流体机械内群液滴趋壁运动及其与边界层拟序结构的相互作用

The droplet-laden flows in fluid machinery always result in an evident deterioration in aerodynamic performance, but less attention has been paid on the underlying mechanism till now. This project is proposed to firstly investigate both the changes in shape and the spatial distribution of the secondary droplets of the consecutive drops impacting on a wet plate by using a high-speed camera and a Tomo-PIV system, to develop a statistical splashing model. Second, it is to explore the mechanism accounting for the preferential concentration of the secondary droplets near the wall and the interaction between the droplets and the coherent structures of the boundary layer under a zero pressure gradient, by the aid of the Tomo-PIV and the LES-VOF (Large Eddy Simulation-Volume of Fluid) numerical simulation method. It is also to study the modulation on coherent vortex structures by the dispersed droplets and the liquid film on the wall and the variations of the modes of energy transfer between eddies at different sizes. Third, it is to investigate the variations in velocity profile and shear stress distribution on the wall caused by the existence of the droplets and the liquid film on the wall, and to obtain a modified wall function considering the impact of droplets. Fourth, a computational model or approach to accurately simulate high Reynolds number flows based on the Euler-Lagrange method is to be developed by embedding the statistical splashing model and the modified wall function. Furthermore, the computational model is to be verified and improved by comparing the simulating results with the test characteristics of a fan with droplet-laden flows and its gas-liquid two-phase flow fields measured by the high-speed camera. This project will reveal the underlying mechanisms for the extra flow losses due to the droplet-laden flows in fluid machinery, and is beneficial for the improvement of the aerodynamic performance of fluid machinery with gas-liquid two-phase flows.

液滴群流动会导致流体机械气动性能明显降低，但对其内在机理的研究匮乏。本项目拟利用高速摄影和层析PIV，首先对串行液滴与湿润壁面碰撞的液滴形态变化和二次液滴的空间分布进行测量和数值分析，以建立相应的飞溅模型。其次，利用层析PIV和LES-VOF数值计算方法，对群液滴的趋壁行为及其与零压梯度平板边界层的拟序结构的相互作用进行探索，研究液滴的弥散及壁面液膜对边界层结构的调制作用和不同尺度涡之间的能量传递模式的变化。第三，研究受到液滴和液膜影响的平板表面切应力及速度分布的变化规律，提出考虑群液滴影响的壁面函数。第四，利用研究获得的壁面函数和液滴与壁面碰撞的飞溅模型，建立基于欧拉-拉格朗日方法的高雷诺数气液两相流高精度数值计算模型，并与喷雾风机的气动性能试验结果和高速摄影流场进行对比验证并加以改进。本项目将揭示气液两相流动导致流体机械气动损失增大的内在机理，为气液两相流体机械气动性能的提高奠定基础。

液滴的近壁流动会导致流体机械气动性能明显降低，但其内在机理的研究匮乏。本项目从对液滴与壁面碰撞过程及其对叶片边界层的影响进行了研究。.首先，实验和数值研究了液滴与带沉积液滴的壁面的碰撞过程。通过测量液滴碰撞力和对应的液滴形态变化，发现液滴碰撞带液滴壁面时，根据雷诺数的不同，碰撞可以分为粘性主导型和惯性主导型，两种碰撞的临界雷诺数在101到107之间。粘性区中无量纲碰撞力峰值随着雷诺数的增大而减小，惯性区中无量纲碰撞力峰值不随雷诺数变化。同时研究了水滴碰撞带液滴壁面时，瑞利射流引起的碰撞力峰值的变化规律，发现带细线状的飞溅现象的瑞利射流伴随着很大的衰减振荡力。.其次，数值研究了带液滴颗粒流动的平板边界层的演化过程。结果显示无论液滴是否掺混进流动壁面附近速度呈现条带状，高低速相间分布。将液滴颗粒掺混进流动后，分析了气相速度和液滴颗粒粒径对平板边界层拟序结构的影响规律。发现在均匀入射模型下，由于壁面边界层附近的粘性底层区域对颗粒的捕集作用，以及壁面涡结构的裹挟，液滴颗粒在低速条纹区富集较为显著。对液滴对平板边界层的湍流调制作用的研究表明，液滴颗粒的加入使得近壁面流向脉动减弱，而对主流区部分影响较为复杂。.再次，改造了原有风洞实验台，使风洞满足低湍流度测试要求。实验研究翼型表面压力分布、升阻力性能曲线和丝线形态。此外，分析了脱体圆柱对翼型边界层的流动控制效果，为流体机械的性能改进提供了基础。