Study On The Law Of Heat And Moisture Transfer In Soil Under Different Heating Methods

The removal of organic pollutants from unsaturated zone has always been a thorny problem.In situ thermal desorption technology,as a relatively effective in-situ remediation technology,has been engineered and widely used in European and American countries for in-situ remediation of organic pollution sites.Many domestic research institutions and enterprises have also conducted research and application of thermal desorption technology.As a carrier for heat exchange in in-situ thermal desorption,soil has a complex structure,and its own thermal conductivity restricts the heat transfer effect.At the same time,the heat and moisture migration involved in the in-situ thermal desorption process changes the thermal conductivity of soil,affecting the distribution of temperature and humidity in soil and the removal of pollutants.Mastering the thermal conductivity of soil and the law of heat and moisture transfer during in-situ thermal desorption is the key to improving the design and repair effect of in-situ thermal desorption.Due to the lack of sufficient understanding and systematic analysis of the characteristics and influencing mechanisms of heat and moisture transfer during in-situ thermal desorption,studying the laws of heat and moisture transfer in soil under different heating methods has important theoretical research significance and engineering application value for the development and promotion of in-situ thermal desorption technology.There are many types of soil.Due to limitations in research time and energy,this article focuses on the study of thermal migration patterns of silty clay and sand with different particle sizes in the Changchun area under different heating methods.Firstly,taking the silty clay in Changchun area as the research object,a combination of experimental and numerical simulation methods is used to study the influencing factors and laws of soil thermal conductivity.To ensure the accuracy and reliability of numerical simulation,a three-dimensional model construction method considering anisotropic dual pore structure was proposed based on the analysis of indoor experimental results and the structural characteristics of three-phase materials in soil.By changing the saturation to simulate the characteristics of unsaturated zone of soil,it is found that the thermal conductivity of anisotropic soil varies greatly in different heat transfer directions,and the pore connectivity and saturation are the decisive factors affecting the anisotropic thermal conductivity.Therefore,due to different heating directions of thermal desorption,there are significant differences in heat transfer efficiency,and a suitable heating method becomes the key to efficient treatment of thermal desorption.Given that the thermal conductivity of soil varies dynamically during the heating process due to the coupling effect of multiple fields,it is not possible to quantitatively evaluate the heating process using the thermal conductivity.To further quantitatively study the impact of different heating methods on soil heat transfer patterns and compare the heat transfer effects of different heating methods,a mechanism sand with quantifiable physical properties parameters was used as the research object to design a sand box heating experimental platform.Through physical model experiments,the heat and moisture transfer and heat transfer ability changes of horizontal heat conduction heating methods were analyzed,and efficient and energy-saving heating parameter design principles were obtained.Finally,the differences in heat and moisture transfer between the specific heat air injection heating method and the heat conduction heating method were analyzed,and a sand column steam injection platform was designed to obtain the heat and moisture transfer law of the steam injection heating method and compare the heat transfer effect of the steam injection parameters.The specific research content and results are as follows:(1)By combining experimental and numerical simulation methods,the influencing factors and mechanisms of soil thermal conductivity are studied.Results indicate that the factors that have a significant impact on the thermal conductivity of soil are saturation,porosity,pore connectivity,and pore connectivity direction.The dual pore structure and liquid phase biased distribution of silty clay result in a segmented increase in thermal conductivity with increasing water content,and the higher the saturation,the greater the thermal conductivity of the soil.The thermal conductivity decreases with the increase of porosity,and compared to the pores within aggregates,the pores between aggregates have a greater impact on the thermal conductivity of the soil model;The smaller the angle between the direction of pore connectivity and the direction of thermal conductivity,the smaller the influence of liquid content on the thermal conductivity in the direction of thermal conductivity.With a 45°angle as the boundary,when the angle exceeds 45°,k_y:k_x(the ratio of vertical thermal conductivity to horizontal thermal conductivity)increases with the increase of liquid content.When the included angle is less than 45°,k_y:k_x decreases with the increase of liquid phase content.As the liquid content increases,the influence of soil anisotropy on thermal conductivity anisotropy gradually decreases.