梢隙空化初生尺度效应及其与梢隙流动结构关系研究

The present project mainly studies the scale effect of tip clearance cavitation inception and the relation with the flow structures. Tip clearance cavitation is a cavitation phenomenon which occurs in the clearance flows between the blade tip and the casing of axial hydraulic machines, such as water pumps, water turbines and marine propellers with duct. The tip clearance cavitation often increases the level of noise and vibration of the system, and leads the severe erosion of the runner blades and the casing. Up to now it is still not possible to accurately predict the onset and development of tip clearance, neither from numerical simulations nor from reduced scale model tests, although there are strong requirements in the industrial application. The difficult may come from the facts that the complexity of both tip clearance flows and cavitation is still not sufficiently explored..In our project the relation between cavitation inception and tip clearance flow structures will be studied mainly by experiments and supplementary by numerical simulations. As tip leakage vortex (TLV) is the main flow structure in the tip clearance flow and cavitation may develop in the core of the TLV and other clearance region where the local pressure is low enough, So the formation and evolution of TLV are key issues to understand the mechanism of tip clearance cavitation inception. In generally TLV is the result of flow separation, shedding and rolls up of the leakage flow in the clearance region which is induced by the pressure difference between the blade pressure and suction sides through. In present project a simplified model is proposed, where the TLV is generated by a typical hydrofoil in a water tunnel in CSSRC and the clearance between the blade tip and the wall can be varied. The 2D-3C-PIV and TR-PIV will employed to measure of and flow structures. According to the measuring result, the characteristics of the velocity and vorticity distribution with wide range of incidence angles, inlet velocities including oscillating flows and tip clearances will be studied in detail. The vortex intensity and dynamics of TLV, as well as the relation to the cavitation inception will be analyzed. At same time numerical simulation will be used to understand the complicated flow structures in tip clearances flows..The study of the scale effect of cavitation inception in the tip clearance flow is an important issue in present project. The series tests of tip clearance cavitation inception will be carried out with different gap sizes, different hydrofoils, different incoming flow conditions including different oscillation incoming flows, as well as different water qualities including air content of water and nuclei distribution. Both methods, observation with high speed camera and noise measurement, are used to detect the cavitation inception. The relations among the cavitation inception number with various non-dimensional parameters which combined by above factors will be summarized. The mechanism of tip clearance cavitation inception will be analyzed based on the experimental measurement and numerical simulation results. Finally the prediction method of tip clearance cavitation inception will be proposed.

梢隙空化是指液体流经叶片梢部与端壁之间时发生的空化现象，广泛存在于以液体为介质的水力机械中，如水泵、水轮机及具有导管的船舶推进器等，梢隙空化会引发噪声增加，振动加剧，民用和国防对准确预报梢隙空化初生均有强烈需求。梢隙流动和空化的双重复杂性使得梢隙空化初生的预报极具挑战性，其中缩比模型可能改变梢隙中的流动特性是引起梢隙空化初生尺度效应的主要因素。本项目以典型水翼梢部与隔板间的流动及空化为研究对象，针对影响梢隙空化初生尺度效应的主要因素，包括水翼模型尺度、间隙宽度、来流条件及水质条件等开展研究工作。研究方法以试验为主，数值模拟为辅，梢隙流场测量采用2D-3C-PIV和TR-PIV技术，空化初生判别采用高速摄像结合噪声测量。重点研究梢隙中的复杂流动结构、梢隙涡特征，及其与空化初生的关系，探究梢隙空化初生机理，建立解决梢隙空化初生尺度效应的新思路。为解决工程上对梢隙空化初生预报奠定理论和试验基础。

梢隙空化是指液体流经叶片梢部与端壁之间时发生的空化现象，广泛存在于以液体为介质的水力机械中，梢隙空化会引发噪声增加，振动加剧和效率降低。本项目以典型水翼梢部与隔板间的流动及空化初生为研究对象，研究方法以试验为主，数值模拟为辅。采用高速摄像空化观测、2D-3C-PIV、内窥式PIV全湿流场测量及DES数值模拟方法，重点研究了梢隙中的复杂流动结构、梢隙涡特征，及其与空化初生的关系，建立解决梢隙空化初生尺度效应的新思路。在此基础上对近年来提出的水翼梢部悬臂式沟槽的流动结构开展了试验研究。.流场测量和数值模拟表明，水翼的梢隙流动主要梢部端面分离涡和吸力面导边下游的梢涡涡组成，分离涡与梢涡在下游融合形成所谓的梢泄涡。随着间隙宽度的减小不论是梢涡空化还是端面分离空化都先逐渐增强，但随着间隙的减小，梢涡空化逐渐消失，同时端面空化也迅速减弱。同时梢隙涡强度随着攻角的增加而变大，随着来流雷诺数增大，梢隙涡强度增大，端面片空化和梢泄涡空化都相应的增强。初生空化试验和数值模拟结果发现，梢隙空化初生数随间隙的减小先增大后减小，在某一间隙宽度最易产生空化。针对梢隙空化初生尺度效应，开展了不同来流雷诺数、不同间隙比，不同水翼攻角的系列试验研究和数值模拟，在获得这些因素影响梢隙空化初生的基本规律的基础上，通过量纲分析和回归分析方法，考虑间隙尺度分别提出大间隙和小间隙情况下的梢隙流动空化初生预报方法；梢隙涡和梢隙空化抑制研究表明，在水翼梢部加装悬臂格栅，尤其是在压力面凸出的格栅可以有效减弱梢隙涡的强度，从而抑制梢隙空化的发生。.由于民用和国防对准确预报梢隙空化初生均有强烈需求，本项目的研究成果可以用于在实际工程设计之初开展相关空化性能的评估工作，对梢隙空化的抑制研究为解决工程上对梢隙空化初生预报和抑制梢隙空化奠定理论和试验基础。.项目发表论文15篇，其中SCI论文9篇。