Study Of Temperature Distribution In Rod Bundle By Using Laser Diagnostic Technology

The thermal hydraulic characteristics of reactor fuel assembly play an important role in the safe operation of nuclear reactor.The spacer grid of fuel assembly can induce swirling flow,increase lateral flow and the fluid mixing in sub-channels.Thus the temperature distribution and mixing characteristics in rod bundles with spacer grid were investigated in the paper,by using the Laser Induced Fluorescence(LIF)technique.That is helpful to improve the thermal hydraulic design and safety performance in reactor system.In order to meet the visual experiment requirements,an experimental facility was designed and conducted.Based on the real fuel assembly of pressurized water reactor,the test section was assembled.The matching index of refraction is adopted,so stainless steel rods were chosen as heating rod,and the other rods were FEP tubes.The laser generator and high speed camera were moved accurately by three-dimensional lifting platform to realize the construction of laser plane and the collection of experimental images.A post-processing method was proposed to eliminate deviation in visual angle and size of images.Based on the spatial transformation,the original images were matched by selecting eigenvalues.Using calibration method of point to point,temperature fields were reconstituted in different section and height downstream of the spacer grid.The stability and temperature sensitivity of two dyes were analyzed,whose spectral characteristics were obtained by spectrograph.Variation of fluorescence intensity with temperature and average temperature sensitivity were obtained.And dyes concentration were confirmed in the experiment.To guarantee sufficient excitation light and prevent light bleaching,optimal laser power density is 6800 W/m~2 to 8300 W/m~2.Temperature distribution characteristics in different heating forms and sections were studied in rod bundle by LIF technique.Temperature distribution clouds were reconstructed in stationary flow and pulsating flow,and the data were taken in different locations of the rods.The qualitative and quantitative results show that the temperature cloud graph can directly display the temperature distribution trend.And the mixing effect can lead to temperature uniformly near the spacer grid.Along with the increase of height,the temperature of the channel near the heating rod is significantly higher than that of other places.Comparing the temperature distribution of stationary flow and pulsating flow,the temperature in the sub-channel is lower than the gap.The temperature distribution of fluctuating flow is periodic.The mixing characteristics were quantitatively evaluated by the axial temperature difference and temperature fluctuation.The influences of the mixing action,flow fluctuation and heating effect on temperature fluctuation were analyzed.The mixing characteristics of rod bundle were displayed directly by mixing of cold and hot fluid.In conclusion,laser induced fluorescence technique can be used to measure whole field of temperature distribution in the rod bundle.Based on the technique,the temperature distribution and mixing effect can be analyzed qualitatively and quantitatively,which is helpful to provide verification and validation for numerical calculation,as well as the optimization design of spacer grid.