mechanical parameters of polyethylene and polyurethane were obtained by uniaxial tensile and compression tests,and the mechanical parameters of soil and interface were measured by indoor geotechnical test and interfacial shear test.The results show that the mechanical properties of polyurethane are greatly affected by temperature,and the 50 mm thick polyurethane can meet the requirements of the design specification.Then,based on the site conditions,a finite element model of the straight pipe section of multi-layer heating pipe/soil interaction is established,the model parameters are assigned with the measured material properties,and the anchor section is intercepted as the calculation domain and the corresponding load is applied.The results show that the stress of the water supply pipe is greater than that of the return pipe,the stress at the top of the pipe is greater than the stress at the bottom,the maximum equivalent value is located near the center line of the top of the pipe,the embedded depth has little effect on the stress of the steel pipe,and the working temperature difference and the internal pressure of the pipe are the main loads of the heating pipeline.Finally,a section of elbow at the engineering site is selected to establish a numerical model,and the thermal coupling analysis of the elbow section is carried out.The results show that the stress of the elbow section is greater than that of the straight pipe arm,the inner stress is greater than the outer stress,and the maximum value is located in the inner lower position.Increasing the wall thickness of high-pressure pipes can effectively reduce pipeline stress,and reducing the working temperature difference of high-temperature pipes has a more obvious effect on stress reduction.The research conclusion of this paper provides a reference for the design of large-diameter direct buried heating pipelines,and also provides a research basis for the revision of technical standards for direct buried heating pipelines.