基于掺气方法的低压水射流分散特性研究

Lower working pressure adaptation in sprinkler irrigation, is one of the development directions in agriculture water saving in recent times. When sprinklers work at low pressure condition, usually too much water concentrate at the end of the range, causing serious uneven water distribution. The aeration jet method is applied in this project to improve the water distribution, and make the sprinkler work normally even below the minimum working pressure. The project will be carried out experimentally and theoretically, combined with numerical simulation method. The dispersion characteristics of aeration jet under low pressure will be investigated. This include the measurement and analysis of raindrop size, study on radius of throw and trajectory of two-phase jet, study on water distribution and the raindrop impact force on ground etc.. The influence of entrained air content on jet dispersion characteristics will be further studied. In addition the air-water jet breakup will be studied, and the bubbles evolutionary process of formation, development and destruction will be further studied. This will lead to the understanding of the inner mechanism of water distribution improvement under lower pressure conditions by aeration jet method. A mathematical model of gas-liquid two phase flow in sprinkler is expected to be established and be solved by programming. Air-water jet structure is expected to be theoretically studied. The project is aimed at water and energy saving by the aeration jet method which improves water distribution under lower working pressure conditions. The project research content will enrich and improve the two phase jet theory, and promote the application of jet technology.

农业节水中喷灌发展方向之一是采用低压喷灌。喷头在低工作压力时则会出现水量向射程末端集中的现象，产生严重的水量分布不均匀，为了改善水量分布，课题提出射流掺气的方法，使喷头在低于原来最小工作压力时仍能进行喷灌作业。课题将采用试验、理论分析与数值模拟结合的研究方法，展开低压下掺气水射流的分散特性研究，包括雨滴粒径测量与分析，两相射流的射程和轨迹线研究，水量分布研究，雨滴对地面打击力分析等。课题将深入研究掺气量变化对射流分散特性的影响；深入研究两相射流碎裂过程，掺入的气泡形成、发展、破裂的演化过程，以期揭示掺气对改善低压情况下喷头水量分布的内在机理。课题预期建立喷头内部气液两相流数学模型并编程求解，预期展开水气两相射流结构的理论研究。课题旨在通过掺气方法实现低压水射流均匀分散，达到节能节水的目的，课题研究内容将丰富和完善两相射流理论，促进射流技术的应用。

降低固定式喷灌喷灌系统的供水压力能达到节能的效果并减少投资，但喷头的工作压力降低至一定程度将会发生喷洒水量分布不均匀。为实现低压下均匀喷洒，在PY20和PY30摇臂喷头的基础上提出一种掺气结构，研制出新形式的掺气摇臂喷头和新的气液射流喷洒方式。对气液两相射流的分散特征开展研究，并应用于田间组合喷灌。采用激光雨滴谱仪测量雨滴粒径结果表明，掺气射流的射程末端中位直径d50小于不掺气情况，射程中段d50大于不掺气情况，射程中部大雨滴粒径的出现改善了喷头径向水量分布向末端集中的不利状况。同轴插入摇臂喷头流道内的掺气管是掺气喷头的主要部件，掺气管从喷嘴内逐渐移出喷嘴外时，抽吸能力逐渐增大，移出喷嘴外2mm为推荐的合适值。研究发现，掺气摇臂喷头结构也可用于抽吸液体，可多功能应用于喷洒叶面肥。建立了水射流轨迹计算的数学模型，结合试验数据获得模型中的阻力增长系数b合适取值为0.01。基于高速摄影法和图像处理的方法，得到不同工作压力下，喷嘴出口的掺气段长度、射流破碎长度与射流的射程之间的拟合经验式，用于对比分析气液两相射流的特征长度值。对射流外流场进行三维数值模拟，得到不同工作压力、不同喷嘴尺寸时射流外流场的液相体积分布，分析得出射流的破碎长度值，采用PIV试验进行射流流场观测，获得流场断面速度分布，数值模拟与实验值数据相符合，射流破碎长度且与高速摄影试验值作对比也符合的较好。开展了掺气摇臂喷头室外草坪组合喷灌试验，试验结果表明：相同工作压力下对比原摇臂喷头，采用射流掺气的方法提高了组合喷灌水量分布均匀系数，可降低喷头工作水压力50至100kPa的范围。掺气水射流的研究为喷灌节能节水提供了新途径，同时为喷头的多功能应用奠定了理论基础。