Study on jet mixing pesticide concentration field measurement with planar laser induced fluorescence

Abstract: Online mixing provide the required concentration of pesticides by injecting the required amount of solution in real time. The concentration field measurement is important for evaluating the effect of online mixing. The plane laser induced fluorescence (PLIF) method is suitable for the concentration field measurement of online mixing with the advantages such as no-touching, no influence on the mixing field, quickly and accurately. In order to measure the jet mixing concentration field for agricultural spraying, a jet mixing device and an auxiliary measurement device of PLIF concentration field were developed in this paper. The jet mixing device included round jet pipe, jet nozzle, mixing tube and other components. This paper mainly focused on the measurement of the mixture concentration field of the small flow nozzles. For this purpose, the inlet diameters of the mixing tubes were D=1.0, 1.5 and 2.5mm and the outlet diameters of jet nozzles were d=0.6, 0.8 and 1.0mm. To be beneficial to pesticide inhalation, the selected combination was D=1.5mm and d=0.8mm in the experiments. Then experiments based on PLIF measurement devices were carried out. Firstly, the concentration calibration experiments were carried out. 6 different concentrations of Rhodamine 6G homogeneous mixture were formulated to calibrate the concentration test. The measured image was filtered by smoothing filter to eliminate the influence of the measurement noise. According to the calibration of total average gray value, the concentration difference between each local area was large, and the gray values of the edge area were obviously smaller than those of the centre one. The calibration based on grid division was able to handle the problem of uneven gray level of the measured image, and a more uniform subarea concentration could be obtained. The calibration effect of 2 calibration methods was compared. The first method was the total average gray calibration（TAC）one which taken the global average gray to calibrate the concentration. The second was grid-based calibration（GBC） one which divided the measurement field into grids and calibrated the concentration grid by grid. The measurement results showed that both the longitudinal and lateral grayscale differences could be smoothed by GBC and more balanced subregional concentrations could be obtained. The difference between the maximum and minimum average gray value based on 8×14 GBC was only 25.22% of that of TAC, which meaned that the calibration accuracy could be improved greatly by GBC. Secondly, the measurement of concentration field with water inlet pressure 0.1-0.6 MPa was carried out with 3 different inhalation concentrations (3.125, 1.000 and 0.250 mg/L). The calibration coefficients of each grid by GBC calibration were used to calculate the average concentrations and coefficient of variation of the measured pictures. The measurement results of different inhalation concentration at different pressure showed that the pressure had a certain effect on average concentration and coefficient of variation in general. Both the average concentrations and the coefficients of variation were relatively larger at pressure 0.1 and 0.2 MPa. Both the average concentrations and the coefficients of variation were smaller at pressure 0.3-0.6 MPa, which indicated that the uniformity of the mixture increased with the increase of pressure in general. The variation trend of the average concentration of the longitudinal and lateral grids showed that the concentration of the end of the mixed tube was higher and the longitudinal mean variation coefficient of the end was larger at 0.1 and 0.2 MPa. The mixing uniformity of the mixing tube was reduced as near the pipe wall. In general, the maximum and minimum concentration difference between different grids was larger at lower pressure, that was, the mixing uniformity was poor at lower pressure. The study showed that the PLIF method is suitable for the concentration field measurement, and the measurement precision can be improved with GBC method and that the pressure may influence the concentration field.