射流混药器改进提高混药均匀性及动态浓度一致性

Improved jet mixer injection nozzle enhancing pesticide mixing uniformity and dynamic concentration consistency

提高喷嘴直接注入式变量喷雾系统中混药器浓度一致性与均匀性同样重要。该文根据多孔板穿过流脉动衰减原理提出了夹层孔管式新型混药器,并以普通射流混药器为参照进行在线混合试验,基于图像进行在线混合瞬时均匀性及动态浓度一致性分析。结果表明:单视角图像误差较小,算法适用性强;载流流量及混合比的增大能提升均匀性及一致性;与射流混药器相比,夹层孔管式混药器试验条件下(800 mL/min<载流流量<2 000 mL/min,4:100<药水混合比<10:100)平均瞬时不均匀性指数从16.70降至14.76,并在药水混合比>9∶100或载流流量>1 400 mL/min时普遍降至约13.00,达到均混效果;其虽难以保证所有工况下混合均匀,却显著提高了脉动注入时混合液浓度一致性,平均动态浓度不一致性值从0.039降至仅0.011,试验条件下只要混合比>5∶100,或注药频率>5.10 Hz,动态浓度不一致性指数均低于0.020,满足浓度一致性要求;由于夹层孔管式混药器相对射流混药器在瞬时均匀性上的优化没有动态浓度一致性明显,且夹层孔管式混药器的瞬时不均匀性指数值与动态浓度不一致性值相关性仅为0.684,低于射流混药器的0.848,因此未来夹层孔管式混药器的进一步优化应以均匀性为主要目标,即使提高浓度一致性也可能提高其混合均匀性。

The pesticide injections in nozzle direct injection system(NDIS) applied in plant protection machinery generally have flow-rate inconsistency characteristics, which cause fluctuations of mixing concentration and inaccuracy of spraying. The purposes in this study were to evaluate the inline mixing efficacy comprehensively based on index of dynamic inconsistency of the mixing concentration and the inline static mixing uniformity in NDIS. A new inline mixer known as the layered mixer(mixer B) with special structures(perforated tubes) to weaken concentration inconsistency and a traditional jet mixer(mixer A) were presented to carry out the direct injection and mixing experiments under variable working conditions. Moreover, image processing methods to quantify mixture concentration discontinuities based on calculating coefficient of variation(CV) values of mean pixels brightness in ROIs was proposed. Static mixing non-uniformity index to identify static mixing non-uniformity based on principal component analysis(PCA) was also proposed. These 2 indices were used to scientifically measure the inline mixing efficacy of the mixers. The relative differences between the calculation results of horizontal viewing and the vertical viewing images were only 2.86% for static mixing non-uniformity index values and 5.37% for CV values, indicating that the single horizontal viewing images could be used to obtain higher accuracy. The threshold of static mixing non-uniformity index for judging whether the mixture was uniform or non-uniform was set to be 13.00 as there weren’t non-scatted viscous pesticides remaining in the detection tube when the static mixing non-uniformity index values dropped below it. In addition, the threshold for determining the concentration consistency was set to be only 2% because there weren’t apparent brightness differences between ROIs at different moments when the CV values were less than it. The increase of carrier flow rate and the mixing ratios could generally improve the static uniformity and dynamic concentration consistency of inline mixing. However, when it was under the condition of a large mixing ratio, the static mixing uniformity of the jet mixer would decline due to the pesticide accumulation in the side wall of the jet mixer. The average static non-uniformity index of the proposed mixer here reduced from 16.70 to 14.76, and reduced to about 13.00 only when mixing ratios was higher than 9:100 and carrier flow rate was higher than 1400 m/min, which could achieve uniform mixing efficacy completely. Although it was difficult for the new mixer to guarantee high static mixing uniformity under all working conditions, it could still significantly improve the dynamic consistency of the mixing concentration under the pulsation injection conditions with the average CV value decreased from 0.039 to only 0.011. Moreover, when the working condition could meet the requirements of injection frequency higher than 5.10 Hz or the mixing ratios was higher than 5:100, the CV values would decline to 0.020, which could fully satisfy the dynamic consistency requirements. The linear correlations between the static uniformity and dynamic consistency of jet mixer and layered mixer were 0.848 and 0.684, respectively. Therefore, a more efficient inline pesticide mixer should be proposed especially when the pesticide injection pulse was strong. The independent sample test results showed that the optimization of the static uniformity of the proposed mixer was less than that of the concentration consistency, which indicates that the new mixer structure has significant concentration pulsation attenuation efficacy. To conclude, further optimization on static uniformity of the proposed mixer should be proposed.