Experiment And Numerical Simulation Of Heat Transfer In A Rectangular Channel With Partial Blockage

Plat-type fuel assembly is widely used in nuclear reactors due to its superior advantage such as compact structure,high heat transfer efficiency.Under the long-term neutron irradiation and high temperature environment,the fuel cladding maybe cause partial swelling,deformation and even bubble in the rectangular channel,which may cause the coolant channel blockage accident.This leads to a decrease of the coolant in the blocked channel,and then the temperature of the fuel element will go up.Severe blockage can cause that the coolant boil and generate large amounts of steam,which threatens the integrity of the fuel element.The research is carried out by combining experimental analysis,theoretical analysis and numerical simulation verification aiming at the phenomenon of partial blockage in the rectangular channel.To study the partial blockage of plate-type fuel assembly.Several types of test section were designed such as the test section with one bubble,the test section with longitudinal three bubbles,the test section with transverse three bubbles and the test section with no bubble.An experimental loop was built up to carry out the relevant experiment.Firstly,by processing and analyzing the wall temperature data near the bubble,it was obtained that as the Reynolds number of water flow increases,the wall temperature decreased and the average Nusselt number of the rectangular channel increased.The bubbles not only induce eddy currents but also destroy the thermal boundary layer,which enhance the flow heat transfer.In addition,the transverse three bubbles enhance the heat transfer.Compared with one bubble,longitudinal three bubble has a low speed backflow zone in the bubble gap,which weakens the flow heat transfer enhancement ability.The heat transfer enhancement effect is more obvious in the transverse three bubble condition with greater blockage.The current heat transfer model is inaccurate to predict the heat transfer in the rectangular channel with partial blockage,the maximum error is only 14.6% when the average Nusselt number calculated by SiderTate formula comparing with the average Nusselt number calculated by the experimental date.Considering the enhancement of the heat transfer in the flow channel by the size and spacing of the bubbles as well as the arrangement of the bubbles,the heat transfer formula was modified.The maximum error of prediction on convective heat transfer of the modified formula in case with one bubble was reduced from 35.4% to 19.8%,The maximum error of prediction on convective heat transfer of the modified formula in case with transverse three bubbles was reduced from 38.5% to21.3%,The maximum error of prediction on convective heat transfer of the modified formula in case with longitudinal three bubbles was reduced from47.8% to 8.7%.The software of STAR-CCM+ was used to carry out numerical simulation,and the results showed that the flow heat transfer effect of the bubble was enhanced due to the influence of flow field changes,and a lowtemperature area appeared around the bubble,the low temperature area develops from about double diameter of the bubble in front of the central point of the first bubble.Compared with the single bubble cases,the longitudinal three bubbles cases have a similar impact on the downstream area of the bubble.Compared with the experimental results,the stronger inlet segment effect of the numerical simulation makes the flow heat transfer around the bubble stronger and the wall temperature around the bubble lower.