叶顶间隙流动中的涡结构实验研究

Experiments of the vortical structure morphology in a tip clearance flow

叶顶间隙流动中的旋涡结构是引发导管桨、水轮机等水力机械叶顶间隙空化问题的主要因素之一。与常规船用螺旋桨(叶顶无端壁)上的梢涡流动不同,叶顶间隙流动受叶片载荷分布和间隙尺度(叶片梢部与端壁的距离)共同作用。为了研究叶顶间隙流动中的旋涡结构特性及其影响因素作用机理,本文采用二维NACA0024翼型在空泡机理水筒中,开展了基于旋涡空化观测的叶顶间隙流动显示实验和基于2D-3C PIV的叶顶间隙流场测量实验。通过空化观测实验,获得了叶顶间隙流动中的旋涡整体分布状态,以及间隙宽度、雷诺数和载荷系数等对旋涡强度的影响规律。通过流场测量实验,获得了局部全湿流场(无空化状态)中的旋涡分布规律以及其随不同影响因素的变化规律。结合流动显示实验和流场测量实验,研究了叶顶间隙流动中的旋涡结构形态以及形成机理,并初步阐明了间隙宽度、雷诺数和载荷系数对叶顶间隙流动旋涡强度的影响规律。研究表明,在NACA0024翼型叶顶间隙流动中存在两种主要稳定涡结构,分别是梢分离涡和梢泄涡;稳定涡结构是引起叶顶间隙流动空化的主要因素;翼型攻角和间隙宽度的大小是影响叶顶间隙流动中旋涡结构强度的主要因素。

Vortical structures in tip clearance flow are prone to induce the flow cavitation for hydraulic machineries such as ducted propellers and hydroturbines.Unlike the tip vortical flow in conventional vessel propellers,the tip clearance flow is affected by the load distribution on the blade and the gap size between the blade tip and the end wall.To study the characteristics of vortical structures in tip clearance flow,the visualization by vortex cavitation bubbles and the velocity measurement by 2 D-3 C PIV are carried out for the NACA0024 straight airfoil in a cavitation tunnel.The global distribution of vortical structures in the tip clearance flow and the influence of gap width,Reynolds number,and load coefficient on the vortex strength are obtained by cavitation observation experiment.In addition,the vortex distribution in local flow fields without cavitation and its variation with different influence factors are obtained by velocity measurement.By combining the flow visualization and velocity measurement results,the morphology of vortical structures and their formation mechanism are analyzed.And the effects of the gap width,Reynolds number,and load coefficient on the vortex strength are illuminated preliminarily.Results show that there are two types of steady vortical structures in the tip clearance flow,i.e.,the tip separated vortex and tip leakage vortex,which are responsible for inducing the cavitation of tip clearance flow.The angle of attack and gap size of the airfoil will affect the strength of vortices in the tip clearance flow significantly.