Influence Of Buried Thermal Pipeline Leakage On Transfer Process Of Soil Heat And Moisture

As a pipeline system that transports and distributes the heat medium from the urban centralized heat sources to heat users,the urban buried thermal pipeline plays a pivotal role in the entire heating system.Therefore,identifying leaking places and determining the amount of leakage quickly and accurately will help reduce the harm caused by pipe leakage.After the heat pipes leak,the surrounding soil temperature and humidity will change significantly along with the duration of the leak,and the final feedback is the abnormal surface temperature of the area above the leak.Based on the current research status and engineering practice,the use of infrared thermal imaging coupling soil temperature and humidity detection method can not only accurately identify the leakage point but also determine the amount of leakage.At present,however,there are few studies on the heat and mass transfer mechanism inside the soil after the leakage of buried thermal pipes,and the laws of soil heat and moisture transfer under local active leakage are not well studied.The research work for this practical problem is as follows.(1)In the theoretical analysis based on the typical actual thermal pipeline leakage phenomenon and the known soil heat and moisture transfer traits,the scaling criterion was derived,and the buried pipeline leakage scale-reduction experiment platform was designed and built according to the scaling criterion.The comparison with scaling criteria deduced by predecessors and field test data verifies that this scaling criterion has high accuracy and reliability.(2)Based on the above-mentioned experimental table,the comparison method was used to analyze in detail the characteristics of soil heat and moisture transfer of normal/leaking pipes.The experimental results showed that under normal circumstances,the moisture transfer inside the soil was caused by thermal driving,and the increase in temperature mainly depends on the thermal diffusion effect of the soil.After the pipeline leaks,there was a strong convection heat and mass transfer phenomenon between the leaking working fluid and the unsaturated soil,which caused a rapid increase in soil moisture and temperature.When the leaking working fluid migrates in a non-radial direction,the influence of gravity on it was significantly enlarged.(3)With the method of controlling variables,the influence of pipeline depth and leakage direction on the characteristics of soil heat and moisture transfer was studied.The experimental results showed that at the same location,the smaller the buried depth,the greater the soil heating rate and the higher the temperature,and the smaller the resistance of the leaking working fluid wetting front in the process of moving inside the soil,the greater the working fluid transfer rate,and the higher the moisture content.When the leakage hole is inclined at 45°,the displacement rate of the wetting front of the leaking working fluid in the radial direction was significantly reduced.(4)Using the porous medium multiphase flow coupled fluid heat transfer module,the dynamic change process of soil heat and moisture transfer with or without leakage of buried pipelines was simulated and studied.First,the verification of the numerical model was carried out,and the numerical results were basically consistent with the experimental data and the degree of agreement was relatively high.Then,the dynamic characteristics of active leakage and no leakage were analyzed.Finally,combined with the experimental results,the difference between soil moisture content and temperature changed with time in the case of active leakage was analyzed.In the experiment,the uneven distribution of soil pores and the thin layer of soil above the surface sensor all had a certain impact on the experimental results,causing the deviation between the numerical simulation and the experimental measurement results.