【作者】 金文；

【导师】 王元； 张鸿雁；

【作者基本信息】 西安建筑科技大学 ， 环境科学， 2010， 博士

【摘要】 微灌是一种高效节水灌溉技术,滴头是微灌系统的核心部件之一,滴头内流道结构特征直接关系到滴头的性能,决定着微灌系统的优劣。揭示微灌滴头微尺度通道内流运动规律,对优化滴头流道结构设计、提高滴头性能具有重要意义。采用Micro-PIV技术分别对600μm×6000μm和800μm×800μm两种方形断面的直通道和平角、尖角和圆角三种齿形微通道在Re=80-300区间内流流场进行实验研究。获得了各流动工况下的流场流线图、速度矢量分布图及速度云图。实验采用Mini:YAG双脉冲激光器、14位灰阶的CCD相机、10倍显微物镜、3μm荧光示踪粒子和仅允许610nm红光透过的滤光镜相配合,通过解决相机与PIV系统的匹配问题,提高了图像信噪比,获取了清晰的微尺度流场粒子图像。并且,通过图像处理技术获得了各流动工况下的流场流线图、速度矢量分布图及速度云图。在图像处理中使用多次测量取平均的方法消除示踪粒子的布朗运动影响,运用系综互相关算法获取微尺度流场分布。实验发现,微尺度流道的壁面粗糙度对内流流动有较大影响,且流道尺度越小,壁面粗糙度的影响越显著；三种齿形微通道内流流场在流道转向内侧都存在低速回流区,内含一个完整的涡；而在平角齿形微通道的左顶角部位另有一个较小范围的低速回流区,也有涡存在；在尖角齿形微通道的顶角区域有较大范围的回流区,并且内部存在大小不同、上下叠加、方向相反的两个涡,下部的大涡结构具有规律性,而叠加于上部的小涡流动结构不稳定,流速更低。低速回流区是固体颗粒容易沉积的区域,是造成堵塞的主要原因。采用在商用CFD软件-Fluent,通过人为设定壁面高相对粗糙度和将壁面粗糙元抽象为多孔介质两种方法对直管微通道流动进行建模,分别匹配Fluent提供的标准k-ε模型、RNG k-ε模型、realizablek-ε模型、标准k-ω模型和sst k-ω模型,逐一在Re=100和Re=300流动条件下,对600μm×600μm和800μm×800μm方形断面直管微通道内流流场进行数值模拟,并将计算结果与相同工况下直管微通道Micro-PIV实验结果进行了定量对比分析。结果表明：将微尺度通道壁面粗糙元抽象为多孔介质模型,采用Realizablek-ε两方程湍流模型能够更好的模拟微尺度管流流动,其中多孔介质层的厚度采用微尺度通道的壁面粗糙元厚度进行设定,多孔介质的粘性阻力系数和惯性阻力系数由多孔介质区域内的流态及阻力进行计算。利用推荐的微尺度流动数值计算方法,在Re=100和Re=300流动条件下,对平角、尖角和圆角三种不同齿形结构的微灌滴头内流流场进行模拟计算。通过将模拟计算结果与相同工况下Micro-PIV实验测量结果的分析比较,证明了该微尺度模拟方法适用于复杂微通道内流流场的计算。由此进一步建立多种尺度下的平角、尖角和圆角三种齿形流道结构的滴头模型,在Re=100、Re=200和Re=300流动条件下,采用推荐的微尺度流动数值方法进行模拟计算,将计算结果从微通道内流回流区分布,不同流道尺度、不同雷诺数下压力降变化,以及流态指数、流量系数比较等几个方面进行分析评价。结果表明：尖角齿形的流道结构作为微灌滴头内流道在工作性能上具有一定优势,但其尖角顶端的低速小涡区域容易沉积固体颗粒,因此对尖角齿形流道进行局部结构改造。从减少低速回流区和增大流动阻力两方面综合考虑,对尖角齿形流道的微灌滴头做出两种改造方案,一是斜削齿尖方法,二是圆弧削齿尖方法,对改造的流道分别建立滴头流道模型并数值计算其内流流动分布,通过计算结果评价分析获知,改造后的流道结构有助于提高微灌滴头工作性能。本研究得到国家“十五”科技攻关项目(2004BA901A21-4)的资助。更多还原

