基于能量加权的喷灌液滴等效指标计算方法

    A calculation method of droplet equivalent indicators of sprinkler irrigation based on energy-weighting

    • 摘要: 等效液滴速度和粒径是衡量喷头喷洒质量和液滴破碎程度的重要参数。为改进和发展液滴等效指标计算方法,该研究提出一种能量加权的液滴等效指标计算方法;基于5种类型喷头喷洒液滴数据,对比能量加权方法与其他类型方法计算结果的特点和差异;建立能量加权等效指标与其他类型指标的回归关系。结果表明:1)能量加权等效方法计算的液滴等效速度和等效粒径可以表征出能量贡献率高的液滴的特征;2)经验公式计算的液滴终端速度与能量加权等效液滴速度存在较好的指数回归关系,和体积有关的等效粒径与能量加权等效液滴粒径存在较好的指数回归关系,决定系数均大于0.8;3)基于能量加权的等效液滴动能可以较好地估计单位时间和单位面积的降水动能,对数回归关系的决定系数为0.84。研究结果可为从能量角度反映液滴的平均特征提供思路。

       

      Abstract: Droplet velocity and particle size determine droplet kinetic energy and have a major impact on soil erosion, crop leaf strike and farm microclimate. The equivalent droplet velocity and diameter describe the average characteristics of the droplet and they are important parameters for measuring the spray quality of the nozzle and the degree of droplet fragmentation. Traditional droplet testing methods can only measure number and particle size, so the equivalent method previously used cannot calculate the average droplet characteristics in terms of droplet energy. With the widespread use of optical instruments in sprinkler tests, droplet velocities can also be measured and the corresponding methods for calculating droplet equivalent indicators should be improved and developed. This study proposed an energy weighted method for calculating droplet equivalent indicators. Based on droplet data sprayed by five types of sprinkler nozzles, the characteristics, and differences between the calculation results of the energy weighted method and other types of methods were compared. The regression relationship between energy weighted equivalent indicators and other types of indicators was established. The results showed that: 1) The energy weighted equivalent droplet diameter was the largest, followed by the equivalent method related to droplet volume, and the smallest is the equivalent method related to droplet number. The overall trend in droplet equivalent diameter related to energy and volume along the radial direction was relatively similar, but there were significant differences at the proximal end of the spray. This is because although a larger proportion of smaller droplets occupy the near end of the spray water, there are also a certain number of large droplets which, after being ejected from the nozzle outlet, fall steeply to the ground without being sufficiently broken up. Large droplets carry a greater kinetic energy and contribute more to the energy at the measurement point, so the energy weighted droplet equivalent diameter is more biased in favour of these large droplets. 2) The droplet equivalent velocity and equivalent diameter calculated by the energy weighted equivalent method can characterise droplets with a high energy contribution. The histogram of the distribution of the number of droplet velocities showed that droplets with velocities less than 6 m/s occupied a large proportion of the droplets. The IWOB nozzle, for example, had a number weighted equivalent velocity of 3.93 m/s, corresponding to a droplet number accumulation frequency of 29.7%, but an energy accumulation frequency of only 4.5%. The energy weighted equivalent droplet velocity was 4.55 m/s. The number of droplets less than this velocity carries about 43.8% of the energy and the number of droplets greater than this velocity is about 21.4%. This velocity was between the maximum velocity and the number-weighted equivalent droplet velocity and may represent the velocity characteristics of a large droplet carrying more energy. 3) There was a good exponential regression between the droplet terminal velocity calculated by the empirical formula and the energy weighted equivalent droplet velocity, and there was a good exponential regression between the equivalent droplet diameter related to volume and the energy weighted equivalent droplet diameter, with correlation coefficients greater than 0.80. 4) The energy weighted equivalent droplet kinetic energy provided a better estimate of the kinetic energy of precipitation per unit time and area, with a coefficient of determination of 0.84 for the logarithmic regression relationship. The results of the study may provide ideas for reflecting the average characteristics of droplets from an energy perspective.

       

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