Study On Multi-field Coupling Heat And Mass Transfer During The Process Of Gas Thermal Remediation Of Soil

In situ soil thermal remediation is an effective remediation technology for decomposing soil pollutants.Increasing soil temperature rise rate and improving temperature distribution are important to in situ soil thermal remediation technology.This paper has studied the soil heat transfer law in the process of gas thermal repair of soil to further accurately reveal the soil heat and mass transfer regular pattern.The gas heating system and soil heat and mass transfer model are established theoretically,and the relationship between the burner outlet temperature and flow and various parameters is studied numerically,as well as the change law of the heating well outlet temperature and heating power;The temperature rise law of in-situ thermal repair soil is experimentally studied.The effects of moisture diffusion conditions,heating methods,and site thermal design on soil heat transfer characteristics are studied numerically,and the temperature field distribution and temperature curve changes in different periods of soil heating are obtained.Theoretical models of gas heating systems and heat and moisture transfer in unsaturated soils are built.The mathematical models of the burner and heating well of the heating system are established,and the influence of different parameters on the power of the natural gas heating system is studied.The results show: the heating power first increases and then stabilizes with the increasing of the natural gas flow;The increase of the excess air coefficient leads to a decrease in the temperature difference between the heating well and the soil,an excessively fast flow rate will cause insufficient heat exchange between the thermal well and the soil and eventually reduce the heating power of natural gas heating system;The heating power of the natural gas heating system increases as the increasing of the pipe diameter,pipe diameter ratio and the heating well depth.Soil temperature variation during the process of in situ thermal desorption is experimental studied.The power of the heating wells,the size of the heated soil and the water seepage speed are designed based on the similarity principle.A test system is developed and a test rig is set up.The experimental study is carried out in the range of heating power 6 ~ 8 k W,water content 15% ~ 30%,and side seepage number 0 ~ 2.The results show: soil temperature changes present three stages,heat-up stage: temperature rises from ambient temperature to boiling point of water?boiling stage: water evaporates and temperature maintains boiling point temperature?superheating stage: soil temperature continues to rise;The longer the distance from the heating wells,the slower the rate of temperature increase and the longer the duration of each phase;When the distance from the heating rod is the same,the temperature increase rate of the measuring point inserted deeper into the soil is faster,which is caused by the heat dissipation from the upper surface of the soil.It can be seen that the surface insulation has a greater impact on the soil temperature increase;The increase of heating power,the decrease of the initial water content and the number of lateral seepages will reduce the duration of the three stages,and the final heating temperature will increase to varying degrees;And predicting the heating period of in-situ soil remediation technology engineering application.The soil heat transfer characteristics during heating soil process are numerically studied.A systematic study of soil heat transfer characteristics under different moisture diffusion conditions,different heating conditions,and different site thermal designs was carried out.The results show: As the water seepage speed increases,the duration of the boiling stage and the superheating stage increases and the duration of the superheating stage is affected more significantly.At the same time,the water seepage could weaken the temperature increase rate of the nearby area,which is not conducive to soil heat transfer;The increase of the initial water content in the soil increases the duration of the heat-up stage and the boiling stage,which has a greater impact on the evaporation section and has no effect on the superheated section;Under the negative pressure of the extraction well,the fluid in the soil is transferred to the extraction well,which causes the temperature field in the soil to shift.The high temperature area is more concentrated between the heating wells and the extraction well.When the heating power is a constant,the average soil temperature increases linearly.When the heating power is a constant,the average soil temperature growth rate gradually slows down;Excessive the heating wells spacing causes high-temperature areas to be concentrated around the soil.Too small heating wells spacing can cause the surrounding soil to fail to reach the target temperature.At the same time,when the heating wells depth is 3.5 meters and 4 meters,the bottom soil cannot get sufficient heat.When the heating wells spacing is 2 meters,the heating well depth is 4.5 meters,the soil heat transfer effect is the best,and the temperature distribution is more uniform.This paper provides favorable theoretical support for the soil temperature rise law and the influence of heat transfer characteristics,and it is of great significance for the development of in situ soil thermal remediation technology.