摘要
为了深入掌握轴流泵叶顶区湍流特性,采用大涡模拟方法对某一模型轴流泵内部非定常流动进行了数值模拟。根据时域和频域特性图,分析间隙内压差和泄漏速度之间的关联现象,讨论了叶顶间隙内泄漏流的瞬态特性。根据三维泄漏涡结构,揭示了轴流泵叶顶区不同类型的涡系,叶顶泄漏涡带在剪切层内涡丝动力的驱动下逐渐变长,然后与射流剪切层分离;叶顶间隙内涡团的瞬态变化大于叶顶泄漏涡的周期性变化,导致剪切层内的小尺度涡的生成周期时间较短,其在主泄漏涡带上方形成了小尺度泄漏流涡带。从叶顶轴平面的涡结构可发现,随着弦长系数的增大,剪切层内的分离涡不断被分离并且被叶顶泄漏涡卷吸,在主泄漏涡向相邻叶片压力面的运动过程中,其涡量不断减小,并且在转轮室端壁面附近不断诱导各种尺度的涡产生。
Numerical analysis of the unsteady flow in an axial flow pump was conducted to understand deeply the characteristics of turbulence in the tip region via LES （large eddy simulation） in ANSYS CFX. Such an understanding was critical to predict and eventually control cavitation and noise as well as vibration in liquid handling systems such as pumps and propellers, and improve their performance. Compared with the conventional numerical methods, LES provided the most promising and feasible alternative to compute the unsteady velocity and pressure fields. In this paper, LES method with large mesh-size requirement was used for studying the transient characteristics of the tip leakage flow and leakage vortex. Some significant conclusions were obtained by the simulation. The responsibility of different flow rate for variation of average head and efficiency calculated by LES had little discrepancy with the experimental values in the low-head axial flow pump model, especially at flow rate condition. A combination of the time-domain and frequency-domain graphs of pressure difference coefficientand leakage velocity coefficient at different chord sections were presented in this paper. It was found that the mutual promotion and restriction relationship between pressure difference and leakage velocity in the tip region was obvious, which led to the unsteady characteristics of the tip leakage flow. The tip leakage dynamics mentioned above could help others understand the underlying mechanisms of low-pressure fluctuations and tip-leakage vortex oscillations. Then, different types of tip vortexes were seen according to three-dimensional structure of leakage vortex, including the corner vortex generated by the flow separation near the pressure side, separated vortex shed from the tip into the shear layer, tip leakage vortex A formed due to the interaction between the leakage flow and the mainstream, as well as the swirl in the tip clearance. The main leakage vortex strip absorbed vortex filaments shed from the tip in the shear layer, which could provide the power for the generation of the main tip leakage vortex. The main leakage vortex was separated from the shear layer, meanwhile there was the phenomenon of “pinch off”, and the main leakage vortex strip was shortened gradually because of the dissipation of its movement and the integration of the mainstream. Due to the faster transient change of the small-scale vortex in the gap compared with the tip leakage vortex, the re-generation cycle of vortex filaments in shear layer was shortened, and then the secondary leakage vortex strip was created above the main vortex strip. From the in-plain leakage vortex structure with mean streamlines and vorticity contours, it could be seen that the separation vortex in shear layer constantly was separated and then rolled up by the leakage vortex with the increasing of chord coefficient, which could drive the main leakage vortex to move forward, however, the vorticity of the main leakage vortex decreased during the moving process towards the pressure surface of the neighboring blade and the counter-rotating induced vortexes were generated constantly, at the same time, the scope of the main leakage vortex expanded a lot, and a large fraction of the wake downstream of the leakage vortex were produced, which could affect the flow field in the passage and enhance the instability of the flow field.
出处
《农业工程学报》
EI
CAS
CSCD
北大核心
2015年第11期74-80,共7页
Transactions of the Chinese Society of Agricultural Engineering
基金
国家自然科学基金面上项目(51479083)
江苏省优势建设学科资助
关键词
泵
泄漏
湍流
大涡模拟
叶顶
泄漏涡
瞬态特性
pumps
leakage
turbulent flow
large eddy simulation
tip
leakage vortex
transient characteristics