【Abstract】 Micro-irrigation is a high efficient and water-saving irrigation technology. Emitter is one of the key components of micro-irrigation system. Structural characteristics of internal flow passage are directly related to emitter’s performance and micro-irrigation system’s quality. Therefore, research on internal flow field of micro channel in micro-irrigation emitter has important significance to optimization design of internal flow passage’s structural and performance improvement of emitter.Using Micro-PIV technique, inflow field were measured in straight and jagged micro-channels with cross section of 600μm×600μm and 800μm×800μm under the conditions of the Reynolds number of 80-300. Clear images of particles were obtained by combining Mini:YAG double pulse laser,14-digit grey scale CCD camera, 10x microscope,3μm fluorescence tracer particles and light filtering lens allowing 610nm red light permeation. Using image processing techniques, Flow field streamlined diagrams、velocity vector distribution map and velocity cloud chart were obtained under all kinds of flow conditions. The influence of Brownian motion was eliminated through the average of obtained images. Flow field distribution of micro channel was obtained by the ensemble cross-correlation algorithm. Experiments exposed that the wall roughness of micro channel has a greater effect on internal flow field. The smaller the size of micro channel was, the greater the wall roughness affected. There existed a low speed vortex cavity at the swerving inside with a complete vortex in three jagged micro-channels. However, there existed a smaller scale low speed vortex cavity with a vortex at the left vertex of flat-jagged micro-channel. Furthermore, there are two vortexes, big intact vortex and superposition vortex on the jag peak of cusp-jagged micro-channel, and compared with the former, and the latter is lower speed, converse and unsteady. The impure substance in low-speed region is prone to deposit and block, hence the structure of micro-channels could be altered.The essay utilized "Fluent" numerical analytic computing software to design the optimum micro-scales flowing simulating plan. The designing adopted micro-scales affect manipulating approaches setting wall roughness element and the porous medium simulating wall roughness element. By utilization of standard k-εmodel, RNG k-εmodel, realizable k-εmodel, standard k-ωmodel and sst k-ωmodel provided by Fluent software, the experiment executed relatively numerical simulations on the rectangle cross section micro-scales water flowing with section size of 600μm×600μm and 800μm×800μm in the case of Re=100 and Re=300. The comparison of velocity field computed from various numerical analyses and the fitting degree of Micro-PIV experimental outcome was done. The results showed that better flow field of micro channel could be obtained by the optimum case that designing case via the porous medium simulating wall roughness element under the solution of realizable k-s. Thickness of the porous medium was set by wall roughness element of micro channel. Viscous resistance coefficient and inertial resistance coefficient were calculated by fluid state and resistance within the porous medium.Using recommend numerical simulating method of flow field of micro channel, internal flow field of micro channel in micro-irrigation emitter was numerical simulated in three jagged micro-channels under the conditions of the Reynolds number of 100 and 300. By the comparison of velocity field computed from various numerical analyses and the fitting degree of Micro-PIV experimental outcome, we can concluded that this numerical simulating method of micro channel was suitable for the calculation of complex internal flow field of micro channel. Using recommend numerical simulating method of flow field of micro channel, further emitter models in three jagged micro-channels were built under the conditions of the Reynolds number of 100,200 and 300. The analysis and evaluation of results were got at the aspects of distribution of recirculation zone of internal flow field of micro channel, different flow channel scales, pressure drop under the conditions of different Reynolds number, fluid index and flow coefficient. The results showed that cusp-jagged micro-channels had advantages in performance as internal channel of micro-irrigation emitter. However, the impure substance in low-speed region is prone to deposit and block. Partial structure of cusp-jagged micro-channels should be modificated. Considering the reduction of low-speed region and the increase of flow resistance, two rehabilitation programs were built of cusp-jagged micro-channels in micro-irrigation emitter. One of them was to rake cut tooth tip and the other was to cut tooth tip by circular arc. Base on micro channel in micro-irrigation emitter after transformation, numerical model was built and internal flow field was calculated. By evaluation and analysis of results, conclusion could be reached that flow channel structure after transformation improved the performance of micro-irrigation emitter.更多还原