Study On The Heat And Mass Transfer In Soil With Phase Change And Simulation Of The Growing Of Plant Roots System

In the present thesis, the mass flow and energy flow in the Resources-Environment-Plant System (REPS) are general analyzed based on the flow and heat and mass transfer in porous media. From the point of view of the system, several thermo-physical problems, such as the heat and mass transfer in porous soil under steady environment or real natural environment condition, effects of temperature and temperature gradient on the moisture migration in unsaturated soil, the coupled transport of moisture, heat and salt in cropped soil, and so on, are emphatically studied. In addition, we present two-dimensional biological modelling and three-dimensional visual simulation to describe the growth of plant root intuitively.A mathematical model to describe the heat transfer and moisture migration in moisture-layered soil is built based on continuity-media dynamics and the local-volume averaging method. Subsequently, we carry through steady and non-steady numerical calculation under steady environmental condition and obtain the temperature field, moisture field, and evaporation rate field in porous soil. The effect of dry-saturated layer on the heat and mass transfer in soil is also analyzed. The numerical simulations show that: Starting from the occurrence of dry soil layer (DSL), water evaporation front moves from soil top surface to the interface between unsaturated layer and dry layer. Both the decrease of water evaporation intensity and the increase of soil temperature are remarkably and the thickness of dry soil region develops slowly.Under natural condition, the variations of temperature, moisture content and water evaporation intensity in soil are obtained by use of numerical method, and corresponding experiment is conducted in a cylindrical soil bed. The result shows that under periodical-changing ambient temperature and solar radiation, soil temperatures and water evaporation rate exhibit a one-day-period fluctuation and the amplitude of soil temperature decreases rapidly with soil depth. Under natural condition, soil moisture content decrease during daytime but slightly increases at night. Furthermore by reason of the converse direction of soil temperature gradient during daytime and at night, evaporation and condensation process occurs in unsaturated soil region alternatively, and observably the higher temperature gradient value, the more intensive the water evaporation or vapor condensation is. Along with the development of dry soil region, the intensity of water evaporating from soil surface decreases gradually and soil temperature increases remarkably. Comparing experiment results with the present calculating results, we find they are satisfactorily consistent, which shows that the theoretical model developed in this paper has good accuracy and reliability.On the bases of non-equilibrium thermodynamic theory and in view of Secret effect, the modelling and experiment study of temperature effect on the moisture migration in unsaturated soil is conducted. Taking the temperature effect into account, a modified mathematical model to describe the heat and moisture transfer in soil is developed. The measured and calculated results indicate that temperature plays an important role in liquid water transport in soil, and the influence of temperature gradient on vapor diffusion is very obvious.The influence of plant roots' modality on the heat and moisture transfer in soil is great. Basing on measured result, a two-dimension biological modeling of wheat root growth is processed with the evolution idea whilst water content, moisture gradient, temperature, soil fertility and air penetrating property are defined as the dominating environment factors in plant root growing. According to the simulated wheat root distribution in soil, the effect of water absorption of root system on the heat and moisture migration in unsaturated soil is analyzed. Moreover, the comparisons of heat and mass transfer between in cropped soil-bed and in bare soil-bed are also conducted.According to the measured results about root system di...