A Study on Flow in a Rod Bundle Influenced Strongly by Buoyant Force

对棒束中的热羽进行了实验研究，以测量子通道之间的湍流热流，并提供有关缝隙处热羽行为的信息。用Ar离子激光器同时测量速度和温度，而不干扰流场。制作了光纤LDV，并将其插入外表面带有镍铬薄膜加热器的玻璃管中，激光通过该加热器进入流体中。速度信息由碳化硅颗粒散射光获得，温度由罗丹明B溶液的荧光变化估计。在Re=50-400和Gr_&lt；DELTAT&GT；=300-1300的条件下，测量了不同功率偏斜条件下缝隙中心的湍流热流密度。实验结果表明，实验得到的湍流热流密度小于预测值。因此，对于钠冷反应堆的应用，预计湍流热流可能会小得可以忽略不计。湍流热流密度随Re和Gr&lt；DELTAT&Gt；实验观察到了负湍流热流，即逆流换热。温度和速度随时间的变化表明，速度和温度之间的相位差产生了反流。位相差被认为是湍流热通量小于预测值的原因。本研究中使用的速度和温度同时测量系统运行良好，不仅在棒束研究中显示出它的能力，而且在一般热流体研究中也显示出它的能力。

The experimental study of the thermal plumes in a rod bundle has been performed to measure the turbulent heat flux between subchannels and to provide information on the thermal plume behavior at the gap. An Ar-ion laser was used to measure velocity and temperature simultaneously without disturbing the flow field. A fiber LDV was made and inserted into a glass pipe with a Nichrome film heater on its outer surface, through which the laser beam was introduce into the fluid. The velocity information was obtained from the light scattered from SiC scattering particles and the temperature was estimated from the change of fluorescence of Rhodamine B solution. The turbulent heat flux at the gap center was measured under the conditions of Re=50-400 and Gr_<DELTAT>=300-1300 for various power skews. The experimental results show that the turbulent heat flux obtained experimentally was for smaller than the predicted values. Therefore, for application to the sodium cooled reactors it was expected that the turbulent heat flux might be negligibly small. The turbulent heat flux increased with Re and Gr_<DELTAT>. Negative turbulent heat flux, that is, counterflux heat transfer was observed. The change of temperature and velocity with time showed that the phase difference between velocity and temperature brought the counterflux. The phase difference is considered to be a reason for smaller turbulent heat flux than the predicted values. The simultaneous measurement system for velocity and temperature used in the present study operated quite well and is expected to display its ability not only in rod bundles but also in general thermal fluid study.