| The research in this dissertation is supported by the National Key Research and Development Projects of Control Technology and Equipment for Precision Irrigation and Fertilization of Field Crops(No.2017YFD0201502),and the Special Scientific Research Fund of Agriculture Public Welfare Profession of China-Comprehensive Technical Scheme of Water-saving,Fertilizer-saving and Pesticide-saving for Wheat,Corn and Rice in Yellow and Huai Valleys in China(No.201503130).Solid fertilizer is the most widely used fertilizer in China.The solid fertilizer was usually pre-dissolved as solution for storage when it was applied in the fertigation system,and then the fertilizer solution was injected into the irrigation system by the fertilization device and transported to the field.This was not only time-consuming and labor-intensive,but also required manual proportioning of fertilizer concentration.Therefore,it has important scientific research significance and engineering application value to develop a solid fertilizer mixing-application device with functions of adding,dissolving and applying,and to study the fertilization performance and application technology mode of the device.This research was conducted based on the methods of theoretical analysis,experiment and numerical simulation.The dissolution characteristics of solid fertilizer was investigated.The key components such as the fertilizer-feeding component and the fertilizer-mixing component were developed.The solid fertilizer mixing-application device was produced and its fertilization performance was investigated.The fertilizer-water mixing laws in irrigation pipe were investigated,and the application technology mode of the solid fertilizer mixing-application device was proposed.The main research and contents were as follows:(1)The dissolution characteristics of solid fertilizer and the variation law of fertilizer solution concentration produced by dissolution were carried out.The dissolution process of solid fertilizer was analyzed theoretically,and the key factors that affected the dissolution rate were investigated.The calculation model for the dissolution rate of solid fertilizer was established.The effect of water-fertilizer mixing mode on the stability of fertilizer concentration was investigated by experiment.The concentration variation model when adding water and fertilizer simultaneously was established.The results showed that the dissolution rate of solid fertilizer decreased with time,which was not business with the amount of water.It was positively correlated with the amount of fertilizer,the saturated solubility of fertilizer and the stirring speed,and negatively correlated with the particle size of fertilizer.The stability of fertilizer concentration was the highest when water and fertilizer were added at the same time.The accuracy of the concentration variation model was high,and the average deviation between the calculation and test results was 6.09%.(2)The input mechanism of the solid fertilizer fertigation equipment was designed and studied.The fertilizer-feeding component was designed,the effects of screw structure,motor speed and fertilizer loose density on fertilizer-feeding flow rate were investigated,and the regulation model of fertilizer-feeding flow rate was constructed.The experiments were carried out to investigate the effect of the filter bucket structure on the fertilizer dissolution rate,and the effects of the stirring speed,the fertilizer-feeding flow rate and the inlet water flow rate on the fertilizer concentration uniformity.The results showed that the optimal structure of the filter bucket was300 mm in diameter and 100 mesh.The fertilizer concentration uniformity increased with the increase of the stirring speed and the inlet water flow rate,decreased with the increase of fertilizer-feeding flow rate.The optimal stirring speed was 400 r/min.(3)Two output mechanisms were proposed for sprinkler irrigation and drip irrigation systems,and the structural optimization methods of the output mechanisms were studied.The structure of the Venturi applicator was optimized with the goal of maximizing the fertilizer suction flow rate.The losses of the reducer section,throat and expanding section of the Venturi applicator were theoretically analyzed,and the theoretical calculation model for the loss of inlet and outlet of the Venturi applicator was established.The internal flow of the Venturi tube was simulated by means of CFD,and the loss calculation model was revised according to the simulation results.The optimal structure parameters of the Venturi applicator were obtained based on the modified loss calculation model.The results showed that the average deviation between the losses calculated by the modified loss calculation model and the simulation results was 4.43%.The optimal reducing angle of the Venturi fertilizer applicator was 19°-28°,and the expanding angle was 6°-11°.The optimal throat diameters were 5,6 and 7 mm when the flow rates through the Venturi applicator were 1.5,2.0 and 2.5 m~3/h,respectively.The optimal throat length and the optimal diameter of the fertilizer suction pipe were equal to the throat diameter.The suction flow of the optimal Venturi applicator was 56.3%higher than that before optimization in same differential pressure.(4)The fertilization performance of the solid fertilizer mixing-application device was investigated.The prototype of the solid fertilizer mixing-application device was designed,and the automatic control system of the prototype was built.According to the working principle of the prototype,the relationship model between the diameter and height of the mixing cavity and the storage cavity were established,and the time coordination strategy of the prototype operation was constructed.Experiments were carried out to study the adjustment accuracy of the fertilizer-feeding flowrate and the inlet water flowrate,the detection accuracy of the fertilization concentration,as well as the influence of fertilization amount and fertilization time on fertilization accuracy.The fertilization uniformity of the solid fertilizer mixing-application device was compared with the pressure differential tank,the injection flow rate was compared with the proportional fertilization pump.The results showed that the average adjustment errors of fertilizer-feeding flow rate and inlet water flow rate were 2.96%and 3.50%,respectively,and the average detection error of fertilization concentration was 5.62%.The fertilization concentration deviation was less than 3%and the fertilizer amount and fertilization time would not affect the fertilization accuracy of the mixing-application device when the added solid fertilizer can be completely dissolved.The maximum fertilization rate(fertilization amount/fertilization time)to ensure uniform fertilization of the device was about 60 kg/h.The fertilization uniformity of the solid fertilizer mixing-application device was much better than that of the differential pressure fertilizing tank,and slightly lower than that of the proportional fertilization pump,but the injection flow rate was about 6 times that of the proportional fertilization pump.(5)Water-fertilizer mixing law in irrigation pipe was investigated.The process of water-fertilizer mixing in pipe was simulated based on the component transport model.The effects of pipe size,water and fertilizer flow rate and physical properties of fertilizer solution on mixing speed and uniform mixing length were explored.The relationship model between uniform mixing length and water flow rate,fertilizer flow rate,irrigation pipe diameter,fertilizer pipe diameter,fertilizer viscosity,and fertilizer concentration were established by dimensional analysis.The results showed that the frequency of fertilizer injection would not affect the mixing speed and uniform mixing length.The larger the fertilizer flow rate and the irrigation pipe diameter,the smaller the fertilizer pipe diameter and the water flow rate,the faster the mixing speed.The uniform mixing length increased with the increase of the fertilizer pipe diameter,the irrigation pipe diameter,the water flow rate and the fertilizer viscosity,and decreased with the increase of the fertilizer flow rate and the fertilizer concentration.The average deviation between the results calculated by the uniform mixing length calculation model and the experimental results was 1.95%.(6)Two application technology modes of the solid fertilizer mixing-application device were proposed.The installation positions of solid fertilizer mixing-application devices in the two irrigation systems were investigated by combining the arrangements of the sprinkler and drip irrigation pipeline,the calculation model of water-fertilizer uniform mixing length,and the irrigation and fertilization regime of sprinkler irrigation for wheat and drip irrigation for cucumber.The results showed that system,the fertilizer injection port on the irrigation main pipe should be at least 7 m away from the end of the main pipe when the solid fertilizer mixing-application device was used in the sprinkler irrigation.The fertilizer injection port on the irrigation pipe should be at least 3.5 m away from the first drip irrigation belt when the device was used in the greenhouse drip irrigation system. |
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