Study On Thermodynamic Properties Of Buried Pipe Under Thermo-hydraulic Coupling Effect

With the acceleration of urbanization process,the thermal pipeline engineering,as the lifeline engineering,is widely distributed in various geological conditions.Because of its high temperature of heat medium,external load and complexity of the environment,the leakage of thermal pipeline in the soft soil may cause serious accidents,such as a great threat to the urban environment and the safety of people.At present,the research of the thermal pipeline,mostly using the finite element simulation,is focused on tube's physical properties at room temperature and the pipe's temperature effects in permafrost area.In this paper,the thermal pipeline under constant and variable temperature is also studied on stress of pipe,soil settlement,soil temperature field and other aspects by model test and finite element analysis.(1)The thermodynamic characteristics of the thermal pipeline and soil are investigated by measuring the temperature,pore pressure,settlement and axial and radial stress under constant temperature(80 ?)and intermittent operation conditions(80 ? ? naturally cooling).The results show:(1)The temperature produced by pipeline is transferred to the soil by heat conduction.The closer distance pipeline of soil,the higher the temperature and the excess pore water pressure induced by temperature are.The excess pore water pressure produced by the hierarchical load can be dissipated and the negative pore pressure is produced after naturally cooling.(2)The soil expanses at heating stage and then settles at loading phase.Under the intermittent operation,The settlement under the intermittent operation is greater than that under constant temperature,which shows that the plastic deformation of soil with higher degree of consolidation is occoured by naturally cooling.(3)The pipe's axial stress and radial stress controlled by temperature can be obviously changed with the temperature change of the heating medium.The pipe is heated and expanded to produce tensile stress under high temperature,and then its stress is restored to initial state after cooling.The compressive stress at tube top is occoured under external load.(2)The stress of thermal pipeline under the effect of temperature-load is studied.The heating medium is set at constant temperature(80 ?)and intermittentoperation conditions(80 ? ?naturally cooling)to simulate the actual operation situation of thermal pipeline and the hierarchical load(from 20 kg to 1600kg)is applied in the range of 0.6m2.The results show:(1)The stress of the pipe is controlled by the temperature of heating medium and the stress caused by the external load is smaller than the stress caused by temperature.(2)The axial stress of pipe top decreases under constant temperature and variable temperature with the external load increasing,because the tensile stress caused by temperature can be offset by the compressive stress which is caused by section moment.On the other hand,the radial stress of the pipe increases with the increase of the load,which is due to the higher the load and the temperature,the greater the radial stress of pipe.(3)The radial stress of the tube side after the heating medium heating and cooling to a certain temperature is smaller than that at a constant temperature.This is due to the soil around the pipe what changes with the changing of heating medium can be reconsolidated after cooling.The extrusion force caused by the reconsolidation soil can offset expansion of the tube side.(3)The Abaqus finite element model of tube-soil,based on the model test,is established and the reliability of the model calculation is verified by comparing the result and model test results.The results show:(1)The temperature of the soil decreases with the increasing of distance from the pipe in both constant and intermittent temperature.The ultimate settlement caused by external load increases with the diameter increasing.The settlement of the soil under intermittent operation is larger than that under constant temperature.(2)Stress of pipe mainly controlled by temperature increases and decreases rapidly with the heating medium heating and cooling.(3)The radial stress of pipe enlarges with the temperature and load increasing.The axial stress increases with the temperature increasing and load decreasing.Tube heat medium with higher temperature,stress of pipe will be larger,under load top tube axial stress decreases,the tube side of the radial stress increases,and the deflection in tube actual pipeline engineering,which increases the load effects,the tube body compared with two-dimensional finite element model the stress,the stress changes affected by the upper load of buried pipeline is more significant.