| With the acceleration of urban energy infrastructure construction,the coverage of central heating network in China is expanding continuously,and the scale of buried thermal pipelines is becoming larger.The frequent leakage accidents of pipelines not only affect the regional thermal supply,but also cause potential safety hazards and economic losses.However,because of the invisible characteristic of buried pipelines,it is difficult to judge the leakage directly.At present,infrared thermography is often used to detect ground temperature anomalies,but it is impossible to distinguish whether the ground temperature anomalies are caused by pipeline leakage or thermal insulation breakage.Therefore,this paper studied the migration process of heat and moisture in soil under the condition of buried thermal pipeline leakage,and analyzed characteristic differences of soil temperature and humidity distribution before and after leakage.Firstly,this paper simplified and analyzed the migration process of heat and moisture in soil under the condition of pipeline leakage,determined the correlation formula between unsaturated soil variable physical parameters and soil moisture content by fitting the test data,and established a complete mathematical model,which laid a theoretical foundation for the numerical simulation of this study.Then,according to the similarity criterion,a small-scale experiment platform of buried thermal pipeline leakage was built and numerically simulated by using ANSYS software.By comparing the simulation and experiment results,the reliability of the small-scale model was verified.The changes of heat and moisture migration mechanism in soil before and after the leakage were preliminarily analyzed.When the pipeline does not leak,the driving force of heat and moisture migration in soil is the temperature gradient caused by a fixed heat source;when the pipeline leakage occurs,the main driving force of heat and moisture migration in soil is the pressure gradient caused by a pressure flow source.Therefore,the distribution characteristics of soil temperature and humidity are different before and after pipeline leakage,which can be used to locate the leakage area.Subsequently,based on the small-scale model and field measure data,the full-scale model was established and numerically simulated.By comparing the simulation and measure results,the reliability of the full-scale model was verified.The differences of heat and moisture transfer mechanisms in soil under the condition of normal pipeline,damaged insulation layer and local leakage were elaborated.The normal pipe or the breakage position of insulation layer can be regarded as a fixed heat source,while the local leakage position is a pressure flow source with high temperature.Therefore,the distribution characteristics of ground temperature field and soil humidity in the direction of depth are different,which can be used to discriminate pipeline fault types.Finally,the effects of leakage parameters(leakage aperture,leakage position,number of leakage holes),pipeline parameters(water supply pressure,pipe diameter,buried depth)and soil parameters(resistance coefficients,porosity)on migration characteristics of heat and moisture in soil were discussed by using the full-scale model.The results show that the local mass flow rate of leakage outlet,the velocity vector direction of leakage outflow and the soil environmental parameters for migration of leakage working medium jointly affect the migration process of heat and moisture in soil under leakage condition.Therefore,the distribution characteristics of soil temperature and humidity with time and depth are different,which can be used to evaluate pipeline leakage.This paper provides a reference basis for discriminating,locating and evaluating pipeline leakage by the combination of temperature and humidity,which is of great utility value to the development of buried pipeline leak detection work and lays a theoretical foundation for the development of buried pipeline leak diagnosis technology. |
