Design and experiment of low pressure Venturi injector based on double fertilizer inlets

Abstract: Venturi injector has been widely used in the integrated irrigation system of water and fertilizer because of its advantages of low cost, simple structure, no external power and so on. In order to reduce the critical inlet pressure of the Venturi injector and make it suitable for low pressure irrigation and fertilization system, a kind of Venturi injector with double fertilizer inlets was designed. The design was based on the study results that increasing the numbers of fertilizer inlets can improve fertilizer absorption capability of Venturi tube. A total of 4 structural parameters (throat contraction ratio, contraction angle, diffusion angle and ratio of throat length and diameter) were required to be determined for the Venturi injector. A total of 16 combinations with the 4 structural parameters were designed by using the orthogonal method. The CFD simulation was used to simulate the fertilizer absorption performance of each scheme. According to the simulation results, the optimum combination of structural parameters was throat contraction ratio 0.3, contraction angle 20o, diffusion angle 8o and ratio of throat length and diameter 1.1. According to the optimum combination of structural parameters, a prototype of Venturi injector was manufactured by the 3D print technique for an experiment. The experiment was carried out in College of Modern Agricultural Engineering of Kunming University of Science and Technology, China. The water inlet pressure from 0 to 0.15 MPa was realized by adjusting the difference in height between the bucket and the Venturi injector and there were 8 points chosen from 0 to 0.15 MPa. The vertical distance between the liquid level and the fertilizer inlets was set to 500 mm. During the experiment, the fertilizer concentration was calculated. The experimental results showed that the measured values agreed well with the simulated values with the root-mean-square error of 0.22 L/min for fertilizer suction amount, 0.96% for flow ratio of inlet, 0.93% for fertilizer concentration, and 0.68% for fertilizer absorption efficiency. The change in measured and simulated fertilizer absorption performance with the inlet pressure was similar. The CFD simulation was used for compare single and double inlets injector with the same structural parameters and the results showed that under the same inlet pressure, fertilizer suction amount, flow ratio of inlet, fertilizer concentration and fertilizer absorption efficiency of the double inlets increased by 90%, 85%, 80% and 80%, respectively, compared to single fertilizer inlet. The measured critical inlet pressure was 0.007 MPa for the double inlets injector. When the inlet pressure was 0.05 MPa, the fertilizer concentration was 13.6%. Compared with the existing Venturi injector, the Venturi injector with double fertilizer inlets need much lower water inlet pressure to obtain the equal or higher fertilizer absorption performance and it is more suitable for low pressure irrigation system.