非饱和黄土水热耦合数值模拟研究
发布时间：2019-03-28 12:03
【摘要】：非饱和黄土通常水分含量较低,在研究水运移过程中,往往忽略了气态水迁移的作用。中国西北地区黄土地下水位很低,但诸如城市和机场硬化路面以下土层含水率却较高,这种现象已经不能使用经典的热和液态水迁移理论来解释。通过分析文献资料和揭示现场“锅盖”效应现象的研究需要,提出了一种用于模拟非饱和黄土中水分和热耦合的新思路,该理论认为水分迁移受到温度梯度和基质吸力梯度的双重作用,其不仅考虑了土中液态水的迁移和热量的传递,还考虑了液态水和气态水之间的相变转换(蒸发和冷凝)及相变潜热和显热。通过建立非饱和黄土模型和水分迁移质量和能量守恒方程,改变初始条件和边界温度,并使用数值软件Comsol Multiphysics对其进行模拟,得出了气态水和液态水在水分迁移过程中的运移基本规律和相对贡献大小及其条件,以及在不同温度梯度、温度水平、初始含水率、迁移时间和土水特性参数等情况下,水分分布变化规律。通过研究得出如下结果:在温度梯度作用下,土体中的水分主要以气态水分形式由高温端向低温端运移,当气态水在低温端凝结成液态水时,水分又会在吸力梯度作用下以液态水形式向高温端运移;土体模型(20厘米)中温度场在10小时后就基本达到了稳定,温度梯度不再变化;随着土体温度梯度、温度水平和迁移时间的逐渐增大,高(低)温端水分迁出(入)量越大,土中含水率增量为0的临界点距高温端越远,水分流失区域越大,总水分迁移量也越大;随着土体初始含水率逐渐增大,高温端水分迁出量逐渐增大;而低温端水分迁入量先增大,当含水率达到某一值时,气态水的迁移通道越来越小,水分迁入量减小,水分流失区域逐渐减小,因此,土体中总水分迁移量是先增大后减小,土中含水率增量为0的临界点距高温端越近,临界点附近曲线变化趋势越发突变。随着与进气值有关的参数的不断增大,高(低)温端水分迁出(入)量越大,土中含水率分布曲线在高(低)温端处越陡,含水率增量为0的临界点距高温端越近,临界点附近曲线变化趋势较大;随着与土水特征曲线减湿斜率有关的参数的逐渐增大,高(低)温端水分迁出(入)量越小,土中含水率分布曲线越平缓,含水率增量为0的临界点距高温端越远。
[Abstract]:The water content of unsaturated loess is usually low, and the role of gaseous water migration is often neglected in the study of water migration. The loess groundwater level in northwest China is very low, but the soil moisture content below the hardened pavement in cities and airports is relatively high. This phenomenon can no longer be explained by the classical thermal and liquid water transfer theory. Based on the analysis of the literature and the need to reveal the phenomenon of "pot cover" effect in the field, a new idea for simulating the coupling of moisture and heat in unsaturated loess is put forward. According to the theory, water transfer is affected by both temperature gradient and matrix suction gradient, which not only takes into account the transfer of liquid water and heat transfer in soil, but also takes into account the effect of temperature gradient and matrix suction gradient. The phase transition (evaporation and condensation) and latent and sensible heat between liquid and gaseous water are also considered. Through the establishment of unsaturated loess model and conservation equation of moisture transfer mass and energy, the initial conditions and boundary temperature are changed, and the numerical software Comsol Multiphysics is used to simulate it. The basic law and relative contribution of gaseous water and liquid water in the process of water migration are obtained, and under the conditions of different temperature gradient, temperature level, initial moisture content, migration time and soil water characteristic parameters, etc. Law of change of water distribution. The results are as follows: under the action of temperature gradient, the moisture in the soil moves mainly from the high temperature end to the low temperature end in the form of gaseous water, and when the gaseous water condenses at the low temperature end to liquid water, The water will move to the high temperature end in the form of liquid water under the action of suction gradient. In the soil model (20 cm), the temperature field is stable after 10 hours, and the temperature gradient is no longer changed. With the gradual increase of soil temperature gradient, temperature level and migration time, the greater the amount of water migration (inflow) at the high (low) temperature end, the farther the critical point with the increment of water content 0 in the soil from the high temperature end, and the larger the water loss area is. The total water migration also increased; As the initial moisture content of soil increases gradually, the water mobility at the high temperature end increases gradually. When the moisture content reaches a certain value, the migration channel of gaseous water becomes smaller and smaller, the water migration volume decreases, and the water loss area decreases gradually. Therefore, the total water migration amount in the soil increases first and then decreases, and when the water content reaches a certain value, the migration channel of gaseous water becomes smaller and smaller. The closer the critical point of soil moisture content increment is to the high temperature end, the more abrupt the change trend of the curve near the critical point is. With the increasing of the parameters related to the inlet value, the greater the water migration (inflow) amount at the high (low) temperature end, the steeper the water content distribution curve in the soil at the high (low) temperature end, and the nearer the critical point at which the moisture content increment is 0 is closer to the high temperature end. The curve near the critical point has a great trend of change; With the increasing of the parameters related to the moisture reduction slope of the soil-water characteristic curve, the smaller the water migration (inflow) amount at the high (low) temperature end is, the more smooth the moisture content distribution curve in the soil is, and the farther the critical point at which the moisture content increment is 0 is from the high-temperature end.
