Effects of diffusion section structure on vortex characteristics and fertilizer absorption performance of non-axisymmetric Venturi injector

Abstract: Fertilization equipment is one of the important components of low-pressure irrigation systems. A Venturi injector is widely used in the agricultural irrigation system, due to its simple structure, low price, and convenient use. The non-axisymmetric Venturi injector normally presents a better fertilizer absorption performance than the axisymmetric one. There are many studies on the response relationship between the fertilizer absorption performance and structural parameters of the Venturi injector. But, only a few studies are focused on the response relationship between the vortex characteristics and structural parameters. Therefore, this study aims to explore the influence mechanism of vortex on the non-axisymmetric Venturi injector in the low-pressure irrigation system, in order to improve the fertilizer absorption performance. An analysis was made to compare the vortex characteristics, fertilizer absorption performance, and the response relationship between fertilizer absorption performance and vortex characteristics of non-axisymmetric Venturi injectors with arc diffusion section and non-axisymmetric Venturi injectors with linear diffusion section. A test platform was developed to evaluate the fertilizer absorption performance. The fertilizer inlet of the non-axisymmetric Venturi injector was set as the atmospheric pressure, where the outlet was 0.04 MPa, and the water inlet was controlled between 0.07 and 0.25 MPa. A pressure gradient was taken for each 0.02 MPa to carry out the experimental test of fertilizer absorption performance. At the same time, the computational fluid dynamics software was selected to carry out the vortex analysis and visualization of the flow field. The mathematical modeling, meshing and simulation of Venturi injectors were carried out to compare with the physical test. The Tecplot post-processing software was used to analyze the numerical simulation, where the spline curve was used to locate the vortex. The results show that the diffusion section of the non-axisymmetric Venturi injector as an arc structure was conducive to reducing the vortex area, and the local energy loss in the vortex area, in order to improve the energy conversion efficiency of the non-axisymmetric Venturi injector. The vortex area and intensity decreased by 28.41%-42.37%, and 6.64%-35.65%, respectively, in the non-axisymmetric Venturi injector with the arc diffusion section under the same difference between inlet and outlet pressure. Once the outlet pressure was 0.04 MPa, the rate of fertilizer absorbed by the non-axisymmetric Venturi injector with the linear diffusion section increased gradually with the increase of inlet and outlet pressure difference, and finally tended to be stable. However, when the non-axisymmetric Venturi injector with the arc diffusion reached the maximum amount of fertilizer absorption, the amount of fertilizer absorption decreased with the increase of inlet and outlet pressure difference. The non-axisymmetric Venturi injector with the arc diffusion section presented a 50.64%-103.22% and 48.15%-98.25% increase in the amount and efficiency of fertilizer absorption, respectively, compared with the linear diffusion section. There was a gradually reduced increase in the amount of fertilizer absorption and its efficiency with the increase of the inlet and outlet pressure difference. The vortex region presented a significant effect on the fertilizer absorption performance of the non-axisymmetric Venturi injector (P≤0.05). The center point of the vortex region was gradually downward and backward with the increase of inlet and outlet pressure difference. When the inlet and outlet pressure difference was 0.03 MPa, the distance between the vortex boundary and the throat of the non-axisymmetric Venturi injector with the linear diffusion section was 0, indicating that the direct impact of the vortex on the formation of negative pressure in the throat. The vortex in the non-axisymmetric Venturi injector in the arc diffusion section was far away from the throat, without a direct effect on the formation of negative pressure in the throat. However, there was a gradually decreased distance between the vortex boundary and the throat of the non-axisymmetric Venturi injector in the arc diffusion section. The influence of the vortex on the throat increased gradually with the increase of the inlet and outlet pressure difference. A decrease in the area or intensity of the vortex area and an increase in the distance from the vortex boundary to the throat can greatly contribute to the better performance of fertilizer absorption in the non-axisymmetric Venturi injector. The finding can provide a strong reference for the structural design of non-axisymmetric Venturi injectors.