不等频率换热管流弹失稳特性实验研究

Experimental Study on Fluid-Elastic Instability of the Heat-Exchange Tube with Unequal Frequencies

蒸汽发生器是压水堆核电站一、二回路的枢纽,承担将一回路冷却剂热量传递给二回路,产生高压蒸汽推动汽轮机组转动发电的重要作用.蒸汽发生器内部换热管呈现倒U形的结构,容易受到流弹失稳的作用而产生大幅度振动,导致管束的失效、破裂,造成经济损失.在其内部换热管的弯管段位置,弯管段通常利用防振条夹持的方式进行支撑,造成在夹持面的面内、面外方向换热管固有频率出现差异.伴随防振条的松弛与失效,会造成弯管段面内方向固有频率低于横流向.利用水洞实验方法,研究节径比1.33、转角三角形排列条件下,面内方向与面外方向固有频率比值分别为50%、60%、70%、80%的单根弹性管的流弹失稳特征,并与两方向固有频率一致换热管的实验结果进行对比.实验结果表明:等频率管在流速较低时呈现整体地小幅度振动,随着流速增大,顺流向、横流向均出现流弹失稳的现象;相较于等频率弹性管,不等频率弹性管也会出现由旋涡脱落主导的振动现象,但在旋涡脱落激振的过程中锁定区更长,振动幅度更大,且不等频率管仅在横流向出现流弹失稳现象,顺流向振幅微弱;在实验条件范围内可以认为,不等频率弹性管出现旋涡脱落诱发振动现象时的流速与弹性管在顺流向上的固有频率正相关;换热管的流弹失稳临界流速由换热管横流向的固有频率控制.

The steam generator is the center of the primary and secondary circuits of the pressurized water reac-tor(PWR)type of nuclear power plant.It plays an important role in transferring the heat of the primary circuit coolant to the secondary circuit to generate high-pressure steam that drives the rotation of the steam turbine unit to produce electricity.The heat-exchange tubes inside the steam generator which form an inverted U-shaped structure are easily affected by fluid-elastic instability and thus produce a large amount of vibration,which renders the tubes susceptible to wear.This could eventually lead to the failure of the steam generator,resulting in economic losses.The typical use of anti-vibration strips to support the elbow section of the internal heat-exchange tubes produces a difference in the natural frequency of tubes between the in-plane and out-of-plane directions of the clamping surface.As the anti-vibration strips relax and fail,the natural frequency in the in-plane direction of the elbow section will be lower than that in the transverse direction.In this paper,the water tunnel experiment method is used to study the fluid-elastic instability of a single flexible tube,with a pitch diameter ratio of 1.33 and a triangular arrangement.The natural fre-quency ratios of the in-plane direction and the out-of-plane direction are 50%,60%,70%,and 80%,respectively.The results of the experiment show that with a low flow velocity,the equal-frequency tube exhibits a small overall vibration.With an increase in the flow velocity,the phenomenon of fluid-elastic instability appears in both the streamwise and transverse directions.Compared with the flexible equal-frequency tube,the unequal-frequency tube produces a longer locking zone and a larger vibration amplitude in the process of vortex shedding excitation.The phenomenon of fluid-elastic instability occurs in the transverse direction only.Within the scope of the experimental conditions,whenever the vortex-induced vibration phenomenon occurs,the flow velocity is positively correlated to the natural frequency in the streamwise direction.In the transverse direction,the critical velocity of the fluid-elastic instability of the heat-exchange tube is controlled by the natural frequency.