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Numerical Simulation Of Coupled Soil Water And Heat Transfer Under Non-Isothermal Conditions

Posted on:2019-04-11Degree:DoctorType:Dissertation
Country:ChinaCandidate:R RenFull Text:PDF
GTID:1313330569979383Subject:Hydraulic engineering
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In semi-arid area of northern China,there are very strong seasonal transformation and temperature variation.The migration of moisture and heat in soil is a coupling process which is restrained,mutual promotion and influence.The beneficial coupling between water and heat can render a soil fertility and plays a role in water conservation and retention.The change of soil moisture and temperature in the field makes the soil water-heat coupling migration process extremely complex.In the face of such a complex multivariable system,it is difficult to deal with its interaction mechanism only by experimental research.In this paper,the mathematical models of one,two and three-dimensional soil water-heat coupling transfer under non-isothermal conditions were studied and established by means of numerical simulation combined with theoretical analysis,and laboratory tests combined with field tests.The main findings are as follows:(1)Under the precondition of ignoring the gas influence,the mathematical model of one-dimensional vertical soil water-heat transfer was established.In the process of establishing the mathematical model,the interaction between soil water movement and heat transfer,and the effect of root water uptake on soil water-heat transfer were mainly considered.The mathematical model was discretized by implicit difference scheme,then the computer program of the model was compiled,and the discrete equation group was solved by the method of cross circulation.An experiment on surface irrigation of winter wheat planted in a soil column was performed.The moisture content and temperature of the soil in the root zone were measured during the entire growing period,and obtained data were used to evaluate the proposed model.Results show:In the simulation period,the average relative error and the mean absolute error of soil water content are 1.84%and0.0039 cm~3/cm~3,respectively;the average relative error and the mean absolute error of soil water temperature are 2.48%and 0.1941?,respectively.Those results show the high accuracy of the model in predicting the actual changes in soil water content and temperature as a function of time and soil depth.Moreover,the model can accurately reflect changes in soil water-heat transfer in different periods.(2)The influence of heat flux on water movement and the influence of root water uptake on the movement of water and heat transfer were studied by the proposed one-dimensional mathematical model.The results show that the simulation results of the model which considering the effect of heat flow on water movement and the effect of root water uptake on heat transfer are better than the model which considering only one of the factors.(3)On the premise of ignoring the effects of biological or chemical action and soil solute potential on water flow and heat flux,the mathematical model of two-dimensional soil water-heat coupling transfer was established.This mathematical model was established focused on the influence of heat transfer on soil water movement and the effect of soil water movement on heat transfer.The alternating direction implicit difference method(e.g.ADI method)was used to solve the model,the discrete equation group was solved by the cross circulation method and then the computer program was compiled.As a new type of irrigation method suitable for fruit forest in northern semiarid hilly area,compared with the traditional method of ground irrigation,the main characteristic of water storage single pit irrigation is that it can lead the water directly into the root zone of the middle and deep layer.Moreover,according to the characteristics of water storage pit irrigation,the single pit irrigation can be reduced to two dimensional infiltration problem.Therefore,an experiment on water storage single pit irrigation under different conditions of soil water and temperature indoor was developed.The moisture content and temperature of the different soil profile in different time were measured,and obtained data were used to evaluate the proposed two-dimensional model.According to the principle of mass conservation,the mathematical model of water level variation with time in water storage single pit was established.The boundary conditions of the mathematical model were determined,according to the characteristics of the water storage single pit.Results show:The average relative error and the root mean square error between the simulated value and the measured value of the water level in the pit are 2.817%and 3.789%,respectively;the average relative error and the root mean square error between the simulated value and the measured value of the1h and 1d moist front are less than 8.496%and 10.34%,respectively;in different soil profile,the average relative error and the root mean square error between the simulated value and the measured value of soil moisture content are less than9.441%and 13.81%,respectively;in different soil profile,the average relative error and the root mean square error between the simulated value and the measured value of soil temperature are less than 1.667%and 1.824%,respectively.Those results show the high accuracy of the model in predicting the water level in pit and the actual changes in soil water content and temperature as a function of time and soil depth.(4)The characteristics of water and heat distribution in irrigated soil under non-isothermal condition were studied by using the two-dimensional mathematical model of soil water and heat transfer in the water storage pit.The results showed that,in the early stage of irrigation,the high value area of soil moisture content showed a tendency to expand gradually with the time of irrigation.However,in the process of soil moisture redistribution and after the end of irrigation water infiltration,the maximum moisture content is decreasing,although the moist body is still expanding.At the beginning of irrigation,the low value area of soil temperature also showed a trend of gradual expansion,and its distribution shape was similar to that of the high value area of soil moisture content.However,with the decrease of irrigation water,the soil temperature decreased gradually and the soil temperature increased in the process of soil moisture redistribution.(5)Under the precondition of ignoring the gas influence,a three-dimensional mathematical model was established for the coupled transfer of soil water and heat under non-isothermal conditions.In the process of establishing the mathematical model,the interaction between soil water movement and heat transfer was considered.The finite element method was used to solve the model,and then the computer program was compiled.Since the water storage pit irrigation method is three-dimensional irrigation,an experiment on water storage single pit irrigation(pit deep 40cm)of apple tree was performed.In this experiment,the soil moisture content and temperature were measured in the root zone during the entire growing period,and obtained data were used to evaluate the proposed three-dimensional model.Boundary conditions of the model were determined by the mass and energy conservation.And the effects of root water uptake,surface evaporation,pit wall evaporation,and the actual field solar radiation,weather changes on boundary conditions were taken into account.The change of water level in impoundment pit was simulated by mass conservation principle.Results show:The average relative error,maximum relative error,and root mean square error between the simulated value and the measured value of soil water content were 9.651%,16.032%and 10.867%,respectively;The average relative error,maximum relative error,and root mean square error between the simulated value and the measured value of soil temperature were 3.902%,9.653%and 5.031%,respectively.The process of soil moisture redistribution and heat transfer in different time and soil profile can be effectively reflected by three-dimensional soil moisture pattern and temperature graph.Those results show the high accuracy of the model in predicting the actual changes in soil water content and temperature as a function of time and soil depth.Moreover,the model can accurately reflect the characteristics of three-dimensional moisture and heat transfer in soil under water storage pit irrigation.(6)In this paper,the three-dimensional soil water-heat transfer mathematical model under non-isothermal conditions was studied,and the characteristics of soil water and heat distributions under different pit depths(pit deep 20?40?60cm)were investigated.The results show that the most suitable pit depth of the 3 years apple tree is 40cm;the relationship between soil water movement and heat transfer is mutual coupling.In practice,according to this approach,the technical factors of water storage pit irrigation can be selected by the proposed three-dimensinal mathematical model.In this paper,the mathematical models of one?two and three-dimensional soil water-heat coupling transfer under non-isothermal conditions enrich and perfect the study on soil water-heat coupling transfer.The crop growth soil management mechanism can be developed by the proposed mathematical models.
Keywords/Search Tags:numerical simulation, water-heat coupling migration, implicit difference method, ADI method, finite element method, water storage pit irrigation
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