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Theoretical And Experimental Investigation On Thermophysical Problems In Resources-Environment-Plant System

Posted on:2005-07-21Degree:DoctorType:Dissertation
Country:ChinaCandidate:A W FanFull Text:PDF
GTID:1103360152967485Subject:Engineering Thermal Physics
Abstract/Summary:PDF Full Text Request
In the present paper, we put forward the concept of Resources-Environment- Plant System and give an analysis of the mass flow and energy flow in this system. In our studies we focuse on several thermophysical problems, such as, the daily variation of soil temperature, effects of atmospheric temperature and soil temperature on plant growth and development, impacts of environmental factors on the simultaneous transfer of heat and moisture in cropped soil, the oxygen concentration field in cropped soil and the temperature effect on soil moisture transport and so on. The details are shown below:1. On the bases of neglecting convection and phase-change heat transfer modes, and assuming that the uncultured soil was a semi-infinite uniform medium, an analytical solution to the daily fluctuation of soil temperature and a correlation between the daily amplitude of soil temperature and soil depth are obtained by solving the one-dimensional transient heat conduction equation. The results show that soil temperatures exhibit a one-day-period fluctuation with the daily variation of ground surface temperature. Meanwhile the amplitude of soil temperature decreases rapidly in an exponent function manner with soil depth and the peak-value-lag-time has a linear correlation with soil depth. Further investigation demonstrates that soil thermal diffusivity has a significant influence on the daily amplitude and the peak-value-lag-time. The depth impacted by the variation of ground surface temperature increases with the increase of soil thermal diffusivity, while the lag time of soil temperature decreases with the increase of soil thermal diffusivity. Finally, we carried out measurements of the daily variations of soil temperature at different depths and found that the daily variation of soil temperature could be fitted by three-order Fourier's series.2. By employing the mathematical model established to describe heat and mass transfer in porous media, simulative comparisons on the responses of soil temperature under different conditions, such as, summer and winter, ambient wind velocity of 1m/s and 4m/s and ambient relative humidity of 35% and 85% have been conducted respectively. The numerical simulations show that: (1) Under the actions of periodical changes of ambient temperature and solar radiation, soil temperatures responses inconsistently under different conditions. (2) The impact on soil temperature imposed by ambient temperature and solar radiation decreases with the increase of soil depth and the variation period of soil temperature turns long increasingly, which implies that the temperature hysteresises is distinct. (3) During the most part of a day in summer, soil temperatures in shallow layers are higher than those in deeper layers, whereas it is reversed in winter. (4) When wind speed is high, heat and mass transfer between the soil surface and the surroundings are enhanced, the higher the wind speed, the more the water evaporation, thereby the more heat is taken away from soil, which leads to a greater decrease of soil temperature, especially in the surface layer. (5) Due to the effect of water evaporation in dry climate, soil temperature at each point is lower than that in humid climate, which is especially obvious in shallow layers of the soil.3. The effects of atmospheric temperature and soil temperature on the growth and development were experimentally studied in Rudbeckia hirta, a biennial herbaceous flower. The results show that greenhouse culture could improve the growth of the overground part and hinder the development of the root system of Rudbeckia hirta. When the soil was heated, the overground part of the Rudbeckia hirta grew better than the unheated one's, while it is reversed for the root system.4. We built a two-dimensional root water uptake function and then developed a seven-field mathematical model with ten variables to describe the simultaneous transfer of heat and moisture in a cylindrical soil bed with plants growing in a greenhouse. In order to investigate the influences of environmenta...
Keywords/Search Tags:soil, plant, heat, moisture, oxygen, transfer, model, simulation, experiment
PDF Full Text Request
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