【学位授予单位】：西安理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TU444
本文编号：2448840
[Abstract]:The water content of unsaturated loess is usually low, and the role of gaseous water migration is often neglected in the study of water migration. The loess groundwater level in northwest China is very low, but the soil moisture content below the hardened pavement in cities and airports is relatively high. This phenomenon can no longer be explained by the classical thermal and liquid water transfer theory. Based on the analysis of the literature and the need to reveal the phenomenon of "pot cover" effect in the field, a new idea for simulating the coupling of moisture and heat in unsaturated loess is put forward. According to the theory, water transfer is affected by both temperature gradient and matrix suction gradient, which not only takes into account the transfer of liquid water and heat transfer in soil, but also takes into account the effect of temperature gradient and matrix suction gradient. The phase transition (evaporation and condensation) and latent and sensible heat between liquid and gaseous water are also considered. Through the establishment of unsaturated loess model and conservation equation of moisture transfer mass and energy, the initial conditions and boundary temperature are changed, and the numerical software Comsol Multiphysics is used to simulate it. The basic law and relative contribution of gaseous water and liquid water in the process of water migration are obtained, and under the conditions of different temperature gradient, temperature level, initial moisture content, migration time and soil water characteristic parameters, etc. Law of change of water distribution. The results are as follows: under the action of temperature gradient, the moisture in the soil moves mainly from the high temperature end to the low temperature end in the form of gaseous water, and when the gaseous water condenses at the low temperature end to liquid water, The water will move to the high temperature end in the form of liquid water under the action of suction gradient. In the soil model (20 cm), the temperature field is stable after 10 hours, and the temperature gradient is no longer changed. With the gradual increase of soil temperature gradient, temperature level and migration time, the greater the amount of water migration (inflow) at the high (low) temperature end, the farther the critical point with the increment of water content 0 in the soil from the high temperature end, and the larger the water loss area is. The total water migration also increased; As the initial moisture content of soil increases gradually, the water mobility at the high temperature end increases gradually. When the moisture content reaches a certain value, the migration channel of gaseous water becomes smaller and smaller, the water migration volume decreases, and the water loss area decreases gradually. Therefore, the total water migration amount in the soil increases first and then decreases, and when the water content reaches a certain value, the migration channel of gaseous water becomes smaller and smaller. The closer the critical point of soil moisture content increment is to the high temperature end, the more abrupt the change trend of the curve near the critical point is. With the increasing of the parameters related to the inlet value, the greater the water migration (inflow) amount at the high (low) temperature end, the steeper the water content distribution curve in the soil at the high (low) temperature end, and the nearer the critical point at which the moisture content increment is 0 is closer to the high temperature end. The curve near the critical point has a great trend of change; With the increasing of the parameters related to the moisture reduction slope of the soil-water characteristic curve, the smaller the water migration (inflow) amount at the high (low) temperature end is, the more smooth the moisture content distribution curve in the soil is, and the farther the critical point at which the moisture content increment is 0 is from the high-temperature end.
【学位授予单位】：西安理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TU444
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