(2)Based on the physical model experiment of sand box,the heat transfer law of sand medium during horizontal heat conduction heating in dry sand is studied.The experimental results indicate that the main heat transfer method for dry sand is heat conduction,and the particle size of the sand medium and heating temperature have a significant impact on the heating effect.The area of the heating zone is calculated by the closed area of the 25℃isothermal coil.When dry sand with different particle sizes is heated,the time for the area of the medium heating zone to reach stability and the area of the heating zone with the same heating time are different.The area of the coarse sand heating zone reaches stability,it takes the shortest time,the area of the heating zone with the same heating time is the largest,while the area of the fine sand heating zone takes the longest to reach stability,and the area of the heating zone with the same heating time is the smallest.The ratio of heating area to energy consumption for the same heating time(3 hours)is defined as the heating efficiency.The experimental results show that when the heating temperature increases,the heating efficiency will significantly decrease,and the heating efficiency of coarse sand decreases the most with the increase of temperature.After stopping heating,the area of the heating zone will continue to increase within a certain range,which is called"thermal inertia".The higher the heating temperature,the larger the particle size of the medium,and the longer the duration of"thermal inertia"and the increase in the area of the heating zone.Therefore,when using intermittent heating method,fully utilizing the"thermal inertia"law can reduce average power consumption and increase the heating area per unit energy consumption.(3)Based on the physical model experiment of the sand box,the heat and moisture transfer law of wet sand with different moisture contents during the horizontal heat conduction heating process is studied.The experimental results indicate that moisture content,sand particle size,and heating temperature have a significant impact on the heat moisture migration.The upper part of the heating rod has the largest heat transfer distance,followed by the horizontal direction,and the lower part has the smallest heat transfer distance.When the water content is 10%,calculate the area of the heating zone with the closed area of the 35℃isothermal coil.The maximum area of the upper heating zone can reach 60%of the total heating zone area.The largest area of the heating zone is fine sand,and the smallest area of the heating zone is coarse sand.For the closed area of the 25℃isothermal coil,the area of the heating zone is calculated.The largest area of the heating zone is coarse sand,and the smallest area of the heating zone is fine sand.This phenomenon is caused by the difference between the particle size of the sand medium and the moisture diffusion and capillary conduction.The heating zone area of fine sand increases with the increase of water content,while the heating zone area of medium sand medium first decreases and then increases with the increase of water content,while the heating zone area of coarse sand medium gradually decreases with the increase of water content.The smaller the sand particle size,the higher the heating temperature and moisture content,and the greater the heat transfer capacity value.The heat transfer capacity of sand with a moisture content of 10%at a heating temperature of 170℃is 1.2 times that at 110℃,which is 30 times its thermal conductivity.In terms of heating efficiency and energy consumption,the heat conduction heat transfer method has the highest thermal efficiency and the lowest energy consumption.The higher the water content,the smaller the medium particle size,and the more obvious the moisture heat mutual feedback effect.The main heat transfer method is heat moisture migration.At this time,the higher the energy consumption,the lower the electricity utilization rate.The shape of the heating contour during the heating process reflects the heat transfer mode of the medium.Usually,the shape of the heating zone is symmetrically distributed about the perpendicular of the heating rod.The shape of the heating zone can be roughly divided into three types:O-shaped,V-shaped,andΛ-shaped.The proportion of the upper heating zone area to the total heating zone area(50%)is used as an indicator to determine the convective heat transfer intensity.The shape of the isotherm line and the changes in the gradient of the isotherm line are comprehensively utilized as the basis for determining the heat transfer mode,providing a basis for rational design of heating parameters.In heterogeneous formations,the heating source is located in formations with higher water content and smaller particle size to obtain a larger heating zone area.By comparing the sand box heat conduction heating with hot air injection heating,it was found that the heating zone area and heating influence distance of the hot air heating method were significantly increased compared to the heat conduction heating method.(4)A sand column steam injection experiment was conducted to study the heat and moisture transfer law in sand under the heating method of hot steam injection.The experimental results indicate that the highest temperature and energy consumption of medium heating depend on the injection temperature.Increasing the steam injection temperature will increase the overall temperature,temperature gradient,and heating rate of the sand column,with little impact on the temperature stabilization time.During the injection of steam into the wet sand column,there were two obvious heating processes.The vaporization flow rate was a significant factor affecting the first heating time,and the second heating was caused by changes in water content in the sand column.The steam injection flow rate has a significant impact on the heating rate and temperature stability time,but has a smaller impact on energy consumption.