EXPERIMENTAL INVESTIGATION OF CHARACTERISTIC FREQUENCY IN UNSTEADY HYDRAULIC BEHAVIOUR OF A LARGE HYDRAULIC TURBINE

The features of unsteady flow such as pressure variation and fluctuation in a large hydraulic turbine usually lead to the instability of operation.This article reports the recent in site investigation concerning the characteristic frequencies in pressure fluctuation,shaft torsional oscillation and structural vibration of a prototype 700 MW Francis turbine unit.The investigation was carried out for a wide load range of 200 MW-700 MW in the condition of water head 57 m-90 m.An extensive analysis of both time-history and frequency data of these unsteady hydraulic behaviours was conducted.It was observed that the pressure fluctuation in a draft tube is stronger than that in upstream flow passage.The low frequency with about one third of rotation frequency is dominative for the pressure fluctuation in part load range.Also the unsteady features of vibration of head cover and torsional oscillation of shaft exhibited the similar features.Numerical analysis showed that the vibration and oscillation are caused by vortex rope in the draft tube.In addition,a strong vibration with special characteristic frequency was observed for the head cover in middle load range.The pressure fluctuation in the draft tube with the same frequency was also recorded.Because this special vibration has appeared in the designed normal running condition,it should be avoided by carefully allocating power load in the future operation.  

利用微动法快速探测岩溶塌陷区覆盖层厚度

为了能在大尺度范围内获得岩溶塌陷典型调查区覆盖层的厚度，采用微动法对柳州市柳城县东泉公社幅岩溶塌陷重点调查区进行了野外观测，并对调查区内已知的56处钻孔进行微动观测，提取特征频率，结合钻孔资料得出覆盖层厚度与特征频率的函数关系满足h =62.5/f。利用此函数关系，结合岩溶塌陷调查区野外微动实测数据提取的特征频率反演出覆盖层厚度。结果表明：该方法的尝试初步见效，经济快捷、适应性强、值得推广，特别适用于大尺度范围内调查岩溶塌陷区域覆盖层厚度分布，结合研究区的地质信息，可以在区域内对岩溶塌陷易发区做一个简单的评价。