Numerical analysis and test on cavitation of jet mixing apparatus

Abstract: The jet mixing apparatus (JMA) is a vitally important part for mixing water with a pesticide, including a working nozzle, suction inlet, diffuser, thumb lock, case, end cap, one-way ball, inserts, etc. The Jet Mixing Apparatus is a simple device with no moving parts, where a high velocity flow (water) is used to pump a second fluid (pesticide). It was broadly used in large plant protection machinery. Its main property is efficiency and stability of the mixing ratio. Cavitation is a physical phenomenon in a Jet Mixing Apparatus happening at low pressure, seriously affecting the performance and wasting energy. In order to acquire the characteristic curve of the relation on the mixing ratio and pressure ratio, experimental and numerical analyses were used to measure the mass flow rate of working, intake, and mixed fluid. The test was conducted in the Key Laboratory of Modern Agricultural Equipment in accordance with the JB/T9782-1999 general test method for plant protection machinery. The outlet pressure was regulated to different levels (0.25, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.35 MPa) by a throttle valve, an electronic scale for mass flow rate, a U mercury manometer for vacuum, which was at 2.0 MPa working pressure and normal atmosphere intake. The computational fluid dynamics (CFD)software ANSYS fluent 15.0 was used for numerical simulation of the cavitation. The Zwart-Gerber-Belamri cavitation model in mixture model was adopted to capture cavitation, and obtained the internal static pressure distribution and gas distribution contour under different outlet pressures. Water was set as the main phase, with density of 1000 kg/m3, and dynamic viscosity of 0.001 kg/(m∙s). Water vapor was set as the second phase, with density of 0.02558 kg/m3, dynamic viscosity of 1.26×10-6, and the bubble radius of 0.01mm. Cavitation pressure was set 3 540 Pa. The two inlet boundary condition was set at pressure-inlet, turbulence intensity of 2%, and hydraulic diameter of 14 mm. The outlet boundary condition was set at pressure-outlet, turbulence intensity of 2%, and hydraulic diameter of 9 mm. A double precision solver and pressure velocity coupling algorithm was adopted. The pressure equation was discrete with two-order upwind, and other equations with the QUICK method. Calculation of residual was set for 10-6, using hybrid initialization to initialize. The experimental values and the simulated values were compared for fitting analysis, and the mathematical relationship between the experimental values and the simulated values was established. The fitting coefficient R² was 0.9618, which verified the accuracy of the model. The results showed that JMA has poor performance even backflow when the pressure ratio was greater than 0.6. The static pressure on the central axis had no significant difference in a working nozzle at the different outlet pressures, negative pressure appeared at the nozzle exit, and the negative pressure zone increased with the decrease of the pressure ratio. The mixing ratio was negatively correlated with the pressure ratio when the pressure was between 0.4 to 0.6. The mixing ratio and pressure ratio were independent when the pressure ratio was less than 0.4, which was the cavitation mixing ratio. The cavitation happened at the outlet pressure of 0.8 MPa, and the lower the outlet pressure, the more severe the cavitation, which was under working pressure 2.0 MPa, and suction pressure zero. Numerical and experiment research of cavitation is a meaningful area for research for improving the efficiency of a jet mixing apparatus.