考虑旋转效应的转轮圆盘结构模态特性研究

Modal Characteristics of Runner Disk Structures Considering Rotation Effect

摘要:

水泵水轮机转轮在频繁启停和变工况运行过程中易发生复杂振动问题,其模态特性直接关系到机组运行安全与稳定性。为探究旋转效应对转轮圆盘结构模态特性的影响,基于声固耦合数值模拟方法,分析了圆盘结构在不同转速及流体环境下的模态特性。结果表明,空气中,旋转效应会使圆盘节径模态由驻波分裂为前行波和后行波模态,节径数越大,模态频率对转速变化越敏感;节圆模态基本不受旋转效应影响;节径节圆耦合模态发生频率分裂,耦合模态振型不完全被旋转效应覆盖,模态位移出现先减小后增大的变化。但由于旋转效应在低密度空气中等效作用不明显,圆盘频率随转速呈现小幅度变化,频率分裂现象不明显。由于圆盘结构与夹带水体之间的相对转动及旋转效应的作用,水中旋转圆盘的节径模态产生明显的频率分裂现象,其中转频为8Hz时,(2,0)模态后行波频率增加了8.11Hz,约占静止圆盘频率的5.9%;(3,0)模态前行波频率减小了9.92Hz,约占静止圆盘频率的2.9%。与空气中旋转圆盘不同,前行波与后行波频率随转速呈线性变化,其频率差随转速线性增大;频率下降率和附加质量系数也随转速呈线性变化。不同流体环境对旋转圆盘模态特性具有不同影响。海水中前行波与后行波频率分裂差小幅度增大,为16.33Hz。含气水中,声速为1000m/s,密度为980kg/m3,圆盘固有频率小幅度增加,前行波与后行波之间的频率分裂差与纯水中相比变化很小,为16.03Hz。研究结果可为水泵水轮机转轮等旋转圆盘状结构的振动特性分析与结构优化设计提供理论支撑。

Abstract:

During frequent start-stop operations and variable working conditions, pump-turbine runners are prone to complex vibration phenomena, and their modal characteristics are directly related to the operational safety and stability of the unit. To investigate the influence of rotation effects on the modal characteristics of disk structures, a fluid-structure interaction numerical approach based on acoustic- structure coupling was employed to analyze the modal behavior of a disk under different rotational speeds and fluid environment. The results showed that, in air, rotation effects caused nodal diameter modes of the disk to split from standing waves into forward and backward traveling wave modes. As the number of nodal diameters increased, the modal frequencies became more sensitive to variations in rotational speed. In contrast, nodal circle modes were scarcely affected by rotation effects. For coupled nodal diameter- nodal circle modes, frequency splitting occurred, and the coupled mode shapes were not completely dominated by rotation effects, resulting in modal displacements that firstly decreased and then increased. However, due to the weak equivalent effect of rotation in low-density air, the disk frequencies exhibited only slight variations with rotational speed, and the frequency splitting phenomenon was not pronounced. Due to the combined influence of relative rotation between the disk structure and the surrounding water as well as rotation effects, the nodal diameter modes of a rotating disk in water exhibited pronounced frequency splitting. Specifically, at a rotational frequency of 8 Hz, the backward traveling wave frequency of the (2,0) mode was increased by 8. 11 Hz, accounting for approximately 5. 9% of the stationary disk frequency. The forward traveling wave frequency of the ( 3, 0 ) mode was decreased by 9. 92 Hz, accounting for approximately 2. 9% of the stationary disk frequency. Unlike the rotating disk in the air, the frequencies of the forward and backward traveling waves varied linearly with rotational speed, and the frequency difference between them was increased linearly as the rotational speed increased. Meanwhile, both the frequency reduction rate and the added mass coefficient also showed linear growth with rotational speed. Different fluid environments exerted distinct influences on the modal characteristics of rotating disks. In seawater, the frequency splitting difference between the forward and backward traveling waves increased slightly to 16. 33 Hz. In aerated water, the speed of sound was 1 000 m/ s, and the density was 980 kg / m3, the natural frequencies of the disk was increased slightly, and the frequency splitting difference between the forward and backward waves was similar to that in pure water, at 16. 03 Hz. The findings can provide theoretical support for vibration characteristic analysis and structural optimization design of rotating disk-like structures such as pump-turbine runners.